Many different gradient waveforms can be used to scan k-space and to obtain a desired image. The most common approach, called two-dimensional Fourier transform imaging (2D FT), is to scan through k-space along several horizontal lines covering a rectilinear grid in 2D k-space. See Figure 12.1 for a schematic of the k-space traversal. The horizontal grid lines are acquired using 128 to 256 excitations separated by a time TR, which is determined by the desired contrast, RF flip angle, and the T1 of the desired components of the image. The horizontal-line scans through k-space are offset in ky by a variable area y-gradient pulse, which happens before data acquisition starts. These variable offsets in ky are called phase encodes because they affect the phase of the signal rather than the frequency. Then for each ky phase encode, signal is acquired while scanning horizontally with a constant x gradient.
FIGURE 12.1 The drawing on the left illustrates the scanning pattern of the 2D Fourier transform imaging sequence. On the right is a plot of the gradient and RF waveforms that produce this pattern. Only four of the Ny horizontal k-space lines are shown. The phase-encode period initiates each acquisition at a different ky and at −kx (max). Data are collected during the horizontal traversals. After all Ny k-space lines have been acquired, a 2D FFT reconstructs the image. Usually 128 or 256 ky lines are collected, each with 256 samples along kx . The RF pulse and the z gradient waveform together restrict the image to a particular slice through the subject.
