Local SAR in adults and children at 7T MR: Realistic estimation by the using of simulations

The evaluation of the local Specific Absorption Rate (SAR) is a major concern in ultra high field (UHF) Magnetic Resonance (MR) systems. In fact, at UHF, the energy deposition due to the radio-frequency (RF) field increases and its distribution inside the subject becomes extremely inhomogeneous and subject dependent. Local SAR measurements are not available in present MR systems; thus, electromagnetic simulations must be performed for RF fields and SAR analysis. In this study we resort to 3D full wave numerical electromagnetic simulations for investigating the dependence of the local SAR at 7 T with respect to the subject size in two different scenarios: i) surface coil loaded by adults and children calves; ii) volume coil loaded by adults and children heads. Concerning the first scenario, it has been found that the maximum local SAR decreases with decreasing load size: this holds true if the RF magnetic fields (B₁⁺) for the different load sizes are scaled so to achieve the same B₁⁺ slice average value.