After the introduction of ultrasound imaging, computed tomography (CT) and Magnetic Resonance Imaging (MRI) were introduced for disease diagnosis and management. Ultrasound imaging was the first effective soft tissue imaging modality used in diagnostic radiology as it provided tomographic views of the anatomy. The AAPM and Medical Physics have contributed substantially to training of physicists and engineers, medical practitioners, technologists, and the public.

A brief review of medical ultrasound imaging and therapy is presented, with an emphasis on the contributions of medical physicists, the American Association of Physicists in Medicine (AAPM) and its publications, particularly its journal Medical Physics. For this reason, a topical review, rather than a primarily chronological one is presented. The physics of medical ultrasound has evolved over many fronts throughout its history. Therapeutic ultrasound, in addition to the physiotherapy practiced for many decades, is just coming into its own as a major tool in the long progression to less invasive interventional treatment. We believe that at the other extreme, high-end systems will continue to compete with all other imaging modalities in imaging departments to be the method of choice for various applications, particularly where safety and cost are paramount.

It is becoming increasingly the front end imaging modality serving often as an extension of the physician's fingers. Ultrasound has been the greatest imaging modality worldwide for many years by equipment purchase value and by number of machines and examinations.

Ultrasound physics 101