Phase-encoded imaging

Just as we obtain the spin density distribution q(i), and hence the projection of the investigated object, in phase encoded imaging, we can reconstruct P(R, A) by... 

Figure 1.7. Surface plots of the spectral-spatial experiments, (a) Pulsed spectral-spatial frequency encoded experiment, (b) cw analogue of (a), (c) Pulsed spectral-spatial phase encoded image. (Ewert et al 1991). Reproduced with permission.

Figure Bl.14.1. Spin warp spin-echo imaging pulse sequence. A spin echo is refocused by a non-selective 180° pulse. A slice is selected perpendicular to the z-direction. To frequency-encode the v-coordinate the echo SE is acquired in the presence of the readout gradient. Phase-encoding of the > -dimension is achieved by incrementmg the gradient pulse G...

Once a slice has been selected and excited, it is necessary to encode the ensuing NMR signal with the coordinates of nuclei within the slice. For each coordinate (x andy) this is achieved by one of two very closely related means, frequency encoding or phase encoding [1]. In this section we consider the fonner and in the next, the latter. In tlie section after that we show how the two are combined in the most coimnon imaging experiment. 

Figure Bl.14.7. Chemical shift imaging sequence [23], Bothx- andj -dimensions are phase encoded. Since line-broadening due to acquiring the echo in the presence of a magnetic field gradient is avoided, chemical shift infonnation is retained in tire echo.

A problematic artifact associated with MRI arises when the imaged subject moves duriag acquisition of the / -space data. Such motion may result ia a discontiauity ia the frequency-encoded or phase-encoding direction data of / -space. When Fourier transformed, such a discontiauity causes a blurred band across the image corresponding to the object that moved. Such an artifact ia an image is referred to as a motion artifact. 

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 ... 

The recipe for the first building block of NMR imaging, the phase encoding, thus goes like this apply a phase gradient of effective area k acquire the signal S(k) repeat for a number of different equidistant values of k perform the inverse... 

Fig. 1.9 Schematic plot of a basic three-dimensional imaging pulse sequence with frequency encoding along one axis (read), and phase encoding along the two remaining orthogonal directions. The choice of directions is arbitrary, as is the position of the phase gradients within the sequence.

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. 1.16 Typical motion-compensated, velocity encoding imaging sequence. The basic scheme from Figure 1.11 is amended by additional compensating gradient pulses for the read, phase and slice directions. The velocity encoding pair of gradient pulses of duration 8 and separation A is drawn on a separate line in reality, it is applied along any of the three...

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.

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