Magnetic-resonance-guided Focused Ultrasound Surgery in the Treatment of Oncology Patients — Fundamentals and Review of Early Clinical Applications

Magnetic-resonance-guided focused ultrasound surgery (MRgFUS) is a novel non-invasive therapeutic technique combining an MR imaging device and a focused ultrasound system, also called high-intensity focused ultrasound (HIFU). Concentration of ultrasound beams produces a highly localised (and controlled) increase in temperature (range 60–90oC) at the focal concentration (sonication) spot and thermal ablation of the tissue. MR allows for precise 3D segmentation of the target tissue and realtime measurement of the temperature at the selected target and surrounding areas. Following injection of a local anaesthetic at the treatment site, the patient lies on a therapeutic table inside the MR system. Acoustic coupling between the transducer and the body and local cooling is achieved by degassed water circulation and gel pads. The treatment takes two or four hours depending on the indication. Recovery time is about one hour after completion of the outpatient procedure. MRgFUS treatment is accomplished using a closed-loop strategy comprising three steps (see Figure 1). The first stage is 3D planning of the procedure, which includes identification of the desired target and definition of position, size and physical features (energy, frequency, duration) of sonication spots. Sonication spots are ellipsoidal, ranging from 1 to 10mm in diameter and from 10 to 50mm in length. The energy delivered ranges from 1,000 to 2,500J in bursts lasting 15–25 seconds. Sonications overlap to ensure that no tumour is left. This also includes consultation of a previous computer simulation to determine energy levels both at the target and in adjacent areas that need protecting. The second stage is the treatment stage, and involves delivery of the energy. Realtime temperature mapping yields the actual values achieved and provides active feedback during the procedure, allowing changes to be made to the sonication parameters if needed. The third stage of treatment is realtime evaluation of the results and a post-treatment contrast MR study. This allows for precise 3D evaluation of the degree of target destruction achieved. All three steps are performed in the same treatment session. Since there is no dose limit, the procedure can be repeated in the same or different sessions if required, depending on lesion size, type or location.