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1. In a modern scanner, during acquisition, X-rays are produced continuously.
2. A series of projections through the patient are collected at discrete angles (e.g. every 0.5°).
3. If there are 900 detectors, that’s 900 * (360 * 2) = 648,000 measurements per rotation.
4. Think of the fan beam as composed of many narrow pencil beams, one to each detector.
5. Each detector, therefore, samples a point on the projection that represents the total attenuation along the pencil beam path from source to detector.
6. For every 360° rotation, an individual voxel is traversed by one or more pencil beams for every measurement taken.
7. The attenuation of each voxel, therefore, contributes to the measured transmission for a large number of the ray sums.
8. An algorithm is then used to reconstruct the images using the CT attenuation data for each voxel.


1. Bushberg, J. T. (2012). The essential physics of medical imaging. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. Find it at Amazon
2. Heggie, J. C., Liddell, N. A., & Maher, K. P. (1997). Applied imaging technology. Melbourne: St. Vincents Hospital.

Ⓒ A. Manickam 2018

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