1. In an x-ray system comprising a digital detector defining a first region suitable for generating data useful for creating a patient x-ray image and a second region less suitable for generating such data than said first region, apparatus for testing the detector comprising: a source of x-rays; a phantom located between said source and at least a portion of the second region so that said detector generates detector test data in at least a portion of the second region in response to said x-rays; and a processor arranged to measure at least one parameter responsive to at least a portion said test data, store a first value of said one parameter at one point of time, make a comparison of said first value with a second value of said one parameter generated at a second point in time later than said first point of time; and generate a result signal representing the results of said comparison.