Introduction
Computed tomography (CT) produces cross-sectional images of the body by measuring how much X-ray beams are attenuated as they pass through tissues from many directions, then reconstructing these measurements mathematically into a slice image. This topic covers the physical principle, the main scanner components, image reconstruction, the parameters that control image quality and dose, the way images are displayed and read, and the basic clinical role of CT.
Physical basis of CT
Conventional radiography projects all structures along the beam onto a single plane, so overlapping organs obscure each other. CT solves this by collecting many projections around the patient and calculating the linear attenuation coefficient of each small volume element (voxel). For display, these values are converted to the Hounsfield unit (HU), a scale normalised to water:
- Water is defined as 0 HU.
- Air is about -1000 HU, so it appears black.
- Fat is negative, roughly -100 HU.
- Most soft tissues and unenhanced blood lie close together, roughly +20 to +60 HU, which is why plain CT has limited soft-tissue contrast without contrast agents.
- Bone ranges from a few hundred HU up to about +1000 HU or more.