Johannes Petzold^{1}, Bernd Ittermann^{1}, and Frank Seifert^{1}

^{1}Physikalisch-Technische Bundesanstalt (PTB), Berlin, Germany

Different methods for SAR control (SAR limit, phase-agnostic SAR limit, amplitude limit) are evaluated in a simulation study of parallel transmit (pTx) body coil systems (2,4, or 8 channels) at different B_{0} (0.5T, 1.5T, 3T). Criteria are the necessary safety factors to accommodate for subject variability, and the resulting mean(B_{1}^{+}). Due to the high safety factor necessary for a phase-based SAR limit, its mean(B_{1}^{+}) performance becomes effectively the same as the much simpler to implement amplitude limit.

In this simulation study, three SAR control modes are compared in terms of safety and mean($$$B_1^+$$$) performance. Field strengths$$$\,B_0\leq3T$$$ are investigated, where pTx gains renewed interest as a mean to mitigate implant hazards

Ten human voxel models (nine from the virtual population

Six voxel models from 50–80kg and without obvious artifacts were used for anchor-model analysis. For all combinations of channel count and$$$\,B_0$$$, 10g-averaged Q-matrices as well as whole body- and head-Q-matrix were computed and divided by their respective IEC limits

A complex excitation vector $$$u$$$ is safe, if one of the following conditions is fulfilled for each VOP $$$k$$$:

where $$$|\,\cdot\,|$$$ denotes the element-wise absolute value, and $$$\alpha$$$ is the maximum single-channel amplitude $$$\alpha\equiv\min_k[(\sum_{ij}|Q_{ij}^{(k)}|)^{-1/2}]$$$

Out of the six remaining voxel models,$$$\,p$$$ were selected as anchors ($$$1\leq\,p\,\leq5$$$) and a different one as the target. 10000 random excitation vectors (half with random amplitudes and phases, half with equal amplitudes and random phases) were scaled such that the respective safety limit is approached for a given set of anchor simulations. These vectors are used to calculate the normalized peak spatial SAR (psSAR) of the selected target simulation. This is done for all 186 combinations of one target and$$$\,p$$$ anchor simulations. Mean($$$B_1^+$$$) was assessed with an overall safety limit consisting of all VOPs multiplied with the maximum normalized psSAR of$$$\,p=5$$$.

Anchor-safe excitations can still create unsafe psSAR in the unknown target model, with highest values under SL control and lowest for AL (Fig.2). IEC limits can be exceeded up to 3.4 times and this overshoot depends mainly on the channel count but not on $$$\,B_0$$$. For SL, psSAR overshoot and hence the safety factor clearly increases with channel count, for PASL and AL they tend to decrease. With higher anchor model count, the maximum normalized psSAR in the target simulation drops (Fig.3). Taking the respective safety factors into account, a higher channel count can result in a lower mean($$$B_1^+$$$) performance (Fig.4).

In the investigated implant-free case, a 2-channel system with SL provides the best mean $$$B_1^+$$$ performance. Once implants in various conceivable configurations are added, higher channel counts might be needed for best mitigation and $$$B_1^+$$$ performance. Here, the AL appears advantageous due to its robustness and simplicity. The model dependance is low (<30% except for 3T, 2ch) and SAR supervision much simpler as only amplitudes (or power levels) need to be checked. In addition, AL has the nice feature that an upper bound for psSAR can be calculated. For 0.5T and 1.5T, such amplitude limit might even be derived just from the patient’s mass. For 3T, the picture is less clear. For SL, any extrapolation too far beyond the investigated anchor simulations appears to be critical. Anchor models close to the given patient are presumably needed for good $$$B_1^+$$$ performance as using the VOPs of an 120kg adult would result in a bad performance for a 30kg child.

- Bardati F, Borrani A, Gerardino A, Lovisolo GA. SAR optimization in a phased array radiofrequency hyperthermia system. IEEE Trans. Biomed. Eng. 1995;42:1201–1207 doi: 10.1109/10.476127.
- Eichfelder G, Gebhardt M. Local specific absorption rate control for parallel transmission by virtual observation points. Magn. Reson. Med. 2011;66:1468–1476 doi: 10.1002/mrm.22927.
- Petzold J, Ittermann B, Seifert F. Robustness of pTx safety concepts to varying subjects and subject positions. In: Proc. Intl. Soc. Mag. Reson. Med. 29. ; 2021. p. 116.
- Silemek B, Seifert F, Petzold J, et al. Rapid safety assessment and mitigation of radiofrequency induced implant heating using small root mean square sensors and the sensor matrix Q s . Magn. Reson. Med. 2021:mrm.28968 doi: 10.1002/mrm.28968.
- Winter L, Seifert F, Zilberti L, Murbach M, Ittermann B. MRI‐Related Heating of Implants and Devices: A Review. J. Magn. Reson. Imaging 2020:jmri.27194 doi: 10.1002/jmri.27194.
- Eryaman Y, Guerin B, Akgun C, et al. Parallel transmit pulse design for patients with deep brain stimulation implants. Magn. Reson. Med. 2015;73:1896–1903 doi: 10.1002/mrm.25324.
- McElcheran CE, Yang B, Anderson KJT, Golenstani-Rad L, Graham SJ. Investigation of Parallel Radiofrequency Transmission for the Reduction of Heating in Long Conductive Leads in 3 Tesla Magnetic Resonance Imaging Xu B, editor. PLOS ONE 2015;10:e0134379 doi: 10.1371/journal.pone.0134379.
- Gosselin M-C, Neufeld E, Moser H, et al. Development of a new generation of high-resolution anatomical models for medical device evaluation: the Virtual Population 3.0. Phys. Med. Biol. 2014;59:5287–5303 doi: 10.1088/0031-9155/59/18/5287.
- Segars WP, Sturgeon G, Mendonca S, Grimes J, Tsui BMW. 4D XCAT phantom for multimodality imaging research: 4D XCAT phantom for multimodality imaging research. Med. Phys. 2010;37:4902–4915 doi: 10.1118/1.3480985.
- Sim4Life 6.0, ZMT Zurich MedTech, Zürich, Switzerland
- Kozlov M, Turner R. Fast MRI coil analysis based on 3-D electromagnetic and RF circuit co-simulation. J. Magn. Reson. 2009;200:147–152 doi: 10.1016/j.jmr.2009.06.005.
- IEC 60601-2-33: Medical electrical equipment – Part 2-33: Particular requirements for the basic safety and essential performance of magnetic resonance equipment for medical diagnosis. International Electrotechnical Commission; 2015.
- Orzada S, Fiedler TM, Quick HH, Ladd ME. Local SAR compression algorithm with improved compression, speed, and flexibility. Magn. Reson. Med. 2021;86:561–568 doi: 10.1002/mrm.28739.
- Seifert F, Cassara A, Weidemann G, Ittermann B. Reliable and robust RF safety assessment of transmit array coils at ultrahigh fields. In: Proc. Intl. Soc. Mag. Reson. Med. 22. ; 2014. p. 4891.

DOI: https://doi.org/10.58530/2022/2555