A new technique for quickly mapping the intensity and distribution of ultrasound waves may be a boon for manufacturers who want to test the effect that device design changes have on intensity distribution. The tool was developed at the UK National Physical Laboratory (NPL) initially to help improve the quality of ultrasound treatment for soft tissue injuries.
The use of ultrasound in physiotherapy accelerates healing of tissue injuries. To be effective, the sound waves should be applied uniformly to the treatment site, but this does not happen in practice, notes NPL in a press release. This can affect quality of treatment and even cause damage. The lab has developed a way to quickly map the intensity and distribution of ultrasound, alerting physiotherapists to sharp hot-spots and allowing them to move the ultrasound head to smooth the intensity or reject it where it could cause more harm than good.
Highly resonant devices, the treatment heads vibrate in a complex pattern, leading to variations in acoustic pressure and acoustic intensity over the area being treated. The resulting so-called hot spots can cause excessive heating and even damage to the tissue.
The tool developed by NPL scientists helps to visualise the distribution and intensity of the acoustic energy through the use of thermochromic crystals. These materials lose their colour when they reach a specific trigger temperature. Importantly, the effect is reversible: the crystals regain their original colour on cooling.
The bottom layer of the tool is made up of the thermochromic crystals embedded in a polyurethane rubber matrix that absorbs sound. The top layer is colourless and is used to trap the heat within the tile. The tile heat produced 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 that 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 within seconds of exposure to the ultrasound.
The tiles can be used to quickly check for treatment head damage, asymmetric beam patterns or hot spots, and more simply to confirm whether the devices are actually working at all. The ability to gain relatively complex information from a simple and cost-effective device, in such a short period of time, should help improve the quality of physiotherapy ultrasound treatments, according to NPL. It could also lead to improvements in the design of ultrasound systems.