Physical Basis

The nuclei of hydrogen in water and fat molecules behave like small spinning bar magnets. When placed in a strong uniform magnetic field (greater than 0.01 tesla or 100 gauss), they execute a circular motion, or precession, weakly aligning to form a net nuclear polarization nearly parallel to the external magnet field. If a short pulse of radio frequency (rf) energy (radio wave), precisely tuned to the precession frequency of the water and fat proton nuclear magnets, is applied, the nuclei absorb a small amount of energy, change their alignment, and then gradually return to their previous equilibrium positions. In responding to the radio wave, the net nuclear magnetization generates a small voltage: the nuclear magnetic resonance signal. This can be detected and recorded electronically.

Two parameters T1 and T2, also known as respectively longitudinal and transverse relaxation times, govern the behavior of the electronic signal detected. The relaxation times are a function of the immediate environment of the resonating protons and vary in the different biologic tissues. For example, free water exhibits long T1 and T2 values and fat exhibits short T1 and relatively short T2 values. Image generation requires a large number of repetitions of the sequence that produces the nuclear resonance signal. The time between repetitions is called TR. For technical reasons, a two-pulse sequence is used to generate a particular type of signal called a spin echo. The spacing between pulses is labeled TE/2, and an echo occurs at a time TE. There are important relationships between the intensity of the nuclear magnetic resonance echo and TR, T1, TE, and T2. Based on the effects of the different relaxation times (T1 and T2), two types of imaging are carried out. Short repetition times (TR) between successive cycles of rf excitation pulses and short echo times (TE) produce stronger signals from tissues with relatively short T1 times, such as fat, especially bone marrow. Hence, weighting favoring short T1 results from pulse sequences using short TR and short TE. On the other hand, longer TRs eliminate much of the T1 signal difference between fat and water so that further manipulation of the rf pulse spacing (longer TE) will enhance signals from tissues with long T2 times, such as edema fluid. Long TR, TE pulse sequences preferentially weight long T2 tissues. Thus, the two basic methods of MRI scanning are labeled T1-weighted and T2-weighted imaging.

To construct an image from the tissue-specific signals of an object (for example, a patient), it is necessary to apply small, spatially inhomogeneous, three-dimensional magnetic fields called gradients. They modify the signal decay of the nuclear magnetization and spatially tag the hydrogen nuclear magnets in the object for mapping the image. The actual reconstruction of an image is complex, just as in CT scanning, and requires a relatively fast computer with a large memory. The ultimate result is a two-dimensional, medically diagnostic, cross-sectional body image that is displayed on a video monitor and recorded on film or digitally stored on a hard disk or magnetic tape as a permanent record. In most gray-scale imaging formats, the strongest signal corresponds to maximum brightness on the black-and-white monitor. Therefore, in heavily T2-weighted images, water appears bright, whereas fat appears intermediate gray. On the other hand, on T1-weighted images, fat appears bright and water appears dark (Fig. 29Z.:l). T.a.b!e...,29.Z.-..1. lists the expected signal intensities for several biologic tissues in these two cases. Many newer and more sophisticated imaging pulse sequences have evolved from this imaging framework, such as chemical fat suppression, inversion recovery, short T1 inversion recovery (STIR), fluid attentuation inversion recovery (FLAIR), gradient echo recall, magnetization transfer, rapid acquisition with relaxation enhancement (RARE), echo planar, and diffusion gradient. For a more complete discussion of the technical aspects and clinical applications of MRI, readers are referred to more specialized texts. 56

FIG. 297-1. Examples of Tl-weighted and T2-weighted magnetic resonance images. A. Tl-weighted coronal image: note that the urine in the bladder ( arrow), which is essentially water with a long T1, appears dark, whereas the femoral bone marrow, subcutaneous, and perivesical fat (short T1 ) appear bright. B. T2-weighted axial image of the same patient as in A: note that the urine, which has a long T2, now appears bright, whereas the surrounding fat with its shorter T2 is intermediate in brightness.

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