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CT BEAM-FILTERING/HARDENING

Physics

CT BEAM-FILTERING/HARDENING

1. Filtration in the tube and housing removes unwanted lower energy photons.
2. Lower energy X‐rays are attenuated more than higher energy X‐rays and contribute to dose but not image quality.
3. Addition of bow-tie filter aims to to achieve a more uniform signal at the detectors, better image quallity with lower surface dose.

BOW-TIE FILTER
4. Since the patient cross-section is usually elliptical, X-rays at the periphery tend to pass through less tissue.
5. To compensate for this difference add a bow-tie filter that is thin in the centre and thick at the edges to artificially harden the beam at the edge.
6. Attenuation occurs by photoelectric effect and Comptom scatter. Scatter can be problematic as it decreases signal:noise ratio.


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REFERENCES

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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