Ettore Flavio Flavio Meliado^1,2,3, Alexander A.J. Raaijmakers^1,2,4, P.R. A.J. Luijten¹, and C.A.T. A.J. van den Berg^2,5
1Department of Radiology, University Medical Center Utrecht, Utrecht, Netherlands, ²Computational Imaging Group for MR diagnostics & therapy, Center for Image Sciences, University Medical Center Utrecht, Utrecht, Netherlands, ³Tesla Dynamic Coils BV, Zaltbommel, Netherlands, ⁴Biomedical Image Analysis, Dept. Biomedical Engineering, Eindhoven University of Technology, Eindhoven, Netherlands, ⁵Department of Radiotherapy, Division of Imaging & Oncology, University Medical Center Utrecht, Utrecht, Netherlands

Recognize potential hazardous situations far from the modeled MR examination scenarios is crucial for RF safety applications and especially for deep-learning-based approaches. Last year we presented a Bayesian deep-learning approach for local SAR assessment. This approach allowed accurate local SAR estimations and returned reliable feedbacks on the error/uncertainty of the estimates for the MR examination scenario observed during training. However, it also showed the dangers of using the predicted uncertainty to identify out-of-training MR examination scenarios. In this study, we propose a simple approach to detect/reject potential erroneous local SAR predictions due to out-of-training transmit array and/or anatomical variations.

Benjamin Michael Hardy^1,2, Rana Banik^1,3, Xinqiang Yan^1,4, and Adam W Anderson^1,3,4
1Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, TN, United States, ²Department of Physics and Astronomy, Vanderbilt University, Nashville, TN, United States, ³Department of Biomedical Engineering, Vanderbilt University, Nashville, TN, United States, ⁴Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Nashville, TN, United States

Shorter wavelengths and higher frequencies at UHF contribute to complicated electric field distributions and higher power absorption in the human head leading to potential safety concerns. Scattering parameters, local and global SAR, and B₁⁺ fields are calculated in an 8-channel surface loop Tx array simulated over 70 head-and-shoulder models of 10 tissue compartments. RF shimming methods and pulse sequences are analyzed over each model to demonstrate local and global SAR variation in a population. Patient proximity, coil loading and design, patient composition, and RF shim weights contribute to the variations in SAR and B₁⁺ experienced by each subject.

Natalie Schoen¹, Frank Seifert¹, Gregory J. Metzger², Oliver Speck³, Bernd Ittermann¹, and Sebastian Schmitter^1,2
1Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Berlin, Germany, ²Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States, ³Department for Biomedical Magnetic Resonance, Otto-von-Guericke University Magdeburg, Magdeburg, Germany

Respiratory motion impacts peak spatial specific absorption rate (psSAR) at cardiac UHF MR imaging as shown previously for individual$$$B_1^+$$$shim vectors. Here two saftey supervision modes were compared: a local SAR control mode (SCM) and a channel-wise power control mode (PCM).

Results show amplitude and location changes of the psSAR between inhale and exhale and this effect depends on the coil setup, element type and breathing pattern. Respiratory-induced psSAR variations are lower when applying PCM compared to SCM, while PCM is more conservative with respect to the total power allowed.

Jessica A. Martinez¹, Alessandro Arduino¹, Oriano Bottauscio¹, and Luca Zilberti^1
1Advanced Materials Metrology and Life Science, Istituto Nazionale di Ricerca Metrologica, Torino, Italy

Specific absorption rate (SAR) values can be obtained using MRI data to calculate the electrical conductivity via Electric Properties Tomography (EPT) and by indirectly obtaining the electric field by the B1⁺ field. However, only using B1⁺ will result in incorrect SAR values. In this work, we propose the calculation of a tissue-specific correction factor to mitigate the inaccuracy of SAR.

Natalia Dudysheva^1,2, Alexis Amadon¹, Alexandre Vignaud¹, Redha Abdeddaim³, and Franck Mauconduit^1
1CEA, NeuroSpin, Paris-Saclay University, CNRS, Gif-sur-Yvette, France, ²Multiwave Imaging, Marseille, France, ³Fresnel Institute, Aix-Marseille University, CNRS, Marseille, France

This work promotes the RF coil elaboration facilitating its in-vivo testing phase. To exclude patient risk, in-vivo studies with each new coil configuration require prior SAR verifications and commission approval lengthening the design process. The so-called “Restricted SAR” (rS) protocol offers a solution to safely use homemade coils in vivo at any development stage without prior validation. This protocol constrains transmitted RF power and automatically respects the SAR safety limits, whatever the SAR distribution. Here, we propose the extended rS protocol with flexible sequence parametrization. This adjustability offers better B₀ mapping coverage, optimal B₁⁺-map estimation, and higher spatial image resolutions.

Natalia Dudysheva^1,2, Nicolas Boulant¹, Alexandre Vignaud¹, and Franck Mauconduit^1
1CEA, NeuroSpin, Paris-Saclay University, CNRS, Gif-sur-Yvette, France, ²Multiwave Imaging, Marseille, France

In this work, we propose to extend the capacities of ultra-low SAR protocol used in coil design. A developed restricted SAR protocol holds the worst-case SAR below the threshold by constraining the total transmitted RF power. Since SAR reflects tissue heating, we exploit a conservative thermal model to derive relaxed SAR constraints depending on acquisition time. This enables to extend the options available for sequences played under this mode, including fat suppression, higher image resolution, or decreased TR.

Shaihan J Malik^1,2, David J Carmichael¹, Jeffrey W Hand¹, and Joseph V Hajnal^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

Electromagnetic and thermal simulations of a range of models (body mass 14-70 kg) were performed for a 7T (297MHz) birdcage head transmit coil. Head averaged SAR (hdSAR) was found to be the limiting factor for all cases, and a (weak) negative linear correlation with body mass was observed (i.e. hdSAR increases with decreasing body mass). Thermal simulations with fixed blood temperature (assumes good thermoregulation) predict temperature increases remain within guidelines if SAR limits are respected. If blood temperature is allowed to vary then elevated core temperatures are predicted for subjects with smaller body mass even when SAR limits are respected.

Maya Delbany¹, Michel Luong^1,2, Laurent Morel³, Jean-Pierre Adam³, Jean-Christophe Joly³, and Vincent Gras^1
1Université Paris-Saclay, CEA, CNRS, BAOBAB, NeuroSpin, 91191, Gif sur Yvette, France, ²CEA, DRF, IRFU, Gif sur Yvette, France, ³CEA, DAM, Gramat, France

In this study, a 3T MRI database is used to create numerical children’s head models in order to study the variability of the specific absorption rate in the head of children at 7T using a transmit array head coil. These numerical models are integrated in a homemade multi-transmit RF coil model and then used to simulate SAR maps in the head using a finite difference time domain solver called GORF.

Simon Schmidt¹, Arcan Erturk¹, Xiaoxuan He¹, Yigitcan Eryaman¹, and Gregory J Metzger^1
1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States

To realize the increased pTx functionality of a 16-channel transceiver dipole array for body imaging at 10.5T, a validation process was implemented.  After quantifying the uncertainty of the RF arrays electromagnetic simulations a total safety factor was determined enabling the implementation of real-time local SAR monitoring on the scanner through use of virtual observation points (VOP).  In vivo data was acquired with the VOP enabled power monitoring demonstrating local RF shimming performance with and without local SAR constrained optimizations.

Zainul Ihsan¹, Michael Schwarz², Mubashir Husain², and Gregor Schaefers^1,2
1MR:comp GmbH, Gelsenkirchen, Germany, ²MRI-STaR - Magnetic Resonance Institute for Safety, Technology and Research GmbH, Gelsenkirchen, Germany

Numerical simulation was performed to evaluate tissue damage due to magnetically induced (MI) force in 3T Magnetic Resonance (MR) environment for multi-configuration passive medical devices. Static magnetic and mechanical simulations were employed to determine the impact of MI force on the tissue surrounding the implant. The simulation results were used to select the worst-case for passive medical devices and for quantitative assessment of the tissue damage.

Johannes Petzold¹, Bernd Ittermann¹, and Frank Seifert^1
1Physikalisch-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 B₀ (0.5T, 1.5T, 3T). Criteria are the necessary safety factors to accommodate for subject variability, and the resulting mean(B₁⁺). Due to the high safety factor necessary for a phase-based SAR limit, its mean(B₁⁺) performance becomes effectively the same as the much simpler to implement amplitude limit.

Filiz Yetisir¹, Esra Abaci Turk^1,2, Elfar Adalsteinsson^3,4,5, Patricia Ellen Grant^1,2,6, and Lawrence L Wald^4,7,8
1Fetal-Neonatal Neuroimaging and Developmental Science Center, Boston Children's Hospital, Boston, MA, United States, ²Department of Pediatrics, Boston Children's Hospital, Boston, MA, United States, ³Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, ⁴Harvard-MIT Health Sciences and Technology, Cambridge, MA, United States, ⁵Institute for Medical Engineering and Science, Massachusetts Institute of Technology, Boston, MA, United States, ⁶Department of Radiology, Boston Children's Hospital, Boston, MA, United States, ⁷Athinoula A. Martinos Center for Biomedical Imaging, Harvard Medical School, Boston, MA, United States, ⁸Department of Radiology, Massachusetts General Hospital, Boston, MA, United States

RF shimming (RFS) improves the transmit field for fetal MRI, however, fetal safety is understudied. Previous simulations studied the SAR_local of the standard imaging mode (CP mode) of each subject to inform the safety of RFS. We evaluated two local SAR management strategies which utilize subject-specific models and use either the individual’s CP mode SAR_local as a limit for RFS or the maximum CP mode SAR_local value across 7 subjects. We evaluated the B₁⁺ performance inside the fetus for each strategy. Using the maximum CP mode SAR_local across the population as the SAR limit greatly improves RFS performance. 

Dheyaa Alkandari¹ and Steven M Wright^2
1Electrical Engineering, Kuwait University, Abdullah Al Mubarak Al Sabah, Kuwait, ²Texas A&M University, College Station, TX, United States

Evidence indicates that excess RF power from coil’s associated circuitry could cause injuries to patients. Slot elements have been investigated earlier and demonstrated good potential when used for multi-channel RF coils. Here we investigate the potential of using the extended metallization of slot elements to shield stray electric field generated from RF coil associated circuitry. RF heating experiment was designed to compare the temperature of the phantom with and without using the slot’s extended metallization to shield the associated circuitry. Experimental results demonstrated an observable reduction in the phantom temperature resulting from extending the slots metallization.

paul nobre¹, Gwenaël Gaborit^2,3, Raphael Sablong¹, Nadege Courjal⁴, Florent Behague⁴, Antoine Coste⁴, Adrien Godet⁴, Miguel Suarez⁴, Lionel Duvillaret³, and Olivier Beuf^1
1Univ. Lyon, INSA-Lyon, Université Lyon 1, UJM-Saint Etienne, CNRS, Inserm, CREATIS, UMR 5220, U1206, Villeurbanne, France, ²Université de Savoie, IMEP-LAHC, UMR 5130, Le Bourget-du-Lac, France, ³KAPTEOS, Sainte-Hélène-du-Lac, France, ⁴Institut FEMTO-ST, UMR CNRS 6174, Université Bourgogne Franche-Comté, Besançon, France

The coaxial cables connecting coils to the MRI are subject to many unwanted interactions with RF pulses and with patients’ tissues. Currents flowing on the shielding of the cable can lead to local SAR increase and RF-induced burns. Despite their limitations and the amount of effort to discard them, there is still no satisfying alternative. One of the issue for competing technologies is to safely provide on coil power supply without cables inside the MRI. In this work, we propose to evaluate the feasibility of passive optical conversion and transmission of the signal to the MRI console.

Giuseppe Carluccio¹, Eros Montin¹, Riccardo Lattanzi¹, and Christopher Michael Collins^1
1Center for Advanced Imaging Innovation and Research (CAI2R), New York, NY, United States

We developed and compared two different Deep-Learning (DL) based approaches to approximating temperature in subject-specific body models. The first involved use of a 2D U-net to predict temperature throughout the body on a slice-by-slice basis, and the second involved use of a 3D U-net to predict temperature in the 3D body. The 3D approach greatly outperformed the 2D approach, and was very fast.