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Reducing Radiation Exposure from Survey CT Scans

Jennifer C. O'Daniel1, Donna M. Stevens2 and Dianna D. Cody2

1 Department of Radiation Physics, Unit 94, The University of Texas M. D. Anderson Cancer Center, Houston, TX 77030.
2 Department of Imaging Physics, Unit 56, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Blvd., Houston, TX 77030.



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  		Fig. 1 —Ion chamber in-air at isocenter for survey CT scan radiation measurement. Inverse-square correction factors were applied to determine entrance exposure for 22.5-cm-diameter patient.

 


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  		Fig. 2 —Typical survey CT scan of ion chamber. Chamber is in center of scanning region.

 


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  		Fig. 3 —Diagram of hypothetical 22.5-cm-patient setup. Measurements were obtained at isocenter, and source-to-isocenter distance was known. We used inverse-square correction factors to determine anteroposterior (0°) and posteroanterior (180°) entrance exposures for a hypothetical 22.5-cm adult patient and a hypothetical 14-cm pediatric patient, to compare our results to typical adult chest radiograph entrance exposure (16 mR or 4.1 x 10-6 C/kg).

 


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  		Fig. 4 —Set-up for measurement of beam width. Kodak X-Omat V or X-Omat TL ready-pack film (Eastman Kodak Company) is suspended on foam block at isocenter. Table, positioned below and away from film, can move without interfering with film position.

 


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  		Fig. 5 —Survey CT scan mean entrance exposures with default scanner settings. Horizontal dotted line marks exposure from one typical chest X-ray (16 mR or 4.1 x 10-6 C/kg). Asterisk symbols denote pediatric survey CT scans that were not truly default but had been adjusted to minimum settings 1 year before the initiation of this study.

 


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  		Fig. 6 —Mean entrance exposure for survey CT scans at minimum X-ray tube current indicated for each scanner model. Horizontal dotted line marks exposure from one typical chest X-ray (16 mR or 4.1 x 10-6 C/kg). Whenever available, 120 kVp and 80 kVp X-ray tube voltages were used. For two CT scanner models (Philips PQ5000 and Philips Ultra Z), only 130 kVp X-ray tube voltage was available for survey CT scans. For one CT scanner model (Philips Mx8000 IDT 16), only 120 kVp and a 0° tube position were available.

 

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