Slice selection phase

Closer examination of equation B 1,14,3 reveals that, after the slice selection pulse, the spin isocln-omats at different positions in the gradient direction are not in phase. Rather they are rotated by i exp jyC. )tind... 

In this section, we will describe three building blocks of NMR imaging phase encoding, frequency encoding and slice selection. All three are related to the signal by the fourth equation ... 

Fig. 1.11 Typical basic three-dimensional negative intensity directly before the actual imaging sequence with slice selection, frequen- read gradient. The shape of the 180° rf pulse cy encoding and phase encoding in three ortho- is drawn schematically to indicate that a soft gonal directions. The compensating lobe for pulse is used, the read gradient is drawn as a rectangle with...

Fig. 1.21 Echo Planar Imaging (EPI) pulse sequence. Gradient-echo based multiple echoes are used for fast single-shot 2D imaging. Slice selection along Gs and frequency encoding along C, are utilized. Phase encoding is realized using short blipped gradient pulses along Gp.

As a final example, we mention the combination of solid-state spectroscopy (Chapter 15) with imaging. The imaging of solids is difficult because the 7Ys are often too short compared with the time it takes to switch and apply field gradients for slice selection and phase and frequency encoding, while in addition extremely large gradient amplitudes are required to overcome the broad line widths. The time to ramp... 

FIGURE 14.2 Pulse sequence for a two-dimensional NMR image in the xy plane, with slice selection along z. The value of the phase-encode gradient Gx is altered in successive repetitions, as indicated. The rf pulses are frequency selective and define the slice while gradient Gz is applied. Within one value of Gx there may be several repetitions with different values of frequency to obtain data from multiple slices. Data are acquired from the spin echo while the readout gradient Gy is applied. 

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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