Computed Tomography (CT) uses a fan-shaped, thin X-ray beam that passes through the body at multiple angles. As the X-ray tube and the detector array continuously rotate 180° around the patient, they collect X-ray transmission measurements. These data points are grouped into a sinogram and digitized into pixels. Each pixel’s grayscale value represents its linear attenuation coefficient — its ability to absorb X-ray photons depending on tissue density.
Values are normalized to water, set to 0 Hounsfield Units (HU):
- Air ≈ −1000 HU
- Soft tissue ≈ +30 to +60 HU
- Bone ≈ +1000 to +3000 HU
1 Major system components
- X-ray generator. A cathode (heating coil) generates electrons; an anode produces the X-ray photons.
- Filters & collimators. Collimators shape the beam to reduce unnecessary radiation. Filters such as the bowtie filter harden the beam and compensate for the body’s thicker center and thinner periphery.
- Detector. Scintillating materials (rare-earth ceramics) indirectly convert X-rays to visible light, then to an electronic signal.
- Slip ring. Eliminates the need to “rewind” the gantry between rotations, enabling continuous spiral multi-slice scanning.