Parallel transmit pulse design for saturation homogeneity (PUSH) for magnetization transfer imaging at 7T

Abstract

Purpose This work proposes a novel RF pulse design for parallel transmit (pTx) systems to obtain uniform saturation of semisolid magnetization for magnetization transfer (MT) contrast in the presence of transmit field B1+ inhomogeneities. The semisolid magnetization is usually modeled as being purely longitudinal, with the applied B1+ field saturating but not rotating its magnetization; thus, standard pTx pulse design methods do not apply. Theory and Methods Pulse design for saturation homogeneity (PUSH) optimizes pTx RF pulses by considering uniformity of root‐mean squared B1+, B1normalrms, which relates to the rate of semisolid saturation. Here we considered designs consisting of a small number of spatially non‐selective sub‐pulses optimized over either a single 2D plane or 3D. Simulations and in vivo experiments on a 7T Terra system with an 8‐TX Nova head coil in five subjects were carried out to study the homogenization of B1normalrms and of the MT contrast by acquiring MT ratio maps. Results Simulations and in vivo experiments showed up to six and two times more uniform B1normalrms compared to circular polarized (CP) mode for 2D and 3D optimizations, respectively. This translated into 4 and 1.25 times more uniform MT contrast, consistently for all subjects, where two sub‐pulses were enough for the implementation and coil used. Conclusion The proposed PUSH method obtains more uniform and higher MT contrast than CP mode within the same specific absorption rate (SAR) budget.

Efficient optimization of chemical exchange saturation transfer MRI at 7 T using universal pulses and virtual observation points

“…We note that the use of subject specific TPs optimized under VOP energy management constraints has been reported recently for magnetization transfer imaging 28 with similar performance as that presented in this manuscript. From a practical point of view, the approach introduced here, the use of UPs and VOPs, brings a substantial improvement as it considerably simplifies the workflow in a clinical setting.…”

“…As the efficiency of the solution depends on the variability of $$$ {\mathrm{B}}_1^{+} $$$ profile in the optimization region, the performance can change drastically from one region of interest to another. As mentioned by Leitao et al, 28 optimized pTx methods (and it is also the case of the one presented here) can greatly benefit from the development of coils with a larger number of transmission channels or increased complementarity between channels. The presented implementation takes advantage of pTx only for the CEST presaturation module but the excitation can also be performed in pTx mode in order to enhance the flip‐angle homogeneity and the SNR as in Libert et al 40 In vivo imaging is inevitably susceptible to subject motion, which can have deleterious effects on CEST contrast maps 41 .…”

“…The first pulse optimization result was that increasing the number of modes beyond two does not significantly improve $$$ {\mathrm{B}}_{1,\mathrm{RMS}}^{+} $$$ homogeneity, as observed previously by Leitão et al 28 We therefore only focused on the optimization of two modes. Typical TPs optimized for hydroxyl‐weighted CEST acquistions for a given volunteer are shown in Figure 2A.…”

“…In the Multiple interleaved mode saturation (MIMOSA) method, 26 the authors use a combination of two modes presenting spatial complementary without taking into consideration the exact $$$ {\mathrm{B}}_1^{+} $$$ spatial distributions. An additional gain in saturation quality has been obtained by using pre‐acquired $$$ {\mathrm{B}}_1^{+} $$$ maps to find the optimal parameters for individual modes by solving an optimization problem, 27,28 with the drawback of increasing the complexity and the workload during the exam. Importantly, current common practice consists in monitoring the Specific Absorption Rate (SAR) by using a very conservative power threshold, provided by the vendor.…”

B1+$$ {B}_1^{+} $$‐correction of magnetization transfer saturation maps optimized for 7T postmortem MRI of the brain

“…However, if identical experimental conditions are achieved across the entire brains and between acquisitions (e.g. by using parallel transmit‐based approaches 17 ), $$$ \mathrm{MTsat} $$$ provides semi‐quantitative maps, proportional to the tissue macromolecular pool fraction. Alternatively, $$$ \mathrm{MTsat} $$$ values can be corrected for their dependence on the MT saturation pulse properties (such as shape, length, and amplitude).…”