Effects of anatomical differences on electromagnetic fields, <scp>SAR</scp>, and temperature change

Abstract

Electromagnetic field simulations are increasingly used to assure RF safety of patients during MRI exams. In practice, however, tissue property distribution of the patient being imaged is not known, but may be represented with a pre-existing model. Repeatedly, agreement in transmit magnetic (B₁⁺) field distributions between two geometries has been used to suggest agreement in heating distributions. Here we examine relative effects of anatomical differences on B₁⁺ distribution, Specific Absorption Rate (SAR) and temperature change (ΔT). Numerical simulations were performed for a single surface coil positioned adjacent a homogeneous phantom and bovine phantom, each with slight geometric variations, and adjacent two different human body models. Experimental demonstration was performed on a bovine phantom using MR thermometry and B₁⁺ mapping. Simulations and experiments demonstrate that B₁⁺ distributions in different samples can be well correlated, while notable difference in maximum SAR and ΔT occur. This work illustrates challenges associated with utilizing simulations or experiments for RF safety assurance purposes. Reliance on B₁⁺ distributions alone for validation of simulations and/or experiments with a sample or subject for assurance of safety in another should be performed with caution.