Volumetric measurement of artificial pure ground-glass nodules at low-dose CT: Comparisons between hybrid iterative reconstruction and filtered back projection

Purpose To compare hybrid iterative reconstruction (HIR) with filtered back projection (FBP) in the volumetry of artificial pure ground-glass nodules (GGNs) with low-dose computed tomography (CT). Materials and methods Artificial GGNs (10 mm-diameter, 523.6 mm³, -660 HU) in an anthropomorphic chest phantom were scanned by a 256-row multi-slice CT with three dose levels (10, 30, 100 mAs). Each scan was repeated six times. Each set was reconstructed by FBP and HIR at 0.625-mm thickness. The volumes of artificial GGNs placed at the lung apex and middle lung field of the chest phantom were measured by two observers. Semi-automated measurements were performed by clicking the cursor in the center of GGNs, and manual measurements were performed by tracing GGNs on axial section. Modification of the trace was added on a sagittal or coronal section if necessary. Measurement errors were calculated for both the FBP and HIR at each dose level. We used the Wilcoxon signed rank test to identify any significant difference between the measurement errors of the FBP and HIR. Inter-observer, intra-observer, and inter-scan variabilities were evaluated by Bland Altman analysis with limits of agreements given by 95% confidence intervals. Results There were significant differences in measurement errors only at the lung apex between FBP and HIR with 10 mAs in both the semi-automated (observer 1, -37% vs. 7.2%; observer 2, -39% vs. 1.9%) and manual methods (observer 1, -29% vs. 7.5%; observer 2, -30% vs. 1.1%), respectively (P < 0.05). HIR provided each variability equal to or less than one half of that of FBP at 10 mAs in both methods. In the semi-automated method, the inter-observer and intra-observer variabilities obtained by HIR at 10 mAs were -11% to 17% and -6.7% to 6.7%, whereas those for FBP at 10 mAs were -29% to 30% and -38% to 20%, respectively. The inter-scan variability for FBP at 100 mAs vs. HIR at 10 mAs was -9.5% to 11%, and that for FBP at 100 mAs vs. FBP at 10 mAs was -73% to 32%. In the manual method, the inter-observer and intra-observer variabilities for HIR at 10 mAs were -14% to 22% and -9.8% to 22%, and those for FBP at 10 mAs were -45% to 36% and -31% to 28%, respectively. The inter-scan variability for FBP at 100 mAs vs. HIR at 10 mAs was -7.4% to 23%, and that for FBP at 100 mAs vs. FBP at 10 mAs was -52% to 26%. Conclusion HIR is superior to FBP in the volumetry of artificial pure GGNs at lung apex with low-dose CT.