Fluid-filled structures have what at their borders?

• A. No border defined
• B. Intense echoes
• C. Weak echoes
• D. Shadowing

Answer: (B)

Fluid-filled structures create a sharp boundary because of a strong change in acoustic impedance at the fluid–tissue interface. This mismatch reflects many of the ultrasound waves, so the edge appears as a bright, intense border, while the inside stays mostly anechoic. The wall often stands out as a thin, well-defined line, helping to differentiate the structure from surrounding tissue. In contrast, a border that isn’t defined would suggest a less distinct boundary, shadowing is produced by dense or calcified objects rather than simple fluid interfaces, and weak echoes at the border wouldn’t reflect the typical bright edge seen with a fluid-tissue boundary.