Abdominal Physics
What to know
Frequency governs the trade-off between resolution and penetration: higher frequencies improve axial and lateral resolution but attenuate faster. Axial resolution equals half the spatial pulse length, and wavelength in soft tissue (mm) is approximately 1.54 divided by frequency in MHz, so 5 MHz gives about 0.31 mm. Lateral resolution is always poorer than axial.
Artifacts are named by mechanism: shadowing behind stones, gas, or bone; posterior acoustic enhancement behind fluid; reverberation and comet-tail behind clips and gas; mirror images across the diaphragm; edge shadowing from refraction; and speed-error artifacts through fat.
Doppler physics: the shift is proportional to transmitted frequency, blood velocity, and the cosine of the angle. Keep the angle at 60 degrees or less. Aliasing occurs when the shift exceeds the Nyquist limit (half the pulse repetition frequency); raise the scale or shift the baseline. Power Doppler is angle-independent but gives no direction.
ALARA governs output; mechanical and thermal indices should be kept low, especially in prolonged or obstetric scanning.
Traps
Posterior enhancement is not a lesion. Gas shadowing is dirty and obscures deeper structures. Aliasing indicates exceeding the scale, not necessarily turbulence, although turbulence broadens the spectrum.
Rule of thumb
Wavelength (mm) ≈ 1.54 / frequency (MHz); axial resolution ≈ half the spatial pulse length.