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How little of an element can be detected is not pinned down.","posed_since":"2008","precise":"Determine the minimum detectable concentration of K-edge elements (iodine at 33.2 keV, gadolinium at 50.2 keV, and higher-$Z$ labels such as tantalum, gold and bismuth) in a 30 cm diameter water phantom at clinical dose, as a function of detector energy resolution, bin count and photon flux. An answer is a detection-limit formula in mg per cm^3 with its dependence on these quantities.","problem_ref":"930874c36a968325068f34df74a2c7b9308fb8335791479d0f0816a4a8ac6a4e","references":"","settled_by":"Phantom measurements across concentration and dose compared against a task-based detectability calculation using the measured detector response.","status_note":"","title":"Detection limit for K-edge contrast 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