Lydia Jean Bardwell Speltz^1,2, Yunhong Shu², Myung-Ho In², Nolan Meyer^1,2, Erin Gray², Diana Lanners², Yihe Hua³, Robert E Watson², John Huston III², Thomas KF Foo³, and Matt A Bernstein^2
1Mayo Clinic Graduate School of Biomedical Sciences, Mayo Clinic, Rochester, MN, United States, ²Department of Radiology, Mayo Clinic, Rochester, MN, United States, ³GE Global Research, Niskayuna, NY, United States

Many implanted devices are labeled MR Conditional, meaning specific, labeled conditions must be met to ensure safe scanning. We developed a software tool for use with a high-performance, compact 3T (C3T) scanner that verifies relevant MR conditions can be met at the location of the device (e.g., abdomen), even if those conditions are exceeded at the level of the anatomy being scanned (e.g., brain).  MR parameters assessed include main magnetic field strength, gradient slew rate, RF amplitude (B1+²), and dB/dz. The limited extent of the fields with C3T suggests high-performance exams can sometimes be obtained without compromising patient safety.

Emre Kopanoglu^1
1CUBRIC, School of Psychology, Cardiff University, Cardiff, United Kingdom

Pulses designed using patient-specific B₁⁺-maps are inherently patient position dependent, while safety models (available on scanners) used for local SAR supervision are not. The effect of this positional mismatch on SAR estimation was investigated for 1-/2-/3-/4-/5-spoke pulses. The results showed substantial underestimation of local SAR: the actual local SAR at off-centre positions was observed to be up to 4.6-fold higher compared to the peak estimated using a centred model. This behaviour was worse for more spokes and consistent across slices between cerebellum and crown. Using multiple distributed models reduced the likelihood of SAR underestimation, but at the cost of over-restrictiveness.

Negin Behzadian¹ and Shiloh Sison^2
1Research and Development, Abbott, Sylmar, CA, United States, ²Research and Development, Abbott, Sunnyvale, CA, United States

Exposure to the RF energy of an MRI shall be limited to prevent potential harm to biological tissues. Whole-body SAR, averaged over any 6-minute interval, shall be limited to 2W/kg and 4W/kg under the limits of Normal and First Level Controlled Operating Mode, respectively, and is further restricted to twice the operating mode limit over any 10-second period. Our study investigates whether similar short-duration assessments are necessary for the alternative RF exposure metric of B_1+RMS, in the context of RF heating safety of cardiac protocols with leaded cardiac implants.

Sydney Nicole Williams¹, Sarah Allwood-Spiers², Paul McElhinney¹, Yuehui Tao³, John E. Foster², Shajan Gunamony^1,4, and David A. Porter^1
1Imaging Centre of Excellence, University of Glasgow, Glasgow, United Kingdom, ²MRI Physics, NHS Greater Glasgow & Clyde, Glasgow, United Kingdom, ³Siemens Healthcare Ltd., Glasgow, United Kingdom, ⁴MR CoilTech Limited, Glasgow, United Kingdom

Parallel transmit (pTx) can reduce B₁⁺ field inhomogeneity present at 7T, but needs additional consideration for specific absorption rate (SAR) monitoring due to the superimposed electromagnetic fields. For self-built pTx coils, validation includes comparing electromagnetic field simulations with experimental B₁⁺ maps and thermometry, previously presented for static pTx in a self-built 8Tx/32Rx coil. We present full-waveform pTx validation with two further tests: comparison of measured and prescribed RF waveforms, and calculation of local SAR with virtual observation points (VOPs) for all pTx pulses with comparison to scanner predictions and measurements. After validation, dynamic pTx is performed in vivo.

Weiman Jiang¹, Fan Yang¹, and Kun Wang^1
1GE Healthcare, Beijing, China

This work proposed a novel SAR characterization method based on an equivalent circuit model and the circuit’s frequency response analysis. Comparing to well recognized pulse energy method defined in NEMA MS 8, this method doesn’t need the flux loop fixed on transmit coil. Meanwhile, it can monitor SAR accurately. The Root Mean Square (RMS) error and maximum error of the novel method relative to the method in NEMA MS 8 are 4.96% and 9.47% respectively. This method could avoid design complexity of integrating the flux loop and has potential to be easily realized in most MRI scanners.

Stephan Orzada¹, Thomas M. Fiedler¹, Harald H. Quick^2,3, and Mark E. Ladd^1,2,4,5
1Medical Physics in Radiology (E020), German Cancer Research Center (DKFZ), Heidelberg, Germany, ²Erwin L. Hahn Institute for MRI, University Duisburg-Essen, Essen, Germany, ³High-Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany, ⁴Faculty of Physics and Astronomy, University of Heidelberg, Heidelberg, Germany, ⁵Faculty of Medicine, University of Heidelberg, Heidelberg, Germany

For parallel transmit systems, control of local SAR is very important to ensure safety. For pulse calculation and online supervision, compression of the SAR matrices is used to reduce calculation effort. The original clustering method by Eichfelder et al. was later outperformed by a method proposed by Lee et al. We propose an enhancement to Lee’s algorithm that further increases compression efficiency, speed and flexibility by iteratively reducing the overestimation.

Alireza Sadeghi-Tarakameh¹, Nur Izzati Huda Zulkarnain¹, Noam Harel¹, and Yigitcan Eryaman^1
1Center for Magnetic Resonance Research (CMRR), University of Minnesota, Minneapolis, MN, United States

We validate a previously proposed temperature prediction workflow for a complete DBS Systems undergoing MRI. Accuracy of the workflow is investigated for different termination conditions using an extension cable and an implantable pulse generator (IPG) device.  The workflow accurately predicted the temperature time-course during the MRI scan for different trajectories and terminations.

Xin Huang¹, Vick Chen¹, and Shiloh Sison^1
1Abbott Laboratories, Sunnyvale, CA, United States

To determine the appropriate peak B1+ field levels, an RF peak B1+ survey is performed on commercial 3T MR systems loaded with ASTM phantom filled with saline. The peak B1+ averaged over center slab inside the 3T commercial MR systems can be as high as 27 µT. The 3T MRI safety assessment should include values up to this level.

Yuqing Wan¹, Nathan Ooms², Paul Nguyen¹, and Guangqiang (Jay) Jiang^1
1Axonics Modulation Technologies, Irvine, CA, United States, ²Purdue University, West Lafayette, IN, United States

Implant device manufacturers often specify a RF exposure limit based on specific absorption rate (SAR) or B1+rms and a continuous active scan time in the device magnetic resonance imaging (MRI) labelling. Due to the restriction of the labeled SAR, MR sequence parameters may need to be adjusted to reduce RF power, resulting in prolonged scan time. The patient examination duration may be extended by up to 67% when compared to standard protocols. The overall session may be further prolonged due to additional wait time requirements from certain implanted devices, which can result in great inconvenience and extra cost to patients.

Gaurav Verma¹, Paul Min², MyungHo In², Jungho Cha³, Akbar Alipour¹, Charlotte Elorette⁴, Lazar Fleysher⁵, Priti Balchandani¹, Helen Mayberg³, and Ki Sueng Choi^3
1Radiology, Icahn School of Medicine at Mount Sinai, New York, NY, United States, ²Radiology, Mayo Clinic, Rochester, MN, United States, ³Center for Advanced Circuit Therapeutics, Icahn School of Medicine at Mount Sinai, New York, NY, United States, ⁴Neuroscience, Icahn School of Medicine at Mount Sinai, New York, NY, United States, ⁵Diagnostic, Molecular and Interventional Radiology, Icahn School of Medicine at Mount Sinai, New York, NY, United States

Radio frequency (RF) safety and image quality testing was performed in the presence of deep brain stimulation (DBS) electrodes using gel acrylamide phantoms and two 3T MRI scanners. Two electrodes, (Abbott and Medtronic) were tested with sequences including GRE, T₁-weighted magnetization-prepared rapid gradient echo (MPRAGE), and T₂-weighted turbo spin-echo (TSE). Safety testing showed low heating when electrodes were aligned along the B₀ field (Z-axis) of the magnet, but higher heating when the electrodes were angled to the main field. Image quality testing showed smaller ferromagnetic susceptibility artifacts when electrode was parallel with B₀ field and normal to transmit field.

Vincent Gras¹ and Nicolas Boulant^1
1DRF/Joliot/Neurospin, CEA - Université Paris Saclay, Gif sur Yvette, France

Parallel transmission to date is the most promising technology to tackle the RF field inhomogeneity problem in MRI at ultra-high field. The Virtual Observation Point compression technique was a cornerstone in the field to reduce drastically the number of SAR matrices to handle in exam supervision and RF pulse design. After its original discovery, it was further improved to boost the compression. This work describes a reformulation of the problem which simplifies substantially the numerical search and allows reducing further the number of matrices by a factor ~5 or more.

Marcus Prier^1,2, Max Joris Hubmann^1,2, Enrico Pannicke^1,2, and Oliver Speck^1,2
1Otto-von-Guericke University, Magdeburg, Germany, ²Research Campus STIMULATE, Magdeburg, Germany

A low cost, stand-alone RF power monitor was developed that fulfills the requirements given in the standard 60601-2-33 and is based on electrical standard components. It consists of two dual directional couplers, two RMS envelope filters and a microcontroller. A fixed power limit can be programmed or patient dependent power limits can be communicated from a host computer. Evaluation measurements show power measurement and RFPA blanking switching times accuracies with an error less than 1%. A sampling rate of 130kSamples/s indicates usability for relatively short or moderate complex shaped RF waveforms.

Felipe Godinez^1,2, Greig Scott³, Joseph V Hajnal^1,2, and Shaihan J Malik^1,2
1Biomedical Engineering Department, School of Biomedical Engineering and Imaging Sciences, King's College London, London, United Kingdom, ²Centre for the Developing Brain, School of Biomedical Engineering and Imaging Sciences, King's College London, London, United Kingdom, ³Department of Electrical Engineering, Stanford, Stanford, CA, United States

Imaging guidewires in MRI is known to have heating hazards associated. In this work we show initial data that supports the use of local Tx coils over whole body coils for the safe imaging of standard guidewires with MRI. This was tested in vitro using parallel transmit (PTx) techniques to generate a scenario for a proper comparison between the two coil types. It was found that a local Tx coil produces less heating at the guidewire tip, than a body coil under comparable conditions.  

Jason Meyers¹, David Prutchi¹, and Ramez Shehada^2
1Impulse Dynamics (USA) Inc., Marlton, NJ, United States, ²Medical Technology Laboratories, La Mirada, CA, United States

The MR environment poses a tissue heating hazard to patients with cardiac IPGs (implantable pulse generators), such as pacemakers and defibrillators, due to RF currents circulating in the loop formed by the IPG, a transvenous lead, and the tissue.  Heating will be limited by the impedance of this loop at the MR frequency (63.87 MHz for 1.5T). The IPG input impedance (a portion of this loop) was measured in six MR-Conditional IPG’s.  All have a comparable, low impedance, suggesting that device manufacturers are not intentionally adding impedance and that some interchangeability may be possible without changing RF-induced heating.

Hassan B Hawsawi^1,2, Ozlem Ipek^3,4, David W Carmichael^5,6, and Louis Lemieux^1
1Clinical and Experimental Epilepsy, University College London, London, United Kingdom, ²Administration of Medical Physics, King Abdullah Medical City, Makkah, Saudi Arabia, ³King’s College London, London, United Kingdom, ⁴CIBM-AIT, Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland, ⁵Department of Biomedical Engineering, King’s College London, London, United Kingdom, ⁶Developmental Imaging and Biophysics Section, UCL Great Ormond Street Institute of Child Health, London, United Kingdom

Electromagnetic (EM) simulations offer the possibility of assessing RF-induced heating around intracranial EEG (icEEG) electrodes across a variety of placement scenarios in a shorter time than experimental phantom-based temperature measurements. However, given the sub-millimetric dimensions of the wires and contacts the range of spatial scales spans many orders of magnitude, leading to high computational demands. In this study, we assessed the effect of model simplification for a typical 8-contact depth EEG electrode on the estimated heating patterns.

Mark Smith¹, Harry Hu², Ram Krishnamurthy¹, John Pitts³, and Mai-Lan Ho^1
1Nationwide Children's Hospital, Columbus, OH, United States, ²Hyperfine, Dublin, OH, United States, ³Hyperfine, Cleveland, OH, United States

In 2020, a portable 64mT ultra-low field MRI system designed for point of care bedside use received 510K clearance (Hyperfine, Guilford, CT).  Our pediatric institution (Nationwide Children’s Hospital) has acquired one of these systems to meet neuroimaging needs for critically ill NICU or PICU patients who cannot tolerate transport to the MRI department.  Prior to scanning patients, safety testing for displacement and heating was conducted on monitoring hardware that will be connected to the patient during the bedside MRI.  The monitoring hardware was found safe to stay connected to the patient during the bedside MRI.

Finya Ketelsen^1,2, Vincent Hammersen¹, Kevin Kröninger², and Gregor Schaefers^1,3
1MRI-STaR - Magnetic Resonance Institute for Safety, Technology and Research GmbH, Gelsenkirchen, Germany, ²TU Dortmund University, Dortmund, Germany, ³MR:comp GmbH, Testing Services for MR Safety & Compatibility, Gelsenkirchen, Germany

Medical implants of all kind must be tested for RF-induced heating to determine their MR safety and compatibility. Test conditions concerning E-field homogeneity and E-field drift during assessment time are defined in ISO/TS 10974 and ASTM-F2182. This study evaluates the influence of E-field drift and E-field homogeneity on RF-heating. It shows that both effects can lead to a temperature underestimation. Even for small implants whose volume is in a homogenous area within ±1dB, this underestimation can occur. System instability and implant size and position can further increase the underestimation of RF-induced heating and should be carefully considered.

Alix Plumley¹, Philip Schmid¹, and Emre Kopanoglu^1
1Cardiff University Brain Research Imaging Centre, Cardiff, United Kingdom

Subject motion in parallel-transmit (pTx) causes channels’ electric field interference patterns to change, influencing SAR distributions. This can cause safety limits to be exceeded when SAR-constrained pulses are designed for one position. Here, we consider effects of pTx coil dimensions on SAR sensitivity to motion by simulating 6 differently-sized coil models, and evaluating SAR at 19 displaced positions. Our results agree with those previously reported for the similar-sized coil, but SAR sensitivity was generally lower for larger coils, and higher for smaller coils, with maximum motion-induced local-SAR increase of 3.8-fold and 1.6-fold for the smallest and largest coil models, respectively.

Fróði Gregersen^1,2,3,4, Cihan Göksu^2,5, Gregor Schaefers^6,7, Rong Xue^4,8,9, Axel Thielscher^1,2, and Lars Hanson^1,2
1Section for Magnetic Resonance, DTU Health Tech, Technical University of Denmark, Kgs Lyngby, Denmark, ²Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital, Amager and Hvidovre, Denmark, ³Sino-Danish Center for Education and Research, Aarhus, Denmark, ⁴University of Chinese Academic of Sciences, Beijing, China, ⁵High-Field Magnetic Resonance Center, Max-Planck-Institute for Biological Cybernetic, Tübingen, Germany, ⁶MRI-STaR-Magnetic Resonance Institute for Safety, Technology and Research GmbH, Gelsenkirchen, Germany, ⁷MR:comp GmbH, MR Safety Testing Laboratory, Gelsenkirchen, Germany, ⁸State Key Laboratory of Brain and Cognitive Science, Beijing MRI Center for Brain Research, Institute of Biophysics, Chinese Academy of Sciences, Beijing, China, ⁹Beijing Institute for Brain Disorders, Beijing, China

Combining transcranial electrical stimulation (TES) with MRI offers various interesting research opportunities, but also introduces safety concerns. Coupling between the RF field and highly conductive TES leads can lead to skin burns. These safety issues are usually mitigated with the use of safety resistors and controlled lead paths that reduce the power absorbed by the leads. However, these methods introduce practical limitations for combined TES/MRI experiments, such as limited stimulation currents and cable stray fields corrupting MR current density imaging. We overcome these limitations by using low-conductivity silicone-rubber as TES leads. Simulations and temperature measurements are used for safety assessment.

Xin Chen¹ and Michael Steckner^1
1Canon Medical Research USA Inc., Mayfield Village, OH, United States

MRI vendors have been asked to provide B1+ contour plots of unloaded whole body transmit coils in order to show technologists/radiographers where not position a patient for SAR hotspot/RF burn avoidance purposes. Two flaws with this strategy are demonstrated by modeling Duke at the abdominal landmark in the centered and off-set position: 1) B1+ maps correlate poorly to SAR hotspots, 2) loaded B1+ maps are significantly different relative to unloaded B1+ maps. Knowledge of the birdcage coil end-ring position in conjunction with spacing pads will improve RF burn safety outcomes.