Virtual Population-Based Assessment of the Impact of 3 Tesla Radiofrequency Shimming and Thermoregulation on Safety and B1+ Uniformity

Manuel Murbach, Esra Neufeld, Eugenia Cabot, Earl Zastrow, Juan Córcoles, Wolfgang Kainz, and Niels Kuster, Magnetic Resonance in Medicine, Early View, September 2015, online September 24, 2015

Magnetic resonance imaging (MRI) has become a routine clinical diagnostic method. Due to the shorter wavelength at 3T, most MRI scanners implement two-port RF shimming to improve image quality. This may significantly alter the RF absorption pattern in patients. This study investigates the effect of RF shimming on local RF power deposition for a wide range of anatomical models, and subsequently estimates the induced temperature increase as a function of the thermoregulatory state of the patient. The newly proposed model for impaired thermoregulation allows conservative estimations for patient groups with limited physiological responses to heating. Worst-case induced peak temperature reach 42.5°C and 45.6°C in patients with normal and impaired thermoregulation, respectively. RF shimming can significantly increase image quality, and can be considered safe for a broad patient population with normal thermoregulation. Patients with impaired thermoregulation should not be scanned outside of the normal operating mode.

The scientific and technical impact of the study can be summarized as:

  • Broad evaluation of the implications of RF shimming on image quality and RF exposure at 3T
  • Newly proposed model for impaired thermoregulation for conservative temperature estimations
  • Identification of worst-case exposure scenarios 
  • Comparison of SAR/temperature correlations to theoretically derived models 
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