Ultrasound Physics, Instrumentation, and Bioeffects/Safety
Frequency, wavelength, resolution
Axial resolution equals spatial pulse length divided by two, so it improves with higher frequency and fewer cycles per pulse. Lateral resolution depends on beam width at the focal zone. Elevation (slice-thickness) resolution is set by element height and is often the poorest dimension on 2D - a trap when a question asks which resolution is worst. Pediatric patients are small, so 5–12 MHz probes give excellent detail with adequate penetration; the same probe in a large adolescent may underpenetrate.
Doppler physics
Continuous-wave Doppler is not range-gated and has no Nyquist limit - reserve it for high-velocity jets (VSD, aortic stenosis, coarctation). Pulsed-wave Doppler is range-gated but aliases once the shift exceeds PRF/2; remedies are raising PRF (shallower sample, narrower sector, color off), baseline shift, or lowering transmit frequency. Angle error follows cos θ: 20 degrees off-axis costs about 6 percent, 60 degrees halves the measured velocity, so keep the angle at or below 20 degrees. The simplified Bernoulli equation (gradient = 4v squared) assumes a narrow, high-velocity jet and ignores flow acceleration and viscous losses; for LVOT-to-aortic gradients use 4(v2 squared minus v1 squared), and remember it underestimates long tunnel-like obstructions such as coarctation or a diffusely narrow shunt.
Output and bioeffects
ALARA governs every study. The thermal index estimates heating; the mechanical index estimates cavitation risk, which rises with low frequency and with gas bodies or contrast agents. Neonates and fetuses deserve short color and Doppler dwell times and the lowest output consistent with diagnostic images.
Rule of thumb: doubling depth halves the PRF available, so Nyquist drops and deep jets alias sooner - move to CW rather than fighting aliasing.