Ultrasound strain and attenuation imaging

Ultrasound images in medicine usually display the amount of sound reflected back from anatomy (B,C or M-mode scans) or the flow in a vessel (Colour or Power Doppler). The former usually displayed as a grey-scale image, and the latter as a grey-scale image with a red-yellow or blue image superimposed onto it.

Attenuation could be an important diagnostic material parameter, but failure to adjust for variations in attenuation also causes several of the artefacts in B-scans. So called 'shadowing' (dark vertical strips below bright regions) and 'enhancement' (light vertical strips below dark regions) are both caused by varying attenuation. So if we can measure attenuation we could 'correct' conventional B-scans as well as displaying it in it's own right.

One way of reducing shadowing is to display compounded images by steering the ultrasound beam, as in Figure 1. Compounding improves the quality of the ultrasound data (Figure 2) and reduces the shadows, but it does not eliminate them.

Instead of averaging scans from different directions, we can process them to calculate the attenuation in the image. Having done so, we can then display a 'corrected' B-scan, and also the attenuation itself. Examples of this are shown below. At the moment, this only works well in in vitro data - methods for improving performance in in vivo data are an active research topic.

Strain imaging

It is also possible to deduce relative anatomical stiffness by looking at ultrasound data. First we take two B-scans of the same anatomy, but one with a very slightly different probe contact pressure. The contact pressure only has to vary by a very small amount, for instance compare the two B-scans in Figure 7 (a) and (b). We then look at small sections of the original radio-frequency data (on which the B-scan is based), like the black strips in the figure. We can then compare the exact locations of these sections in both B-scans using a very accurate algorithm. This gives us an estimate of relative vertical displacement between the two B-scans, as in Figure 7 (c).

We then look at the gradient of this displacement - this shows the strain at this location in the image. An image of strain (Figure 7 (d)) reveals the relative stiffness of the anatomy. This potentially has significant clinical impact, particularly in spotting small tumours which are often stiffer than the surrounding tissue.

The following are several examples of conventional B-scans and corresponding strain images.