Septal Line Appearances On HRCT In Diffuse Lung Diseases:the Pathologic Basis And Clinical Diagnostic Significance

Objective: The purpose of this study was to determine the septal line appearances on HRCT in 28 cadaveric lungs, the underlying anatomic and pathologic features, and then to evaluate the role of septal lines in diagnosis and differential diagnosis of diffuse lung diseases(DLD).The other aim of this study was to evaluate the image quality of volumetric HRCT reconstructions(VHRCT) and coronal multiplanar reformations(MPR) in the assessment of the secondary pulmonary lobules and interlobar fissures by comparing with direct HRCT scans in 14 cadaveric lungs.Methods: Twenty-eight entire lung specimens were available from autopsy of the patients with coal worker's pneumoconiosis(CWP) and occupational exposure to coal dusts. All lung specimens were inflated and fixed by Heitzman's method, and undergone axial and coronal SSCT/MSCT and HRCT scans. Fourteen of them performed volumetric scan by MSCT and undergone volumetric HRCT reconstructions(VHRCT) and coronal multiplanar reformations. At the same time, CT images of 176 patients, pathologically proved and clinically diagnosed of DLDs, were retrospectively reviewed, with special attention paid to appearances of septal line on HRCT. Gross specimen section(50um~100um slice thickness) and histologic section(5um~8um slice thichness) were performed on 10mm-thickness slices of lung specimen that displayed various linear opacities on HRCT images.Images analysis: First, Kerley's A line, B line and reticular opacity were identified in lung specimens and DLDs. Kerley's B line was classified into regular type and irregular type. The occurrence rate, pattern, distribution and amount of septal line were evaluated in DLDs. The anatomic and pathologic basis of linear opacities were analyzed on the gross specimen section and histologic section. Second, image quality of HRCT, VHRCT and MPR were rated by consensus of two reviewers who paid special attention to visualization of the following three normal structures: the interlobular septa, lobular core and the interlobar fissures. The normal structures in the same slice image obtained from HRCT, VHRCT and MPR were scored subjectively as 0, absent; 1, present and vague; 2, present and clear. The difference in image quality among HRCT, VHRCT and MPR was statistically analyzed using Wilcoxon Matched-Pairs Signed-ranks Test. AΡvalue of less than 0.05 was considered to be significant.Results:1. Septal line appearances on HRCT in 28 cadaveric lungs were precisely correlated with the gross and histologic findings. We found that: Kerley's A lines were seen in 17 of 28 lung specimens.There was no significant difference in the distribution between the upper and the lower lung area. They originated from the hilum and extended to the visceral pleura, or never extended to it. The anatomic basis of A line was a combination of two or more thickened interlobular septas(82.4%) and incomplete fibrotic septa among segments or subsegments(17.6%). The linear opacities at pathologic analysis were due to deposits of coal dusts and fibrosis, edema and inflammation, thickening of wall vessel assoiated with fibrosis in interlobular septa.Kerley's B lines were seen in 23 of 28 lung specimens. Regular type and irregular type B line could coexisted in the same case. Predominant regular type was seen in 11 of 28 lung specimens, including two cases with stageⅠandⅡCWP and 9 cases without CWP. Predominant irregular type was seen in 12 of 28 lung specimens, including two cases with stageⅠ, 7 cases with stageⅡ, 1 case with stageⅢ, and two cases without CWP. All short linear opacities predominantly distributed in subpleural regions. Regular type of B line correlated pathologically with edema, inflammation, slight fibrosis and small amount of deposits of coal dusts. Irregular type of B line correlated pathologically with serious fibrosis, clusters of coal dusts, confluence of fibrosis and pulmonary emphysema.Reticular opacities were seen in 13 of 28 lung specimens. Their distributions were predominantly in central lung area. Reticular opacities were owing to the outline of secondary pulmonary lobule formed by thickening of interlobular septa. Pulmonary edema and inflammatory cells can be seen in the diseased septal line.2. Septal line appearances were analyzed on HRCT in 176 DLDs,the findings were as follows:Kerley's A lines were present in 11 of 176 patients(6.3%). They occurred in cases of interstitial pulmonary edema, lymphangitic carcinomatosis, sarcoidosis and pulmonary alveolar proteinosis.Kerley's B lines were present in 94 of 176 patients(53.4%). The occurrence of them was frequent in cases of lymphangitic carcinomatosis, interstitial pulmonary edema, sarcoidosis, CWP and fibrosis. Regular type of them was seen mainly in edema and inflammation. Irregular type of them was seen mainly in lymphangitic carcinomatosis, sarcoidosis, CWP and fibrosis.The average number of Kerley's B lines was great in lymphangitic carcinomatosis, interstitial pulmonary edema and fibrosis caused by connective tissue disease and idiopathic pulmonary fibrosis.Reticular opcity was seen in 16 of 176 patients(9.1%).They occurred in pulmonary edema, lymphangitic carcinomatosis, SARS, physicochemical factors induced lung injury and pulmonary infection."Crazy paving"sign was present in 15 of 176 patients(8.5%). The linear network in the crazy-paving pattern(n=7) was believed to be owing to thickening of interlobular septa.3. The image quality of HRCT, VHRCT and MPR were rated.The score values for direct HRCT scans were 6 points(10 cases), 5 points(3 cases), 4 points(1 cases). The score values for VHRCT were 6 points(10 cases), 4 points(1 cases), 3 points(3 cases). The score values for MPR were 3 points(1 cases), 2 points(8 cases), 1 point(2 cases), 0 point(3 cases). The differences of image quality between MPR and HRCT or VHRCT were statistically significant(Ρ=0.001). The image quality of VHRCT was equal to that of HRCT(Ρ=0.063).Conclusions:1. The anatomic basis of Kerley's A line is interlobular septa itself or combination of several interlobular septas. In Cadaveric lung, septal line caused by lymphatic dilatation in the interlobular septa and around deep parenchymal veins and anastomatic lymphatic channels plays a lesser role.2. The septal line at pathologic analysis were due to deposits of coal dusts and fibrosis, edema and inflammation, thickening of wall vessel assoiated with fibrosis in interlobular septa.3. Regular, irregular type of Kerley's B line can coexist in different stage of CWP and occupational exposure to coal dusts, and due to different extent of fibrosis and deposits of coal dusts.4. A variety of septal lines are a nonspecific findings seen in DLD,but may be helpful to narrow the differential diagnosis of DLD based on the different appearance of them.5. The image quality of VHRCT was equal to that of HRCT in lung specimen, but MPR obtained from image data using 1.25mm×8i MSCT is unsuitable for evaluating lobular structures and linear opacities because of poor image quality. Direct HRCT scan is the best method for evaluating lobular structures and subtle lung abnormalities.