Last year, writing in the 50th anniversary issue of our sister journal, Peter Wells, distinguished research professor at Cardiff University in Wales, argued that ultrasonic imaging stands alongside X-ray CT and MRI as "one of the most disruptive medical applications of physics and engineering, having profoundly changed the practice of medicine in the last 50 years". He concluded by adding that "the prospects are indeed brighter than ever before" (Phys. Med. Biol. 51 R83).

Wells would seem to have a point. Ultrasound accounts for around one in four of all medical imaging procedures worldwide, while the rise and rise of 3D ultrasound - in obstetrics, cardiac applications and interventional procedures - is hardly suggestive of a modality in the dumps (see Making waves: the 3D ultrasound pioneer). Quite the opposite: Frost & Sullivan, a technology consultancy, reckons the European market for diagnostic ultrasound systems will be worth around $1.1 billion by 2013 - up from $719 million in 2006.

Hot stuff

If the commercial and clinical prospects for diagnostic ultrasound look robust, the same can also be said for therapeutic ultrasound, an emerging family of technologies that's developing a momentum all of its own. In broad terms, therapeutic ultrasound shows promise in treating a variety of diseases, including stroke, Parkinson's disease, internal bleeding and cancer. The payback: short recovery times and few side-effects.

At the forefront of this therapeutic revolution is high-intensity focused ultrasound (HIFU), a non-invasive cancer treatment that uses ultrasound to destroy tumours. Right now, HIFU is only used clinically to induce thermal ablation of the target site - i.e. high-intensity ultrasound beams are focused within an area of malignant tissue, heating it up to a high enough temperature to instantly destroy the tumour cells. What's more, because the threshold ultrasound intensity is only reached at the focal point, overlying and surrounding healthy tissue is left unharmed.

Progress is encouraging. According to a report published earlier this year by US consulting company MedDevice Concepts, around 100,000 patients have to date been treated with HIFU. This includes 50,000 patients in China treated for benign and malignant tumours and 14,000 Europeans treated for prostate cancer, as well as 30,000 patients in 45 other (non-US) countries undergoing adipose-tissue (fat cells) removal.

In the works, however, are all manner of variations on the HIFU theme - albeit most of them early-stage R&D just now. In August, for example, medicalphysicsweb reported how researchers at Duke University (Durham, NC) are applying ultrasound in such a way as to induce mechanical damage to tumour cells (see HIFU shakes up cancer treatment). In this approach, the sound waves effectively shake the tumour until its cell membranes rupture, releasing intracellular proteins, which in turn trigger an immune response. As well as targeting the primary tumour, the Duke team thinks that breaking apart the tumour cells "may have an even more significant impact in suppressing cancer metastasis by waking up the immune system".

Similar principles are being applied in a different clinical context - the ultrasound-mediated treatment of ischaemic stroke - by ImaRx Therapeutics (Tucson, AZ) in partnership with Philips Medical Systems (the Netherlands). ImaRx's "SonoLysis" technology exploits ultrasound-activated microbubbles, with or without a thrombolytic drug, to break up blood clots and restore blood flow to oxygen-deprived tissues (see Can bubbles blow blood clots to bits?). With SonoLysis, the submicron bubbles penetrate the blood clot so that when ultrasound is applied to the clot region it causes expansion, contraction and cavitation of the bubbles - processes that effectively "shake" the clot apart.

Sound medicine

Meanwhile, scientists at the University of Washington (Seattle, WA) are applying HIFU's heating power not to destroy tumours, but to stop bleeding (a process known as haemostasis). Shahram Vaezy, associate professor of bioengineering at the University of Washington, and colleagues have already demonstrated HIFU's potential as a safe, effective way to mend punctured blood vessels and repair lacerations to the liver and spleen and have now turned their attention to wound repair in the lung (see Stitch-free wound repair? Try ultrasound).

The Seattle team's work on haemostasis will feature in a presentation at the 154th annual meeting of the Acoustical Society of America (ASA) in New Orleans, LA, later this month. A raft of other papers will showcase the considerable progress that's being made with therapeutic ultrasound in diverse clinical applications. Among the headline themes are the use of ultrasound to promote uptake of chemotherapy drugs in brain tissue; the combination of MR image guidance and HIFU to treat tumours; and the development of acoustic "miniscalpels" that deliver power to deep-lying tissues without heating.

If the ASA meeting is anything to go by, the development effort underpinning therapeutic ultrasound looks to be in rude health. In fact, the prospects for clinical progress appear "brighter than ever before".