Computed Tomography (CT scan) is least accurate for diagnosts of:

D i.e. 1 cm size Gall stones CT scan has a very limited role in diagnosis of gall stone (Cholelithiasis) as only a minority (20-60%) of gall bladder stones are visibleQ which show calcification. Most (70%) stones are cholesterol stones and 93% of them are radiolucent). CT scan, however, can evaluate complications such as pancreatitis, pericholecystic fluid, abscess or perforation. Ultrasound is the most accurate modality for the diagnosis of gall bladder stonesQ, which appear as echogenic foci producing acoustic shadows. Stone mobility is also identified (although not essential for making diagnosis). Small stones are differentiated from small polyps by the demonstration of mobility or the presence of an acoustic shadow. Non visualization of gall bladder on US may be d/t previous Acoustic Shadow cholecystectomy, nonfasting, abnormal gall bladder position, emphysematous cholecystitis or because the gall bladder' is filled with stones. The latter can be identified by double-arc shadow sign = hypoechoic line between two echogenic lines in gall bladder fossa (i.e. 2 parallel curved echogenic lines seperated by a thin anechoic space with dense acoustic shadowing distal to the deeper echogenic line). - CT scan is one of the most accurate procedure in detection of lesions in pancreas (including tail), retroperitoneum (eg paraaoic lymph nodes) and liver (focal lesions like aneurysm, hemangioma etc).