Interactive Taxonomy of MR Imaging Sequences

An Interactive Taxonomy of MR Imaging Sequences
RSNA, 2006

http://radiographics.rsna.org/content/26/6/e24/suppl/DC1

Steady state

Steady-State MR Imaging Sequences: Physics, Classification, and Clinical Applications
Govind B. Chavhan, MD, DNB, Paul S. Babyn, MD, Bhavin G. Jankharia, MD, Hai-Ling M. Cheng, PhD and Manohar M. Shroff, MD

http://radiographics.rsna.org/content/28/4/1147.full?sid=f59ba1cb-6f2f-4609-aa0b-52f4d569e8e3

Diffusion Imaging

http://radiology.rsna.org/content/217/2/331.full?sid=0852fa87-4270-43d3-b8af-eb1026644ee2
State of the Art
Diffusion-weighted MR Imaging of the Brain
Pamela W. Schausiefer, MD, P. Ellen Grant, MD and R. Gilberto Gonzalez, MD, PhD

http://www.ajnr.org/cgi/content/full/29/4/632?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=diffusion&andorexactfulltext=and&searchid=1&FIRSTINDEX=0&sortspec=relevance&resourcetype=HWCIT

Diffusion Tensor MR Imaging and Fiber Tractography: Theoretic Underpinnings

P. Mukherjeea, J.I. Bermana, S.W. Chunga, C.P. Hessa and R.G. Henrya
a From the Department of Radiology, University of California, San Francisco, San Francisco, Calif

http://coolmristuff.wordpress.com/

http://coolmristuff.wordpress.com/

ARRS GoldMiner

RRS GoldMiner (http://GoldMiner.arrs.org/) is another radiology-centric search engine whose niche is providing the radiologist with direct access to images selected only from peer-reviewed journals. GoldMiner was created by Charles E. Kahn, Jr, MD, of the Medical College of Wisconsin (Milwaukee, Wis) and is offered as a free service of the American Roentgen Ray Society (4). Its collection was recently markedly expanded to more than 170 000 images and over 225 journals by partnering with BioMed Central, a free database of peer-reviewed scientific articles. Like Yottalook, GoldMiner performs a concept-based search that expands the user's search query beyond simple word-matching. The GoldMiner service can be useful in assembling a presentation or in scanning for look-alike images when struggling with a problem case. The Web site features a clean design, with each row of the display containing a thumbnail image, as well as the source article's title and journal link (Fig 4), though it would be helpful to be able to view more images per page, as in the Yottalook thumbnails-only view. A click on the thumbnail image directs you to the full-resolution image on the journal's Web site and the full text of the article it came from. A PowerPoint (Microsoft, Redmond, Wash) slide containing both the image and appropriate reference notation can often be downloaded through these links. This online access is possible because the journals typically make their content fully available on the Web within 12–24 months after publication. One can filter the results by modality, patient age, and patient sex from a set of pull-down tabs that also indicate how many image results are available in each category.
A newer offering from the same site is GoldMiner Global. Alhough still in a developmental (beta) status, this multilingual interface enables users to submit search terms in any of 10 languages to the GoldMiner image database

Google ,Yotta

By automatically stripping out results unrelated to medical imaging, Yottalook vastly increases the utility of the various specialized search services that you may already be familiar with in Google. For example, Google Book Search allows you to search within the actual text of an enormous number of hard-copy books, both recent and out of print. The digitized copy of the book appears onscreen opened to the page of interest, with your search terms already marked in yellow highlighting. The preview mode allows you to jump between chapters or flip page by page through surprisingly extensive sections of each book. Yottalook optimizes this already impressive service by listing only radiology texts in the search results. A search for “glioblastoma” is thus reduced from an overwhelming 1306 books in the native Google Book Search, including numerous patient survival guides and basic science texts, to a more manageable 168 medical imaging texts in Yottalook Books (Fig 3). Similarly, when one searches in the Yottalook customization of Google Scholar, one's results are already prefiltered to include only the imaging literature.
Yottalinks” are special sites that the Yottalook editors have designated as particularly high yield for particular radiology search topics. For example, if one queries “renal failure,” a Yottalink to an online glomerular filtration rate (GFR) calculator appears in a separate section of the results page.

There are numerous additional features to be discovered on the Web site. After using it just a few times, I believe that many radiologists will opt to make Yottalook their first-line search rather than Google or other general search engines.

There are several ways of making access to Yottalook's search results even easier. One can make Yottalook the Web browser's default homepage, add a Yottalook button to the Google search toolbar, or incorporate it into one's own customized Google “mash-up” homepage (iGoogle). Yottalook will be showing up in other contexts as well, since it is available for developers to include in their own Web sites and has already been integrated as the search engine of the RSNA Web site (http://www.rsna.org).

Yottalook

Screenshot from Yottalook Web search. Note the numerous radiology-specific filtering options to enhance the relevance of a search query.
Image searching is robustly implemented in Yottalook, with over 500 000 entries indexed. A detailed listings view displays thumbnail images with adjacent image captions and online journal links. A convenient thumbnails-only view squeezes up to 50 images on a single page (Fig 2). As the cursor passes over the various images, a pop-up text box displays the appropriate caption. By clicking on the “Bookmark” button, you can store the image collection in your browser's favorites list or store and share it online through a number of different social bookmarking sites.

NETASSETS

The massive collections of radiology links
on portal sites such as AuntMinnie
.com (http://www.auntminnie.com)
and RadiologyEducation.com (http:
//radiologyeducation.com) are a testament
to the enthusiasm with which
radiologists have embraced the Internet
as an “e-learning” platform (1).
The ideal is clear: fast relevant answers
from reliable sources on beautifully
illustrated Web sites that are
easy to navigate. However, when one
is hunting for a quick answer during a
hectic day or a lonely overnight shift,
the deficiencies of the Web for pointof-
care decision support become glaring.
Although general search engines
have improved, one still often encounters
too much irrelevant information
for the radiologist, including advertisements,
patient-oriented information,
and outdated or unreliable material.
Luckily, we have now entered the
age of the “radiology-centric” search.

Yottalook: “Y’oughta Look”
Fusing proprietary technologies with a
highly customized Google search engine,
the developers of Yottalook (http:
//www.yottalook.com) at iVirtuoso (Washington,
DC) have optimized the radiology
search experience by incorporating
a semantic, or concept-based, approach
(2). A user’s query is analyzed with
reference to various medical ontologic
taxonomies, including an enhanced version
of Radlex, the radiology lexicon developed
with sponsorship from the Radiological
Society of North America
(RSNA). The search engine can consequently
expand the query to all possible
relevant terms and index the results in a
way that enables intelligent filtering. For
example, if one were to enter “PE” into
the Yottalook search box, the site understands
that this could refer to pulmonary
embolism, preeclampsia, or pericardial
effusion and allows one to
choose appropriately. The semantic
search engine cleverly recognizes synonyms
such as gallstones and cholelithiasis
and utilizes hierarchic relation between medical concepts to return the most comprehensive and relevant search results. For example, when one enters the disease category “phakomatosis” in Yottalook, the related terms tuberous sclerosis, neurofibromatosis, and ataxia-telangiectasia are offered as additional search options

Radiology-specific filters are another key strategy by which Yottalook enhances the search experience. By restricting the scope to such relevant categories as teaching files, journal articles, continuing medical education materials or to specific modalities such as computed tomography (CT) or magnetic resonance (MR) imaging, one can much more quickly get to the right answer (Fig 1). The “Anatomy” filter will preferentially return search queries from dedicated online atlases such as Gray's Anatomy and journal articles likely to have high-yield anatomic content, such as one example titled “The Cystic Duct: Normal Anatomy and Disease Processes” (3) from an online education exhibit in RadioGraphics. Combining the various filters is possible in order to make even more specific searches practical. For example, if one were trying to update one's practice's MR imaging protocol for carotid dissection, a Yottalook search could be performed with the “MRI” and “Protocol” filters chosen. Top results would include the links to various institutions' Web sites that have made their protocols freely available online (eg, Massachusetts General Hospital Neuroradiology—MRI Protocols, http://www.mghneuroradiology.org/NewFiles/mrip.html). If one were to choose the filter for a specific hardware manufacturer, only journal articles that listed the appropriate equipment in their methods section would be listed.

high-density diffuse optical tomography (DOT)

Radiologists Use Light To Scan The Inner Workings Of The Brain
December 1, 2007 — Radiologists have developed a new device to understand brain activity. It is a collection of fiber optic cables attached to a flexible cap placed atop the head. The cables send near-infrared light through the skull and into the brain, where it is diffused or scattered before it is collected by receiver cables. The device is able to interpret the light to measure blood circulation and the amount of oxygen in that blood, which helps explain brain activity.

When doctors want to find out what's going on inside a baby's brain it usually requires, noisy or dangerous equipment and babies sitting completely still.
But, new technology is now giving researchers a fascinating look inside an infant’s brain in a much easier way.
Radiologists are using a new technique to see what parts of a baby’s brain are working during any given task. Their method is baby-friendly with no exposure to radiation or loud machines.
“It has a more wearable cap so it can be placed in infants heads while they sit in their parents lap,” Joseph Culver, Ph.D., Washington University Medical School said. Culver and his colleagues improved a brain imaging technique called high-density diffuse optical tomography.
It measures how much blood and oxygen are in the brain.
“It’s similar to taking a flashlight and putting it on one side of your hand and looking at the light come through your hand so the light has traveled through your hand and the light that you detect on the other side tells you something about what’s inside your hand,” Dr. Culver said. Fiber optic cables on the cap shine light on the baby’s brain. The light scatters revealing blood flow related to brain activity in a 3D tomographic image. You can see it in action, when a patient watches a flickering light; a similar rotating pattern shows up in the brain’s blood flow.
“There’s an increase in blood flow to that area and that allows us to map that neuron activity,” Dr. Culver said.
Future uses for the cap include researching brain development in the tiniest of babies … or monitoring a baby’s brain during surgery.
BACKGROUND: Researchers have developed a new brain imaging technique for infants called high-density diffuse optical tomography which helps them to study the developing infant brain. This should help treat infant brain injuries by being able to monitor them in their incubators, and help scientists learn important basics about developing brains. The new scanner is quieter, and portable because it is much smaller – about the size of a small refrigerator – than typical MRI or CT scan machines. The developers are working to make the unit even smaller, about the size of a microwave.
THE PROBLEM: Scientists have been able to study brain scans of infants while they are asleep or sedated using functional MRI (magnetic resonance imaging). Ideally researchers would like to scan their brains while sitting on a parent’s lap or interacting with their environment. fMRI requires the patient to be inserted into a tightly confined passage through a large, noisy magnet; most infants find it upsetting and difficult to sit still in that environment. In the same way CT scans involve large, loud equipment, and also expose patients to levels of x-rays considered unsafe for infants.
HOW IT WORKS: The high-density diffuse optical tomography (DOT) uses harmless light from the near-infrared light spectrum. Unlike X-rays or ultrasound, near-infrared light passes through bone easily, so scientists can use the diffusing light to determine blood flow and oxygenation in the blood vessels of the brain. When these characteristics increase, it indicates that the area of the brain they are scanning is contributing to a mental task. To scan a patient, scientists attach a flexible cap that covers the exterior of the head above the region of interest. Inside the cap are fiber optic cables. Some of those cables shine light on the head and by determining the way the light is scattered, researchers can learn more about brain activity. Light passes out of one fiber optic cable, goes through the tissue, and is received by another cable. Based on its interpretation of the diffusion data, the machine creates a 3D image based on whether the red blood cells have lots of oxygen or less oxygen to determine brain activity.
WHAT IS fMRI: Magnetic resonance imaging uses radio waves and a strong magnetic field to take clear and detailed pictures of internal organs and tissues. fMRI uses this technology to identify regions of the brain where blood vessels are expanding, chemical changes are taking place, or extra oxygen is being delivered. These are indications that a particular part of the brain is processing information and giving commands to the body. As a patient performs a particular task, the metabolism will increase in the brain area responsible for that task, changing the signal in the MRI image. So by performing specific tasks that correspond to different functions, scientists can locate the part of the brain that governs that function.
The American Association of Physicists in Medicine and The Optical Society of America contributed to the information contained in the TV portion of this report.

Nanoparticles Cross Blood-Brain Barrier To Enable 'Brain Tumor Painting

Nanoparticles Cross Blood-Brain Barrier To Enable 'Brain Tumor Painting'
ScienceDaily (Aug. 4, 2009) — Brain cancer is among the deadliest of cancers. It's also one of the hardest to treat. Imaging results are often imprecise because brain cancers are extremely invasive. Surgeons must saw through the skull and safely remove as much of the tumor as they can. Then doctors use radiation or chemotherapy to destroy cancerous cells in the surrounding tissue.

Researchers at the University of Washington have been able to illuminate brain tumors by injecting fluorescent nanoparticles into the bloodstream that safely cross the blood-brain barrier -- an almost impenetrable barrier that protects the brain from infection. The nanoparticles remained in mouse tumors for up to five days and did not show any evidence of damaging the blood-brain barrier, according to results published this week in the journal Cancer Research.
Results showed the nanoparticles improved the contrast in both MRI and optical imaging, which is used during surgery.
"Brain cancers are very invasive, different from the other cancers. They will invade the surrounding tissue and there is no clear boundary between the tumor tissue and the normal brain tissue," said lead author Miqin Zhang, a UW professor of materials science and engineering.
Being unable to distinguish a boundary complicates the surgery. Severe cognitive problems are a common side effect.
"If we can inject these nanoparticles with infrared dye, they will increase the contrast between the tumor tissue and the normal tissue," Zhang said. "So during the surgery, the surgeons can see the boundary more precisely.
"We call it 'brain tumor illumination or brain tumor painting,'" she said. "The tumor will light up."
Nano-imaging could also help with early cancer detection, Zhang said. Current imaging techniques have a maximum resolution of 1 millimeter (1/25 of an inch). Nanoparticles could improve the resolution by a factor of 10 or more, allowing detection of smaller tumors and earlier treatment.
Until now, no nanoparticle used for imaging has been able to cross the blood-brain barrier and specifically bind to brain-tumor cells. With current techniques doctors inject dyes into the body and use drugs to temporarily open the blood-brain barrier, risking infection of the brain.
The UW team surmounted this challenge by building a nanoparticle that remains small in wet conditions. The particle was about 33 nanometers in diameter when wet, about a third the size of similar particles used in other parts of the body.
Crossing the blood-brain barrier depends on the size of the particle, its lipid, or fat, content, and the electric charge on the particle. Zhang and colleagues built a particle that can pass through the barrier and reach tumors. To specifically target tumor cells they used chlorotoxin, a small peptide isolated from scorpion venom that many groups, including Zhang's, are exploring for its tumor-targeting abilities. On the nanoparticle's surface Zhang placed a small fluorescent molecule for optical imaging, and binding sites that could be used for attaching other molecules.
Future research will evaluate this nanoparticle's potential for treating tumors, Zhang said. She and colleagues already showed that chlorotoxin combined with nanoparticles dramatically slows tumors' spread. They will see whether that ability could extend to brain cancer, the most common solid tumor to affect children.
Merely improving imaging, however, would improve patient outcomes.
"Precise imaging of brain tumors is phenomenally important. We know that patient survival for brain tumors is directly related to the amount of tumor that you can resect," said co-author Richard Ellenbogen, professor and chair of neurological surgery at the UW School of Medicine. "This is the next generation of cancer imaging," he said. "The last generation was CT, this generation was MRI, and this is the next generation of advances."
Other co-authors are Omid Veiseh, Conroy Sun, Chen Fang, Narayan Bhattarai, Jonathan Gunn of the UW's department of materials science and engineering; Forrest Kievit and Kim Du of UW bioengineering; Donghoon Lee of UW radiology; Barbara Pullar of the Fred Hutchinson Cancer Research Center; and Jim Olson of the Fred Hutchinson Cancer Research Center and Seattle Children's Hospital.
The research was funded by the National Institutes of Health, the Jordyn Dukelow Memorial Fund and the Seattle Children's Hospital Brain Tumor Research Endowment.
http://www.sciencedaily.com/images/2009/08/090803185714.jpg

Relaxation

As we have seen, perturbation of the magnetization by application of a short RF pulse tips it away from the longitudinal axis and generates a transverse component. If this magnetization is allowed to precess freely, there is a regrowth of the longitudinal magnetization called longitudinal relaxation, and destruction of the transverse magnetization called transverse relaxation. Relaxation is described by exponential time constants: T1 for longitudinal and T2 for transverse relaxation. Exponential processes are those whose rate of change depends on how far they have left to go; the closer they get to their final value, the more slowly they approach it. T1 is defined as the time taken for the longitudinal magnetization to relax from 0 to 63% of the equilibrium magnitude. T2, or the transverse relaxation time constant, is the measure of the time that the transverse magnetization takes to relax (decay) to 37% of its initial magnitude. T2 decay occurs because individual spins (usually referred to as isochromats) rotate at slightly different rates due to their chemical environment, and eventually they get out of sync and begin to point in random directions and cancel each other. This process is referred to as "dephasing," because the spins acquire different phases in the range 0 to 360°. In practice, field inhomogeneities can be another source of transverse relaxation, other than the chemical environment. The dephasing due to this process is accounted for by another time constant, T2'. The total transverse relaxation time constant (T2*) then is the reciprocal sum of T2 and T2' and is given by:


Both the longitudinal and transverse relaxations are modeled by exponential functions (Fig. 5-3) given by the Bloch equations, which are stated without further justification:


Mz
longitudinal magnetization
Mz0
longitudinal magnetization at t = 0+, i.e., immediately after the RF pulse
M0
equilibrium magnetization
Mxy
transverse magnetization.
Different tissues have different relaxation time constants, and these form the basis of some of the contrast mechanisms in MRI.

Magnetization1

So, in order to rotate M away from the z direction:
The magnetic field of the RF pulse must lie somewhere in the x-y plane (this is the plane perpenicular to M)
The B1 field must stay synchronous with the magnetization by rotating at the same (Larmor) frequency as M (Fig. 5-2A).3 Otherwise, the effect of the B1 field would average out and produce no net rotation.
In this case, as B1 tips M off axis, the motion of M relative to B1 is characterized by a simple rotation around the B1 axis (Fig. 5-2B). At the Larmor frequency, the motion of M describes a cone-shaped "precession" around B1 so that M lies within the y-z plane. The precessional frequency of M around B1 is γB1 and the angle α that M rotates through is given by

where t is the duration for which the RF field of strength B1 is applied (see Fig. 5-2B). α is generally referred to as the "flip angle."

Magnetization

From a practical point of view, we need only concern ourselves with the "net magnetization vector," M (Fig. 5-1), which is the vector sum of all the individual spins (small arrows) aligned parallel to the main magnetic field, B0. At equilibrium, M is steady along the axis of B0 (conventionally the "z" axis) and experiences no tendency to rotate off axis; i.e., there is no twisting effect or "torque." This is because the main field and the magnetization vector are parallel; in order for there to be a torque, the magnetization must be at least partially perpendicular to the main field. So, when aligned along z, the magnetization is essentially static and does not generate a detectable signal in the presence of the static external field. In order to be detectable, the magnetization must have a net component which lies within the transverse (x-y) plane. If rotated away from z, the magnetization will experience a torque, proportional to its component in the x-y plane, and will precess coherently about B0 and be detectable in the x-y plane by a suitable probe. The Larmor frequency is the resonant frequency with which the spins rotate in the presence of a magnetic field; it is specific for each nuclear species and, for any given nucleus, it increases proportionately with the strength of the applied external magnetic field. At 1.5 Tesla, the Larmor frequency for hydrogen nuclei (protons) is approximately 63.75 megahertz (MHz); at 3.0 Tesla, it is approximately 127.8 MHz. Recall that a changing magnetic field always implies a changing electric field, and vice versa. The changing electric field constitutes a sinusoidally alternating voltage in the transverse plane, and this is detectable by a probe. How might M be rotated partially or fully into the transverse plane? The rotation of the magnetization vector, M, is accomplished by the magnetic field of an RF pulse, generally designated the B1 field. Recall that radio waves are part of the spectrum of electromagnetic radiation, and that all electromagnetic waves are composed of electric and magnetic fields perpendicular to each other, and oscillating sinusoidally at a specific frequency. The magnetic field of the RF wave, B1, can be thought of as adding to the main field, B0, but in a perpendicular direction. Now we have M and B1 perpendicular to each other, so B1 exerts a torque on M. The direction of the torque is at all times perpendicular both to B1 and M, and is defined by a "vector product" as detailed below (don't worry if you're not familiar with the vector product).

ARTIFACTS IN MR IMAGING

MRI Organisations

Related Organizations

SMRT, Section for Magnetic Resonance Technologists

ISMRM, The British Chapter

ISMRM, The German Chapter

Academy of Molecular Imaging

American Association of Physicists in Medicine

American Board of Medical Physics

American College of Medical Physics (ACMP)

American College of Radiology

American Heart Association

American Liver Society

American Medical Association (AMA)

American Registry of MRI Technologists (ARMRIT)

American Registry of Radiologic Technologists

American Roentgen Ray Society

American Society of Radiologic Technologists

Association of Managers of Magnetic Resonance Laboratories

British Association of MR Radiographers (BAMRR)

Canadian Association of Medical Radiation Technologists

European Chinese Society for Clinical Magnetic Resonance

European Congress of Radiology (ECR)

European Society of Magnetic Resonance in Medicine and Biology

Health Professions Network

Intersocietal Commission for the Accreditation of Magnetic Resonance Laboratories (ICAMRL)

International Consortium for Medical Imaging Technology

International Society of Magnetic Resonance (ISMAR)

International Society of Radiographers and Radiological Technicians (ISRRT)

Japan Society of Magnetic Resonance in Medicine

Korean Society of Magnetic Resonance in Medicine (KSMRM)

Magnetic Resonance Managers Society (MRMS)

NMR Information Server

Organization for Human Brain Mapping

Radiological Society of North America

Society for Cardiovascular Magnetic Resonance

Society of Interventional Radiology

Society of Non-Invasive Imaging in Drug Development

Sociedad Mexicana de Radiología e Imagen (SMRI)

Swiss Radiology Society

www.brainmapping.org

MR Physics

http://www.simplyphysics.com/MAIN.HTM


http://www.simplyphysics.com/page2_1.html

MRI GLOSSARY

AC - Alternating Current is a continuously changing flow of electrons that alternates its polarity at a periodic rate.

ACQUISITION - the process of measuring and storing image data.

ACQUISITION MATRIX - the total number of independent data samples in the frequency (f) and phase (f) directions.

ACQUISITION TIME - the period of time required to collect the image data. This time does not include the time necessary to reconstruct the image. ADC - analog-to-digital converter

ALIASING (WRAP AROUND ARTIFACT) - the phenomenon resulting from digitizing fewer than two samples per period in a periodic function. Aliasing can occur in MR imaging whenever the area of anatomy extends beyond the field of view. These areas extending beyond the field of view boundaries are aliased back into the image to appear at artifactual locations.

ALTERNATING CURRENT (AC) - a current that continuously changes in magnitude and direction. In the US the current changes at a frequency of 60 Hz.

AMPLITUDE - the signal height. The greater the amplitude of the signal, the larger the number of protons in the image and the brighter it will appear.

ANALOG - being continuous, or having a continuous range of values.

ANALOG-TO-DIGITAL CONVERTER (ADC) - a system that receives analog input data and produces digital values at its output. Used by the MRI scanner to convert the received signal into a format more compatible with the computer systems.

ANTENNA - a device that enables the sending and/or receiving of electromagnetic waves. See also Transmitter, Receiver Coils and Surface Coils.

ARCHIVING - the storage of image and patient data for future retrieval.

ARRAY PROCESSOR - a dedicated computer system used to perform Fourier transformations to accelerate the processing of the received numerical data relative to the MR imaging process.

AVERAGING - see Signal Averaging.

AXIAL - a plane, slice or section made by cutting the body or part of it at right angles to the long axis. If the body or part is upright, the cut would be parallel to the horizon. B or Bo - a conventional symbol for the constant magnetic field produced by the large magnet in the MR scanner. B1 - the conventional symbol used for identifying the radio frequency (RF) magnetic field.

BANDWIDTH (BW) - an all-inclusive term referring to the preselected band or range of frequencies which can govern both slice select and signal sampling.

CHEMICAL SHIFT - a variation in the nominal Larmor frequency for a particular isotope within the imaging volume. The amount of shift introduced is directly proportional to the strength of the magnetic field, and is specified in parts per million (ppm) of the resonant frequency.

CINE - a series of rapidly recorded multiple images taken at sequential cycles of time and displayed on a monitor in a dynamic movie display format. This technique can be used to show true range of motion studies of joints and parts of the spine.

CIRCLE OF WILLIS - a large network of interconnecting blood vessels at the base of the brain that when visualized resembles a circle.

CLAUSTROPHOBIA - a psychological reaction to being confined in a relatively small area.

CNR - contrast-to-noise ratio.

COHERENCE - the act of maintaining a constant phase relationship between oscillating waves or rotating objects.

CONTRAST - the relative difference of signal intensities in two adjacent regions of an image. Image contrast is heavily dependent on the chosen imaging technique (i.e., TE, TR, TI), and is associated with such parameters as proton density and T1 or T2 relaxation times.

CONTRAST REVERSAL - an image phenomenon where the darks become bright, and the brights become dark. This is usually most prevalent in sequences utilizing an extended TR.

CONTRAST-TO-NOISE RATIO (CNR) - the ratio of signal intensity differences between two regions, scaled to image noise. Improving CNR increases perception of the distinct differences between two clinical areas of interest.

CORONAL - a plane, slice or section made by cutting across the body from side to side and therefore parallel to the coronal suture of the skull.

CROSSTALK - an artifact introduced into images by interference between adjacent slices of a scan. This artifact can be eliminated by limiting the minimum spacing between slices.

CRYOGEN - a cooling agent, typically liquid helium or liquid nitrogen used to reduce the temperature of the magnet windings in a superconducting magnet. dB/dt - The rate of change of the magnetic field. This shows the ratio between the amount of change in amplitude of the magnetic field (dB) and the time it takes to make that change (dt). The value of dB/dt is measured in Tesla per second (T/s). DC - direct current.

DEPHASING - the fanning out or loss of phase coherence of signals within the transverse plane. See also T2.

DIPOLE - a magnetic field characterized by its own north and south magnetic poles separated by a finite distance.

DIRECT CURRENT (DC) - a continuous current that flows in only one direction.

DISPLAY MATRIX - the total number of pixels in the selected matrix, which is described by the product of its phase and frequency axis.

DOMAIN THEORY - a theory of magnetism which assumes that groups of atoms produced by movement of electrons align themselves in groups called"domains" in magnetic materials.

DTPA - Diethylenetriaminepentaacetic acid - Gadolinium chelating (chemical bonding) agent that solves the problem of toxicity

ECHO PLANAR IMAGING (EPI) - the utilization of rapid gradient reversal pulses of the readout gradient resulting in a series of gradient echo signals to reduce fast dephasing or signal loss.

ECHO TIME - see TE.

ECHO TRAIN - a series of 180° RF rephasing pulses and their corresponding echoes for a Fast Spin Echo (FSE) pulse sequence.

ETL - Echo Train Length

EDDY CURRENT - an induced spurious electrical current produced by time-varying magnetic fields. Eddy currents can cause artifacts in images and may seriously degrade overall magnet performance.

ELECTROMAGNET - a type of magnet that utilizes coils of wire, typically wound on an iron core, so that as current flows through the coil it becomes magnetized. See also Resistive Magnet, Superconducting Magnet.

ELECTRON SPIN RESONANCE (ESR) - the response of electrons to electromagnetic radiation and magnetic fields at discrete frequencies. EPI - echo planar imaging. See also Echo Planar Imaging.

EQUILIBRIUM - a state of balance that exists between two opposing forces or divergent forms of influence.

EXCITATION - delivering (inducing, transferring) energy into the "spinning" nuclei via radio-frequency pulse(s), which puts the nuclei into a higher energy state. By producing a net transverse magnetization an MRI system can observe a response from the excited system.

FARADAY SHIELD (Faraday Cage) - an electrically conductive screen or shield that reduces or eliminates interference between outside radio waves and those from the MRI unit.

FAST SCANNING - a specialized technique usually associated with short TR, reduced flip angle and repeated 180° rephasing pulses.

FAST SPIN ECHO (FSE) - a fast spin echo pulse sequence characterized by a series of rapidly applied 180° rephasing pulses and multiple echoes, changing the phase encoding gradient for each echo.

FAT SATURATION (FAT-SAT) - A specialized technique that selectively saturates fat protons prior to acquiring data as in standard sequences, so that they produce negligible signal. The pre-saturation pulse is applied prior to each slice selection. This technique requires a very homogeneous magnetic field and very precise frequency calibration. See also Fat Suppression.

FAT SUPPRESSION - the process of utilizing specific parameters , commonly with STIR (short TI inversion recovery) sequences, to remove the deleterious effects of fat from the resulting images. See also STIR.

FDA - the United States Food and Drug Administration FID - see Free Induction Decay

FIELD OF VIEW (FOV) - defined as the size of the two or three dimensional spatial encoding area of the image. Usually defined in units of cm2.

FFT (Fast Fourier Transform) - a particularly fast and efficient computational method of performing a Fourier Transform, which is the mathematical process by which raw data is processed into a usable image.

FIELD ECHO (FE) (also known as GRADIENT ECHO) - echo produced by reversing the direction of the magnetic field gradient to cancel out the position-dependent phase shifts that have accumulated due to the gradient.

FLAIR FLuid Attenuated Inversion Recovery

FLARE Fast Low-Angle Recalled Echoes

FLIP ANGLE (FA) - the angle to which the net magnetization is rotated or tipped relative to the main magnetic field direction via the application of an RF excitation pulse at the Larmor frequency. The Flip Angle is used to define the angle of excitation for a Field Echo pulse sequence.

FLOW COMPENSATION - a function of specific pulse sequences, i.e., CRISP¿ (Complex Rephasing Integrated with Surface Probes) spin echo, wherein the application of strategic gradient pulses can compensate for the objectionable spin phase effects of flow motion.

FLUX - invisible lines of force that extend around a magnetic material. The greatest density is at the two poles of the magnet.

FLUX DENSITY - the number of lines of force per unit area of a magnetic material.

FOURIER TRANSFORM (FT) - a mathematical procedure used in MRI scanners to analyze and separate amplitude and phases of the individual frequency components of the complex time varying signal. Fourier transform analysis allows spatial information to be reconstructed from the raw data.

FOV - See Field Of View.

FREE INDUCTION DECAY (FID) - if transverse magnetization of the spins is produced, e.g., by a 90É RF pulse, a transient MR signal at the Larmor frequency results that decays toward zero with a characteristic time constant of T2*. This decaying signal is the FID.

FREQUENCY - the number of cycles or repetitions of any periodic wave or process per unit time. In electromagnetic radiation, it is usually expressed in units of hertz (Hz), where 1 Hz = 1 cycle per second.

FREQUENCY ENCODING - the process of locating an MR signal in one dimension by applying a magnetic field gradient along that dimension during the period when the signal is being received.

FRINGE FIELD - a term usually relating to the extents of the magnetic field surrounding the magnet. Safety requirements dictate that the distances of particular field strengths from the magnet must be known, and that potentially unsafe areas must be indicated with appropriate warning signs. Access to areas with field strengths of 5 gauss and higher must be strictly controlled. FSE - See Fast Spin Echo. Gx, Gy, Gz - the conventional symbols for the three orthogonal magnetic gradients. The subscripts designate the conventional spatial direction of the gradient.

GADOLINIUM (Gd) - gadolinium is a non-toxic paramagnetic contrast enhancement agent utilized in MR imaging. When injected during the scan, gadolinium will tend to change signal intensities by shortening T1 in its surroundings.

GATING - timing the acquisition of MR data to physiological motion in order to minimize motion artifacts (e.g., cardiac gating, respiratory gating).

GAUSS - a unit of magnetic field strength that is approximately the strength of the earth's magnetic field at its surface (the earth's field is about 0.5 to 1G). The value of 1 gauss is defined as 1 line of flux per cm2. As larger magnetic fields have become commonplace, the unit gauss (G) has been largely replaced by the more practical unit tesla (T), where 1 T = 10,000 G. GHOSTING - an image artifact primarily associated with the phase direction.

GRADIENT COILS - three paired orthogonal current-carrying coils located within the magnet which are designed to produce desired gradient magnetic fields which collectively and sequentially are superimposed on the main magnetic field (Bo) so that selective spatial excitation of the imaging volume can occur. Gradients are also used to apply reversal pulses in some fast imaging techniques.

GRADIENT ECHO (GE) - see Field Echo.

GRADIENT MAGNETIC FIELD - A small linear magnetic field applied in addition to (superimposed on) the large static magnetic field in an MRI scanner. The strength (amplitude) and direction of the gradient fields change during the scan, which allows each small volume element (voxel) within the imaging volume to resonate at a different frequency. In this way, spatial encoding may be performed.

GYROMAGNETIC RATIO (g) - a constant for any given nucleus that relates the nuclear MR frequency and the strength of the external magnetic field. It represents the ratio of the magnetic moment (field strength) to the angular momentum (frequency) of a particle. The value of the gyromagnetic ratio for hydrogen (1H) is 4,258 Hz/Gauss (42.58 MHz/Tesla).

HERTZ - the standard unit of frequency equal to 1 cycle per second. The larger unit megahertz (MHz) = 1,000,000 Hz.

HOMOGENEITY - uniformity of the main magnetic field.

HYDROGEN DENSITY (H+) - the concentration of Hydrogen atoms in water molecules or in some groups of fat molecules within tissue. Initial MR signal amplitudes are directly related to H+ density in the tissue being imaged.

IMAGE (DATA) ACQUISITION TIME - the time required to gather a complete set of image data. The total time for performing a scan must take into consideration the additional image reconstruction time when determining how quickly the image(s) may be viewed.

IMAGE RECONSTRUCTION - the mathematical process of converting the composite signals obtained during the data acquisition phase into an image.

INHOMOGENEITY - lack of homogeneity or uniformity in the main magnetic field.

INVERSION RECOVERY (IR) - an imaging sequence that involves successive 180É and 90É pulses, after which a heavily T1-weighted signal is obtained. The inversion recovery sequence is specified in terms of three parameters, inversion time (TI), repetition time (TR) and echo time (TE).

INVERSION TIME (TI) - the time period between the 180° inversion pulse and the 90° excitation pulse in an Inversion Recovery pulse sequence. ISOTOPE - Atomic nuclei that contain the same number of protons, but differ in the number of neutrons in the nucleus of the atom for the element concerned. K-SPACE - a data acquisition matrix containing raw image data prior to image processing. In 2DFT, a line of data corresponds to the digitized NMR signal at a particular phase-encoding level.

LARMOR EQUATION - an equation that states that the frequency of precession of the nuclear magnetic moment is directly proportional to the product of the magnetic field strength (Bo) and the gyromagnetic ratio (g). This is stated mathematically as å = g Bo.
LARMOR FREQUENCY - the frequency at which magnetic resonance in a nucleus can be excited and detected. The frequency varies directly with magnetic field strength, and is normally in the radio frequency (RF) range.

LATTICE - in MRI, the magnetic and thermal environment through which nuclei exchange energy in longitudinal (T1) relaxation.

LONGITUDINAL MAGNETIZATION - the component (MZ) of the net magnetization vector in the direction of the static magnetic field. After RF excitation, this vector returns to its equilibrium value at a rate characterized by the time constant T1.

LONGITUDINAL RELAXATION - return of longitudinal magnetization to its equilibrium value after excitation due to the exchange of energy between the nuclear spins and the lattice.

LONGITUDINAL RELAXATION TIME - the time constant, T1, which determines the rate at which excited protons return to equilibrium within the lattice. A measure of the time taken for spinning protons to re-align with the external magnetic field. The magnetization will grow after excitation from zero to a value of about 63% of its final value in a time of T1.

MAGNETIC GRADIENT - one of three linear magnetization waveforms superimposed on the main magnetic field at specific times within a pulse sequence to select the imaging region or provide necessary spatial localization information. A magnetic gradient is defined as the amount and direction of the linear rate of change of the magnetic field in space. MAGNETIC FIELD - magnetic lines of force which extend from a north polarity and enter a south polarity to form a closed loop around the outside of a magnetic material.

MAGNETIC MOMENT - a measure of the net magnetic properties of an object or particle. A nucleus with an intrinsic spin will have an associated magnetic dipole moment so that it will interact with a magnetic field (as if it were a tiny bar magnet).

MAGNETIC RESONANCE - the absorption or emission of energy by atomic nuclei in an external magnetic field after the application of RF excitation pulses using frequencies which satisfy the conditions of the Larmor equation.

MAGNETIC RESONANCE ANGIOGRAPHY (MRA) - MR image visualization of selected vascular structures, such as the Circle Of Willis or the carotid arteries.

MAGNETIC RESONANCE SPECTROSCOPY (MRS) - an MR technique wherein a sample is placed in a strong, very uniform, magnetic field, and stimulated with RF electromagnetic energy. If the field is uniform over the volume of the sample, "similar" nuclei will contribute a particular frequency component to the detected response signal irrespective of their individual positions in the sample. Since nuclei of different elements resonate at different frequencies, each element in the sample contributes a different frequency component. A chemical analysis can then be conducted by analyzing the MR response signal into its frequency components.

MAGNETIC SUSCEPTIBILITY - the extent to which a material becomes magnetized when placed within a magnetic field. Differences in magnetic susceptibilities at tissue borders are a frequent cause of MRI artifacts.

MAGNETIZATION VECTOR (Mz) - the integration of all the individual nuclear magnetic moments which have a positive magnetization value at equilibrium versus those in a random state.

MAXIMUM INTENSITY PROJECTION (MIP) - a processing method for MRA images. A MIP is a record of a maximum intensity ray (generated through a mathematical algorithm) as it passes through an angiographic volume. Each point in an MIP represents the highest intensity experienced in that location on any partition within the imaging volume.

MR IMAGING - the use of magnetic resonance principles in the production of diagnostic views of the human body where the resulting image is based upon three basic tissue parameters (proton density, T1 relaxation time, T2 relaxation time) and flow characteristics. MRA - See Magnetic Resonance Angiography. MRS - See Magnetic Resonance Spectroscopy.

MULTI-ANGLE OBLIQUE - the ability to display anatomical structures in a variety of planes from the data acquired in just one scan.

MULTI-ECHO IMAGING - imaging using a series of echoes acquired as a train following a single excitation pulse. In spin-echo imaging, each echo is formed by a 180É pulse. Typically, a separate image is produced from each echo of the train.

MULTI-SLICE IMAGING - an imaging technique in which the repetition period (TR) is utilized for acquiring additional slices in other layers or planes.

NET MAGNETIZATION VECTOR - a vector which represents the sum of all of the contributions of the magnetic moments within the magnetic field; the magnitude and direction of the magnetization resulting from this collection of atomic nuclei.

NEUTRON - an uncharged neutral particle located in the nucleus of most atoms which serves as a stabilizer.

NEX - number of excitations. See also Number of Excitations, Signal Averaging.

NMR SIGNAL - the electromagnetic signal in the radio-frequency range produced by the precession of the transverse magnetization of the spins. The rotation of the transverse magnetization induces a voltage in a receiving antenna (coil) which is amplified and demodulated by the receiver circuits.

NOISE - an undesirable background interference or disturbance that affects image quality. NSA - the number of signal averages performed during the scan. See also NEX and Signal Averaging.

NUCLEAR SPIN - also known as inherent spin, this defines the intrinsic property of certain nuclei (those with odd numbers of protons and/or neutrons in their nucleus) to exhibit angular momentum and a magnetic moment. Nuclei that do not exhibit this characteristic will not produce an NMR signal.

NUCLEUS - the core or center part of an atom, which contains protons having a positive charge and neutrons having no electrical charge, except in the common isotope of hydrogen, where the nucleus is a single proton.

NUMBER OF EXCITATIONS - an indicator of how many times each line of k-space data is acquired during the scan.

OBLIQUE - a plane or section not perpendicular to the xyz coordinate system, such as long and short axis views of the heart.

ORTHOGONAL - a plane or section perpendicular to the xyz coordinate system.

OSCILLATION - rhythmic periodic motion.

PARAMAGNETIC SUBSTANCE - a substance with weak magnetic properties due to its unpaired electrons. Researchers are developing certain paramagnetic materials, such as gadolinium, as MRI invasive contrast media.

PARTIAL VOLUMING - a loss of resolution due to excessively large voxels, typically caused by slices that are too thick.

PERMANENT MAGNET - a magnet design that utilizes blocks of ferromagnetic materials (permanent magnets) to generate a magnetic field between the two poles of the magnet. There is no requirement for additional electrical power or cooling, and the iron-core structure of the magnet leads to a limited fringe field and no missile effect. Due to weight considerations, permanent magnets are usually limited to maximum field strengths of 0.3T.

PHANTOM - an artificial object of known dimensions and properties that is used to test or monitor an MRI systems homogeneity, imaging performance and orientation aspects.

PHASE - an angular relationship describing the degree of synchronism between two sinusoidal waveforms of the same frequency.

PHASE COHERENCE - a term describing the degree to which precessing nuclear spins are synchronous.

PHASE CONTRAST - an MRA technique utilizing the change in the phase shifts of the flowing protons in the region of interest to create an image.

PHASE ENCODING - the process of locating an MR signal by altering the phase of spins in one dimension with a pulsed magnetic field gradient along that dimension prior to the acquisition of the signal. As each signal component has experienced a different phase encoding gradient pulse, its exact spatial reconstruction can be specifically and precisely located by the Fourier transformation analysis. Spatial resolution is directly related to the number of phase encoding levels (gradients) used.

PIXEL - acronym for a picture element, the smallest discrete two-dimensional part of a digital image display.

PLANAR IMAGING - a method of scanning in which the data is collected simultaneously from an entire layer.

PRECESSION - comparatively slow gyration of the axis of a spinning body so as to trace out a cone. Caused by the application of a torque tending to change the direction of the rotation axis and continuously directed at right angles to the plane of the torque. The magnetic moment of a nucleus with spin will experience such a torque when inclined at an angle to the magnetic field, resulting in precession at the Larmor frequency.

PRESATURATION (PRE-SAT) - a specialized technique employing repeated RF excitation of structures adjacent to the ROI for the purpose of reducing or eliminating their phase effect artifacts.

PROTON - a positively charged particle located in the nucleus of an atom. The number of protons in the nucleus governs the chemical properties of that element.

PROTON DENSITY - the concentration of mobile Hydrogen atoms within a sample of tissue. See also Hydrogen Density.

PROTON DENSITY WEIGHTED IMAGE - an image produced by controlling the selection of scan parameters to minimize the effects of T1 and T2, resulting in an image dependent primarily on the density of protons in the imaging volume.

PULSE PROGRAMMER - the computer-controlled component of the MRI scanner that determines the timing of the pulse sequence parameters of the scan, such as echo time, pulse amplitude, phase and frequency.

PULSE SEQUENCE - a preselected set of defined RF and gradient pulses, usually repeated many times during a scan, wherein the time interval between pulses and the amplitude and shape of the gradient waveforms will control NMR signal reception and affect the characteristics of the MR images.

QUENCH - an event which can only occur in superconducting magnets, it is caused by a loss of superconductivity; a rapid increase in the resistivity of the magnet, which generates heat that results in the rapid evaporation of the magnet coolant (liquid helium). This evaporated coolant is a hazard that requires emergency venting systems to protect patients and operators. A quench can cause total magnet failure.

RADIO FREQUENCY - an electromagnetic wave with a frequency that is in the same general range as that used for the transmission of radio and television signals. Abbreviated RF. The RF pulses used in MR are commonly in the 1-100 megahertz range, and their principle effect upon a body is potential tissue heating caused by absorption of the applied pulses of RF energy.

READOUT GRADIENT - magnetic field gradient applied during the period when the receiver components are on. The application of this gradient, which is active during the period when the echo is being formed, results in the frequency encoding of the object being imaged. See also Frequency Encoding.

RECEIVER - the portion of the MRI equipment that detects and amplifies the RF signals picked up by the receiver coil. Includes a preamplifier, NMR signal amplifier, and demodulator.

RECEIVER COIL - a coil , or antenna, positioned within the imaging volume and connected to the receiver circuitry that is used to detect the NMR signal. In certain applications, the same coil can be used for both transmission and reception. Receiver coils types include solenoidal, planar, volume, quadrature and phased array coils.

RECONSTRUCTION - the mathematical process by which the displayed image is produced from the raw k-space data obtained from the receiver circuitry, typically utilizing Fourier transformation and selective filtering.

REGION OF INTEREST (ROI) - the area of anatomy being scanned that is of particular importance in the image.

RELAXATION TIME - after excitation the spins will tend to return to their equilibrium distribution in which there is no transverse magnetization and the longitudinal magnetization is at its maximum value and oriented in the direction of the static magnetic field. After excitation the transverse magnetization decays toward zero with a characteristic time constant T2, and the longitudinal magnetization returns toward equilibrium with a characteristic time constant T1.

REPETITION TIME (TR) - the amount of time that exists between successive pulse sequences applied to the same slice. It is delineated by initiating the first RF pulse of the sequence then repeating the same RF pulse at a time t. Variations in the value of TR have an important effect on the control of image contrast characteristics. Short values of TR (< 1000 ms) are common in images exhibiting T1 contrast, and long values of TR (> 1500 ms) are common in images exhibiting T2 contrast. TR is also a major factor in total scan time. See also TR.

REPHASING - the process of returning out-of-phase magnetic moments back into phase coherence. Caused either by rapidly reversing a magnetic gradient (Field Echo) or by applying a 180É RF pulse (Spin Echo). In the spin-echo pulse sequence this action effectively cancels out the spurious T2* information from the signal.

RESISTIVE MAGNET - a common type of magnet that utilizes the principles of electromagnetism to generate the magnetic field. Typically large current values and significant cooling of the magnet coils is required. Resistive magnets fall into two general categories - iron-core and air-core. Iron-core electromagnets provide the advantages of a vertically-oriented magnetic field, and a limited fringe field with little, if-any, missile effects due to the closed iron-flux return path. Air-core electromagnets exhibit horizontally oriented fields, which have large fringe fields (unless magnetically shielded) and are prone to missile effects. Resistive magnets are typically limited to maximum field strengths of approximately 0.6T.

RESONANCE - a large amplitude vibration in a mechanical or electrical system caused by a relatively small periodic stimulus with a frequency at or close to a natural frequency of the system. The exchange of energy at a particular frequency between two systems. ROI - see Region Of Interest.

SAGITTAL - a plane, slice or section of the body cutting from front to back through the saggital suture of the skull, and continued down through the body in the same direction, dividing it into two parts, then turning one half to view it from its cut surface.

SAMPLING - the conversion of analog signals to discreet digital values through a preselected measurement process. SAR - see Specific Absorption Rate.

SATURATION RECOVERY - a little-used pulse sequence that generates a predominately proton density dependent signal, basically employing a 90° RF excitation pulse, with a very long repetition time. This procedure allows the saturated spins to return to equilibrium before the next pulse is activated.

SELECTIVE EXCITATION - controlling the frequency spectrum (bandwidth) of an RF excitation pulse while imposing a gradient magnetic field on spins so that only a desired region will have a suitable resonant frequency to be excited. SCAN TIME - a description of the total time required to acquire all the data needed to produce the programmed image. See also Acquisition Time, Image (Data) Acquisition Time.

SHIM COILS - coils positioned near the main magnetic field that carry a relatively small current that is used to provide localized auxiliary magnetic fields in order to improve field homogeniety. See also Shimming

SHIMMING - The process of improving field homogeniety by compensating for imbalances in the main magnetic field of an MRI system. This can be accomplished through a combination of passive (mechanical) shimming (e.g., adding or removing steel from the magnets poles) and active shimming (the use of shim coils) to fine-tune the magnetic field.

SIGNAL AVERAGING - a signal-to-noise improvement method that is accomplished by taking the average of several FIDs made under similar conditions. This is also referred to as the number of excitations (NEX) or the number of acquisitions. The approximate amount of improvement in signal-to-noise (S/N) ratio is calculated as the square root of the number of excitations ( ).

SIGNAL-TO-NOISE RATIO (S/N, SNR) - The ratio between the amplitude of the received signal and background noise, which tends to obscure that signal. SNR, and hence image quality, can be improved by such factors as increasing the number of excitations, increasing the field of view, increasing slice thickness, etc. SNR also depends on the electrical properties of the patient being studied and the type of receiving coil used.

SLICE - the term describing the planar region or the image slice selection region.

SLICE ENCODING - relates to the addition of phase encoding steps for 3D volumetric imaging.

SLICE SELECTION - exclusive excitation of spins in one slice performed by the coincident combination of a gradient magnetic field and a narrow bandwidth or slice selective RF pulse at a specific Larmor frequency.

SLICE THICKNESS - the thickness of an imaging slice. Since the slice profile is not sharply edged, the distance between the points at half the sensitivity of the maximum (full width at half maximum) is used to determine thickness.

SMART - the acronym for Slice-specific, Multi-Angle, multi-Resolution, multi-Thickness scanning. This function allows the operator to individually customize the thickness, field-of-view and position of each slice in a multi-angle study. SNR - see Signal-To-Noise Ratio

SPATIAL RESOLUTION - the ability to define minute adjacent objects/points in an image, generally measured in line pairs per mm (lp/mm).

SPECIFIC ABSORPTION RATE - an RF exposure concern that describes the potential for heating of the patient's tissue due to the application of the RF energy necessary to produce the NMR signal. The RF induced heat load can be directly related to the

SAR (Specific Absorption Rate), which is defined as the RF power absorbed per unit of mass of an object, and is measured in watts per kilogram (W/kg).

SPIN - the property exhibited by atomic nuclei that contain either an odd number of protons or neutrons, or both.

SPIN-ECHO (SE) - re-appearance of the NMR signal after the FID has apparently died away, as a result of the effective reversal (rephasing) of the dephasing spins by techniques such as specific RF pulse sequences or pairs of field gradient pulses, applied in time shorter than or on the order of T2. Proper selection of the TE time of the pulse sequence can help control the amount of T1 or T2 contrast present in the image. Also a pulse sequence type that usually employs a 90° pulse, followed by one or more 180° pulses.

SPIN-LATTICE RELAXATION TIME - see T1 and Longitudinal Relaxation Time.

SPIN-SPIN RELAXATION TIME - see T2 and Transverse Relaxation Time.

STEADY-STATE FREE PRECESSION - the name for any field echo or gradient echo sequence in which a non-zero steady state develops for both transverse and longitudinal components of magnetization. If the RF pulses are close enough together, the MR signal will never completely decay, implying that the spins in the transverse (x-z) plane never completely dephase. STIR - the acronym for Short TI Inversion Recovery. A specialized application of the Inversion Recovery pulse sequence that sets the inversion time (TI) of the sequence at 0.69 times the T1 of fat, thereby suppressing the fat in the image. See also Fat Suppression.

SUPERCONDUCTIVE MAGNET - a magnet whose field is generated by current in wires made of a superconducting material such as niobium-titanium, that has no resistance when operated at temperatures near absolute zero(-273°C, -459°F). Such magnets must be cooled by, for example, liquid helium. Superconducting magnets typically exhibit field strengths of >0.5T and have a horizontal field orientation, which makes them prone to missile effects without significant magnetic shielding. See also Quenching.

SURFACE COIL- a type of receiver coil which is placed directly on or over the region of interest for increased magnetic sensitivity. These coils are specifically designed for localized body regions, and provide improved signal-to-noise ratios by limiting the spatial extent of the excitation or reception. T - tesla T1 - spin-lattice longitudinal relaxation time. The characteristic time constant for spins to realign themselves with the external magnetic field after excitation.

T1 WEIGHTED - an image created typically by using short TE and TR times whose contrast and brightness are predominately determined by T1 signals.

T1 RELAXATION - see Longitudinal Relaxation Time.

T2 - spin-spin or transverse relaxation time. The time constant for loss of phase coherence among spins oriented at an angle to the static magnetic field due to interactions between the spins. Results in a loss of transverse magnetization and the MRI signal.

T2* ("T-two-star") - the time constant for loss of phase coherence among spins oriented at an angle to the static magnetic field due to a combination of magnetic field inhomogeneities and the spin-spin relaxation. Results in a rapid loss of transverse magnetization and the MRI signal.T2* < T2.

T2 WEIGHTED - an image created typically by using longer TE and TR times whose contrast and brightness are predominately determined by T2 signals. TAU (t) - the interpulse times (time between the 90° and 180° pulse, and between the 180° pulse and the echo) used in a spin echo pulse sequence. TE (Echo Time) - represents the time in milliseconds between the application of the 90° pulse and the peak of the echo signal in Spin Echo and Inversion Recovery pulse sequences.

TE (Echo Time) - represents the time in milliseconds between the application of the 90° pulse and the peak of the echo signal in Spin Echo and Inversion Recovery pulse sequences.

TESLA (T) - the preferred unit of magnetic flux density. One tesla is equal to 10,000 gauss. The Tesla unit value is defined as a field strength of 1 Weber per meter 2, where 1 Weber represents 1 x 108 (100,000,000) flux lines.

THREE DIMENSIONAL IMAGING (3DFT) - a specialized imaging technique that uses computer processing to combine individual slice acquisitions together to produce an image that represents length, width and height. TI (Inversion Time) - the time between the initial (inverting) 180° pulse and the 90° pulse used in inversion recovery pulse sequences.

TIME OF FLIGHT (TOF) - and MRA technique relying solely on the flow of unsaturated blood into a magnetized presaturated slice. The difference between the unsaturated and presaturated spins creates a bright vascular image without the invasive use of contrast media.

TIP ANGLE - angle between the net magnetization vector before and after an RF excitation pulse. Small tip angles allow a decrease in TR, which is used to decrease scan time in Field Echo pulse sequences. See Flip Angle.

TR (Repetition Time) - the amount of time that exists between successive pulse sequences applied to the same slice. See also Repetition Time.

TRANSAXIAL - a plane perpendicular (rotated 90°) to the long axis of the human body. See also Axial.

TRANSCEIVER COIL - an MRI surface coil that acts as both transmitter and receiver.

TRANSMITTER - the portion of the MR scanner that produces the RF current and delivers it to the transmitting coil (antenna). The RF signal produced by the transmitter is used to excite the protons in the imaging volume.

TRANSVERSE MAGNETIZATION - component of the net magnetization vector at right angles to the main magnetic field. Precession of the transverse magnetization at the Larmor frequency is responsible for the detectable NMR signal. In the absence of externally applied RF energy, the transverse magnetization will decay to zero with a characteristic time constant of T2, or more strictly T2*.

TRANSVERSE RELAXATION TIME - the time constant, T2, which determines the rate at which excited protons reach equilibrium, or go out of phase with each other. A measure of the time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field due to interaction between spins, resulting in a reduction in the transverse magnetization. The transverse magnetization value will drop from maximum to a value of about 37% of its original value in a time of T2.

TUNING - the process of adjusting the transmitter and receiver circuitry so that it provides optimal signal performance at the Larmor frequency. A properly tuned scanner will produce images with a higher signal- to-noise ratio, and therefore improved diagnostic versatility.

TWO-DIMENSIONAL IMAGING (2DFT) - the Fourier transformation process reconstructs the detected frequency and phase encoded image information (which are rotated 90° from each other) into a usable image.

VECTOR - a quantity that has both magnitude and direction and that is commonly represented by an arrow. The length of the line segment represents the magnitude, and its orientation in space represents its direction. Vector quantities can be added to or subtracted from one another.

VELOCITY - speed in a particular direction.

VELOCITY ENCODING (VENC) - a specialized technique used fro encoding flow velocities.

VISCOSITY - a property of a fluid or semi-fluid that affects its mobility, and therefore its intensity in an image.

VOLUMETRIC IMAGING - a specialized technique where all the MR signals are collected from the entire tissue sample and imaged as a whole entity. Compare with slice select.

VORTEX FLOW - area within a blood vessel where the blood is suddenly accelerated, then rapidly decelerated. This would be commonly seen in blood passing through a vascular stenosis (narrowing), and becomes a factor in MRA.

VOXEL - volume element; the element of the three-dimensional space corresponding to a pixel, for a given slice thickness.

MRI TUTOR

http://www.mritutor.org/mritutor/index.html


Teaching Files

http://www.mritutor.org/mriteach/

ISMRM

http://www.ismrm.org/

The International Society for Magnetic Resonance in Medicine is a nonprofit professional association devoted to furthering the development and application of magnetic resonance techniques in medicine and biology. The Society holds annual scientific meetings and sponsors other major educational and scientific workshops.

MRI Artifacts

http://www.mritutor.org/mritutor/artifact.htm

BASICS

http://www.medical.siemens.com/siemens/en_US/gg_mr_FBAs/files/MAGNETOM_World/MR_Basics/Magnets_Spins_and_Resonances.pdf

Rotating frame of reference

http://www.youtube.com/watch?v=BNXSllytZtI

Story of MRI

The story of MRI starts in about 1946 when Felix Bloch proposed in a Nobel Prize winning paper some rather new properties for the atomic nucleus.
He stated that the nucleus behaves like a magnet. He realized that a charged particle, such as a proton, spinning around its own axis has a magnetic field, known as a magnetic momentum.
He wrote down his finding in what we know as the Bloch Equations.
It would take until the early 1950s before his theories could be verified experimentally.
In 1960 Nuclear Magnetic Resonance spectrometers were introduced for analytical purposes.
During the 1960s and 1970s NMR spectrometers were widely used in academic and industrial research. Spectrometry is used to analyze the molecular configuration of material based on its NMR spectrum

Some bits of History

Human brain imaging with functional magnetic resonance is among the most recent developments in a field that came into existence barely 20 years ago. Today, scientists use fMRI to follow changes in brain activity as stroke patients start to regain lost abilities, with the aim of developing better strategies for treatment and therapy. They use fMRI to investigate the developing neural networks for language, hearing, vision, and motor systems in an infant listening to her mother's voice. They can also try to understand the subtle abnormalities in brain activation of children diagnosed with attention deficit hyperactivity disorder, as well as the memory difficulties of patients suffering from schizophrenia.
The underlying phenomenon that makes all this possible is known as nuclear magnetic resonance, and the path to its discovery started with early investigations into the nature of the atom. Although the idea of the atom dates to the ancient Greeks, gaining objective knowledge of it--and of its constituent parts--has come about in just the last hundred years or so. In 1897, physicist J. J. Thomson at Cambridge University in England discovered the electron followed by Ernest Rutherford's discovery of the atomic nucleus. Over the next few decades, a number of brilliant theoretical physicists--including Max Planck, Niels Bohr, Erwin Schrodinger, and Werner Heisenberg--built on one another's work to advance our understanding of the structure and properties of the atom and atomic particles. In so doing, they revolutionized physics, producing a new language and theory known as quantum mechanics.
THE EXPERIMENTS OF I. I. RABI
In 1929, Isidor Isaac Rabi began teaching quantum mechanics at Columbia University. Over the next decade his research group used a technique called molecular beam resonance to study the magnetic properties of atoms and molecules. At the time of Rabi's experiments, physicists knew that the atomic nucleus is composed of two types of particles, positively charged protons and neutral particles called neutrons. Surrounding this nucleus in a kind of fuzzy cloud are negatively charged electrons. Physicists also had discovered that electrons, protons, neutrons--and in many cases the nuclei themselves--behave as though spinning about their axes, just like planets. This results in a property called spin angular momentum, which has both magnitude and direction. Such a spinning particle generates a magnetic field and associated "magnetic moment"--acting like a tiny bar magnet with north and south poles. When placed in a strong external magnetic field, the "magnetic moment" of a nucleus tends to align with (parallel to) or against (antiparallel to) the external field. Parallel alignment corresponds to a lower energy state than antiparallel alignment.
Rabi's experiments involved passing a beam of lithium chloride molecules through a vacuum chamber and manipulating the beam with different magnetic fields. By studying how the magnetic field affected the path of the molecules, he could learn about the magnitudes of the magnetic moment of the nucleus. With the appropriate stimulus, he predicted, the magnetic moments of the nuclei could be induced to flip, or change their orientation relative to the magnetic field. In 1937, following a suggestion by Dutch physicist Cornelius J. Gorter, Rabi and his group added a new wrinkle to their experiments: They bathed the molecular beam in radio waves--electromagnetic signals in the radiofrequency, or broadcasting, range--while varying the magnetic field strength.
They did this because of another feature of spinning particles. In an external magnetic field, atomic particles spin with a wobble called precession, much like a child's top wobbles about the vertical when it is tipped slightly. The magnetic moment of an atomic nucleus wobbles at a characteristic frequency that depends on its type (hydrogen vs. lithium, for instance) and also on its environment. For example, increasing the magnetic field strength increases this frequency, while decreasing the field strength lowers it.
Rabi and his team adjusted the magnetic field strength until they induced the magnetic moments of the nuclei to flip, which occurs when the frequency of the radio signal matches the nuclei's characteristic precessional frequency. When this match--the resonance frequency--occurs, a nucleus absorbs energy from the radio signal that is precisely equal to the difference between its two energy states and thus jumps to the higher state. A flip also occurs when a nucleus emits that energy in falling back from the higher to the lower energy state. Whether the nucleus was excited to the higher energy state or falling to the lower one, Rabi could detect the transition. His technique is now called magnetic resonance or, more precisely, molecular beam magnetic resonance.
Rabi's group applied the new technique to deduce unprecedented details of the internal interactions of molecules. They discovered a series of resonances within a single molecule that allowed them to "see" how individual atoms are bound together and how their nuclei are affected by neighboring atoms. These extraordinary experiments and the development of molecular beam magnetic resonance as a technique for studying the magnetic properties and internal structure of molecules, atoms, and nuclei resulted in Rabi winning the 1944 Nobel Prize in physics.
Within a few months of these experiments, Rabi's group would try a variation: manipulating the radio frequency rather than the strength of the magnetic field. This method--which spreads the resulting signals into a spectrum, much as visible light is spread by passing through a prism--is the basis for radio frequency spectroscopy, which would revolutionize chemical analysis and prove a vital component in the development of magnetic resonance scanning as a tool for medical diagnosis.
A DIFFERENT KIND OF RESONANCE
The outbreak of World War II interrupted work on nuclear magnetic resonance, but the postwar years saw an explosion of progress. In the United States, two groups of physicists independently set out to develop a simpler method for observing the magnetic resonance in the nuclei of molecules in liquids and solids instead of the isolated molecules of Rabi’s experiments. At Harvard University, Edward Purcell, whose group included Henry Torrey and Robert Pound, led the research. At Stanford University, Felix Bloch led a team that included William Hansen and Martin Packard.
Both Purcell and Bloch chose to study the proton --the nucleus of the hydrogen atom (H). Because the hydrogen nucleus is composed of a single proton, it has a significant magnetic moment. Hydrogen would turn out to be the most important element for MRI because of its favorable nuclear properties, nearly universal presence, and abundance in the human body as part of water (H2O). Purcell's group used a two-pound block of paraffin wax as their hydrogen source; Bloch's group used a few drops of water contained in a glass sphere. The two research groups placed the samples in a magnetic field and waited for their nuclei to reach thermal and magnetic equilibrium, a magnetized state in which slightly more of the nuclei are aligned parallel to the external field than antiparallel to it. Then, as Rabi's team had done, the research teams applied radio waves in an effort to get the magnetic moments of the nuclei in the samples to flip. Purcell and Bloch hoped to detect magnetic resonance by observing the energy that precessing nuclei absorbed or gave to the radio frequency field when the resonance condition was satisfied.
In 1945, within three weeks of each other, both groups managed to create the conditions necessary to observe the phenomenon. Their experiments demonstrated what is technically known as nuclear magnetic resonance in condensed matter (now shortened to NMR) as distinguished from Rabi's discovery, molecular beam magnetic resonance. In 1952, Bloch and Purcell shared the Nobel Prize in physics for these experiments.
Research in NMR now leaped ahead. The researchers who surrounded the Purcell and Bloch labs quickly began using NMR spectroscopy to investigate the chemical composition and physical structure of matter. One of the first advances in the course of this work was measuring quantities called relaxation times, T1 and T2. T1 is the time it takes the nuclei in test samples to return to their natural alignment; T2 is the duration of the magnetic signal from the sample. One of Purcell's first graduate students, Nicolaas Bloembergen, who had arrived at Harvard from the Netherlands in 1946, played a key role with Pound and Purcell in this research. Bloembergen was the first researcher to measure relaxation times accurately and, along with Purcell and Pound, also measured how they changed in a variety of liquids and solids. Fortunately for future research and applications, relaxation times could be measured in seconds or fractions of seconds, making NMR a practical research tool.
Bloembergen, Purcell, and Pound published a paper in 1948 that became extremely influential in several branches of physics. The manipulation of relaxation times has provided a powerful method in chemistry and biology for analyzing the structure of molecules--and as other researchers would learn later, is essential for producing the contrast needed to image useful images of tissues in the human body.
LISTENING FOR ECHOES
In the late 1940s, Henry Torrey at Rutgers University and, independently, Erwin Hahn at the University of Illinois took a new step forward in NMR by applying pulses of strong radio waves to the sample instead of a single continuous wave. They first observed transient NMR signals during the application of long pulses. Following Hahn's later observations that transient NMR signals could be measured after the application of short pulses, the pulse technique became an important method for physicists and chemists investigating atoms and molecules.
Additionally, Hahn discovered a phenomenon known as "spin echo," which proved important for measuring relaxation times. At first he attributed these seemingly spurious signals to a misfiring of his electronics. After more study he recognized that they were caused by the speeding up and slowing down of the spinning nuclei due to variations in the local magnetic fields. By applying two or three short radio pulses and then listening for the echoes, Hahn found that he could obtain even more detailed information about nuclear spin relaxation than he could with a single pulse.
Pulsed NMR and spin echoes would play a crucial role in the development of magnetic resonance imaging two decades later. At the time, however, the idea of using NMR to make images simply did not occur to scientists using NMR spectra in physics and chemistry. In any event, before NMR could become a practical imaging tool, some further steps were needed. One important aid was a new pulse method called Fourier transform NMR, first proposed by Russell Varian of Varian Associates in the late 1950s. At about the same time, Richard E. Norberg and Irving Lowe at Washington University in St. Louis showed experimentally and theoretically how one could get all the results available from continuous wave experiments by mathematical manipulation of the signals produced in a pulse experiment. However, at that time the mathematical step needed to analyze the pulse data (a technique called Fourier transformation) was impractical because of the limitations of the computers available.
In the late 1960s, Richard Ernst and Weston Anderson, while working at Varian Associates, were studying the complex many-line NMR spectra of interest to chemists. It was a slow process searching by trial and error for the frequencies to produce all the many lines of the spectra. They realized that simultaneously broadcasting a range of radio frequencies at the atoms in the sample and then performing Fourier analysis of the resulting pulse signal could give all the results of the continuous wave method. This technique was thousands of times faster than the old method, and it also allowed researchers to observe signals only one-tenth as strong. By then advances in computers had made Fourier transformation practical. It now became possible to use NMR to analyze very small samples of a material or to identify very rare atoms in larger samples. In 1991, Ernst won the Nobel Prize in chemistry for his contributions to the development of high-resolution NMR spectroscopy.
THE SCIENCE OF IMAGING
Critical to making MRI a reality was the advent of the high speed computers needed to handle the enormous quantity and complexity of the computations involved in imaging. In addition to the necessary computing power, three other developments contributed to the birth of MRI. One was the work of British electronics engineer Godfrey Hounsfield, who in 1971 built an instrument that combined an x-ray machine and a computer and used certain principles of algebraic reconstruction to scan the body from many directions--manipulating the images to produce a kind of cutaway view of the interior. Unknown to Hounsfield, South African nuclear physicist Allan Cormack had published essentially the same idea in 1963, using a reconstruction technique called the Radon transform. Although Cormack's work was not widely circulated, in 1979 he and Hounsfield shared the Nobel Prize in physiology or medicine for the development of computerized tomography, or CT. The principles underlying CT are the foundation of many sophisticated imaging methods in use today.
The other two developments essential to MRI were related to nuclear magnetic resonance. One was the conceptualization of NMR as a medical diagnostic tool; the other was the invention of a practical method for producing useful images from NMR data.
As early as 1959, J. R. Singer at the University of California, Berkeley, proposed that NMR could be used as a non-invasive tool to measure in vivo blood flow. Then in 1969, Raymond Damadian, a physician at Downstate Medical Center in Brooklyn, New York, began to think of a way to use the technique to probe the body for early signs of cancer. In a 1970 experiment he surgically removed fast-growing tumors that had been implanted in lab rats and showed that the tumors' NMR signals differed from those of normal tissue. Damadian published the results of his experiments in 1971 in the journal Science.
An essential technical advance that opened up the ensuing widespread application of NMR to produce useful images was due to chemist Paul Lauterbur, who was then at the State University of New York at Stony Brook. In 1971, he watched a chemist named Leon Saryan repeat Damadian's experiments with tumors and healthy tissues from rats. Lauterbur concluded that the technique was insufficiently informative for locating and diagnosing tumors and went on to devise a practical way to use NMR to make images.
Lauterbur's groundbreaking idea was to superimpose on the spatially uniform static magnetic field a second weaker magnetic field that varied with position in a controlled fashion, creating what is know as a magnetic field gradient. At one end of a sample the graduated magnetic field would be strong, becoming weaker in a precisely calibrated way down to the other end. Because the resonance frequency of nuclei in an external magnetic field is proportional to the strength of the field, different parts of the sample would have different resonance frequencies. Thus, a given resonance frequency could be associated with a given position. Moreover, the strength of the resonance signal at each frequency would indicate the relative size of volumes containing nuclei at different frequencies and thus at the corresponding position. Subtle variations in the signals could then be used to map the positions of the molecules and construct an image. (Today's magnetic resonance imaging devices impose three sets of electromagnetic gradient coils on the subject to encode the three spatial coordinates of the signals.)
Across the Atlantic in Britain, Peter Mansfield at the University of Nottingham, England, had a similar idea. He was looking into using NMR to obtain structural details of crystalline materials. In work published in 1973, Mansfield and his colleagues also used a field gradient scheme. In 1976, Mansfield developed a MRI technique known as echo-planar imaging, which can rapidly scan a whole brain.
Meanwhile, Lauterbur's results, published in 1973, included an image of his test sample: a pair of small glass tubes immersed in a vial of water. Working with the small NMR scanner he had created (and using a technique called back projection borrowed from CT scanning), he continued to image small objects, including a tiny crab scavenged by his daughter from the Long Island beach near his home. By 1974, using a larger NMR device, he produced an image of the thoracic cavity of a living mouse. Mansfield, for his part, had imaged a number of plant stems and a dead turkey leg by 1975, and by the next year he had captured the first human NMR image--a finger. Damadian also was at work producing images. In 1977, he produced an image of the chest cavity of a live man.
By the early 1980s the flurry of activity around MRI had given rise to a burgeoning commercial enterprise. ("Nuclear" had been quietly dropped from the name in the meantime because of its unfavorable connotations.) Advances in high-speed computing and superconductive magnets allowed researchers to build larger MRI machines with enormously improved sensitivity and resolution and made possible many new applications.
FROM STRUCTURE TO FUNCTION
A phenomenal tool for imaging the anatomy and structure of living tissue, MRI was greatly enhanced in the 1980s and 1990s by the development of its ability to capture an organism in action--to study function. The breakthrough that led to functional MRI, fMRI as it is known, came in the early 1980s, when George Radda and colleagues at the University of Oxford, England, found that MRI could be used to register changes in the level of oxygen in the blood, which in turn could be used to track physiological activity. The principle behind BOLD (for blood oxygen level dependent) contrast imaging had been described some 40 years earlier by Linus Pauling. In 1936, Pauling and Charles D. Coryell, both then at the California Institute of Technology, published a paper describing the magnetism of hemoglobin, the oxygen-carrying pigment that gives red blood cells their color. Much earlier, in 1845, English physicist and chemist Michael Faraday, the discoverer of electromagnetic induction, investigated the magnetic properties of dried blood and made a note to himself: "Must try recent fluid blood." As it happened, Faraday never got around to it, leaving it to Pauling and Coryell more than 90 years later. The two chemists found that the magnetic susceptibility of fully oxygenated arterial blood differed by as much as 20 percent from that of fully deoxygenated venous blood.
In 1990, Seiji Ogawa of AT&T's Bell Laboratories reported that, in studies with animals, deoxygenated hemoglobin, when placed in a magnetic field, would increase the strength of the field in its vicinity, while oxygenated hemoglobin would not. Ogawa showed in animal studies that a region containing a lot of deoxygenated hemoglobin will slightly distort the magnetic field surrounding the blood vessel, a distortion that shows up in a magnetic resonance image.
At about the same time, other investigators also were studying these effects in humans. In 1992, for example, a number of researchers, including Ogawa, John W. Belliveau at Massachusetts General Hospital, and Peter Bandettini at the Medical College of Wisconsin, published results of studies of the brain's response to sensory stimulation using functional MRI techniques. Among other uses, fMRI currently can help guide brain surgeons away from critical areas of the brain, detect signs of stroke, and elucidate the workings of the brain.
Today what Rabi began has become a multibillion dollar industry. MRI scanning and spectroscopy are widely used diagnostic imaging technologies in medicine, and as new techniques and ever more powerful machines have come online in the past few years, the speed and precision of MRI and fMRI have increased dramatically.
None of this would have been possible without the nearly four decades of basic research following Rabi's first detection of nuclear magnetic resonance. Those decades included crucial discoveries by physicists and chemists interested in studying the magnetic properties of atoms and molecules, in seeing how they interact, and in elucidating their basic structures. As George Pake, Purcell's second graduate student, put it in 1993, "Without the basic research, magnetic resonance imaging was unimaginable."