A CT (Computed Tomography) scan is a medical imaging technique that uses X-rays and computer processing to produce detailed cross-sectional images (slices) of internal organs, bones, and soft tissues. Unlike a conventional X-ray, which produces a single 2D shadow image, a CT scan builds a full 3D picture of the body's interior.
The technique was developed in the early 1970s and has become one of the most powerful diagnostic tools in modern medicine.
X-ray Source and Detector Array: The patient lies on a motorised table that moves through a large, ring-shaped machine called a gantry. Inside the gantry, an X-ray source and an array of detectors are mounted opposite each other.
Rotation: The X-ray source emits a fan-shaped beam of X-rays. The source and detector array rotate 360° around the patient, capturing hundreds of X-ray projections from different angles.
Attenuation Data: As X-rays pass through the body, different tissues absorb (attenuate) them to different degrees:
Computer Reconstruction: The detectors measure the varying intensities of transmitted X-rays at each angle. A computer applies mathematical algorithms (such as filtered back projection) to reconstruct a detailed cross-sectional (tomographic) image from this raw data.
The reconstructed CT image is built from tiny 3D volume elements called voxels (volume pixels).
| Tissue | Hounsfield Units (HU) |
|---|---|
| Air | −1000 |
| Fat | −100 to −50 |
| Water | 0 |
| Soft tissue | +20 to +80 |
| Bone | +400 to +1000 |
By mapping HU values to a grey scale, the computer produces images where different tissues are clearly distinguishable.
| Feature | Conventional X-ray | CT Scan |
|---|---|---|
| Image type | 2D shadow | 3D cross-sectional slices |
| Soft tissue contrast | Poor | Excellent |
| Overlapping structures | Yes | Eliminated |
| Depth information | No | Yes |
| Radiation dose | Low | Higher |
CT scans are particularly superior for imaging soft tissues because they can distinguish between tissues with very similar densities — something a conventional X-ray cannot do.
A CT scanner rotates an X-ray source and detector 360° around the patient. A computer processes the attenuation data from multiple angles using back-projection algorithms to reconstruct detailed cross-sectional images. Each image element (voxel) is assigned a Hounsfield Unit value, allowing different tissues to be identified and distinguished.