T1 weighted image (T1WI). The T1 relaxation times for water and fat in the body range from 100 to 2000 msec. The primary source of signal attenuation in a spin-echo image is the progressive saturation of the magnetic moment by the repetitive slice selection pulses.

The signal from fat is bright, whereas image intensities from areas of muscle and fluid are lower. Cartilage, ligaments and tendons appear very dark. Bone marrow is also bright due to its fat content

Short Tau Inversion Recovery Images (STIR). Fat appears as high signal on T1WI and T2WI and may obscure the pathology which may also appear high on T2WI. To overcome this problem fat saturation imaging is needed. Fat-Sat sequences are acquired at a T1 where it nullifies the T1 of the fat. This is often achieved by introducing an inversion pulse which makes the signal from fat null before the RF pulse is applied.

The images show very dark fat and bone marrow, whereas water and edema fluid appear high, producing acceptable contrast.

Marrow abnormalities are best evaluated with fat suppression techniques such as fat-suppressed proton-density–weighted imaging or with short-inversion-time inversion recovery (STIR) sequences (1,500/20; inversion time msec = 100–150). However, susceptibility to gradient inhomogeneity makes fat suppression techniques less optimal than STIR techniques in imaging the ankle and foot. Cartilage abnormalities can be visualized with two-dimensional or three-dimensional (3D) gradient-echo sequences.

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