In a method and apparatus for correcting image data from a medical imaging scan of a subject, into which subject a specified amount of an imaging substance has been introduced, a region of the image data, containing an anomalous proportion of the imaging substance introduced, is identified. For the identified region a regional value of a variable in the image data associated with the imaging substance is determined. The regional value is used to determine the proportion of the substance in the region, and the proportion is subtracted from the specified amount of the imaging substance.
