[Therapies by focused ultrasound]. / Thérapies par ultrasons focalisés.

Many techniques of thermotherapy have emerged over the last several years in the field of oncology using different types of physical agents, including ultrasound. Only ultrasound can target deep seated lesions non-invasively without need for percutaneous probe insertion. Depending on their utilization, it is possible to select either thermal effects, in a continuous mode, at low temperature (allowing thermo-induced biological effects) or at high temperature (allowing thermoablation), or mechanical effects, in a pulsed mode, at low energy level (allowing biological effects) or at high energy levels (histotripsy). Thermoablation by focused ultrasound is now developing fast for applications in many organs. It gained a well defined role for the treatment of prostatic cancer and uterine leiomyoma but needs to be better evaluated in other organs such as the breast. Treatment of abdominal tumors must still be considered as experimental as long as problems related to acoustic interfaces (produced by ribs and gas) and movement correction are not resolved. Biological applications of focused ultrasound are currently being explored and have a great long term potential.

Magnetic resonance temperature imaging.

Continuous, real-time, 3D temperature mapping during a hyperthermic procedure may provide (i) enhanced safety by visualizing temperature maps in and around the treated region, (ii) improved efficiency by adapting local energy deposition with feedback coupling algorithms and (iii) therapy end-points based on the accumulated thermal dose. Non-invasive mapping of temperature changes can be achieved with MRI and may be based on temperature dependent MRI parameters. The excellent linearity of the temperature dependency of the proton resonance frequency (PRF) and its near-independence with respect to tissue type make the PRF-based methods the preferred choice for many applications, in particular at mid- to-high field strength (> or =0.5 T). The PRF methods employ RF-spoiled gradient echo imaging methods and incorporate fat suppression techniques for most organs. A standard deviation of less than 1 degrees C, for a temporal resolution below 1 s and a spatial resolution of approximately 2 mm is feasible for immobile tissues. Special attention is paid to methods for reducing artifacts in MR temperature mapping caused by intra-scan and inter-scan motion and motion and temperature-induced susceptibility effects in mobile tissues. Real-time image processing and visualization techniques, together with accelerated MRI acquisition techniques, are described because of their potential for therapy guidance.