MultiBand imaging simultaneously excites and encodes multiple imaging slices, and teases them apart with effective usage of the degrees of freedom in modern multi-channel receive arrays. This talk will focus on principles of excitation, encoding and reconstruction of this volumetric acquisition scheme, which has improved the sampling efficiency of functional and diffusion weighted imaging, and found promising applications in structural and quantitative imaging.
MultiBand (MB) or Simultaneous MultiSlice (SMS) imaging encodes information from multiple imaging slices simultaneously. This is performed by exciting multiple imaging slices at the same time, which causes the acquired signal to include contributions from all of the excited locations. Parallel imaging reconstruction techniques (1,2) are then employed to disentangle the individual slices from the acquired “collapsed” slice data.
For most conventional line-by-line (aka spin-warp) acquisitions, scan time reduction achievable by standard parallel imaging and MB approaches is similar, but there are core differences between the two acceleration techniques. In standard “in-plane” parallel imaging, data are undersampled by skipping some of the phase encoding lines to reduce the acquisition time. Since the encoded slice/volume experiences less noise averaging due to the reduction in acquired k-space information, in-plane acceleration leads to sqrt(R) reduction in signal-to-noise ratio (SNR). Here, R denotes the acceleration rate, i.e. the ratio of the number of skipped lines to the sampled ones. This reduction in SNR is intrinsic to the in-plane accelerated acquisition, and is further compounded by “geometry factor (g-factor)” that stems from the imperfect parallel imaging reconstruction, as will be discussed shortly.
MB acceleration, on the other hand, does not suffer from this intrinsic sqrt(R) loss of SNR. This is because the k-space data of each imaging slice are still fully encoded, and each of the slices still experience the same duration of noise averaging as they would without MB acceleration. In this sense, MB/SMS acceleration does not “undersample” k-space. However, the collapsed slice group still needs to be disentangled using the degrees of freedom in multi-channel receiver coils to obtain images of the individual slices. As in all parallel imaging techniques, the limited degrees of freedom in the coil sensitivity information impacts this reconstruction problem. The mathematical noise amplification due to the ill-conditioning of the parallel imaging reconstruction is captured by the g-factor (1).
To improve the conditioning of this inverse problem, it is possible to shift the excited slices relative to each other in the phase-encoding direction, so that the collapsed voxels come from physically distant locations. This helps improve the orthogonality of the coil sensitivity information of the collapsing voxels, thereby mitigating g-factor penalty on retained SNR and increasing the achievable MB factor. Slice shifting can be performed in a couple of different ways. Controlled Aliasing in Parallel Imaging (CAIPI) (3,4) is an umbrella term for such MB acquisitions, whereby the relative slice shifts are obtained by using radiofrequency (RF) pulses with tailored phase. Considering a simple two-slice MB acquisition with spin-warp encoding, this entails using an RF excitation with an alternating phase pattern of {0, pi, 0, pi, …} for the second slice, while the phase of the first slice is not modulated. Due to Fourier shift theorem, a phase ramp in k-space leads to a field of view (FOV) shift in image space. Capitalizing on this idea, this CAIPI scheme shifts the second slice by FOV/2, thereby improving the parallel imaging capability.
For echo planar imaging (EPI), a single RF excitation is utilized to encode all lines within the k-space plane, so an alternating RF phase scheme between k-space lines is not applicable. Still, an FOV shift can be achieved by creating a phase ramp across the slices using the Gz gradient. This idea forms the basis of blipped-CAIPI technique (5), which has allowed MB acceleration techniques to be utilized in EPI acquisitions without voxel tilting artifacts. MB enabled EPI has allowed rapid functional and diffusion weighted MRI acquisitions, and found applications in large scale neuroimaging studies such as the Human Connectome Project (6).
In-plane parallel imaging in EPI acquisition does not lead to significant scan time reduction, but allows reduction in geometric distortions that stem from main field inhomogeneity and blurring due to T2* relaxation. As such, it may be beneficial to perform in-plane and MB acceleration at the same time to find a good trade-off between artifact reduction and faster sampling. Interestingly, MB imaging (potentially in conjunction with in-plane acceleration) admits a 3D k-space representation, which has allowed the unification of reconstruction for 3D and SMS encoded acquisitions (7).
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