Title: On radio-frequency implant safety in parallel transmission MRI
Authors: Petzold, Johannes, Physikalisch-Technische Bundesanstalt (PTB), Fachbereich 8.1, Biomedizinische Magnetresonanz, ORCID: 0000-0001-9503-0998
Contributors: HostingInstitution: Physikalisch-Technische Bundesanstalt (PTB), ISNI: 0000 0001 2186 1887
Resource Type: Text / Dissertation
Publisher: Physikalisch-Technische Bundesanstalt (PTB)
Rights: Download for personal/private use only, if your national copyright law allows this kind of use.
Relationships: IsPartOf: ISSN 2941-1297
IsIdenticalTo: ISBN 978-3-944659-35-0
Dates: Available: 2024-06-07
Created: 2024-04
File: Download File (application/pdf) 17.91 MB (18779926 Bytes)
MD5 Checksum: 2d745f4100553c55bd2a6f107fe3ecb0
SHA256 Checksum: 7bd3e933dd6877a01b574bc0f94d03deb3f9275bf031c3ac6f1532579dbcff98
Keywords magnetic resonance ; electromagnetic simulation ; parallel transmission ; patient model uncertainty ; position uncertainty ; RF safety ; safety factor ; active implantable ; medical devices ; implant safety ; RF heating ; virtual sensor
Abstract: The rising count of patients bearing an active implantable medical device (AIMD) are often hindered from having magnetic resonance (MR) exams because MR-unsafe implants can only be scanned outside of their regulatory approval, while MR conditional implants require the use of thresholds from complex manufacturer- and implant-specific guidelines. This degrades the MR performance because of over-conservative limits or could even endanger patients if the limit is not strict enough. It is known that parallel transmission (pTx)-systems can generate safe optimised radio-frequency (RF)-shims that exploit the full MR performance.

This thesis contributes to implant RF safety in pTx MR by describing an implant safety concept that separates native pTx safety of the patient without implant from implant safety that is assessed with an implant-integrated sensor.

The concept’s prerequisite – the RF safety of pTx for the native case – is first demonstrated in silico at the example of a pTx body coil that is driven in different configurations at 1 - 16 channels with 0.5 T, 1.5 T and 3 T. The average B1+-field (mean(B1+)) of pTx is up to 30 % higher than for the single channel circular polarised (CP) mode when accommodating model uncertainty with a safety factor and the same International Electrotechnical Commission (IEC) specific absorption rate (SAR) limits are applied. Position uncertainty can similarly be addressed with a safety factor. Furthermore, it was found that neglecting phase information leads to minor mean(B1+) drops of 3 – 20 %, depending on the number of channels and B0 compared to the common SAR-controlled mode, in exchange for a reduced complexity.

The theoretic applicability of the proposed safety concept is shown by simulating a spinal cord stimulator dummy implant in a second step. The ability to calibrate feasible sensor types against the established hazard measures SAR and temperature is demonstrated. The potential of pTx in mitigating implant hazards rises with channel count with up to 3 times the mean(B1+) of the CP mode for an implant-caused temperature rise limit of 2 K at 16 channels for 1.5 T and 3 T.

This thesis finishes with the description of a process for implant manufacturers on how to calibrate a sensor signal against an implant hazard.
Series Information: PTB-Bericht Diss-6
Citation: Petzold, J., 2024.On radio-frequency implant safety in parallel transmission MRI. Dissertation, Otto-von-Guericke-Universität Magdeburg. Braunschweig: Physikalisch-Technische Bundesanstalt. PTB-Bericht Diss-6. ISBN 978-3-944659-35-0. Verfügbar unter: https://doi.org/10.7795/110.20240412