Safety in MRI
 

Tuesday 2 June 2015

Alan Leewood¹, Sharath Gopal¹, Kerim Genc², Steve Cockram³, Philippe Young³, Jeff Crompton⁴, and Josh Thomas^4
1MED Institute, Inc., West Lafayette, IN, United States, ²Simpleware Ltd, VA, United States, ³Simpleware Ltd, Devon, United Kingdom, ⁴AltaSim Technologies, LLC, OH, United States

A full-body human anatomy and multiphysics finite element analysis (FEA) model is developed to evaluate RF-induced heating from MR imaging. The FEA approach provides a coupled transient thermal solution and incorporates high fidelity geometric features at minimal computational cost, thereby overcoming two significant limitations of the finite difference time domain (FDTD) method. Specific absorption rates (SAR) from the study are in good agreement with published results based on the FDTD method; maximum temperature occurs at a location that is consistent with previous simulation and experimental results. This work will be extended to add passive implants (e.g. stents and orthopedic devices) into the anatomy.

Lukas Winter¹, Eva Oberacker¹, Celal Özerdem¹, Yiyi Ji¹, Florian von Knobelsdorff-Brenkenhoff^1,2, Gerd Weidemann³, Frank Seifert³, and Thoralf Niendorf^1,2
1Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrück Center for Molecular Medicine, Berlin, Germany, ²Experimental and Clinical Research Center (ECRC), a joint cooperation between the Charité and the Max-Delbrueck Center for Molecular Medicine, Berlin, Germany, ³Physikalisch Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany

En route to broader clinical UHF-MR studies it is essential to gain a better insight into the interaction of passive conducting implants with radiofrequency fields. The conclusions drawn from earlier studies are valuable but constrained to the very specific experimental setup used. Recognizing the need for an universal and rapid approach for SAR assessment, this work derives and validates an analytical expression for fast SAR estimation of coronary stents. Our findings can potentially be translated to any patient, coronary stent type, RF coil configuration and RF transmission regime.

Sabrina Klix¹, Antje Els¹, Katharina Paul¹, Andreas Graessl¹, Celal Oezerdem¹, Oliver Weinberger¹, Lukas Winter¹, Christof Thalhammer¹, Till Huelnhagen¹, Jan Rieger¹, Heidrun Mehling², Jeanette Schulz-Menger^2,3, and Thoralf Niendorf^1,2
1Berlin Ultrahigh Field Facility (B.U.F.F.), Max Delbrück Center for Molecular Medicine (MDC), Berlin, Germany, ²) Experimental and Clinical Research Center (ECRC), a joint cooperation between the Charité Medical Faculty and the Max-Delbrueck-Center, Berlin, Germany, ³HELIOS Klinikum Berlin-Buch, Dept. of Cardiology and Nephrology, Berlin, Germany

This study examines the subjective acceptance during UHF-CMR in a cohort of healthy volunteers who underwent a cardiac MR examination at 7.0T.

Richard Williamson¹, Michael Childers¹, Tushar Dharampal¹, Shiloh Sison¹, Amber Durica¹, Gabriel Mouchawar¹, and John Nyenhuis^2
1St. Jude Medical, Sylmar, California, United States, ²Purdue University, West Lafayette, Indiana, United States

This abstract demonstrates that for some implantable devices, torque testing per ASTM F2213 as a part of MR conditional safety assessment, may not be necessary as a conservative magnetic torque value may be calculated using the measured magnetically induced force per ASTM F2052 along with the physical properties of the device. Two conservative upper-bound equations (Tier 1 and Tier 2) are derived and used to estimate the magnetically induced torque for four implantable devices.

Gerd Weidemann¹, Frank Seifert¹, Werner Hoffmann¹, and Bernd Ittermann^1
1Physikalisch-Technische Bundesanstalt, Braunschweig und Berlin, Germany

A toroidal time domain current clamp with fiber optic readout was developed for the measurement of the RF currents induced by MR investigations in the protruding ends of implanted conductive structures with diameters up to 10 mm in situ in an MR scanner. Tests with wires inside a body phantom show that even when all parts of the wires are 30 cm outside the body coil of a 3T MR scanner currents of up to 0.1 A are measured at 1 kW transmitter power.

Jong-Min Kim¹, Jang-Gyu Cha², Ji-Young Hwang³, Seung-Eun Jung⁴, Hyunn-Kyoon Lim⁵, Do-wan Kim⁶, Kwang-Su Kim⁶, Sung-Jin Kang², Han-Joong Kim¹, Suchit Kumar¹, Junyong Park⁷, Chulhyun Lee⁷, and Chang-Hyun Oh^1
1Electronic and information engineering, Korea University, Seongbuk-Gu, Seoul, Korea, ²Department of Radiology, Soonchunhyang University Bucheon Hospital, Seoul, Korea,³Department of Radiology, Ewha Women's University Mokdong Hospital, Seoul, Korea, ⁴Department of Radiology, The Catholic University of Korea St. Mary's Hospital, Seoul, Korea,⁵Korea Research Institute of Standards and Science, Daejeon, Korea, ⁶Korean Institute of Accreditation of Medical Imaging, Seoul, Korea, ⁷The MRI Team, Korea Basic Science Institute, Chungcheongbuk-do, Korea

The quality evaluation schemes such as the ACR methods1 are good enough to decide whether the MRI system is useful for clinical application based on certain measurement parameters showing the image quality. In ACR method, 11 slices of MR images are usually acquired on the ACR phantom and they are used to evaluate the 7 items (geometric accuracy, high-contrast spatial resolution, slice thickness accuracy, slice position accuracy, image intensity uniformity, percent-signal ghosting, and low-contrast object detectability). However, there are several limitations of ACR method like observer-dependent, time consuming, and accurate numerical ratings on the system performance. In this study, 3 items (vessel conspicuity, brain tissue contrast, SNR) in addition to ACR method is proposed. For semi-automatic and quantitative MR system classification, all of above-mentioned items are evaluated numerically by using MATLAB (Mathwork, Inc., MA).

Mélina Bouldi^1,2, Olivier David^1,2, Stephan Chabardes^2,3, Alexandre Krainik^2,3, and Jan M Warnking^1,2
1Université Grenoble Alpes, Grenoble Institut des Neurosciences, Grenoble, Rhône-Alpes, France, ²U836, Inserm, Grenoble, Rhône-Alpes, France, ³CHU de Grenoble, Rhône-Alpes, France

The MR safety of a vagus nerve stimulator was assessed. Phantom experiments with a 3T whole-body transmit coil showed heating by >3K at the electrodes. E-field maps from electromagnetic simulations without the device show prominent hot spots close to the path of the VNS electrode, both in phantom and realistic human body models. This configuration, not recommended by the device manufacturer, indeed presents a significant risk, with predicted high variability of actual heating in patients dependent on the exact position of the electrode at E-field hot-spots. Simulations may be able to assess risks better than phantom experiments in this context.

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

Electrical Properties Tomography technique, which utilizes measurable B1 fields to reconstruct the local electrical properties of biological tissues, can be used to estimate local SAR on a subject-specific basis. The purpose of the present work is to validate such B1-based local SAR estimation by using previously proposed gEPT algorithm. B1 measurement was obtained in phantoms containing gelatin and biological tissues at 7T with a multi-channel transmit/receive coil, and local SAR distribution was predicted for a specific heating protocol. SAR results were subsequently converted into temperature changes which in turn were validated against temperature changes measured by MRI Thermometry.

Ting Song¹, Maria Ida Iacono¹, Leonardo M. Angelone¹, and Sunder Rajan^1
1Center for Devices and Radiological Health, U.S. Food and Drug Administration, Silver Spring, MD, United States

The purpose of this study is to perform a retrospective analysis of data related to RF-induced heating tests of medical implants provided in U.S. FDA submissions. We report interim data on 55 small passive medical implants for 1.5T and 3.0T. The data show that there is no clear length dependence for devices ¡Ü120 mm even when the data are stratified by the magnetic field, device type, and test vendor. Most devices less than 50 mm in length showed less than 2.6 oC of temperature change per WB-SAR except for one device that showed 4.5 oC of temperature change per WB-SAR.

Pallab K Bhattacharyya¹, Howard Goldman², Mark J Lowe¹, Adrienne Quirouet², and Stephen E Jones^1
1Imaging Institute, Cleveland Clinic, Cleveland, OH, United States, ²Glickman Urological Institute, Cleveland Clinic, Cleveland, OH, United States

RF heating testing during pelvic and lumbar scans of Medtronic Interstim II (Model 3058) implantable pulse generator (IPG) connected to Medtronic Quadipolar Nerve Stimulator Lead (Model 3889) at 1.5T Siemens Aera scanner with receive-only coils were performed. Temperatures of the electrodes were measured by using fiber optic sensors with fluoroptic monitoring with the IPG and lead placed inside an ASTM gel phantom. No electrode heating was observed when the lead was connected with the IPG in any of the scans, while considerable heating was observed when the IPG was disconnected and taken out of the phantom.

Peyman Shokrollahi¹, Wendong Wang², Adam C. Waspe³, James M. Drake³, and Andrew A. Goldenberg^1
1Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, ON, Canada, ²School of Mechanical Engineering, Northwestern Polytechnical University, Xi'an, Shaanxi, China, ³Hospital for Sick Children, Toronto, ON, Canada

Obtaining haptic information and controlling the force produced by actuators is one of the key issues in performing a successful operation by surgical robots. The behaviour of MRI-compatible actuators such as ultrasonic motors (USM) in an MR environment needs to be characterized. A one DoF MR-compatible robot was built to measure the torque produced by a USM. The axial force was measured with a piezo-resistive sensor through transferring the torque using a screw. Inside and outside MR-environment forces were compared through statistical analysis. The measured force was almost doubled while the motor was running during the MR scan.

Simon Reiß¹, Andreas Bitzer², and Michael Bock^1
1Radiology - Medical Physics, University Medical Center Freiburg, Freiburg, Germany, ²Biolab Technology AG, Zürich, Switzerland

The accurate and localized measurement of electric fields in MRI is very important to assess RF safety hazards. In this work we present a purely optical sensor which is substantially smaller than existing dipoles to measure E-fields with high spatial resolution in a clinical MR system. This sensor allows for MR-safety inspections of RF coils and even for E-field assessment of small metallic structures such as implants or catheters in the MRI environment.

Thomas Lottner^1,2, Mathias Nittka¹, Theresa Bachschmidt^1,3, Heiko Meyer¹, and Wolfgang Nitz^1,2
1Siemens Healthcare, Erlangen, Germany, ²University of Regensburg, Regensburg, Germany, ³Experimental Physics 5, University of Würzburg, Würzburg, Germany

Changing the B1 polarization has shown to reduce artifacts due to induced currents in metallic implants. The presented work aims to show that the right elliptical polarization not just improves image quality, but furthermore decrease the SAR value near the implant. Numerical simulations were performed on a human male with a simplified implant model placed in his right femoral bone. The optimal polarization was approximated by an analytical model. The optimized and the circular polarization were compared and a significant increase of B1 homogeneity and decrease in local SAR was shown for the elliptical polarization.

Mahdi Abbasi^1,2, Gregor Schaefers¹, Amin Douiri¹, and Daniel Erni^2
1MR:comp GmbH, Gelsenkirchen, NRW, Germany, ²General and Theoretical Electrical Engineering (ATE), University of Duisburg-Essen, Duisburg, NRW, Germany

The quantitative analysis of induced E-field due to presence of medical catheter implants during MRI become particularly critical if the implant is only semi immersed into the human body where a part remains outside. As examples, deep brain electrodes are partly immersed into the brain and various EEG and ECG electrodes as zero immersing configurations. As the result of the numerical simulations, SAR distribution in the surroundings of the tip of the generic implants is obtained for 3 configurations under test: partly immersed insulated/uninsulated and fully immersed uninsulated generic implant. Temperature measurements are done for all worst case lengths and two adjacent lengths in an in-house 64 MHz RF laboratory system.

Guillaume Ferrand¹, Michel Luong¹, Alexis Amadon², and Nicolas Boulant^2
1DSM/IRFU/SACM, CEA-Saclay, Gif-sur-Yvette, France, ²DSV/I2BM/Neurospin, CEA-Saclay, Gif-sur-Yvette, France

In high field MRI, a phased array coil comprising a large number of transmit elements appears more attractive with regard to local SAR reduction and to RF inhomogeneity mitigation. However, the linear combination of electric fields errors makes the total SAR error much more complex to estimate. Here, we propose mathematical tools and hypotheses to estimate the standard deviation of the SAR produced by multiple transmit elements, knowing the standard deviation of the error on the electrical field produced by each element. Thanks to these tools, the best margins can be applied to the final SAR, to ensure the safety of the patient in in-vivo experiments, without overconservative measures.

Nahla M H Elsaid¹, Adrian I Nachman^2,3, Weijing Ma², Tim P DeMonte⁴, and Michael L G Joy^1,2
1IBBME, University of Toronto, Toronto, Ontario, Canada, ²Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, Canada, ³Department of Mathematics, University of Toronto, Toronto, Ontario, Canada, ⁴FieldMetrica Inc., Toronto, Ontario, Canada

This is a numerical study showing the effect of anisotropy on the accuracy of conductivity values from two different types of conductivity imaging methods. One is anisotropic based method versus another that is isotropic based. The comparison is made using a validation approach that requires a forward solver which imports the anisotropic conductivity or the isotropic conductivity according to the method to be validated.

Özlem Ipek¹, Joao Jorge^2,3, Frederic Grouiller⁴, Wietske van der Zwaag¹, Lijing Xin², and Rolf Gruetter^2,5
1CIBM-AIT, EPFL, Lausanne, Vaud, Switzerland, ²LIFMET, EPFL, Lausanne, Vaud, Switzerland, ³Bioengineering, University of Lisbon, Lisbon, Portugal, ⁴CIBM, Geneva University Hospital, Geneva, Switzerland, ⁵Radiology, University of Lausanne, Lausanne, Vaud, Switzerland

An exact model of a 64-channel EEG cap and the RF-coil was simulated, and B1+ and SAR distributions across the human head evaluated with/without the presence of the EEG cap. Simulation results were complemented by experimental B1+ and temperature measurements from a human subject with simultaneous EEG acquisition, using both RF volume and surface coils at 7T MR. No RF-safety concern is raised for simultaneous EEG-fMRI at 7T MR with the current setup. However, with the introduction of the EEG cap, a pronounced RF shielding effect is found, which leads to a decrease in the RF efficiency in the subject.

Gerd Weidemann¹, Frank Seifert¹, Werner Hoffmann¹, Harald Pfeiffer¹, and Bernd Ittermann^1
1Physikalisch-Technische Bundesanstalt, Braunschweig und Berlin, Germany

A method to derive the radiated power of 7T transmit coils is developed and applied to an 8-channel pTx coil to determine the radiated power as a function of steering conditions. It is based on fiber optic time domain E-field vector measurements inside a cylindrical single mode waveguide which mimics the patient tunnel of a typical 7T scanner. The maximum radiated power was found for the CP-mode of the array coil. Although, the portion of the radiated power of the specific coil studied is low it can be higher for other coil designs and will become more prominent for higher fields.

Malek I. Makki^1
1MRI Research, University Children Hospital Zurich, Zurich, Switzerland

Inward spiral-trajectory technique was used as slice-select excitation to acquire reduced FOV volumetric 3D fast gradient recalled echo and achieve ultra-high spatio-temporal resolution images at high SNR and free of blurring artefact.

Rebecca Krimins^1,2, Larry Gainsburg³, Amanda Lauer⁴, Meiyappan Solaiyappan², and Dara Kraitchman^1,2
1Center for Image-Guided Animal Therapy, Johns Hopkins University, Baltimore, Maryland, United States, ²Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University, Baltimore, Maryland, United States, ³Mid-Atlantic Veterinary Neurology and Neurosurgery, Catonsville, Maryland, United States, ⁴Department of Otolaryngology and Center for Hearing and Balance, Johns Hopkins University, Baltimore, Maryland, United States

The auditory brainstem response test is the accepted test for detecting hearing loss in dogs. Current standards in veterinary medicine do not require hearing protection for canines undergoing MRI. The purpose of this study was to determine whether routine MRI studies at 1.5T cause hearing loss in client-owned dogs. In addition, the authors assessed whether or not hearing protection can be beneficial.

Jacco A de Zwart¹, Peter van Gelderen¹, Qi Duan¹, Natalia Gudino¹, Cem M Deniz², Leeor Alon², and Jeff H Duyn^1
1Advanced MRI, LFMI, NINDS, National Institutes of Health, Bethesda, MD, United States, ²Dept. of Radiology & Sackler Institute of Graduate Biomedical Sciences, NYU School of Medicine, New York, NY, United States

A non-linear least squares fitting based method for MR thermometry is proposed and developed for coil performance and RF-safety assessment using recently developed sucrose-doped agarose-water phantoms. The method, which acquires a minimum of four gradient-echo signals, allows reliable measurement of RF-heating-related shifts in water proton resonance frequency (PRF) in the presence of confounding proton signals from the doping material. Calibration experiments show water-PRF temperature dependence of -0.009 ppm·K^-1 for these agarose-based phantoms, similar to literature values for tissue (~-0.01 ppm·K^-1). However, otherwise identical gelatin-based gels show reduced water-PRF temperature dependence (-0.006 ppm·K^-1), possibly due to spin exchange.

Youngseob Seo¹ and Min-Jae Kang^1
1Center for Medical Metrology, Korea Research Institute of Standards and Science, Daejeon, Korea

MR manufacturer-reported specific absorption rate (SAR) values on clinical MR imaging systems are uncertain. Radiofrequency heating during MRI scan is an important safety concern. We measured SAR values and temperature elevation at 1.5 and 3 T.

Shivaprasad Ashok Chikop¹, Antharikshanagar Bellappa Sachin Anchan¹, Arush Arun Honnedevasthana¹, Shaikh Imam¹, and Sairam Geethanath^1
1Medical Imaging Research Center, Dayananda Sagar Institutions, Bangalore, Karnataka, India

Spatially localized Tissue Fingerprinting (STiF) provides Non-MR parametric maps (NPM) based on tissue properties like electrical conductivity, thermal conductivity, tissue density, heat capacity, elasticity and heat generation rate. These maps reflect structural, functional and pathological conditions of the tissues and they can be used for SAR calculation in EM modeling and analysis which are based on tissue types. StiF relates MR data with NPM through Magnetic Resonance Fingerprinting so as to map a range of MR parameters to a range of non-MR parameters via tissue characteristics, similar to MRF which maps signal evolution to MR parametric maps.

Zidan Yu¹, Sherman Xuegang Xin^1,2, and Christopher Collins^1
1Bernard and Irene Schwartz Center for Biomedical Imaging, New York University School of Medicine, New York, New York, United States, ²Biomedical Engineering, Southern Medical University, Guangzhou, Guangdong, China

Due to the high likelihood for a patient implanted with a pacemaker to take an MRI exam, and the significant concern about the SAR around the lead tip of an implanted pacemaker, the feasibility of employing high permittivity materials for reducing the SAR is explored here. The results indicated that it is promising to reduce heating around the lead tip with use of dielectric materials in a situation where a patient with an implanted pacemaker undergoes an MR scan of head and a large birdcage coil is used in transmission but a local receive coil is used in reception.

Steffen Sammet^1,2 and Christina Louise Sammet^3,4
1Department of Radiology, University of Chicago Medical Center, Chicago, Illinois, United States, ²Department of Radiology, The Ohio State University, Columbus, Ohio, United States,³Department of Radiology, Ann & Robert H. Lurie Children’s Hospital of Chicago, Chicago, Illinois, United States, ⁴Department of Radiology, Northwestern University, Chicago, Illinois, United States

This educational electronic poster outlines the design of a globally applicable educational MR safety module with a concise multiple-choice exam for instructing medical students about basic MR and patient related safety. The MR safety course can be implemented as a traditional didactic lecture, interactive, or self-administered online. The goal of the course is to make sure that medical students worldwide get a basic understanding of MR principles and safety considerations. It can be used universally by all medical school programs and will help to ensure consistent quality of teaching materials and MR safety standards.

Hongbae Jeong¹, Konstantinos Papoutsis¹, Peter Jezzard¹, and Aaron T. Hess^2
1FMRIB Centre, University of Oxford, Oxford, Oxfordshire, United Kingdom, ²Department of Cardiovascular Medicine, University of Oxford, Oxford, Oxfordshire, United Kingdom

In ultra-high-field magnetic resonance imaging, there are two specific needs for EM simulation: one is the estimation of transmit and receive magnetic field distributions and the other is the estimation of specific absorption rate. In this study, we compare and evaluate two methods of simulating an eight channel parallel transmit transceiver coil via generated B1+ fields and SAR distribution in a 7T cylindrical phantom and human head model. Method 1 is the conventional tuning method whereby one voltage source is placed on each element and manually tuned using explicit capacitor values and Method 2 only uses voltage sources.

Iva Petkovska¹, Bobby Kalb¹, John Hur¹, Peter Ott², Kusum Lata², Parinita Dherange², Isabel Oliva¹, Shannon Urbina¹, Hina Arif¹, Surya Chundru¹, James Costello¹, and Diego Martin^1
1Medical Imaging, University of Arizona, Tucson, AZ, United States, ²Sarver Heart Center, University of Arizona, Tucson, AZ, United States

MRI in patients with cardiac implantable electronic devices could be performed safely on a case-by-case basis and only if the site is staffed with individuals with the appropriate radiology and cardiology knowledge and expertise, including cases involving directed cardiothoracic imaging.

Deborah Anne Langman¹, Eric Aliotta^1,2, Dan Margolis¹, J. Paul Finn^1,2, and Daniel B Ennis^1,2
1Radiological Sciences, UCLA, Los Angeles, CA, United States, ²Biomedical Physics IDP, UCLA, Los Angeles, CA, United States

3T MRI exams for patients with pacemakers and ICDs is contraindicated, largely due to concerns that lead-tip heating is worse at 3T. Our objective was to characterize in vitro pacemaker lead-tip heating during a range of 3T MRI exams compared to 1.5T. 3T lead-tip heating was found to be lower than 1.5T lead-tip heating near the center of the B1-coil and higher near the periphery, whereas for 1.5T the highest heating was near the center of the B1-coil. Our reluctance to scan patients with pacemakers or ICDs at 3T may be misguided under some conditions and deserves greater consideration.

BuSik Park¹, Amir Razjouyan², Leonardo Angelone², and Sunder s Rajan^3
1FDA/CBER, Silver Spring, MD, United States, ²FDA/CDRH/OSEL, MD, United States, ³Div. of Biomedical Physics, FDA/CDRH, Silver Spring, MD, United States

We present numerical simulations at 128 MHz of specific absorption rate (SAR) in a virtual female model with a breast expander implant. The analysis was performed using four different body positions landmarked to isocenter (i.e., shoulder, heart, sternum, and abdomen). The 10g-averaged SAR (SAR10g) with and without the breast expander implant was evaluated at selected coronal planes (i.e., center of the implant and 37.5 mm above). The simulation results showed that the highest SAR10g changes with the breast expander implant were in the saline volume of the implant. The SAR10g within the implant decreased, compared to background whereas it increased in some of the surrounding regions of the implant. However, the maximum SAR10g (SAR10g-peak) in a selected region of interest (ROI) around the implant was still less compared to the peak value over the entire body (i.e., 26.0 W/kg vs. 41.2 W/kg), indicating no significant effect of the implant on the overall ratio SAR10g-peak/whole-body averaged SAR.