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Comparison of B1 Field-Calibrated Specific Absorption Rate Estimation Between a Whole-Body and a Compact 3T MR Scanner

Y Shu , N Huber , S Tao , M In , J Trzasko , J Huston , M Bernstein*, Mayo Clinic, Rochester, MN


(Monday, 7/30/2018) 3:45 PM - 4:15 PM

Room: Exhibit Hall | Forum 8

Purpose: Accurate estimation of the RF-power deposition, and hence the Specific Absorption Rate(SAR), is an important MRI safety aspect¹. One way to lower the SAR is to limit RF exposure to a localized body region. A novel compact 3T(C3T) MRI scanner has been developed to image the brain and extremities². It uses a smaller RF transmit coil (37.6x40cm²) than that (60-70x60-70cm²) of a whole-body scanner. By adapting methods published for whole-body scanners³, we measured average SAR on the C3T with phantom calorimetry experiments and compared it with a conventional whole-body system. For a more accurate SAR estimation, the B1 maps were used to calibrate actual flip angle (FA), so that the comparison results were normalized.

Methods: A 3D-printed head phantom filled with half-normal saline was scanned. A 60-minute, 4-echo spin-echo sequence was acquired with identical parameters including nominal B1+RMS, displayed SAR, FA, TR, etc. The applied average head SAR was derived from temperature rise measured in the phantom. B1 mapping was performed using the double-angle methodâ?´. The root-mean-square(RMS) values of the FA were measured, and the transmit gain values recorded. These were used to correct the calculated SAR value by ensuring the actual B1+RMS values were normalized.

Results: Through B1 map calibration, the calculated SAR was adjusted to reflect the true RF deposition with the same actual FA in the phantom. The comparison shows that the SAR on the whole-body scanner is 23% higher than that of the C3T. This is consistent with previous results obtained with a cylindrical phantomâ?µ.

Conclusion: The localized head SAR measurement on the C3T was calibrated using calorimetry with B1 mapping correction for inaccurate FA. The comparison between the C3T and the whole-body scanner demonstrated the benefit of using a smaller RF coil to reduce SAR, which can benefit high-SAR applications (e.g., simultaneous multislice imaging).


Calibration, MRI, Quality Control


IM- MRI : Quality Control

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