Thoracic and Cardiac Imaging / Imagerie cardiaque et imagerie thoracique
RSNA Expert Consensus Statement on
Reporting Chest CT Findings Related to
COVID-19: Interobserver Agreement
Between Chest Radiologists
Danielle Byrne, FFR (RCSI)
, Siobhan B. O’ Neill, FFR (RCSI)
Nestor L. Mu
¨ller, MD, PhD
, C. Isabela Silva Mu
¨ller, MD, PhD
John P. Walsh, FFR (RCSI)
, Sabeena Jalal, MBBS, MSc
William Parker, MD
, Ana-Maria Bilawichm, MD
and Savvas Nicolaou, MD
Purpose: To assess the interobserver variability between chest radiologists in the interpretation of the Radiological Society of
North America (RSNA) expert consensus statement reporting guidelines in patients with suspected coronavirus disease 2019
(COVID-19) pneumonia in a setting with limited reverse transcription polymerase chain reaction testing availability. Methods:
Chest computed tomography (CT) studies in 303 consecutive patients with suspected COVID-19 were reviewed by 3 fellowship-
trained chest radiologists. Cases were assigned an impression of typical, indeterminate, atypical, or negative for COVID-19
pneumonia according to the RSNA expert consensus statement reporting guidelines, and interobserver analysis was per-
formed. Objective CT features associated with COVID-19 pneumonia and distribution of findings were recorded. Results: The
Fleiss kappa for all observers was almost perfect for typical (0.815), atypical (0.806), and negative (0.962) COVID-19 appearances
(P< .0001) and substantial (0.636) for indeterminate COVID-19 appearance (P< .0001). Using Cramer V analysis, there were very
strong correlations between all radiologists’ interpretations, statistically significant for all (typical, indeterminate, atypical, and
negative) COVID-19 appearances (P< .001). Objective CT imaging findings were recorded in similar percentages of typical cases
by all observers. Conclusion: The RSNA expert consensus statement on reporting chest CT findings related to COVID-19
demonstrates substantial to almost perfect interobserver agreement among chest radiologists in a relatively large cohort of
patients with clinically suspected COVID-19. It therefore serves as a reliable reference framework for radiologists to accurately
communicate their level of suspicion based on the presence of evidence-based objective findings.
Objectif : E
´valuer la variabilit´
e inter-observateurs entre radiologistes thoraciques concernant les lignes directrices de la
eclaration de consensus d’experts de la Radiological Society of North America (RSNA) pour le signalement des patients ayant
une pneumonie COVID-19 suspect´
ee dans un ´
e des tests de d´
epistage par rt-PCR est limit´
ethodes : Des ´
etudes de tomodensitom´
etries (TDM) thoraciques effectu´
ees chez 303 patients cons´
ecutifs suspects de
COVID-19 ont ´
ees par 3 radiologistes thoraciques form´
es dans le cadre d’un fellowship. Chaque cas a ´
e selon sa
forme (typique, ind´
e, atypique ou n´
egatif) pour la pneumonie COVID-19 en suivant les lignes directrices pour leur sig-
ees de la d´
eclaration de consensus d’experts de la RSNA; une analyse inter-observateurs a ensuite ´
eristiques objectives des TDM associ´
`la pneumonie COVID-19 et la r´
epartition des constatations ont ´
esultats : Le kappa de Fleiss pour tous les observateurs a ´
e presque parfait pour toutes les formes typiques (0,815), atypiques
Department of Radiology, Vancouver General Hospital, British Columbia, Canada
University of British Columbia, Vancouver, British Columbia, Canada
Department of Radiology, Delfin Clinic, Salvador, Bahia, Brazil
Danielle Byrne, FFR (RCSI), Department of Radiology, Vancouver General Hospital, Vancouver, British Columbia, Canada V5Z 1M9.
Canadian Association of
ªThe Author(s) 2020
Article reuse guidelines:
(0,806) et n´
egatives (0,962) de COVID-19 (P< 0,0001) et substantiel (0,636) pour les formes ind´
ees de COVID-19 (P<
0,0001). L’analyse V de Cramer a montr´
`s fortes corr´
elations entre toutes les interpr´
etations des radiologistes, avec une
signification statistique pour toutes les formes (typiques, ind´
ees, atypiques et n´
egatives) de COVID-19 (P< 0,001). Les
constatations objectives a
`l’imagerie par TDM ont ´
ees avec des pourcentages semblables de cas typiques par tous les
observateurs. Conclusion : La d´
eclaration de consensus d’experts de la RSNA pour le signalement des constatations a
thoracique en rapport avec la COVID-19 d´
emontre une concordance substantielle a
`presque parfaite entre les radiologistes
thoraciques dans une cohorte relativement importante de patients ayant une infection COVID-19 suspect´
ee cliniquement. Il s’agit
donc d’un cadre de r´
erence fiable permettant aux radiologistes de communiquer avec exactitude leur niveau de soupc¸on en se
basant sur la pr´
ements objectifs reposant sur des donn´
2019n-CoV, COVID-19, CT, lung diseases, pneumonia
Coronavirus disease 2019 (COVID-19) is a novel infectious
disease caused by severe acute respiratory syndrome corona-
virus 2. It was first identified in the human population in
and spread rapidly around the world reaching
pandemic status in March 2020.
Reverse transcription poly-
merase chain reaction (RT-PCR) of respiratory specimens is
the most widely used method for diagnosing COVID-19.
However, in many clinical settings, RT-PCR testing may not
be readily available, be limited to hospitalized patients, or may
be initially falsely negative.
Furthermore, patients with mild
respiratory symptoms may be reluctant to travel to a PCR test-
ing site or hospital.
Although most radiological societies and professional orga-
nizations have recommended against performing chest com-
puted tomography (CT) in the workup of patients with
suspected COVID-19, in resource-poor countries where
RT-PCR is not widely available, a tentative diagnosis or exclu-
sion is often based on clinical and CT findings alone. Addi-
tionally, several retrospective studies have shown that CT has
greater sensitivity (86%-98%) and lower false-negative rate
The sensitivity of nasopharyngeal swabs
ranges from 42%to 71%,
with false negatives observed
more frequently early in the course of the disease.
This has led to the widespread use of chest CT in the
workup of patients with suspected COVID-19 particularly in
the setting of negative RT-PCR and high clinical suspicion.
COVID-19 results in a spectrum of chest CT manifestations,
which evolve over time ranging from peripheral predominant
ground-glass opacities to an organizing pneumonia reaction
with additional findings including crazy-paving and
more diffuse ground glass, thus overlapping with imaging
features of a variety of other disease processes including other
infections, drug reaction, and inhalational exposure.
there are a number of etiologies with imaging findings that
overlap with those of COVID-19, the inclusion of COVID-19
in the differential diagnosis of the radiology report may lead
to unwarranted anxiety among referring physicians and
patients. Optimal interpretation and reporting of the chest
CT and clear, unambiguous communication with the referring
physician are essential for optimal patient care as well as
community and health care worker safety during the
An expert consensus statement on reporting chest CT find-
ings related to COVID-19, endorsed by the Society of Thoracic
Radiology, the American College of Radiology, and the Radi-
ological Society of North America (RSNA), was published to
assist radiologists in recognizing and describing lung imaging
findings in a standardized manner in patients under investiga-
tion for COVID-19 pneumonia and provide clarity in commu-
nication with other health care providers.
The purpose of this
study was to assess the interobserver variability between chest
radiologists in the evaluation of CT scans in a cohort of patients
with respiratory symptoms and suspected COVID-19 pneumo-
nia using the RSNA expert consensus reporting guidelines in a
setting with limited RT-PCR testing availability.
Methods and Materials
Approval was obtained from the institutional review board, and
the need for informed consent was waived for this retrospective
study. A total of 303 consecutive patients (158 males and 145
females) with a median age of 49 years (range, 18-96) who
presented with respiratory symptoms were suspected to have
COVID-19 infection and underwent CT chest imaging between
March 2, 2020, and March 16, 2020, were included. Due to the
resource-limited nature of the hospital, reliable RT-PCR data
were not available.
Chest CT studies were acquired on a 32-slice single-source CT
scanner (Siemens Somatom Scope) using a standardized CT
technique: 110 kVp, 345 mA max, pitch of 1.5, 0.6 seconds
rotation time, and 1 mm scan thickness. Inspiratory phase CT
chest examinations were acquired with patients in the supine
position without administration of intravenous contrast.
Computed tomography images were extracted from the picture
archiving and communication system, anonymized, and
2Canadian Association of Radiologists’ Journal XX(X)
imported onto a secure browser-based viewing system for CT
scans (MD.ai, available at https://www.md.ai/). The software dis-
played cross-sectional CT images only with soft tissue and lung
kernel and allowed for dynamic scrolling, window width–win-
dow level adjustment, panning, and zoom. Three fellowship-
trained chest radiologists, each with >10 years experience reading
chest CT examinations (S.B.O.N., A.-M.B., and C.I.S.M.) per-
formed diagnostic interpretation on all included CT studies.
Observers, blinded to all patient information including RT-PCR
result, recorded the presenceor absence and distribution of typical
COVID-19 imaging features (see Table 1) and scored each study
according to the RSNA expert consensus reporting guidelines as
to whether a study was typical, indeterminate, atypical, or nega-
tive for the presence of COVID-19 pneumonia.
Statistical analysis was performed using SPSS statistics version
25 (IBM). To quantify interobserver agreement, Cramer V and
Fleiss kappa were determined across observers. Kappa values
were obtained by reporting scores of each observer (according
to the RSNA expert consensus statement on reporting chest CT
findings related to COVID-19) to the median of the other 2
observers. Interobserver agreement was considered slight for a
kappa value of 0.01 to 0.20, fair for 0.21 to 0.40, moderate for
0.41 to 0.60, substantial for 0.61 to 0.80, and almost perfect for
0.81 to 1.00.
The result of a Cramer’s V-test lies between 0
and 1 and is interpreted as following: 0, no association; 0.05 to
0.1, weak; 0.1 to 0.15, moderate; 0.15 to 0.25, strong; >0.25,
The Fleiss kappa for all observers was almost perfect (0.815)
for typical COVID-19 appearance (P< .0001), substantial
(0.636) for indeterminate COVID-19 appearance (P< .0001),
and almost perfect for atypical (0.806) and negative (0.962)
COVID-19 appearances (see Table 2). Using Cramer V analy-
sis, there were very strong correlations between all radiolo-
gists’ interpretations, statistically significant for all (typical,
indeterminate, atypical, and negative) COVID-19 appearances
(P< .001; see Table 3). Of the cases with overall typical ima-
ging appearance for COVID-19, specific imaging findings
Table 1. Frequency Distribution of Typical COVID-19 Findings.
Rad 1 (%) Rad 2 (%) Rad 3 (%)
Typical cases out of total (303) 140 (46.2%) 160 (52.8%) 139 (45.9%)
% of typical cases
Ground glass total 137 (97.9%) 157 (98.1%) 130 (93.5%)
Round 124 (88.6%) 154 (96.3%) 124 (89.2%)
Peripheral 137 (97.9%) 156 (97.5%) 133 (95.7%)
Crazy paving total 55 (39.3%) 37 (23.1%) 44 (31.7%)
Round 31 (22.1%) 32 (20%) 40 (28.8%)
Peripheral 55 (39.3%) 40 (25%) 43 (30.9%)
Consolidation 91 (65%) 95 (59.4%) 72 (51.8%)
Round 49 (35%) 90 (56.3%) 61 (43.9%)
Peripheral 80 (57.1%) 104 (65%) 81 (58.3%)
Peribronchovascular 45 (32.1%) 43 (26.9%) 48 (34.5%)
Perilobular 70 (50%) 49 (30.6%) 29 (20.9%)
Consolidation with reverse
55 (39.3%) 20 (12.5%) 33 (23.7%)
Posterior distribution 140 (100%) 160 (100%) 139 (100%)
Bronchial dilatation 5 (3.6%) 11 (6.9%) 6 (4.3%)
Table 2. Inter-Rater Agreement Between Chest Radiologists According to the RSNA Expert Consensus Statement on Reporting Chest CT
Findings Related to COVID-19.
COVID-19 appearance Agreement Fleiss kappa Standard error Pvalue Interpretation
Typical COVID-19 appearance
Radiologist 1 and radiologist 2 and radiologist 3 0.815 <.0001 Almost perfect agreement
Radiologist 1 and radiologist 2 90.1% 0.803 0.034 <.0001 Almost perfect agreement
Radiologist 1 and radiologist 3 91.1% 0.821 0.033 <.0001 Almost perfect agreement
Radiologist 2 and radiologist 3 91.1% 0.823 0.032 <.0001 Almost perfect agreement
Indeterminate COVID-19 appearance
Radiologist 1 and radiologist 2 and radiologist 3 0.636 <.0001 Substantial agreement
Radiologist 1 and radiologist 2 89.1% 0.597 0.063 <.0001 Moderate agreement
Radiologist 1 and radiologist 3 89.4% 0.668 0.054 <.0001 Substantial agreement
Radiologist 2 and radiologist 3 89.8% 0.641 0.058 <.0001 Substantial agreement
Atypical COVID-19 appearance
Radiologist 1 and radiologist 2 and radiologist 3 0.806 <.0001 Almost perfect agreement
Radiologist 1 and radiologist 2 98.0% 0.823 0.071 <.0001 Almost perfect agreement
Radiologist 1 and radiologist 3 98.3% 0.830 0.074 <.0001 Almost perfect agreement
Radiologist 2 and radiologist 3 97.7% 0.762 0.086 <.0001 Substantial agreement
Negative for pneumonia
Radiologist 1 and radiologist 2 and radiologist 3 0.962 <.0001 Almost perfect agreement
Radiologist 1 and radiologist 2 98.3% 0.960 0.018 <.0001 Almost perfect agreement
Radiologist 1 and radiologist 3 98.7% 0.968 0.016 <.0001 Almost perfect agreement
Radiologist 2 and radiologist 3 98.3% 0.960 0.018 <.0001 Almost perfect agreement
Abbreviations: CT, computed tomography; RSNA, Radiological Society of North America.
Byrne et al 3
including ground glass, consolidation, and crazy-paving were
recorded in similar percentages of cases by the observers (see
Table 1). A posterior predominance of distribution was
observed in all typical cases (100%). Examples of typical
COVID-19 CT appearances are presented in Figures 1 to 6.
COVID-19 may result in a wide spectrum of CT imaging find-
ings, which in isolation or combination can be of variable sig-
nificance. Findings may vary depending on the severity of
disease, time point in the infection, or the presence of back-
ground or coexistent lung disease. As a novel infection, by
definition, experience of interpreting radiologists is limited,
with continuously evolving described imaging features requir-
ing time to establish diagnostic confidence. Given the potential
implications of a COVID-19 diagnosis to the patient, exposed
health care workers, and the wider community, concise and
unambiguous radiology reporting is of paramount importance.
The RSNA expert consensus statement on reporting chest CT
findings related to COVID-19 was published to facilitate report-
ing of CT imaging studies acquired during the COVID-19 pan-
The statement’s main purpose was to provide
radiologists with standardized reporting language to apply to
patients under investigation for COVID-19 and to allow them
to assign an evidence-based impression of their level of suspi-
cion for COVID-19 infection, thus facilitating clear communi-
cation with referring physicians.
Currently, there is a paucity
of data on the initial experience utilizing the RSNA expert con-
sensus statement guidelines in clinical practice.
Our results show that there is overall good agreement for
typical, indeterminate, atypical, and negative imaging
Table 3. Correlation Between Chest Radiologists According to the RSNA Expert Consensus Statement on Reporting Chest CT Findings
Related to COVID-19.
COVID-19 appearance Cramer V Pvalue Interpretation
Typical COVID-19 appearance
Radiologist 1 and radiologist 2 0.810 <.001 Very strong correlation
Radiologist 1 and radiologist 3 0.821 <.001 Very strong correlation
Radiologist 2 and radiologist 3 0.831 <.001 Very strong correlation
Indeterminate COVID-19 appearance
Radiologist 1 and radiologist 2 0.611 <.001 Very strong correlation
Radiologist 1 and radiologist 3 0.669 <.001 Very strong correlation
Radiologist 2 and radiologist 3 0.665 <.001 Very strong correlation
Atypical COVID-19 appearance
Radiologist 1 and radiologist 2 0.823 <.001 Very strong correlation
Radiologist 1 and radiologist 3 0.842 <.001 Very strong correlation
Radiologist 2 and radiologist 3 0.774 <.001 Very strong correlation
Negative for pneumonia
Radiologist 1 and radiologist 2 0.960 <.001 Very strong correlation
Radiologist 1 and radiologist 3 0.968 <.001 Very strong correlation
Radiologist 2 and radiologist 3 0.960 <.001 Very strong correlation
Abbreviations: CT, computed tomography; RSNA, Radiological Society of North America.
Figure 1. Bilateral posterior and peripheral predominant ground-glass opacities in (A) (short arrows), with a slightly more rounded appearance
in (B) (short arrows).
4Canadian Association of Radiologists’ Journal XX(X)
appearances according to the RSNA expert consensus state-
ment reporting guidelines among expert chest radiologists in
suspected COVID-19 cases where RT-PCR was largely una-
vailable. The RSNA expert consensus statement guidelines
provide both standardized language and impression cate-
gories, which will likely reduce reporting variability and
improve clarity in report interpretation among referring
physicians. The CO-RADS (COVID-19 Reporting and Data
System) is an additional recently described categorical report-
ing system to assess the degree of lung involvement by
COVID-19 on chest CT in patients with moderate to severe
symptoms with categories 1 to 5 according to increasing sus-
picion on CT.
The CO-RADS system was not assessed in
this study but reports very good performance in predicting
Figure 2. Examples of bilateral peripheral posterior predominant consolidation with perilobular morphology consistent with organizing
pneumonia reaction pattern. Examples of perilobular arcades are demonstrated in (A) (short arrows) and areas of subpleural sparing in (B)
(arrowheads). Magnified image of perilobular arcades demonstrated in (C).
Figure 3. Examples of rounded peribronchovascular ground-glass opacities, slightly ill-defined in (A) and (B) (short arrows) and more confluent
and well demarcated in (C) (long arrows).
Byrne et al 5
COVID-19 patients with substantial interobserver agreement
on initial clinical application.
Commonly reported imaging features with greater specifi-
city for COVID-19 pneumonia include peripheral and bilat-
eral ground glass and/or multifocal ground glass with rounded
morphology with or without consolidation or visible intralob-
ular lines (‘‘crazy paving’’) and features of organizing pneu-
monia including the reverse halo sign.
opacities and consolidation were recorded in a high percent-
age of typical COVID-19 cases among observers in our study
with good agreement (see Table 1), emphasizing the impor-
tance of identifying these features on CT in patients with
suspected COVID-19 where RT-PCR results may be delayed
or unavailable. Posterior predominance of distribution
been reported in COVID-19 patients and was almost uni-
formly observed in the typical cases in this study. Bronchial
dilatation in affected areas
is less commonly seen in this
study in line with recent literature.
may be normal early on and subsequently depict an acute lung
injury response to an infectious insult, with peripheral predo-
minant multifocal ground glass with or without consolidation
followed by organizing pneumonia reaction pattern.
Due to the highly infectious nature of COVID-19 with esti-
mated reproduction number (R0) of 2 to 2.53, rapid and
Figure 4. Spectrum of ground-glass opacities with peripheral predominance in (A) and (B) (short arrows).
Figure 5. Spectrum of organizing pneumonia reaction pattern (short arrows) ranging from mild in (A), with peripheral crescentic, perilobular
consolidation and central ground glass consistent with the ‘‘reverse halo’’ sign (long arrow) demonstrated in the left lower lobe in (B) and
parenchymal distortion demonstrated in (C) and (D).
6Canadian Association of Radiologists’ Journal XX(X)
accurate diagnostic methods are needed to identify, isolate, and
treat the patients in a timely fashion, which could reduce mor-
tality rates and the risk of public contamination. In cases where
RT-PCR is initially negative, typical findings on CT may
encourage maintenance of infection control mechanisms and
prompt repeat testing.
In cases of high clinical suspicion
but negative RT-PCR, a combination of repeated swab tests
and CT imaging may be useful to reach the diagnosis and assist
With many countries beginning to ease lock-
down restrictions and health care systems returning to near pre-
pandemic levels of operation, it is important that radiologists
remain vigilant to the typical COVID-19 CT imaging findings
with the almost inevitable anticipated second wave of the pan-
demic. Such typical CT findings should be kept in mind for the
foreseeable future, particularly in the outpatient CT reporting
setting, as it is known that asymptomatic patients with COVID-
19 can have abnormalities on CT.
Our study has several limitations. Reverse transcription
polymerase chain reaction was unavailable in the majority of
cases and therefore we were only able to assess interobserver
agreement according to the RSNA expert consensus statement
guidelines in suspected COVID-19 cases. Furthermore, find-
ings could not be correlated with timing of onset of infection,
severity of symptoms, or patient outcome due to lack of avail-
able clinical data.
The role of chest CT imaging in the era of COVID-19 continues
to evolve with a particularly important role in cases with high
clinical suspicion but negative or unavailable RT-PCR. We
have shown that the RSNA consensus statement on reporting
chest CT findings related to COVID-19 has good interobserver
agreement among expert chest radiologists in a relatively large
cohort of patients with suspected COVID-19 and unavailable
RT-PCR testing. The guidelines provide a framework to which
radiologists can easily refer and language which can easily be
applied in daily clinical practice in order to accurately commu-
nicate their level of suspicion for COVID-19 based on the
presence of evidence-based objective imaging findings.
D. Byrne and S. B. O’ Neill contributed equally to this manuscript.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to
the research, authorship, and/or publication of this article.
The author(s) received no financial support for the research, author-
ship, and/or publication of this article.
Danielle Byrne, FFR (RCSI) https://orcid.org/0000-0002-8168
C. Isabela Silva Mu
¨ller, MD, PhD https://orcid.org/0000-0003
Figure 6. Examples of peripheral predominant ground-glass opacities with superimposed intralobular and interlobular septal thickening (short
arrows) consistent with crazy paving (A-C). D, Magnified image of the left lung in (C) more clearly demonstrates intralobular and interlobular
septal thickening (long arrow) superimposed on peripheral ground-glass opacification (arrowhead).
Byrne et al 7
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