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Ultrasonics

Working Group 1.62

Thermo-acoustic sensor

Ultrasound exposure measurements for medical ultrasound systems are essential as regards aspects of safety and quality assurance. Spatial-peak temporal-averaged output intensities of ultrasound machines which have to be declared by the manufacturer according to IEC 61157 or for government approval are commonly derived from hydrophone measurements. In the case of modern multiple-mode diagnostic equipment the required hydrophone measurements are quite cumbersome and expensive. Much effort is necessary to find out the parameter settings and operation modes that produce the highest intensities, and synchronisation of the measurement system to all ultrasound pulses incident at the position of interest is a difficult task.

Thermoacoustic sensors as developed by the working group are a very simple and low-cost alternative for the determination of local temporal-averaged intensities. Here, inclusion of all contributing pulses and temporal averaging is done by the sensor inherently without the need for synchronisation to individual pulses and pulse sequences. The measurement technique is based on the transformation of the incident ultrasonic energy into heat inside a small-sized cylindrical absorber. The absorber is in part thermally insulated by an air-filled housing, and only the front face is in contact with the surrounding water to allow the ultrasound wave to get into the absorber. Due to this contact, the temperature of the front face is equal to that of the surrounding water, while at the rear side the temperature increases. Part of the heat produced inside the absorber permanently flows through the front face to the water. After a certain time of insonation, thermal equilibrium will appear between the heat produced by ultrasound absorption and the heat given off to the surrounding water. From the temperature enhancement at the rear side of the absorber at equilibrium measured by a small resistor temperature probe, the intensity averaged over the sensor cross section can be determined if the temperature-to-intensity transfer factor in dependence on the ultrasound frequency is known from previous calibration measurements. Use of this measurement technique can reduce the expenses for regulatory testing and may improve the results and the reliability of output parameters of diagnostic ultrasound equipment.

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