Radiological findings for diagnosis of SARS-CoV-2 pneumonia (COVID-19)

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Salient features:

This article focuses on:

  • Introduction
  • Aim
  • Radiological findings
  • Radiological findings according to the stage of the disease
  • Radiological findings according to age groups
  • Correlation between reverse transcriptase polymerase chain reaction results and chest computed tomography
  • Comparison between radiological findings of COVID-19 pneumonia and other viral pneumonias
  • Conclusion

Detailed summary:

Introduction:

  • COVID-19 infection can develop as a mild, moderate, or severe illness, including severe pneumonia, acute respiratory distress syndrome (ARDS), sepsis, and septic shock.
  • The incubation period ranges from about 5 days (interval: 4–7 days) to a maximum of 12–13 days.
  • Mild cases may develop flu-like symptoms: high fever, myalgia, fatigue, and respiratory symptoms, especially dry cough, with possible progression to pneumonia.
  • The onset is usually less abrupt than in the flu, and upper respiratory symptoms seem minor or absent.
  • The main source of infection is other patients already infected with SARS-CoV-2.
  • This is transmitted through respiratory droplets (aerosols > 5 m) over short distances (1.5–2 m) when patients cough, speak or sneeze, and through contaminated hands when in close contact with the mouth, nose, or bulbar conjunctiva.
  • Prolonged contact represents the highest risk, with casual contacts being less likely to cause contagion.
  • There can be infections from asymptomatic patients and even from people in the incubation period of the disease.
  • Transmission through infected surfaces is likely, with SARS-CoV-2 recently reported to remain on plastic and stainless steel up to 72 h.
  • The diagnosis of COVID-19 disease is made through reverse transcription polymerase chain reaction [RT-PCR]) with the detection of nucleic acids of SARS-CoV-2 or by the virus’s genetic sequencing.
  • Pharyngeal or nasopharyngeal smear samples, sputum, stools, or blood may be used. The virus can also be isolated in airway epithelial cell cultures.
  • Therefore, RT-PCR screening is regarded as the gold standard laboratory test for the diagnosis of COVID-19.
  • False negatives have been documented in some cases, likely due to inappropriate viral sample material or technical problems during nucleic acid extraction.
  • The incidence of COVID-19 has been observed to be higher in men than in women, for which the influence of the X chromosome and sex hormones on innate and adaptive immunity.
  • The respiratory system is the first to be affected by COVID-19, so in suspicious cases it is common to request a chest X-ray (CXR) as the first imaging test.
  • However, its diagnostic performance in the initial stages of the disease is limited, since it has been reported that pathological findings identifiable on chest computed tomography (CCT) may not be detected on X-rays.
  • CCT is a very valuable tool to diagnose COVID19 infection, both in the initial assessment of lung involvement and for its follow-up.

Aim:

To identify the characteristics of this rapidly progressive viral pneumonia onimaging tests, bothonCXRand CCT. The characteristic imaging findings ofthe different stages ofthe disease, the evolution of the findings associated with the progression of the disease or with clinical improvement, and the variation in the prevalence of the different patterns according to the age of the patients.

Radiological findings :

  • Ground-glass opacities, either alone or in combination with pulmonary consolidations, is the most common finding on CCT.
  • Other findings include interlobular septal thickening, bronchiectasis , pleural thickening, crazy-paving pattern, bilateral lower lobe involvement and peripheral and posterior location predominance.
  • The term ground-glass opacities (GGO) describes the parenchymal opacification of the lung which produces a smaller increase in attenuation compared to consolidation, so that, despite the increase in density, the pulmonary vessels and the bronchial walls continue to be differentiated from the affected parenchyma.
  • GGO represent a partial invasion of the airspace, they are less opaque than the consolidations.
  • Consolidation refers to the occupation of the airspace by pathological products (pus, water, blood, etc.).
  • Consolidation occurs as a homogeneous increase in pulmonary parenchymal attenuation (increased density) that obscures the margins of vessels and airway walls.
  • It may exhibit the air-bronchogram sign, which refers to the visualization of air-filled bronchial lumens within a pulmonary parenchymal opacity.
  • The crazy-paving pattern is characterized by a inter and intralobular septal thickening superimposed on GGO, simulating a crazy-paving floor, a finding that is also identified much more easily on CCT than on CXR.
  • The lower sensitivity of CXR for GGO detection the most common finding of COVID-19 pulmonary infection that may go unnoticed, has been postulated as the reason for the low sensitivity of CXR compared to CCT in the initial diagnosis ofthis infection.
  • The predominantly peripheral distribution is more accurately detected in the CCT studies compared to the CXR studies, with bilateral pneumonia being the most common finding identified in CXR.
  • Pleural effusion, pericardial effusion, mediastinal lymphadenopathy, cavitation, halo sign, and pneumothorax were detected less commonly in affected patients.
  • Cavitation is an air-filled space following necrosis of the parenchyma, in this case inflammatory, which is visualized as an area of radiolucency or low attenuation within a lung consolidation.
  • The halo sign is a ground-glass opacity around a nodule or mass.
  • Acute pulmonary thromboembolism in COVID-19 has been reported as a rare complication.
  • Another rare form of presentation is the pattern of organized pneumonia, including the reverse halo sign consisting of a ground-glass area surrounded by a complete or nearly complete consolidation ring.

Radiological findings according to the stage of the disease

  • Lung lesions have been found to be detected in CCT studies from even before the onset of symptoms to day 14 after the onset of symptoms, with an average of 4 days.
  • In a study by Jin et al. the most common CCT scan finding was to observe single or multiple GGOs, patchy consolidations, lung nodules with surrounding ground-glass opacity and air bronchogram.
  • The second or early stage, related to the first and second day after the onset of symptoms, included the majority of patients, and was characterized by single or multiple GGOs and the combination of GGOs with interlobular septal thickening (crazy-paving).
  • In the third or rapid progression stage, between days 3 and 7 from the onset of symptoms, large confluent consolidations were observed with air bronchogram.
  • The fourth consolidation stage corresponds to the second week of the symptomatic period, in which there is a reduction in the size and density of lung consolidations.
  • In the fifth stage or that of dissipation, 2–3 weeks from clinical onset, pulmonary opacities are patchier and bronchial wall thickening and reticulation due to intra and interlobular septal thickening are more commonly observed.
  • The study by Pan et al. included a follow-up CCT, which showed that GGO, consolidations and septal thickening increased in the majority of patients (85%) who experienced disease progression, suggesting fibrotic changes.
  • Several studies have reported that this interlobular septal thickening occurs only secondary to GGO and consolidations, so that as the disease progresses, the prevalence of the reticular pattern due to septal thickening increases.
  • Patients who showed pulmonary nodules on the initial CCT also showed an increase in their number and size, or their fusion, during follow-up.
  • A decrease in the attenuation of other nodules was also identified in the same follow-up study.
  • According to another study GGO with a lower number of affected lobes predominated in the early stages and, as the disease progressed, there was an increase in this pattern’s involvement, an increase in the affected lung lobes and the occurrence of pulmonary consolidations.
  • On average, it was observed that CCT findings are more significant on day 10 of the disease and that, after day 14, 25% of the patients showed an improvement in radiological findings, including a decrease in the number of affected lobes, a resolution of the crazy-paving pattern and the consolidations.
  • In the studies of patients admitted to the ICU, the most commonly described findings were multilobar and subsegmental consolidations.
  • The occurrence of pleural effusion has been reported as a complication of late stages of the disease, probably secondary to heart failure in the context of ARDS.
  • In pregnant women, the usual progression reported also starts with GGO in the initial stages, which as the disease progresses develops into consolidations and crazy-paving patterns, with resolution of these findings in the final stages.

Radiological findings according to age groups

  • Song et al. carried out a study in which patients were classified into 2 groups according to their age, patients younger than 50 and patients older than 50.
  • In the youngest group of patients, 77% had GGO, and only 23% had lung consolidations, while in the group of patients over 50 years, GGO were present in 55% and consolidations in 45%, difference that was statistically significant between both groups (p < 0.001).
  • Atypical findings were also more common in older patients.

Correlation between reverse transcriptase polymerase chain reaction results and chest computed tomography

  • A limited number of patients with COVID-19- compatible radiological findings on CTT have been reported to have negative RT-PCR results in the first few days.
  • The first TCT study showed GGO that later evolved to foci of consolidation. RTPCR results in these patients turned positive between days 2 and 8.

Comparison between radiological findings of COVID-19 pneumonia and other viral pneumonias

  • In the case of SARS-CoV-2 positive patients, lung opacities had a peripheral location in 100% of the cases, while this was only 31.8% in the negative.
  • The number of affected lung segments and lobes was also higher in the case of positive cases.
  • In COVID-19 cases, it was more common to find the air bronchogram sign and reticular pattern, while in negative patients, it was more common to identify centrilobular nodules.
  • Pneumonia caused by the varicella-zoster virus is characterized by halo-sign nodules and GGOs in both lungs.
  • Influenza A virus infection is characterized by multiple areas of consolidation and diffuse ground-glass opacities, similar to the presentation of COVID-19 pneumonia.

Conclusion :

  • CCT has proven to be an imaging technique with particularly good results to identify lung involvement caused by SARS-CoV-2 (COVID-19), even in asymptomatic patients.
  • Ground-glass opacities, isolated or in combination with lung consolidations, is the most common radiological finding.
  • The radiological findings that correlate with clinical improvement occur after 2 weeks of favorable symptom progression and include a gradual consolidation resolution, with a decrease in the number of lesions and the number of affected lung lobes.
  • Some studies show that CCT findings can vary according to the age of the patients, with consolidations being more common in older patients and GGO in younger patients.
  • CCT can be a highly valuable diagnostic technique because it can detect the characteristic findings of the disease.

Reference Link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7190522/

 

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