engleski

A new thermal method for measuring the output power of medical ultrasound transducers employing the pyroelectric effect

Ultrasound power represents a key measurand relating to the safe use of medical ultrasound. The measurement of the radiation force experienced by a special target (absorbing or reflecting) that completely intercepts the acoustic beam has become the standardized method of determining acoustic output power. Whilst an excellent understanding of both the measurement protocol and the important sources of measurement uncertainty has developed, the small magnitude of the radiation force makes it difficult to apply for measurement of powers less than a few 10’s of mW, within a clinical environment. There is therefore a need for a new measurement technique of the required accuracy at low powers which can be easily applied. This paper describes a new measurement method developed at NPL that is a thermal technique, based on the conversion of acoustic energy into heat. The energy within the ultrasound beam is absorbed within a layer of a special polyurethane rubber material, whose absorption is sufficiently high to ensure deposition occurs within a few mm of the water-absorber interface. The rate of change of temperature at the surface is monitored using the pyroelectric voltage generated within a thin membrane of piezoelectric polymer, intimately bonded to the absorber and sufficiently large to intercept the whole beam. The change in the pyroelectric waveform generated by the sensor at times immediately following Switch ON and Switch OFF of the transducer is proportional to the delivered ultrasound power. This paper describes the new method, along with an evaluation of its performance through measurements on a range of applied fields and an underpinning theoretical model. Studies point to the importance of the acoustic and thermal properties of the absorbing backing layer in controlling the sensor response and have revealed several key features of the method: its high sensitivity, suggesting instruments capable of measuring powers as low as a few mW, and the possibility of measuring acoustic intensity, through employment of a sensor whose active area is small in relation to the acoustic pressure distribution. Currently, acoustic intensity can only be derived using hydrophones, by making fundamental assumptions regarding the plane-wave nature of the acoustic field.

ultrasound power; acoustic intensity; thermal method; pyroelectric effect

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