Repeated low-dose computed tomography in current and former smokers for quantification of emphysema

Texture-based Quantification of Centrilobular Emphysema and Centrilobular Nodularity in Longitudinal CT Scans of Current and Former Smokers

RATIONALE AND OBJECTIVES

The effect of smoking cessation on centrilobular emphysema (CLE) and centrilobular nodularity (CN), two manifestations of smoking-related lung injury on computed tomography (CT) images, has not been clarified. The objective of this study is to leverage texture analysis to investigate differences in extent of CLE and CN between current and former smokers.

MATERIALS AND METHODS

Chest CT scans from 350 current smokers, 401 former smokers, and 25 control subjects were obtained from the multicenter COPDGene Study, a Health Insurance Portability and Accountability Act-compliant study approved by the institutional review board of each participating clinical study center. Additionally, for 215 of these subjects, a follow-up CT scan was obtained approximately 5 years later. For each CT scan, 5000 circular regions of interest (ROIs) of 35-pixel diameter were randomly selected throughout the lungs. The patterns present in each ROI were summarized by 50 computer-extracted texture features. A logistic regression classifier was leveraged to classify each ROI as normal lung, CLE, or CN, and differences in the percentages of normal lung, CLE, and CN by study group were assessed.

RESULTS

Former smokers had significantly more CLE (P <0.01) but less CN (P <0.001) than did current smokers, even after adjustment for important covariates such as patient age, GOLD stage, smoking history, forced expiratory volume in 1 second, gas trapping, and scanner model. Among patients with longitudinal CT scans, continued smoking led to a slight increase in CLE (P = 0.13), whereas sustained abstinence from smoking led to further reduction in CN (P <0.05).

CONCLUSIONS

The proposed texture-based approach quantifies the extent of CN and CLE with high precision. Differences in smoking-related lung disease between longitudinal scans of current smokers and longitudinal scans of former smokers suggest that CN may be reversible on smoking cessation.

Quantitative thoracic CT techniques in adults: can they be applied in the pediatric population?

With the rapid evolution of the multidetector row CT technique, quantitative CT has started to be used in clinical studies for revealing a heterogeneous entity of airflow limitation in chronic obstructive pulmonary disease that is caused by a combination of lung parenchymal destruction and remodeling of the small airways in adults. There is growing evidence of a good correlation between quantitative CT findings and pathological findings, pulmonary function test results and other clinical parameters. This article provides an overview of current quantitative thoracic CT techniques used in adults, and how to translate these CT techniques to the pediatric population.

Changes in volume-corrected whole-lung density in smokers and former smokers during the ITALUNG screening trial

PURPOSE

To evaluate with a volume-corrected whole-lung approach changes in lung density over 2 years consistent with progression of pulmonary emphysema in smokers and former smokers enrolled in the ITALUNG trial of lung cancer screening using low-dose computed tomography (LDCT).

MATERIALS AND METHODS

A total of 103 subjects (mean age 63±4 y with a pack-year history of at least 20) underwent 2 whole-lung LDCT examinations 2 years apart. Visual assessment was made independently by 2 experienced observers on the initial LDCT examination with a 0 to 4 grading system for each of 6 regions (right and left upper, mid, and lower lung). The whole-lung 15th percentile of attenuation coefficient and relative area (RA) at -910 HU, both corrected to the individual lung volume (Perc15v and RA910v), were measured on the 2 LDCT examinations. The intrasubject variability of Perc15v and RA910v was previously determined in 32 other subjects of the trial examined using the same scanner and technique twice over a 3-month interval for suspicious nodules.

RESULTS

The 2 operators agreed on the presence of mild to severe emphysema (visual score ≥1 in at least 1 region) at initial LDCT examination in 24 (23%) of the 103 subjects. Fifteen subjects (15%) showed a Perc15v change between the 2 examinations exceeding the lower 95% limit of agreement, indicating progression of emphysema with a mean difference in lung density of -14.7%±2.6%. Ten of the 15 were identified as showing emphysema progression by RA910v as well. No association was observed between progression of emphysema and visual evidence of emphysema at initial LDCT examination, smoking status, or pack-years at baseline, or intervening changes in smoking habits.

CONCLUSION

Once variations in inspiratory lung volumes are taken into account, changes in lung density over 2 years consistent with progression of pulmonary emphysema in elderly smokers and former smokers are uncommon.

Variation in quantitative CT air trapping in heavy smokers on repeat CT examinations

Objectives

To determine the variation in quantitative computed tomography (CT) measures of air trapping in low-dose chest CTs of heavy smokers.

Methods

We analysed 45 subjects from a lung cancer screening trial, examined by CT twice within 3 months. Inspiratory and expiratory low-dose CT was obtained using breath hold instructions. CT air trapping was defined as the percentage of voxels in expiratory CT with an attenuation below −856 HU (EXP₋₈₅₆) and the expiratory to inspiratory ratio of mean lung density (E/I-ratio_MLD). Variation was determined using limits of agreement, defined as 1.96 times the standard deviation of the mean difference. The effect of both lung volume correction and breath hold reproducibility was determined.

Results

The limits of agreement for uncorrected CT air trapping measurements were −15.0 to 11.7 % (EXP₋₈₅₆) and −9.8 to 8.0 % (E/I-ratio_MLD). Good breath hold reproducibility significantly narrowed the limits for EXP₋₈₅₆ (−10.7 to 7.5 %, P = 0.002), but not for E/I-ratio_MLD (−9.2 to 7.9 %, P = 0.75). Statistical lung volume correction did not improve the limits for EXP₋₈₅₆ (−12.5 to 8.8 %, P = 0.12) and E/I-ratio_MLD (−7.5 to 5.8 %, P = 0.17).

Conclusions

Quantitative air trapping measures on low-dose CT of heavy smokers show considerable variation on repeat CT examinations, regardless of lung volume correction or reproducible breath holds.

Key Points

• Computed tomography quantitatively measures small airways disease in heavy smokers.

• Measurements of air trapping vary considerably on repeat CT examinations.

• Variation remains substantial even with reproducible breath holds and lung volume correction.

Robust, standardized quantification of pulmonary emphysema in low dose CT exams

RATIONALE AND OBJECTIVES

The aim of this study was to present and evaluate a fully automated system for emphysema quantification on low-dose computed tomographic images. The platform standardizes emphysema measurements against changes in the reconstruction algorithm and slice thickness.

MATERIALS AND METHODS

Emphysema was quantified in 149 patients using a fully automatic, in-house developed software (the Robust Automatic On-Line Pulmonary Helper). The accuracy of the system was evaluated against commercial software, and its reproducibility was assessed using pairs of volume-corrected images taken 1 year apart. Furthermore, to standardize quantifications, the effect of changing the reconstruction parameters was modeled using a nonlinear fit, and the inverse of the model function was then applied to the data. The association between quantifications and pulmonary function testing was also evaluated. The accuracy of the in-house software compared to that of commercial software was measured using Spearman's rank correlation coefficient, the mean difference, and the intrasubject variability. Agreement between the methods was studied using Bland-Altman plots. To assess the reproducibility of the method, intraclass correlation coefficients and Bland-Altman plots were used. The statistical significance of the differences between the standardized data and the reference data (soft-tissue reconstruction algorithm B40f; slice thickness, 1 mm) was assessed using a paired two-sample t test.

RESULTS

The accuracy of the method, measured as intrasubject variability, was 3.86 mL for pulmonary volume, 0.01% for emphysema index, and 0.39 Hounsfield units for mean lung density. Reproducibility, assessed using the intraclass correlation coefficient, was >0.95 for all measurements. The standardization method applied to compensate for variations in the reconstruction algorithm and slice thickness increased the intraclass correlation coefficients from 0.87 to 0.97 and from 0.99 to 1.00, respectively. The correlation of the standardized measurements with pulmonary function testing parameters was similar to that of the reference (for the emphysema index and the obstructive subgroup: forced expiratory volume in 1 second, -0.647% vs -0.615%; forced expiratory volume in 1 second/forced vital capacity, -0.672% vs -0.654%; and diffusing capacity for carbon monoxide adjusted for hemoglobin concentration, -0.438% vs -0.523%).

CONCLUSIONS

The new emphysema quantification method presented in this report is accurate and reproducible and, thanks to its standardization method, robust to changes in the reconstruction parameters.

Inter-scan repeatability of CT-based lung densitometry in the surveillance of emphysema in a lung cancer screening setting

PURPOSE

To investigate the inter-scan repeatability of CT-based lung densitometry protocols in the surveillance of emphysema in a lung cancer screening setting.

MATERIALS AND METHODS

Fifty-two healthy subjects who underwent low-dose chest CT and subsequent follow-up scan within a 16 month interval were retrospectively evaluated. Inter-scan repeatabilities were assessed for 9 different CT-based lung densitometry protocols with standard deviation (SD) of inter-scan differences. Susceptibility to inspiratory level was additionally assessed for each protocol, and volume adjustment (VA) was applied in order to evaluate the potential improvement of repeatability after compensating the influence of inspiratory level.

RESULTS

A wide variation of inter-scan repeatability was observed among the evaluated protocols showing a difference of up to a factor of 9. Susceptibility of inspiratory level was found to be highly associated with the inter-scan repeatability of densitometric protocols. The application of VA could substantially reduce the influence of inspiratory level for all protocols, which results in an improvement of repeatability up to 51%. The order of repeatability among the protocols remained unchanged after VA. The resulting two best protocols in terms of inter-scan repeatability were RA970 and Perc1 which showed SD of 0.8% and 5.5 HU, respectively.

CONCLUSIONS

Lung densitometry protocols produce different levels of repeatability for an asymptomatic population, each being influenced by inspiratory level to a different degree. For surveillance of emphysema in a lung cancer screening setting, RA970 and Perc1 may be the most suitable protocols, in which the application of VA needs to be included as a critical part.