It is assumed that diagnostic ultrasound as used for imaging the fetus in utero is harmless for the baby, despite the fact that only 20-40% of the energy transmitted through the maternal skin is used to form the image. The power levels used in most forms of diagnosis are sufficiently low (~20mW) that its safety is a reasonable assumption. However, if the incident acoustic power is significantly increased, then the absorbed energy may be used to therapeutic advantage. This is the case for physiotherapy ultrasound where temperature rises of ~2-3oC are sought, for hyperthermic treatments of cancer where an increase of 6-8 oC is required, and for High Intensity Focused Ultrasound (HIFU) where temperatures in excess of 56oC are needed. Apart from tissue heating, ultrasound produces mechanical effects as it passes through tissue. These mechanical effects are also harnessed for use in therapy.
In HIFU, a high power ultrasound beam is focused at a distance from its source using either geometric or electronic means. A tight ellipsoidal focus can be formed, typically at low megahertz frequencies, ~ 1.5 – 2 cm long and 1.5 – 2mm in diameter. The primary aim of the HIFU exposure is to induce a temperature of at least 56oC for at least 1 second, as this leads to thermal ablation (coagulative necrosis) of the tissue. This gives a very sharp boundary between live and dead cells at the edge of the ablated volume. HIFU thus has the potential to provide a non-invasive treatment of deep-seated tumours. HIFU has been shown to be a useful technique for the treatment of tumours of the prostate, kidney, liver, breast, pancreas and bone. First HIFU treatments have recently been carried out in the brain. In this talk, HIFU and its current clinical status will be described. The challenges of accurate beam calibration and dosimetry will be discussed, and the topic of acoustic cavitation in this context will be addressed.
Gail ter Haar is a Physicist, with a DSc in clinical medicine. She is Professor of Therapeutic Ultrasound at the Institute of Cancer Research, and a Visiting Professor at Oxford. Her interests lie mainly in developing therapeutic applications of ultrasound for cancer (especially high intensity focused ultrasound, HIFU). In 2007 she was awarded the NHS Healthcare Scientist of the Year award for Innovation for her work on HIFU therapy. She is founding President of the International Society for Therapy Ultrasound (ISTU). Gail is an honorary member of BMUS, honorary fellow of the American Institute for Ultrasound in Medicine, and fellow of the Acoustical Society of America and IPEM. She is Deputy Editor of “Ultrasound in Medicine and Biology”, associate editor of “Ultrasonics” and on the editorial boards of International Journal of Hyperthermia & Journal of Therapeutic Ultrasound. Gail has written 5 books and 32 book chapters, and over 180 peer reviewed research papers.
About the Medical Physics Lunchtime Seminar:
The Medical Physics Lunchtime Seminar is normally held on the last Monday of every month (except Bank holidays) between 1 and 2 pm. The seminar series is open to all and features
principal investigators in and collaborators with the Department of Medical Physics and Bioengineering at UCL and UCLH. Staff and students within the department are strongly encouraged to attend to find out more about the various exciting activities going on in the department. The seminar series aims to introduce the career path of the speakers, cultivate a stronger identity within the department, share current research fields with the wider UCL community, and to inspire the next generation of scientists and students.