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.