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Zanovello U, Fuss C, Arduino A, Bottauscio O. Efficient prediction of MRI gradient-induced heating for guiding safety testing of conductive implants. Magn Reson Med 2023; 90:2011-2018. [PMID: 37382200 DOI: 10.1002/mrm.29787] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/19/2023] [Revised: 05/29/2023] [Accepted: 06/15/2023] [Indexed: 06/30/2023]
Abstract
PURPOSE To propose an efficient numerical method to predict the temperature increase of an implantable medical device induced by any linearly polarized homogeneous magnetic field, according to the ISO 10974 methodology for testing of gradient-induced device heating. THEORY AND METHODS The concepts of device-specific power and temperature tensors are introduced to mathematically describe the electromagnetic and thermal anisotropic behavior of the device, from which the device heating for an arbitrary exposure direction can be predicted. The proposed method is compared to a brute-force approach based on simulations, and validated by applying it to four reference orthopedic implants with a commercial simulation software. RESULTS The proposed method requires about 5% $$ \% $$ of the time required by the brute-force approach, and 30% $$ \% $$ of the memory occupancy. The temperature increase predicted by the proposed method over a range of incident magnetic field exposures deviated from brute-force direct simulations by less than± $$ \pm $$ 0.3% $$ \% $$ . CONCLUSION The proposed method allows efficient prediction of the heating of an implantable medical device induced by any linearly polarized homogeneous magnetic field using a small fraction of the simulations required by the brute-force approach. The results can be used to predict the worst-case orientation of the gradient field, for subsequent experimental characterization according to the ISO 10974 standard.
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Affiliation(s)
- Umberto Zanovello
- Metrologia dei materiali innovativi e scienze della vita, Istituto Nazionale di Ricerca Metrologica, Torino, Italy
| | | | - Alessandro Arduino
- Metrologia dei materiali innovativi e scienze della vita, Istituto Nazionale di Ricerca Metrologica, Torino, Italy
| | - Oriano Bottauscio
- Metrologia dei materiali innovativi e scienze della vita, Istituto Nazionale di Ricerca Metrologica, Torino, Italy
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2
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Guryev GD, Milshteyn E, Giannakopoulos II, Lattanzi R, Wald LL, Adalsteinsson E, White JK. MARIE 2.0: A Perturbation Matrix Based Patient-Specific MRI Field Simulator. IEEE Trans Biomed Eng 2023; 70:1575-1586. [PMID: 36383593 PMCID: PMC10689076 DOI: 10.1109/tbme.2022.3222748] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
High static field MR scanners can produce human tissue images of astounding clarity, but rely on high frequency electromagnetic radiation that generates complicated in-tissue field patterns that are patient-specific and potentially harmful. Many such scanners use parallel transmitters to better control field patterns, but then adjust the transmitters based on general guidelines rather than optimizing for the specific patient, mostly because computing patient-specific fields was presumed far too slow. It was recently demonstrated that the combination of fast low-resolution tissue mapping and fast voxel-based field simulation can be used to perform a patient-specific MR safety check in minutes. However, the field simulation required several of those minutes, making it too slow to perform the dozens of simulations that would be needed for patient-specific optimization. In this paper we describe a compressed-perturbation-matrix technique that nearly eliminates the computational cost of including complex coils (or coils and shields) in voxel-based field simulation of tissue, thereby reducing simulation time from minutes to seconds. The approach is demonstrated on a wide variety of head+coil and head+coil+shield configurations, using the implementation in MARIE 2.0, the latest version of the open-source MR field simulator MARIE.
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Prisciandaro J, Zoberi JE, Cohen G, Kim Y, Johnson P, Paulson E, Song W, Hwang KP, Erickson B, Beriwal S, Kirisits C, Mourtada F. AAPM Task Group Report 303 endorsed by the ABS: MRI Implementation in HDR Brachytherapy-Considerations from Simulation to Treatment. Med Phys 2022; 49:e983-e1023. [PMID: 35662032 DOI: 10.1002/mp.15713] [Citation(s) in RCA: 9] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/03/2021] [Revised: 04/11/2022] [Accepted: 05/05/2022] [Indexed: 11/05/2022] Open
Abstract
The Task Group (TG) on Magnetic Resonance Imaging (MRI) Implementation in High Dose Rate (HDR) Brachytherapy - Considerations from Simulation to Treatment, TG 303, was constituted by the American Association of Physicists in Medicine's (AAPM's) Science Council under the direction of the Therapy Physics Committee, the Brachytherapy Subcommittee, and the Working Group on Brachytherapy Clinical Applications. The TG was charged with developing recommendations for commissioning, clinical implementation, and on-going quality assurance (QA). Additionally, the TG was charged with describing HDR brachytherapy (BT) workflows and evaluating practical consideration that arise when implementing MR imaging. For brevity, the report is focused on the treatment of gynecologic and prostate cancer. The TG report provides an introduction and rationale for MRI implementation in BT, a review of previous publications on topics including available applicators, clinical trials, previously published BT related TG reports, and new image guided recommendations beyond CT based practices. The report describes MRI protocols and methodologies, including recommendations for the clinical implementation and logical considerations for MR imaging for HDR BT. Given the evolution from prescriptive to risk-based QA,1 an example of a risk-based analysis using MRI-based, prostate HDR BT is presented. In summary, the TG report is intended to provide clear and comprehensive guidelines and recommendations for commissioning, clinical implementation, and QA for MRI-based HDR BT that may be utilized by the medical physics community to streamline this process. This report is endorsed by the American Brachytherapy Society (ABS). This article is protected by copyright. All rights reserved.
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Affiliation(s)
| | | | - Gil'ad Cohen
- Memorial Sloan-Kettering Cancer Center, New York, NY
| | | | - Perry Johnson
- University of Florida Health Proton Therapy Institute, Jacksonville, FL
| | | | | | - Ken-Pin Hwang
- The University of Texas MD Anderson Cancer Center, Houston, TX
| | | | - Sushil Beriwal
- Allegheny Health Network Cancer Institute, Pittsburgh, PA
| | | | - Firas Mourtada
- Sidney Kimmel Cancer Center at Thomas Jefferson University Hospital, Philadelphia, Pennsylvania
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4
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Arduino A, Bottauscio O, Chiampi M, Zanovello U, Zilberti L. A contribution to MRI safety testing related to gradient-induced heating of medical devices. Magn Reson Med 2022; 88:930-944. [PMID: 35344605 PMCID: PMC9314691 DOI: 10.1002/mrm.29235] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/12/2021] [Revised: 01/22/2022] [Accepted: 02/24/2022] [Indexed: 12/26/2022]
Abstract
PURPOSE To theoretically investigate the feasibility of a novel procedure for testing the MRI gradient-induced heating of medical devices and translating the results into clinical practice. METHODS The concept of index of stress is introduced by decoupling the time waveform characteristics of the gradient field signals from the field spatial distribution within an MRI scanner. This index is also extended to consider the anisotropy of complex bulky metallic implants. Merits and drawbacks of the proposed index of stress are investigated through virtual experiments. In particular, the values of the index of stress evaluated for realistic orthopedic implants placed within an ASTM phantom are compared with accurate heating simulations performed with 2 anatomic body models (a man and a woman) implanted through a virtual surgery procedure. RESULTS The manipulation of the proposed index of stress allows to identify regions within the MRI bore where the implant could affect the safety of the examinations. Furthermore, the conducted analysis shows that the power dissipated into the implant by the induced eddy currents is a dosimetric quantity that estimates well the maximum temperature increase in the tissues surrounding the implant. CONCLUSION The results support the adoption of an anisotropic index of stress to regulate the gradient-induced heating of geometrically complex implants. They also pave the way for a laboratory characterization of the implants based on electrical measurements, rather than on thermal measurements. The next step will be to set up a standardized experimental procedure to evaluate the index of stress associated with an implant.
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Affiliation(s)
| | | | - Mario Chiampi
- Istituto Nazionale di Ricerca Metrologica (INRIM), Torino, Italy
| | | | - Luca Zilberti
- Istituto Nazionale di Ricerca Metrologica (INRIM), Torino, Italy
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5
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Rubia-Rodríguez I, Zilberti L, Arduino A, Bottauscio O, Chiampi M, Ortega D. In silico assessment of collateral eddy current heating in biocompatible implants subjected to magnetic hyperthermia treatments. Int J Hyperthermia 2021; 38:846-861. [PMID: 34074196 DOI: 10.1080/02656736.2021.1909758] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023] Open
Abstract
Purpose: Bearing partially or fully metallic passive implants represents an exclusion criterion for patients undergoing a magnetic hyperthermia procedure, but there are no specific studies backing this restrictive decision. This work assesses how the secondary magnetic field generated at the surface of two common types of prostheses affects the safety and efficiency of magnetic hyperthermia treatments of localized tumors. The paper also proposes the combination of a multi-criteria decision analysis and a graphical representation of calculated data as an initial screening during the preclinical risk assessment for each patient.Materials and methods: Heating of a hip joint and a dental implant during the treatment of prostate, colorectal and head and neck tumors have been assessed considering different external field conditions and exposure times. The Maxwell equations including the secondary field produced by metallic prostheses have been solved numerically in a discretized computable human model. The heat exchange problem has been solved through a modified version of the Pennes' bioheat equation assuming a temperature dependency of blood perfusion and metabolic heat, i.e. thermorregulation. The degree of risk has been assessed using a risk index with parameters coming from custom graphs plotting the specific absorption rate (SAR) vs temperature increase, and coefficients derived from a multi-criteria decision analysis performed following the MACBETH approach.Results: The comparison of two common biomaterials for passive implants - Ti6Al4V and CoCrMo - shows that both specific absorption rate (SAR) and local temperature increase are found to be higher for the hip prosthesis made by Ti6Al4V despite its lower electrical and thermal conductivity. By tracking the time evolution of temperature upon field application, it has been established that there is a 30 s delay between the time point for which the thermal equilibrium is reached at prostheses and tissues. Likewise, damage may appear in those tissues adjacent to the prostheses at initial stages of treatment, since recommended thermal thresholds are soon surpassed for higher field intensities. However, it has also been found that under some operational conditions the typical safety rule of staying below or attain a maximum temperature increase or SAR value is met.Conclusion: The current exclusion criterion for implant-bearing patients in magnetic hyperthermia should be revised, since it may be too restrictive for a range of the typical field conditions used. Systematic in silico treatment planning using the proposed methodology after a well-focused diagnostic procedure can aid the clinical staff to find the appropriate limits for a safe treatment window.
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Affiliation(s)
| | - Luca Zilberti
- Istituto Nazionale di Ricerca Metrologica (INRIM), Turin, Italy
| | | | | | - Mario Chiampi
- Istituto Nazionale di Ricerca Metrologica (INRIM), Turin, Italy
| | - Daniel Ortega
- IMDEA Nanoscience, Madrid, Spain.,Institute of Research and Innovation in Biomedical Sciences of the Province of Cádiz (INiBICA), University of Cádiz, Cádiz, Spain.,Condensed Matter Physics department, University of Cádiz, Cádiz, Spain
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6
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Arduino A, Zanovello U, Hand J, Zilberti L, Brühl R, Chiampi M, Bottauscio O. Heating of hip joint implants in MRI: The combined effect of RF and switched-gradient fields. Magn Reson Med 2021; 85:3447-3462. [PMID: 33483979 PMCID: PMC7986841 DOI: 10.1002/mrm.28666] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2020] [Revised: 12/09/2020] [Accepted: 12/09/2020] [Indexed: 12/20/2022]
Abstract
PURPOSE To investigate how the simultaneous exposure to gradient and RF fields affects the temperature rise in patients with a metallic hip prosthesis during an MRI session. METHODS In silico analysis was performed with an anatomically realistic human model with CoCrMo hip implant in 12 imaging positions. The analysis was performed at 1.5 T and 3 T, considering four clinical sequences: turbo spin-echo, EPI, gradient-echo, and true fast imaging sequence with steady precession. The exposure to gradient and RF fields was evaluated separately and superposed, by adopting an ad hoc computational algorithm. Temperature increase within the body, rather than specific absorption rate, was used as a safety metric. RESULTS With the exception of gradient-echo, all investigated sequences produced temperature increases higher than 1 K after 360 seconds, at least for one body position. In general, RF-induced heating dominates the turbo spin-echo sequence, whereas gradient-induced heating prevails with EPI; the situation with fast imaging sequence with steady precession is more diversified. The RF effects are enhanced when the implant is within the RF coil, whereas the effects of gradient fields are maximized if the prosthesis is outside the imaging region. Cases for which temperature-increase thresholds were exceeded were identified, together with the corresponding amount of tissue mass involved and the exposure time needed to reach these limits. CONCLUSION The analysis confirms that risky situations may occur when a patient carrying a hip implant undergoes an MRI exam and that, in some cases, the gradient field heating may be significant. In general, exclusion criteria only based on whole-body specific absorption rate may not be sufficient to ensure patients' safety.
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Affiliation(s)
| | | | - Jeff Hand
- School of Biomedical Engineering and Imaging SciencesKing’s College LondonLondonUnited Kingdom
| | - Luca Zilberti
- Istituto Nazionale di Ricerca Metrologica (INRIM)TorinoItaly
| | - Rüdiger Brühl
- Physikalisch‐Technische BundesanstaltBraunschweig and BerlinGermany
| | - Mario Chiampi
- Istituto Nazionale di Ricerca Metrologica (INRIM)TorinoItaly
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Humphreys CA, Smith C, Wardlaw JM. Correlations in post-mortem imaging-histopathology studies of sporadic human cerebral small vessel disease: A systematic review. Neuropathol Appl Neurobiol 2021; 47:910-930. [PMID: 34037264 DOI: 10.1111/nan.12737] [Citation(s) in RCA: 15] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2020] [Revised: 03/29/2021] [Accepted: 05/02/2021] [Indexed: 11/30/2022]
Abstract
AIMS Sporadic human cerebral small vessel disease (SVD) commonly causes stroke and dementia but its pathogenesis is poorly understood. There are recognised neuroimaging and histopathological features. However, relatively few studies have examined the relationship between the radiological and pathological correlates of SVD; better correlation would promote greater insight into the underlying biological changes. METHODS We performed a systematic review to collate and appraise the information derived from studies that correlated histological with neuroimaging-defined SVD lesions. We searched for studies describing post-mortem imaging and histological tissue examination in adults, extracted data from published studies, categorised the information and compiled this narrative. RESULTS We identified 38 relevant studies, including at least 1146 subjects, 342 of these with SVD: 29 studies focussed on neuroradiological white matter lesions (WML), six on microinfarcts and three on dilated perivascular spaces (PVS) and lacunes. The histopathology terminology was diverse with few robust definitions. Reporting and methodology varied widely between studies, precluding formal meta-analysis. PVS and 'oedema' were frequent findings in WML, being described in at least 94 and 18 radiological WML, respectively, in addition to myelin pallor. Histopathological changes extended beyond the radiological lesion margins in at least 33 radiological WML. At least 43 radiological lesions not seen pathologically and at least 178 histological lesions were not identified on imaging. CONCLUSIONS Histopathological assessment of human SVD is hindered by inconsistent methodological approaches and unstandardised definitions. The data from this systematic review will help to develop standardised definitions to promote consistency in human SVD research.
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Affiliation(s)
| | - Colin Smith
- Centre for Clinical Brain Sciences, University of Edinburgh, Edinburgh, UK
| | - Joanna M Wardlaw
- Centre for Clinical Brain Sciences, University of Edinburgh, Edinburgh, UK.,UK Dementia Research Institute at The University of Edinburgh, Edinburgh, UK.,Row Fogo Centre for Research into Ageing and the Brain, Edinburgh, UK
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8
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Delcey M, Bour P, Ozenne V, Ben Hassen W, Quesson B. A fast MR-thermometry method for quantitative assessment of temperature increase near an implanted wire. PLoS One 2021; 16:e0250636. [PMID: 33983935 PMCID: PMC8118538 DOI: 10.1371/journal.pone.0250636] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2020] [Accepted: 04/09/2021] [Indexed: 11/18/2022] Open
Abstract
PURPOSE To propose a MR-thermometry method and associated data processing technique to predict the maximal RF-induced temperature increase near an implanted wire for any other MRI sequence. METHODS A dynamic single shot echo planar imaging sequence was implemented that interleaves acquisition of several slices every second and an energy deposition module with adjustable parameters. Temperature images were processed in real time and compared to invasive fiber-optic measurements to assess accuracy of the method. The standard deviation of temperature was measured in gel and in vivo in the human brain of a volunteer. Temperature increases were measured for different RF exposure levels in a phantom containing an inserted wire and then a MR-conditional pacemaker lead. These calibration data set were fitted to a semi-empirical model allowing estimation of temperature increase of other acquisition sequences. RESULTS The precision of the measurement obtained after filtering with a 1.6x1.6 mm2 in plane resolution was 0.2°C in gel, as well as in the human brain. A high correspondence was observed with invasive temperature measurements during RF-induced heating (0.5°C RMSE for a 11.5°C temperature increase). Temperature rises of 32.4°C and 6.5°C were reached at the tip of a wire and of a pacemaker lead, respectively. After successful fitting of temperature curves of the calibration data set, temperature rise predicted by the model was in good agreement (around 5% difference) with measured temperature by a fiber optic probe, for three other MRI sequences. CONCLUSION This method proposes a rapid and reliable quantification of the temperature rise near an implanted wire. Calibration data set and resulting fitting coefficients can be used to estimate temperature increase for any MRI sequence as function of its power and duration.
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Affiliation(s)
- Marylène Delcey
- IHU Liryc, Electrophysiology and Heart Modeling Institute, Fondation Bordeaux Université, Pessac-Bordeaux, France
- Centre de recherche Cardio-Thoracique de Bordeaux, U1045, Univ. Bordeaux, Bordeaux, France
- INSERM, Centre de recherche Cardio-Thoracique de Bordeaux, U1045, Bordeaux, France
- Siemens Healthcare, Saint-Denis, France
| | - Pierre Bour
- IHU Liryc, Electrophysiology and Heart Modeling Institute, Fondation Bordeaux Université, Pessac-Bordeaux, France
- Centre de recherche Cardio-Thoracique de Bordeaux, U1045, Univ. Bordeaux, Bordeaux, France
- INSERM, Centre de recherche Cardio-Thoracique de Bordeaux, U1045, Bordeaux, France
| | - Valéry Ozenne
- IHU Liryc, Electrophysiology and Heart Modeling Institute, Fondation Bordeaux Université, Pessac-Bordeaux, France
- Centre de recherche Cardio-Thoracique de Bordeaux, U1045, Univ. Bordeaux, Bordeaux, France
- INSERM, Centre de recherche Cardio-Thoracique de Bordeaux, U1045, Bordeaux, France
| | | | - Bruno Quesson
- IHU Liryc, Electrophysiology and Heart Modeling Institute, Fondation Bordeaux Université, Pessac-Bordeaux, France
- Centre de recherche Cardio-Thoracique de Bordeaux, U1045, Univ. Bordeaux, Bordeaux, France
- INSERM, Centre de recherche Cardio-Thoracique de Bordeaux, U1045, Bordeaux, France
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9
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Nguyen BT, Pilitsis J, Golestanirad L. The effect of simulation strategies on prediction of power deposition in the tissue around electronic implants during magnetic resonance imaging. ACTA ACUST UNITED AC 2020; 65:185007. [DOI: 10.1088/1361-6560/abac9f] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
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10
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Zilberti L, Zanovello U, Arduino A, Bottauscio O, Chiampi M. RF-induced heating of metallic implants simulated as PEC: Is there something missing? Magn Reson Med 2020; 85:583-586. [PMID: 32936504 DOI: 10.1002/mrm.28512] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2020] [Revised: 08/17/2020] [Accepted: 08/18/2020] [Indexed: 01/06/2023]
Affiliation(s)
- Luca Zilberti
- Istituto Nazionale di Ricerca Metrologica, Torino, Italy
| | | | | | | | - Mario Chiampi
- Istituto Nazionale di Ricerca Metrologica, Torino, Italy
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11
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A health and safety model for occupational exposure to radiofrequency fields and static magnetic fields from 1.5 and 3 T MRI scanners. HEALTH AND TECHNOLOGY 2020. [DOI: 10.1007/s12553-019-00379-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
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12
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Wang Y, Wang Q, Liu J, Zeng Q, Zheng J, Kainz W, Chen J. On the development of equivalent medium for active implantable device radiofrequency safety assessment. Magn Reson Med 2019; 82:1164-1176. [DOI: 10.1002/mrm.27785] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2018] [Revised: 03/06/2019] [Accepted: 04/04/2019] [Indexed: 12/31/2022]
Affiliation(s)
- Yu Wang
- Department of Electrical and Computer Engineering University of Houston Houston Texas 77204‐4005
| | - Qingyan Wang
- Department of Electrical and Computer Engineering University of Houston Houston Texas 77204‐4005
| | - Jingshen Liu
- Department of Electrical and Computer Engineering University of Houston Houston Texas 77204‐4005
| | - Qi Zeng
- Department of Electrical and Computer Engineering University of Houston Houston Texas 77204‐4005
| | - Jianfeng Zheng
- Department of Electrical and Computer Engineering University of Houston Houston Texas 77204‐4005
| | - Wolfgang Kainz
- Division of Biomedical Physics U.S. FDA Silver Spring Maryland 20993
| | - Ji Chen
- Department of Electrical and Computer Engineering University of Houston Houston Texas 77204‐4005
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13
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Destruel A, Fuentes M, Weber E, O'Brien K, Jin J, Liu F, Barth M, Crozier S. A numerical and experimental study of RF shimming in the presence of hip prostheses using adaptive SAR at 3 T. Magn Reson Med 2019; 81:3826-3839. [PMID: 30803001 DOI: 10.1002/mrm.27688] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2018] [Revised: 01/17/2019] [Accepted: 01/20/2019] [Indexed: 12/25/2022]
Abstract
PURPOSE Parallel transmission techniques in MRI have the potential to improve the image quality near metal implants at 3 T. However, current testing of implants only evaluates the risk of radiofrequency (RF) heating in phantoms in circularly polarized mode. We investigate the influence of changing the transmission settings in a 2-channel body coil on the peak temperature near 2 CoCrMo hip prostheses, using adaptive specific absorption rate (SAR) as an estimate of RF heating. METHODS Adaptive SAR is a SAR averaging method that is optimized to correlate with thermal simulations and limit the temperature to 39°C near hip implants. The simulated peak temperature was compared when using whole-body SAR, SAR10g , and adaptive SAR as a constraint for the maximum allowed input power. Adaptive SAR was used as a fast estimate of temperature to evaluate the trade-off between good image quality and low heating near the hip implants. Electromagnetic simulations were validated by simulating and measuring B1 maps and electric fields in a phantom at 3 T. RESULTS Simulations and measurements showed excellent agreement. Limiting whole-body SAR to 2 W/kg and SAR10g to 10 W/kg resulted in temperatures up to 49.3°C and 40.7°C near the hip implants after 30 minutes of RF exposure, respectively. Predictions based on adaptive SAR limited the temperature to 39°C, and allowed to improve the B1 field distribution while preventing peak temperatures near the hip implants. CONCLUSION Significant RF heating can occur at 3 T near hip implants when parallel transmission is used. Adaptive SAR can be integrated in RF shimming algorithms to improve the uniformity and reduce heating.
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Affiliation(s)
- Aurelien Destruel
- School of Information Technology and Electrical Engineering, University of Queensland, St. Lucia, Australia.,Centre for Advanced Imaging, University of Queensland, St. Lucia, Australia
| | - Miguel Fuentes
- School of Information Technology and Electrical Engineering, University of Queensland, St. Lucia, Australia.,Population Health Research on Electromagnetic Energy, School of Public Health and Preventive Medicine, Monash University, Clayton, Australia
| | - Ewald Weber
- School of Information Technology and Electrical Engineering, University of Queensland, St. Lucia, Australia
| | - Kieran O'Brien
- Centre for Advanced Imaging, University of Queensland, St. Lucia, Australia.,Siemens Healthcare, Brisbane, Australia
| | - Jin Jin
- Siemens Healthineers USA, Los Angeles, California
| | - Feng Liu
- School of Information Technology and Electrical Engineering, University of Queensland, St. Lucia, Australia
| | - Markus Barth
- Centre for Advanced Imaging, University of Queensland, St. Lucia, Australia
| | - Stuart Crozier
- School of Information Technology and Electrical Engineering, University of Queensland, St. Lucia, Australia
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14
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Liorni I, Neufeld E, Kühn S, Murbach M, Zastrow E, Kainz W, Kuster N. Novel mechanistic model and computational approximation for electromagnetic safety evaluations of electrically short implants. Phys Med Biol 2018; 63:225015. [PMID: 30418958 DOI: 10.1088/1361-6560/aae94c] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
This paper addresses unresolved issues related to the safety of persons with conductive medical implants exposed to electromagnetic (EM) fields. When exposed to EM fields compatible with the reference limits-in particular <100 MHz-implants may enhance local fields and energy absorption to values much higher than the basic restrictions that are considered safe. A mechanistic model based on transfer functions has been postulated for elongated active implants at magnetic resonance imaging (MRI) frequencies and used as a basis for standards dealing with MRI implant safety. However, this mechanistic model is inconsistent with the behavior observed for electrically short implants, such as abandoned leads in MRI or active implants under low-frequency exposure conditions (e.g. wireless power transfer). In this paper, a new mechanistic model for electrically short implants is proposed that allows implant safety assessment to be decomposed into separate steps. Per tip-shape, it requires only a single simulation or measurement of the implant exposed under (semi-)homogeneous conditions. To validate the approach, predictions of the mechanistic model were compared to results of numerical simulations for electric- and magnetic-field exposures. The impact of parameters such as tissue properties, length, tip shape, and insulation thickness on safety- and compliance-relevant quantities was studied. Validation involving an anatomically detailed computational human body model with a realistic implant at multiple locations under electric and magnetic exposures resulted in prediction agreement on the order of 7% (maximal deviation <15%). The approach was found to be applicable for electrical lengths up to 20% of the effective wavelength and can be used to derive suitable testing procedures as well as to develop safety guidelines and standards.
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Affiliation(s)
- Ilaria Liorni
- Foundation for Research on Information Technologies in Society (IT'IS), Zeughausstrasse 43, 8004 Zurich, Switzerland. Author to whom any correspondence
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15
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Destruel A, O'Brien K, Jin J, Liu F, Barth M, Crozier S. Adaptive SAR mass-averaging framework to improve predictions of local RF heating near a hip implant for parallel transmit at 7 T. Magn Reson Med 2018; 81:615-627. [PMID: 30058186 DOI: 10.1002/mrm.27379] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/25/2018] [Revised: 04/26/2018] [Accepted: 05/07/2018] [Indexed: 12/26/2022]
Abstract
PURPOSE Magnetic resonance imaging is used increasingly to scan patients with hip prostheses. We evaluated the reliability of 10 g-averaged specific absorption rate (SAR10g ) to predict radiofrequency (RF) heating in tissues surrounding a hip implant at 7 T in an 8-channel pTx hip coil. A new adaptive SAR mass-averaging method is proposed to improve the correlation between the distribution of mass-averaged SAR and that of tissue temperature. METHODS Currently, RF safety standards for implants are based on temperature instead of SAR, as SAR has not been introduced with regard to exposure scenarios with implants. In this manuscript, however, adaptive SAR is proposed for fast and reliable exposure evaluation with implants, after its correlation with tissue temperature is verified. A framework to calculate adaptive SAR mass-averaging was introduced, which uses a different averaging mass in tissues surrounding the implants and was designed to prevent the temperature from exceeding 39ºC. Predictions from SAR10g and adaptive SAR were compared with thermal simulations. RESULTS The SAR10g method failed to predict both the location and amplitude of heating in tissue near the metal implants. In some cases, the temperature far exceeded 39ºC even when SAR10g was only 70% of the maximum allowed 10 W/kg. The distributions of adaptive SAR and temperature matched in most of the configurations, and the temperature remained below 39ºC when adaptive SAR was constrained. CONCLUSION Adaptive SAR can accurately monitor RF heating and could be used for parallel transmit at 7 T to supplement current standards.
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Affiliation(s)
- Aurelien Destruel
- School of Information Technology and Electrical Engineering, University of Queensland, Australia.,Centre for Advanced Imaging, University of Queensland, Australia
| | - Kieran O'Brien
- Centre for Advanced Imaging, University of Queensland, Australia.,Siemens Healthcare Pty Ltd, Brisbane, Australia
| | - Jin Jin
- School of Information Technology and Electrical Engineering, University of Queensland, Australia.,Siemens Medical Solutions USA, Malvern, Pennsylvania.,Institute for Neuroimaging and Informatics, University of Southern California, Los Angeles, California
| | - Feng Liu
- School of Information Technology and Electrical Engineering, University of Queensland, Australia
| | - Markus Barth
- Centre for Advanced Imaging, University of Queensland, Australia
| | - Stuart Crozier
- School of Information Technology and Electrical Engineering, University of Queensland, Australia
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Erhardt JB, Fuhrer E, Gruschke OG, Leupold J, Wapler MC, Hennig J, Stieglitz T, Korvink JG. Should patients with brain implants undergo MRI? J Neural Eng 2018. [DOI: 10.1088/1741-2552/aab4e4] [Citation(s) in RCA: 59] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
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17
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Silemek B, Acikel V, Oto C, Alipour A, Aykut ZG, Algin O, Atalar E. A temperature sensor implant for active implantable medical devices for in vivo subacute heating tests under MRI. Magn Reson Med 2017; 79:2824-2832. [DOI: 10.1002/mrm.26914] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2017] [Revised: 08/17/2017] [Accepted: 08/17/2017] [Indexed: 11/09/2022]
Affiliation(s)
- Berk Silemek
- National Magnetic Resonance Research Center (UMRAM)Bilkent UniversityAnkara Turkey
| | - Volkan Acikel
- Aselsan, REHIS Power Amplifier TechnologiesAnkara Turkey
| | - Cagdas Oto
- Department of AnatomyAnkara UniversityAnkara Turkey
| | - Akbar Alipour
- National Magnetic Resonance Research Center (UMRAM)Bilkent UniversityAnkara Turkey
- Department of Electrical and Electronics EngineeringBilkent UniversityAnkara Turkey
| | - Zaliha Gamze Aykut
- Department of Molecular Biology and GeneticsBilkent UniversityAnkara Turkey
| | - Oktay Algin
- National Magnetic Resonance Research Center (UMRAM)Bilkent UniversityAnkara Turkey
- Department of RadiologyAtaturk Training and Research HospitalAnkara Turkey
| | - Ergin Atalar
- National Magnetic Resonance Research Center (UMRAM)Bilkent UniversityAnkara Turkey
- Department of Electrical and Electronics EngineeringBilkent UniversityAnkara Turkey
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