Material and Method: We included 200 cases who had thorax CT scans for various reasons. A multislice CT scanner was used for the study. Diffuse hyperaeration was evaluated for each case's thorax CT. Our study group comprised 161 patients with bilateral diffuse hyperaeration while the control group comprised 39 patients without radiologically detectable hyperaeration . Density measurements were done for lungs accepting -910 HU and less as hyperaeration. Statistical analyses was applied to both groups for thorax AP / lateral diameters (thoracic index), trachea lateral /AP diameters (tracheal index), right main bronchus lateral/AP diameters and left main bronchus lateral/AP diameters (bronchial index) using SPSS 16.0. Mann-Whitney U test was used for comparison the two groups.

Results: There was significant discrepancy between two group's thoracic index, tracheal index and bronchial index measurements (p<0.05). ROC analysis showed that treshold value for thorax is 0,825. The patients with increased thorax AP diameters within the control group had higher median ages, concluding age has an effect on the increased thorax AP diameter.

Conclusion: Thoracic index is effected by hyperaeration. Hyperaeration cause saber sheath trachea configuration. Thoracic index values of 0.825 and higher favor increased thoracic AP diameter.

Even though the average diameters of trachea and bilateral main bronchus are previously demonstrated within the normal individuals, the AP-tolateral diameter ratio of the thorax is unknown in the normal population. As a consequence instead of a quantitative manner, the increase in the AP diameter of the thorax is being reported relatively.

Increased anterior-posterior (AP) diameter of the trachea also known as the ''saber-sheath'' configuration is reported in the COPD patients in the literature. The 95% of the patients with saber-sheath trachea are reported to have COPD as well^2-5. Increased intrathoracic tracheal diameter is a consequence of compression of the mediasten and lateral walls of the trachea by increased volume of the both lungs. The increased AP diameter of the thorax is also known to accompany the hyperaeration of the lungs⁶.

In this study our purpose is to demonstrate the AP-tolateral diameter ratio of the thorax as thoracic index, tracheal index and bronchial diameters in the normal individuals as well as in the patients who have diffuse hyperaerated lungs.

A multislice CT scanner (64 slice Toshiba Aquilion, Chicago, USA) was used for the study. CT was performed on a 64-row detector scanner at 120 kVp, 155 mA during inspiration covering from the apex to the basal segments of the lungs with contrast iodine injection in supine position. The slice thickness was 1 mm with 0.5 mm slice gap.

Air trapping was evaluated for each case with thorax CT. Our study group comprised 161 patients with bilateral diffuse hyperaeration while the control group comprised 39 patients without radiologically detectable hyperaeration. Density measurements were done for lungs accepting -910 HU and less as hyperaeration⁷. Bilateral flattened diaphragm or lower position of the diaphragm, retrosternal and intercostal widening promoted diffuse hyperaeration as well.

The measurements were done semi-automatically via the already existing work station’s computer programs. AP and lateral thoracal diameters were measured at heart’s maximum transverse diameter level. Bones were included in the measurements of thoracal AP and lateral diameters (Figure 1).

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Figure 1: Axial CT slice shows the measurement of thorax AP and lateral diameters.

Trachea’s lateral and AP diameters were measured 2 cm proximal to the carina whereas main bronchus diameters (AP and lateral) were measured 1.5 cm distal to carina level. Coronal multiplanar reformatted (MPR) images were used to measure trachea’s diameters (Figure 2a, 2b). An arrow has been replaced 2 cm proximal to trachea where lateral diameter was measured in coronal plane as well as AP diameter in axial plane. Coronal MPR images were used to measure bilateral bronchus diameters (Figure 2c, 2d). Lateral diameter of right main bronchus was measured in coronal plane 1.5 cm distal to carina and this level was marked with an arrow as well as right main bronchus AP diameter in the axial plane. Lateral diameter of left main bronchus was measured in coronal plane 1.5 cm distal to carina and this level was marked with an arrow as well as left main bronchus AP diameter in the axial plane.

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Figure 2a: Trachea lateral diameter measurement on coronal MPR image.

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Figure 2b: Trachea AP diameter measurement on axial CT image.

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Figure 2c: Right main bronchus lateral diameter measurement on coronal MPR image

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Figure 2d: Right main bronchus AP diameter measurement on axial CT image

Patients in both study and control group were evaluated for age, sex and thorax CT findings. Statistical analyses was applied to both groups for thorax AP/lateral diameters (thoracic index), trachea lateral/AP diameters (tracheal index), right main bronchus lateral/AP diameters and left main bronchus lateral/AP diameters (bronchial index) using SPSS 16.0. Mann-Whitney U test was used to find if there was significant discrepancies between two group’s trachea, right and left main bronchus diameter ratios (Table 1). Logistic regression analysis was also applied to find out the effect of age among the control group patients with increased thoracic index.

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Table 1: Comparison between two groups’ thoracal index, tracheal index and two main bronchus bronchial index with Mann-Whitney U test

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Table 2: Age, sex, mean thoracal index, mean tracheal index, mean bronchial index and saber sheath percentages of two groups

There was significant discrepancy between two group’s thoracic, tracheal and bronchial index (p<0.05). The threshold was statistically significant due to its p value 0,016 (<0,05). Chi-square test was used to evaluate the presence of significant discrepancy of the the threshold values between two groups. Chi-square=6,969, Sd=1, p=0,008, <0,05 ROC analysis showed that threshold value for thorax is 0,825. Based on this value positive predictive value is 56,4% and negative predictive value is 66.5% . There was significant discrepancy between two group’s threshold values (p<0,05).

Thoracic index was found >0,825 in 56.4% and <0,825 in 43.6% of the hyperaerated cases(n=39). The thoracal index was >0,825 in 33.5% within the control group (n=161). The risk of AP / lateral diameter ratio >0,825 was 2,5 times greater in the hyperaerated group (Figure 3a, 3b).

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Figure 3a: Axial CT slice of a 62 years old male patient with diffuse hyperaerated lungs. Thoracic index (thorax AP/lateral diameter ratio) in this patient was increased (0.98).

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Figure 3b: Axial CT slice in the same patient shows increased trachea lateral/AP diameter(tracheal index as 0.55, saber-sheath trachea)

Logistic regression analysis was applied to find out the effect of age among the control group patients whose thorax diameter ratios were greater than 0,825. We didn’t find any significant discrepancies when comparing the median and mean ages of the study group’s patients with or without increase in thorax AP diameters. However, the patients with increased thorax AP diameters within the control group had higher median and mean ages, concluding age has an effect on the increased thorax AP diameter.

Only one of the 161 patients comprising the control group had a thoracic index less than 0,6. 12 patients of 39 patients (30,7%) within the study group had saber-sheath trachea.

Greene et al.⁹ showed coexistence of COPD within the 95% of the saber–sheath trachea patients. Our study demonstrated hyperaeration in 12 patients (92,3%) within the 13 saber-sheath trachea deformity which is compatible with Greene et al. The only case who didn't show hyperaeration was a 74 y/o male saber-sheath trachea patient with increased thoracic AP diameter. All the saber–sheath trachea patients found in our study were male. As though it is known as a deformity affecting almost only male population¹².

Our study radiologically demonstrated increased thorax AP diameter and AP/lateral diameter ratio within the diffuse hyperaerated lungs which is compatible with Cassart et al.’s supine CT study’s findings and measurements¹³. However, previously done standing PA and lateral radiography studies showed no evidence of that^14,15.

Radiologically it is known that hyperaeration may lead increased thorax diameter⁶. Our study showed a threshold value of 0,825 for thoracal index moreover demonstrated increased thorax AP/lateral diameters in 22 (56,4%) patients within the study group.

Many studies demonstrated increased AP diameter with increasing age^16,17. In 54 (33,5%) patients of our control group’s 161 patients hyperaeration wasn’t appointed radiologically however increased thorax AP diameter was. 42 of these 54 patients were over 60 y/o. In the control group the average age of patients with thoracic index higher than 0,825 was statistically higher than the patients with thoracic index less than 0,825.

To our knowledge the impact of hyperaeration on the bronchus diameter hasn’t been demonstrated previously in the literature. Our study showed bilateral bronchus lateral/AP diameter ratios were affected by hyperaeration as well as trachea diameter.

One of our limitations is the different numbers of patients in control and study groups which prevented us from statistical comparison. Moreover, our study comprised random patients who had different complaints for a routine thorax CT scan. For this reason precisely our control group doesn’t comprise only healthy adults which may be the main cause of increased thora-cic or tracheal AP diameter. Because recurrent polychondritis, tracheobronchopathia osteochondroplastica, amyloidosis, primary and metastasing tumors, mediastinitis effecting the airways, tracheomalasia are also known possible causes of tracheal narrowing¹⁸.

In conclusion thorax AP/lateral diameter ratio is affected by hyperaeration. Thorax AP/lateral diameter ratio’s threshold is found to be 0.825 and the higher values should favor increased thoracic AP diameter. Tracheal lateral/AP diameter ratio is affected by the hyperaeration. Bilateral main bronchus lateral/AP diameter ratio is affected by hyperaeration.

Particularly in elder patients saber-sheath trachea and increased thoracic AP diameter can be observed radiologically.

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