The term “Doppler imaging” is used in reference to two very different processes. Both take advantage of a phenomenon known as the Doppler effect, in which the frequency of a wave appears to change relative to the position of the observer. One form of Doppler imaging is used to look deep into space to learn about the surface composition of stars. Another form is used to look into the body to gather information about the movement of blood through the circulatory system, and the parts of the body which move, such as the valves of the heart.
When one looks at the Sun, through protective lenses of course, the Sun's surface appears to be uniform. In fact, it's not, as detailed imaging studies have revealed. The surface of the Sun is inconsistent, and in other stars, these inconsistencies can become even more extreme. They are caused by variations in the composition of the surface of the star which lead to hot spots, cold spots, and different energy levels.
With Doppler imaging, researchers can learn about the surface composition of distant stars by studying the ways in which their spectral lines shift as they rotate. This can be used to create a map of the surface which reveals areas with differing levels of activity. Doppler imaging is usually done with the assistance of computer programs which do the hard calculations and generate an image, and it is used to learn more about individual stars and the universe in general.
In the body, Doppler imaging is used to look at the flow of blood through the vessels. Also known as Doppler ultrasound, it utilizes a ultrasound probe which can track the movement of blood with the assistance of the Doppler effect to see whether something is moving towards the probe or away from it. Many machines do color Doppler imaging, overlaying a color map which provides a more complete picture of what is going on than a simpler grayscale image.
One use of Doppler imaging is in monitoring circulation. For example, after a reattachment surgery, it can be used to confirm that the reattached limb is getting an ample blood supply. Doppler imaging can also be used in the monitoring of patients with circulatory problems, identifying areas where circulation is slowing or stopping. It is also used in cardiac studies, where the heart is imaged to see how quickly and effectively it moves blood. Issues such as valve leaks and a sluggish ejection fraction will be revealed in a Doppler imaging study.