1. A computer system operable to identify calcification in a patient image data set showing one or more blood vessels, the computer system comprising: a mass storage device for storing patient image data sets; and a processor unit operable to execute machine-readable instructions to: (a) obtain an image data set from the mass storage device, the image data set comprising voxels each having an intensity value; (b) form the intensity values into a group of intensity values, the intensity values covering an intensity range; (c) define a plurality of intensity thresholds with values spaced across the intensity range and including the end values of the intensity range; (d) for each of the plurality of intensity thresholds: (i) partition the group of intensity values into two sub-groups according to the intensity threshold; and (ii) calculate an information criterion as a function of the intensity threshold; (e) generate an information criterion measure curve, being a plot of the calculated information criteria against intensity threshold; (f) locate a maximum in the information criterion measure curve and set the corresponding intensity threshold to be a calcification threshold; and (g) partition the group of intensity values into two sub-groups using the calcification threshold, thereby identifying the voxels corresponding to the intensity values in the sub-group above the calcification threshold as representing calcification in the patient.