Role of diagnostic imaging in diagnosis by Covid-19


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Submission Date: 2020-12-03
Review Date: 2020-12-17
Pubblication Date: 2020-12-28

Abstract

Abstract:

The diagnosis from Covid-19 provides the set of several examinations such as: clinical examinations, laboratory examinations, radiographic examinations. Using radiological imaging, RX and chest CT, it is possible to evaluate the impairment of lung function and thanks to this aspect it is possible to define the severity and clinical conditions of the patient. In this way, it allows timely therapeutic intervention especially if the patient shows a mild condition in such a way as to avoid the onset of further complications. Chest X-rays allow both an initial assessment of patients and the possibility to perform a differential diagnosis towards other possible causes of lung parenchyma involvement. The CT scan, which highlights the peculiar characteristics of COVID pneumonia, is performed both as diagnostic confirmation and in the patient’s follow-up.

Introduction

The epidemic from Covid-19 represents the first major health emergency of the new millennium worldwide. By the term “Covid-19” we mean the disease caused from the new Coronavirus, which “Co” is for the Corona, “Vi” for the virus, “D” for the disease and “19” indicates the year in which it manifested itself. Coronaviruses are a large family of positive-stranded RNA viruses, with a crown-like appearance under an electron microscope. Betacoronaviruses include SARS-Cov, MERS-Cov and the new Coronavirus called SARS-Cov-2. Specifically, SARS-Cov-2 infection has several stages such as mild, moderate or severe disease. The most frequent symptoms caused by COVID-19 are cough, fever and general malaise. In severe cases, the infection can cause pneumonia, acute respiratory distress syndrome, septic shock, and even death. The presence of lung complications, especially interstitial pneumonia, can be revealed by performing both a chest X-ray and a CT scan.

The thorax RX examination provides information in the most advanced stages of pneumonia where it shows bilateral multifocal alveolar opacities that tend to complete pulmonary opacity. This examination is not performed in the early stages of the disease as being burdened by poor sensitivity in identifying early lung changes could be completely negative. Chest X-rays can be followed either as a standard exam then with the patient standing upright or as a bed X-ray in those patients who fail to keep the station erect or who cannot leave the wards of belonging.

The tomographic study of the chest allows a panoramic evaluation of the pulmonary parenchyma rendering, consequently, this examination more specific, compared to the standard radiographic examination, as it shows the typical characteristics of COVID pneumonia.  In addition to the evaluation of pneumonia, the tomographic examination, in specific cases, can be performed with injection of contrast agent for the evaluation of diffuse thromboembolism, as it has been noted that some COVID19-patients develop an alteration in blood clotting. In the case of contrast-free examination, it is important to ensure that the protocol is finalized for an optimal representation of the interstitium. Regarding the protocol to be used we can define the choice between a standard TC protocol followed by reconstructions appropriate to the diagnosis to be performed or a high resolution TC protocol ( HRCT).

Materials and methods

The x-ray examination involves the acquisition of two projections, PA and left LL , both with a DFP of 180 cm and generally high kV (about 120); the x-ray examination in bed provides, a single projection in AP with a DFP of 100 and between 75 and 85 kV if the examination is performed with grid, if, instead,  it is executed without grid will be used between 65 and 70 kV. X-rays of positive patients may show: increase in interstitial texture, parenchymal thickening with possible pleural effusion.

Fig.1 – The above image shows an important picture in which there are multiple thickening peribroncho-cerebral parenchyma in the right pulmonary perilary, in the right upper and lower pulmonary field and in the left upper pulmonary perilary.

The tomographic examination involves the choice between two protocols: the standard or high resolution. As far as the standard protocol is concerned, the medium-layer scan is performed starting from the bases up to the apex in deep inhalation to contain as much as possible the artifacts from respiratory movement being often patients with respiratory difficulties. If the patients are intubated they will use a rapid protocol with momentary stop of the ventilation system. The display parameters used for the pulmonary parenchyma are a window level of -600 HU and a window width of 1500 HU; for mediastinal structures and soft tissues a window level of 40 HU and a window width of 300 HU. The HRCT protocol is defined as such because it allows the acquisition of larger volumes along the z-axis. The execution of the scan is almost similar to that of the standard TC protocol, the greatest difference is in the thick layer, as with the HRCT technique it will be acquired at a thickness of 1-1.25 mm with advancement of 0.5 mm. Again, a window level of -600 HU and a window width of 1500 HU are used as display parameters for the pulmonary parenchyma; for mediastinal structures and soft tissues a window level of 40 HU and a window width of 300 HU. The most common radiological pictures, found in tomographic imaging, are: Frosted glass opacity or ground glass; Involvement of the inter- or intra-lobular interstice or “crazy Paving”; Areas of parenchymal consolidation.

Fig. 2

In the reported images we find multiple densely pseudonodular parenchymal “ground glass” with distribution “patches”, located mainly in the mantle bilaterally, more evident in the linguistic and basal left where they tend to converge. Minimum and initial thickening of interlobular septa in apical and basal bilateral locations.

Conclusion

Since the onset of the pandemic the role of imaging in the diagnosis and evolution of COVID-19 has been as an instrument comparable to the RT-PCR test. Going to evaluate the advantages and disadvantages of individual methods we can define that: the execution of radiographic imaging is justified in the evaluation of patients in advanced stage of pneumonia, as it is characterized by a low sensitivity will not allow the identification of early lung changes; this is not true for CT because, being endowed with a high sensitivity, allows the evaluation of specific patterns, extending the use of imaging to early stages of disease. During the emergency some works proposed chest radiography as an alternative technique to CT, because, with a lower dose to the patient, allows to obtain information comparable to that obtained with CT. This proposal was rejected because, despite minor exposure, the x-ray examination does not contain the same information content as a CT and would therefore not be justified to perform it as an alternative to the tomographic examination. It seems that the only case in which the RX examination is preferred to the TC is in the assessment of patients in hospital, because the execution of the examinations in bed, through the use of portable equipment, goes to reduce the risk of infection with SARS-VOC-2 by health workers.

References

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