Some scanners still in use are of the third-generation design. The X-ray source and detector array are rigidly fixed to a gantry on either side of the patient. The whole gantry rotates around the patient as the images are taken

The black and white squares within the grid (patient) represent tissues of different densities. At each point on the axial rotation an image is taken of the tissue slice. These images are then transferred to a computer where powerful mathematics is used to produce a final image of the tissue slice

In modern multiple-slice CT scanner design an array of detectors captures multiple ‘slices’ of anatomy in a single acquisition. The X-ray source and the detector array form a unit which rotates around the patient as the CT table moves through the bore of the scanner. The imaging data is therefore essentially acquired in a ‘helix’. The most recent generation of scanners have several hundred detectors and use lower doses to acquire large volumes of imaging data with each rotation and with reduced artefact from patient movement

This is a representation of the Hounsfield Unit scale of CT tissue density. Water is defined as 0 HU and air as -1000 HU. The ‘level’ is the HU (density) at the centre of the ‘window’ and is positioned to optimise detail of particular tissues within the anatomical region imaged. The ‘window’ is the range of units that are displayed within the image greyscale either side of the ‘level’. Outside this range the values are shown as black if of lower density or white if of higher density. Example ‘levels’ and ‘windows’ are shown: ‘soft tissue windows’ (L = 50, W = 300); ‘lung windows’ (L = -600, W = 1200); and ‘bone windows’ (L = 500, W = 1500)