SAR and Temperature Evaluations for B1 Shimming at 7 tesla

Introduction In high field MRI, the increased interactions between the electromagnetic waves and the biological tissues may give rise to heat and thermal hazards. This is clearly apparent for multi-transmit array applications where the electromagnetic fields are heavily manipulated in the subject to be imaged. In this work we provide complete 3D predictions of the B1 + fields, MRI images, energy absorption by the head and corresponding specific absorption rate (SAR) distributions, temperature rise distributions over the whole head for MRI experiments associated with quadrature and multi-transmit (B1 shimming) operations at 7 tesla.. Analysis on the relation between B1 + field distributions, MR images, SAR and ΔT (temperature change due to the RF excitation) distributions is conducted. Methods In our work, the full wave method based on finite difference time domain (FDTD) was used to calculate B1 and the electric fields at 7 tesla. Using B1 shimming mechanisms, we improved B1 + field homogeneity over three dimensional (3D) brain regions while minimizing the total energy absorption as well as the local SAR by the human head. SAR and ΔT calculations were then simulated under the both excitation conditions: B1 shimming and with quadrature excitation. 1.SAR calculation: 10-gm (IEC limit) SAR was evaluated in order to obtain average intensity of the B1 + field over the brain region equal to 2.936 micro Tesla (μT), which is the B1 + field strength needed to produce a flip angle of π/2 with a 2-msec rectangular RF pulse. 2.Bio-heat model: The temperature changes within the head associated with B1 shimming and with quadrature excitation were calculated using the following bio-heat equation: density chip b p P SAR T T B A K t T C , 0 2 ) ( + + − − + ∇ = ∂ ∂ ρ ,where Cp (J/kg oC) denotes the specific heat, K (J/m s oC)