MR Safety

Monday 12 May 2014

Devashish Shrivastava¹, Jinfeng Tian¹, John Hughes¹, and J Thomas Vaughan^1
1University of Minnesota, Minneapolis, MN, United States

In vivo radiofrequency (RF) heating was simulated by solving the first principles based generic bioheat transfer model (GBHTM) in a digital pig due the power deposition from a 3T body coil. The simulations were validated by measuring the heating using fluoroptic probes in anesthetized swine due to the whole-body average power deposition of 2.77 W/kg for an hour in a 3T scanner. The GBHTM predicted the RF heating accurately. The simulations and measurements provided fundamental understanding of the RF energy transport and related heating in vivo to make MRI more powerful as well as improve patient safety.

Subhendra N Sarkar¹, Ron L Alterman², Efstathios Papavassiliou², Douglas L Teich¹, Rafael Rojas¹, Rafeeque A Bhadelia¹, Jeremy Stormann¹, Ines Cabral-Goncalves¹, and David B Hackney^1
1Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, BOSTON, MA, United States, ²Division of Neurosurgery, Beth Israel Deaconess Medical Center, Harvard Medical School, BOSTON, MA, United States

Treatment efficacy for deep brain stimulators (DBS) in medically refractory Parkinson’s is high when surgical planning and further assessments are done by high quality MRI with adequate RF power although there exist severe SAR restrictions for that. Sentinel events after MRI seems rare but neurologic deficits previously attributed to surgery could have been from routine MRI. We have developed and tested the utility of a modified FSTIR sequence within low SAR guidelines on patients with DBS and observed similar tissue contrasts from the low and high SAR sequences with low SAR images being equally useful for diagnosis and neurosurgical planning.

Clare McElcheran¹, Laleh Golestanirad², and Simon Graham^1,2
1Medical Biophysics, University of Toronto, Toronto, ON, Canada, ²Sunnybrook Health Science Centre, Toronto, ON, Canada

Deep Brain Stimulation (DBS), a treatment for movement disorders (eg Parkinson’s Disease) consists of leads and electrodes that send electrical impulses to deep brain nuclei. During MRI, the electric field (E) created by radiofrequency (RF) excitation couples with the leads, amplifies E, and can cause dangerous heating of nearby tissues. Parallel RF transmission (PTX) is investigated to suppress heating by varying the amplitude and phase of each transmit element. In an 8-element PTX with a uniform cylindrical phantom, E is reduced 97% compared to a linear birdcage transmitter, with <10% transmit magnetic field inhomogeneity in the chosen field of view.

Ingmar Graesslin¹, Peter Vernickel¹, Peter Börnert¹, Kay Nehrke¹, Giel Mens², Paul Harvey², and Ulrich Katscher^1
1Philips Research Laboratories, Hamburg, Germany, ²Philips Healthcare, Best, Netherlands

Achieving RF patient safety in parallel transmission is difficult, due to the freedom in tailoring the RF transmit fields. Before and during the scan, its conformity with existing SAR limits has to be verified to ensure patient safety. We developed, implemented, and verified a new comprehensive RF patient-safety-supervision concept that combines real-time global SAR and local SAR supervision with real-time RF supervision. This new concept allows for a significantly increased permissible RF duty cycle, improves the detection of SAR limit violations and patient-unsafe conditions, and reduces the number of false-positive scan interruptions.

Nikolaus Weiskopf¹, David Bradbury¹, Sheila Burns¹, and Janice Glensman^1
1Wellcome Trust Centre for Neuroimaging, University College London, London, United Kingdom

Only few studies are published on the safety of tattoos in MRI. We present an interim report on a prospective study on all healthy volunteers with tattoos who were scanned at our lab. From our sample of 127 volunteers the probability of a tattoo related adverse reaction is estimated to be lower than 4.5% when additional precautions are applied. Although this is the first prospective study on this topic avoiding several potential confounds of previous studies, the particular sample and conditions studied may still limit general conclusions.

Eva Oberacker¹, Lukas Winter¹, Frank Seifert², Jaroslav Marek¹, Gerd Weidemann², Eugen Hofmann³, and Thoralf Niendorf^1,4
1Berlin Ultrahigh Field Facility (B.U.F.F.), Max Delbrück Center for Molecular Medicine, Berlin, Berlin, Germany, ²Physikalisch Technische Bundesanstalt, Berlin, Germany, ³Biotronik AG, Bülach, Switzerland, ⁴Experimental and Clinical Research Center, a cooperation of the Charité Medical Faculty and the Max Delbrück Center for Molecular Medicine, Berlin, Germany

This work performs a careful safety evaluation of RF induced heating of coronary stents including electromagnetic (EM) simulations and E-field measurements. Complex electromagnetic field coupling is investigated depending on stent type, length, location, orientation, vessel diameter and RF coil used. Electromagnetic (EM) and thermal simulations were performed in phantoms and human voxel models and validated in ASTM phantom measurements. The results are transferrable to various RF coil designs and may be utilized to estimate safe RF exposure levels for SAR personalized UHF-MR exams including patients with intracoronary and other vascular stent implants.

Yigitcan Eryaman^1,2, Bastien Guerin², Can Akgun³, Joaquin L. Herraiz¹, Adrian Martin^1,4, Angel Torrado-Carvajal^1,5, Norberto Malpica^1,5, Juan A. Hernandez-Tamames^1,5, Emanuele Schiavi^1,4, Elfar Adalsteinsson^6,7, and Lawrence L. Wald^2,7
1Madrid-MIT M+Vision Consortium in RLE, MIT, Cambridge, Massachusetts, United States, ²Martinos Center for Biomedical Imaging, Dept. of Radiology, MGH, Charlestown, MA, United States, ³Invenshure, MN, United States, ⁴Dept. of Applied Mathematics, Rey Juan Carlos University, Madrid, Spain, ⁵Dept. of Electronic Technology, Rey Juan Carlos University, Madrid, Spain, ⁶Dept. of Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts, United States, ⁷Harvard-MIT Health Sciences and Technology, MIT, Cambridge, MA, United States

We present a pulse design strategy that can be used to safely scan patients with implants. Our strategy is based on utilizing implant friendly modes which are defined as the modes of an array that cancel the local SAR around the implant lead tip. We performed EM simulations using a multi-tissue realistic head model with a generic deep brain stimulator implant. As a result of the pulse design, local SAR at the lead tip is reduced below SAR limits. A uniform axial flip angle distribution is obtained.

Arian Beqiri¹, Francesco Padormo^1,2, Jeff W. Hand¹, Joseph V. Hajnal^1,2, and Shaihan J. Malik^1
1Division of Imaging Sciences and Biomedical Engineering, King's College London, London, United Kingdom, ²Centre for the Developing Brain, King's College London, London, United Kingdom

Cardiac imaging at high field suffers from image quality issues due to greater B₁⁺ inhomogeneities and is frequently limited by SAR, which constrains the speed at which scans can be run without exceeding regulatory limits. By using parallel transmission MRI, we demonstrate the ability to significantly reduce SAR whilst simultaneously improving B₁⁺ homogeneity in order to optimise imaging over the cardiac region for two differently sized subjects. We also show the effects of selecting the wrong SAR model for each subject.

Robert A. Pooley¹, Krzysztof R. Gorny², Christopher P. Favazza², Joel P. Felmlee², Chen Lin³, Matt A. Bernstein², and Robert E. Wharen^4
1Radiology, Mayo Clinic, Jacksonville, Florida, United States, ²Radiology, Mayo Clinic, Rochester, MN, United States, ³Radiology and Imaging Science, IU School of Medicine, Indianapolis, IN, United States, ⁴Neurosurgery, Mayo Clinic, Jacksonville, Florida, United States

Temperature measurements were made in a phantom at the tip of a deep brain stimulator lead. The DBS components were arranged in various configurations and scanned at high and low SAR with body coil transmit. The temperature increase for the lead only and lead + insertion stylet was in the range of 0.5 - 3.2C at high SAR and < 0.1C at low SAR. The full DBS system resulted in heating of 14.4C at high SAR and 0.25C at low SAR. There is increasing interest in evaluating heating of leads with body coil RF transmit, which is contraindicated.

Xiaotong Zhang¹, Pierre-Francois Van de Moortele², Jiaen Liu¹, Sebastian Schmitter², and Bin He^1,3
1Department of Biomedical Engineering, University of Minnesota, Minneapolis, MN, United States, ²Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States, ³Institute for Engineering in Medicine, University of Minnesota, Minneapolis, MN, United States

It has been shown that Electrical Properties (EPs) of biological tissues can be derived from MR-based B1 measurement. A strong appeal for these ‘Electrical Property Tomography’ (EPT) methods is to predict in real-time on a per-subject basis local SAR induced by RF pulsing. To reduce error propagation along the reconstruction, we eliminate the need for computing EPs by introducing the concept of ‘virtual tissue EPs’ (VEPs), tailored to provide max local SAR estimation based on measured B1 maps, with a safety margin. We evaluate the concept on electromagnetic models (EM) and in-vivo data of head imaging at 7T.