Coil
Piece of hardware that can transmit or receive radiofrequency pulses during MRI. All scanners are equipped with a large body coil within the scanner itself. For most musculoskeletal applications, a surface coil is used. This is a smaller coil that can be placed on or around the body part of interest for improved imaging.
Cross talk
Phenomenon that occurs as a result of some “spillover” of radiofrequency excitation between adjacent tissue slices during MRI and results in increased image noise. This effect can be minimized by inserting small “gaps” of non-imaged tissue between adjacent slices.
Echo
Refers to the radiofrequency returning from tissues, which is used to create the final image. Various types of echoes (eg, spin echo, gradient echo) are produced, depending on which pulse sequence is used.
Echo train
Specialized rapid pulse sequences can produce a series of echoes, known as an echo train, in the same amount of time that conventional sequences produce a single echo. The reduction in imaging time is directly proportional to the length of the echo train (typically 2-16). See also fast spin echo.
Fast spin echo (FSE)
Family of pulse sequences that include RARE, fast spin echo (General Electric term), and turbo spin echo (Siemens/Phillips term). These pulse sequences produce images with contrast similar to conventional spin echo sequences, but in less time.
Fat saturation
Certain scanning techniques result in the suppression (reduction) of the signal intensity arising from fat. The two main techniques are inversion recovery imaging and frequency-selective fat suppression.
Field of view
Amount of tissue included on each cross-sectional image. Typically expressed in mm2 or cm2.
Gadolinium
Paramagnetic compound that forms the basis for most MR contrast agents. Its primary effect is to cause increased signal intensity on T1-weighted images within tissues (if administered intravenously) or within a joint (if administered intra-articular after being diluted in saline).
Gap
Small slice of non-imaged tissue inserted between two adjacent imaging slices to reduce cross talk.
Gradient echo
Family of pulse sequences originally developed to produce T2-weighted images in less time than with the spin echo technique. Because of differences between these two types of sequences, gradient echo-T2W images are designated T2*W and are especially useful for imaging ligaments, fibrocartilage, and hyaline articular cartilage. They also are useful for identifying areas of hemorrhage, metal, bone, or air owing to heightened susceptibility effects.
Inversion recovery
Commonly known as STIR (short tau inversion recovery). This technique results in excellent fat suppression and high signal intensity from areas of fluid or edema; it is extremely sensitive for detecting many types of acute pathology.
Matrix
Grid of voxels that compose each MR image. Typical matrix values range from 128 × 128 to 512 × 512 (width × height).
Noise
The quality of an MR image is determined largely by two competing factors—signal and noise. Image noise refers to the background graininess that results from several factors, including the background electrical noise of the imaging system, the presence of the patient in the magnet, the imaging coil used, and other factors. For a given patient, the noise is constant, and maneuvers employed to improve the signal of the image result in an improved signal-to-noise ratio (SNR) and a better image.
Proton density sequence
Pulse sequence that is relatively balanced in terms of T1 and T2 weighting (TR <1000, TE >30). Tissue contrast on these images is based on the number of protons within each tissue, rather than their T1 or T2 relaxation properties. (Fat is relatively bright, whereas fluid is gray because of the higher number of protons per unit volume in the fat).
Pulse sequence
Combination of imaging parameters that are selected at the MRI console to produce images of predictable tissue contrast. The most common families of pulse sequences include spin echo, FSE, gradient echo, and inversion recovery (STIR).
Resolution
Ability to distinguish between two objects. The better the resolution of an image, the easier it is to distinguish objects of increasingly smaller size. Generally, the smaller the voxels in an image, the better the resolution, but this also results in decreased image signal.
Signal average
Number of times each portion of tissue (voxel) is sampled to generate an MR image. Increasing the number of signal averages improves the signal-to-noise ratio of an image, but also prolongs imaging time proportionately.
Slice thickness
Thickness of the MR image. Although each image is projected two-dimensionally on a monitor or film, it represents a 3D slice of tissue, typically ranging from 1 to 10 mm in depth. The smaller the slice thickness, the better the resolution.
Spin echo sequence
Family of pulse sequences that includes T1-weighted, proton density–weighted, and T2-weighted sequences.
STIR (short tau inversion recovery)
See inversion recovery.
Susceptibility
Degree to which a tissue distorts the magnetic fi eld around it. Certain materials, such as surgical hardware, metal fragments, or the iron-containing hemoglobin found within areas of hemorrhage, have large susceptibilities and tend to create artifactual signal loss on MR images. Gradient echo pulse sequences accentuate these artifacts. FSE sequences tend to minimize them.
T1, T2
Inherent properties of tissue that defi ne how a proton would react during MRI. Each tissue has unique T1 and T2 values. As a result, contrast between tissues on an MR image is based primarily on differences in T1 or T2 properties, depending on the imaging parameters selected (ie, T1 or T2 “weighting”).
TE
Also known as echo time, a parameter selected at the imaging console that controls the T2 weighting of an image. A short TE minimizes T2 differences, whereas a long TE maximizes T2 weighting.
TR
Also known as repetition time, an imaging parameter selected at the console that controls the amount of T1 weighting in an image. A short TR maximizes T1 differences, whereas a long TR minimizes T1 weighting.
Turbo spin echo
See fast spin echo.
Voxel
Basic unit of the MR image, this represents a small portion of tissue within the patient that is sampled during the MR examination. The size of each voxel is determined by the fi eld of view, imaging matrix, and slice thickness. Also known as volume elements.
Weighting
Refers to the contrast properties of a particular imaging sequence. This is determined by selecting specifi c scanning parameters at the console that emphasize contrast differences between tissues based on tissue-specifi c properties (eg, T1-weighted or T2-weighted images).
