Gerhard Brinker

Evaluation of MR-Induced Hot Spots for Different Temporal SAR Modes Using a Time-Dependent Finite Difference Method With Explicit Temperature Gradient Treatment

An investigation of magnetic resonance (MR)-induced hot spots in a high-resolution human model is performed, motivated by safety aspects for the use of MR tomographs. The human model is placed in an MR whole body resonator that is driven in a quadrature excitation mode. The MR-induced hot spots are studied by varying the following: the temporal specific absorption rate (SAR) mode (ldquosteady imagingrdquo, ldquointermittent imagingrdquo), the simulation procedure (related to given power levels or to limiting temperatures), and different thermal tissue properties including temperature-independent and temperature-dependent perfusion models. Both electromagnetic and thermodynamic simulations have been performed. For the electromagnetic modeling, a commercial finite-integration theory (FIT) code is applied. For the thermodynamic modeling, a time-domain finite-difference (FD) scheme is formulated that uses an explicit treatment of temperature gradient components. This allows a flux-vector-based implementation of heat transfer boundary conditions on cubical faces. It is shown that this FD scheme significantly reduces the staircase errors at thermal boundaries that are locally sloped or curved with respect to the cubical grid elements.

Sampling and evaluation of specific absorption rates during patient examinations performed on 1.5-Tesla MR systems

It was the purpose of present study, to evaluate a large number of exposure-time courses measured during patient examinations in clinical routine in relation to the current IEC standard and the draft version of the revised standard and, moreover, to investigate whether there is a correlation between the subjective heat perception of the patients during the MR examination and the intensity of RF power deposition. To this end, radiofrequency exposure to 591 patients undergoing MR examinations performed on 1.5-Tesla MR systems was monitored in five clinics and evaluated in accordance with both IEC standards. For each of the 7902 sequences applied, whole body and partial body SARs were estimated on the basis of a simple patient model. Following the examinations, 149 patients were willing to provide information in a questionnaire regarding their body weight and their subjective heat perception during the examination. Although patient masses entered into the MR system were in some cases too high, reliable masses could be estimated by the SAR monitor. In relation to our data, the revision of the IEC standard results in a tightening of the restrictions, but still more than 96% of the examinations did not exceed the SAR limits recommended for the normal operating mode. For the exposure conditions examined, no statistically significant correlation was found between the subjective heat perception of the patients and the intensity of power deposition. Taking advantage of the possibility to compute running SAR averages, MR sequences can be employed in clinical practice for which SAR levels exceed the defined IEC limits, if the acquisition time is short in relation to the averaging period and energy deposition has been low previous to the applied high-power sequence.

Experimental investigation and histopathological identification of acute thermal damage in skeletal porcine muscle in relation to whole-body SAR, maximum temperature, and CEM43 °C due to RF irradiation in an MR body coil of birdcage type at 123 MHz

Abstract 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.