Physics and Theories of Ultrasound

Ultrasound imaging uses acoustic energy to produce vibrations that reflect back to the source (Multi-frequency Linear Probe) to produce a gray scale image known as Pulse-Echo Imaging. These vibrations are measured as cycles per second or hertz (Hz). Human acoustic range is from 60 to 20,000 hertz. MSK Ultrasound acoustic energy is measured in Mega-Hertz (1,000,000) with the ideal range for Foot and Ankle imaging at 10 to 13 MHz. The higher the MHz frequency, the better the image resolution for superficial structures. Deeper structures require a lower MHz frequency with a resultant decrease in image resolution.

The speed of sound varies through different soft tissues and bone. The factors that determine the speed of sound through a specific structure are its density and compressibility (i.e. Acoustic Impedance). Therefore, different anatomical structures and pathological processes will result in changes in the perceived sound waves which allows spatial resolution (i.e. Pulse-echo Imaging).

Reflection of a sound wave is dependent upon the angle of incidence. The Angle of Incidence is determined by the relationship of the transducer probe to the underlying reflecting structure. Image resolution improves when the transducer probe is perpendicular (90 degrees, Angle of Incidence) to the surface of the underlying structure. As this angle decreases (less than 90 degrees) the sound wave will reflect off the structure to be imaged and will not enhance image quality. This is very important to remember when imaging round structures (i.e. Tendon, Bone, etc.), that the transducer probe maintains its perpendicular relationship to the curved surface of the underlying anatomical structure.