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A comparison of various custom-built pTx RF arrays for body imaging at 7T with regard to their transmit efficiency1Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany, 2Faculty of Physics and Astronomy, Heidelberg University, Heidelberg, Germany, 3Erwin L. Hahn Institute for Magnetic Resonance Imaging, Universität Duisburg-Essen, Essen, Germany, 4High Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany, 5Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany, 6Faculty of Medicine, Heidelberg University, Heidelberg, Germany, 7Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States
Synopsis
Keywords: RF Arrays & Systems, RF Arrays & Systems, Ultra highfield, 7T, coil comparison, pTx, transmit efficiency
Motivation: A standardized pTx body array would be highly beneficial for 7T, especially as a step toward achieving an extension of the existing 7T CE label/FDA clearance to whole-body MRI.
Goal(s): To compare the transmit performance (Tx efficiency/coverage) of 5 different existing body coils containing different element types.
Approach: Measure relative and absolute B1+ maps and evaluate them regarding the Tx efficiency and Tx heat-foot coverage.
Results: The B1+ efficiency was similar for all local coil concepts with some differences in heat-foot excitation coverage. The remote array achieved the largest heat-foot excitation coverage and can achieve similarly high flip angles as the local arrays.
Impact: B1+ efficiency was similar for all local coil concepts with some differences in excitation coverage. The remote array achieved the largest excitation coverage and can achieve similarly high flip angles as the local arrays if sufficient transmit power is available.
Introduction
The commercially available Nova 8Tx/32Rx head coil (Nova medical Inc., Wilmington, USA) has become the standard coil for head imaging at 7T using parallel transmit (pTx). In recent years, body imaging has become of increasing interest1,2,3, despite the challenges of inhomogeneous B1+ transmit profiles and SAR limitations4. However, until now, no equivalent standard pTx coil exists for body imaging; instead, most sites have developed their own radiofrequency (RF) arrays over the past decade5. A standardized pTx body array would be highly beneficial i) for 7T sites without RF engineering capabilities, ii) for comparability between sites, and iii) for multi-center studies. Furthermore, to achieve a potential extension of the existing 7T CE label/FDA clearance to whole-body imaging, a standardized RF array for body imaging would be desirable. Hence, in search of identifying such a pTx body array, we initiated a study to compare existing arrays from several sites with regard to their transmit and receive performance. This work presents preliminary results of this study focusing on the transmit performance (Tx efficiency and coverage) of five different 7T pTx body arrays containing different element types from three collaborating sites.Methods
Measurements were performed with an 8x1kW pTx system (except Coil5) on the same 7T system (Magnetom 7T, Siemens, Germany). Five RF arrays from three different sites were evaluated. The first four are local arrays intended to be placed directly on the subject; the fifth is a remote array mounted outside the bore liner, similar to a conventional body coil at lower field strength. Coil1: 8TxRx meander stripline array6 and Coil2: 8Tx32Rx combined meander stripline (TxRx) and loop (Rx) array7 from ELH Essen, Coil3: 8Tx16Rx array8 from PTB Berlin using either 8 loops (Coil3L) or 8 dipoles (Coil3D) only for Tx while receiving with all 16 elements, Coil4: 8TxRx array from DKFZ Heidelberg, which is a replica of the Coil1 without the upgraded new coil housing, and Coil5: 32Tx32Rx remote whole-body array9 from DKFZ, which was operated with a 32x2kW pTx system. All coils were centered on the identical phantom that was subsequently placed in the isocenter (see Fig.1a). After manual B0 shimming, transverse 2D multi-slice relative B1+ maps10 (B1R) were acquired. Phase-only B1+ shimming was performed on the isocenter slice maximizing the B1+ efficiency within a circular region-of-interest (ROI; radius of 6cm, c.f. Fig.2) in the phantom center using Matlab. Subsequently, absolute B1+ maps were acquired in the isocenter using a 2D radial MRF sequence (TE=2.35ms, TR=4.7ms, TA=40s, RF pulse duration=1600μs, voxel size=2x2x5mm, base resolution=128) following previous work11 using i) Cp+, ii) Cp2+, and iii) the efficiency shim in transverse, sagittal and coronal orientation. All scans were obtained with a fixed transmitter reference amplitude of 170V per Tx channel. The max and mean B1+ and efficiency value in the ROI was calculated for the efficiency shim for each coil. Second, the excitation coverage in head-foot (h-f) direction was determined as shown in Fig.1b.Results and Discussion
B1+ magnitude and efficiency maps for all arrays are shown in Fig.2. The local arrays (Coils1-4) yield mean (max) B1+ values in the ROI between 3.82 (4.22)μT/√(kW) and 4.28 (4.70)μT/√(kW), with Coil4D achieving the highest B1+ value (Fig.3). Max efficiency was close to 100% for all coils, mean efficiency ranged between 75.8% and 81.6%, the latter being achieved by Coil3D. As expected, Coil5 (remote 32Tx32Rx whole-body array) revealed lower mean/max B1+ values of 1.93/2.17μT/√(kW) since the elements are located furthest away from the phantom. However, this 2.2-fold less B1+ magnitude can be compensated by the 8-fold higher available total RF power9, thus enabling also high flip angles. Fig.4 shows the Tx coverage along h-f direction for all coils. For the local coils, the largest Tx coverage is achieved by the 8TxRx array from Essen (~283mm) followed by the 8Tx32Rx array from Essen (~274mm) (Fig.5). Notably, the 8TxRx replica array (Coil4) yielded a 12% shorter h-f coverage (~250mm) than Coil1. These three arrays all consist of meander stripline elements6. The loop and dipoles elements from Coil3L/Coil3D reached excitation lengths of ~173mm and ~213mm, respectively. Interestingly, for the 32TxRx array from Heidelberg no substantial Tx variations along h-f where found along the entire FoV (~384mm), which would be highly beneficial for large FoV body applications.Conclusion
This preliminary study investigated the Tx performance of 5 different pTx coils. Overall, the B1+ efficiency was similar for the four local coil concepts, whereas the h-f coverage was highest for the 8TxRx ELH coil. Future work will include commercial RF body arrays and also determine the receive performance to complement the comparison.Acknowledgements
We gratefully acknowledge funding from the German Research Foundation (SCHM 2677/4-1).References
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