K-space raster

This section introduces the reader to the basic principles of MRI and the concept of the k-space raster. The basic MRI pulse sequence, the spin-echo imaging sequence, is described at this point. For more detailed discussion of the background theory of MRI the interested reader should refer to texts by Callaghan5 and Kimmich.6... 

To understand imaging pulse sequences the concept of the k-space raster is used.7 Rewriting eqn (2) for the general case of the variation of resonance frequency with spatial position r we obtain ... 

Fig. 6. (a) Schematic representation of a simple slice-selective 2-D spin-echo pulse sequence. In this pulse sequence the magnetic field gradient (G, ) is varied for successive acquisitions of different rows of the k-space raster, (b) The corresponding k-space raster used to show how we interpret the pulse sequence. Following a sufficient Ti-relaxation period, the sequence is repeated to acquire a second row of the k-space raster. Acquisition of each row of k-space requires a separate r.f. excitation and application of a G,-gradient of different magnitude. 

Thus the imaging experiment is seen as acquisition of data in the time domain, sampling the k-space raster, followed by Fourier transformation to the frequency domain, which in turn is directly related to real space. 

In this example, the gradient applied in the y-direction is referred to as the phase encoding gradient. Phase encoding refers to the action of an applied gradient that is responsible for moving the acquisition through the k-space raster. In this case, the... 

Fig. 14. MBEST-EPI (a) slice-selective pulse sequence and (b) corresponding k-space raster.

While the simple schematic of Fig. 4 allows us to appreciate the concept of obtaining spatial resolution in the measurement, it is almost impossible to understand and design MRI pulse sequences using this approach. Instead, the approach used is that of the so-called k-space raster, introduced by Mansfield (1977). 

Fig. 21. (a) The rotating RARE pulse sequence, and (b) the associated k-space raster. The pulse sequence is a variant of the RARE pulse sequence with the read and phase directions rotated with respect to the laboratory-frame after each echo acquisition such that the orientation of k-space relative to the rotating object remains constant. After each echo acquisition, the system is returned to the centre of k-space, which is not affected by sample rotation. 

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