When a CT scan is made, there is a source of X-rays on one side of the patient and a detector on the other. The rays pass through the patient in principle, but are weakened by different types of tissue. What remains of the radiation is ultimately measured by the detector.
When one such picture is made, the source and the detector move up a little, and the next picture is made. This continues until an image has been made of all sides of the head. A computer algorithm then creates an image that shows where there are tissues with high radiation transmission and where there are tissues with low radiation transmission.
Author: Myrthe Princen (Translated by Thomas von Rein)