Quantitative and qualitative lung parameters in photon-counting CT: comparison of ultra-low-dose with standard low-dose protocols

OBJECTIVES: Photon-counting CT (PCCT) enables dose reduction beyond energy-integrating CT, but ultra-low-dose acquisitions may impair assessment of airways and lung parenchyma. We evaluated the impact of ultra-low-dose (ULD) PCCT on quantitative and qualitative lung parameters. METHODS: In this prospective study, 101 patients underwent same-session low-dose (LD) and ULD chest CT for mixed indications (CTDIvol, 0.64 ± 0.15 vs. 0.11 ± 0.03 mGy). Automated QCT assessed noise index, airway segmentation, airway wall percentage, emphysema index (EI), mean lung density (MLD), and Percentile15 (Perc15) on soft (Br36u) and lung (Bl64u) kernels. Two readers scored image quality, airways, and emphysema extent using Likert scales. RESULTS: ULD increased noise compared with LD (Bl64u: +94.4%; Br36u: +32.8%) and reduced visual image quality (p < 0.001). Automated airway segmentation decreased (-41.4 mean no. of branches, p < 0.001), with fewer visible airway generations (p < 0.001). Visual bronchial wall thickening did not differ (R1 p = 0.89; R2 p = 0.20). Airway wall percentage showed kernel- and generation-dependent differences: whole-tree values did not differ on Br36u (LD-ULD -0.05%; p = 0.83) but were higher at ULD on Bl64u (LD-ULD -3.33%; p < 0.001), with greater differences peripherally. At ULD, MLD and Perc15 were lower than at LD by 12.5 and 19.9 HU, respectively (both p < 0.001), while EI was 6.2% higher. Visual emphysema assessment shifted toward lower severity at ULD (p < 0.001). CONCLUSION: ULD-PCCT enables dose-efficient QCT but introduces limitations: reduced peripheral airway visibility/segmentation with differences in airway wall measurements, biased density-based emphysema quantification, and visual emphysema underestimation at ULD, supporting dose-aware interpretation and further validation of correction strategies. KEY POINTS: Question To study the influence of ultra-low-dose photon-counting chest CT on quantitative and qualitative airway and emphysema assessment compared to low-dose CT. Findings Ultra-low-dose increased noise and reduced peripheral airway segmentation; airway wall metrics showed kernel- and generation-dependent differences, with biased density-based and visual emphysema assessment. Clinical relevance PCCT enables quantitative lung imaging at ultra-low dose, but dose reduction alters airway segmentation and density-based emphysema metrics. ULD-derived QCT therefore requires cautious interpretation and validation of dose-aware correction strategies before routine clinical use.