Bart R. Steensma¹, Janot P. Tokaya², Peter R. S. Stijnman¹, M. Arcan Erturk³, Cornelis A. T. van den Berg¹, and Alexander J. E. Raaijmakers^4
1Center for Image Sciences - Computational Imaging Group, University Medical Center Utrecht, Utrecht, Netherlands, ²TNO, Utrecht, Netherlands, ³Medtronic, Minneapolis, MN, United States, ⁴Biomedical Engineering - Medical Imaging Analysis, Eindhoven University of Technology, Eindhoven, Netherlands

The effect of transmit frequency on the risk of RF heating for elongated metallic implants was investigated through simulations in a phantom and a human model for 21, 64, 128 and 300 MHz. We demonstrate that for uniform E-field exposure, worst-case E-field enhancement at the tip reduces with increasing transmit frequency. However, for realistic E-field exposures, E-field enhancement increases with transmit frequency for similar B₁⁺  levels. For similar head SAR levels, E-field enhancement for worst-case implant length is roughly equal for all transmit frequencies. In all cases implant length is the main determinant of RF heating.

Ehsan Kazemivalipour^1,2,3, Alireza Sadeghi-Tarakameh⁴, Boris Keil⁵, Yiğitcan Eryaman⁴, Ergin Atalar^1,2, and Laleh Golestanirad^3,6
1Electrical and Electronics Engineering Department, Bilkent University, Ankara, Turkey, ²National Magnetic Resonance Research Center (UMRAM), Bilkent University, Ankara, Turkey, ³Department of Radiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, United States, ⁴Center for Magnetic Resonance Research (CMRR), University of Minnesota, Minneapolis, MN, United States, ⁵Institute of Medical Physics and Radiation Protection, Mittelhessen University of Applied Sciences, Giessen, Germany, ⁶Department of Biomedical Engineering, McCormick School of Engineering, Northwestern University, Evanston, IL, United States

We examined the effect of magnetic field strength, and by proxy RF resonance frequency, on RF heating of deep brain stimulation (DBS) lead models during RF exposure covering frequencies from 64MHz (1.5T) to 447MHz (10.5T). We report 1g-SAR at the tips of a cohort of 33 DBS implant models with realistic trajectories when the input power of RF coils was adjusted to impose safety limits based on either B₁⁺ or global head SAR. We observed that coils with higher resonance frequency generated a lower 1g-SAR around implanted leads when the global head SAR was kept constant across different coils.

Paul S Jacobs¹ and Andrew J Fagan^1
1Radiology, Mayo Clinic, Rochester, MN, United States

Electromagnetic simulations were performed to investigate the maximum local 1g-averaged SAR induced in tissue adjacent to orthopedic screws of varying lengths across field strengths ranging from 3T to 7T. Simulated input power values were initially normalized to a constant transmitted power of 1W and further normalized to ensure a constant background B₁+ value. SAR_max was approximately 36% lower than the peak near 4T. The results demonstrate that RF-induced tissue heating near implants can, in certain circumstances, be lower at higher fields.

Mostafa Berangi^1,2, Andre Kuehne¹, and Thoralf Niendorf^1,2
1MRI.TOOLS GmbH, Berlin, Germany, ²Berlin Ultrahigh Field Facility (B.U.F.F.), Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany

Bio-degradable implants have the benefit of eliminating implant removal surgery. Yet, it’s essential to monitor the implant and tissue\bone after implementation. MRI monitoring in the vicinity of the implant might be compromised by tissue heating and transmission field inhomogeneities. To address these challenges here an RF transceiver array is designed, manufactured and evaluated aiming to facilitate 7T MRI in the vicinity of implants. For this purpose, EMF simulations are performed to derive an optimum excitation vector that reduces local RF power deposition while ensuring a proper B₁⁺-field. The RF array is evaluated in phantom experiments and benchmarked with the simulations.

Alessandro Arduino¹, Umberto Zanovello¹, Jeff Hand², Luca Zilberti¹, Rüdiger Brühl³, Mario Chiampi¹, and Oriano Bottauscio^1
1Istituto Nazionale di Ricerca Metrologica, Torino, Italy, ²School of Biomedical Engineering and Imaging Sciences, King’s College London, London, United Kingdom, ³Physikalisch-Technische Bundesanstalt, Braunschweig and Berlin, Germany

Local heating effects in patients carrying orthopaedic implants have been studied mostly as a consequence of radiofrequency fields and, to a less extent, to gradient coil fields, separately. In this study the heating produced by the combined effects of both fields during realistic clinical sequences is numerically simulated in an anatomical human model carrying a CoCrMo hip implant. Risky situations have been identified, also showing that, sometimes, the gradient field heating can be most significant. The analysis suggests that criteria, based on whole-body SAR only, may not be sufficient to ensure patients’ safety.

Berk Silemek¹, Frank Seifert¹, Bernd Ittermann¹, and Lukas Winter^1
1Physikalisch-Technische Bundesanstalt (PTB) Braunschweig and Berlin, Berlin, Germany

The sensor Q Matrix ($$$Q_S$$$) concept is introduced for safe MRI of elongated implants without compromising imaging quality. For this purpose, small and low-cost sensors, parallel transmission (pTx) and the orthogonal projection method are sufficient without the need to perform simulations, safety testing or additional MR imaging. The $$$Q_S$$$  can also be implemented along with other pTx mitigation methods such as null modes. The $$$Q_S$$$ based orthogonal projection method and the null modes method are demonstrated in testbed and MRI experiments.

Flora Guarnotta¹, Jeremie Clement¹, Rachel Sparks¹, and Özlem Ipek^1
1School of Biomedical Engineering and Imaging Sciences, King's College London, London, United Kingdom

Epileptic seizure investigations with sEEG electrodes are limited to regions sampled and this may exclude relevant seizure network areas. Simultaneous EEG/fMRI may better elucidate the relationship whole-brain interactions and sEEG connectivity. However, excessive radiofrequency(RF) heating of implanted electrodes is showstopper. We investigated the role of parallel-transmit(pTx) RF coil arrays at 3 Tesla to control the RF-heating and local signal increases near the electrodes using computational field simulations. The pTx coils more specifically the 16ch was more efficient than the birdcage in terms of controlling the RF-field steering in the brain, which indicated less RF-heating near the contacts of the electrodes.

Ali Caglar Özen^1,2, Simon Reiss¹, Thomas Lottner¹, Dursun Korel Yildirim^3,4, Ozgur Kocaturk^3,5, and Michael Bock^1
1Deptartment of Radiology, Medical Physics, University Medical Center Freiburg, University of Freiburg, Freiburg, Germany, ²German Consortium for Translational Cancer Research Partner Site Freiburg, German Cancer Research Center (DKFZ), Heidelberg, Germany, ³Institute of Biomedical Engineering, Bogazici University, Istanbul, Turkey, ⁴Cardiovascular Branch, Division of Intramural Research, National Heart Lung and Blood Institute, National Institutes of Health, Bethesda, MD, United States, ⁵Transmural Systems, Andover, MA, United States

Interventional devices such as actively visualized catheters or guidewires have a long conductive line between the source and the probe at the tip. Long conductive wires can interract with the electromagnetic fields generated by the transmit coils resulting in RF-induced heating of surrounding tissue. Existing standards do not cover partial immersion and dynamic motion of such devices.  In this study, we propose an automated electric field detection and mapping system that is capable of performing hotspot detection, transfer function(TF) measurement and TF validation using different dipole antennae. RF-induced heating can be evaluated without moving the device, thus eliminating positioning errors.

Manuel Murbach¹, Thomas Doering², and Gregor Schaefers^2,3
1Murbach EMConsulting, Zurich, Switzerland, ²MR:comp GmbH, Gelsenkirchen, Germany, ³MRI-STaR - Magnetic Resonance Institute for Safety, Technology and Research GmbH, Gelsenkirchen, Germany

This study aims at exploring an alternative method (Tier 2.5) to estimate realistic in vivo E-fields at implant locations by including the directivity and length of the implants, which neglects non-relevant E-field components and too localized averaging schemes. The latest revision of the ASTM standard F2182-19e2 requires establishing a relation of the temperature increase near a metallic implant between A) the experimental measurement and B) the in vivo estimation in a representative patient population. The usage of currently standardized methods results in presumably overly conservative estimations, causing implant labels to be up to a factor 10 more restrictive.

Aiping Yao¹, Tolga Goren¹, Theodoros Samaras², Niels Kuster^1,3, and Wolfgang Kainz^4
1IT'IS Foundation, Zurich, Switzerland, ²Department of Physics, Aristotle University of Thessaloniki, Thessaloniki, Greece, ³ITET, ETH Zurich, Zurich, Switzerland, ⁴FDA, Silver Spring, MD, United States

To demonstrate the potential enhanced heating of abandoned (capped), damaged, or broken leads, we estimated in-vitro the deposited lead-tip power for a generic active implantable medical device (AIMD), intact and with wire breaks at regular intervals. We studied break size, a cut and capped lead, and a second wire parallel to the broken wire. The deposited lead-tip power enhancement was up to 30-fold over the intact lead. The presence of a nearby intact, or even broken, wire reduced this enhancement factor to ~3-fold. This shows the risk of extending MR-conditional labeling to broken, damaged, abandoned, or cut AIMD leads.