Conclusions and future prospects

The physics of ultrasonic imaging is quite well understood. The performance of pulseecho grey-scale imaging systems is ultimately limited by the attenuation, nonlinearity and inhomogeneity of tissues and by the need to minimize the possibility of hazard by minimizing the ultrasonic exposure. Despite these limitations, however, there is scope for improvement. The rate of improvement in the capabilities of digital electronic circuits shows no sign of diminishing and, as costs continue to fall, so ultrasonic signal digitization is moving closer to the individual transducer element. The potential benefits of contrast agents are only just beginning to be explored and the reduction in clutter that can be obtained by second harmonic imaging, both without and with contrast agents, is an excellent example of very recent progress. New transducer materials will probably result in greater sensitivity and better noise performance and may make it possible to reduce exposures. Some of these materials may also be used in affordable two-dimensional transducer arrays for three-dimensional image acquisition.

Although there are many similarities between Doppler (i.e., frequency or phase) and timedomain processing for obtaining information about blood flow and tissue motion, the two techniques are often considered to be different and sometimes, in competition. Time-domain processing does have some advantages (e.g., there is no direct equivalent of ambiguity due to the exceeding of the Nyquist limit), but it is computationally more demanding than the Doppler technique. As computing becomes more accessible and less expensive, however, this is becoming less of a consideration. In situations in which the greatest sensitivity is required, colour power imaging has an important low-noise advantage over colour velocity imaging, whether performed by Doppler or time-domain processing.

Three-dimensional imaging improves the perception of anatomical relationships, whether by specialists reviewing complicated situations or by untrained observers with routine cases. There is scope for increasing the rate of three-dimensional image information acquisition by the use of multiple-beam systems with parallel processing. Image segmentation remains a problem, however, and this limits the opportunities for useful image display.

Amongst the specialized imaging methods, endoluminal techniques are already in routine use. Synthetic aperture imaging may become useful, at least until two-dimensional arrays have been further developed, and in microscanning applications. Computed tomography may be used to provide information about tissue refraction and attenuation, for the improvement of traditional pulse-echo images. Elasticity imaging is a very promising technique; it may be developed into quantitative telepalpation.

Although the primary contemporary role of ultrasonic imaging is in diagnosis, the method also has important applications in monitoring the progression and regression of disease, in some areas of screening, and in interventional procedures, both for localization and for guidance. Ultrasonic imaging is likely to become one of the preferred visualization techniques in minimally invasive surgery, because of its high speed and ease of use.

The safety record of ultrasonic imaging is impeccable. There is no reason to suppose that contemporary techniques employ levels of exposure that could cause biological damage, but it is prudent to be cautious and further research is justified.

Acknowledgments

I am grateful to Dr H S Andrews for providing figures 10 and 21. The following figures are from product literature: Advanced Technology Laboratories, figures 7, 9, 15-18; Medison, figure 20; Endosonics, figure 23; and General Electric, figure 26.

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