Ultrasound imaging uses vibrations similar to sound waves; but where sound waves heard by humans have a vibration less than 20 000 vibrations per second the vibrations used in ultrasound are much higher than this.
Medical Ultrasound imaging is a little like the principle behind radar or sonar. A vibration is sent out, and it is either absorbed or reflected by the materials it encounters. Unlike X-rays which tend to give only an on/off contrast between hard and soft materials, an ultrasound will give reasonably detailed surface images, and plainly shows liquids and solid materials. Though the images produced depend at least partly on the skill of the operator it is not too difficult to produce useful 3D images.
Unfortunately Ultrasound has trouble seeing through gas. This is why it uses a gel to make contacts with the skin, and why it is sometimes necessary for patients to fast before some ultrasound examinations.
Though the first ultrasound is known to date from about 1949 the history and development of the technology is rather complex. Doctors and researchers in France, Scotland, Sweden and the USA all independently developed ultrasound systems. These researchers were a mixture of specialists from nuclear and medical backgrounds, with connections to navel applications. The navel use of SONAR, dating to at least 10 years earlier, was of at least some importance to the development of ultrasound.
Unlike many other forms of radiology Ultrasound has no more radiation than normal electrical equipment, and makes no use of X-rays.