Recent advances in biomedical ultrasonic imaging techniques

Hai-Dong Liang, J. Alison Noble, Peter N. T. Wells

Nowadays in the UK, more than one quarter of all clinical imaging procedures use ultrasound and the number of ultrasonic scans that are performed each year exceeds all of those done by X-ray computed tomography, magnetic resonance imaging and radionuclide scanning combined [1]. Doubtless, the situation is similar in the rest of the world, but with ultrasound being even more used in less-developed countries, because of its relatively low cost. Diagnostic ultrasound has come into its ascendency over the last 60 years, mutating from a laboratory curiosity being developed and promoted by very few innovative engineers and visionary clinicians (for instance, in the USA [2], in Japan (see [3]), in Sweden [4] and in the UK [5]) into an indispensible tool in modern routine clinical practice.

Each of the wide range of different imaging modalities has its own individual characteristics. Usually in practice, the choice of the optimal modality is fairly obvious, because it depends on the clinical problem being investigated. Thus, for example, an X-ray is appropriate if a fracture is suspected, because X-rays are good at imaging bones. Likewise, for the management of cancer, positron emission tomography—a type of radionuclide scanning—can be invaluable because of its ability to visualize malignant tissue. The relative advantage of any particular modality lies essentially in the mechanism of the contrast in the images which it produces. In this respect, ultrasonic imaging is rather versatile. Primarily, the image contrast depends on differences in densities and speeds of sound, because these properties determine the scattering and reflectivity of tissue. In addition, flow, motion, strain and elasticity can be estimated ultrasonically and used to produce parametric images, for example, by colour maps superimposed on images of scattering and reflectivity. Moreover, ultrasonic imaging is fast and apparently safe, as well as being well liked by patients. Thus, the technology is particularly valuable in clinical practice in obstetrics and gynaecology, internal medicine, genitourinary studies, cardiology, vascular studies, dermatology and breast disease, but it is not of much use in the investigation of gas-containing or bony structures.

Ultrasonic imaging is a mature medical technology, the evolution of which has been the result of many years of biomedical engineering and clinical research. Moreover, the level of contemporary research activity is great and it covers a vast range of emerging opportunities. Very many ultrasound papers appear in medical journals devoted to the various clinical specialties and these generally report on the expansion of the diagnostic horizons, which results from novel applications and observations with existing—albeit often new—technologies. In organizing this Theme Issue of Interface Focus, however, we sought to compile papers that would be representative of advances in the technologies themselves, sometimes even before their clinical potentials have been explored. Of course, our intention certainly was not to produce what could equally well be a multi-authored book. We wanted each paper to be a scholarly peer-reviewed work in its own right, appealing to specialists in its field; and, at the same time, we wanted the Theme Issue to provide a coherent and timely snapshot of at least most of the main areas of contemporary research, aimed at a multi-disciplinary readership and, thus, to be able to catalyse new research collaborations.

In order to understand the context of the papers in this Theme Issue, the reader will have to have a grasp of the basic principles of ultrasonic imaging. The Theme Issue does not include a primer of this kind and, if one is needed, a good place to start is with the tutorial by Halliwell [6].

The papers which are included—referred to below—fall naturally into several categories, the first of which can be thought of as being concerned with advances in mainstream technologies. These are those on medical ultrasound systems [7], ultrasonic colour Doppler imaging [8], three-dimensional ultrasonic scanning [9], vascular ultrasound for atherosclerosis imaging [10] and quantitative contrast-enhanced ultrasonic imaging [11]. The second category consists of papers on technologies that are just now beginning to impact upon clinical practice: real-time quasi-static ultrasound elastography [12], acoustic radiation force-based elasticity imaging [13] and ultrasonic image analysis and image-guided interventions [14]. The third category consists of papers that discuss contemporary research, which is either not directly concerned with clinical studies (micro-ultrasound for preclinical imaging [15]), or which, although promising, is still seen as being some time away from having an impact: biomedical photoacoustic imaging [16], ultrasound-mediated optical tomography [17], continuous wave ultrasonic Doppler tomography [18] and thermal strain imaging [19]. Since the perceived safety of ultrasonic imaging is often cited as being one of its advantages over most other modalities, there is a paper in the final category that puts this into perspective: ultrasonic imaging: safety considerations [20].

The vagaries of peer review led to some particularly notable omissions. There is no paper on medical ultrasonic transducers. To remedy this, the reader is referred to the classic review by Hunt et al. [21] and to more recent papers by Brown et al. [22] on high-frequency linear arrays, Karaman et al. [23] on two-dimensional matrix arrays and Park et al. [24] on capacitive micromachined ultrasonic transducers. Also, it would have been better if a paper specifically on ultrasonic molecular imaging could have been included; but, by looking elsewhere, the review by Voigt [25] gives many illuminating insights.

Acknowledgements

We are very grateful to our friends and colleagues, all distinguished experts in their own fields, for responding positively to invitations to contribute to this Theme Issue. We also thank the anonymous referees, whose assessments ensured the quality of all the papers and whose insightful comments so markedly improved the final versions of many of them.

Footnotes

  • Received May 3, 2011.
  • Accepted May 16, 2011.

References