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TESTING OF ULTRASOUND TRASDUCERS BY USE OF THERMOCROMIC TILE

Propagation of ultrasound through biological structures produces thermal and nonthermal (mechanical) effects. Those effects can be used in medical application of low intensity ultrasound for physiotherapy [1]. The therapeutic ultrasound administration implies into a variety of energy and time dosage to achieve clinical results, which are associated to the increase of tissue temperature to healing rates. Tissue temperature increases due the intensity levels irradiated through the patient’s body. However, high intensity levels can generate excessive heat what can be dangerous to biological tissue. Thus quality control of physiotherapy ultrasound equipment performance plays a very important role. Acceptability of ultrasound beam for clinical use can be verified measuring uniformity of its acoustic intensity. Beam homogeneity can be quantified by the parameter called beam non- uniformity ratio (BNR), which represent the ratio of the highest intensity in the field to the average intensity. Value of the area over which ultrasonic power is emitted denotes the parameter effective radiating area (ERA). These parameters of ultrasound transducer could change during time and as such should be part of quality assurance program and checked on regular basis. The standardised method for beam uniformity verification is based on the scanning of the acoustic field in water tanks using miniature hydrophones. Unfortunately this method is not very convenient for use in hospital environment. An alternative method based on the use of thermochromic materials was proposed by Butterworth et al. [2]. Method consists in placing an acoustic absorber tile containing a thermochromic pigment in cross- section of the ultrasonic beam, forming an image of the temperature distribution. This method gives information about whole thermal pattern, which is related to the intensity of ultrasound, offering a qualitative measurement. While the thermochromic materials can provide rapid qualitative data to the naked eye, the quality of this data can be improved through image capture and post processing. In this paper the evaluation of this method in clinical conditions was presented. Seven physiotherapy ultrasound transducers clinically used in University Hospital Rijeka were tested by using thermochromic material. A systematic image capture protocol has been applied. Images are taken before and after ultrasonic exposure. All thermochromic images were postprocessed in order to calculate beam parameters ERA and BNR. The ERA and BNR were determined for transducers with frequency in range from 1 MHz to 3.3 MHz, and intensity range of 1W/cm2 to 3W/cm2. Experimental results and the comparisons with specified transducers performance will be presented.

ultrasound; thermocromic material; effective radiating area

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