Patient-specific in vivo local SAR Estimation and Validation

Introduction: Tissue heating and local Specific Absorption Rate (SAR) is a major problem for high field MRI, particularly when using multiple transmit channels. In the framework of Electric Properties Tomography (EPT), the local SAR is estimated based on a B1 map [1]. Recently, first preliminary low resolution in vivo SAR images were obtained [2]. In this study, B1-based local SAR is determined in vivo and validated using FDTD simulations. The impact of neglecting the magnetic field components other than B1 is investigated. The presented approach yields a sufficiently accurate and patient-specific local SAR measurement. Theory: Assuming constant tissue density, the local SAR is given by Eq. (1). Thus, for determining local SAR, electric conductivity σ and the electric field vector E are required. The electric properties σ and ε can be obtained from the complex magnetic field vector H according to Eq. (2) [1]. In Eq. (2), ∂A is a closed path around a 2D area with constant κ = ε − iσ/ω. For a determination of electric properties, all three complex components of H are needed in Eq. (2). Since A is an arbitrarily orientated surface, this additional degree of freedom can be used to further reduce the required input quantities. Writing Eq. (2) for the Ayz surface and integrating along the x-direction yields Eq. (3). Hence, the dielectric properties κ can be calculated exactly from the measured B1 fields only.The electric fields E are related to the magnetic field vector and the electric properties as stated in Ampere’s law. However, to exactly determine E, all three complex components of H are required.