A method for distinguishing between gray matter and white matter starting from a time-dependent computed tomography image data record from a perfusion CT examination is disclosed. In at least one embodiment, a plurality of time-independent images are calculated from the time-dependent image data record, a plurality of threshold histogram analyses are performed in order to determine regions of the brain which can be assigned to one or more types of cerebral matter, and subsequently the region of gray matter is determined from the information obtained in respect of type and region of the cerebral matter using at least one logical combination and at least one exclusion method. A control and computational unit is also disclosed with a storage medium in which a computer program or program module is stored, which executes the described method during operation.