ReviewTherapeutic ultrasound
Introduction
The ability of ultrasound to interact with tissue to produce biological changes has been known for a long time (Wood and Loomis, 1927). Much of the early drive to understand these interactions came from an interest in harnessing ultrasonically induced changes for therapeutic benefit. More recently, the concern has been to understand any possible hazard that may arise from diagnostic ultrasound imaging.
It is convenient to divide therapeutic ultrasound into two classes, applications that use ‘low’ intensity (0.125–3 W cm−2) and those using ‘high’ intensities (≥5 W cm−2). The intention of the lower intensity treatments is to stimulate normal physiological responses to injury, or to accelerate some processes such as the transport of drugs across the skin. The purpose of the high intensity treatments is rather to selectively destroy tissue in a controlled fashion. An alternative classification scheme would be in terms of applications for which the sound is directly coupled to the tissue via a coupling medium, and those for which the ultrasound transducer is coupled to a waveguide terminated with a tool specifically designed for the task required.
Section snippets
Physiological basis for therapeutic ultrasound
Two types of mechanism are commonly invoked to explain the effects produced by therapeutic ultrasound. These are classed as thermal and non-thermal effects. It is, however, often extremely difficult to identify positively the mechanisms involved in producing biological change, and indeed to isolate non-thermal effects from thermal ones.
Physiotherapy
Ultrasound was originally introduced into physiotherapy as an alternative diathermy technique, to compete with hot pack, microwave and radiofrequency heating. Its main use has been in the treatment of soft tissue injuries, but it has also been applied to bone and joint conditions and to accelerate wound healing. Over the years, there has been a move toward reducing the average intensities used, either by reducing the power to the transducer, or by the use of pulsing regimes. This reflects an
Ultrasound for cancer therapy
The potential use of ultrasound for the treatment of cancer was first mentioned in the literature in 1933 when it was reported that it had no specific effect on Ehrlich’s carcinoma (Szent-Gyorgi, 1933). After this, enthusiasm for this approach has come in waves with, for example, the first report of successful application in human skin metastases in 1944 (Horvath, 1944), while a conference in Erlangen in 1944 concluded that enthusiasm for its potential was not backed up by clinical results and
Focused ultrasound surgery
A surgical technique that is to replace a conventional surgical knife or scalpel should be reproducible and controllable in its ability to destroy tissue, it should be able to affect a sharply defined region only, and should preferably be quick. High intensity focused ultrasound beams have most of these qualities. The tissue ablation technique that makes use of such beams is known interchangeably as high intensity focused ultrasound (HIFU) or focused ultrasound surgery (FUS).
The principle
Tool surgery
It is not the remit of this review to provide in-depth consideration of the therapeutic uses of ultrasonically driven tools. Tools have been designed for tissue debulking and aspiration, and for tissue cutting. The use of intravascular ultrasonically driven wire tips have been investigated, both for the acceleration of dissolution of blood clots and for the acceleration of dissolution of clots by thrombolytic drugs (Siegel, 1996, Tachibana and Tachibana, 1996). Specialised tips have been
Conclusions
It is often forgotten that the first uses for ultrasound in the medical field were for therapeutic applications. Despite considerable early interest in its potential to produce beneficial effects, enthusiasm for many of the techniques proposed was not sustained. An increased understanding of the way in which ultrasound interacts with tissue has led to a resurgence of interest in ultrasound therapy, and to a better understanding of the appropriate exposure regimes for the generation of specific
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