MR-guided high intensity focused ultrasound (MRgHIFU) ablation of bone lesions: Impact of the ultrasound focusing on the thermal curves and of dissection needles interposition on the ultrasound field

Objective To evaluate the impact of the ultrasound (US) focusing on the thermal curves in an in vitro (bone and cement) and in vivo experiment [1] . To compare these results with the acoustic and thermal simulation results [2] , [3] . To evaluate the US field distortion secondary to dissection needles. Material and methods MRgHIFU sonications were performed on a bone marrow (F = 1 MHz, P  = 25 W during 90 s), on the ilium of a pig (in vivo) (F = 1 MHz, P  = 79 W during 40 s) and a specimen of cement (F = 2.7 MHz, P  = 20 W during 45 s). Focusing was superficial (at the periosteum or on the cement), deep (in the marrow or behind the cement) or intermediate (cortical bone). An acoustic and thermal simulation (k-wave) was performed using the same model of bone marrow with two values of bone acoustic absorption coefficient (α = 4.7 et 9.9 dB/cm) and compared with the MRI thermometry. For spinal and pelvic lesions, a dissection may be necessary to protect nerves during thermal ablation [4] : therefore, the impact of dissection needles (22 to 13-gauge) on the US field was evaluated. Results A vaster and higher periosteal heating was observed with deep sonication ( Fig. 1 ). The lateral and vertical heating tended to appear with a delay, which can be explained by thermal inertia. These results were best reproduced with the simulation for α=4.7 dB/cm. Simulation with α = 9.9 dB/cm showed a much higher periosteal heating and different thermal curves. Cement and in vivo thermal curves were similar to the bone marrow results. No US field distortion was observed with 22 to 18-gauge needles. Conclusion For the treatment of painful bone metastases, deep focusing may be the most efficient. Simulations can reproduce the in vitro results. However, simulation results depend on tumoral bone α which remains difficult to measure. For spinal or pelvic lesions thermal ablation requiring a protection of neural structures [5] , dissection needles may be placed in the HIFU cone without any consequence at the condition that the liquid injected does not spread in front the US.