Read gradient

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...

Clearly slice selection and phase encoding form part of the preparation segment of the sequence, evolution occurs under the influence of the PGSE pulse pair, and detection occurs in the presence of the read gradient. Given excitation of a single slice, the number of dimensions is reduced to two in k space and one in q space. We may combine all these influences to write... 

Fig. 14. Radiofrequency and gradient pulse sequence for velocity and diffusion imaging in which the molecular motion is measured in the domain of two spatial dimensions (x, y) of a slice selected normal to the z axis. Note that the PGSE pulse pair (g) provides the third dimension—that of motion—while the phase encode (Gj,) and read gradients (G ) provide the first and second spatial dimensions.

The method has been given the acronym MA55iE y for modulus addition using spatially separated echo spectroscopy [Cal5]. Compared to coherent superposition of echoes in the absence of phase fluctuations and a read gradient the method is associated with a loss in signal-to-noise-ratio by a factor of where n is the number of points under the image obtained by Fourier transformation of the echo. This is a small price to pay for access to displacements on the scale of lOnm by NMR. 

Application of the read gradient Gx and acquisition of n data points. 

The basic principle is depicted in Fig. 6.2.16(a) [Cho2]. The rf excitation is a train of small flip-angle pulses forming a DANTE sequence, i.e., the sum of flip angles is 90° (cf. Section 5.3.4). They are applied while the read gradient Gx is tuned on. The response to the DANTE excitation is refocused in a Hahn echo, which itself consists of a train of echoes. This echo train is detected in the presence of the read gradient Gx... 

Fig. 6.2.16 [Cho2] 2D DANTE-imaging. (a) Timing of signals. The rf excitation is a DANTE pulse train applied in a read gradient Gx- The Hahn echo of the response is detected while both the phase-encoding gradient Gy and the read gradient Gx are turned on. The slice-selection gradient is Gj. (b) Traces in k space.

By admitting the second harmonic at three times the gradient modulation frequency, the advantages of schemes (b) and (c) are combined (Fig. 8.2.2(a)) [Codl]. Here the read gradient for frequency-encoding is modulated in amplitude according to... 

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