Читать книгу The Physics and Technology of Diagnostic Ultrasound: A Practitioner's Guide - Robert Gill - Страница 23

The word "scattering" describes the interaction of ultrasound with small structures (such as red cells and capillaries) in the tissues (see Figure 2.9).

Figure 2.9 Scattered energy is sent in all directions. This means that the appearance of a scatterer is independent of the direction of incidence of the ultrasound. This is different to reflection, which is very dependent on the direction of the incident ultrasound.

It differs from reflection in two important ways:

 scattered energy is distributed in all directions, whereas reflected ultrasound goes in a single direction;

 the scattered energy is generally much weaker than reflected energy and so the echoes due to scattering are generally displayed in the image as low- to mid-level grey tones.

If you look at a typical ultrasound image you will see that the majority of the echo information in the image comes from scattering from within tissue, not reflection from interfaces between different tissues. Thus the nature of the scattered echoes and their appearance in the image are very important.

You will also notice that scattering produces a random granular echo texture in the image. To understand why this happens, consider Figure 2.10.

Figure 2.10 At one instant of time the transmitted pulse will "see" a volume of tissue (shown in light blue); the transducer will receive echoes from any scatterers that are within this volume. In soft tissue there will generally be a large number of scatterers within the volume, and the echo signal seen by the transducer will be the sum of the signals from all these scatterers.

This shows that at each instant the echo signal coming from soft tissue is actually the sum of the echoes from a number of individual scatterers that lie within the ultrasound pulse.

Since these scatterers are randomly positioned relative to each other, their echoes will add together randomly. This causes the echo signal received by the transducer to have a random variation in its amplitude. This phenomenon is termed "speckle" and it gives rise to the "echo texture" that we see in ultrasound images.

Speckle is a random process that is only indirectly related to the distribution of the scatterers. To highlight this, consider Figure 2.11. This is an image of an ultrasound "phantom" – a test object made of a gel material containing scatterers and designed to look like liver tissue when scanned. (The strong white echoes come from "point targets" that are used to check measurement accuracy and other aspects of equipment performance; they will be discussed in chapter 10.)

Figure 2.11 Scan of an ultrasound phantom (test object) showing speckle.

Notice how the echo texture varies with depth. Close to the transducer the texture is quite fine-grained whereas at greater depths it is much coarser. The phantom material, however, is uniform throughout the phantom, highlighting the fact that the speckle does not directly reflect a tissue property.