MultiBand/Simultaneous Multi-Slice ImagingHow does Simultaneous Multi-Slice (SMS) imaging work?
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Simultaneous Multi-Slice (SMS) imaging, also known as MultiBand (MB) imaging, employs complex RF-pulses together with parallel imaging coil arrays to acquire several sections along the z-axis simultaneously. This allows a significant reduction in image acquisition time with little signal-to-noise penalty.
As described in a prior Q&A, the historical precursor to modern MB/SMS methods was POMP (Phase Offset MultiPlanar) imaging. The POMP technique used a composite RF-pulse to simultaneously excite two slices, each of which were phase-encoded over only one-half of the total field-of-view (FOV). Phase alternation allowed the two slices to be separately reconstructed without overlap. Unlike SMS, POMP does not utilize parallel imaging technology and is hence applicable to single-channel RF coils.
MultiBand, like POMP, begins with a composite pulse that excites several slices simultaneously. MB pulses can be designed in several ways, but the most straightforward method is to sum several standard RF waveforms with different phase offsets together. The number of simultaneous slices is referred to as the MB or SMS acceleration factor. In current commercial implementations (Siemens' SMS, GE's HyperBand, and Philips' Multi-band SENSE), acceleration factors of 2-4 are typical.
To take advantage of parallel imaging acceleration, MB uses coil encoding together with either gradient- or RF-encoding to resolve data along the slice-select (z)-axis. Because modern coil arrays typically have only a few coil elements in the z-direction, coil sensitivity differences along that axis are rather poor. Accordingly the simultaneously excited slices must be spaced widely apart (typically at least 25−30 mm) as shown in the diagram. An interleaving technique is then used to excite missing slices in the gaps.
To overcome the limitations of coil-encoding, the CAIPIRINHA technique was developed. CAIPIRINHA stands for "Controlled Aliasing In Parallel Imaging Results In Higher Acceleration" and is the subject of a later Q&A. The underlying idea is that to avoid overlap in reconstruction, simultaneously excited slices can be shifted within the phase encoding direction in a "controlled" fashion. Hence slices very close together can be separated, even when a coil has little encoding power in the z-direction. Currently the most widely used method for BOLD and diffusion-weighted SMS imaging is called blipped-CAIPI, so named because of the use of blipped rewinder gradients to keep phase accumulation within the range of ±180º while imparting a constant phase shift between adjacent lines of k-space. Reconstruction of the SMS data can be performed using either SENSE, GRAPPA, or hybrid techniques. Because of the large amount of data rapidly acquired, computation demand is high and image reconstruction times can be long.
The blipped-CAIPI method for echo-planar imaging. A MB pulse simultaneously excites three (3) slices defined together with the slice-select gradient. In-plane k-space is traversed by oscillation of the frequency (readout) gradient with stepping along the phase-encode direction. "Blipped" gradients pulses (red) are applied along the slice-select direction during the readout gradient switching and simultaneously with the phase encoding pulses. This creates the constant required phase modulation between the simultaneously excited slices. Every third blipped pulse is of larger magnitude and opposite polarity serving to rewind the net phase and normalize the gradient moment. The end result is "controlled" aliasing over FOV/3.
Unlike standard parallel imaging acceleration techniques, MB/SMS acceleration results in little to no penalty in signal-to-noise. This is because neither the echo train length, number of phase-encoding steps, nor number of k-space samples has been reduced as occurs in conventional parallel imaging acceleration methods. Eddy currents due to rapid gradient switching may still create Nyquist-like ghosts that affect SMS images. Another artifact resulting in decreased imaging quality in SMS imaging is known as residual aliasing or slice leakage. This reconstruction artifact can be very insidious, manifest only as hazy noise on certain images. It is especially problematic for fMRI studies that depend on the same expected signal from adjacent slices in an acquisition.
References
Barth M, Breuer F, Koopmans PJ, et al. Simultaneous Multislice (SMS) imaging techniques. Magn Reson Med 2016; 75:63-81. (Excellent recent review).
Feinberg DA, Setsompop K. Ultra-fast MRI of the human brain with simultaneous multi-slice imaging. J Magn Reson 2013; 229:90-100.
Hargreaves BA, Cunningham CH. Nishimura DG, Conolly SM. Variable-rate selective excitation for rapid MRI sequences. Magn Reson Med 2004; 52:590-597.
Setsompop K, Gagoski BA, Polimeni JR, et al. Blipped-controlled aliasing in parallel imaging for simultaneous multislice echo planar imaging with reduced g-factor penalty. Magn Reson Med 2012; 67:1210-1224.
Barth M, Breuer F, Koopmans PJ, et al. Simultaneous Multislice (SMS) imaging techniques. Magn Reson Med 2016; 75:63-81. (Excellent recent review).
Feinberg DA, Setsompop K. Ultra-fast MRI of the human brain with simultaneous multi-slice imaging. J Magn Reson 2013; 229:90-100.
Hargreaves BA, Cunningham CH. Nishimura DG, Conolly SM. Variable-rate selective excitation for rapid MRI sequences. Magn Reson Med 2004; 52:590-597.
Setsompop K, Gagoski BA, Polimeni JR, et al. Blipped-controlled aliasing in parallel imaging for simultaneous multislice echo planar imaging with reduced g-factor penalty. Magn Reson Med 2012; 67:1210-1224.
Related Questions
Can you excite two slices at once?
What is CAIPIRINHA? Isn't it some sort of a drink?
Can you excite two slices at once?
What is CAIPIRINHA? Isn't it some sort of a drink?