Validating subject-specific RF and thermal simulations in the calf muscle using MR-based temperature measurements

\u3cp\u3ePurpose: Ongoing discussions occur to translate the safety restrictions on MR scanners from specific absorption rate (SAR) to thermal dose. Therefore, this research focuses on the accuracy of thermal simulations in human subjects during an MR exam, which is fundamental information in that debate. Methods: Radiofrequency (RF) heating experiments were performed on the calves of 13 healthy subjects using a dedicated transmit-receive coil while monitoring the temperature with proton resonance frequency shift (PRFS) thermometry. Subject-specific models and one generic model were used for electromagnetic and thermal simulations using Pennes' bioheat equation, with the blood equilibration constant equaling zero. The simulations were subsequently compared with the experimental results. Results: The mean B \u3csup\u3e+\u3c/sup\u3e \u3csub\u3e1\u3c/sub\u3e equaled 15 µT in the center slice of all volunteers, and 95% of the voxels had errors smaller than 2.8 µT between the simulation and measurement. The intersubject variation in RF power to achieve the required B \u3csup\u3e+\u3c/sup\u3e \u3csub\u3e1\u3c/sub\u3e was 11%. The resulting intersubject variation in median temperature rise was 14%. Thermal simulations underestimated the median temperature increase on average, with 34% in subject-specific models and 28% in the generic model. Conclusions: Although thermal measures are directly coupled to tissue damage and therefore suitable for RF safety assessment, insecurities in the applied thermal modeling limit their estimation accuracy. Magn Reson Med 77:1691–1700, 2017.\u3c/p\u3

Quantitative T2 mapping of the mouse heart by segmented MLEV phase-cycled T2 preparation

Purpose A high-quality, reproducible, multi-slice T2-mapping protocol for the mouse heart is presented. Methods A T2-prepared sequence with composite 90 and 180 radiofrequency pulses in a segmented MLEV phase cycling scheme was developed. The T2-mapping protocol was optimized using simulations and evaluated with phantoms. Results Repeatability for determination of myocardial T2 values was assessed in vivo in n=5 healthy mice on 2 different days. The average baseline T2 of the left ventricular myocardium was 22.5±1.7 ms. The repeatability coefficient for R2=1/T2 for measurements at different days was ¿R2=6.3 s-1. Subsequently, T2 mapping was applied in comparison to late-gadolinium-enhancement (LGE) imaging, to assess 1-day-old ischemia/reperfusion (IR) myocardial injury in n=8 mice. T2 in the infarcts was significantly higher than in remote tissue, whereas remote tissue was not significantly different from baseline. Infarct sizes based on T2 versus LGE showed strong correlation. To assess the time-course of T2 changes in the infarcts, T2 mapping was performed at day 1, 3, and 7 after IR injury in a separate group of mice (n=16). T2 was highest at day 3, in agreement with the expected time course of edema formation and resolution after myocardial infarction. Conclusion T2 prepared imaging provides high quality reproducible T2 maps of healthy and diseased mouse myocardium. Magn Reson Med 72:409-417, 2014. © 2013 Wiley Periodicals, Inc. Copyright © 2013 Wiley Periodicals, Inc