Following regulatory approval of single-transmit MRI at 7 tesla, there is a rapidly growing interest in clinical MRI at this field strength. However, the wider use of diagnostic MRI at 7T will require imaging in parallel-transmit (pTx) mode to reduce B1+ inhomogeneity. Previous work introduced a dual-mode head coil that operates in both transmit modes and this study investigates the use of this coil for the pTx case. It also describes the safety procedure that was followed to ensure safe operation for human scanning using the real-time SAR supervision on a commercial scanner

Allwood-Spiers, Sarah

Foster, John E.

Gunamony, Shajan

McElhinney, Paul

Porter, David A.

Tao, Yuehui

Williams, Sydney

English

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06/10/2020 https://cds.ismrm.org/protected/20MPresentations/abstracts/4038.htmlhttps://cds.ismrm.org/protected/20MPresentations/abstracts/4038.html 1/34038Validation and Safety Approval of a Dual-Mode Head Coil for pTx Applications In Vivo at 7TeslaSydney N. Williams , Sarah Allwood-Spiers , Paul McElhinney , Yuehui Tao , John E. Foster , David A. Porter , and Shajan GunamonyImaging Centre of Excellence, University of Glasgow, Glasgow, United Kingdom, MRI Physics, NHS Greater Glasgow and Clyde, Glasgow, United Kingdom, Imaging Centre of Excellence, Siemens Healthcare Ltd.United Kingdom, Glasgow, United Kingdom, MR CoilTech, Glasgow, United KingdomSynopsisFollowing regulatory approval of single-transmit MRI at 7 tesla, there is a rapidly growing interest in clinical MRI at this field strength. However,the wider use of diagnostic MRI at 7T will require imaging in parallel-transmit (pTx) mode to reduce B1+ inhomogeneity. Previous workintroduced a dual-mode head coil that operates in both transmit modes and this study investigates the use of this coil for the pTx case. It alsodescribes the safety procedure that was followed to ensure safe operation for human scanning using the real-time SAR supervision on acommercial scanner.IntroductionThe higher spatial resolution provided by 7T MRI of the brain leads to improved diagnostic confidence compared to scanning at 3T [1] and there is anincreasing interest in clinical applications. Regulatory approval has been granted in the USA and EU for clinical 7T imaging in single-transmit (1Tx) mode,but this is not yet the case for parallel-transmit (pTx) methods [2], which overcome the poor B1+ homogeneity at higher field strengths [3]. The currentgeneration of commercial 7T systems support both modes of operation, but require separate RF coils in each case. Previous work [4] addressed this issue,by introducing an open-face, dual-mode coil design that can be used for both 1Tx and pTx data acquisition. This study describes the initial application ofthis coil type for pTx scanning in vivo, including details about the safety approval process used to address the complex SAR considerations required forpTx operation.MethodsThe dual-mode head coil consists of 8 transmit and 32 receive elements, and interfaces with both 1Tx and pTx modes of the 7T scanner (MAGNETOMTerra, Siemens Healthcare, Erlangen, Germany). The coil is locally shielded and designed with large eye cutouts to provide an open feel for patient comfortby optimizing the design in CST Studio Suite (Dassault Systemes) [4,5]. Extensive validation was required for local ethics committee approval in pTx mode,as the superimposed RF pulses can generate locally elevated SAR values that are several times greater than the average whole-head value [6].Following a similar heating experiment as Hoffman et al. [7], a sugar-salt-agar phantom was heated for 17min in two experiments using different transmitmodes: circularly polarized (CP) with 23.1W, and a second phase configuration with 21.9W (amplitudes equal across channels). The phase images of agradient echo scan before and after heating were used to estimate temperature change within the phantom [8] and were corroborated with a CSTsimulation.The fidelity of each transmit channel was validated by comparing experimental and simulated single-channel B1+ maps. Experimental maps were collectedin a head-and-shoulders phantom with a presaturation TurboFLASH sequence [9].The SAR monitoring on the 7T Terra is implemented by virtual observation points (VOPs) [6]. These were generated with an overestimation of 10% for ourcoil by concatenating three 1mm isotropic body model simulations (Duke and Ella from the Virtual Family cohort [10] and Gustav from CST), each at threelongitudinal positions. By design, VOPs will always meet or exceed full-body model SAR estimates, and maximum SAR reported by the scanner will beeven more conservative. To test this relationship, the CST- and VOP-based maximum 10g SAR values were estimated for fixed input power and three pTxmodes: CP, CP2+ (fixed amplitude and 90° phase between channels), and the 'worst case' excitation vector generated during VOP compression. Thesevalues were compared to the scanner-reported SAR for the same power. After above tests, the coil was approved by local safety and ethics committees for scanning healthy subjects in pTx mode. To test the functionality of pTx, aconsented volunteer was scanned with the TurboFLASH B1+ mapping protocol (single channel and combined modes), as well as a T1-weighted FLASHand T2-weighted TSE. A CP-mode configuration was compared to an optimized flip angle (FA) magnitude least-squares shim [11] with constraints on flipangle standard deviation and SAR.ResultsFigure 1 shows our dual-mode coil and the corresponding CST simulation of the coil and a home-built head-and-shoulders phantom.Figure 2 compares simulated and measured temperature maps from the heating tests conducted in the sugar-salt-agar gel phantom. The distribution andtemperature rises between simulation and measured data are in good agreement, with differences attributed to the time-scales of CST (instantaneouspower) vs. experiment (power deposition over 17min). Figure 3 shows simulated and experimental complex B1+ maps for the head-and-shoulders phantom. After subtracting the phase from a set reference coil,both magnitude and phase maps are in excellent agreement. Figure 4 contains two tables. Table A reviews maximum local SAR estimations from the CST model, VOP compression, and reported scanner values. Asdesired, the scanner is the most conservative. Table B reports flip angle mean, standard deviation, input power, and SAR for the scan performed in vivo,comparing CP and B1+ shim modes. An added benefit of the pTx mode is that it had lower local SAR than conventional CP. Figure 5 shows the pTx coil results for a CP mode acquisition and B1 shims optimized to the volunteer B1+ map. In both the FLASH and TSE images thereis significant improvement in the left temporal lobe signal that was lost to B1+ inhomogeneity.Discussion and ConclusionAlthough 7T MRI can improve diagnostic confidence, wider clinical application is limited by B1+ inhomogeneity. This strongly motivates to integrate pTxmethods into routine examinations and to explore optimized coil designs that suit clinical requirements. The coil used in this study provides clinical benefit1 2 1 3 2 1 1,41 2 3406/10/2020 https://cds.ismrm.org/protected/20MPresentations/abstracts/4038.htmlhttps://cds.ismrm.org/protected/20MPresentations/abstracts/4038.html 2/3by avoiding the closed design used by other 7T head coils and by operating in both 1Tx and pTx modes. The coil improved image uniformity using B1shimming and passed a rigorous safety approval procedure, verifying that real-time scanner supervision keeps local SAR values within safe limits.AcknowledgementsPatrick Liebig, Rene Gumbrecht, Natasha Fullerton, Tracey Hopkins, Rosemary Woodward, Graeme KeithReferences[1] Springer et al., “Comparison of routine brain imaging at 3 T and 7 T,” Invest. Radiol., 2016.[2] T. Ibrahim et al., “Application of finite difference time domain method for the design of birdcage RF head coils using multi-port excitations,” Mag. Reson.Im., 2000.[3] T. Ibrahim et al., “Effect of RF coil excitation on field inhomogeneity at ultra high fields: a field optimized TEM resonator,” Mag. Reson. Im., 2000.[4] G. Shajan et al., “An 8-channel transmit 32-channel receive 7T head coil for 1Tx and pTx scanner modes,” Proc. Int. Soc. Mag. Reson. Med., 2019.[5] P. McElhinney et al., “Computational simulation of an 8Tx/32Rx open-face head coil for 7 tesla,” Proc. Brit. Chap. Int. Soc. Mag. Reson. Med., 2019.[6] G. Eichfelder et al., “Local specific absorption rate control for parallel transmission by virtual observation points,” Mag. Reson. Med., 2011.[7] J. Hoffmann et al., “Safety testing and operational procedures for self-developed radiofrequency coils,” NMR Biomed., 2015.[8] P. Ehses et al., “MRI thermometry: fast mapping of RF-induced heating along conductive wires,” Mag. Reson. Med., 2008.[9] S. Chung et al., "Rapid B1+ mapping using a preconditioning RF pulse with TurboFLASH readout," Mag. Reson. Med., 2010.[10] A. Christ et al., “The virtual family—development of surface-based anatomical models of two adults and two children for dosimetric simulations,” Phys.Med. Biol., 2012.[11] K. Setsompop et al., “Slice-selective RF pulses for in vivo B1 inhomogeneity mitigation at 7 tesla using parallel RF excitation with a 16-element coil,”Mag. Reson. Med., 2008.FiguresFigure 1A: The dual-mode head coil and the splitter used to interface the coil to the scanner when in 1Tx mode. The large cut-outs allow for fixing a ‘look-out’ mirror which improves patient comfort. Figure 1B: A screenshot of the coil model created in CST and loaded with our home-built head and shoulderphantom filled with tissue equivalent solution.Figure 2: CST simulated temperature map for heating experiment conducted in CP mode (A) and a second phase mode (C). Corresponding experimentaltemperature maps for CP mode (B) and the same second phase mode (D).Figure 3: Simulated B1+ magnitude (A) and phase (C) maps in the head-and-shoulders B1+ phantom. Matching experimental B1+ magnitude (B) andphase (D) maps for the same location in the phantom.06/10/2020 https://cds.ismrm.org/protected/20MPresentations/abstracts/4038.htmlhttps://cds.ismrm.org/protected/20MPresentations/abstracts/4038.html 3/3Figure 4: Table (A) compares the SAR values from simulation, VOPs, and the scanner for three pTx modes. Table (B) shows the simulated normalized rootmean-squared error (NRMSE), simulated flip angles metrics, experimental flip angles, and power/SAR measurements for the CP and B1+ shimmed (pTx)modes scanned in vivoFigure 5: Experimental images for one slice with the pTx coil in CP mode (A) and with B1+ shimming (B). The left column shows the combinedTurboFLASH FA map (FOV=300x300x255mm , matrix=256x256x51, TE=1.75, FA=5°), the middle column T1w FLASH (FOV=195x240x195mm ,matrix=512x416x51, TE/TR=2.2/418ms, FA =30°), and the right column the T2w TSE (FOV=195x240x93.6mm , matrix=512x416x25, ETL=9,TE /TR=57/1000ms, FA =135°). There’s a clear improvement of B1+ in the left temporal lobe using a pTx B1+ shim.3 3ref 3eff refProc. Intl. Soc. Mag. Reson. Med. 28 (2020)4038

Validation and Safety Approval of a Dual-Mode Head Coil for pTx Applications In Vivo at 7 Tesla

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Validation and Safety Approval of a Dual-Mode Head Coil for pTx Applications In Vivo at 7 Tesla

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