Malwina Molendowska¹, Fabrizio Fasano^2,3, Umesh Rudrapatna¹, Ralph Kimmlingen³, Derek K. Jones^1,4, Slawomir Kusmia¹, Chantal M. W. Tax^1,5, and C. John Evans^1
1Cardiff University Brain Research Imaging Centre (CUBRIC), Cardiff University, Cardiff, United Kingdom, ²Siemens Healthcare Ltd, Camberly, United Kingdom, ³Siemens Healthcare GmbH, Erlangen, Germany, ⁴Mary McKillop Institute for Health Research, Faculty of Health Sciences, Australian Catholic University, Melbourne, Australia, ⁵Image Sciences Institute, University Medical Center Utrecht, Utrecht, Netherlands

Increasing-available ultra-strong gradient systems introduce a new challenge in MRI: the unknown interaction of time-varying magnetic fields with the human body. We characterise the physiological effects of deploying 300 mT/m gradients Siemens Connectom system when imaging regions below the neck (i.e., heart and prostate). We show gradient amplitude thresholds for PNS (ramp times < 2ms) are first reached on the Y-gradient. Moreover, landmarking on the heart gives the highest probability of generating magnetophosphenes (all gradient-axes). This study establishes: (i) limitations in greatest system performance; and (ii) that the so-far head-only Connectom system can be safely used in ‘below-the-neck’ applications.

Mathias Davids^1,2,3, Bastien Guerin^1,2, and Lawrence L Wald^1,2,4
1Martinos Center for Biomedical Imaging, Boston, MA, United States, ²Harvard Medical School, Boston, MA, United States, ³Computer Assisted Clinical Medicine, Mannheim, Germany, ⁴Harvard-MIT, Division of Health Sciences and Technology, Cambridge, MA, United States

We apply a fast PNS model evaluated on a Huygens’ surface to characterize PNS performance as a function of patient position and pose in previously described PNS optimized gradients. The coils were initially PNS-optimized for a female body model in a standard brain imaging position. The Huygens’ approach allows us to assess other body positions and demonstrate the PNS benefits/penalties associated with other imaging applications and in both a female and male body model by dramatically speeding up the PNS characterization (to a couple of seconds per body position/orientation). The findings support a broad benefit from the PNS optimized windings.

Luca Zilberti¹, Alessandro Arduino¹, Riccardo Torchio², Umberto Zanovello¹, Fabio Baruffaldi³, Paolo Bettini², Piergiorgio Alotto², Mario Chiampi¹, and Oriano Bottauscio^1
1Istituto Nazionale di Ricerca Metrologica, Torino, Italy, ²Dipartimento Ingegneria Industriale, Università degli Studi di Padova, Padova, Italy, ³Istituto Ortopedico Rizzoli, Bologna, Italy

Peripheral nerve stimulation is a typical source of concern in MRI, which, in principle, could be significantly affected by the presence of a foreign object, like an orthopaedic prosthesis, in the body. This work investigates this possibility through a computational approach, making use of a human model where a knee implant has been placed, realistically, through a “virtual surgery”. Results indicate that the presence of the prosthesis can give rise to local enhancement of the electric field induced by the operation of the gradient coils, hence increasing the risk.

Natalie G Ferris^1,2, Mathias Davids^3,4,5, Valerie Klein^3,5, Bastien Guérin^3,4, and Lawrence L Wald^3,4
1Harvard Graduate Program in Biophysics, Harvard University, Cambridge, MA, United States, ²Harvard-MIT Division of Health Sciences and Technology, MIT, Cambridge, MA, United States, ³A. A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, United States, ⁴Harvard Medical School, Boston, MA, United States, ⁵Computer Assisted Clinical Medicine, Heidelberg University, Heidelberg, Germany

Peripheral Nerve Stimulation (PNS) limits the use of high-performance gradient systems. We exploit nerve membrane hyperpolarization and depolarization in readout gradient waveform design to achieve a higher gradient moment (area) in a given time without initiating PNS.  In a typical symmetric trapezoidal readout waveform, the slew ramps have identical rise times but opposite dB/dt (E-field sign). Asymmetrizing the waveform enables achieving the same gradient moment while emphasizing membrane hyperpolarization over depolarization. The disparate effects of hyperpolarizing and depolarizing pulses are sufficient to impact the nerve’s PNS threshold, potentially increasing the spatial resolution.

Wyger Brink¹, Sahar Yousefi^1,2, Prernna Bhatnagar¹, Marius Staring², Rob Remis³, and Andrew Webb^1
1C.J. Gorter Center, dept. of Radiology, Leiden University Medical Center, Leiden, Netherlands, ²Division of Image Processing, dept. of Radiology, Leiden University Medical Center, Leiden, Netherlands, ³Circuits and Systems, dept. of Microelectronics, Delft University of Technology, Delft, Netherlands

Compliance with RF exposure limits in ultra-high field MRI is typically based on “one-size-fits-all” safety margins to account for the intersubject variability of local SAR. In this work we have developed a semantic segmentation method based on deep learning, which is able to generate a subject-specific body model for personalized RF exposure prediction at 7T.

Sydney Nicole Williams¹, Jürgen Herrler², Patrick Liebig³, Paul McElhinney¹, Shajan Gunamony^1,4, Armin M. Nagel⁵, and David A. Porter^1
1Imaging Centre of Excellence, University of Glasgow, Glasgow, United Kingdom, ²Department of Neuroradiology, University Hospital Erlangen, Erlangen, Germany, ³Siemens Healthineers, Erlangen, Germany, ⁴MR CoilTech Limited, Glasgow, United Kingdom, ⁵Institute of Radiology, University Hospital Erlangen, Erlangen, Germany

We compare two methods for estimating local SAR with virtual observation points (VOPs) in a commercial and a self-built 8Tx/32Rx head coil, respectively. The commercial coil relies on a constant safety factor that reduces the hardware power limits in each transmit channel, where the self-built coil derives the VOPs from coil electromagnetic field (EMF) simulations. We show the use of both coils in vivo with MPRAGE using Universal and subject-specific pTx pulses. The EMF-based VOPs, used with the self-built coil, resulted in a lower local SAR estimate for a similar image quality, providing more flexibility in pulse sequence design.

E.F. Meliadò^1,2,3, A.J.E. Raaijmakers^1,2,4, M. Maspero^2,5, M.H.F. Savenije^2,5, P.R. Luijten¹, and C.A.T. 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

Residual error/uncertainty is always present in the estimated local SAR, therefore it is essential to investigate and understand the magnitude of the main sources of error/uncertainty. Last year we presented a Bayesian deep learning approach to map the relation between subject-specific complex B₁⁺-maps and the corresponding local SAR distribution, and to predict the spatial distribution of uncertainty at the same time. The preliminary results showed the feasibility of the proposed approach. In this study, we show its ability to reliably capture the main sources of uncertainty and detect deviations in the MR examination scenario not included in the training samples.

Alireza Sadeghi-Tarakameh¹, Lance DelaBarre¹, Nur Izzati Huda Zulkarnain¹, Noam Harel¹, and Yigitcan Eryaman^1
1Center for Magnetic Resonance Research (CMRR), University of Minnesota, Minneapolis, MN, United States

We mitigated the radiofrequency heating at the contacts of a DBS electrode by utilizing an implant-friendly (IF) excitation scenario at 7T. IF modes of a 16-channel transmit/receive coil are calculated by minimizing electrode-shaft current close to the contacts. An IF excitation is calculated by shimming the B₁⁺ field in an ROI using individual IF modes of the array. The proposed approach is able to mitigate the shaft current and RF heating at the contacts. 

Bhumi Bhusal¹, Ehsan Kazemivalipour², Jasmine Vu¹, Stella LIn¹, Bach Thanh Nguyen¹, John Kirsch³, Elizabeth Nowac⁴, Julie Pilitsis⁵, Joshua Rosenow¹, Ergin Atalar², and Laleh Golestanirad^1
1Northwestern University, Chicago, IL, United States, ²Bilkent University, Ankara, Turkey, ³Massachusetts General Hospital, Charlestown, MA, United States, ⁴Illinois Bone and Joint Institute, Wilmette, IL, United States, ⁵Albany Medical College, Albany, NY, United States

Though there are many studies reporting RF heating of implants in horizontal MRI scanners, there is almost no literature on vertical scanners that have 90° rotated transmit coils and a fundamentally different distribution of RF fields. Here we evaluate RF heating of deep brain stimulation (DBS) implants during MRI in a 1.2T open-bore vertical scanner compared to a 1.5T horizontal system with both numerical simulations and experimental measurements. We found a significant reduction in RF heating using vertical vs horizontal RF coils which are attributable to the orthogonal orientation of RF electric fields.

Kellen Mulford¹, David Darrow², Sean Moen², Samuel Ndoro², Bharathi D. Jagadeesan³, Andrew W. Grande², Donald R. Nixdorf⁴, and Pierre-Francois Van de Moortele^1
1Center for Magnetic Resonance Imaging, University of Minnesota, Minneapolis, MN, United States, ²Department of Neurosurgery, University of Minnesota, Minneapolis, MN, United States, ³Department of Radiology, University of Minnesota, Minneapolis, MN, United States, ⁴Department of Diagnostic and Biological Science, University of Minnesota, Minneapolis, MN, United States

Pain recurrence following invasive treatments for trigeminal neuralgia is common, yet the treatment decisions in these situations lack objective guidance. The goal of this work was to establish the feasibility of using 7.0-Tesla MRI to visualize treatment related effects from percutaneous radiofrequency rhizotomy procedures. We scanned two patients with trigeminal neuralgia and one human cadaver specimen at 7.0-Tesla after radiofrequency rhizotomy procedures. Both acute and long-term treatment related effects were successfully visualized by our protocol. Future work will recruit a large cohort of trigeminal neuralgia patients to correlate imaging findings with clinical outcomes.