Ultrasound Physics and Instrumentation
The Ultrasound Physics and Instrumentation domain tests whether you can predict image quality and artifact from machine settings, and whether you can choose parameters that answer the clinical question at the lowest output that works. Expect calculations and cause-and-effect reasoning, not slogans.
Core rules
- Soft-tissue attenuation is roughly 0.5 dB/cm/MHz one way, so doubling frequency doubles the loss.
- Axial resolution is about half the spatial pulse length, i.e. λ/2, with λ(mm) = 1.54/f(MHz).
- Lateral resolution equals beam width: best at the focal zone, worse in the near and far fields.
- Frame rate = PRF ÷ lines per frame; depth and a large color box both slow it.
- The Doppler shift depends on cos θ; keep the angle at or below 60° and angle-correct.
- Aliasing appears when the shift exceeds PRF/2 (the Nyquist limit); raise PRF, lower frequency, or increase the angle.
Worked example
A 5 MHz linear probe images a tendon at 3 cm. λ = 1.54/5 = 0.31 mm, so axial resolution is about 0.15 mm. One-way attenuation is 5 × 0.5 = 2.5 dB/cm, roughly 7.5 dB out to 3 cm and 15 dB round trip. Moving to 10 MHz halves λ (0.15 mm) and axial resolution (about 0.08 mm), but doubles attenuation to 30 dB round trip; the system adds gain and noise rises. The sharper image is not automatically the more diagnostic one.
Traps that cost marks
Lateral resolution is not improved by a longer pulse - that is axial. Harmonics narrow the beam and improve lateral resolution and signal-to-noise; they are not a route to better axial resolution. PRF is not frame rate. Do not claim a larger aperture worsens axial resolution, and do not fix aliasing by lowering the PRF.