Centri-lobular nodules are the most common abnormality encountered on HRCT, yet, they have a wide differential diagnosis .
This study recorded the predominant HRCT findings along with the centri-lobular pulmonary nodules of various conditions and compared their different etiologies.
In this study, we had four main etiological diagnosis for centri-lobular nodules that include infection/ inflammatory lung disease (as in viral, granulomatous, fungal diseases and aspiration pneumonitis), inhalation lung disease (as in hypersensitivity pneumonitis, RB, RB-ILD, and post-toxin-bronchiolitis obliterans), autoimmune diseases (as sarcoidosis, rheumatoid arthritis, and non-specific autoimmune diseases), and hemorrhage and lung edema.
Although clinical criteria and exposure to an allergic antigen are used for diagnosis of inhalation lung disease, imaging finding is crucial in supporting the diagnosis . Shobeirian et al.  and Churg et al.  results were similar to this study, and ground glass opacity and reticulations were found to be the most common findings in HRCT followed by fibrosis and air trapping.
RB-ILD is a rare inflammatory lung disorder induced by heavy tobacco smoking . Park et al.  and Sieminska and Kuziemski  results were consistent to this study, and bronchial wall thickening was found to be the major HRCT finding in patients with RB-ILD followed by ground glass opacities.
Bronchiolitis obliterans is associated with the inhalation of toxic gases including nitrogen dioxide, war gas, and sulfur mustard . Bakhtavar et al.  and Travis et al.  studies stated that the most dominant HRCT finding in patients with bronchiolitis obliterans was concluded to be patchy air trapping mostly in the lower lobes and centri-lobular nodules, with multiple segments distribution associated with bronchial wall thickening, bronchiectasis. However, Raghu et al.  study showed that centri-lobular nodules if associated with ground glass opacities even with air trapping were considered inconsistent features in bronchiolitis obliterans. These results were consistent with this study.
The results of this study were in agreement with the previous studies of Shobeirian et al. , Iwasawa et al. , Razavi et al.  and Rossi et al.  concerning the distribution, location, shape, and pathological findings of inhalation lung disease centri-lobular nodules. Correlating with other etiologies, HRCT showed high statistical significance regarding diffuse distribution and upper lobes involvement, as well as associated ground glass opacities and air trapping.
Ryu et al.  illustrated that regarding infection and inflammatory lung diseases, centri-lobular nodules were the main CT features in most cases of bronchiolitis. In a case series report by Nabeya et al. , HRCT findings revealed bronchial wall thickening in 80% of cases and ground glass opacities in 40% of cases. Also, Weinman et al.  study found that bronchial wall thickening followed by ground glass opacities was the most common HRCT finding in viral bronchiolitis.
Multi-segment distribution was most commonly demonstrated in our study. Kim et al.  stated that the anatomical distribution of HRCT findings in bronchiolitis is most commonly in the lower lobes (69%), followed by diffuse distribution (57%), according to the type of the infecting virus.
Following the previous results, Zhu et al.  stated that fungal infection granuloma showed a nodular presentation in 75.0% of cases, followed by consolidation (62.5%), and ground glass appearance (62.5%). Centri-lobular nodules were concluded to characterize fungal lung infection in immunocompromised patients [22, 23].
Giacomelli et al.  concluded that the most common HRCT findings in patients with pulmonary tuberculosis were ground glass opacities with consolidation, followed by centrilobular nodules with tree-in-bud pattern and cavitation. Likewise, to our study, Im et al.  study showed that, in active tuberculosis, the most common CT finding (82–100% of cases) was centri-lobular nodules with segmental distribution, which represents bronchogenic dissemination of the disease.
Similar to this study, Duan et al.  stated that Sarcoidosis imaging findings on HRCT included ground glass opacities, centri-lobular nodules, consolidation, and intrathoracic lymphadenopathy. Zhu et al.  as well reported that HRCT of pulmonary sarcoidosis typically shows nodules in 96.1% of cases in multiple or miliary distribution with variable morphological presentation and with bilaterality in 92.6% of cases mainly in the left lower lobe (85.2%) and right lower lobe (81.5%).
Corresponding to this study, Yilmazer et al.  results showed that while HRCT can be normal in 54% of rheumatoid arthritis patients, the most common HRCT findings were ground glass opacities (42%). While Izumiyama et al.  reported that centri-lobular nodules were reported in 23.6% of rheumatoid arthritis patients examined by HRCT distributed mainly in the middle and lower lobes, bronchiectasis was observed in 17.1% of patients.
Similar to this study, Scheeren et al.  results regarding aspiration pneumonitis showed tree-in-bud nodularity with unilateral or bilateral distribution of centri-lobular nodules on HRCT. Centri-lobular nodules, bronchiectasis, and ground glass opacities along with atelectasis and consolidation were concluded as common features of aspiration pneumonitis.
Similarly, Zakynthinos et al.  HRCT findings in cases of hemochromatosis showed patchy ground glass nodules mainly in the upper lobe , yet it was diffusely distributed in this study.
As Schmidt et al.  and Naidich et al.  studied, the main HRCT features of Langerhans cell histiocytosis were lung cysts and cavitation, followed by centri-lobular nodules. The distribution of the cystic lesion was characteristic involving predilection for mid and upper zones.
In CT chest with centri-lobular pulmonary nodules, multidisciplinary approach should be done to reach the proper diagnosis, it could be summarized as follows: first; relevant history taking, second; determine the radiographic features of nodules including shape, number, location, distribution, and surrounding pulmonary parenchymal associated findings. Then, diagnostic possibilities could be suggested. Lastly, auxiliary approach, mainly laboratory or histopathological assessment, could be done .
The main limitation of the study was the small number of cases over each disease category.