A Technique to Identify Isoattenuating Gallstones with Dual-Layer Spectral CT: An ex Vivo Phantom Study

Background Previously reported dual-energy CT methods for detecting noncalcified gallstones have reduced accuracy for gallstones smaller than 9 mm. Purpose To develop a dual-energy CT method for differentiating isoattenuating gallstones from bile and compare it with previously reported dual-energy CT methods by using a prospective ex vivo phantom reader study. Materials and Methods From May 2017 to May 2018, gallstones were collected from 105 patients (34 men; mean age, 51 years; age range, 18-84 years) undergoing cholecystectomy and placed inside 120-mL vials containing ox bile. The vials were placed inside a water-filled phantom and were scanned with dual-layer dual-energy CT. Thirty isoattenuating gallstones (4.3-24.7 mm in diameter) were evaluated. Conventional CT images, virtual noncontrast images, and monoenergetic images at 200 and 40 keV were created. Segmented images were created by using a two-dimensional histogram of Compton and photoelectric attenuation. Six readers evaluated the presence of isoattenuating gallstones in each image. Intra- and interreader agreement was measured by using percentage agreement, diagnostic performance was evaluated by using mean area under the receiver operating characteristic curve (AUC) estimates and pairwise comparisons, and the agreement of gallstone sizes measured at pathologic examination with those measured on segmented images was compared by using Bland-Altman analysis. Results For all gallstones, segmented images provided the highest mean intrareader (88.1%) and interreader (88.2% and 93.6%) agreements for all readers and reading sessions and the highest overall AUC (0.99; 95% confidence interval [CI]: 0.97, 1.00; adjusted P < .02 for all). For gallstones larger than 9 mm, no significant difference was found between the segmented and monoenergetic AUCs (all P > .94, adjusted P > .05 for all). For gallstones measuring 9 mm or smaller, the segmented images had the highest overall AUC (0.99; 95% CI: 0.97, 1.00; adjusted P < .01 for all). The mean difference in stone sizes was -0.6 mm, with limits of agreement from 2.6 to -3.8 mm. Conclusion Segmented images from Compton and photoelectric attenuation coefficients improve detection of isoattenuating gallstones compared with previously reported dual-energy CT methods. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Matos in this issue.