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In-vivo 7T body MRI with complete abdominal coverage using pTx and a 32-Tx-channel whole-body RF antenna array

Johannes A. Grimm^1,2, Christoph S. Aigner³, Constantin Schorling¹, Sebastian Dietrich³, Stephan Orzada¹, Thomas M. Fiedler¹, Mark E. Ladd^1,2,4, and Sebastian Schmitter^1,3,5
¹Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany, ²Faculty of Physics and Astronomy, Heidelberg University, Heidelberg, Germany, ³Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany, ⁴Faculty of Medicine, Heidelberg University, Heidelberg, Germany, ⁵Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States

Synopsis

Keywords: High-Field MRI, Parallel Transmit & Multiband, whole-body, abdomen, kT-points, free-breathing, 3D, RPE acquisition

Motivation: Flip angle variations or voids are problematic at 7T, especially when large fields of excitation are needed, such as for the entire abdomen.

Goal(s): To homogeneously excite the whole abdomen using dynamic pTx and to investigate the minimal number of kT-points necessary.

Approach: Dynamic pTx is applied in the abdomen using a 32-Tx-channel whole-body array and free-breathing relative B₁⁺ mapping.

Results: The preliminary data indicates that exciting the whole abdomen is feasible and that dynamic pTx yields sufficient FA homogeneity in all subjects.

Impact: The preliminary data indicates that exciting the whole abdomen is feasible and that dynamic pTx yields sufficient FA homogeneity in all subjects.

Introduction

In UHF body MRI, flip angle (FA) variations or voids that arise from the spatially inhomogeneous B₁⁺ transmit profiles are a main challenge. This is already problematic for small organs (i.e. prostate) but especially when large fields of excitation (FoX) are needed, such as for the entire abdomen¹. To address this issue, static parallel transmit (pTx) (B₁⁺phase/magnitude shimming) has been applied with either local transmit (Tx) body arrays or remote Tx arrays^2,3located behind the bore liner similar to Tx coils of clinical 1.5T/3T scanners. Remote Tx arrays enable higher Tx-channel counts, potentially resulting in a larger FoX than most local transmit arrays, and they provide superior patient comfort. When aiming at larger FoX, 7T studies from various groups showed that i) static pTx provides insufficient FA homogeneity^4,5using local arrays, ii) dynamic pTx may be needed (e.g. spokes^4,6, kT-points⁵) for large FoX such as the liver or iii) “time interleaved acquisition of modes” (TIAMO)⁷ may be used in such cases⁸. However, recent work showed that static pTx and a remote 32-Tx-channel body array achieves acceptable homogeneity when exciting the liver in subjects with normal BMI⁹. Thus, the question arises to what extent this whole-body array could benefit from dynamic pTx for 3D FA homogenization of the entire abdomen for applications such as shown by Stijns et al.¹
Thus, this study investigates static and dynamic pTx of the whole abdomen in multiple subjects and demonstrates the application of static and dynamic pTx in the abdomen using a 32-Tx-channel whole-body array.

Methods

Measurements were performed in three subjects (1m/2f, BMI=21.9-25.1 kg/m²) on a 7T MRI (Magnetom 7T, Siemens, Germany) with a prototype 32-channel antenna array integrated into the scanner bore³ for both transmission and reception. According to previous works^9,10 relative 3D B₁⁺maps of all 32 Tx channels covering the entire abdomen were obtained in free-breathing using a radial phase-encoded (RPE) acquisition performed in 23min14s after B₀ shimming (vendor supplied, second order). The parameters for the scans are listed in Figure 1a. The relative B₁⁺ magnitude and phase maps of a representative transversal slice for one subject are shown in Figure 2. From these B₁⁺ maps, 1 to 4 kT-point pulses were calculated in the small tip angle approximation with interleaved greedy and local optimization methods^11,12 for each of the three subjects to excite a region of interest (ROI) covering the volume of the abdomen (Figure 3) with a nominal 10° FA. Each optimization was carried out with 100 random starting phases. Using these RF pulses, 3D GRE scans with RPE acquisition were acquired with up to 4 kT-points for subject 1 and 2. Additionally, a zero shim with magnitude 1 and phase 0 for all channels was obtained.

Results and Discussion

Predicted B₁⁺ maps of all subjects for zero shim and 1 to 4 kT-points for 8 transversal slices are shown in Figure 3. Figure 4 shows GRE acquisitions (receive profile not removed) for subject 1 and 2 in all orientations and the corresponding FA prediction for the transversal slice, which matches. FA predictions and GRE acquisitions show that applying only 1 kT-point (i.e. phase/magnitude shimming) fails to homogeneously excite the whole abdomen. With 2 kT-points, no FA dropouts are observed in the ROI and an acceptable visual homogeneity can be achieved in both the predictions and the GRE scans. However, GRE scans reveal small residual FA variations (Figure 5, white arrows), which might be caused by ΔB₀ as it was not included in the design, yet. Increasing the number of kT-points further does not visibly increase the homogeneity. This can also be qualitatively seen by the coefficient of variation (CV= std/mean), which was calculated in the ROI. For subject 1/2/3, the CV could be reduced from 51.4%/48.4%/48.1% with zero shim to 31.3%/32.8%/26.8% using 2 kT-points. With a higher number of kT-points the CV is only improved slightly for all three subjects to 30.8%/32.7%/26.4% for 4 kT-points, but additional kT-points might provide more significant benefit for high-BMI subjects. The CV is biased by low proton density regions (i.e. air, bones) and could be reduced further by excluding them.

Conclusion

With the 32-channel array, the preliminary data indicates that whole-abdomen shimming is feasible and that only dynamic (not static) pTx achieves sufficient FA homogeneity in the subjects examined. Although further studies with more subjects and larger BMI range are needed, 2 kT-points seem to be a practical solution that provides a good tradeoff between transmit power and FA fidelity for whole-abdomen applications in the low FA regime.

Acknowledgements

We gratefully acknowledge funding from the German Research Foundation (GRK 2260 Bioqic).

References

[1] Stijns RCH, Philips BWJ, Nagtegaal ID, Polat F, de Wilt JHW, Wauters CAP, Zamecnik P, Fütterer JJ, Scheenen TWJ. USPIO-enhanced MRI of lymph nodes in rectal cancer: A node-to-node comparison with histopathology. Eur J Radiol. 2021 May;138:109636. doi: 10.1016/j.ejrad.2021.109636. Epub 2021 Mar 10. PMID: 33721766.

[2] Vaughan JT, Snyder CJ, DelaBarre LJ, Bolan PJ, Tian J, Bolinger L, Adriany G, Andersen P, Strupp J, Ugurbil K. Whole-body imaging at 7T: preliminary results. Magn Reson Med. 2009 Jan;61(1):244-8. doi: 10.1002/mrm.21751.

[3] Orzada S, Solbach K, Gratz M, Brunheim S, Fiedler TM, Johst S, Bitz AK, Shooshtary S, Abuelhaija A, Voelker MN, Rietsch SHG, Kraff O, Maderwald S, Flöser M, Oehmigen M, Quick HH, Ladd ME. A 32-channel parallel transmit system add-on for 7T MRI. PLoS One. 2019 Sep 12;14(9):e0222452. doi: 10.1371/journal.pone.0222452. PMID: 31513637; PMCID: PMC6742215.

[4] Wu X, Schmitter S, Auerbach EJ, Uğurbil K, Van de Moortele PF. Mitigating transmit B 1 inhomogeneity in the liver at 7T using multi-spoke parallel transmit RF pulse design. Quant Imaging Med Surg. 2014 Feb;4(1):4-10. doi: 10.3978/j.issn.2223-4292.2014.02.06. PMID: 24649429; PMCID: PMC3947979.

[5] Runderkamp B, van der Zwaag W, Roos T, Strijkers G, Caan M, Nederveen A. Whole-liver flip angle shimming at 7T using eight-channel parallel transmission kt-points pulses with FPE-DREAM B1+ mapping. Proc. ISMRM 2022 #2868.

[6] Shirvani S., Ding B., Dragonu I., Liebig P., Rua C., Karkouri J., Klomp D., Hess A. T., Rodgers C. T. Initial experience on 7T Terra for human parallel transmit (pTx) liver imaging. Proc. ISMRM (2020) #4288

[7] Orzada S, Maderwald S, Poser BA, et al. RF Excitation Using Time Interleaved Acquisition of Modes (TIAMO) to Address B1 Inhomogeneity in High-Field MRI.

[8] Maatman iT, Ypma S, Block kT, Maas MC, Schulz J, Scheenen TWJ, Radial Stack-of-Stars Abdominal MRI at 7 Tesla. Proc. ISMRM 2023 #1418.

[9] Grimm JA, Aigner SA, Dietrich S, Orzada S, Fiedler TM, Nagel AM, Ladd ME, Schmitter S. In-vivo 3D liver imaging at 7T using kT-point pTx pulses and a 32-Tx-channel whole-body RF antenna array. Proc. ISMRM 2023 #1419.

[10] Dietrich S, Schmitter S. 3D Free-breathing multichannel absolute B1+ Mapping in the human body at 7T. Magn Reson Med. 2021;85(5):2552-2567.

[11] Grissom WA, Vogel MW. Small-tip-angle spokes pulse design using interleaved greedy and local optimization methods. Magn Reson Med 2012;68:1553-1562.

[12] Cao Z, Grissom WA. Array-compressed parallel transmit pulse design. Magn Reson Med. 2016;76(4):1158-69.

Figures

Figure 1: a) Scan parameters for the relative B₁⁺ maps and the GRE acquisitions with zero shim, static pTx, and 2 to 4 kT-points. b) Example kT-points pulse train with 4 kT-points for subject 1 with a total pulse duration of 1ms and a blip duration of 80µs.

Figure 2: Relative B₁⁺ maps of one example slice through the abdomen for all 32 channels showing a) magnitude and b) relative phase of the channels with respect to channel 1. Amplifiers of channels 19, 20, 25, 27 and 32 were not operational during the measurements.

Figure 3: FA predictions for 8 transversal slices through the abdomen with zero shim and 1 to 4 kT-points for subject 1 (f, BMI=21.9kg/m²), subject 2 (m, BMI=25.1kg/m²) and subject 3 (f, BMI=24.1kg/m²). ROI marked in white.

Figure 4: GRE images of example slices through the abdomen in all orientations and corresponding FA predictions for the transversal slice for subject 1 and subject 2 (residual FA variations marked with white arrows). Please note that the location of the measured FA dropouts matches the prediction for the non-optimized shim. Excitation contrast (water/fat) varies between different number of kT-points due to being optimized only on water frequency.

Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)

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DOI: https://doi.org/10.58530/2024/4941