A model to assess SAR for surface coil magnetic resonance spectroscopy measurements

Surface coils are widely used in magnetic resonance (MR) studies due to their superior signal to noise properties. Application of excessive power levels to transmit surface coils may result in local tissue damage. A homogeneous muscle tissue model for the conservative prediction of surface coil specific absorption rate (SAR) is introduced. Based on this model, sequence parameters can be limited to provide operational levels within safety guidelines. It is demonstrated that this model provides worst-case SAR estimates at MR frequencies of 25.75 MHz and 63.6 MHz. The dependence of SAR on model structure and geometry is analysed and conclusions on the relationship between SAR levels and local anatomy are drawn. By making a worst-case assumption for the tissue parameters the model provides safe operational levels for all tissue types. Power-demanding proton-decoupled 31P magnetic resonance spectroscopy experiments are possible based on the SAR estimates provided. To date SAR values are calculated for 1 g of tissue. Changes in regulations to calculate SAR values for 10 g tissue masses, and the according averaging of local SAR over a larger volume, have been proposed by the International Electrotechnical Commission. A comparative study shows that up to 100% more energy may be applied to surface coils if SAR values are determined for 10 g tissue masses rather than 1 g tissue masses.