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Calorimetric Calibration of Specific Absorption Rate (SAR) On a Compact 3T MRI Scanner

Y Shu

Y Shu1*, S Tao1 , D Yeo2 , T Foo2 , P Weavers1 , J Trzasko1 , J Huston1 , M Bernstein1 , (1) Mayo Clinic, Rochester, MN, (2) GE Global Research, Niskayuna, NY,


MO-L-GePD-I-1 (Monday, July 31, 2017) 1:15 PM - 1:45 PM Room: Imaging ePoster Lounge

Purpose: Prediction of specific absorption rate (SAR) is an important safety aspect for human subject scans. Recently, a novel compact 3T (C3T) MRI scanner has been developed for brain and MSK applications. Compared to a conventional RF body coil (60-70x60-70cm² diameterxlength cm²), the C3T scanner is equipped with a smaller RF coil (37.6x40cm²), which substantially reduces the body mass exposed to RF energy. To ensure patient safety, we measured the average head SAR of the transmit coil and compared it with a whole-body scanner, using calorimetry.

Methods: Calorimetric tests were performed on both the C3T and a 3T MR750w whole-body MR scanner. A 4.3-liter cylindrical phantom filled with half-normal saline was scanned. Two thermometry probes were placed inside the phantom to measure the temperature increase during a 30-min 4-echo spin echo sequence. A separate probe was used to measure the ambient temperature. Temperature changes were recorded at a 1 sec interval from the start of the sequence until ~3 hours after the sequence ended. The phantom temperature T (°C) change during this experiment was modeled by:T(t) = T0 e^(-k(t-t0))+ (Ta + S/kc) x (1- e^(-k(t-t0))). First, the cooling constant k was determined using the temperature decay during the cooling period. Second, the estimated k was substituted into the equation, which was then fitted to the data measured when the heating sequence was running to calculate the SAR.

Results: Under the same RF exposure, the measured SAR in the phantom inside an MR750w is 19% higher than that measured for the C3T.

Conclusion: We successfully performed the average head SAR measurement on the C3T system using calorimetry which demonstrated a moderate SAR reduction. The reduced SAR could benefit a variety of applications including simultaneous multislice imaging and aid engineers in refining model-dependent SAR estimates displayed on the scanner console.

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