High-intensity focused ultrasound therapy: an overview for radiologists

Abstract

High-intensity focused ultrasound therapy is a novel, emerging, therapeutic modality that uses ultrasound waves, propagated through tissue media, as carriers of energy. This completely non-invasive technology has great potential for tumor ablation as well as hemostasis, thrombolysis and targeted drug/gene delivery. However, the application of this technology still has many drawbacks. It is expected that current obstacles to implementation will be resolved in the near future. In this review, we provide an overview of high-intensity focused ultrasound therapy from the basic physics to recent clinical studies with an interventional radiologist's perspective for the purpose of improving the general understanding of this cutting-edge technology as well as speculating on future developments.

Fig. 1

Important physical parameters of sound physics and their relations.

Fig. 2

Basic concept of HIFU-induced tissue change by hyperthermia. As US waves are focused onto small spot, acoustic pressure is rapidly elevated near focus where tissue temperatures are also raised to level that is sufficient for thermotherapeutic effects, resulting in coagulation necrosis.

Fig. 3

Relationships between sound pressure, power, energy, and intensity. Sonic intensity, defined as energy passing through unit area within unit time, is derived from plane wave. As seen in equation, intensity is proportional to square of acoustic pressure and is also function of property of medium (density and speed of sound) through which waves are propagated.

Fig. 4

Various methods of focusing US waves: A. Spherically-curved transducer, B. Flat transducer with interchangeable lens, C. Phased-array transducer causing only steering, and D. Phased-array transducer causing steering and focusing at same time.

Fig. 5

Classical thermal lesion formed by focused US surgery (US absorption only) on porcine liver specimen. A. Cigar-shaped thermal lesion is formed at focal zone of US wave pathway (two overlaid triangles) following HIFU single exposure. B. Final thermal lesion after stacking each single lesion. Single lesions are much smaller than clinically common tumors and therefore each thermal lesion should be stacked compactly without leaving intervening viable tissue. This lesion can cover entire pathological lesion as well as has very sharp margin that could be controlled easily.

Fig. 6

Schematic drawing of US-induced gene therapy. When plasmid DNA-containing microbubbles are passed through blood vessels adjacent to diseased cells, insonated US waves rupture microbubbles and release plasmid DNA. Released DNA penetrates into cell through membranes by means of sonoporation.

Fig. 7

Strategy of transcranial focused US therapy.