The piezoelectric effect was described in Chapter 2, in connection with its application, in inverse mode, for generation of acoustic fields. It is also of major importance in its application to acoustic detection. Piezoelectric detectors have been constructed in a wide variety of forms but, most commonly, for medical applications at least, they have the form of thin, parallel-sided plates or films, either plane or appropriately shaped.
For diagnostic use, such detectors, which in practice may also be the radiators or 'projectors' of pulse-echo systems, are usually plates cut to operate in fundamental thickness mode. A modern diagnostic transducer will often be sectioned into a number of elements arranged in a one- or two-dimensional array, but will normally be used with a sufficient number of such elements connected coherently to form an effective aperture equivalent to at least 30 acoustic wavelengths in the propagation medium: the minimum requirement for achieving satisfactory directivity. Diagnostic transducers are normally mounted (e.g. by epoxy resin casting) onto an acoustically matched and attenuating backing block, in order to damp resonances that would otherwise be set up by an incoming acoustic signal, with consequent loss of range resolution. It is common practice also to coat the outer surface of the transducer with one or more 2/4 acoustic matching layers (Section 1.7.3), a procedure analogous to 'blooming' of optical surfaces. This procedure, however, somewhat counteracts the effect of matched backing, in that it is essentially frequency sensitive and imposes a bandwidth limitation.
It cannot be assumed that a transducer constructed in this manner will have a uniform sensitivity over its aperture: the measured sensitivity even of single-element transducers commonly shows considerable variability, with possible fall-off near the perimeter and anomalies associated with unsuspected flaws in the adhesion of matching and backing layers. Such departures from 'ideal' behaviour will affect the spatial directivity pattern of the detector. Further discussion of the behaviour of piezoelectric detectors in diagnostic-type applications is taken up in Chapter 9.
The property of phase coherence over an extended aperture, which is vital for high spatial resolution diagnostic applications, can prove to be a major drawback when attempts are made to use a transducer in applications that require the quantitative detection of received energy in a manner that is independent of its direction of arrival. This problem can arise where a phasesensitive receiving transducer is used for quantitative measurement of propagation properties (e.g. attenuation coefficient) of a material such as human tissue, in which inhomogeneities in refractive index can significantly distort the arriving wave front. In such a situation, components of the wave front arriving at different points on the transducer face may tend to cancel at its electrical output, in a manner that is inappropriate for the exact measurement of total intercepted acoustic power (Busse & Miller 1981b). This provides an illustration of the somewhat differing requirements that can be placed on detectors intended for use in 'imaging' and 'metrology' applications respectively, whilst at the same time signalling a need for caution in the quantitative interpretation of imaging data.
Design criteria for detectors intended to be applied for dosimetry and measurement of material properties will generally differ from those intended for diagnostic use, particularly in relaxation of an extreme requirement for sensitivity: in diagnostic application there is always an underlying requirement to maximise the information derived from a given exposure to a patient. Thus, for narrow-band and multiple spot-frequency measurements on materials and solutions, quartz is a good detector in spite of its relatively low coupling coefficient, whilst for measurements directed at 'exposimetry' (see Section 3.8), where small physical size and uniform frequency response are generally important, it is convenient to use miniature ceramic or PVDF detectors ('hydrophones') in essentally non-resonant conditions.
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