ultrasound
Summary
Ultrasound is sound with frequency above about 20,000 Hz, beyond human hearing. In imaging, pulses reflect off tissue boundaries and the echo time gives depth (d = vt/2). Higher frequency gives finer resolution but less penetration.
Detail
Meaning: Ultrasound is sound with frequency above about 20,000 Hz, the upper limit of human hearing. It is a longitudinal mechanical wave. In imaging, a transducer sends short pulses into the body. Part of each pulse reflects at boundaries between tissues that differ in density and sound speed, and the echoes build the image. Higher frequency gives shorter wavelength and finer resolution, but more attenuation, so less penetration.
Equation: d = vt/2, where d is reflector depth (m), v is the speed of sound in the tissue (m/s), and t is the round-trip echo time (s). The 2 appears because the pulse travels there and back.
On the MCAT: Expect a depth calculation from an echo delay, with v given (about 1540 m/s in soft tissue). Expect a trend question: a higher-frequency probe resolves smaller structures but images less deeply. In Doppler ultrasound, echoes from blood moving toward the probe return at a higher frequency.
Common confusion: Ultrasound is not electromagnetic like X-rays, so it is non-ionizing and cannot travel through a vacuum. Changing the frequency changes the wavelength, not the speed, because speed is set by the medium (λ = v/f).
Sources
- Physics (Cutnell and Johnson)
- Physics: Principles with Applications (Giancoli)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.