Pushing CT and MR Imaging to the Molecular Level for Studying the “Omics”: Current Challenges and Advancements

Figure 6

Schematic description of the differences between MB excitation and CAIPIRINHA. The slice selection gradient would be applied during the RF excitation, where the RF frequency and the slice resonant frequency were the same. Since MB excitation and CAIPIRINHA excited multiple slices simultaneously, the bandwidth of RF excitation would become wider than the single slice excitation. MB excitation modulated the RF excitation pulse with several frequencies to excite multiple slices, so the acquired images would overlap together without FOV shifting (the upper row). Considering the bandwidth of frequency encoding and the removal of the artifacts, the excitation slices should have gaps with several centimeters depending on the gradient strength. Besides, the pixel skew was another factors controlled by the gradient strength [20]. CAIPIRINHA, which also excited multiple slices simultaneously, controlled the aliasing pattern by phase-modulated RF pulses. In the figure, for example, the phase of RF pulse in the second slice was interlacedly altered to shift the overlapping patterns with FOV/2 (the lower row). The black dashed arrow indicated the phase-encoding line without phase alteration in both slices, and the white one did the phase alteration of 𝜋 only in the second slice. It could improve the 𝑔-factor to increase the SNR in image reconstruction. The technology could get rid of the pixel skew in MB excitation and make adjacent slice excitation feasible.