Statistical simulation of SAR variability with geometric and tissue property changes by using the unscented transform

Abstract

PURPOSE: The local specific absorption rate (SAR) is critical to the safety of radio frequency transmit coils. A statistical simulation approach is introduced to address the local SAR variability related to tissue property and geometric variations. METHODS: The local SAR is modeled as the output of a nonlinear transformation with factors that may affect its value being treated as random input variables. Instead of using the Monte Carlo method with a large number of sample points, the unscented transform is applied with a small set of deterministic sample points. A sensitivity analysis is further performed to determine the significance of each input variable. Electromagnetic simulations are carried out by the finite-difference time-domain method implemented on graphic processing unit. RESULTS: The local SAR variability of a 7 Tesla square loop coil for spine imaging and a 16-element brain imaging array as the result of tissue property and geometric changes were examined respectively. SAR limits were determined based on their means and standard deviations. CONCLUSION: The proposed approach is efficient and general for the study of local SAR variability.