Impact of Contrast Enhancement and Virtual Monoenergetic Image Energy Levels on Emphysema Quantification

Abstract

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.

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