1184 Table 2 Repeatability: relative volume difference (%) among three assessments for each reconstructive setting in solid pulmonary nodules <500 mm3 by location and morphology Nodule characteristics Number (%) Relative volume difference (%) 1 mm and soft kernel (A) 2 mm and soft kernel (B) 2 mm and sharp kernel (C) Location Purely intra-parenchymal 129 (64.5) Attached 71 (35.5) Morphology Smooth-round 153 (76.5) Non-smooth-round 47 (23.5) Total 200 (100) 6.0 12.6 6.6 14.1 8.9 23.6 20.4 22.7 21.7 22.5 26.2 54.1 29.4 59.1 37.5 represent real growth but could be caused by the use of thickness is doubled, these two voxels will form one voxel different reconstruction settings. Therefore, consistency in with an average of -450 HU. This voxel will then be the use of reconstruction settings is recommended during excluded from segmentation as it is below the threshold follow up in order to avoid unreliable growth assessments and subsequently lead to a decrease in volume compared and false-positive test results in CT screening programs. with the original one. When the two original voxels have In our study we demonstrated that there is poor CT values of -500 and -200 HU, then the new voxel will be agreement between the volume measurements on CT included in the segmentation and lead to an increase in the reconstructed with different section thicknesses and measured volume. Therefore, the effect of section thickness kernels, which has also been reported in previous studies. on measured volume is dependent on the density of the In two studies including 42 pulmonary metastases and 75 nodules. pulmonary nodules, an increase in section thickness led to Smaller nodules (mean volume 71 mm3) in our study an increase in nodule volume measured [20, 21]. However, were more inclined to be influenced by the partial-volume we found that an increase in section thickness to 2 from effect and had lower CT density than larger nodules (mean 1 mm led to a decrease in the measured volume, which is volume 348 and 1,259 mm3) in the above-mentioned contradictory to previous studies. This can be explained by studies. Consequently, the increase in the section thickness an averaging effect of the CT density of adjoining voxels caused a decrease in the measured volume in our study, when the section thickness increases. For example, if there contradictory to the mentioned studies (Fig. 2). With are two adjoining voxels along the longitudinal axis with respect to the reconstruction kernel, Honda et al. have CT values of -600 and -300 HU, only one voxel will be reported that using a high-spatial frequency reconstruction included in the segmentation by the volumetric software as algorithm led to significantly larger volumes than using the applied threshold was -400 HU. When the section low-spatial frequency algorithms. A low-spatial algorithm Table 3 Effects of reconstruction setting, repeated measures and nodule characteristics on the measured volume by multi-level modelling Regression coefficient (95% confidence interval) P value A: 1 mm thickness and soft kernel B: 2 mm thickness and soft kernel C: 2 mm thickness and sharp kernel 1 2 3 Intra-parenchymal Attached Smooth-round Non-smooth-round B × intra-parenchymal A × attached C × attached B × smooth-round A × non-smooth-round C × non-smooth-round 1.29 (1.03–1.63) 1 1.29 (1.03–1.63) 1 1.01 (1–1.02) 1.01 (1–1.02) 1 1.74 (1.39–2.18) 1 1.83 (1.42–2.37) 1 0.93 (0.88–0.99) 0.90 (0.84–0.95) 1 0.97 (0.90–1.03) 0.91 (0.85–0.98) 0.03 0.03 0.03 0.03 <0.01 <0.01 0.03 0.01 0.36 0.01 Variable Settings Measuring times Location Morphology Setting × location Setting × morphology