Specific absorption rates and signal-to-noise ratio limitations for MRI in very-low magnetic fields

Coil loading experiments were performed to characterize specific absorption rates (SARs) for adult human subjects in uniform linearly-polarized time-varying magnetic fields B from 30 kHz to 1.25 MHz, corresponding to a range of Larmor frequencies f that is relevant to MRI in very-low magnetic fields. For oscillating fields directed perpendicular to the sagittal plane of the human body in the standard anatomical position it was found that SAR = 4.3(1) x 10(-7) (M/L)f(2)B(2), where M and L are the mass and height of the subject and all quantities are expressed in SI base units. The average linear density M/L appearing in this expression was observed to be an excellent anthropomorphic index for characterizing the manner in which SAR depends on the average transverse dimension of the subject normal to the applied field. As anticipated, SAR values over this frequency range were low compared to those observed at higher frequencies, indicating that emerging applications requiring high duty-cycle and/or intense radio-frequency MR tipping pulses will not lead to excessive heating of tissues. Data from these experiments also corroborate and quantify predictions that significant improvements in signal-to-noise-ratios can be achieved through appropriate receive-antenna design. (c) 2012 Wiley Periodicals, Inc. Concepts Magn Reson Part A 40A: 281294, 2012.