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Improving RF efficiency in the brain and the neck at 7T using a novel pTx coil1CUBRIC, School of psychology, Cardiff University, Cardiff, United Kingdom, 2MR Coils, Zaltbommel, Netherlands, 3Department of Radiology, University Medical Center Utrecht, Utrecht, Netherlands
Synopsis
MRI at UHF is promising for a variety of clinical applications however it is challenging because of B0 and B1 inhomogeneities and SAR limitations. To improve the RF efficiency in the brain and the neck, a novel 8Tx/32Rx RF coil has been built by MR coils. Its performance in terms of B1+ intensity and homogeneity at the brainstem/cerebellum and the neck was assessed and compared with the standard Nova Medical 8Tx/32Rx coil. The novel coil allows for a higher B1+ at the brainstem/cerebellum and the neck where B1+ mapping and shimming need further optimisation for ASL application.
Introduction
MRI at ultra-high field (≥7T) is promising for a variety of clinical applications due to the improved intrinsic signal-to-noise ratio (SNR) and susceptibility induced contrast. However, it is technically challenging due particularly to transmit field (B1+) inhomogeneities and Specific Absorption Rate (SAR) limitations. The rapid drop of B1+ toward the brainstem and the neck due to the limited extent of the standard head coils in addition to the large B1+ and B0 inhomogeneities make clinical applications in those regions much more challenging. Imaging brainstem and cerebellum as well as quantifying blood flow using pseudo-Continuous Arterial Spin Labelling (pCASL) are applications in which substantial B1+ is required in both the neck and the brain. To overcome these pitfalls, different approaches have been adopted like designing novel coils1, designing tailored RF pulses2 or parallel transmit (pTx) techniques (RF shimming3,4 and transmit SENSE5,6 ). In this work, the performance of a novel 8Tx/32Rx built by MRCoils (Zaltbommel, The Netherlands) in terms of B1+ intensity and homogeneity at the brainstem/cerebellum and the neck was assessed and compared with the standard Nova Medical 8Tx/32Rx coil (Wilmington, USA).Materials and methods
Experiments were performed on a whole-body Siemens 7 T scanner (Magnetom) equipped with an 8 channel pTx system with which we used the MRCoils 8Tx/32Rx and Nova 8Tx/32Rx coils. Unlike the Nova coil, the MRCoils device is asymmetrical with three transmitting elements around the neck and 5 around the (Fig1). The B1+ shimming and mapping were performed first at the brainstem/cerebellum and then at the neck (arterial spin labelling plane). As ASL requires optimal B1+ at both neck and brain, additional B1+ maps were acquired within the brain. The performance of the MRCoils coil was compared with the Nova coil. To perform those measurements, ten healthy volunteers were imaged. After the standard B0 shimming and frequency adjustment, phase and magnitude images corresponding of the individual transmitting elements were acquired using a Siemens gradient-echo sequence (TE/TR=2.1/100ms, spatial resolution=3.1x3.1x5mm, TA=8s). B1 maps were acquired using a Siemens multi-slice sequence based on turbo-flash readout (TE/TR=1.9ms/10s, spatial resolution=3.1x3.1x5mm, TA=1min40s). The B1+ optimisation was based on phase only RF shimming. The images were transferred to a separate computer on which a Matlab script was used to select ROIs and to calculate phase corrections for each element.Results
Using the novel coil, both sagittal and axial B1 maps before and after RF shimming depicted in Fig2 show a clear improvement of RF homogeneity around brainstem/cerebellum. Compared to Nova coil, an improvement of 65% (117° vs. 70°) of flip angle (FA) was obtained (Fig3). RF shimming at both neck and brain are depicted in Fig4a for sagittal and axial slices. RF shimming significantly enhanced B1+ intensity but homogeneity still needs further improvement. When shimming in the neck, destructive interferences and low B1+ intensities can be noticed on FA maps within the brain. To improve B1+ within the brain, a further RF shimming was necessary as shown in Fig5. Due to its symmetry, Nova coil transmits a more homogeneous B1+ in the brain without RF shimming but it drops rapidly towards the brainstem/cerebellum. For that coil, RF shimming at the neck did not improve the intensity nor the homogeneity of B1+ and drastically deteriorates the latter in the brain (Fig4b).Discussion
RF shimming with the novel coil improved significantly both B1+ intensity and homogeneity at the brainstem/cerebellum compared to the Nova coil. However, despite the improvements, the intensity and the homogeneity of B1+ at the neck were not optimal for applications like pCASL because of the adopted B1+ mapping and shimming methods. Indeed, measuring B1+ maps of individual elements separately suffers from a high degree of uncertainty in measuring FAs when the actual FA or the SNR are low. In addition, phase only B1 shimming is sensitive to field inhomogeneities and low SNR. Using a volumetric sequence like DREAM7 for B1+ mapping in combination with multiple interferometric measurements would improve the sensitivity, reduce T2* effects, and drastically reduce the acquisition time. Performing RF shimming using dedicated algorithms would improve both B1+ intensity and homogeneity within the ROI3,8,9. Methods like pCASL needing optimal B1+ within the neck and brain for the labelling and the imaging respectively, would benefit from dynamic B1 shimming approaches that switch between two sets of shimming coefficients during the same TR.Conclusion
Compared to the 8Tx/32Rx Nova coil, the new coil allows for a higher B1+ at the brainstem/cerebellum and the neck where B1+ mapping and shimming need further optimisation for ASL application.Acknowledgements
We would like to knowledge the Wellcome Trust Strategic Award for funding this research and Fabrizio Fasano (Siemens engineer) for his help.References
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