Ultrasonographic Anatomy

Ultrasound Basics - Echoes & Shadows

• Echogenicity: Describes tissue's ability to reflect ultrasound waves.
  • Anechoic (black): No echoes (e.g., fluid-filled cysts, blood vessels).
  • Hypoechoic (dark grey): Few echoes (e.g., muscle, some tumors).
  • Isoechoic: Same as surrounding tissue.
  • Hyperechoic (bright white): Many echoes (e.g., bone, gallstones, fat, air).
• Key Acoustic Artifacts:
  • Acoustic Shadowing: ↓ signal posterior to strong reflectors (e.g., stones, bone).

    ⭐ Acoustic shadowing is classically seen posterior to gallstones or calcifications.

  • Acoustic Enhancement: ↑ signal posterior to fluid-filled structures (e.g., cysts, bladder).
  • Reverberation: Multiple parallel echoes (e.g., comet tail with gas).
  • Edge Shadowing: Refractive shadow at curved edges.

Abdominal Ultrasonography - Viscera Views

• Liver: Homogeneous; echogenicity > renal cortex.
  • Segments: Couinaud.
  • Views: Subcostal, intercostal.
  • Signs: 📌 Portal triad (Mickey Mouse), 📌 Hepatic veins (Playboy bunny).
• Gallbladder & Biliary Tree:
  • GB wall: <3 mm.
  • CBD: Normal <6 mm (↑ 1 mm/decade >60 yrs; up to 10 mm post-cholecystectomy).
• Spleen: Homogeneous; echogenicity > liver. Length <12-13 cm.
• Pancreas: Echogenicity > liver (↑ with age/fat). Landmark: Splenic vein posteriorly.
• Kidneys: Cortex hypoechoic to liver/spleen. Medulla more hypoechoic than cortex. Sinus fat hyperechoic. Length 9-12 cm.
• Aorta/IVC:
  • Aorta: Pulsatile, non-compressible, diameter <3 cm.
  • IVC: Compressible, respiratory variation.

⭐ The seagull sign on ultrasound refers to the celiac artery and its branches (splenic and common hepatic arteries).

Pelvic & Small Parts Ultrasonography - Beyond Belly

• Pelvic USG: Assesses uterus, ovaries, adnexa, bladder, prostate.
  • Approaches: Transabdominal (TAS) overview; Transvaginal (TVS) detail (female); Transrectal (TRUS) prostate.
  • Uterus: Endometrial thickness (postmenopausal <5mm no HRT). Myometrium.
  • Ovaries: Follicles, cysts, masses. Ovarian volume assessment.
  • Prostate (TRUS): Volume, peripheral zone (commonest Ca site).

  ⭐ The "sliding sign" in pelvic ultrasound is important for assessing adhesions, particularly in endometriosis.

• Small Parts USG: High-frequency linear probes for superficial structures.
  • Thyroid: Isthmus AP diameter <5mm. Nodules (TIRADS), thyroiditis.
  • Scrotum: Testicular torsion, epididymo-orchitis, varicocele (vein diameter >2-3mm with Valsalva), hydrocele.
  • MSK: Tendinopathy, tears, effusions, dynamic assessment.
  • Other: Eye (globe, lens), Neck LNs (morphology).

Doppler Ultrasound & FAST - Flow & Trauma

• Doppler Ultrasound: Assesses blood flow using Doppler effect.

  • Detects direction & velocity. 📌 BART: Blue Away, Red Towards.
  • Applications: DVT, stenosis, perfusion, fetal heart.
  • Types:
    • Color Doppler: Visualizes flow.
    • Power Doppler: ↑ sensitivity for low flow, no direction.
    • Spectral Doppler: Waveform analysis (RI, PI).
      • RI = $(PSV - EDV) / PSV$
      • PI = $(PSV - EDV) / TAMV$

• FAST (Focused Assessment with Sonography for Trauma): Rapidly detects free fluid (e.g., hemoperitoneum) in trauma.

  • Key in ATLS. 📌 4 Ps (Views):
    • Perihepatic (Morrison's Pouch)
    • Perisplenic
    • Pelvic
    • Pericardial (Subxiphoid)
  • eFAST: Adds lung views (pneumo/hemothorax).

  ⭐ In trauma, the most common site for initial free fluid accumulation in the supine patient is Morrison's pouch (hepatorenal space).

High‑Yield Points - ⚡ Biggest Takeaways

• Acoustic impedance mismatch is the basis of ultrasound imaging.
• Structures: hyperechoic (bright, e.g., bone), hypoechoic (darker, e.g., muscle), anechoic (black, e.g., simple cyst).
• Doppler ultrasound assesses blood flow direction and velocity.
• FAST scan rapidly detects free fluid (hemoperitoneum, pericardial effusion) in trauma.
• Transducer choice: high-frequency for superficial (high resolution), low-frequency for deep (good penetration).
• Common artifacts: acoustic shadowing (e.g., gallstones) and posterior acoustic enhancement (e.g., cysts).