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1. Because the beam shape is so different in the penumbra region compared to the center of the beam, the penumbra region is to be avoided.

2. Hence, the penumbra is positioned outside of the active detector arrays—those x-rays essentially strike lead shielding on the sides of the detector assembly or inactive detectors.

3. As the beam width increases, the effective dose to the patient will correspondingly increase due to increase in DAP.


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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