Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

ing time is approximately equal to 192 X 2 X 0.03 seconds = 15.5 seconds. A conventionalspin echo requires a time equal to 3.84 minutes for a TR = 600 msee. Thecompromises of the GRE pulse sequence include SNR losses, magnetic susceptibilityartifacts, and less immunity from magnetic field inhomogeneities. In addition,the multiple slice acquisition with the spin echo sequence reduces the effective timeper image, and thus the comparison of multiple slice acquisition with GRE imagingtime is closer than calculated above, since usually only one slice can be acquired witha very short TR interval. There are several acronyms for gradient recalled echo,including GRASS, FISP, Spoiled GRASS, FLASH, ete.Echo Planar Image AcquisitionEcho planar image (EPI) acquisition is a technique that provides extremely fast imagingtime. Single-shot (all of the image information is acquired within 1 TR interval)or multishot EPI has been implemented. For single-shot EPI, image acquisition typicallybegins with a standard 90-degree flip, then a PEG/FEG gradient applicationto initiate the acquisition of data in the periphery of the k-space, followed by a 180-degree echo-producing RF pulse. Immediately after, an oscillating readout gradientand phase encode gradient "blips" are continuously applied to stimulate echo formationand rapidly fill k-space in a stepped "zig-zag" pattern (Fig. 15-23). Thei _~o_oE_ff_eC:i~::.E----··1i ~~stL--~~.---,~--------GradientEcho[[ky [[[FIGURE 15-23. Single-shot echo planar image (EPI) acquisition sequence. Dataare deposited in the k-space with an initial large PEG application to locate theinitial row position, followed by phase encode gradient "blips" simultaneouswith FEG oscillations to fill the k-space line by line by introducing one-rowphase changes in a zigzag pattern. Image matrix sizes of 64 x 64 and 128 x 64are common for EPI acquisitions.