The effect of lung volume on nodule size on CT

Abstract

Rationale and objectives: We sought to determine how measures of nodule diameter and volume on computed tomography (CT) vary with changes in inspiratory level.

Materials and methods: CT scans were performed with inspiration suspended at total lung capacity (TLC) and then at residual volume (RV) in 41 subjects, in whom 75 indeterminate lung nodules were detected. A fully automated contouring program was used to segment the lungs; followed by segmentation of all nodules and the corresponding lobe using semiautomated contouring in both TLC and RV scans. The percent changes in lung and lobar volumes between TLC and RV were correlated with percent changes in nodule diameters and volumes.

Results: Both nodule diameter and volume varied nonuniformly from TLC to RV-some nodules decreased in size, while others increased. There was a 16.8% mean change in absolute volume across all nodules. Stratified by size, the mean value of the absolute percent volume changes for nodules > or =5 mm and <5 mm were not significantly different (P = .26). Stratified by maximum attenuation, the mean value of the absolute percent volume changes between the TLC and RV series for noncalcified (17.7%, SD = 13.1) and completely calcified nodules (8.6% SD = 5.7) were significantly different (P < .05).

Conclusion: Significant differences in nodule size were measured between TLC and RV scans. This has important implications for standardizing acquisition protocols in any setting where size and, more important, size change are being used for purposes of lung cancer staging, nodule characterization, or treatment response assessment.

Figure 1

This graph plots nodule volume at TLC relative to nodule volume at RV. The dashed line is the line of identity, in which nodule volumes are equivalent between RV and TLC. The solid line represents our results and lies to the left of the line of identity (value of x axis = value of y axis). The fact that there are points on both sides of the identity line indicates a non-uniform direction of change in volume with varying lung volumes.

Figure 2

The percent change in nodule volumes is plotted against the corresponding percent change in lobar volumes. Nodule volume varied non-uniformly between TLC and RV: some volumes decreased, while some increased. The flat line fits our dataset, indicating the absence of correlation between percent change in nodule volume and lobar volume.

Figure 3

Volume rendering of a right upper lobe nodule shows a 21% increase in nodule volume in going from (a) value at TLC (815mm³) to (b) value at RV (987mm³). The rendering captures changes in both the size and shape of the nodule in two different breathhold conditions.

Figure 4

(A, B) Axial images and (C, D) 3D volume rendering of a left upper lobe nodule. The axial images show the segmentation results through the nodule at its greatest diameter at TLC (panel A) and RV (panel B). The nodule exhibits a 24% decrease in volume when going from value at TLC (930mm³) to the value at RV (747 mm³). The volume renderings at TLC (panel C) and RV (panel D) show that the nodule changes in both size and configuration between the two breathhold series.

Figure 5

This graph plots maximum nodule attenuation vs. the absolute value of percent volume change. The non-calcified nodules are represented in this area of maximum HU < 200. The partially calcified nodules are visible in the area between maximum HU ≥ 200 and < 1000 HU, and completely calcified nodules are represented by attenuation ≥ 1000 HU.