The tile consists of two-layers. The bottom layer is made up of the thermochromic crystals embedded in a polyurethane rubber matrix which 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 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. The image below shows the beam pattern from a physiotherapy ultrasound Transducer operating under the following conditions:
- 0.75 MHz – 5 seconds – 1 W/cm2 (nominal)
- 1.5 MHz – 10 seconds – 1 W/cm2 (nominal)
- 3 MHz – 15 seconds – 1 W/cm2 (nominal)
This tile provides information about the distribution of ultrasound energy coming off the transducer head. The system relies on embedding special color changing crystals that respond to heat into tiles. Applying ultrasound to the tiles provides a quick visual representation of the energy being delivered; offering clinicians the ability to gauge the effect on tissues, as well as helping engineers better understand the performance of their transducers.
Bajram Zeqiri, an NPL Science Fellow who led the project, describes 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.
This means that 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.