Synopsis
MRI exam times are typically much longer compared to other imaging techniques, motivating the development of methods for accelerating image acquisition. This lecture will focus on simultaneous multislice acquisitions, providing the background for the initial developments and describing some of the latest state of the art methods.
Simultaneous Slice Excitation
Although simultaneous multislice (SMS) has been around for a long time, several developments were required before it could achieve widespread interest from the MRI community.
SMS excitation was introduced in the 1980’s with the motivation to improve signal-to-noise ratio (SNR). Since at that time only single channel coils were available for signal reception, no actual gain in scanning times could be attained but by exciting $$$N$$$ slices at the same time, it would be possible to improve SNR by a factor of $$$\sqrt{N}$$$. To achieve slice separation, RF phase modulation was performed to either: 1) enable Hadamard encoding, requiring $$$N$$$ acquisitions for separating $$$N$$$ slices acquired with a standard field-of-view (FOV) through appropriate combination (adding/subtracting) of the images1, or 2) to induce a relative spatial shift in the phase-encode direction, requiring $$$N$$$ times more phase encoding steps to resolve an $$$N$$$ times larger virtual FOV2.
The Importance of Parallel Imaging
With the development of multi-channel coils and introduction of parallel imaging reconstruction algorithms, it was finally possible to use SMS to reduce acquisition times. The first demonstration was achieved using a spinal array coil, taking advantage of the spatial arrangement of the coil elements to enable slice separation3. Unfortunately the geometry of the available brain coils was less favourable for SMS application, particularly in the case of axial slices, since coil sensitivity patterns barely varied between slices. In that case, a crucial development was the idea of, once again, using RF phase modulation to induce a relative shift between slices so that pixels with very different coil sensitivities would overlap instead - Controlled Aliasing In Parallel Imaging Results In Higher Acceleration (CAIPIRINHA)4; this step was crucial to enable slice separation while avoiding excessive noise amplification due to unfavourable coil geometry (geometry factor).Combining SMS and Echo Planar Imaging
Although CAIPIRINHA did provide a solution for multi-shot acquisitions, RF phase modulation could not be used to speed-up single-shot Echo Planar Imaging (EPI) acquisitions. The solution in that case was to introduce extra gradient blips applied along the slice-encode direction to introduce the required slice shifts. The first SMS-EPI implementation maintained the same gradient polarity throughout the whole readout, enabling to adjust the shift distances by varying the blip area5. The downside of this method was that it resulted in phase dispersion across the slice thickness, leading to signal loss and image blurring. This issue was addressed by refocusing with subsequent blips, using different patterns to achieve the desired slice shifts (blipped-CAIPI)6.Killer Applications
It was only when application examples of SMS-EPI started to be published that interest in this technique really took off. This was also helped by the introduction of receiver coils with increasingly higher channel numbers, with multi-ring geometries which would guarantee a manageable level of noise amplification following slice separation even without requiring CAIPI shifts. The combination of the two made it feasible to achieve even higher slice acceleration factors.The motivation for applying SMS to EPI was to increase the image acquisition rate. In functional MRI (fMRI) applications the acquisition rate determines the temporal resolution7 while in diffusion MRI (dMRI) it sets the maximum number of diffusion directions and/or weighting levels which can be sampled within a given scan time8. Another important application is Arterial Spin Labelling, where imaging is carried out following inversion of blood magnetization in a different location. Since tagged magnetization suffers relaxation decay, imaging time is limited and application of SMS enables to increase brain coverage9.Multiband RF Pulse Developments
A challenge that remained to be addressed involved the generation of multiband RF pulses. The simplest approach consists of multiplying a single band (SB) RF pulse by a modulation function with spatial frequency components at the desired slice locations, but can easily lead to large B1 amplitudes, exceeding the RF amplifier limits. Although several approaches have been proposed to ameliorate this problem, including time-shifting10 or finding the optimal complex weights for combining the individual SB pulses11, RF pulse lengths tended to be significantly stretched or the flip angle needed to be reduced to to avoid affecting the achievable repetition times (TR), potentially limiting the achievable image contrast. Since in non-EPI sequences RF pulses can take up a large fraction of the TR, recent efforts have been focusing on keeping MB RF pulses shorter while obeying the specific absorption rate (SAR) limits. The state of the art consists of employing joint time-optimal slice-selection gradient and RF pulse design12. It is also of the utmost importance to take into account the achievable hardware performance, namely the gradient temporal bandwidth to avoid slice profile errors13.Acknowledgements
RGN has received funding from the POR Lisboa 2020 program (grant number LISBOA-01-0145-FEDER-029686).References
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