Impact of Contrast Enhancement and Virtual Monoenergetic Image Energy Levels on Emphysema Quantification: Experience With Photon-Counting Detector Computed Tomography.

Purpose: The aim of this study was to evaluate the impact of contrast enhancement and different virtual monoenergetic image energies on automatized emphysema quantification with photon-counting detector computed tomography (PCD-CT).

Material And Methods: Sixty patients who underwent contrast-enhanced chest CT on a first-generation, clinical dual-source PCD-CT were retrospectively included. Scans were performed in the multienergy (QuantumPlus) mode at 120 kV with weight-adjusted intravenous contrast agent. Virtual noncontrast (VNC) images as well as virtual monoenergetic images (VMIs) from 40 to 80 keV obtained in 10-keV intervals were reconstructed. Computed tomography attenuation was measured in the aorta. Noise was measured in subcutaneous fat and defined as the standard deviation of attenuation. Contrast-to-noise with region of interest in the ascending aorta and signal-to-noise ratio in the subcutaneous fat were calculated. Subjective image quality (and emphysema assessment, lung parenchyma evaluation, and vessel evaluation) was rated by 2 blinded radiologists. Emphysema quantification (with a threshold of -950 HU) was performed by a commercially available software. Virtual noncontrast images served as reference standard for emphysema quantification.

Results: Noise and contrast-to-noise ratio showed a strong negative correlation (r = -0.98; P < 0.01) to VMI energies. The score of subjective assessment was highest at 70 keV for lung parenchyma and 50 keV for pulmonary vessel evaluation (P < 0.001). The best trade-off for the assessment of emphysema while maintaining reasonable contrast for pulmonary vessel evaluation was determined between 60 and 70 keV. Overall, contrast-enhanced imaging led to significant and systematic underestimation of emphysema as compared with VNC (P < 0.001). This underestimation decreased with increasing VMI-energy (r = 0.98; P = 0.003). Emphysema quantification showed significantly (P < 0.05) increased emphysema volumes with increasing VMI energies, except between 60-70 keV and 70-80 keV. The least difference in emphysema quantification between contrast-enhanced scans and VNC was found at 80 keV.

Conclusion: Computed tomography emphysema quantification was significantly affected by intravenous contrast administration and VMI-energy level. Virtual monoenergetic image at 80 keV yielded most comparable results to VNC. The best trade-off in qualitative as well as in quantitative image quality evaluation was determined at 60/70 keV.