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The purpose of this study was to quantify how reducing x-ray beam intensity (i.e., mAs) affects lesion
detection performance in abdominal CT examinations. A simulation package (Syngo Explorer) was used to
reconstruct 4-mm thick CT images of a patient undergoing a standard abdominal exam. Simulations were
performed at four x-ray beam intensities of 100%, 70%, 50%, and 25%. Four observers were used to
perform a series of two Alternate Forced Choice (2-AFC) experiments that measure the lesion contrast
(I92%) corresponding to a detection accuracy of 92%. Four lesion sizes were used ranging from 5 mm to 12
mm. Results were plotted as log(I92%) versus log(mAs) to quantify how changes in x-ray intensity affect
lesion detection, as well as log(I92%) versus log(size) to generate contrast-detail curves. The fitted slope of
noise in reconstructed images as a function of relative CT x-ray beam intensity was -0.25, which is about
half the value of -0.5 expected for an ideal quantum noise limited imaging system. For lesion sizes between
5 mm and 10 mm, slopes of log(I92%) versus log(mAs) curves were very similar for all four observers, and
ranged between -0.10 and -0.17. For 5 mm sized lesions, doubling the x-ray beam intensity improved
detection performance by about 13%, whereas for 7 and 10 mm lesions, doubling the x-ray intensity
improved detection performance by about 7%. For the 12 mm lesion there were no consistent patterns for
all four readers, which may be related to the lack of a standardized viewing distance. The average slope for
the four contrast detail curves was -0.41 ± 0.09, which is substantially less than the value of -1.0 predicted
for an ideal observer operating with a quantum noise limited images. For our abdominal CT images,
doubling of the lesion size resulted in improvements in lesion detection of ~ 30%.
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