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Patients

We retrospectively searched the hospital and radiology information system of our hospital
for patients diagnosed with right-sided aortic arch, including situs inversus viscerum, who underwent chest CT from April 2003 to September 2014. We excluded patients who
did not have thoracic spine osteophytes (Fig. 1). Our Institutional Review Board approved this study and waived the need for informed
consent from the patients.

Fig. 1. Flow chart of patient selection process.

CT technique

Chest CT was performed with various scanners as follows, GE CT9800 (GE Healthcare America),
SIEMENS SOMATOM PLUS40 (Siemens Medical Solutions, Forch- heim, Germany), PHILIPS
Brilliance CT40, PHILIPS Brilliance CT64 (Philips Healthcare, Netherlands), TOSHIBA
Aquilion 16, TOSHIBA Aquilion ONE, TOSHIBA Aquilion CX (Toshiba, Tokyo, Japan). Images
of the entire lung were produced with varying section thickness, depending on time
period it was performed: 10 mm, 2003–2004; 7 mm, 2003–2004; and 5 mm, 2006 onward.
Lung window images were reconstructed with high-frequency algorithm (width, 1,600 HU;
level, ?600 HU).

Image interpretation

Two radiologists, one with 12 years’ experience and the other with 15 years’ experience
in reading chest CT scans, independently reviewed the reviewed CT scans and evaluated
the following factors: position of thoracic osteophytes relative to the midline of
the vertebral body (left, right, and both sides), thickness of thoracic osteophytes
(Fig. 2), and presence or absence of focal pulmonary opacities adjacent to the osteophytes.
We evaluated the only localized lung shadow in contact with the osteophytes, in order
to exclude the dependent opacities in the posterior region of the lung on supine CT,
by confirming that there was no abnormal shadow in the subpleural region except for
the site of osteophytes.

Fig. 2. Method of osteophyte measurement. A 74-year-old man with a left-sided thoracic osteophyte
associated with pulmonary opacity. Cumulative thickness of the osteophyte (two-headed arrow) and pulmonary opacity (reticular type) was 6 mm.

For patients with several thoracic osteophytes, the thickest osteophyte adjacent to
the lung field was chosen as representative.

Focal pulmonary opacities were evaluated for thickness and categorized according to
the morphological type, reticular or linear shadow. The reticular shadow was defined
as fine netlike arrangement of interstitial thickening. The linear shadow was defined
as an elongated thin or thick line parallel to the pleura. We adopted the type of
shadow which accounted for more than half when both reticular and linear patterns
were seen on CT. Disagreement between the two observers was resolved by consensus.

Depending on the presence or absence of lung shadows, we divided osteophytes into
two groups: those with pulmonary opacity (Group A) and those without pulmonary opacity
(Group B).

Statistical analysis

The abovementioned variables were compared between the two groups using Mann–Whitney
U test for numeric values and Chi square test or Fisher’s exact test for categorical
values. A P value of 0.05 was considered statistically significant. Statistical analysis was
performed using Statistical Package for the Social Sciences (SPSS), version 22.0 for
Windows (IBM, Japan).