<prompt> Computed Tomography CT scan is least accurate for diagnosts of:1cm size Aneurysm in the Hepatic Aery1cm size Lymph node inthe para-aoic region1cm size Mass in the tail of pancreas1cm size Gall stones  <response>: 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 dt 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. 