2.4. MRgFUS sonications and MR thermometry

MRgFUS heating sonications were performed using the T-Type phantoms inside a Siemens 3 T Prisma^FIT scanner (Siemens Healthcare, Erlangen, Germany). The phantoms (N = 2) were placed in the testing column seen in Figure 1(a) and suspended above the MRI-compatible 256-element phased-array transducer described above. An 8-channel flexible receive coil array was placed around the testing column to provide sufficient imaging signal-to-noise ratio. The transducer was coupled to the face of the phantom with degassed, deionized water. An MR-compatible power generator (Image Guided Therapy, Pessac, France) was used to control the MRgFUS system. For each T-Type phantom, three sonications were applied at the geometric focus of the transducer (20.84 s, 50.1 ± 0.1 acoustic W each). Approximate 5-minute cooling intervals were allowed between sonications. The thermal response of each sonication was monitored in real time using 3D MR temperature imaging (3D GRE with segmented EPI readout; TR/TE: 38/11 ms; resolution: 1.5 × 1.5 × 3 mm; FOV: 80 × 128 × 20 mm; flip angle: 15°; acquisition time: 5.5 s/volume; ETL: 7). All MR temperature imaging (MRTI) data were interpolated to 0.5-mm isotropic resolution [31] before analysis.