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(Database last updated on Mar 27, 2024)
| ID Number | 1512 | |
| Study Type | In Vivo | |
| Model | Thermal Analysis & Regulation (catch all) | |
| Details | Thermal Analysis & Regulation (catch all) AUTHORS' ABSTRACT: Carluccio et al. 2013 (IEEE #5271): We present an approach to performing rapid calculations of temperature within tissue by interleaving, at regular time intervals, 1) an analytical solution to the Pennes (or other desired) bioheat equation excluding the term for thermal conduction and 2) application of a spatial filter to approximate the effects of thermal conduction. Here, the basic approach is presented with attention to filter design. The method is applied to a few different cases relevant to magnetic resonance imaging, and results are compared to those from a full finite-difference (FD) implementation of the Pennes bioheat equation. It is seen that results of the proposed method are in reasonable agreement with those of the FD approach, with about 15% difference in the calculated maximum temperature increase, but are calculated in a fraction of the time, requiring less than 2% of the calculation time for the FD approach in the cases evaluated. AUTHORS' ABSTRAT: Murbach et al. 2014 (IEEE #5892): PURPOSE: This article investigates the safety of radiofrequency induced local thermal hotspots within a 1.5T body coil by assessing the transient local peak temperatures as a function of exposure level and local thermoregulation in four anatomical human models in different Z-positions. METHODS: To quantize the effective thermal stress of the tissues, the thermal dose model cumulative equivalent minutes at 43°C was employed, allowing the prediction of thermal tissue damage risk and the identification of potentially hazardous MR scan-scenarios. The numerical results were validated by B1 (+) - and skin temperature measurements. RESULTS: At continuous 4 W/kg whole-body exposure, peak tissue temperatures of up to 42.8°C were computed for the thermoregulated model (60°C in nonregulated case). When applying cumulative equivalent minutes at 43°C damage thresholds of 15 min (muscle, skin, fat, and bone) and 2 min (other), possible tissue damage cannot be excluded after 25 min for the thermoregulated model (4 min in nonregulated). CONCLUSION: The results are found to be consistent with the history of safe use in MR scanning, but not with current safety guidelines. For future safety concepts, we suggest to use thermal dose models instead of temperatures or SAR. Special safety concerns for patients with impaired thermoregulation (e.g., the elderly, diabetics) should be addressed. AUTHORS' ABSTRACT: Nadobny et al. 2015 (IEEE #5893): Purpose: This study is an investigation of the relationship between several characteristic parameters and acute thermal damage in porcine skeletal muscle. Material and methods: Fourteen pigs under injection anaesthesia were placed into a magnetic resonance body coil and exposed for different time durations to different specific energy absorption rate (SAR) levels at 123 MHz. Local temperatures were measured using four temperature sensors. Sensors 1-3 were placed in skeletal muscle and one sensor was placed in the rectum. Sensors 1 and 2 were placed in hot-spot areas and sensor 3 was placed at the periphery of the animals. The pigs were exposed to whole-body SAR (SAR-wb) between 2.5 W/kg and 5.2 W/kg for 30 or 60 min. Three animals received no SAR. After each experiment, muscle samples adjacent to the positions of sensors 1-3 were taken for frozen section analysis. Three characteristic parameters were chosen for investigation: SAR-wb, maximum sensor temperature (T-max), and cumulative equivalent minutes at 43 °C (CEM43 °C). Results: Histopathological criteria were established to detect acute thermal tissue damage in frozen sections such as widening of intercellular space between the muscle fibres and loss of glycogen. Clear tissue damage thresholds were found for T-max and CEM43 °C, though not for SAR-wb. For all animals with high thermal exposure, damage was also found for muscle samples adjacent to the peripheral sensor 3. Conclusions: Both T-max and CEM43, are able to predict thermal damage in porcine muscle. However, CEM43 is the less ambiguous parameter. The reasons for the occurrence of the aforementioned damage at low local temperatures at the animals' periphery remain unclear and further investigations are needed. AUTHORS' ABSTRACT: van Rhoon et al. 2013 (IEEE #5894): OBJECTIVE: To define thresholds of safe local temperature increases for MR equipment that exposes patients to radiofrequency fields of high intensities for long duration. These MR systems induce heterogeneous energy absorption patterns inside the body and can create localised hotspots with a risk of overheating. METHODS: The MRI + EUREKA research consortium organised a "Thermal Workshop on RF Hotspots". The available literature on thresholds for thermal damage and the validity of the thermal dose (TD) model were discussed. RESULTS/CONCLUSIONS: The following global TD threshold guidelines for safe use of MR are proposed: 1. All persons: maximum local temperature of any tissue limited to 39 °C 2. Persons with compromised thermoregulation AND (a) Uncontrolled conditions: maximum local temperature limited to 39 °C (b) Controlled conditions: TD < 2 CEM43°C 3. Persons with uncompromised thermoregulation AND (a) Uncontrolled conditions: TD < 2 CEM43°C (b) Controlled conditions: TD < 9 CEM43°C The following definitions are applied: Controlled conditions A medical doctor or a dedicated trained person can respond instantly to heat-induced physiological stress Compromised thermoregulation All persons with impaired systemic or reduced local thermoregulation KEY POINTS: " Standard MRI can cause local heating by radiofrequency absorption. " Monitoring thermal dose (in units of CEM43°C) can control risk during MRI. " 9 CEM43°C seems an acceptable thermal dose threshold for most patients. " For skin, muscle, fat and bone,16 CEM43°C is likely acceptable. AUTHORS' ABSTRACT: Neufeld et al. 2015 (IEEE #6011): To maximize diagnostic accuracy and minimize costs, magnetic resonance imaging (MRI) scanners expose patients to electromagnetic exposure levels well above the established maximum, but in a well-controlled environment. In this paper, we discuss a novel safety assessment model that offers maximum flexibility while ensuring no local tissue damage due to radiofrequency induced heating occurs. This model is based on the cumulative equivalent minutes at 43 °C (CEM43) thermal dose concept, which naturally considers exposure duration, tissue sensitivity and the transient nature of heating, and permits rapid assessment of exposure safety of a given MRI scan using information about the transient specific absorption rate (SAR). It builds upon theoretical considerations (e.g., relating peak temperatures in the presence and absence of local thermoregulation) as well as data extracted from simulations involving anatomical models (e.g., to determine the characteristic time of temperature changes). The model is capable of predicting CEM43 for patients with either uncompromised thermoregulation or absent thermoregulation. The model predictions approximate detailed simulations well and results illustrate the importance of adequately considering changes in perfusion. The model presented herein offers an MRI safety assessment approach that overcomes problems associated with traditional SAR-based limits. Its limitations and the associated uncertainties are discussed together with remaining open questions. |
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| Findings | ||
| Status | Completed With Publication | |
| Principal Investigator | ||
| Funding Agency | ????? | |
| Country | UNITED STATES | |
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