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Anecdotally, approximately 25% of all ultrasound transducers in clinical use have some form of undetected fault or structural defect1. Almost all of these faults may be attributed to ‘wear and tear’ and accidental damage. Predominantly, the type of issues are: failure of the crystal elements in the acoustic array, delamination of the lens, perforation of the lens and ingress of aqueous gel, and cable damage at the entry point to the scanhead. Traditionally, action to replace the transducer only occurs once a visible deterioration of the ultrasound image is noticeable by the technologists. Even then, experienced operators who have become conscious of the costs associated with probe replacement, will endeavour to optimise the image by adjusting the system controls or by repositioning the probe. Consequently it is important for the Sonographer to understand the relationship between transducer performance and resultant clinical data. For many years a qualitative test using a tissue equivalent phantom was the only reference available to a service engineer. |
However, phantom generated images were arbitrary in situations where a deterioration of spatial resolution was reported and rarely accurately repeatable. Further, the cause of degradation could not be isolated to the system or the probe.1 In modern arrays of 128 elements or greater, as few as two consecutive elements can result in a noticeable impact on the acoustic beam. As the number of ‘dead’ or desensitised elements increases, the degradation of image resolution increases significantly and consequently the efficacy of the examination is compromised. Currently, the only quantitative test for determining the integrity of an acoustic array is a digital transducer analyser, which interrogates the signal path of each individual crystal and analyses the returned echo waveform. The results are presented in a ‘sensitivity histogram’ which displays the acoustic performance of each individual crystal in the array (see fig. 2).
The analyser simulates the "front end" of the digital ultrasound platform and also measures : capacitance, pulse width at 20dB, centre frequency, fractional bandwidth and the frequency spectra of each individual element. Cumulatively, this information together with a high powered microscope examination of the probe structure, provides a comprehensive data set from which an accurate diagnostis can be made to determine the condition of the transducer.