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A review is made of the performance of X-ray systems applied to the imaging of dilute amounts of iodine using subtraction methods. Relevant performance criteria are defined and desired performance levels are derived. For a general purpose system these are iodine sensitivity of 1 mg/cm2, cancellation of 10 g/cm2 soft tissue and 2 g/cm2 bone to be equivalent to iodine sensitivity, capability of imaging through 25 cm soft tissue, spatial resolution exceeding 1 lp/mm and acceptable imaging time depending on the application. Conceptual aspects of several subtraction techniques are discussed including temporal, generalized temporal, dual-energy K-edge, dual-energy non-K-edge, three-energy K-edge, and hybrid subtraction. Experimental performance of several representative area and linear detector systems is tabulated according to subtraction algorithm. A table of references is provided.