Smith R A
Division of Radiation Science and Acoustics, National Physical Laboratory, Teddington, Middlesex, UK.
Phys Med Biol. 1989 Nov;34(11):1593-607. doi: 10.1088/0031-9155/34/11/007.
A basic requirement for making measurements of medical ultrasonic fields using small sensors is that the sensor should be smaller than the ultrasonic wavelength. Until recently, the smallest commercially-available PVDF membrane hydrophone sensor had a diameter of 0.5 mm, which is larger that the wavelength in water for frequencies above 3 MHz. Thus many measurements have been made with hydrophones which are strictly too large. In this situation, averaging of the acoustic pressure over the active element can cause an underestimate of the spatial-peak acoustic pressure level. In the past, this error was estimated using theoretical models of the beam profile. However, these models make basic assumptions about both the ultrasonic field and the directional response of the hydrophone--assumptions which may not be valid in all diagnostic ultrasonic fields. GEC-Marconi membrane hydrophones with diameters as small as 0.1 mm have now been used to check these theories for diagnostic fields. This paper shows that the error resulting from the use of too large a hydrophone can be up to three times that predicted by current theories. Possible new correction methods are discussed for use in some situations. In other cases the errors can only be reduced by using these new hydrophones, particularly when the acoustic waveform is distorted by nonlinear propagation.
使用小型传感器测量医学超声场的一个基本要求是传感器应小于超声波长。直到最近，市售最小的聚偏二氟乙烯（PVDF）膜水听器传感器直径为0.5毫米，对于频率高于3兆赫的情况，该直径大于水中的波长。因此，许多测量是使用严格来说太大的水听器进行的。在这种情况下，对有源元件上的声压进行平均会导致空间峰值声压级被低估。过去，这种误差是使用波束轮廓的理论模型来估计的。然而，这些模型对超声场和水听器的方向响应都做了基本假设——这些假设在所有诊断超声场中可能并不成立。现已使用直径小至0.1毫米的GEC - 马可尼膜水听器来检验这些关于诊断场的理论。本文表明，使用过大水听器所产生的误差可能高达当前理论预测值的三倍。文中还讨论了在某些情况下可能使用的新校正方法。在其他情况下，只有使用这些新水听器才能减小误差，尤其是当声波形因非线性传播而失真时。
