Ultrasound Physics and Instrumentation: Resolution, Doppler, and Artifacts
Resolution, attenuation, and artifacts
Sound travels at approximately 1540 m/s in soft tissue. Higher frequency means shorter wavelength, better axial resolution, and worse penetration - the central trade-off behind most physics items. Axial resolution equals spatial pulse length divided by two; a 3-MHz transducer with a 3-cycle pulse yields about 0.77 mm. Lateral and elevational resolution depend on beam width, not pulse length, and improve with focusing and harmonic imaging.
The Doppler equation relates frequency shift to velocity and the cosine of the angle; alignment within 20° keeps error under about 6%. Aliasing occurs when the Doppler shift exceeds the Nyquist limit (PRF/2). Three fixes: raise the scale/PRF, shift the baseline, or switch to continuous wave. Frame rate equals PRF divided by lines per frame, so increasing depth or sector width degrades temporal resolution - critical when timing rapid events.
Recognize the artifacts: reverberation and comet tail from strong reflectors, shadowing behind calcium, enhancement behind fluid, mirror image behind a strong interface, side-lobe and range-ambiguity clutter, twinkling behind crystals.
Bioeffects: mechanical index rises with peak negative pressure and falls with frequency; Doppler delivers more energy than 2D, so apply ALARA.
Rule of thumb: if the image looks better but the measurement got worse, check your settings - harmonic imaging, higher frequency, and added gain improve the picture but can shift numbers.