The acoustic output of diagnostic ultrasound scanners 3
repetition frequency (prf) remains constant. Thus, with this proviso, control settings that maximize pr will be those that also maximize Ispta. A large pr, and hence Ispta, is usually produced if the operator-controlled focus (which acts in the scan plane) is set close to the (fixed) elevation focus, since this increases the strength of focussing in a three-dimensional sense. However, as discussed above, setting the focus (or range-gate in the case of spectral Doppler) to a greater depth may well increase the transmission aperture and drive voltage, and hence produce even greater pressures and intensities near the scan-plane focus. Only practical measurement can establish which of these two effects will produce the greatest pr and Ispta. In spectral Doppler mode, a high Doppler frequency scale setting, or the selection of “high prf” mode, is likely to produce a higher power and Ispta. In themselves, these prf-related controls would not be expected to affect pr values, but manufacturers sometimes arrange for drive voltages, and hence pressure amplitudes, to be reduced for safety reasons if a high prf is selected. A short range-gate is likely to give higher pr values, since drive voltages are usually reduced as gate length increases, again for safety reasons. The effect of range-gate length on power and Ispta is difficult to predict, for the same reason. The use of harmonic imaging modes is often accompanied by higher pr (to generate more non-linearity) leading to greater output power and higher Ispta.
The highest pr in spectral Doppler mode is commonly associated with the shortest range-gate, and lowest prf. A high Doppler frequency scale setting, or the selection of “high prf” mode, is likely to produce a higher power and Ispta
3.5 Acoustic output values 3.5.1 Independent measurements of acoustic outputs Measurements of acoustic exposure parameters from diagnostic ultrasound systems have long been of interest for assessing their acoustic safety. The first surveys of acoustic outputs to include real-time B-scan array systems (c.f. static B-scanners) were published in 1978 (Carson et al., 1978) and 1985 (Duck et al., 1985), when this technology was relatively new. Although the number of array systems studied in these reports is small (two and four respectively) and some of the measurement methods differ from those now used, the values are of interest because they are so much lower than those reported more recently for current systems. These early reports are discussed in more detail later. The most comprehensive surveys of acoustic output values for ultrasound systems using real-time transducers were published or carried out in the 1990s (Duck and Martin, 1991; Henderson et al., 1995; Whittingham, 2000). These surveys were carried out in the UK by NHS medical physics departments, independently of the equipment manufacturers and relate to systems in active clinical use at the time of measurement. The measurement methods used were based on the principles described above, i.e. using a PVDF membrane hydrophone in a water bath to measure acoustic pressure and a RFB to measure acoustic power. In all 3 surveys, the active element of the hydrophone used was 0.5 mm in diameter. In these surveys, the spatial-peak values of pressure and the intensity parameters given were those measured in water at the point in the acoustic field where they achieved their 33
Several surveys of acoustic exposure parameters were carried out in the 1990s. These reported peak values of pressure and intensity measured in water under worst-case conditions