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Alon L, Deniz CM, Carluccio G, Brown R, Sodickson DK, Collins CM. Effects of Anatomical Differences on Electromagnetic Fields, SAR, and Temperature Change. CONCEPTS IN MAGNETIC RESONANCE. PART B, MAGNETIC RESONANCE ENGINEERING 2016; 46:8-18. [PMID: 27134586 PMCID: PMC4847547 DOI: 10.1002/cmr.b.21317] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/29/2023]
Abstract
Electromagnetic field simulations are increasingly used to assure RF safety of patients during MRI exams. In practice, however, tissue property distribution of the patient being imaged is not known, but may be represented with a pre-existing model. Repeatedly, agreement in transmit magnetic (B1+) field distributions between two geometries has been used to suggest agreement in heating distributions. Here we examine relative effects of anatomical differences on B1+ distribution, Specific Absorption Rate (SAR) and temperature change (ΔT). Numerical simulations were performed for a single surface coil positioned adjacent a homogeneous phantom and bovine phantom, each with slight geometric variations, and adjacent two different human body models. Experimental demonstration was performed on a bovine phantom using MR thermometry and B1+ mapping. Simulations and experiments demonstrate that B1+ distributions in different samples can be well correlated, while notable difference in maximum SAR and ΔT occur. This work illustrates challenges associated with utilizing simulations or experiments for RF safety assurance purposes. Reliance on B1+ distributions alone for validation of simulations and/or experiments with a sample or subject for assurance of safety in another should be performed with caution.
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Affiliation(s)
- Leeor Alon
- Department of Radiology, New York University School of Medicine, New York, NY, USA, 10016
- Center for Advanced Imaging Innovation and Research (CAIR), New York University School of Medicine, New York, NY, USA, 10016
- NYU WIRELESS, NYU-Poly Brooklyn Campus, Brooklyn, NY, 11201
| | - Cem Murat Deniz
- Department of Radiology, New York University School of Medicine, New York, NY, USA, 10016
- Center for Advanced Imaging Innovation and Research (CAIR), New York University School of Medicine, New York, NY, USA, 10016
- NYU WIRELESS, NYU-Poly Brooklyn Campus, Brooklyn, NY, 11201
| | - Giuseppe Carluccio
- Department of Radiology, New York University School of Medicine, New York, NY, USA, 10016
- Center for Advanced Imaging Innovation and Research (CAIR), New York University School of Medicine, New York, NY, USA, 10016
| | - Ryan Brown
- Department of Radiology, New York University School of Medicine, New York, NY, USA, 10016
- Center for Advanced Imaging Innovation and Research (CAIR), New York University School of Medicine, New York, NY, USA, 10016
| | - Daniel K. Sodickson
- Department of Radiology, New York University School of Medicine, New York, NY, USA, 10016
- Center for Advanced Imaging Innovation and Research (CAIR), New York University School of Medicine, New York, NY, USA, 10016
- NYU WIRELESS, NYU-Poly Brooklyn Campus, Brooklyn, NY, 11201
| | - Christopher M. Collins
- Department of Radiology, New York University School of Medicine, New York, NY, USA, 10016
- Center for Advanced Imaging Innovation and Research (CAIR), New York University School of Medicine, New York, NY, USA, 10016
- NYU WIRELESS, NYU-Poly Brooklyn Campus, Brooklyn, NY, 11201
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52
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Winter L, Oberacker E, Paul K, Ji Y, Oezerdem C, Ghadjar P, Thieme A, Budach V, Wust P, Niendorf T. Magnetic resonance thermometry: Methodology, pitfalls and practical solutions. Int J Hyperthermia 2015; 32:63-75. [DOI: 10.3109/02656736.2015.1108462] [Citation(s) in RCA: 132] [Impact Index Per Article: 14.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
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Murbach M, Neufeld E, Cabot E, Zastrow E, Córcoles J, Kainz W, Kuster N. Virtual population-based assessment of the impact of 3 Tesla radiofrequency shimming and thermoregulation on safety and B1 + uniformity. Magn Reson Med 2015; 76:986-97. [PMID: 26400841 DOI: 10.1002/mrm.25986] [Citation(s) in RCA: 36] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/07/2015] [Revised: 07/20/2015] [Accepted: 08/21/2015] [Indexed: 01/22/2023]
Abstract
PURPOSE To assess the effect of radiofrequency (RF) shimming of a 3 Tesla (T) two-port body coil on B1 + uniformity, the local specific absorption rate (SAR), and the local temperature increase as a function of the thermoregulatory response. METHODS RF shimming alters induced current distribution, which may result in large changes in the level and location of absorbed RF energy. We investigated this effect with six anatomical human models from the Virtual Population in 10 imaging landmarks and four RF coils. Three thermoregulation models were applied to estimate potential local temperature increases, including a newly proposed model for impaired thermoregulation. RESULTS Two-port RF shimming, compared to circular polarization mode, can increase the B1 + uniformity on average by +32%. Worst-case SAR excitations increase the local RF power deposition on average by +39%. In the first level controlled operating mode, induced peak temperatures reach 42.5°C and 45.6°C in patients with normal and impaired thermoregulation, respectively. CONCLUSION Image quality with 3T body coils can be significantly increased by RF shimming. Exposure in realistic scan scenarios within guideline limits can be considered safe for a broad patient population with normal thermoregulation. Patients with impaired thermoregulation should not be scanned outside of the normal operating mode. Magn Reson Med 76:986-997, 2016. © 2015 Wiley Periodicals, Inc.
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Affiliation(s)
| | | | | | - Earl Zastrow
- IT'IS Foundation, Zurich, Switzerland.,Swiss Federal Institute of Technology (ETH), Zurich, Switzerland
| | - Juan Córcoles
- Department of Electronic and Communication Technology, Universidad Autónoma de Madrid (UAM), Escuela Politécnica Superior, Madrid, Spain
| | - Wolfgang Kainz
- US Food and Drug Administration (FDA), Center for Devices and Radiological Health (CDRH), Silver Spring, Maryland, USA
| | - Niels Kuster
- IT'IS Foundation, Zurich, Switzerland.,Swiss Federal Institute of Technology (ETH), Zurich, Switzerland
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55
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Kyriakou A, Neufeld E, Werner B, Székely G, Kuster N. Full-wave acoustic and thermal modeling of transcranial ultrasound propagation and investigation of skull-induced aberration correction techniques: a feasibility study. J Ther Ultrasound 2015; 3:11. [PMID: 26236478 PMCID: PMC4521448 DOI: 10.1186/s40349-015-0032-9] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/29/2014] [Accepted: 07/05/2015] [Indexed: 01/09/2023] Open
Abstract
Background Transcranial focused ultrasound (tcFUS) is an attractive noninvasive modality for neurosurgical interventions. The presence of the skull, however, compromises the efficiency of tcFUS therapy, as its heterogeneous nature and acoustic characteristics induce significant distortion of the acoustic energy deposition, focal shifts, and thermal gain decrease. Phased-array transducers allow for partial compensation of skull-induced aberrations by application of precalculated phase and amplitude corrections. Methods An integrated numerical framework allowing for 3D full-wave, nonlinear acoustic and thermal simulations has been developed and applied to tcFUS. Simulations were performed to investigate the impact of skull aberrations, the possibility of extending the treatment envelope, and adverse secondary effects. The simulated setup comprised an idealized model of the ExAblate Neuro and a detailed MR-based anatomical head model. Four different approaches were employed to calculate aberration corrections (analytical calculation of the aberration corrections disregarding tissue heterogeneities; a semi-analytical ray-tracing approach compensating for the presence of the skull; two simulation-based time-reversal approaches with and without pressure amplitude corrections which account for the entire anatomy). These impact of these approaches on the pressure and temperature distributions were evaluated for 22 brain-targets Results While (semi-)analytical approaches failed to induced high pressure or ablative temperatures in any but the targets in the close vicinity of the geometric focus, simulation-based approaches indicate the possibility of considerably extending the treatment envelope (including targets below the transducer level and locations several centimeters off the geometric focus), generation of sharper foci, and increased targeting accuracy. While the prediction of achievable aberration correction appears to be unaffected by the detailed bone-structure, proper consideration of inhomogeneity is required to predict the pressure distribution for given steering parameters Conclusions Simulation-based approaches to calculate aberration corrections may aid in the extension of the tcFUS treatment envelope as well as predict and avoid secondary effects (standing waves, skull heating). Due to their superior performance, simulationbased techniques may prove invaluable in the amelioration of skull-induced aberration effects in tcFUS therapy. The next steps are to investigate shear-wave-induced effects in order to reliably exclude secondary hot-spots, and to develop comprehensive uncertainty assessment and validation procedures.
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Affiliation(s)
- Adamos Kyriakou
- Foundation for Research on Information Technologies in Society (IT'IS), Zeughausstrasse 43, Zürich, 8004 Switzerland ; Swiss Federal Institute of Technology (ETH) Zürich, Rämistrasse 101, Zürich, 8092 Switzerland
| | - Esra Neufeld
- Foundation for Research on Information Technologies in Society (IT'IS), Zeughausstrasse 43, Zürich, 8004 Switzerland
| | - Beat Werner
- Center for MR-Research, University Children's Hospital, Steinwiesstrasse 75, Zürich, 8032 Switzerland
| | - Gábor Székely
- Swiss Federal Institute of Technology (ETH) Zürich, Rämistrasse 101, Zürich, 8092 Switzerland
| | - Niels Kuster
- Foundation for Research on Information Technologies in Society (IT'IS), Zeughausstrasse 43, Zürich, 8004 Switzerland ; Swiss Federal Institute of Technology (ETH) Zürich, Rämistrasse 101, Zürich, 8092 Switzerland
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56
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Bottauscio O, Cassarà AM, Hand JW, Giordano D, Zilberti L, Borsero M, Chiampi M, Weidemann G. Assessment of computational tools for MRI RF dosimetry by comparison with measurements on a laboratory phantom. Phys Med Biol 2015; 60:5655-80. [DOI: 10.1088/0031-9155/60/14/5655] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Carluccio G, Bruno M, Collins CM. Predicting long-term temperature increase for time-dependent SAR levels with a single short-term temperature response. Magn Reson Med 2015; 75:2195-203. [PMID: 26096947 DOI: 10.1002/mrm.25805] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2015] [Revised: 05/20/2015] [Accepted: 05/21/2015] [Indexed: 11/10/2022]
Abstract
PURPOSE Present a novel method for rapid prediction of temperature in vivo for a series of pulse sequences with differing levels and distributions of specific energy absorption rate (SAR). THEORY AND METHODS After the temperature response to a brief period of heating is characterized, a rapid estimate of temperature during a series of periods at different heating levels is made using a linear heat equation and impulse-response (IR) concepts. Here the initial characterization and long-term prediction for a complete spine exam are made with the Pennes' bioheat equation where, at first, core body temperature is allowed to increase and local perfusion is not. Then corrections through time allowing variation in local perfusion are introduced. RESULTS The fast IR-based method predicted maximum temperature increase within 1% of that with a full finite difference simulation, but required less than 3.5% of the computation time. Even higher accelerations are possible depending on the time step size chosen, with loss in temporal resolution. Correction for temperature-dependent perfusion requires negligible additional time and can be adjusted to be more or less conservative than the corresponding finite difference simulation. CONCLUSION With appropriate methods, it is possible to rapidly predict temperature increase throughout the body for actual MR examinations.
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Affiliation(s)
| | - Mary Bruno
- New York University School of Medicine, New York, New York, USA
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58
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Simonis FFJ, Petersen ET, Lagendijk JJW, van den Berg CAT. Feasibility of measuring thermoregulation during RF heating of the human calf muscle using MR based methods. Magn Reson Med 2015; 75:1743-51. [PMID: 25977138 DOI: 10.1002/mrm.25710] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2014] [Revised: 02/04/2015] [Accepted: 03/09/2015] [Indexed: 11/06/2022]
Abstract
PURPOSE One of the main safety concerns in MR is heating of the subject due to radiofrequency (RF) exposure. Recently was shown that local peak temperatures can reach dangerous values and the most prominent parameter for accurate temperature estimations is thermoregulation. Therefore, the goal of this research is testing the feasibility of measuring thermoregulation in vivo using MR methods. THEORY AND METHODS The calves of 13 volunteers were scanned at 3 tesla. A Proton Resonance Frequency Shift method was used for temperature measurement. Arterial Spin Labeling and phase contrast scans were used for perfusion and flow measurements respectively. The calves were monitored during extreme RF exposure (20 W/kg, 16 min) and after physical exercise. RESULTS Temperature increases due to RF absorption (range of the 90th percentile of all volunteers: 1.1-2.5°C) matched with the reference skin temperature changes. Increases in perfusion and flow were defined on the whole leg and normalized to baseline. Perfusion showed a significant increase due to RF heating (ratio compared with baseline: 1.28 ± 0.37; P < 0.05), the influence of exercise was much greater, however (2.97 ± 2.45, P < 0.01). CONCLUSION This study represents a first exploration of measuring thermoregulation, which will become essential when new safety guidelines are based on thermal dose.
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Affiliation(s)
- Frank F J Simonis
- Department of Radiotherapy, Imaging Division, University Medical Center Utrecht, Utrecht, the Netherlands
| | - Esben T Petersen
- Department of Radiotherapy, Imaging Division, University Medical Center Utrecht, Utrecht, the Netherlands
| | - Jan J W Lagendijk
- Department of Radiotherapy, Imaging Division, University Medical Center Utrecht, Utrecht, the Netherlands
| | - Cornelis A T van den Berg
- Department of Radiotherapy, Imaging Division, University Medical Center Utrecht, Utrecht, the Netherlands
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Neufeld E, Fuetterer M, Murbach M, Kuster N. Rapid method for thermal dose-based safety supervision during MR scans. Bioelectromagnetics 2015; 36:398-407. [DOI: 10.1002/bem.21919] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/22/2014] [Accepted: 03/28/2015] [Indexed: 11/08/2022]
Affiliation(s)
| | | | - Manuel Murbach
- IT'IS Foundation; Zurich; Switzerland
- Swiss Federal Institute of Technology Zurich (ETHZ); Zurich; Switzerland
| | - Niels Kuster
- IT'IS Foundation; Zurich; Switzerland
- Swiss Federal Institute of Technology Zurich (ETHZ); Zurich; Switzerland
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60
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Gholipour A, Estroff JA, Barnewolt CE, Robertson RL, Grant PE, Gagoski B, Warfield SK, Afacan O, Connolly SA, Neil JJ, Wolfberg A, Mulkern RV. Fetal MRI: A Technical Update with Educational Aspirations. CONCEPTS IN MAGNETIC RESONANCE. PART A, BRIDGING EDUCATION AND RESEARCH 2014; 43:237-266. [PMID: 26225129 PMCID: PMC4515352 DOI: 10.1002/cmr.a.21321] [Citation(s) in RCA: 47] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/08/2023]
Abstract
Fetal magnetic resonance imaging (MRI) examinations have become well-established procedures at many institutions and can serve as useful adjuncts to ultrasound (US) exams when diagnostic doubts remain after US. Due to fetal motion, however, fetal MRI exams are challenging and require the MR scanner to be used in a somewhat different mode than that employed for more routine clinical studies. Herein we review the techniques most commonly used, and those that are available, for fetal MRI with an emphasis on the physics of the techniques and how to deploy them to improve success rates for fetal MRI exams. By far the most common technique employed is single-shot T2-weighted imaging due to its excellent tissue contrast and relative immunity to fetal motion. Despite the significant challenges involved, however, many of the other techniques commonly employed in conventional neuro- and body MRI such as T1 and T2*-weighted imaging, diffusion and perfusion weighted imaging, as well as spectroscopic methods remain of interest for fetal MR applications. An effort to understand the strengths and limitations of these basic methods within the context of fetal MRI is made in order to optimize their use and facilitate implementation of technical improvements for the further development of fetal MR imaging, both in acquisition and post-processing strategies.
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Affiliation(s)
- Ali Gholipour
- Department of Radiology, Boston Children's Hospital, Boston, Massachusetts, USA
| | - Judith A Estroff
- Department of Radiology, Boston Children's Hospital, Boston, Massachusetts, USA
| | - Carol E Barnewolt
- Department of Radiology, Boston Children's Hospital, Boston, Massachusetts, USA
| | - Richard L Robertson
- Department of Radiology, Boston Children's Hospital, Boston, Massachusetts, USA
| | - P Ellen Grant
- Department of Radiology, Boston Children's Hospital, Boston, Massachusetts, USA
| | - Borjan Gagoski
- Department of Radiology, Boston Children's Hospital, Boston, Massachusetts, USA
| | - Simon K Warfield
- Department of Radiology, Boston Children's Hospital, Boston, Massachusetts, USA
| | - Onur Afacan
- Department of Radiology, Boston Children's Hospital, Boston, Massachusetts, USA
| | - Susan A Connolly
- Department of Radiology, Boston Children's Hospital, Boston, Massachusetts, USA
| | - Jeffrey J Neil
- Department of Radiology, Boston Children's Hospital, Boston, Massachusetts, USA
| | - Adam Wolfberg
- Boston Maternal Fetal Medicine, Boston, Massachusetts, USA
| | - Robert V Mulkern
- Department of Radiology, Boston Children's Hospital, Boston, Massachusetts, USA
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Winter L, Oberacker E, Özerdem C, Ji Y, von Knobelsdorff-Brenkenhoff F, Weidemann G, Ittermann B, Seifert F, Niendorf T. On the RF heating of coronary stents at 7.0 Tesla MRI. Magn Reson Med 2014; 74:999-1010. [PMID: 25293952 DOI: 10.1002/mrm.25483] [Citation(s) in RCA: 48] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2014] [Revised: 09/11/2014] [Accepted: 09/11/2014] [Indexed: 12/16/2022]
Abstract
PURPOSE Examine radiofrequency (RF) induced heating of coronary stents at 7.0 Tesla (T) to derive an analytical approach which supports RF heating assessment of arbitrary stent geometries and RF coils. METHODS Simulations are performed to detail electromagnetic fields (EMF), local specific absorption rates (SAR) and temperature changes. For validation E-field measurements and RF heating experiments are conducted. To progress to clinical setups RF coils tailored for cardiac MRI at 7.0T and coronary stents are incorporated into EMF simulations using a human voxel model. RESULTS Our simulations of coronary stents at 297 MHz were confirmed by E-field and temperature measurements. An analytical solution which describes SAR(1g tissue voxel) induced by an arbitrary coronary stent interfering with E-fields generated by an arbitrary RF coil was derived. The analytical approach yielded a conservative estimation of induced SAR(1g tissue voxel) maxima without the need for integrating the stent into EMF simulations of the human voxel model. CONCLUSION The proposed analytical approach can be applied for any patient, coronary stent type, RF coil configuration and RF transmission regime. The generalized approach is of value for RF heating assessment of other passive electrically conductive implants and provides a novel design criterion for RF coils.
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Affiliation(s)
- Lukas Winter
- Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrueck Center for Molecular Medicine, Berlin, Germany
| | - Eva Oberacker
- Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrueck Center for Molecular Medicine, Berlin, Germany
| | - Celal Özerdem
- Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrueck Center for Molecular Medicine, Berlin, Germany
| | - Yiyi Ji
- Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrueck Center for Molecular Medicine, Berlin, Germany
| | - Florian von Knobelsdorff-Brenkenhoff
- Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrueck Center for Molecular Medicine, Berlin, Germany.,Experimental and Clinical Research Center (ECRC), a joint cooperation between the Charité Medical Faculty and the Max-Delbrueck Center for Molecular Medicine, Berlin, Germany
| | - Gerd Weidemann
- Physikalisch Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
| | - Bernd Ittermann
- Physikalisch Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
| | - Frank Seifert
- Physikalisch Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
| | - Thoralf Niendorf
- Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrueck Center for Molecular Medicine, Berlin, Germany.,Experimental and Clinical Research Center (ECRC), a joint cooperation between the Charité Medical Faculty and the Max-Delbrueck Center for Molecular Medicine, Berlin, Germany
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62
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Griffin GH, Anderson KJ, Celik H, Wright GA. Safely assessing radiofrequency heating potential of conductive devices using image-based current measurements. Magn Reson Med 2014; 73:427-41. [DOI: 10.1002/mrm.25103] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/10/2013] [Revised: 12/09/2013] [Accepted: 12/09/2013] [Indexed: 11/08/2022]
Affiliation(s)
- Gregory H. Griffin
- Department of Medical Biophysics; Faculty of Medicine, University of Toronto; Toronto Canada
- Imaging Research; Sunnybrook Research Institute; Toronto Canada
| | | | - Haydar Celik
- Imaging Research; Sunnybrook Research Institute; Toronto Canada
| | - Graham A. Wright
- Department of Medical Biophysics; Faculty of Medicine, University of Toronto; Toronto Canada
- Imaging Research; Sunnybrook Research Institute; Toronto Canada
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