Ultrasonography is the most useful radiologic test for diagnosing acute cholecystitis. It has a sensitivity and specificity of 95%. In addition to being a sensitive test for documenting the presence or absence of stones, it will show the thickening of the gallbladder wall and the pericholecystic fluid. Focal tenderness over the gallbladder when compressed by the sonographic probe (sonographic Murphy's sign) also is suggestive of acute cholecystitis. Biliary radionuclide scanning (HIDA scan) may be of help in the atypical case. Lack of filling of the gallbladder after 4 hours indicates an obstructed cystic duct and, in the clinical setting of acute cholecystitis, is highly sensitive and specific for acute cholecystitis. A normal HIDA scan excludes acute cholecystitis. Ref: Oddsdottir M., Pham T.H., Hunter J.G. (2010). Chapter 32. Gallbladder and the Extrahepatic Biliary System. In F.C. Brunicardi, D.K. Andersen, T.R. Billiar, D.L. Dunn, J.G. Hunter, J.B. Matthews, R.E. Pollock (Eds), Schwaz's Principles of Surgery, 9e.