DETECTION OF SIMULATED LUNG NODULES WITH COMPUTED RADIOGRAPHY - EFFECTS OF NODULE SIZE, LOCAL OPTICAL-DENSITY, GLOBAL OBJECT THICKNESS, ANDEXPOSURE

Rationale and Objectives. We quantified differences in the detection o f simulated lung nodules on computed radiographs on the basis of varia tions in nodule size, local contrast, body habitus (global contrast), and exposure. Methods. A step-wedge phantom was developed to simulate the attenuation ranges of the lung, retrocardiac, and subdiaphragmatic regions of the adult human chest. Additional Lucite wedges were used to simulate two different body thicknesses and to provide variable str uctural noise. Soft-tissue-equivalent nodules of 3-mm and 5-mm diamete r that resulted in 10% differences in attenuation from lung equivalenc e were embedded in lung-equivalent material. By superimposing the shee ts in various positions, 84 unique nodule configurations containing ei ght nodules per image were exposed on a computed radiography system. C omputed radiographs were acquired at two different exposures approxima ting standard exposure and underexposure. For each resulting phantom i mage, seven observers scored the presence or absence of a nodule withi n individual cells of a 5 x 5 grid matrix. Results. True-positive frac tions for 3-mm-diameter nodules were very low across all conditions. T rue-positive fractions for 5-mm-diameter nodules varied from 0.23 to 0 .98. Significant differences in the conspicuity of 5-mm nodules depend ed on differences in phantom thickness and differences in the location s of nodules within lung-, retrocardiac-, or subdiaphragmatic-equivale nt regions. Accuracy in detecting nodules was significantly lower at l ower exposures when nodules were located in the subdiaphragmatic-equiv alent region. Conclusion. On computed radiographs, small nodules (5-mm diameter) can be reliably detected when they are located in areas of high or moderate surrounding local contrast, such as the lung or media stinal regions. Detection of nodules decreases in regions of lower opt ical density corresponding to the subdiaphragmatic regions of the ches t. The decrease in nodule detectability is greatest under conditions t hat simulate large body thickness and underexposure.