Severe COVID-19 pneumonia: Perfusion analysis in correlation with pulmonary embolism and vessel enlargement using dual-energy CT data.

BACKGROUND: Gas exchange in COVID-19 pneumonia is impaired and vessel obstruction has been suspected to cause ventilation-perfusion mismatch. Dual-energy CT (DECT) can depict pulmonary perfusion by regional assessment of iodine uptake.
OBJECTIVE: The purpose of this study was the analysis of pulmonary perfusion using dual-energy CT in a cohort of 27 consecutive patients with severe COVID-19 pneumonia.
METHOD: We retrospectively analyzed pulmonary perfusion with DECT in 27 consecutive patients (mean age 57 years, range 21-73; 19 men and 8 women) with severe COVID-19 pneumonia. Iodine uptake (IU) in regions-of-interest placed into normally aerated lung, ground-glass opacifications (GGO) and consolidations was measured using a dedicated postprocessing software. Vessel enlargement (VE) within opacifications and presence of pulmonary embolism (PE) was assessed by subjective analysis. Linear mixed models were used for statistical analyses.
RESULTS: Compared to normally aerated lung 106/151 (70.2%) opacifications without upstream PE demonstrated an increased IU, 9/151 (6.0%) an equal IU and 36/151 (23.8%) a decreased IU. The estimated mean iodine uptake (EMIU) in opacifications without upstream PE (GGO 1.77 mg/mL; 95%-CI: 1.52-2.02; p = 0.011, consolidations 1.82 mg/mL; 95%-CI: 1.56-2.08, p = 0.006) was significantly higher compared to normal lung (1.22 mg/mL; 95%-CI: 0.95-1.49). In case of upstream PE, EMIU of opacifications (combined GGO and consolidations) was significantly decreased compared to normal lung (0.52 mg/mL; 95%-CI: -0.07-1.12; p = 0.043). The presence of VE in opacifications correlated significantly with iodine uptake (p<0.001).
CONCLUSIONS: DECT revealed the opacifications in a subset of patients with severe COVID-19 pneumonia to be perfused non-uniformly with some being hypo- and others being hyperperfused. Mean iodine uptake in opacifications (both ground-glass and consolidation) was higher compared to normally aerated lung except for areas with upstream pulmonary embolism. Vessel enlargement correlated with iodine uptake: In summary, in a cohort of 27 consecutive patients with severe COVID-19 pneumonia, dual-energy CT demonstrated a wide range of iodine uptake in pulmonary ground-glass opacifications and consolidations as a surrogate marker for hypo- and hyperperfusion compared to normally aerated lung. Applying DECT to determine which pathophysiology is predominant might help to tailor therapy to the individual patient´s needs.

Introduction

Oxygenation of patients suffering from COVID-19 pneumonia is reported to be difficult due to a ventilation-perfusion mismatch [Am J Respir Crit Care Med. ">1-4]. Over the course of the SARS-CoV-2 pandemic we had to learn that COVID-19 is associated with a high risk of thrombotic and embolic events notably in the second, hyperinflammatory stage of disease [5-12]. Autopsy studies showed endothelial injury and widespread thrombosis with microangiopathy [13, 14]. Thus, ventilation-perfusion mismatch is thought to be mainly due to dead space ventilation. Therefore, aggressive anticoagulation is meanwhile accepted part of therapy [15]. Somewhat contradictory to the outlined pathophysiology is the fact that vessel enlargement within pulmonary opacifications is a frequent finding on CT indicating rather hyperaemia than vessel occlusion [16-18]. Ventilation-perfusion mismatch might therefore also be caused by shunting [1, 3].

Dual-energy computed tomography (DECT) pulmonary angiography is an established tool to assess the regional distribution of pulmonary perfusion [19-21]. Several groups applied DECT to analyze pulmonary perfusion patterns in COVID-19 pneumonia [22-26]. The results are inhomogeneous. Si-Mohamed et al. found lobes with predominant GGO (early phase of disease) to be hyperperfused and lobes with predominant consolidation (late phase of disease) to be hypoperfused [22]. On the contrary, Ridge saw a decreasing amount of perfusion defects after day 14 [23]. Grillet et al. reported hyperperfusion of both ground glass opacities (GGO) and consolidation [24]. Lang et al. reported about a mixture of hypo- and hyperperfused opacifications [26]. The latter two groups did not provide any data about the phase of disease. Different disease phases, extremely inhomogeneous patient groups with different portions of mildly and severely diseased patients, different modes of analysis (subjective versus objective) and missing correlation with upstream pulmonary embolism might be the reasons for the conflicting results.

The purpose of this study was the objective analysis of pulmonary perfusion using DECT in a cohort of 27 consecutive patients with severe COVID-19 pneumonia including correlation with vessel enlargement and pulmonary embolism.

Materials and methods

From April 1st, 2020 all CTs of the chest in patients with severe COVID-19 pneumonia (dyspnea, respiratory frequency ≥ 30/min, blood oxygen saturation ≤ 93%, partial pressure of arterial oxygen to fraction of inspired oxygen ratio < 300, and / or lung infiltrates > 50% within 24 to 48 hours) [27] were performed contrast-enhanced using dual-energy mode. These prospectively acquired data were retrospectively evaluated. The study was approved by the institutional ethics committee (ethics committee of the Regensburg University Medical Center, IRB nr. 20-1784-104). Written informed consent was waived. All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and / or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Study population

The inclusion criteria were consecutive adult patients (≥ 18 years old) with RT-PCR-proven SARS-CoV-2 infection and COVID-19 pneumonia who had a contrast-enhanced DECT of the chest performed between April 1st and May15th 2020 at our university tertiary care medical center. Image quality of all examinations was rated before analysis. Exclusion criteria were non-diagnostic CTs due to technical artifacts. In case of more than one CT per patient, the first CT only was analyzed. Patients were identified by means of a full-text database query of all CT-scans performed between April 1st and May 15th 2020 using the term “*COVID*” OR *SARS* in the Radiological Information System (RIS, Nexus.med RIS, Version 8.42, Nexus, Villingen-Schwenningen, Germany) and by searching the electronic patients records for SARS-CoV-2 RT-PCR results. Patient characteristics (age, gender, date of symptom onset, referring ward and ventilation and / or ECMO status) and anticoagulation and corticosteroid therapy status at the time of the dual-energy CT were extracted from electronic patient records. CT imaging parameters including contrast medium injection modus were extracted from the DICOM protocols of the scans.

CT protocol

All patients underwent contrast-enhanced DECT of the chest with or without additional scans of the head and / or abdomen. All DECT examinations were performed in supine position during end-inspiratory hold using a second generation dual-source CT scanner (Siemens Somatom Definition Flash; Siemens Healthcare, Erlangen, Germany). The contrast medium was iohexol (Accupaque 350, GE Healthcare Buchler, Braunschweig, Germany) administered at a flow rate of 3–5 mL/s. 70 mL were injected for examinations of the chest only. 100 mL were injected for chest examinations in combination with other body regions. The scan was triggered with a bolus-tracking technique. For this, a region of interest (ROI) was placed into the main pulmonary artery. The scan started with a scan delay of 5 seconds after reaching a threshold of 100 Hounsfield Units (HU). All DECT were performed with 100 kVp (tube A) and 140 kVp with tin filter (tube B) with a quality reference mA setting of 150 mA (for tube A) using the combined angular and longitudinal automatic tube current modulation technique (CARE Dose 4D, Siemens Healthcare, Erlangen, Germany).

Image postprocessing

From each DECT data set perfusion images were calculated using a dedicated dual-energy postprocessing software for assessment of pulmonary perfused blood volume (PBV) (syngo.CT Dual Energy, version VB30A, Siemens Healthcare, Erlangen, Germany). The preset parameters for tissue recognition were minimum -960 HU and maximum -600 HU. These values are the default setting for evaluation of normally aerated lung and do not allow for analysis of opacified lung. Thus, the maximum was manually set to +150 HU to also include ground glass opacities and consolidation into the analysis. Normalization was performed by placing a dedicated ROI into the main pulmonary artery. Perfusion images were reconstructed as maximum intensity projections (MIP) in three planes (axial, coronal, sagittal) with a slice thickness of 5 mm and an increment of 5 mm. For colour-coding the look-up table (LUT) overlay „Spectrum 10”was applied. Distribution of colours were set in such a way that another colour was displayed for an enhancement increase of 8 HU each. Moreover, multiplanar reconstructions (MPR) applying lung (B60f) and soft-tissue (I31f/2) kernel were generated from weighted average images (60% from the 140 kVp and 40% from the 100 kVp data). MPRs were reconstructed in three planes (axial, coronal, sagittal) with a slice thickness of 1 mm and an increment of 0.75 mm. The images were sent to a picture archiving and communication system (PACS, Syngo Imaging, Siemens Healthcare, Erlangen, Germany).

Image analysis

In a first step, overall image quality considering motion and beam hardening artifacts, contrast-noise ratio and opacification of pulmonary arteries of all dual-energy CT examinations meeting the inclusion criteria was subjectively assessed by two readers in consensus: one board-certified general radiologist with 12 years of experience (JR) and one subspeciality thoracic radiologist with 15 years of experience (FP), using a four-point scale: 1, excellent, no artifacts; 2, good, mild impairment of image quality without impairment of evaluability; 3, satisfactory, moderate impairment of image quality without impairment of evaluability; and 4, inadequate, severe artifacts, not evaluable for pulmonary perfusion analysis. Dual-energy image analysis was performed in consensus by a subspecialty thoracic radiologist with 15 years of experience (FP) and a board-certified general radiologist with 12 years of experience (JR). Both readers were blinded to clinical patient status. Colour-coded pulmonary perfused blood volume (PBV) images were displayed side by side with MPR in lung and soft-tissue window. The colour-coded PBV maps were displayed as axial maximum intensity projections at 5 millimeters reconstruction slice thickness. MPRs were analyzed regarding the presence and distribution of GGO and consolidation. The ROIs for consolidation and GGO were drawn as large as possible. Mixed areas were intentionally excluded in order to collect data purely representing consolidation and GGO, respectively. Pulmonary embolism (PE) was recorded. The diameter of vessels within opacifications was evaluated. Vessel enlargement was defined as clearly larger vessel diameter compared to vessels of the same generation in normally aerated lung tissue. Also, pulmonary embolism (PE) upstream of opacification was recorded.

Iodine uptake (mg/mL lung tissue) was measured 1. by placing a ROI in normally aerated lung, 2. by placing a ROI in each of up to three separate areas of GGO and 3. by placing a ROI in each of up to three separate areas of consolidation. The decision which areas of GGO and consolidation were analyzed was made according to the colour-coded PBV map. Opacifications with least, medium and most intense enhancement were selected in order to record the entire range of perfusion of GGO and consolidation, respectively.

Statistical analysis

Categorical data are presented as absolute and relative frequencies, while continuous data are presented using mean, standard deviation (SD) and range (min-max). Differences in the iodine uptake between normally aerated lung and opacified lung (separated into GGO and consolidation), between vessel enlargement and no vessel enlargement, and between iodine uptake downstream to pulmonary embolism and no pulmonary embolism, were analyzed by using linear mixed models. To account for repeated measurements of patients within the GGO and consolidation areas, patients nested within the respective area were added as a random factor to the models. Estimated means and corresponding 95%-confidence intervals are presented as effect estimates. A p-value <0.05 was considered statistically significant for all analyses. All analyses were performed by using SAS (Version 9.4, The SAS institute, Cary, NC) and the procedure proc mixed.

Results

Patient population

29 patients were eligible. The CTs of 2 patients were excluded due to severe technical artifacts (classified as score of 4 in overall image quality assessment, see below). Thus 27 patients (19 men (70%) and 8 women (30%)) were included in the study. Mean age was 57 years (SD 13 years, range 21–73 years). The mean interval from symptom onset to the date of the DECT was 23.4 days (SD 12.8 days; range: 7–62 days). At the time of DECT, 26 patients (96%) were treated on the intensive care unit, and one patient (4%) was treated on the general ward. 24 patients (89%) were mechanically ventilated, and 9 patients (33.3%) were on extracorporeal membrane oxygenation. The clinical indication for DECT was critical worsening of symptoms in 25 patients (92.6%) and suspicion of pulmonary embolism in 5 patients (18.5%). At the time of DECT, all 27 patients had at least prophylactic anticoagulation therapy, with 16 patients (59.3%) receiving full-dose therapeutic anticoagulation and 2 patients (7.4%) receiving half-therapeutic dose anticoagulation. At the time of DECT, 8 patients (29.6%) received corticosteroid therapy. 20 DECT examinations were combined scans of several body regions (head, abdomen). 7 scans were chest examinations only. Patient demographics are summarized in Table 1.

Table 1

Patient demographics.

Patient demographics
Age (years), mean (SD, range)	57 (13, 21–73)
Gender	19 m (70%), 8 w (30%)
Days from symptom onset to date of DECT, mean (SD, range)	23.4 (12.8, 7–62)
Patients on intensive care unit	26 (96%)
Patients mechanically ventilated	24 (89%)
Patients on general ward	1 (4%)

Overall image quality of the evaluated dual-energy CTs was excellent (score of 1) in 7 cases (24.1%), good (score of 2) in 16 cases (55.2%), satisfactory (score of 3) in 4 cases (13.8%) and inadequate (score of 4) in 2 cases (6.9%). The two cases with score 4 were excluded from analysis. The estimated mean iodine uptake of normally aerated lung was 1.22 mg/mL (95%-CI 0.95, 1.49). Three different areas of GGO could be analyzed in all 27 scans, resulting in 81 measurements. As for consolidation, three measurements could be made in 25 scans and two measurements in 2 scans, resulting in 79 measurements (Table 2). Fig 1 illustrates the range of iodine uptake of both GGO and consolidation displayed as difference to normally aerated lung for each scan.

Table 2

Overview of measurements of iodine uptake and CT findings.

				Iodine uptake (mg/mL)
Patient#	Age (years)	Gender	Interval from symptom onset to CT (days)	Normal lung	GGO #1	VE GGO #1	PE GGO #1	GGO #2	VE GGO #2	PE GGO #2	GGO #3	VE GGO #3	PE GGO #3	CO #1	VE CO #1	PE CO #1	CO #2	VE CO #2	PE CO #2	CO #3	VE CO #3	PE CO #3
1	68	m	16	0.3	0.5	+	-	1	+	-	0.8	+	-	1.5	+	-	0.2	-	+	0.7	-	-
2	65	m	23	0.3	1.5	+	-	0.4	-	-	1.5	+	-	1.8	+	-	1.4	+	-	0.1	-	-
3	68	w	24	2.1	2.7	+	-	2.7	+	-	2.5	+	-	1.9	-	-	1.8	-	-	2.1	+	-
4	60	m	23	1.2	1	-	-	0.4	-	-	0.7	-	-	1.5	+	-	0.3	-	-	0.6	-	-
5	68	w	16	1.6	1.3	-	-	1.1	-	-	2.6	+	-	1.4	-	-	2.2	+	-	3.3	+	-
6	63	m	17	1.3	1.4	+	+	1.9	+	+	2.6	+	-	0.3	+	+	0.3	-	+	0.1	-	+
7	73	w	7	0.9	2.5	+	-	1.7	+	-	2.3	+	-	0.8	+	+	1.8	+	-	2.2	+	-
8	64	w	15	1.6	1.6	+	-	1.7	+	-	2.1	+	-	0.9	-	-	1.8	+	-	1.9	+	-
9	36	m	22	1.5	2.4	+	-	4.2	+	-	2.6	+	-	2.1	-	-	4.3	-	-	0.2	-	+
10	68	m	29	0.1	2.4	+	-	3.1	+	-	1.4	-	-	0.1	-	-	2.2	-	-	2.3	-	-
11	39	m	10	1.5	2.6	+	-	1.6	+	-	2.4	+	-	1.2	+	-	2	-	-	3	+	-
12	57	m	8	1.7	1.5	+	-	1.4	+	-	1.8	+	-	1.3	+	-	1.2	+	-	X	X	X
13	31	m	8	1	0.7	-	-	2	+	-	1.7	+	-	1.2	-	-	1.5	+	-	1.5	-	-
14	51	m	62	1.5	1.1	-	-	1.5	+	-	1.6	+	-	2.9	+	-	0.8	-	-	0.7	-	-
15	64	w	52	1.2	1.6	+	-	2.1	+	-	3	+	-	2.3	+	-	2.6	+	-	1.8	-	-
16	62	m	25	0.7	0.8	+	-	0.8	+	-	1.3	+	-	0.6	-	-	1.7	+	-	0.5	-	-
17	73	m	29	0.7	1.2	-	-	1.6	+	-	1	-	-	0.8	-	-	1.3	+	-	1.6	+	-
18	69	m	13	1.1	0.8	-	-	0.5	-	-	1.1	+	-	1.4	-	-	2	+	-	0.4	-	+
19	55	m	19	2.2	2.4	-	-	1	+	-	2.2	+	-	1.6	-	-	2.4	+	-	2.5	+	-
20	55	m	25	0.9	0.9	-	-	1.8	+	-	2.3	+	-	2.5	+	-	1.7	+	-	0.9	-	-
21	44	w	15	1.1	2.3	+	-	1.6	+	-	2.2	+	-	3	+	-	2.8	+	-	3.6	+	-
22	57	m	40	1.9	2.8	+	-	2.2	-	-	1.7	-	-	4	+	-	1.7	-	-	1.4	-	-
23	55	m	36	1.3	0.7	-	-	0.3	-	-	1.6	+	-	1	-	-	1.6	+	-	1	+	-
24	51	m	22	1.8	1.9	-	-	3.8	+	-	3.4	+	-	3.4	+	-	3.2	+	-	3.4	+	-
25	67	m	31	0.4	1.3	+	-	2	+	-	1.5	+	-	1.5	+	-	1.7	+	-	0.7	-	-
26	21	w	19	2.2	2.9	+	-	3.8	+	-	2.1	-	-	5.1	+	-	0.2	-	-	X	X	X
27	51	m	26	0.9	1	-	-	1	+	-	1.5	+	-	0.9	-	-	2.4	+	-	2	+	-

f: female.

m: male.

ROI: Region of interest, measurement area.

#: Number (of patient, or of measurement area, respectively).

GGO: Ground glass opacification.

VE: Vessel Enlargement proximal to ROI.

PE: Pulmonary Embolism proximal to ROI.

CO: Consolidation.

+: positive finding.

-: negative finding.

x: missing value.

Fig 1

a. Difference of maximum and minimum of iodine uptake (mg/mL; x-axis) in ground-glass opacification compared to normally aerated lung. Each horizontal line represents one patient (y-axis; order of patients from top to bottom as in Table 2). b. Difference of maximum and minimum of iodine uptake (mg/mL; x-axis) in consolidation compared to normally aerated lung. Each horizontal line represents one patient (y-axis; order of patients from top to bottom as in Table 2).

Pulmonary perfused blood volume (PBV) without upstream pulmonary embolism

There was a considerable inhomogeneity of iodine density in opacifications even when focusing on opacifications without upstream PE (Table 3). The estimated mean iodine uptake for GGO and consolidation without evidence of upstream PE was higher than that in normally aerated lung: GGO 1.77 mg/mL (95%-CI: 1.52–2.02), consolidation: 1.82 mg/mL (95%-CI:1.56–2.08) (shown in Figs 2–5). The difference of the estimated mean iodine uptake values of normally aerated and opacified lung was significant for both GGO (p = 0.011) and consolidation (p = 0.006). In contrast, no significant difference was seen regarding the iodine uptake of ground glass and consolidation (p = 0.791).

Table 3

Iodine uptake of opacifications compared to normally aerated lung.

Iodine uptake*	Ground glass			Consolidation
	total	Vessel enlargement	Pulmonary embolism	total	Vessel enlargement	Pulmonary embolism
increased	60	52	2	48	36	0
equally	5	4	0	4	1	0
decreased	16	3	0	27	6	7
total	81	59	2	79	43	7

* Relative to iodine uptake in normally aerated lung.

Fig 2

Iodine uptake in normally aerated lung and ground-glass opacifications (two different patients, a) and b). Ground-glass opacifications demonstrated a higher iodine density as compared to normally aerated lung.

Fig 5

Iodine uptake (mg/mL) of normally aerated lung (estimated mean: 1.22; 95%-CI: 0.95–1.49), areas of ground-glass opacification (estimated mean: 1.77; 95%-CI: 1.52–2.02), and consolidation (estimated mean: 1.82; 95%-CI: 1.56–2.08) without upstream pulmonary embolism.

Iodine uptake in normally aerated lung and consolidation.

Three reconstructions of the identical slice position, a. MPR in lung window, b. colour-coded PBV MIP with thresholds for tissue recognition set to minimum -100 HU and maximum + 150 HU for visualization of iodine density in consolidation and c. colour-coded PBV MIP with thresholds for tissue recognition set to minimum -960 HU and maximum -600 HU for visualisation of iodine density in normally aerated lung. Parameter settings were separated for this illustration only in order to enable a clear depiction of perfusion differences by optimizing the range of the look-up table to tissue density. Consolidations (arrows) demonstrated a higher iodine density as compared to normally aerated lung (see colour-coding scale).

Iodine uptake in normally aerated lung and opacifications with and without upstream pulmonary embolism.

Four reconstructions of the identical slice position, a. MPR in lung window, b. MPR in soft tissue window, c. colour-coded PBV MIP with thresholds for tissue recognition set to minimum -960 HU and maximum -400 HU for visualisation of iodine density in normally aerated lung and GGO and d. colour-coded PBV MIP with thresholds for tissue recognition set to minimum -100 HU and maximum +150 HU for visualization of iodine density in consolidation. Parameter settings were separated for this illustration only in order to enable a clear depiction of perfusion differences by optimizing the range of the look-up table to tissue density. Extensive diffuse GGO is present. Several consolidations are seen in the posterior segments of both lower lobes. Subsegmental pulmonary embolism is identified in the right lower lobe (arrowhead). Iodine density of lung tissue downstream of PE (bold arrows) is decreased while iodine uptake in GGO and in particular in consolidations (thin arrows) without upstream PE is increased compared to normally aerated lung (see colour- coding scale).

PBV without upstream pulmonary embolism and vessel enlargement

Vessel enlargement (VE) was present in 57/79 (72.2%) of GGO areas and in 41/72 (56.9%) of consolidated areas without upstream PE. When VE was present without upstream PE, GGO was hyperperfused in 50/57 (87.7%) and equally or hypoperfused compared to normally aerated lung in 4/57 (7%) and 3/57 (5.3%), respectively. When VE was present, consolidation was hyperperfused in 36/41 (87.8%) and equally or hypoperfused compared to normally aerated lung in 1/41 (2.4%) and 4/41 (9.8%), respectively. Estimated mean iodine uptake of areas of GGO demonstrating VE was 2.01 mg/mL (95%-CI: 1.74–2.28) and estimated mean iodine uptake of areas of consolidation with accompanying VE was 2.28 mg/mL (95%-CI: 1.98–2.57). Without accompanying VE, the estimated mean iodine uptake of areas of GGO and of consolidation was 1.15 mg/mL (95%-CI: 0.79–1.50) and 1.21 mg/mL (95%-CI: 0.9–1.53). Statistical analysis showed that the estimated mean iodine uptake of opacified lung areas (GGO and consolidation) with VE was significantly higher as compared to estimated mean iodine uptake of areas of GGO and consolidation with normal vessel width (p<0.001) (shown in Fig 6). The estimated mean iodine uptake of opacified lung areas (GGO and consolidation) without VE did not differ from that of normally aerated lung.

Fig 6

Iodine uptake (mg/mL) of normally aerated lung (estimated mean: 1.22; 95%-CI: 0.95–1.49), areas of ground-glass opacification without and with vessel enlargement (estimated mean: 1.15; 95%-CI: 0.79–1.15; and 2.01; 95%-CI: 1.74–2.28, respectively), and areas of consolidation without and with vessel enlargement (estimated mean: 1.21; 95%-CI: 0.90–1.53; and 2.28; 95%-CI: 1.98–2.57, respectively).

PBV with upstream pulmonary embolism

PE upstream to the measurement ROI was present in 2/81 (2.5%) of GGO and in 7/79 (8.9%) of consolidated areas. The estimated mean iodine uptake of areas with GGO and consolidation (combined) without and with evidence of upstream PE was 1.79 mg/mL (95%-CI: 1.62–1.97) and 0.52 mg/mL (95%-CI: -0.07–1.12), respectively. The difference was significant (p<0.001).

Discussion

Since December 2019 the world suffers from the new Coronavirus SARS-CoV 2 induced airway disease. Despite extensive scientific work-up the pathophysiology of lung injury caused by COVID-19 pneumonia is subject of ongoing controversy [Am J Respir Crit Care Med. ">1-4]. The discussion is mainly driven by the observation of an unusual ventilation pattern in terms of a combination of preserved lung compliance with severely impaired gas exchange. It was hypothesized that SARS-CoV 2 infection leads to significant vascular obstruction due to pulmonary embolism and / or in-situ thrombosis [28]. The resulting dead-space ventilation could explain the observed hypoxaemia. Meanwhile this hypothesis has been supported by autopsy results in several case series showing extensive microthrombi [13, 14]. However, an alternative explanation for hypoxaemia is significant shunting. Vessel enlargement is frequently observed within the opacified lung parenchyma of patients with COVID-19 pneumonia [16-18]. This feature seems to be unique for SARS-CoV 2 and has not been shown for other types of pneumonia. Several study groups evaluated lung perfusion based on the analysis of DECT data [22–26, 29]. The results are inconsistent probably due to different study setups and even more so inhomogeneous study populations and disease stages. However, meanwhile it is well known that COVID-19 presents with a complex pathophysiological dynamic traversing the stages of early infection, pulmonary manifestation, thrombo-inflammation and hyperinflammation [30-JAMA Intern Med. 2020 ">37]. Thus it seems not to be surprising that pulmonary blood flow is subject to fluctuations during course of disease.

We systematically evaluated a cohort of 27 patients with RT-PCR proven COVID-19 pneumonia being treated in a tertiary care university hospital. All except one patient required intensive care, 89% were mechanically ventilated. Thus our cohort represents the subset of advanced and severe disease stage. We focused on the subgroup of severely diseased patients in order a) to establish a homogeneous study group in terms of stage of lung injury and b) to investigate a study group at risk with a strong need for further pathophysiological insight. DECT demonstrated a wide range of perfusion characteristics of pulmonary opacifications (both GGO and consolidation) with some being hypo- and others being hyperperfused. Increased iodine uptake was associated with an increase of intralesional vessel diameter. This feature has been described as typical for COVID-19 pneumonia [16-18]. Expectedly, opacifications downstream of pulmonary embolism demonstrated decreased iodine uptake.

Our results raise new aspects regarding the nature of lung injury caused by SARS-CoV-2 in the advanced stage. On one hand, we saw opacifications that were hypoperfused as compared to normally aerated lung. This finding is in line with the investigations of Lang et al. and Si-Mohamed et al. and supports the hypothesis of dead-space ventilation as one cause of impaired gas exchange [22, 25, 26].

On the other hand, we were able to demonstrate that a considerable fraction of opacifications is hyperperfused despite being diminished or non-ventilated. This finding suggests the presence of a functional intrapulmonary right-to-left shunt. Estimation of the amount of hyperperfused lung tissue on CTs performed without dual-energy technique might be possible by evaluation of the prevalence of vessel enlargement the presence of which correlated with hyperperfusion. An in-depth pathophysiological discussion is beyond the scope of the article. However, a direct effect of SARS-CoV-2 on the vascular autoregulation has been discussed and attributed to the virus’ affinity to the ACE-2 receptor [38]. Thus, hyperaemia is possibly mediated by vasodilation or failed hypoxic vasoconstriction [2]. Also, neo-angiogenesis was described in COVID-19 pneumonia in a recently published autopsy series [13]. Our findings might help to tailor the therapeutic management to the individual patient´s situation. Patients with predominant hypoperfusion / deadspace ventilation and/or pulmonary embolism will likely benefit from inhalative vasodilators (e.g. NO or prostacycline) which on the other hand might be deleterious for patients with mainly hyperperfused opacifications and vessel enlargement.

Limitations of our study are as follows: The sample size was rather small and the study design was retrospective. Thus our results have to be confirmed in future studies. More importantly, the conclusions drawn for clinical management have to be carefully evaluated. Selection of measurement areas for perfusion analysis was subjective and the ROIs represented a sample of the entire volume of opacifications. However, placing of ROIs were guided by the colour-coded iodine map enabling registration of the entire range PBV. Artificial intelligence-enhanced computing algorithms could enable the registration of iodine uptake over the entire extent of opacifications. Such a software, however, was not available at our institution. The DECTs were acquired with different protocols regarding iodine delivery rate. This was due the fact that DECTs from clinical routine were analyzed. However, results were presented and analyzed with regard to the relative difference of iodine uptake in opacifications compared to normally aerated lung. This way an intraindividual normalization was established.

In summary, we were able to demonstrate that opacifications in patients with severe COVID-19 pneumonia can be both hypoperfused and hyperperfused. Indeed, the mean iodine uptake within pulmonary opacifications in our cohort was higher than that in normally aerated lung. Thus, impaired gas exchange might be caused by a combination of dead-space ventilation and functional intrapulmonary right-to-left shunt. Of note, intralesional vessel enlargement correlated with iodine uptake. Further research including correlation of perfusion imaging with ventilation parameters, minimal-invasive shunt assessment via right heart catheterization and histopathology is mandatory for a better understanding of the pathophysiology of COVID-19 pneumonia.

Conclusion

Assessment of lung perfusion with DECT in advanced COVID-19 pneumonia revealed opacifications to be both hypo- and hyperperfused. Thus, a combination of dead-space ventilation and functional intrapulmonary right-to-left shunt might contribute to impairment of gas exchange. Applying DECT to determine which pathophysiology is predominant might help to tailor therapy to the individual patient´s needs.

(XLSX)

Click here for additional data file.

3 Feb 2021

PONE-D-20-38572

Advanced COVID-19 Pneumonia: Perfusion Analysis in Correlation with Pulmonary Embolism and Vessel Enlargement using Dual-Energy CT Data

PLOS ONE

Dear Dr. Poschenrieder,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

As you will recognize from the comments of the reviewers both raised points of critique, especially regarding methodology and discussion of findings.

Please submit your revised manuscript within 2 months. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

We look forward to receiving your revised manuscript.

Kind regards,

Rudolf Kirchmair

Academic Editor

PLOS ONE

Journal Requirements:

When submitting your revision, we need you to address these additional requirements.

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at

https://journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and

https://journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

2. Your ethics statement should only appear in the Methods section of your manuscript. If your ethics statement is written in any section besides the Methods, please delete it from any other section.

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Partly

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: No

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: This is a retrospective study evaluating DECT derived iodine uptake in 27 patients with severe COVID-19. The results show a predominance of hyperperfusion in opacified areas which correlates with vessel enlargement.

Major remarks:

Material and Methods

L66: Patients. How did you clinically define “advanced” COVID-19 pneumonia and “severe” COVID-19 pneumonia? Please explain and keep only one of these surrogates.

L79: How many patients were excluded because of non-diagnostic CTs due to technical artifacts?

L84: Anticoagulation status. Please specify.

L92: CT protocol. Since most patients (89%) were mechanically ventilated, how did you manage to perform CT scans during end-inspiration? Please specify.

L130: Vessel enlargement. You did not separate arterial from venous vessels. Please denote.

L135: As shown in figures 3 and 4, the colour-coded PBV maps are very inhomogeneous. Did the 2-dimensional axial plane ROI contain the entire geographic area of GGO and consolidation? Please explain in more detail. In COVID-19, mixed areas of GGO with some band-like consolidations are frequently observed. Did you exclude these mixed areas? A 3D segmentation of GGO and consolidation would be technically possible and the parameters of these VOI could have been evaluated as well.

Results

L157: Mean interval from symptom onset to the date of the DECT ranged considerably from 7-62 days. What was the clinical indication for DECT? Critical worsening of symptoms? Suspicion of pulmonary embolism? Please give more details on the overall clinical situation and treatment (e.g. corticosteroids, oxgen therapy, ECMO, …). It would be also of great interest to compare early scans, e.g. within 14 days of symptoms from late scans.

L163: Please provide information on image quality. What was the HU obtained in the pulmonary trunk? Image quality in pulmonary CTAs may vary substantially. Motion artifacts are frequently observed in mechanically ventilated ICU patients, as well as in critical non-intubated patients who frequently have problems in holding their breath. Please give more details.

Reviewer #2: Poschenrieder and colleagues present an interesting retrospective study including a cohort of 27 patients with severe COVID-19 pneumonia, describing/evaluating lung perfusion on dual energy CT through the iodine uptake measure.

The main finding in this study was that opacifications in the subset of critically ill COVID-19 patients can be both hypoperfused and hyperperfused. Authors hypothesize that this critical finding is the combination of dead-space ventilation and intrapulmonary right-to-left shunt, respectively, and thus explaining the COVID-19 pattern with severely impaired gas exchange.

I have the following comments and suggestions:

Major points:

- Keywords are missing in the manuscript.

- Materials and Methods, Image Analysis: please explain deeper how many experienced thoracic radiologist read each imaging test, if it was blinded to patient status or not, and the kappa index between radiologists for the variables (vessel enlargement, opacifications, but mainly for pulmonary embolism).

- Discussion: It should be interesting that authors explain the potential clinical impact of their finding (i.e. anticoaugulation, anti-infllammatory).

- Discussion: although the discussion is quite well written, I consider that authors should reconsider important limitations of the methodology used (i.e. simple size; limitations of observational studies to infer causality, ...).

Minor points:

- Introduction, line 40: it is said that “aggressive anticoagulant is meanwhile accepted part of therapy”. Although it has been purposed by some authors, the reference used by authors (Bikdeli B, et al. J Am Coll Cardiol 2020) did not recommend this management, and this is a very controversial point with very low certainty (WHO document: Clinical Management of COVID-19: interim guidance).

We strongly recommend the authors to replace this reference by recent clinical trial that could support this sentence: Tacquard C, Mansour A, Godon A, et al, Impact of high dose prophylactic anticoagulation in critically ill patients with COVID-19 pneumonia, CHEST (2021), doi: https://doi.org/10.1016/j.chest.2021.01.017.

- Introduction, line 60: please correct “the reasons for the conflicting results”.

- Materials and Methods, line 125: please correct “consensus”.

- Discussion, line 227: please correct “… evaluated a cohort of ….”

- Discussion, line 250: please correct “… SARS-Cov-2 ….”

- Discussion, line 264: please correct “… that opacifications in the subset ….”

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

7 Apr 2021

Hereafter we are pleased to respond to each point raised by the academic editor and the reviewers:

Journal Requirements

1. We thoroughly reviewed our manuscript text including the manuscript title and adjusted it to PLOS ONE style requirements.

2. We removed the redundant ethics statement from the statement section at the end oft he mansucript. As you suggested, the ethics statement now appears only in the methods section of our manuscript.

Reply to review comments

@Reviewer #1:

Material and Methods:

L66: Patients. How did you clinically define “advanced” COVID-19 pneumonia and “severe” COVID-19 pneumonia? Please explain and keep only one of these surrogates.

Response: Thank you very much for this comment. We used the specifications „advanced“ and „severe“ interchangeable. The definition of severe pneumonia is outlined in the MM section. We now replaced „advanced“ by „severe“ in the whole manuscript text and title for the sake of consistency.

L79: How many patients were excluded because of non-diagnostic CTs due to technical artifacts?

Response: During the survey period from April 1st and May 15th 2020, 29 patients with COVID-19 pneumonia underwent dual-energy CT and thus met the inclusion criteria. Image quality of all examinations was rated before analysis. Two patients had to be excluded due to technical artifacts („zebra artifacts“), so that finally 27 patients could be included for pulmonary perfusion analysis. We added this information as part of a more precise description of patient selection and image quality assessment.

L84: Anticoagulation status. Please specify.

Response: Reviewing our medical files, we now described in detail in the MM section how anticoagulation status was assessed. We also added the respective data to the results section of our manuscript.

L92: CT protocol. Since most patients (89%) were mechanically ventilated, how did you manage to perform CT scans during end-inspiration? Please specify.

Response: Thank your very much for this important remark. All dual-energy CTs were performed performing an inspiratory-hold maneuver. We added this information.

L130: Vessel enlargement. You did not separate arterial from venous vessels. Please denote.

Response: We are grateful for this important comment which gives us the opportunity to clarify. To the best of our knowledge differentiation between arterial and venous vessel enlargement was not done in literature so far. Although this would be an interesting point, the value for clarifying pathophysiology would probably be limited. Vessel enlargement can occur both in pulmonary embolism / in-situ thrombosis and hyperaemia. We believe that pulmonary blood volume is a better parameter to separate these conditions. This was one of the main objectives of our study. According to our results vessel enlargement reflects rather hyperaemia of affected lung areas than thrombus formation /embolism.

L135: As shown in figures 3 and 4, the colour-coded PBV maps are very inhomogeneous. Did the 2-dimensional axial plane ROI contain the entire geographic area of GGO and consolidation? Please explain in more detail. In COVID-19, mixed areas of GGO with some band-like consolidations are frequently observed. Did you exclude these mixed areas? A 3D segmentation of GGO and consolidation would be technically possible and the parameters of these VOI could have been evaluated as well.

Response: Thank you for this comment. The colour-coded PBV maps were displayed as axial maximum intensity projections at 5 millimeters reconstruction slice thickness. The ROIs for consolidation and GGO were drawn as large as possible. We intentionally exlcuded mixed areas in order to collect data purely representing consolidation and GGO, respectively. Unfortunately, the software did not provide the technical requirements for VOI-based three-dimensional analysis of dual-energy CT data.

Results

L157: Mean interval from symptom onset to the date of the DECT ranged considerably from 7-62 days. What was the clinical indication for DECT? Critical worsening of symptoms? Suspicion of pulmonary embolism?

Response: Thank you very much for this comment. The clinical indication for DECT was critical worsening of symptoms in 25 out of 27 patients (92.6 %) and suspicion of pulmonary embolism in 5 out of 27 patients (18.5 %). We added this information to the results.

Please give more details on the overall clinical situation and treatment (e.g. corticosteroids, oxgen therapy, ECMO, …).

Response: According to the reviewer´s point of criticism we specified clinical data and now report treatment data in detail.

It would be also of great interest to compare early scans, e.g. within 14 days of symptoms from late scans.

Response: Thank you very much for addressing this interesting issue. The vast majority of our patients underwent dual-energy CT in a late stage of disease, with only 5 patients out of 27 (18.5 %) being scanned within 14 days after symptom onset, versus 22 out of 27 patients (81.5 %) being examined later than 14 days after symptom onset. Thus, a comparison of these two groups unfortunately was not possible.

L163: Please provide information on image quality. What was the HU obtained in the pulmonary trunk? Image quality in pulmonary CTAs may vary substantially. Motion artifacts are frequently observed in mechanically ventilated ICU patients, as well as in critical non-intubated patients who frequently have problems in holding their breath. Please give more details.

Response: Thank you very much for bringing up this important point. As mentioned above 2 out of 29 dual-energy CTs had to be excluded from our study due to heavy technical artifacts („zebra artifacts“) which obviated evaluation of pulmonary perfusion pattern. We now amended and re-structurized the materials and methods section of our manuscript. As part of this, we systematically evaluated overall image quality considering motion and beam hardening artifacts and contrast-noise ratio. Overall image quality was subjectively assessed by two readers in consensus, one board-certified general radiologist with 12 years of experience (JR) and one subspeciality thoracic radiologist with 15 years of experience (FP), using a four-point scale. The results of this additional evaluation were added to the results section of our manuscript.

@Reviewer #2:

Major points:

- Keywords are missing in the manuscript.

Response: We added the keywords.

- Materials and Methods, Image Analysis: please explain deeper how many experienced thoracic radiologist read each imaging test, if it was blinded to patient status or not, and the kappa index between radiologists for the variables (vessel enlargement, opacifications, but mainly for pulmonary embolism).

Response: Thank you very much for your this important comment which gives us the opportunity to clarify. The image analysis was done by two readers in consensus, a board-certified general radiologist with 12 years of experience (JR) and a subspeciality thoracic radiologist with 15 years of experience (FP). Both readers were blinded to patient status. We amended the materials and methods section.

- Discussion: It should be interesting that authors explain the potential clinical impact of their finding (i.e. anticoagulation, anti-inflammatory).

Response: Thank you very much for addressing the potential clinical impact of our findings. We discussed this point extensively with our clinical colleagues who had encouraged us to do this study in the first place. Our findings might help to tailor the therapeutic management to the individual patient´s situation. Patients with predominant hypoperfusion /deadspace ventilation and /or pulmonary embolism will likely benefit from inhalative vasodilators (e.g. NO or prostacycline) which on the other hand might be deleterious for patients with mainly hyperperfused opacifications and vessel enlargement. We amended th discussion by these considerations.

- Discussion: although the discussion is quite well written, I consider that authors should reconsider important limitations of the methodology used (i.e. simple size; limitations of observational studies to infer causality, ...).

Response: We now amended the limitations paragraph in the discussion according to your helpful suggestions.

Minor points:

- Introduction, line 40: it is said that “aggressive anticoagulant is meanwhile accepted part of therapy”. Although it has been purposed by some authors, the reference used by authors (Bikdeli B, et al. J Am Coll Cardiol 2020) did not recommend this management, and this is a very controversial point with very low certainty (WHO document: Clinical Management of COVID-19: interim guidance).

We strongly recommend the authors to replace this reference by recent clinical trial that could support this sentence: Tacquard C, Mansour A, Godon A, et al, Impact of high dose prophylactic anticoagulation in critically ill patients with COVID-19 pneumonia, CHEST (2021), doi: https://doi.org/10.1016/j.chest.2021.01.017.

Response: Thank you very much for correcting us and providing new data addressing this critical point. We replaced the respective reference by the suggested one.

- Introduction, line 60: please correct “the reasons for the conflicting results”.

Response: Thank you very much for your alertness. We apologize for this oversight and corrected it.

- Materials and Methods, line 125: please correct “consensus”.

Response: Thank you very much for your alertness. We apologize for this oversight and corrected it.

- Discussion, line 227: please correct “… evaluated a cohort of ….”

Response: Thank you very much for your alertness. We apologize for this oversight and corrected it.

- Discussion, line 250: please correct “… SARS-Cov-2 ….”

Response: Thank you very much for your alertness. We apologize for this oversight and corrected it.

- Discussion, line 264: please correct “… that opacifications in the subset ….”

Response: Thank you very much for your alertness. We apologize for this oversight and corrected it.

17 May 2021

Severe COVID-19 pneumonia: perfusion analysis in correlation with pulmonary embolism and vessel enlargement using dual-energy CT data

PONE-D-20-38572R1

Dear Dr. Poschenrieder,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Rudolf Kirchmair

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: (No Response)

Reviewer #2: The authors have provided adequate responses to most of my concerns. I congratulate the authors for it.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Gerlig Widmann

Reviewer #2: Yes: Alberto García-Ortega

31 May 2021

PONE-D-20-38572R1

Severe COVID-19 pneumonia: perfusion analysis in correlation with pulmonary embolism and vessel enlargement using dual-energy CT data

Dear Dr. Poschenrieder:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Prof Rudolf Kirchmair

Academic Editor

PLOS ONE