Ultrasound Physics and Instrumentation
Wave basics
Ultrasound travels at an assumed 1540 m/s in soft tissue, so wavelength in millimeters equals 1.54 divided by frequency in MHz. Higher frequency means shorter wavelength, better resolution, and worse penetration; that trade-off underlies most physics questions.
Resolution
Axial resolution equals spatial pulse length divided by two, and spatial pulse length depends on wavelength and the number of cycles in the pulse - damping shortens the pulse and improves it. Lateral resolution improves with focusing and higher frequency and degrades away from the focal zone. Elevational (slice-thickness) resolution is the worst of the three and cannot be corrected by the operator. Attenuation is roughly 1 dB/cm/MHz, and the half-value layer is the depth at which intensity falls by half.
Doppler and frame rate
Pulse repetition frequency is limited by depth (approximately 77,000 ÷ depth in cm), and the Nyquist limit is half the PRF; velocities above it alias. The fix is to raise the scale or PRF, narrow the color box, or switch to continuous wave, which does not alias because there is no range gating. Frame rate equals PRF divided by lines per frame, so deeper depth, wider sector, and a larger color box all slow the frame rate.
Knobology and traps
Receiver gain, time-gain compensation, compression/dynamic range, reject, and focus change display appearance, not the acoustic beam. Tissue harmonics transmit at f and receive at 2f, improving lateral resolution and reducing clutter. A classic trap: confusing gain with power, or believing that increasing frequency will improve penetration.
Rule of thumb
For a two-cycle pulse, axial resolution in millimeters is about 1.54 ÷ frequency in MHz - roughly 0.3 mm at 5 MHz.