During ultrasound examination, a structure that is filled with fluid (e.g., a cyst) shows posterior acoustic enhancement — the tissue deep to the cyst appears brighter than the adjacent tissue at the same depth. This artefact occurs because:
- A Refraction of sound at the curved posterior wall of the cyst redirects the beam
- B Reverberation echoes within the cyst reinforce signals from deeper structures
- C The fluid attenuates sound less than the adjacent soft tissue, so more sound energy passes through and is reflected back from deeper structures ✓
- D The cyst creates a standing wave that amplifies adjacent echoes
Correct answer: C. The fluid attenuates sound less than the adjacent soft tissue, so more sound energy passes through and is reflected back from deeper structures
Explanation
Posterior acoustic enhancement (increased through-transmission) occurs because fluid has very low acoustic attenuation compared to solid tissue. The time-gain compensation (TGC) of the ultrasound machine is calibrated for average soft tissue attenuation; since the fluid attenuates less, more sound energy emerges from the deep wall of the cyst and reaches deeper structures, producing artificially bright echoes deep to the cyst. This is a useful sign to confirm cystic content.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.