Recent trends in ultrasound technology that can be expected to develop further are the use of higher frequencies, often linked with intracavitary transducers, and exploitation of the power of modern microprocessors. Probes operating at frequencies as high as 30 MHz are being developed for specialised purposes such as intravascular ultrasound, intraductal probes for the pancreas and biliary tree, and even higher-frequency devices for examination of the skin and experimental small animal work.
Fast microprocessors allow better control of beam profiles and open the way to encode pulses so that their echoes can be distinguished from noise to improve image contrast, or be allowed to overlap but still be separated out for display, overcoming the conventional limitations of the PRF (see under 'Ultrasound Methods', above). This increases the frame rate and allows multiple focusing to be applied on transmit, as well as making it possible to run Doppler and imaging simultaneously at fast frame rates. More sophisticated image processing methods such as automatic time gain compensation, speckle reduction and clutter suppression can be applied in real time. An important trend is the development of plane wave ('ultrafast') imaging in which the transmitted wave is unfocussed; all the elements of the array are active on receive and the focussing is performed electronically. The advantage over the traditional line-by-line approach is a massive increase in frame rate (as high as 20,000 fps for small fields of view). This can be used to capture fast processes, as in tracking the shear waves in elastography and the motion of fetal heart valves and in Doppler. It can also be traded against multipulse techniques that offer improvements in fields such as microbubble detection.
3D reconstruction is now a real-time operation thanks to the combination of 2D arrays and fast signal processors. It is of proven value in obstetrics where the 3D display of complex fetal anatomy facilitates recognition of developmental anomalies, especially around the face; here the sharp difference between the echo-poor liquor and the tissue echoes makes segmentation (the automatic separation of surfaces) relatively simple. In many other potential applications, re-slicing is more promising than 3D display, for example to display the coronal (C-plane) to reveal the infiltrating structure of breast carcinomas.
Elasticity imaging or elastography is a new method that is interesting because of the very high contrast it offers between masses, especially tumours, and the host tissue. The principle is simple: image before and after applying a distorting force (stress) that moves the tissues by a few millimetres, and create an image of the tissue's response (strain) by comparing the two (Fig. 1). In principle, any imaging method can be used: ultrasound has the advantage that the transducer can be used to apply the stress and of working in real time while magnetic resonance has also been used successfully. For ultrasound, the transducer is usually used to apply the stress manually and the strain is detected by tracking the speckle pattern as it changes during probe-induced distortion. Another approach is to use the acoustic radiation force (impulse ARFI) to move the tissues and either detect the resulting distortion by speckle tracking or by detecting the shear wave that is set up—its speed is related to Young’s modulus. An advantage of this technique is that the results are quantitative. The information used to create the images is similar to that gained from clinical palpation except that it is much more sensitive, especially to deeper structures. It has already found clinical applications in the breast, liver and prostate, and is an active research area.
FIGURE 1 ■ Elastography. The echogenic lesion in the left pane has the appearances of a haemangioma (arrowhead). In the elastogram in the right pane, it is seen as a blue region against the liver's mainly green coloration; this indicates that the lesion is stiffer than the liver.
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