CT (Computed Tomography) scan
Medical Imaging - OCR A-Level Physics
Key Definition
CT (Computed Tomography) scan
An imaging technique that uses multiple X-ray beams taken from many angles around the body, processed by computer to produce cross-sectional (slice) images.
An imaging technique that uses multiple X-ray beams taken from many angles around the body, processed by computer to produce cross-sectional (slice) images.
- The X-ray tube and detectors rotate around the patient, taking many projections at different angles.
- A computer uses back-projection algorithmsMathematical methods used in CT scanning to reconstruct a 3D image from multiple X-ray projections taken at different angles. to reconstruct a 2D cross-sectional image (slice) from the multiple 1D projections.
- Multiple slices can be combined to create a 3D imageA three-dimensional reconstruction of internal anatomy, built from multiple 2D cross-sectional slices. of internal structures.
- CT gives much better soft-tissue contrastThe ability to distinguish between different soft tissues (e.g. muscle vs fat) in medical imaging. than a plain X-ray because each pixel's attenuationThe reduction in intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. of radiation (such as X-rays) as it passes through a material, due to absorption and scattering. is measured independently.
- Disadvantages: higher radiation doseA measure of the biological effect of ionising radiation on tissue, measured in sieverts (Sv). than a single X-ray (multiple exposures), expensive, and cannot image in real time.
Examiner Tips and Tricks
- CT vs plain X-ray is a common comparison question.
- Key advantages of CT: cross-sectional images (no overlap of structures), better soft-tissue differentiation, 3D reconstruction possible.
- Key disadvantage: much higher radiation dose.