S evere acute respiratory failure is the dominant cause of death in patients with coronavirus disease 2019 (CO-VID-19) (1). The pathophysiology and imaging features of severe COVID-19 pneumonia have been the focus of considerable interest from the outset of the pandemic. In early disease, widespread ground-glass opacification predominates at thoracic CT (2-6) and is supposedly associated with highly compliant lungs and disrupted vasoregulation (7). Vascular dysregulation is believed to be consequence of exaggerated activation of inflammatory and coagulation cascades (termed immunothrombosis) (1,(8)(9)(10)(11)(12). Later in the course of disease, CT more commonly shows consolidation and fibrosis associated with lower lung compliance (13).There is growing evidence from radiologic and pathologic studies of a significant vasculopathy in COVID-19 pneumonia (14-17); in a recent study of postmortem lungs in COVID-19, there were widespread microthromboses and striking new vessel formation (16). Furthermore, based on qualitative analyses, a number of studies have highlighted the potential role of dual-energy CT pulmonary angiography (DECTPA) (15,(18)(19)(20)(21). Accordingly, in the present study, we aimed to evaluate the relationships between a quantitative measure of perfusion at DECTPA (relative perfused blood volume [PBV], ie, PBV relative to pulmonary artery enhancement [PBV/PAenh]) ( 22) and (a) disease duration, (b) right ventricular dysfunction (RVD) at echocardiography, (c) d-dimer levels, and (d) obstruction score (23) in patients with severe COVID-19 pneumonia. A secondary aim was to compare PBV/PAenh in COVID-19 pneumonia to that of healthy volunteers.

Breathlessness is common in patients after COVID-19 [1]. Patients may have an isolated impairment of gas transfer (diffusion of the lung for carbon monoxide, DLCO) at lung function testing, often without obvious interstitial lung disease or classical pulmonary emboli (PE) on imaging. Iodine maps from post-COVID patients undergoing dual energy computed tomography (DECT) demonstrate hypoenhancement in areas of normal lung parenchyma [2] ( figure 1 ). We hypothesized that in breathless patients recovering from COVID-19, low DLCO would correlate with a CT marker of lung perfusion, measured using DECT-derived iodine enhancement, including in patients where parenchymal disease was absent. As an even more specific indicator for the pulmonary vascular compartment, we hypothesized that KCO (DLCO corrected for alveolar volume) would even better correlate with DECT perfusion, and more so than forced vital capacity (FVC) and CT measures of interstitial lung involvement.

Purpose: To assess if dual-energy computed tomographic pulmonary angiography (DECTPA) derived lobar iodine quantification can provide an accurate estimate of lobar perfusion in patients with severe emphysema, and offer an adjunct to single-photon emission CT perfusion scintigraphy (SPECT-PS) in assessing suitability for lung volume reduction (LVR).Materials and Methods: Patients with severe emphysema (forced expiratory volume in 1 s <49% predicted) undergoing evaluation for LVR between May 2018 and April 2020 imaged with both SPECT-PS and DECTPA were included in this retrospective study. DECTPA perfused blood volume maps were automatically segmented and lobar iodine mass was estimated and compared with lobar technetium (Tc99m) distribution acquired with SPECT-PS. Pearson correlation and Bland-Altman analysis were used for intermodality comparison between DECTPA and SPECT-PS. Univariate and adjusted multivariate linear regression were modelled to ascertain the effect sizes of possible confounders of disease severity, sex, age, and body mass index on the relationship between lobar iodine and Tc99m values. Effective radiation dose and adverse reactions were recorded.Results: In all, 123 patients (64.5 ± 8.8 y, 71 men; mean predicted forced expiratory volume in 1 s 32.1 ± 12.7%,) were eligible for inclusion. There was a linear relationship between lobar perfusion values acquired using DECTPA and SPECT-PS with statistical significance (P < 0.001). Lobar relative perfusion values acquired using DECTPA and SPECT-PS had a consistent relationship both by linear regression and Bland-Altman analysis (mean bias, −0.01, mean r 2 0.64; P < 0.0001). Individual lobar comparisons demonstrated moderate correlation (r = 0.79, 0.78, 0.84, 0.78, 0.8 for the right upper, middle, lower, left upper, and lower lobes, respectively, P < 0.0001). The relationship between lobar iodine and Tc99m values was not significantly altered after controlling for confounders including symptom and disease severity, age, sex, and body mass index.Conclusions: DECTPA provides an accurate estimation of lobar perfusion, showing good agreement with SPECT-PS and could potentially streamline preoperative assessment for LVR.

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