Mar 11, 2013

Whole-Body and Local RF Absorption in Human Models as a Function of Anatomy and Position within 1.5T MR Body Coil

Manuel Murbach, Esra Neufeld, Wolfgang Kainz, Klaas P. Pruessmann, and Niels Kuster, Magnetic Resonance in Medicine, Volume 71, Issue 2, pp. 839-845, February 2014, online February 25, 2013

We investigated the correlation between large-scale anatomical features (e.g., height, weight) and radiofrequency (RF) energy deposition during magnetic resonance imaging (MRI). Six anatomical human models (three adults and three children) were evaluated in different MR imaging positions (head to knees) within a 1.5 T body coil. The specific absorption rate (SAR), the pivotal parameter used to quantify absorbed RF, increases with the radial dimension of the patient and, therefore, with the large-scale anatomical properties. For a fixed B1+ incident field, the whole body SAR (wbSAR) can be up to 2.5 times higher (local SAR up to 7 times) in obese adult models compared to child models. When the exposure is normalized to 4 W/kg wbSAR, the local SAR can well exceed safety limits for local transmit coils and shows inter-subject variations of up to a factor of three. The correlations between anatomy and induced local SAR are weak for normalized exposure, but strong for a fixed B1+ field, suggesting that anatomical properties could be valuable for rapid SAR predictions. This study demonstrates that a representative virtual human population is indispensable for the investigation of local SAR levels.  

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

  • Accurate computational human models are excellent tools for studying the influence of anatomy on MR exposure.
  • Rapid SAR prediction based on large-scale anatomical properties of the patient is feasible.
  • RF absorption is quantized as a function of anatomy.
  • Worst-case MR exposure scenarios and numerical evidence of very high local SAR hotspots that depend on model and position can be identified.