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PATIENT SIZE AND X-RAY TRANSMISSION IN BODY CT

Ogden, Kent; Huda, Walter; Scalzetti, Ernest M.; Roskopf, Marsha L.

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Abstract— Physical characteristics were obtained for 196 patients undergoing chest and abdomen computed tomography (CT) examinations. Computed tomography sections for these patients having no evident pathology were analyzed to determine patient dimensions (AP and lateral), together with the average attenuation coefficient. Patient weights ranged from approximately 3 kg to about 120 kg. For chest CT, the mean Hounsfield unit (HU) fell from about −120 HU for newborns to about −300 HU for adults. For abdominal CT, the mean HU for children and normal-sized adults was about 20 HU, but decreased to below −50 HU for adults weighing more than 100 kg. The effective photon energy and percent energy fluence transmitted through a given patient size and composition was calculated for representative x-ray spectra at 80, 100, 120, and 140 kV tube potentials. A 70-kg adult scanned at 120 kVp transmits 2.6% of the energy fluence for chest and 0.7% for abdomen CT examinations. Reducing the patient size to 10 kg increases transmission by an order of magnitude. For 70 kg patients, effective energies in body CT range from ∼50 keV at 80 kVp to ∼67 keV at 140 kVp; increasing patient size from 10 to 120 kg resulted in an increase in effective photon energy of ∼4 keV. The x-ray transmission data and effective photon energy data can be used to determine CT image noise and image contrast, respectively, and information on patient size and composition can be used to determine patient doses.

*SUNY Upstate Medical University, Department of Radiology, 750 E Adams Street, Syracuse, NY 13210.

Manuscript received 17 January 2003;

revised manuscript received 9 October 2003, accepted 27 January 2004

For correspondence or reprints contact: K. Ogden at the above address, or email at ogdenk@mail.upstate.edu.

© 2004 by the Health Physics Society