The problem of determining the input of some physical process, given the output, by inverting a model of this process that uses input parameters to predict output observations. A wide range of planning, scanning, and surveying applications involve solving inverse problems. For example, for a CT scanner, there is a model that – given parameters regarding the location and density of body parts – can predict what observations will be made by any sensor receiving X-rays. A CT scanner collects these observations, and then has to determine the corresponding body parameters. See also forward problem.