Testing Tiles Designed for Ultrasound Treatment of Soft Tissue Injuries

Ideally, the sound waves should be applied uniformly to the treatment site, but it is well known that this does not occur typically in practice. This can affect quality of treatment and even cause damage.

The UK National Physical Laboratory (NPL; Teddington, UK) has developed a way to quickly map the distribution and intensity of ultrasound, allowing treatment heads to be used to administer the treatment more effectively. The application will signal physiotherapists to sharp “hot-spots,” allowing them to move the head to smooth the intensity or discard it where it could cause more harm than good. It also has potential for manufacturers, who could rapidly evaluate the effect that design alterations have on the intensity distribution.

Piezoelectric-based treatment heads, during treatment, transform electrical energy to mechanical energy, creating the vibrations needed to produce the ultrasound waves. These are transmitted into the target tissue with the aid of a thin layer of coupling gel. The treatment heads actually vibrate in a complex pattern, partly because of the fact that they are extremely resonant devices. This leads to variations in acoustic pressure and acoustic intensity over the treated region, resulting in “hot-spots,” which can cause over-heating and even damage to the tissue. Without carrying out the complicated and time-consuming process of mapping the acoustic field, it is very difficult to know precisely where the acoustic energy is going.

NPL scientists have devised an answer to this hurdle by developing a simple tool to help visualize the distribution and intensity of the acoustic energy. The approach works by using crystals that are thermochromic (they lose their color when heated up above a specific trigger temperature). Importantly, the effect is reversible; the crystals regain their original color on cooling.

The tool consists of two-layers; the bottom layer is comprised of the thermochromic crystals encapsulated in a polyurethane rubber matrix, which absorbs sound. The top layer is colorless and is employed to capture the heat within the tile. The tile heat generated by the acoustic energy is quickly and evenly trapped, and the crystals turn white as they reach the trigger temperature. This then produces a pattern on the tile, which represents the temperature distribution generated by the treatment head, which in turn relates to the spatial distribution of the acoustic intensity. The pattern can be clearly visible after only 10 seconds of exposure to the ultrasound.

Bajram Zeqiri, an NPL science fellow who led the project, described how you would test an ultrasound treatment head with the tiles. “In clinical practice the new ‘imager’ tiles would be used in much the same way you would treat a patient: by applying coupling gel to the treatment head, coupling it to the tile, switching on for typically 10 seconds, and then removing and observing the resulting image.”

The tiles can be used to quickly monitor for treatment head damage, asymmetric beam-patterns (hot-spots), and more simply to validate whether the devices are actually working at all. The capability to gain comparatively complicated data from a simple and cost-effective device, in such a short period of time, should help improve the quality of physiotherapy ultrasound treatments.

Related Links:
UK National Physical Laboratory