Quantitative computed tomography (CT) has become a valuable tool in assessing lung diseases, offering precise insights into airway alterations that are otherwise challenging to visualise. While traditionally applied to chronic conditions such as COPD and interstitial lung disease, its use in acute infections has expanded significantly since the onset of the COVID-19 pandemic. Inflammatory processes within the lung, particularly airway wall thickening and parenchymal changes, can now be investigated with advanced imaging techniques like spectral CT. A recent study published in European Radiology explored the feasibility of using spectral CT to distinguish between COVID-19 and non-COVID-19 pneumonia, focusing on airway wall thickening and contrast enhancement.
Distinctive Patterns of Airway Changes
In this retrospective single-centre study, data from 331 individuals, including lung-healthy controls, COVID-19 patients and non-COVID-19 pneumonia cases, were analysed using spectral CT imaging. Airway parameters were assessed using spectral imaging at different energy levels to calculate the slope of the spectral attenuation curve (λHU), which serves as a surrogate for iodine-based contrast enhancement. Airway wall thickening, particularly in generations 5 to 10 of the bronchial tree, was significantly more pronounced in both pneumonia subtypes compared to controls. However, this thickening was quantitatively similar between COVID-19 and non-COVID-19 groups in the arterial phase, suggesting that structural wall changes are a common response to infectious inflammation regardless of the pathogen involved.
Interestingly, median maximum airway wall attenuation, while elevated in both disease groups, was markedly higher in COVID-19 patients during the venous phase. These findings underline the shared inflammatory response in the airways but hint at additional vascular mechanisms in COVID-19 that influence contrast dynamics. The increased airway wall thickness and attenuation values confirm the sensitivity of spectral CT to detect subtle inflammatory changes, even when traditional imaging may not differentiate between infection types.
Airway Wall Contrast Enhancement in COVID-19
A key differentiator identified in this study was the extent of airway wall contrast enhancement, particularly in the arterial phase. The slope of the spectral attenuation curve (λHU5-10) was significantly higher in COVID-19 pneumonia than in non-COVID-19 pneumonia and healthy controls. This suggests increased arterial blood flow or neovascularisation in the bronchial walls of COVID-19 patients. The enhancement differences were not observed in the venous phase, supporting the hypothesis that COVID-19 triggers a unique early-phase vascular response, possibly driven by peribronchial neovascular changes.
This vascular pattern aligns with histological observations of dilated and abnormal bronchial vessels in post-mortem COVID-19 specimens. The early enhancement captured by arterial-phase imaging indicates that spectral CT may serve as a non-invasive tool for identifying such vascular phenomena. The findings further support the utility of λHU5-10 as a specific marker for COVID-19-related airway vascular changes, which are not as pronounced in other pneumonia types.
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Technical and Methodological Considerations
The study ensured comparability across groups by standardising contrast administration and evaluating contrast enhancement in key vascular structures. While some differences were noted in descending aorta opacification, particularly in the arterial phase, these were attributed to physiological variations such as BMI rather than methodological flaws. The consistency of contrast protocols supports the reliability of observed differences in airway parameters.
However, the study also acknowledges certain limitations. The retrospective design and use of standard clinical protocols limited the ability to optimise scan timing for airway wall enhancement. Furthermore, the non-COVID-19 pneumonia group included various pathogen types, potentially introducing variability that could mask more subtle inter-group differences. While the assumption that higher λHU values reflect neovascularisation is supported by existing literature, direct histological validation remains absent. Additionally, the simplified linear approach to calculating λHU from two energy levels, though practical, does not fully capture the complex behaviour of iodine attenuation in vivo.
Spectral CT enables the quantification of airway wall thickening and contrast enhancement with high sensitivity. The technology effectively detects shared inflammatory changes in COVID-19 and non-COVID-19 pneumonia, yet it also reveals a distinct pattern of early arterial-phase contrast enhancement in COVID-19 patients. This differentiation is likely linked to unique vascular responses, potentially including neovascularisation, associated with COVID-19. The slope of the spectral attenuation curve (λHU5-10) emerges as a promising imaging biomarker for detecting these vascular changes. While further research is needed to validate and refine these findings, particularly with histological correlation and standardised scan timing, spectral CT holds substantial promise for improving the characterisation of infectious lung diseases and guiding targeted clinical management.
Source: European Radiology
Image Credit: iStock
