doppler ultrasound

Meaning: A transducer emits ultrasound, sound above human hearing, and moving red blood cells reflect it. The moving cells first act as a moving receiver and then as a moving source, so the echo returns at a shifted frequency. Motion toward the transducer raises the echo frequency, and motion away lowers it. The size of the shift reveals the blood speed.

Equation: Δf = 2fv cosθ / c, where Δf is the frequency shift (Hz), f is the emitted frequency (Hz), v is the speed of the reflector (m/s), θ is the angle between the beam and the direction of motion (rad), and c is the speed of sound in tissue (m/s). It holds when v is much smaller than c.

On the MCAT: Passages give the beam frequency and a measured shift and ask for blood speed, or ask whether the echo frequency rises or falls as flow approaches or recedes. Expect proportional reasoning: doubling v or f doubles Δf, and a larger angle lowers it.

Common confusion: Students assume the shift gives speed regardless of beam direction. Only the velocity component along the beam counts, so at θ = 90° cosθ = 0 and no shift appears. The speed of sound in the tissue stays the same; the echo's frequency and wavelength change.