IEC TR 62649:2010 pdf download – Requirements for measurement standards for high intensity therapeutic ultrasound (HITU) devices
3.2 Very high pressures
Pressures above the cavitation threshold for the measurement medium (usually water) can produce bubbles as dissolved gas is drawn out of solution. Three main problems may then arise: first, the bubbles formed may partly shield the sensor from the ultrasound field; secondly, violent bubble activity can damage or destroy the sensor. The occurrence of both of these effects can be minimised by removing dissolved gas and particulate matter from the measurement medium, but it may be difficult to maintain sufficient purity for a prolonged period.
Thirdly, because of the high pressure levels involved, a proportionately larger fraction of the pressure spectrum is distributed into higher harmonics compared to a bubble-less medium. There is also the risk of direct mechanical effects on the sensor itself due to large compressional and tensional forces. This is most likely to be a problem when there are weak points between different components of the sensor (for instance, if there is delamination of the glue layer in a bilaminar hydrophone).
3.3 Very high intensities Energy absorbed from the ultrasound beam heats the sensor and this may affect its performance or even destroy it. For instance, the sensitivity of a membrane hydrophone can change if it is heated close to its Curie temperature. For polyvinylidenefluoride (pvdf), the most widely used hydrophone material, depolarisation occurs progressively with time at temperatures above about 70 °C and almost immediately at 1 1 0 °C. The thinness of membrane hydrophones will offer some protection against thermal damage because heat is very quickly lost to the surrounding medium. However, the sensitivity of pvdf hydrophones is temperature dependent and this change will be an additional source of uncertainty. Probe hydrophones may face greater risk and absorbing radiation force balance targets will certainly be damaged unless great care is taken to dissipate the absorbed energy. Heating can be reduced by generating low duty cycle toneburst ultrasound rather than continuous-wave. However, HITU transducers are generally only weakly damped and, consequently, may take many acoustic cycles for the pressure ‘ring-up’ at the start of the toneburst; there is an equivalent ‘ring-down’ at the end of the toneburst. This must be accounted for by scaling results from toneburst to the c.w. situation. In addition, since typically 30-50 % of the electrical energy is dissipated within the transducer, its temperature and properties will change with time during operation. Using toneburst mode will reduce this self-heating and may lead to significant differences in acoustic output compared to the c.w. case.
3.4 Strong focusing In a focused field, two important plane-wave assumptions are not valid. Firstly, the particle velocity is not strictly in phase with the pressure, meaning that the local intensity is not truly proportional to the square of the pressure; hence, there is an increased uncertainty when deriving the intensity from a pressure measurement with a hydrophone. Secondly, the radiation force on a target placed in the field is no longer determined solely by the properties of the target and the total ultrasound power. The geometry of the field also plays a role, especially for the widely-used conical reflecting targets; absorbing targets are preferable provided that they are not damaged by excessive heating.
There is also a third effect which relates to the directional response of a hydrophone. In a plane wave, the hydrophone can be aligned so that the wave is incident in the preferred direction for the hydrophone (usually perpendicular to the plane of the sensing element). In a focused field, the pressure at the hydrophone can be considered as the superposition of wavelets with a relative phase and an angular distribution which are determined by the transducer geometry and its distance from the hydrophone. An ideal hydrophone would respond equally to wavelets from any direction and the output signal would be proportional to the sum of the wavelets. A real hydrophone, on the other hand, has a sensitivity which depends on the angle of incidence of the wavefront and the output voltage therefore depends on a weighted summation of the wavelets. This means that the output voltage waveform is different in magnitude and shape from the pressure waveform. This distortion increases with the large physical apertures and short focal lengths frequently used for therapeutic applications. Furthermore, the non-ideal nature of real transducers which have amplitude and phase variations across their apertures introduce additional complexities into the measurement process. There is no information available on the measurement uncertainties in cases where the field is generated by two or more widely separated transducers, or where the point of measurement lies within or close to the volume defined by the surface of the transducer or transducers.IEC TR 62649 pdf download.
