Answer: Adenomyomatosis of the Gallbladder

There should be some gallbladder wall thickening with the almost pathognomonic “ring-down” or comet-tail artifact. Why? This is a disease of deposition of cholesterol crystals in the Rokitansky-Aschoff sinuses.The US beam causes the crystals to vibrate and thus they give off (aka reflect back) decreasing harmonics of the resonant frequency. These harmonics yield the appearance of multiple reflective surfaces deep to the crystal.. aka “ring-down”

This should be distinguished from the so called “dirty shadowing” of emphysematous cholecystitis - notice you can kind of see through it (like bowel - aka air), but it is not usually focal and much more diffuse

picture borrowed from: http://radiopaedia.org/cases/emphysematous-cholecystitis

reference: http://radiopaedia.org/articles/adenomyomatosis-of-the-gallbladder