Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FEG n n n IL.-ct--Echo ,I.16 ms 32 ms 48 ms 64 ms~--~I!V~--r---JL--~..,----tt--:- Effective-.. TE .Echo train = 4Effective TE = 16 msFIGURE 15-20. Conventional fast spin echo (FSE)uses multiple 180-degreerefocusing RF pulses per TR interval with incremental changes in the PEGto fill several views in the k-space. This example illustrates an echo trainlength of four, with an "effective" TE equal to 16 msec (when the centralarea of the k-space is filled). The reversed polarity PEG steps reestablishcoherent phase before the next gradient application. Slightly different PEGstrengths are applied to fill adjacent areas during each TR, and this continuesuntil all views are recorded. As shown, data can be mirrored usingconjugate symmetry to reduce the overall time by another factor of two.corresponding FSE with an echo train length of 4 (Fig. 15-20) requires about 2.1minutes. Specific FSE sequences for T2 weighting and multiecho FSE are employedwith variations in phase reordering and data acquisition. FSE is also known as"turbo spin echo," or "RARE" (rapid acquisition with refocused echoes).Inversion Recovery AcquisitionThe inversion recovery pulse sequence uses an initial ISO-degree RF pulse to excitethe slice of tissue by inverting the spins. A 90-degree pulse is applied after a delayTI (inversion time) to convert the recovered longitudinal magnetization into thetransverse plane. A second ISO-degree pulse is applied at TE/2 to invert the spinsand produce an echo at a time TE after the 90-degree pulse (Fig. 15-21). DuringRF excitation, the SSG is applied to localize the spins in the desired plane. Phaseand frequency encoding occur similarly to the spin echo pulse sequence. As the TRis typically long (e.g., ~3,000 msec), several slices can be acquired within the volumeof interest. STIR (short tau inversion recovery) and FLAIR (fluid attenuatedinversion recovery) pulse sequences are commonly used.