In the first of this series on the basics of using MSK ultrasound, we discussed how you can orientate yourself to the transverse/longitudinal image on ultrasound. I thought it would be useful to next discuss the appearance of different tissues. This is another key aspect to help orientation, as when you can appreciate the relationship of tendons to bones, and tie that together with your anatomical knowledge you can quickly develop a blueprint in your mind of where you are and what you are looking at.
I will cover the key tissues that you will encounter
Bone is represented as a very bright structure and appears 'hyperechoic'. It creates a significant acoustic impedence mismatch and therefore is very reflective and shows as bright white (hyperechoic) on the image. No sound waves can pass through bone and therefore deep to it will always be dark.
Muscle presents as hypoechoic, with some internal signals as a result of collagen fibres. The echotexture of normal skeletal muscles consists of a relatively dark or 'hypoechoic' background reflecting muscle fascicles along with linear hyperechoic strands related to fibroadipose septa (perimysium).
Normal tendons as seen in Figure 3 and 4 with the achilles, show tightly packed hyperechoic lines representing the fibrisl of the tendon. On the transverse image the fibrillar pattern is presented as multiple hyperechoic dots in a tightly packed bundle.
Ultrasound demonstrates nerves as 'honeycomb' or 'pepper pot' like structures composed of hypoechoic spots embedded in a hyperechoic background. They appear distinctly different to tendons in a transvere image as you can see in Figure 6. A nerve is more difficult to distinguish in a longitudinal view as seen in Figure 7, but can be found with careful probe skills and will appear more hypoechoic than surrounding tendons. In Figure 7, the nerve can be seen as a dark line superficial to the deeper flexor tendons of flexor digitorum.
Fluid presents has an anechoic appearance on ultrasound (Figure 8), and can be confirmed with dynamic interrogation as it should respond to pressure as seen in Figure 9 below, where a shoulder effusion is moved dynamically.
Figure 9 - Dynamic interrogation of shoulder effusion
As always, please comment and post your thoughts on this article or the topic in general!