BRIEF OVERVIEW OF MRI PHYSICS

Tissues contain many hydrogen nuclei (protons), including those found in water. The presence of hydrogen nuclei in water and the central role of edema in the inflammatory process enable use of MRI as a powerful means of mapping tissue damage. When an animal is placed in a strong magnetic field, the hydrogen nuclei line up parallel to the main magnetic field (Figure 125-2). When a radiofrequency coil transmits a radiofrequency pulse (see Figure 125-3, A), the parallel alignment of hydrogen nuclei (protons) is changed (Figure 125-3, B). When the radiofrequency pulse is turned off (Figure 125-3, C), hydrogen nuclei release the added energy and return to an orientation parallel to the main magnetic field (see Figure 125-3, C). This released energy (signal) is measured by the radiofrequency coil (now acting as an antenna) to produce the image.

Figure 125-2 A magnet (A) causes hydrogen nuclei (protons) to be repositioned from a normal random pattern to parallel alignment within the main magnetic field (B). When an animal is placed in a strong magnetic field (C), the hydrogen nuclei align with the magnetic field.

(From MRI Made Easy. Berlex Laboratories, Wayne, NJ, 2006.)

Figure 125-3 A, The hydrogen nuclei line up parallel to the main magnetic field, and the radiofrequency (RF) coil applies a pulse. B, The RF pulse causes the hydrogen nuclei to move from a parallel to a perpendicular orientation. C, When the RF pulse is turned off, the hydrogen nuclei release the added energy and return to their original alignment parallel with the main magnetic field. The released energy is received by the RF coil and is used to produce the image.

(From MRI Made Easy, Berlex Laboratories, Wayne, NJ, 2006.)

All tissues release this added energy at different rates because of the unique concentration of hydrogen nuclei they contain, particularly in the form of water. These tissue-specific proton concentrations and their distinct patterns of energy release are the central features that provide tissue discrimination in the MR image. In essence, MRI produces a hydrogen plot of the tissue imaged within the magnetic field. Low signal is black (low content of hydrogen nuclei, e.g., water), and high signal is white (high content of hydrogen nuclei, e.g., water).

Typically, three scanning sequences are used during all MRI studies: T1 weighted (T1W), T2 weighted (T2W), and short Tau inversion recovery (STIR). Tissues such as fat that release energy quickly are highlighted on T1W images (Figure 125-4), and tissues containing water that release energy slowly are highlighted on T2W and STIR sequences. A T1W image is produced when the radiofrequency coil collects image information immediately, and a T2W image is produced when image information is collected after a short delay (Figure 125-5). The signal produced by fat can be pronounced and overwhelm the remainder of the image. A STIR image is produced by a protocol that minimizes the signal produced by fat.

Figure 125-4 Sagittal section of the foot in a T1-weighted image. Notice the high (white) signal produced by fat in the center of the middle phalanx. Because healthy tendons contain little water or hydrogen, they appear black in all magnetic resonance imaging sequences.

(Courtesy Hallmarq Veterinary Imaging, Ltd.)

Figure 125-5 Same image as in Fig. 125-4. Notice the signal change by tissue in the same sagittal section of the digit with the T1-weighted (T1), T2-weighted (T2), and short Tau inversion recovery (STIR) images. The high signal from fat (for example, within the bone marrow) in the T1 image becomes black in the STIR image, and the gray line in the coffin joint in the T1 image becomes white in the STIR image.

(Courtesy Hallmarq Veterinary Imaging, Ltd.)

Fat in bone marrow and water in edematous tissues are the contrast extremes in MRI. In a T1W image, fat produces a white signal and water produces a black (dark) signal (see Figure 125-5). In a T2W image, fat produces a darker (less white) signal and water produces a white (lighter) signal.

It is less important to commit to memory, for example, the fact that tendon is dark in a T1W image and more important to recognize that high signal (white) in T1W, T2W, and STIR images allows aging of a lesion. The hydrogen in water (edema) from an acute lesion is best seen in STIR and then in T2W images; T2W and STIR images reveal tissue fluid from acute injuries. The presence of a signal increase (white) in T1W but not T2W and STIR suggests chronic or degenerative injury. Whereas nuclear scintigraphy indexes the metabolic state of bone structures, MR images represent the metabolic state of both hard and soft tissues in the context of the chronicity of disease.

Typically, images are collected in three planes (Figure 125-6