Instrumentation, Technique, Artifacts, and Interventional Procedures
Equipment and technique
Choose the highest-frequency transducer that still provides adequate penetration: 5–9 MHz transvaginal for pelvic detail, 3.5–5 MHz transabdominal for larger or deeper structures. The focal zone belongs at the level of interest, and overall gain and time-gain compensation should be adjusted so fluid is anechoic without losing far-field detail. Frame rate falls with increased depth, color box size, and compounding; keep the color box small. Apply ALARA at all times, watching the thermal and mechanical indices during first-trimester scanning and pulsed Doppler, where exposure matters most.
Artifacts to recognize
Acoustic shadowing behind stones, calcification, and gas; posterior enhancement behind fluid; reverberation and comet-tail artifact from cholesterol crystals or small calcifications; ring-down from gas; mirror-image artifact from a strong reflector such as the diaphragm or bladder wall; edge shadowing from refraction at curved structures; slice-thickness artifact making a thin structure look filled with echoes; and speed-error artifact displacing structures deep to fat. Color Doppler adds aliasing above the Nyquist limit, flash artifact from motion, and blooming beyond true vessel caliber. Twinkling artifact behind calcifications can mimic flow.
Interventional procedures
Amniocentesis is performed from about 15 weeks with a 22-gauge needle under continuous guidance, discarding the first 1–2 mL to reduce maternal contamination. Chorionic villus sampling is done at 10–13 weeks, transabdominally or transcervically. Fetal transfusion treats anemia; vesicoamniotic and thoracoamniotic shunts decompress obstructed systems. Saline infusion sonohysterography distends the cavity for polyps and adhesions, and is avoided with active infection or suspected pregnancy.
Rule of thumb
When a structure appears unexpectedly deep or duplicated, change the angle and frequency before calling pathology - most of these findings are physics, not anatomy.