Computed Tomography
A plain X-ray image is one projection, so structures at different depths overlap. CT reduces this overlap by reconstructing one thin section from many directions.
Many directions reconstruct one 2D section
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- Select one thin section of the body.
- Take many X-ray measurements through that section from different angles.
- A computer combines the measurements into a 2D attenuation map of the section.
Key idea
One CT section is reconstructed from many projections. It is not one ordinary X-ray photograph viewed from above.
Repeat adjacent sections to build a 3D image
- Move to the next thin section.
- Repeat the many-angle scan and reconstruction.
- Compile all reconstructed 2D sections into a 3D image (9702/42/O/N/25 Q10(c)).
| Method | Measurements | Result |
|---|---|---|
| plain X-ray | one projection | overlapping 2D shadow image |
| one CT section | many directions | one reconstructed 2D map |
| complete CT scan | many adjacent sections | combined 3D image |
Your turn9702/42/O/N/25 Q10(c)
Describe how CT produces a three-dimensional image of a structure. [3]
Show worked answer
- Scan the structure in thin sections.
- Take many scans of each section from different angles.
- Repeat for all sections and compile the data into a 3D image.