13 - Physics of Ultrasound

Summary

Ultrasound provides unique advantages in the diagnostic imaging of patients in the ED. A comprehensive understanding of the physical principles supporting this modality is not mandatory for incorporation into an emergency medicine practice. However, an appreciation for several fundamental concepts and a solid grasp of the system controls will allow improved image acquisition at the bedside and facilitate precise image interpretation.

PRINCIPLES OF ULTRASOUND

The fundamental principal of diagnostic ultrasound relies on the transmission of sound into the patient's body and reception of reflected sound — which is then displayed as data for interpretation. The sound energy used in diagnostic ultrasound generally ranges from 2 to 13 MHz, far outside the range detectable by the human ear (20–20,000 Hz). A simple analogy to assist in understanding the basic principle is the use of sonar, in which sound waves are emitted and the sonar device awaits the return of these impulses. Based on an assumed rate of travel, the sonar device may then determine the distance of objects by the time lapse from emission to return of a pulse of sound.

The modern use of diagnostic ultrasound can be traced to early use in the 1950s. Although the application of these early systems differs significantly from the units at use in today's EDs, several important principles can be understood using this early technology.