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Development of mri rf exposure risk probability for patients/workers based on local temperature safety considerations.

The rf exposure in and around mri will be thoroughly assessed with respect to local sar and temperature rises. All results will be validated in experimental phantoms and in-vivo.

PROJECT BACKGROUND: Magnetic resonance imaging (MRI) has established itself as a routine technique in medical diagnostics. Its advantages over conventional computer tomography are improved image contrast, particularly for soft tissues, and reduced health risk for patients and personnel due to the absence of ionising radiation. Nevertheless, measures need be taken in order to prevent adverse effects from the static and gradient magnetic fields. Furthermore, the strength of the magnetic RF field must be carefully controlled in order to limit heating of the patient's body due to absorption of RF power. The current edition of the MRI safety standard IEC 60601-2-33 defines exposure limits for the absorbed power averaged over the exposed body region of the patient or the worker (whole body or partial body SAR) or averaged over any 10g of body tissue (local SAR). Modern and upcoming MRI scanners (1.5T - 7T) use RF (Radio Frequency) fields in the VHF (Very High Frequency) and UHF (Ultra High Frequency) range. At these frequencies, the field distribution inside the human body is highly complex. It strongly depends on the tissue distribution and on the shape and posture of the body. Whereas the MR scanner uses the patient weight to estimate the whole body or partial body SAR and to display it to the operator, no methods exist to correlate this value with the local SAR. For workers, exposure is estimated as a function of the patient SAR and the magnetic RF field outside the bore. Numerical simulations of the SAR distribution during an MRI scan which have recently been conducted by different research labs and MRI manufacturers have repeatedly shown hot spots inside the patient's body which significantly exceed the exposure limits of the local SAR during normal operation mode of the MRI scanner. In spite of the long history of the safe use of MRI scanners in routine examinations, it remains open whether the induced temperatures are high enough to cause temporary or permanent tissue damage. The reduction of the RF power seems an apparent remedial measure to prevent the violation of the exposure limits. This, however, would lead to significant deterioration of the imaging quality and would therefore impair the applicability of MRI as a diagnostic tool. The objective of this proposal is to provide the information necessary for maximum benefits to the patient from MRI technologies with minimal risk for the patient while workers are posed to minimal or no risk. OBJECTIVES: The goals of the project are - to develop the methodology for the numerical and experimental assessment of the locally induced SAR and tissue heating during MR examinations; - to determine the probability of tissue damage due to induced heat as a function of the scanning scenario; - to derive risk/benefit curves as well as general exposure limits for future editions of IEC 60601-2-33 which do not restrain imaging quality while providing maximum safety of patients and workers. RATIONALE: Different MR scanner and RF coil types for B0 fields from 1.5T to 7T will be identified, including typical patient scanning positions for various kinds of examinations and typical positions of workers during the scans. This evaluation will be used to define an initial matrix of exposure scenarios for SAR and temperature assessment during the further course of the project. These will be evaluated in a combined numerical and experimental approach [Kuster et al., 2006]. For the numerical simulations of the SAR and the temperature, different anatomical models of adults and children will be used. Several high-resolution models of different age groups, body heights and sexes are available to the project team. These models can be used for combined EM and thermal simulations and allow numerical resolutions in the sub-millimetre range. They distinguish more than 50 different tissue types. Since the electrical load of the RF coil and the field distribution inside the patient significantly depend upon the body mass and on the dielectric tissue properties, the additional development of a strongly obese adult model is intended. The dielectric parameters of fat tissue are mainly characterised by low conductivity and permittivity. Therefore, the impact of an obese body on the loading and the field distribution can be expected to be different from that of an average patient. For all simulations, the Finite-Difference Time-Domain method will be used. Experimental tools allowing accurate measurements of the incident magnetic and electric RF fields inside and in the environment of MRI scanners of up to 7T as well as SAR and temperature measurements in experimental phantoms will be developed. Typical incident fields and their distributions will be evaluated for a number of MRI scanners. The SAR distribution in the anatomical models will be characterised for typical body positions during MR scans using different coil types (birdcage, TEM, open bore, partial body coils, surface coils, etc.). The conditions (tissue distribution, incident field properties) leading to hot spots inside the body will be identified. A correlation between the maximum local SAR, the whole body SAR and the incident B1-field will be established for the identified scanning scenarios. The induced tissue heating will be simulated for the most relevant scenarios with high local SAR. Advanced thermal modelling considering anisotropic blood perfusion, temperature exchange by larger blood vessels, thermal regulation processes, etc. [Neufeld et al., 2006] will be applied to evaluate the maximum heating in different types of organs and tissues. In order to estimate the risk of tissue damage, the CEM43-CT90 concept will be applied. CEM43-CT90 describes the correlation between tissue damage and exposure temperature and time. Parameters for the relevant tissue types will be compiled from a literature study. Based on the acquired knowledge, we will evaluate various scan conditions which allow a maximum RF sequence while preventing tissue damage. Since simulations will be used to quantify the induced exposure conditions, diligent experimental validation of the numerical models and techniques is of paramount importance. The equipment developed for SAR and temperature measurements together with the appropriate experimental phantoms will first be used to validate the numerical predictions in well-controlled phantom configurations. Next, the numerical predictions for local hotspots and temperature increases will be tested in pig cadavers, i.e. more complex anatomy as phantoms. Finally, in a separate series of experiments with anaesthetised pigs, the predicted CEM43-CT90 will be validated for in-vivo conditions. The final validation for the predicted CEM43-CT90 tissue damage levels will be conducted in pigs in-vivo. We will also clarify the feasibility of a possible follow-up human volunteer study on the maximum temperature increases in hotspots using functional MRI or other non-invasive measurements that might be conducted in a follow-up study if needed. A comprehensive uncertainty and variability analysis following the approach described in [Kuster et al., 2006] will allow assessment of the induced tissue heating for a sufficiently large amount of the population. Based on this analysis, exposure limits for patients and workers for the upcoming editions of IEC 60601-2-33 will be derived and presented to the respective standardisation working groups. All results will be published at conferences and in scientific literature, and will be provided to the ICNIRP and the WHO (World Health Organisation). PROJECT WORK PLAN: Partner: IT’IS Foundation Country: CH Ref. Nr.: 1 Partner: Erasmus Medical Center Country: NL Ref. Nr.: 2 Partner: INTEC Country: B Ref. Nr.: 3 Partner: Aristotle University Thessaloniki (RCL) Country: GR Ref. Nr.: 4 Partner: SPEAG Country: CH Ref. Nr.: 5 Partner: Philips Medical Systems Country: NL Ref. Nr.: 6 Partner: Siemens Medical Solutions Country: D Ref. Nr.: 7 Partner: FDA - CDRH Country: USA Ref. Nr.: 8 Partner: Biomedizinische Technik (ETHZ/UNIZH) Country: CH Ref. Nr.: 9 Partner: Charite Berlin Country: D Ref. Nr.: 10 Partner: Zurich MedTech Country: CH Ref. Nr.: 11 WORK PACKAGE LIST: Nr.: WP1 Title: Classification of MRI FGUs Partners Responsible: 9, 6, 7 Pers. Months: 3 Start Month: 1 End Month: 2 Nr.: WP2 Title: Anatomical Models Partners Responsible: 8, 1, 2, 10 Pers. Months: 15 Start Month: 3 End Month: 12 Nr.: WP3 Title: Development of Experimental & TCAD Tools Partners Responsible: 5, 1, 8, 9 Pers. Months: 24 Start Month: 1 End Month: 9 Nr.: WP4 Title: Characterisation of Exposure Partners Responsible: 1, 9, 6, 7 Pers. Months: 3 Start Month: 10 End Month: 11 Nr.: WP5 Title: Numerical SAR Evaluation Partners Responsible: 3, 1, 5 Pers. Months: 30 Start Month: 3 End Month: 18 Nr.: WP6 Title: Thermal Simulations Partners Responsible: 4, 2, 1, 10 Pers. Months: 24 Start Month: 3 End Month: 18 Nr.: WP7 Title: CEM43-CT90 Partners Responsible: 2, 8, 10 Pers. Months: 6 Start Month: 3 End Month: 22 Nr.: WP8 Title: Experimental Validation Partners Responsible: 1, 9 Pers. Months: 3 Start Month: 10 End Month: 12 Nr.: WP9 Title: Experimental Validation In-Vivo Partners Responsible: 10, 7, 1, 2 Pers. Months: *) Start Month: 3 End Month: 24 Nr.: WP10 Title: Uncertainty Evaluation Partners Responsible: 1, 3, 4, 2, 10 Pers. Months: 6 Start Month: 12 End Month: 22 Nr.: WP11 Title: Consolidation and Dissemination Partners Responsible: all Pers. Months: 10 Start Month: 12 End Month: 24+ Nr.: WP12 Title: Development of RF Birdcage Test Environment Partners Responsible: 1, 11 Pers. Months: 18 Start Month: 3 End Month: 17 Total Pers. Months: 124 *) handled by contract between Partners 7 and 10 DESCRIPTION OF THE WORK PACKAGES: * WP1 - Classification of MRI Field Generating Units: Background information: Partners 9, 6, 7, 1, 4, 5, 8 and 10 have long-standing experience in the design of MR tomographs. Objectives: - Determination of the most important RF coil designs (Birdcage, TEM, etc.) and frequencies. - Assessment of typical exposure positions during MR examinations (children and adults). Milestone: classification of exposure scenarios. * WP2 - Completion of Anatomical Models: Background information: - Partners 1, 4, 5 and 8 have developed new segmentation techniques for achieving full 3D EM & thermal human models. - Models of various adults (Visible Human, Japanese man/woman, virtual family man/woman) and of children (virtual family: 2 children, ZonMw: infants and adolescents) are already available or under development within the framework of different research projects. - The development of a model of an obese adult is proposed in this project. - An advanced poser software allowing changes of the body posture of the phantoms is under development; within this project, the phantoms will be enhanced with special requirements for preservation of the thermal properties. - Enhanced models with discrete vessel models are under development. Objectives: - development of MR protocols for whole body scans of an obese adult in an appropriate scanner. - approval by ethics committee, recruitment of volunteer. - MR scans of volunteer. - segmentation of the MR images, model generation. - development of MR protocols for whole body scans of a pig. - approval, MR scans and segmentation of the pig. Milestones: anatomical high-resolution models ranging from children to obese adults as well as for a pig. * WP3 - Development of Experimental & TCAD Tools for Field and Temperature Evaluation: Background information: - Partner 5 is the pacesetter in RF near-field assessment systems, measurements and simulations. - SEMCAD X is currently the most advanced simulation tool in terms of features (EM & T solvers), precision and speed, with an advantage of at least a factor of 50 compared to REMCOM, CST; the thermal solvers provide unique features such as anisotropic cooling by blood perfusion and discrete vessel networks. - DASY5, EASY4, iSAR and their E, H, SAR and T probes are the most accurate tools for nearfield measurements. Their market share for compliance testing of mobile phones exceeds 95%. Most government laboratories have acquired this technology (USA (FDA, FCC), Canada, Japan, Korea, Taiwan, China, etc.). - Within this project, Partner 5 together with Partners 1, 9 will adapt this near-field measurement technology for operation within MRI environments and will extend the numerical tool for more effective EM simulations of small details within MRI scanners as well as produce an improved thermal simulator that enables the prediction of temperature increases and tissue damage in-vivo with known uncertainty. Objectives: - development of isotropic electric and magnetic field sensors for measurements in MRI environments. - development of SAR probes and temperature probes. - development of calibration techniques at MRI frequencies. - development of novel extensions for effective simulations of fine details in MRI scanners. - development of novel techniques for determining reliable temperature predictions under different conditions as well as integration of the CEM43-CT90 model. Milestones: MR compatible measurement equipment enabling accurate measurement of the incident and induced fields to which patients and workers are exposed; MR compatible equipment for the measurement of temperature rise. * WP4 - Experimental Evaluation of the EM Exposure of Patients and Workers Objectives: - measurements of the EM fields of selected MRI scanners at UniZ, Erasmus MC, Siemens MS and Philips MS. - comparison with predicted exposures based on WP1. - development of EM field exposure distributions and a matrix of scenarios to be used in WPs 5 - 6. Milestones: Characterisation of the distributions of the EM fields to which patients and workers are exposed as the basis for the numerical analysis; matrix of exposure scenarios to be used in WPs 5 – 6. * WP5 - Numerical SAR Evaluation Background Information: - Partners 1, 3, 4 and 5 gained vast experience in SAR evaluation by providing the scientific basis for wireless standards (e.g., IEEE 1528, IEC 62209, etc., within the EUREKA projects SARSYS, SARSYS BWP, etc.). - They have significantly contributed towards understanding the interaction of electromagnetic fields and biological tissue. This includes the development of the basic physical mechanisms of the absorption of EM energy in the human body, the development of simulation software and anatomical models and the design of in-vivo and in-vitro exposure setups for bio-experiments. - Partners 1 and 8 actively take part in the development of RF safety standards within the various working groups of IEEE/ICES and IEC. - Partners 1, 4 and 5 are active members of the COST action on EM assessments (e.g., COST 281). - Partners 1, 6, 7, 8 and 10 already have experience in SAR evaluation within MRI. Objectives: - numerical evaluation of SAR for exposure scenarios as defined in WPs 1 and 4 and based on the models developed and defined in WP2. - quantification of the SAR distribution at hot spots. - identification of an absorption mechanism for the quantification of hot spots. - possible correlation between local SAR, whole body SAR and B1. Milestones: identification of hot spot effects, quantification and spatial distributions of local maxima as a function of whole or partial body SAR, B1, RF power and coil dimensions. * WP6 - Thermal Simulations Background Information: - Partners 1, 2, 4 and 10 have long-term experience in the numerical modelling and measurement of temperature distributions in the human body. - Partners 6, 7, 8, 9 and 10 have experience in temperature measurements in MRI. - Partners 1, 2, 4 and 10 developed advanced numerical methods for the assessment of RF induced heating. Novel techniques, e.g., for blood flow cooling or thermoregulation, could be developed and successfully applied to the improvement of traditional hyperthermia treatment methods, e.g., Nadobny et al. in 2007. - These tools and applications will be further enhanced in this project together with the improved human models (WP2). Objectives: - enhanced numerical simulation tools for temperature prediction, including tin wires, the tensorial and discrete vessel network for blood flow, etc. - numerical assessment of SAR induced temperature distribution for the most relevant scenarios identified in WP5. - evaluation of the impact of different thermal models (anisotropy, DIVA, thermal regulation, etc.). Milestone: range of possible induced deltaT. * WP7 - CEM43-CT90: Background Information: - Partners 2, 10 are two of the leading centres for hyperthermia and therefore have the experience and the necessary network for the application of the CEM43-CT90 model in the context of this project. Partner 8 will support this consortium with its expertise. Objectives: - Definition of criteria for non-hazardous heating as well as for tissue damage. - Definition of maximum temperature values for all tissue types. - Development of scan parameter limits to prevent exceeding the CEM43-CT90 values. Milestone: rationale for tolerable tissue heating and corresponding absolute T values; appropriate scan parameters based on CEM43-CT90 values. * WP8 - Experimental Validation in Phantom: Background Information: - Partners 1, 2, 3 and 4 have developed comprehensive validation techniques for the wireless industry. These techniques are applied in the dosimetry of portable wireless devices, and techniques will be adopted and extended for this project. - Partners 1, 2 and 9 will conduct the validations at UNIVERSITAETSSPITAL ZUERICH. Objectives: - Validation of predicted E-fields/SAR in homogeneous phantoms by measurements in different commercial MR scanners. - In-vivo validation of the predicted temperature increase during MR scans of a volunteer. Milestones: validated numerical methods and models for simulation of the exposure in different RF coils. * WP9 - Experimental Validation In-Vivo Background Information: - Partner 10 has long-standing experience in MR-guided hyperthermia and the development/use of numerical methods for the calculation of SAR and temperature distributions. - Partner 10 has long-standing experience in experimental validation in animal models. - Partner 7 will provide an experimental MR birdcage coil suitable for heating experiments in animals. - Partner 1 will assist Partner 10 in the segmentation of the pig scans. - Partners 1 and 2 will support Partner 10 with their expertise. Objectives The objective of this Work Package is validation in-vivo of the numerically predicted temperature increase found in W6 as a function of the peak spatial SAR as well as the tissue damage determined based on the CEM43-CT90 model in anaesthetised pigs. Milestones: - Ethical approval by the BUNDESAMT FUER GESUNDHEIT UND SOZIALES, GERMANY. - Generation of the numerical birdcage model. - Generation of pig models. - Determination of typical positions of the hot spots using calculations in pig models based on high-resolution CT scans. Study of the dependence of the hot spots on segmentation models and/or positioning in the MR coil. - 20 heating experiments in pigs: 6 in freshly sacrificed and 14 in narcotised live pigs: § before each experiment, catheters will be inserted into numerically predicted hot-spot locations, and, during the experiments, the actual temperature values will be measured by means of temperature probes placed into the catheters; § the precise positions of the temperature probes will be detected using high-resolution whole-body CT scans; § a-posteriori numerical analysis will be performed based on measured data. - Post-experimental histological investigations in selected tissues to study the dependence of the tissue damage on the applied MR power levels. - Comparison with the results of WPs 6 and 7. * WP10 - Uncertainty Evaluation: Background Information: - Partners 1, 3, 4, 5 and 8 have pioneered the uncertainty analysis in dosimetry and developed the uncertainty budget for the wireless industry. They will adapt the technique for MRI within this project. Objectives: - Development of the uncertainty matrix. - Estimation of the maximal SAR and deltaT distributions for the entire patient population. - Uncertainty assessment of this distribution. Milestone: probability distribution of risk of local tissue damage during MR scans * WP11 - Consolidation and Dissemination - Dissemination of the results in peer-reviewed journals and at conferences. - Presentation of the results to the respective standardisation bodies IEC 62B / MT40. - Dissemination of the results to implant manufacturers. Milestones: information allowing the optimisation of benefits / risks in MRI applications, improved standard with minimal scanning restrictions, a basis for the reliable safety evaluation of patient implants. * WP12 – Development of RF Birdcage Test Environment Background Information: - Partners 1 & 5 have been active in the definition, development and characterisation of well defined test environments for wireless devices. - Partner 1 has experience in conducting comprehensive numerical validations. - Partner 13 in conjunction with partners 1 and 5 will design and validate a generic test birdcage for experimental exposure assessment. Objectives: To take patient derived knowledge and utilise it in developing a generic test environment based on a birdcage MRI body coil that can be used to evaluate the MRI safety and compatibility of medical implants to the RF field. The birdcage will provide a standardised test environment which is well characterised. A numerical model of the birdcage will allow direct comparison of modelling and measurement and the ability to perform a direct comparison between devices. Milestones: - Development of a generic specification of the birdcage body coil. - Mechanical drawings of the birdcage with optimised field patterns and performance. - The integration of excitation and control electronics. - Fully characterised birdcage. - Validated test birdcage and numerical model for evaluation of implants. References: [Kuster et al., 2006] Niels Kuster, Veronica Berdinas-Torres, Neviana Nikoloski, Michael Frauscher, and Wolfgang Kainz, 'Methodology of detailed dosimetry and treatment of uncertainty and variations for in-vivo studies', Bioelectromagnetics, vol. 27, no. 5, pp. 378–391, July 2006. [Neufeld et al., 2006] Esra Neufeld, Theodoros Samaras, Nicolas Chavannes, and Niels Kuster, 'New model to simulate EM induced temperature distributions and the influence of blood flow', in Proceedings of the 28th Annual Meeting of the Bioelectromagnetics Society, pp. 430-431, June 2006, Cancun, Mexico. [Nadobny et al. 2007] Nadobny J., Szimtenings M., Diehl D., Stetter E., Brinker G., Wust P.: 'Evaluation of MR-Induced Hot-Spots for Different Temporal SAR Modes Using a Time-Dependent Finite Difference Method With Explicit Temperature Gradient Treatment,' IEEE Trans. Biomed. Eng., 2007, in press, available as preprint version in ieeexplore.ieee.org/: Digital Object Identifier: 10.1109/TBME.2007.893499. Keywords: Standardisation, Exposure Risk Probability, Medical Technology.
Acronym: 
MRI+
Project ID: 
4 144
Start date: 
01-12-2007
Project Duration: 
42months
Project costs: 
1 360 000.00€
Technological Area: 
Sensor Technology related to measurements
Market Area: 
Other diagnostic

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