DISCUSSION
Secondary pulmonary is the basic unit of pulmonary anatomic
function, which has been playing an important role in
HRCT(High-resolution computed tomography). The imaging features of
secondary pulmonary lobular structure run through all stages of the
image performance of COVID-19. According to the anatomical differences
of pulmonary lobules, Fleischner and Heitzman proposed that the lung can
be divided into peripheral pulmonary cortex and central pulmonary
medulla(4,5). Webb put forward the concept of cortical lobule,
highlighting the anatomical characteristics of pulmonary cortical
lobule. The characteristics of the pulmonary cortical lobule also lay
the anatomical basis for the distribution of COVID-19 under the pleura
(6,7).Therefore, it is critical to discern the characteristics of
lobules and their distribution for accurate diagnosis of COVID-19.
The CT features of viral pneumonia, such as lobular size, fusion lobule,
thickening of intralobular interstitium and interlobular septum, that
have been respectively reported in the related literatures of influenza
and SARS(Severe acute respiratory syndrome)(8-12). In particular,
COVID-19 has a virus homology with SARS. The image of COVID-19 is highly
similar to the lobular image feature reported by SARS(13-16).
Therefore, the evaluation method and scoring standard of five categories
of secondary pulmonary lobular structure image based on the anatomical
structure of secondary pulmonary lobule were proposed for the first time
in this study. The lobular size sign in COVID-19 often shows GGO or
consolidation of 1-3centimeters, which is consistent with the results of
the lobular size of SARS by Wong. (13). The fusion lobular sign is
defined as the shape of wedge-shaped, fan-shaped, irregular or bat wing
with the size between 3 centimeters and lung lobe, which does not follow
the lung segments or lobe. In the early stage, the interlobular septal
thickening sign is generally not obvious, only to make the edge of the
lesion clearer, and transit to grid shape in the advanced or the
recovery stage. The interlobular interstitial thickening sign likes
“paving stone sign” which is very characteristic, and is very specific
in the diagnosis of COVID-19. The interlobular vascular thickening sign
is more common in the peripheral pulmonary cortex lobular blood vessels,
particularly in the early stage of COVID-19.
From the perspective of image thinking, CT images of different types
COVID-19 mainly include typical and atypical CT features, and the
evaluation of dynamic stages. In this study, combined with the typical
imaging diagnostic standards of COVID-19 that were reported by domestic
imaging diagnosis guidelines and some literatures (16-18), as well as a
summary of 103 confirmed cases, 82(79.61%) single-stage and 12(11.65%)
multi-stage cases of the typical COVID-19 have three characteristics.
These three typical performances of COVID-19 are consistent with some
reports (19-21), but the ideas and concerns of authors are based on the
image signs of secondary pulmonary lobular structure, and the analysis
from density, size and distribution is more in line with the traditional
image thinking mode. As a new infectious disease, COVID-19 has not been
fully recognized so far. In our study, we found that the image
manifestations of atypical cases are mostly GGO or consolidation,
including single or mixed GGO, thickening of bronchovascular bundle with
peripheral GGO, mixed GGO in lobes or segments, and halo sign or anti
halo sign in a few cases. These findings are consistent with the
relevant literature (20-22).
In this study, there are two types of differential diagnosis of
COVID-19: infectious disease(other viral pneumonia, bacterial pneumonia,
mycoplasma infection, cryptogenic organizing pneumonia, eosinophilic
pneumonia) and non-infectious disease(pulmonary hemorrhage, pulmonary
edema and traumatic wet lung). To some extent, the imaging
manifestations of these diseases may overlap with COVID-19, so it is
great significance to combine the imaging signs of secondary pulmonary
lobular structure for differential diagnosis between COVID-19 and
non-COVID-19. However, there are some limitations in this study:Firstly,
there are few cases of non-COVID -19, resulting in partial statistical
results bias.Secondly, there is no differential diagnosis between
COVID-19 and other relatively certain diseases.Thirdly, in COVID-19
cases, the correlation between secondary pulmonary lobular structural
signs and related laboratory indicators was not studied, such as
C-reactive protein. This is also the research direction of the next step
after increasing the number of samples.