Performance and use of current sheet antennae for RF-hyperthermia of a phantom monitored by 3 tesla MR-thermography

Several MR-compatible current sheet antennae (CSA) of different height (h) (16 cm (l) x 8 cm (w) x 1-5 cm (h)) were built for simulated RF (96 MHz) hyperthermia of a medium-sized (12l) tissue-equivalent phantom inside a 3 tesla whole body tomograph. Prior to use, efficiencies of the CSA were determined by network analysis and by calorimetry. Depending on the height h of the CSA and on the thickness d(bolus) of the water bolus used for RF-coupling of the CSA to the lossy medium, their efficiency varied between 20-70% and the CSA with h = 3 cm was selected for simulated RF hyperthermia. During heating, spatial temperature distributions (20-42 degrees C) of five slices (voxel size 2 x 2 x 10mm(3)) were recorded intermittently within 4 s/slice by measuring the temperature dependent shift of the (1)H resonance frequency (125.32 MHz). A phased array consisting of two identical CSA produced distinctly different spatial temperature distributions at 0 and 180 degrees phase difference between both RF channels feeding the antennae. Within a one-dimensional heat diffusion model, the specific absorption rate (SAR) of the electromagnetic wave generated by a single antenna was deduced from the experimental data resulting in a penetration depth (1/e(2)) of approximately 4 cm.