Current Status of Computed Tomography in Novel Coronavirus Disease 2019 Pneumonia

Introduction

The novel coronavirus, which causes severe acute respiratory syndrome (SARS) like illness, is a newly found zoonotic virus originating from the bats.¹ This virus, now called, SARS coronavirus-2 (SARS-CoV) belongs to the subgenus Sarbecovirus and family Coronaviridae.² These are single-stranded RNA viruses and bats are natural hosts and reservoirs.² After the initial outbreak in China in late December 2019, the disease has spread worldwide with over 3 million cases infected and over 200,000 dead in just 4 months and numbers continue to rise.³

The SARS-CoV2 virus predominantly affects the respiratory system and the symptoms differ from the other diseases caused by coronavirus family like SARS and Middle East Respiratory Syndrome (MERS).⁴ The disease affects all ages. About 37% of the infected patients had associated comorbidities such as hypertension, cardiovascular disease, and diabetes mellitus.⁵ The common clinical manifestations include fever, dry cough, dyspnea, and myalgia.^4,5 Laboratory findings are nonspecific with elevation of C-reactive protein, erythrocyte sedimentation rate, and lactate dehydrogenase.⁵

In view of the highly contagious nature of the disease and the absence of any definitive treatment, early diagnosis is necessary to isolate the infected person.⁶ The diagnosis is confirmed by reverse-transcriptase polymerase chain reaction (RT-PCR) on the respiratory or blood samples by identifying the RNA gene. However, it has a sensitivity over 90%.^7,8 The real-time RT-PCR takes 5 to 6 hours to provide the final result.⁹ Radiological imaging is a critical component of the diagnostic workup, monitoring of disease progression, and follow-up in coronavirus-related pulmonary syndromes. Although initially it was suggested that imaging, particularly with computed tomography (CT) scan, plays an important role in the diagnosis of SARS-CoV2 infection, its specificity is low (25%) and may be normal (►Fig. 1).^6,7 The typical imaging features of coronavirus disease 2019 (COVID-19) pneumonia are also nonspecific and can be seen in many other diseases.¹⁰ Hence, it is not routinely recommended.

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Fig. 1

Normal appearance. (A) Schematic diagram of a terminal alveolus showing normal structures. (B) Computed tomography scan (lung window) of normal lungs.

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Pathology

Although most patients have mild symptoms, lung involvement is found in most patients, even in those without symptoms.¹¹ The pathological changes in the lungs in SARS-CoV2 are found to be similar to that of SARS and MERS.^12,13 The main pulmonary histological findings include deposition of hyaline membrane, damage to the alveolar epithelial cells, exudates of fibrin, and diffuse hyperplasia of type II pneumocytes, all of which are the features of diffuse alveolar damage.¹⁴ The disease progresses from mild exudative phase involving alveolar edema, proteinaceous exudate, focal pneumocyte hyperplasia, and patchy infiltration of inflammatory cells to consolidation, when there is proliferation of fibroblasts with clusters of extracellular matrix and fibrin within the alveoli (►Figs. 2 and 3).¹⁴ These changes match with the imaging findings of ground glass opacities (GGOs) and consolidation. Intra-alveolar neutrophilic infiltration may be seen in the event of a superimposed bacterial bronchopneumonia. The preferential involvement of lungs in most cases has been attributed to the presence of high concentration of angiotensin converting enzyme 2 within the pneumocytes that are specifically utilized by the virus.¹⁵

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Fig. 2

Early exudative phase. (A) Schematic diagram showing changes at the level of alveolus in the early phase. (B) Axial computed tomography scan (lung window) showing ground glass opacities in both lungs.

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Fig. 3

Late exudative phase. (A) Schematic diagram showing changes at the level of alveolus in the late phase. (B) Axial computed tomography scan (lung window) showing consolidation with air bronchogram in the left lower lobe.

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Precautions While Performing CT Scan

All CT scans when done for a SARS-CoV2 positive patient should be done by a technologist after donning the personal protective equipment.¹⁶ Two technologists are needed for performing a CT scan, one to position the patient and the other in the console room.¹⁷ Entry of personnel into the CT room should be restricted. The scans should be properly scheduled to later part of the day after the routine other cases, unless it is an emergency.

After the CT scans have been performed, the CT room should be properly sanitized.¹⁷ The high-frequency contact surfaces should be wiped with a cloth soaked in alcohol-based disinfectants. The protocol for decontamination is to mop the floor and to wipe the CT scanner, initially with water mixed with detergent followed by sodium hypochlorite solution. Subsequently, the room is fumigated with formaldehyde for 30 minutes. The CT suite should be completely closed for 1 hour and properly ventilated.

Imaging Features on Computed Tomography

The noncontrast CT scan of the chest is performed using the standard technique. The patient is in supine position and the scan is performed from the level of thoracic inlet to the level of costophrenic angles, with breath held in end-expiration.¹¹ Multidetector CT scan is preferred with images reconstructed at slice thickness of 1.5 mm. Intravenous contrast is usually not required.¹⁸ The iodinated contrast enhanced CT is done in specific situations like hemoptysis or when we need to see pulmonary vasculature. Both lung and mediastinal windows should be evaluated along with multiplanar reconstructions.

Various findings on CT have been described that assist in the diagnosis of SARS-CoV2 pneumonia. These can be broadly classified into alveolar or parenchymal, airway, pleural, and other organ changes.^6,11,19,20

Current Status of CT Scan for SARS-CoV2 Infection

The RT-PCR test for SARS-CoV2 infection is highly sensitive with sensitivity rates ranging from 93 to 100%.³⁰ The results are usually available within 6 to 12 hours, and within a day in all of the patients. The CT scan has a sensitivity and specificity in the range of 80 to 90% and 60 to 70%, respectively, for the diagnosis of SARS-CoV2 infection.⁶ Out of the patients who were RT-PCR positive, the CT scan showed findings in 61 to 97% in various studies.³¹

Since abnormal lung CT findings can be present even in asymptomatic patients, CT scan may have a role in a clinically suspected case with negative RT-PCR.²³ CT scan currently has not been recommended for routine use in the diagnosis of SARS-CoV2 pneumonia. Further, routine use of CT scan will overwhelm the resources and will reduce the throughput for non-COVID cases as the machine will be closed frequently for sanitization. The radiology team should be aware of all precautions and strategies to minimize the risk of infection among staff and patients.³²

Since the chest imaging is an important component of patient management in individuals with COVID-19, further investigations are required to expand understanding of the imaging findings throughout the disease course.³² The experiences with SARS and MERS show that follow-up imaging should be performed in individuals recovering from COVID-19 to look for evidence of chronic involvement of the lungs (i.e., interlobular thickening, air trapping or fibrosis).³³

Conclusion

SARS-CoV2 is a zoonotic viral disease predominantly involving the respiratory system. The real-time RT-PCR is highly sensitive and is diagnostic in > 95% cases. Routine imaging of the chest with CT scan for diagnosis is not currently indicated. In the current prevalence of the disease, incidental detection of COVID-19 pneumonia on CT examinations not directly performed for this has critical importance since radiologically visible COVID-19 pneumonia is associated with possible viral transmission.³⁴ However, early reports have stated that initial imaging might show normal findings in 15% of individuals, so a normal chest imaging examination does not exclude the infection.³³ Knowledge of the spectrum of findings on the CT scan, although non-specific, may assist the radiologist to suggest a diagnosis if found incidentally in non-suspected patients. Using CT diagnostically is not known to provide clinical benefit(s) and could lead to false security if results are negative.³⁵ The objective quantification of lung involvement manually or by artificial intelligence at the time of diagnosis might have prognostic value and an effect on the choice of therapy.³⁶