12.10.2009
Thermal sensors developed at PTB can be favourably applied to the measurement of intensity profiles of modern diagnostic ultrasound devices and complement the conventional hydrophone measurement technique.
Ultrasound exposure measurements for medical ultrasound systems are essential as regards the aspects of patient safety and quality assurance. International standards require the characterization of output beams regarding, for instance, power, intensity, peak pressure, and lateral dimension. Spatial-peak temporal-average intensities are particularly important, since they are are linked to a potential thermal hazard for the patient. Usually, local intensity parameters are derived from hydrophone measurements. However, in the case of modern multi-mode diagnostic ultrasound systems, the necessary measurement procedures can become technically very difficult, time consuming and expensive. Instead of running the actual operation mode as it is used in medical practice, exposure measurements in these cases may only be possible in special engineering modes and only by the manufacturer.
The practical applicability of a more simple and low-cost thermal intensity sensor technique developed at PTB has now been demonstrated by detailed comparison measurements. The recent miniaturization of the sensors offers a spatial resolution of 0.6 mm, which is similar to commonly used hydrophones. The method has shown to be an adequate alternative to the determination of ultrasound time-averaged intensity distributions, i.e. the measurement of axial and lateral beam profiles. The advantages of the thermal sensor technique are especially evident for intensity measurements in the case of scanning and combined modes of the diagnostic device, where the synchronization between hydrophone measurements and the complex pulse emission pattern can be difficult if not impossible. Since the thermal sensor technique is inherently temporal-averaging, there is no longer a need for the determination of the individual repetition rates of all the different pressure waveforms occurring at each measurement location. Even upcoming diagnostic systems which may support operation modes without any fixed pulse repetition or scan repetition rates, can easily be characterized using the novel method.
Figure 1: Comparison of axial intensity beam plots measured with the thermal sensor with hydrophone-based results for a convex array transducer operated in pulse Doppler mode with different focal settings zroi. Uncertainties in the range of ±20% to ±30% are usual and accepted for intensity values derived from hydrophone measurements. The deviations obtained were smaller than these uncertainties, except for short distances z where multiple reflections lead to disturbances.
Contact person:
Dr. V. Wilkens, FB 1.6, AG 1.62, e-mail: Volker.Wilkens@ptb.de