ultrasound imaging
Summary
Ultrasound imaging builds a picture from echoes of high-frequency sound pulses (above 20,000 Hz) reflected at boundaries between tissues of different acoustic impedance. Echo delay gives depth, d = vt/2; higher frequency gives finer resolution but less penetration.
Detail
Meaning: Ultrasound imaging sends pulses of high-frequency sound (above 20,000 Hz; diagnostic scans use MHz) into the body and builds an image from the echoes. A piezoelectric transducer emits the pulse and detects the echo. Sound partly reflects at a boundary between media of different acoustic impedance (Z = ρv, density times sound speed); a bigger mismatch gives a stronger echo. Gel removes air at the skin, which would reflect nearly all the sound.
Equation: d = vt/2, where d is the depth of the boundary (m), v is the speed of sound in the tissue (m/s), and t is the round-trip time of the echo (s).
On the MCAT: Compute depth from an echo time and a given sound speed. Predict that higher frequency means shorter wavelength and finer resolution but stronger attenuation, so less penetration. Doppler shift of echoes from moving blood gives flow speed; flow toward the probe raises the detected frequency.
Common confusion: Ultrasound is a mechanical sound wave, not an electromagnetic one, so it needs a medium and is not ionizing like X-rays. Students also forget the factor of 2: the measured time covers the trip there and back.
Sources
- College Physics
- Physics: Principles with Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.