Eur Radiol (2007) 17: 2505–2511 DOI 10.1007/s00330-007-0644-8 UROGENITAL Alexander Chapman Gail ter Haar Received: 30 January 2007 Revised: 9 March 2007 Accepted: 23 March 2007 Published online: 1 May 2007 # Springer-Verlag 2007 Thermal ablation of uterine fibroids using MR-guided focused ultrasound-a truly non-invasive treatment modality Abstract Uterine fibroids are a sig- nificant source of morbidity for women of reproductive age. Defini- tive treatment has traditionally been a hysterectomy, but increasingly women are not prepared to undergo such an invasive procedure for a benign and usually self-limiting condition. Although a number of minimally in- vasive techniques are now available, focused ultrasound has a considerable advantage over them as it is completely non-invasive and does not require an anaesthetic. Improvements in imaging techniques, particularly magnetic reso- nance imaging (MRI), have enabled the accurate planning, targeting and monitoring of treatments. We review the early experience of focused ultra- sound surgery for the treatment of fibroids, and, in particular, the results of the recent phase I, II and III multi- centre clinical trials. These trials and other studies which demonstrate that MR-guided focused ultrasound abla- tion is feasible, safe and appears to have an efficacy that is comparable with other treatment modalities are described. This technique has the advantages of being non-invasive and being deliverable as an out-patient procedure. Keywords Focused ultrasound . Fibroids . Leiomyoma . HIFU . MR- guided . Ablation Introduction Ultrasound is a pressure wave that operates at a frequency above the threshold for human hearing. Medical diagnostic ultrasound operates at frequencies of ~2–20 MHz, while therapeutic applications usually employ 0.5–5 MHz. In focused ultrasound ablation (FUS, also referred to as high intensity focused ultrasound, HIFU) the ultrasound energy is brought to a tight focus within a target volume. If the energy concentrated within the focus is sufficient to raise the tissue temperature above 56°C during a 1 s or longer exposure, instantaneous coagulative necrosis can be achieved. A single 2-s exposure results in an ellipsoidal lesion, typically 1.5–2 cm in length and 1.5–2 mm in diameter. As the ultrasound wave propagates through the tissues, energy is lost due to absorption and scatter. The motion of the tissue in response to the ultrasound pressure wave leads to frictional heating. In the absence of blood flow, the rate of heating is determined by the absorption coefficient of the tissue, and the intensity and frequency of the wave. At the intensity levels used for focused ultrasound ablation, beam propagation is non-linear. The higher harmonic compo- nents of the fundamental frequency of the beam generated by the non-linearity are absorbed more strongly than the fundamental itself, thus leading to greater heating than would be observed for purely linear propagation. The peak rarefaction (negative) pressures of the ultra- sound wave may draw gases out of solution in the tissue, leading to the formation of bubbles. The activity of bubbles within an ultrasound field is known as acoustic cavitation and can be classified “non-inertial” or “inertial”. The type of cavitation depends on bubble size compared to the resonant size at the incident ultrasound frequency. Non- inertial cavitation refers to the sustained oscillations of the bubbles, while inertial cavitation refers to the expansion and then violent collapse of bubbles. The heating consequences of cavitation results both from the scattering A. Chapman (*) . G. ter Haar Joint Department of Physics, The Royal Marsden Hospital: Institute of Cancer Research, Downs Road, Sutton, Surrey, SM2 5PT, UK e-mail: alex.chapman@icr.ac.uk e-mail: gail.terhaar@icr.ac.uk A. Chapman Department of Urology, The Royal Marsden Hospital: Institute of Cancer Research, Downs Road, Sutton, Surrey, SM2 5PT, UK