Ultrasound
Ultrasound
This domain tests ultrasound physics, artifacts, abdominal and pelvic sonography, vascular Doppler, and obstetric emergency diagnosis. Image optimization and artifact recognition are central.
Core rules
- Frequency trades penetration for resolution: higher frequency gives better resolution but less depth. Doppler shift depends on angle; a 60-degree angle is optimal.
- Artifacts: shadowing behind calcifications, posterior enhancement behind cysts, reverberation from strong reflectors, twinkling behind stones, and anisotropy in tendons.
- FAST examines pericardium, perihepatic, perisplenic, and pelvic spaces. DVT ultrasound uses compression; noncompressibility is diagnostic.
- Obstetric: gestational sac is visible transvaginally at about 5 weeks; fetal heart activity by 6–7 weeks. Ectopic pregnancy shows an empty uterus with adnexal mass and free fluid.
Worked example
A 28-year-old with 7 weeks of amenorrhea and pelvic pain has a positive hCG. Transvaginal ultrasound shows an empty uterus, a 2 cm left adnexal mass, and free fluid. This is a presumed ectopic pregnancy; urgent gynecology referral is required. A pseudogestational sac would be central and not produce an adnexal mass.
Traps
Calling an anisotropy artifact a tear; missing a small pneumothorax on lung ultrasound; misdiagnosing a corpus luteum cyst as ectopic; failing to use Doppler in testicular torsion; measuring the fetus by the wrong parameter.