All ultrasound works by placing a probe on the body that emits sound waves into the tissue, listens for the return echo, and generates an image based on the data collected. It is very similar to radar, or sonar, used by submarines to navigate underwater. Conventional ultrasound presents a two-dimensional image, or "slice" of a three-dimensional object, such as a fetus or an internal organ.
Recently scientists have been able to build ultrasound machines that can produce a three-dimensional image. This is done by sending pulses of sound all across the body's surface to take several different 2D images, or slices, of the area. These scans are then combined by a computer to form 3D images. This enables doctors to get a better view of the object being examined. 3D ultrasound is ideal for early detection of cancerous and benign tumors; detecting breast lesions; imaging a fetus in the womb; and visualizing blood flow in various organs. Four-dimensional ultrasound takes the technique one step further, adding the element of time to the 3D images. The result is live-action images of the unborn child or any internal organ.
Finally, there is Doppler ultrasound. When the object reflecting the ultrasound wave is moving, it changes the frequency of the echoes. The frequency gets higher if the sound is moving toward the sound's source and lower if it is moving away from it. How much the frequency changes depends upon how fast the object being imaged is moving, so Doppler ultrasound can be used to calculate how fast an object is moving. For instance, the technique has been used to measure the rate of blood flow through the heart and major arteries.