Comment on New Technology - Ultrasound Elastography

Table of Contents

Jeffrey C. Bamber

Ancient cultures extending back many thousands of years used palpation to assess the mechanical properties of tissues, and thus detect and characterise disease or injury. Hippocrates, for example, is reported to have advised that the first issue to address when presented with a patient who has sustained a head injury is locating the wound and determining whether the cranium underneath is strong or weak. Palpation was used to determine "whether the bone is denuded of flesh or not." The ancient Egyptians, for example, also used palpation; the technique is mentioned in the Edwin Smith Papyrus, the world's earliest known example of medical literature, dated around 1900BC.

Simple palpation continues to be of value in modern medicine, both practised by doctors and as a technique for self-examination, but palpation is limited to a few accessible tissues and organs, and the interpretation of the information sensed by the fingers is highly subjective. Ultrasound elastography aims to display images that are related to a broad range of parameters that describe the spatial and temporal variations in tissue viscoelasticity. It does so by processing time-varying echo data to extract the spatial and/or temporal variation of a stress-induced tissue displacement or strain. There is a range of signal processing techniques for extracting the tissue displacements or strains, such as searching for the peak of the cross-correlation between two echo patterns, seeking the time shift that maintains zero phase change, or even measuring the phase change itself, which is Doppler. There are many ways of applying the stress (force), and there are many approaches to extracting properties for image display. The variations combinations and refinements of these possibilities provides ample opportunity for ongoing research, much of which is discussed each year at the International Conference on the Ultrasound Measurement and Imaging of Tissue Elasticity, now in its 7th year, with past proceedings available on the web site http://www.uth.tmc.edu/schools/med/rad/elasto/conference/.

In recent years the method of elastography, in an early form, has emerged as an option on a number of commercial ultrasound systems, and is starting to prove clinically valuable in many areas, particularly for example in assisting breast cancer diagnosis, or in guiding minimally invasive treatment of prostate disease. This is a key moment in the success of any new technique, where the method may be evaluated by new users outside of the laboratories in which it was developed, and elastography is performing well. Reports at major conferences such as RSNA and ECR are coming from groups achieving successes that confirm earlier studies carried out by those more experienced in elastography, in using it as an adjunct to conventional ultrasound for improving the assessment of breast abnormalities.

The history of the development of ultrasound elastography did not jump from palpation to where we are now with commercial implementation; this transition has taken place over a 30 year period, with contributions from workers in France and Belgium during the 1970s, who pioneered the assessment of tissue stiffness by observation of M-mode features during palpation, then European and Japanese who did the same for B-mode "dynamic features" during the 1980s, followed by workers from various countries during the 1980s who developed algorithms for processing ultrasonic echoes for the measurement of tissue displacement, and eventually of strain or strain-rate during the 1990s. The current commercially available "freehand elastography" derives its name from the use of a hand-held ultrasound transducer to induce tissue strain by applying gentle pressure to the surface of the body, or to use internal cardiovascular pulsations as the source of stress, while the system displays either a grey-scale image of tissue strain alongside the conventional echo image, or the two are combined using a colour overlay for strain. The term freehand elasticity imaging was first used by the author in 1996, and arguably the first elasticity image made in this way was shown at the annual conference of the American Institute for Ultrasound in Medicine in 1988.

The usefulness of elastography, even in its present form, is likely to improve quite quickly, as we learn to take advantage of the information that it provides and as new technology allows it to move into three dimensions, which will have a substantial impact on the quality of elastograms as well as ability to interpret elastographic artefacts. Nevertheless, in its current form it would remain a strongly subjective technique and would continue, as with palpation, to require interpretive skills to be learnt. There are good reasons to believe that a more quantitative and objective analysis will lead to clinically more valuable measures of tissue composition, function or state, with images that are easier to interpret. This is where the future of elastography is leading, employing advanced models of tissues and their mechanical behaviour to convert strain data to images of tissue properties such as shear modulus, compressibility, nonlinearity, anisotropy, friction at mechanical discontinuities, as well as properties that determine viscoelastic and poroelastic behaviour (related to microvascular and interstitial fluid flow). It is tempting to consider that some of the dreams past, of the subject of tissue characterisation, may yet come to fruition.

An important subgroup of such advanced methods makes use of shear wave propagation. Conventional echography uses ultrasound, which travels mostly as a longitudinal wave. The mechanical properties mentioned above are governed by the shear properties, especially the shear modulus, and a measurement of the speed with which a shear wave propagates provides quite a direct measurement of the shear modulus. Because shear waves in soft tissue travel about one thousand times slower than longitudinal waves, ultrasound images can be used to observe the passage of a shear wave and hence its speed, so long as the ultrasound echoes can be generated frequently enough. One commercial device has already existed for a number of years that uses such concepts to characterise liver disease; a "thumper" on the surface of the abdomen launches a shear wave into the body and ultrasound A-scans (at thousands of repetitions per second) are used to measure its speed of propagation in the liver, which is strongly related to degree of fibrosis. Advanced (parallel receive) electronics is now making it possible to acquire whole echograms at thousands of frames per second, so that it is possible to make full images of the shear modulus. Alternatively, it is possible to "slow down" the shear waves, by having two interfering continuous shear wave fields generated by vibrating sources, so that even a conventional scanner can be used to follow their motion.

A final option in the current "bag of tricks" is to generate the stress using ultrasound radiation force. A short burst of highly focused ultrasound creates a small (few tends of microns) transient displacement of the tissue, which can be detected using the kinds of echo signal processing employed in elastography. This can potentially be used create elastograms of deep or fast-moving tissue, such as the heart or liver, or even tissue beyond rigid obstructions such as the skull. Two commercial manufacturers are due to release products of this type later in 2008. Meanwhile, research in elastography seems set to continue to gain pace, building on the technical and clinical success to date.

Jeffrey C. Bamber

Member of EFSUMB Publications Committee, Head of Ultrasound and Optical Imaging, Joint Department of Physics, Institute of Cancer Research and Royal Marsden Hospital, Downs Road, Sutton, Surrey, SM2 5PT, UK.