Temperature and SAR calculations for a human head within volume and surface coils at 64 and 300 MHz

Abstract

Purpose: To examine relationships between specific energy absorption rate (SAR) and temperature distributions in the human head during radio frequency energy deposition in MRI. Materials and Methods:A multi-tissue numerical model of the head was developed that considered thermal conductivity, heat capacity, perfusion, heat of metabolism, electrical properties, and density. Calculations of SAR and the resulting temperature increase were performed for different coils at different frequencies.Results: Because of tissue-dependant perfusion rates and thermal conduction, there is not a good overall spatial correlation between SAR and temperature increase. When a volume coil is driven to induce a head average SAR level of either 3.0 or 3.2 W/kg, it is unlikely that a significant temperature increase in the brain will occur due to its high rate of perfusion, although limits on SAR in any 1 g of tissue in the head may be exceeded. Conclusion:Attempts to ensure RF safety in MRI often rely on assumptions about local temperature from local SAR levels. The relationship between local SAR and local temperature is not, however, straightforward. In cases where high SAR levels are required due to pulse sequence demands, calculations of temperature may be preferable to calculations of SAR because of the more direct relationship between temperature and safety.

Sensitivity‐enhanced 13C MR spectroscopy of the human brain at 3 Tesla

“…5). According to Faraday's law, a homogeneous RF coil might suggest equally distributed RF heating; however, local SAR can be higher due to the applied electric fields and dielectric resonances (7). Temperature maps of a phantom using the birdcage coil showed that this was indeed the case (Fig.…”

“…This design is a substantial improvement over conventional butterfly coils for 1 H decoupling. However, within the sensitive field of surface coils, the local SAR will vary greatly and may exceed safety guidelines (7). Since the B 1 field of the two slightly overlapping 1 H surface coils decays linearly with respect to distance (8), the use of 1 H decoupling is limited to a small region of the brain in order to stay within local SAR guidelines.…”

“…Since the B 1 field of the two slightly overlapping 1 H surface coils decays linearly with respect to distance (8), the use of 1 H decoupling is limited to a small region of the brain in order to stay within local SAR guidelines. Recently it was shown that volume coils with homogeneous B 1 fields have lower local SAR values than quadrature surface coils (7). Therefore, it would be more efficient to use volume coils for 1 H decoupling rather than surface coils.…”

System Stability of a 3T-MRI During Continuous EPI Scan

“…This change was specific to the volunteer s data by the 3T. According to the references, [27][28][29][30] body temperature increased by the heating effect of radio-frequency RF pulse [27][28][29] and become higher over time. 29 Moreover, the relationship between cerebral blood flow CBF and artificial Further study will need to clarify the origin of this phenomenon, and the design of the new phantom whose fluidity can simulate biological tissues is required.…”

A Boundary-Value-Free Reconstruction Method for Magnetic Resonance Electrical Properties Tomography Based on the Neumann-Type Integral Formula over a Circular Region

“…Therefore, EP maps are expected to be useful for the diagnosis of malignant tissues. In addition, conductivity maps can be used to calculate the specific absorption rate (SAR) distribution [5], which is the heating criterion of tissues in recent high field MRI systems.…”