1181 normal-dose chest CT, and no data are available from low- three settings were the same and large enough to cover the dose chest CT so far. Furthermore, all these studies focused complete lung cross-section. on the agreement between the volumes measured with the Each low-dose chest CT examination was reconstructed different reconstruction settings, but none of them addressed using three different settings immediately after imaging with the repeatability of each individual reconstruction setting. the following three combinations of section thickness/ For reconstruction settings with a lower repeatability, increment and kernel: (A) 1.0 mm/0.7 mm and a soft agreement is also expected to be lower. The purpose of this kernel (Siemens B30 filter), (B) 2.0 mm/1.4 mm and a soft study was to evaluate volume measurement variability in kernel (Siemens B30) and (C) 2.0 mm/1.4 mm and a pulmonary nodules detected with low-dose chest CT with sharp kernel (Siemens B60). Setting A was the standard three reconstruction settings including the repeatability and setting for the screening project [9], settings B and C were to quantify the effect of reconstruction settings on the volume used for this particular study. Since a thicker section is less measurement. effective for nodule detection, only a 2 mm section thickness was chosen in our study [23]. The Siemens B30 kernel is the standard soft-tissue reconstruction kernel, and B60 is the Patients and methods bone reconstruction kernel, widely used in high resolution chest CT at normal dose levels. Nodule selection This study is a side study of the Dutch-Belgian multi-centre Volumetric analysis randomised low-dose CT lung cancer screening trial NELSON [9]. The NELSON study has been approved by To evaluate the effect of reconstruction settings on the Minister of Health of the Netherlands and the ethics volumetric measurement variability, each nodule was committees of all four participating hospitals. Informed measured three consecutive times on every reconstructed consent was obtained from all participants. The original CT image by one radiologist with 8 years of experience in approval and informed consent for the screening study chest CT. Digital workstations (Leonardo, Siemens Medical included the ability to use data for future research, Solutions, Erlangen, Germany) were used for nodule including the current side study. volumetric analysis, with U.S. Food and Drug Administration This side study concerned an analysis of category II (FDA) approved, commercially available software for semi- 3 and III solid nodules (15–500 mm , effective diameter automated volume measurements (LungCare, Somaris/5, 3.1–9.8 mm) according to the NELSON classification VB28B-W, Siemens Medical Solutions). [9]. Participants screened in the Groningen NELSON After the nodule was marked with a mouse click, the study centre between September 2007 and February program automatically defined the volume of interest 2008 (n = 562, 556 males, 6 females), were consecu- (VOI) around the nodule. A 3D template was generated, tively enrolled in this side study when they were which represented the nodule in its optimal way, if possible diagnosed with non-calcified, solid nodules with a without inclusion of surrounding structures. A second volume smaller than 500 mm3. For every enrolled mouse click initiated the automated volume measurement, participant, all qualified nodules were included. Finally, which included the following steps [14]: first, a fixed 79 male and 3 female participants with 200 nodules attenuation threshold of −400 HU was applied to extract a were included in the analysis, having a mean ± SD age 3D-connected “structure of interest”. This “structure of of 64 ±5 years. interest” can also include surrounding structures such as vessels or parts of the chest wall. Apart from this process, a small spherical 3D template originating from the click Imaging point was gradually expanded and its cross-correlation with the segmented nodule was computed for each step. The Low-dose chest CT examinations were performed by using peak value of the cross-correlation curve was determined 64 detector-row helical CT (Somatom-64, Siemens Med- and an empirical cut-off value close to the peak value was ical systems, Forchheim, Germany) with the following used to separate the nodule from its potential adjacent parameters: 0.5 s tube rotation, 32 × 0.6 mm collimation structures. Finally, segmentation was completed by fusing with z-sample system and 28.8 mm table feed per rotation the optimal 3D template and the “structure of interest”. The (pitch = 1.5). A caudal-cranial direction without contrast segmented nodule was then shown in yellow on the was used. Images were obtained from the level of the lung volume-rendered display as the VOI. If the reader was not bases (posterior recesses) to the lung apex with the help of a satisfied with the fit of the VOI, a manual modification of scout view. Exposure settings were 20–30 mAs and 100– the segmentation by increasing or decreasing the volume 140 kVp depending on the weight of the participant. This could be performed. Manual adjustment of the VOI was not corresponds to a dose-length-product <107 mGy·cm and an performed in this side study in order to avoid reader- effective radiation dose <1.6 mSv. The fields-of-view of all induced volume measurement variability.