Space Echo Planar Imaging

FIGURE 14.3 Pulse sequence for echo-planar imaging. Slice selection is carried out as in Fig. 14.2. The readout gradient Gy is rapidly switched in direction in synchrony with application of a pulsed phase-encode gradient G as indicated. Data are acquired while Gy is turned on but Gx is off. 

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Three sampling strategies are now introduced echo planar imaging (EPI), rapid acquisition with relaxation enhancement (RARE), and low excitation angle imaging. The first two are based on the sampling of more than one line of k-space for each r.f. excitation, and the third uses rapid multiple r.f. excitations. 

Fig. 5.4.2 [Ljul] Trajectories in k space, (a) Line-scan method, (b) Back-projection method, (c) Modified back-projection method, (d) Fourier imaging, (e) Echo-planar imaging.

Figure Bl.14.4. k-space representations of the (a) spin-echo imaging pulse sequence in figure Bl.14.1. (b) echo-planar imaging sequence in figure Bl. 14.3 and (c) back-projection imaging. In (a) and (b) the components of the wave vectors and represent frequency and phase encoding, respectively. For back-...

As a consequence of the Maxwell equations, linear field gradients are accompanied by additional spatially dependent field components. Then, an asymmetric coil generates concomitant field terms of zeroth and first order in space. The formalism was used experimentally to compensate for artifacts observed in three different imaging methods an image shift in standard echo planar imaging (EPI), an echo shift in diffusion-weighted EPI, and a phase shift in a flow quantification technique based on phase contrast images. 

The fast spin-echo (rapid acquisition relaxation enhanced (RARE)) sequence is the spin-echo equivalent of echo planar imaging, and its structure is shown in Figure 2E. Contrast is manipulated by choosing at which point in the echo train the central point of fe-space is acquired. 

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