Ultrasound
Core framework
Ultrasound questions mix physics, artifacts, and clinical applications. Know frequency versus resolution and penetration: higher frequency gives better spatial resolution but less penetration. Axial resolution improves with higher frequency and shorter pulse; lateral resolution depends on beam width and focus. Time-gain compensation compensates for depth attenuation; the Nyquist limit (PRF/2) explains aliasing on Doppler.
Artifacts
Posterior acoustic enhancement behind a cyst, shadowing behind a stone or calcification, reverberation (comet tail, ring-down), mirror image, and edge shadowing. Anisotropy affects tendons and nerves. Twinkling artifact behind a stone on color Doppler is a helpful clue.
Clinical applications
OB: gestational sac visible at about 5 weeks, yolk sac at 5.5 weeks, embryo with cardiac activity at 6 weeks (transvaginal). First-trimester dating uses crown-rump length. Ectopic pregnancy: empty uterus with adnexal mass and free fluid. Thyroid: ACR TI-RADS uses composition, echogenicity, shape, margin, and echogenic foci to assign TR1–TR5, with FNA thresholds based on size. DVT: noncompressible vein. Carotid: ICA versus ECA by waveform and anatomy.
Traps and rule of thumb
Traps: confusing shadowing (stone) with enhancement (cyst); forgetting aliasing is fixed by increasing PRF or lowering the baseline; assuming a simple cyst needs aspiration; attributing a normal-sized appendix on ultrasound as definitive exclusion.
Rule of thumb: "enhancement means fluid, shadowing means calcium or gas." For TI-RADS, size thresholds drive biopsy: TR4 nodules are sampled at ≥1.5 cm and TR5 at ≥1 cm.