AJR: Brain CT rad dose varies by scanner

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Patients are subject to different organ radiation doses in the lens of the eye and brain depending on CT scanner assignment, according to a study published in the August edition of American Journal of Roentgenology (AJR).

Researchers utilized anthropomorphic phantoms in order to determine whether patients are subject to variable radiation doses based on scanner assignment for routine CT scanning of the brain, as radiation doses can be influenced by protocol design and the type of CT scanner, wrote lead author Tracy A. Jaffe, MD, of the department of radiology at Duke University Medical Center in Durham, N.C., and colleagues.

The researchers placed 20 metal oxide semiconductor field effect transistor dosimeters in the brain of an anthropomorphic phantom designed to simulate a medium-sized adult male and then scanned the phantom scanned three times according to the routine brain protocols on four different scanners at the medical center.

Jaffe and colleagues measured absorbed organ doses for skin, cranium, brain, lens of the eye, mandible and thyroid. In addition, effective dose was calculated on the basis of the dose-length product recorded on each scanner, the authors explained.
Statistically significant differences between scanners with respect to dose were recorded for brain and lens of the eye, determined the researchers, and absorbed doses were lowest on the single-detector scanner. Moreover, the highest doses were found on the 4-MDCT scanner and a 64-MDCT scanner not capable of gantry tilt, with effective dose ranging from 1.22 to 1.86 mSv.

Skin doses ranged from 0.33 to 3.61 cGy, cranium from 2.57 to 3.47 cGy, brain from 2.34 to 3.78 cGy, lens from 2.51 to 5.03 cGy, mandible from 0.17 to 0.48 cGy and thyroid from 0.03 to 0.28 cGy, wrote the researchers. 

“The radiology community is becoming increasingly concerned with the radiation dose attributable to CT... We found that at a single institution, patient assignment to a specific CT scanner can lead to significant variation in absorbed organ dose,” said the authors. Specifically, variations in detector configuration, scanner design and clinically optimized protocols can contribute to differences in absorbed organ doses in the brain and lens of the eye, they concluded.

“On scanners without gantry tilt, CT of the brain should be performed with careful head positioning and shielding of the orbits,” advised Jaffe. “These precautions are especially true for patients who need repeated scanning and for pediatric patients.”

The authors expressed plans to extend their study by comparing doses from single-detector CT and MDCT after dose reduction techniques have been put in place.