Lung Volume Calculation in Preclinical MicroCT: A Fast Geometrical Approach

Abstract

In this study, we present a time-efficient protocol for thoracic volume calculation as a proxy for total lung volume. We hypothesize that lung volume can be calculated indirectly from this thoracic volume. We compared the measured thoracic volume with manually segmented and automatically thresholded lung volumes, with manual segmentation as the gold standard. A linear regression formula was obtained and used for calculating the theoretical lung volume. This volume was compared with the gold standard volumes. In healthy animals, thoracic volume was 887.45 mm³, manually delineated lung volume 554.33 mm³ and thresholded aerated lung volume 495.38 mm³ on average. Theoretical lung volume was 554.30 mm³. Finally, the protocol was applied to three animal models of lung pathology (lung metastasis and transgenic primary lung tumor and fungal infection). In confirmed pathologic animals, thoracic volumes were: 893.20 mm³, 860.12 and 1027.28 mm³. Manually delineated volumes were 640.58, 503.91 and 882.42 mm³, respectively. Thresholded lung volumes were 315.92 mm³, 408.72 and 236 mm³, respectively. Theoretical lung volume resulted in 635.28, 524.30 and 863.10.42 mm³. No significant differences were observed between volumes. This confirmed the potential use of this protocol for lung volume calculation in pathologic models.

Keywords: lung; microCT; preclinical imaging; quantification; volume.

Figure 1

Thoracic volume measuring process. (A) Axial reorientation of the thorax. (B) Short basis measure line placement at carina level (dot line) and height distance measurement (stripes and dots line) in coronal view. (C) Height measure (stripes and dots line) and big basis line placement at diaphragmatic level (stripes line). (D) Representative figure of thoracic truncated cone overlaid on thorax µCT.

Figure 2

Manually delineated and thresholded lung volume segmentations. (A) Manual delineation of lung volume, with ROI (red) covering the lung overlaid on transversal µCT image of the lung. Note the inclusion of intrapulmonary soft tissue. (B) Thresholded segmentation of aerated lung tissue (red ROI) applying a −700/−400 Hounsfield Units threshold. (C) Thresholded segmentation of aerated lung tissue (yellow ROI) with −700/−300 Hounsfield Units threshold. Note the different lung tissue inclusion in the segmentation between (B) and (C), depending on the applied threshold. (D,E) Three-dimensional rendering of the manually delineated segmentation (blue) and the thoracic volume superimposed (yellow). The inclusion of structures such as the heart in the thoracic volume is visually evident.

Figure 3

Thoracic, manually delineated and thresholded volumes from healthy mice group and Bland-Altman plot. (A) Distribution of mean values. (B) Correlation between thoracic and manually delineated lung volumes (R² = 0.7126). (C) Correlation between manual and threshold volumes (R² = 0.9526). (D) Correlation between thoracic and threshold lung volumes (R² = 0.6871). (E) Correlation between manually delineated lung volume and theoretical lung volume (R² = 0.8573). In all the cases, the correlation is statistically significant (p-values shown in Table A3). At the bottom, the Bland-Altman plot, where all the points except one are included in the range of 95% of confidence for the theoretical volume minus gold standard volume. The x-axis shows the average measurement of lung volume using both methods while y-axis shows the difference between the theoretical lung volume and manually delineated lung volume. The red line represents the average difference in measurements.

Figure 4

Representative lung microCT images of the different animal models used. (A) Healthy animal. Note the absence of intrapulmonary soft tissue structures except vessels, heart (white star) and the first part of the diaphragm (white asterisk). (B) Transgenic lung tumor model with a solitary lung tumor (white arrow) in contact with heart burden (white star). (C) Metastatic lung cancer model. Presence of multiple soft tissue structures compatible with lung tumors. (D) Fungal infection model. Presence of multiple hyperdense structures (fungal abscesses, asterisks) and a consolidated lung tissue dorsal to the heart (area delimited by a white dashed line).

Figure 5

Thoracic, manually delineated and thresholded lung volumes of pathologic groups. (A) Mean values of volumes from the three calculations in metastatic model, transgenic lung tumor model and fungal infection model) (B) Correlation between thoracic and manually delineated lung volumes including the data from the different animal models (mm³), R²: 0.619. (C) Correlation between manually delineated and thresholded lung volumes in the same batch of data (mm³), R²: 0.242. (D) Correlation between thoracic and threshold lung volumes (mm³), R²: 0.038. Note the lack of correlation between threshold volume and the other volumes, while there is a significant correlation between thoracic and manually delineated volumes, as in the healthy group. All the statistical values are displayed in Table A3.