Study population
This retrospective study was approved by the ethics committee and performed in accordance with the Declaration of Helsinki. The cohort was prospectively enrolled. Written informed consent was obtained from all the subjects prior to participation. From June 2009 to August 2011, consecutive COPD patients meeting the same inclusion criteria as those of previous studies were recruited [9, 11]: (1) clinical diagnosis of pure COPD based on clinical course, symptoms, chest CT examination, and pulmonary function test (PFT) with post-bronchodilator inhalation; (2) exclusion of other respiratory diseases such as acute respiratory infection, bronchiectasis, or any type of interstitial lung disease; (3) current or former smokers; (4) ≥ 20-year-old adults who gave informed consent, including for additional x-ray exposure within tolerant range compared to conventional chest radiography; (5) no status of pregnancy, potential pregnancy, or lactation; (6) scheduled for conventional chest radiograph; (7) ability to follow instructions for tidal or forced breathing. All the COPD subjects were classified into spirometry grades from 1 to 4 by global initiative for chronic obstructive lung disease (GOLD) with corresponding percent predicted forced expiratory volume in one second (%FEV1) [21]. COPD mild group was defined as GOLD 1 or 2 (i.e., %FEV1 ≥ 50), and COPD severe group was defined as GOLD 3 or 4 (i.e., %FEV1 < 50) in this study.
On the other hand, healthy volunteers as a control group were also recruited from May 2013 to February 2014. The following inclusion criteria were the same as those of previous studies [9, 11]: (1) ≥ 20-year-old adults who gave informed consent, including for additional x-ray exposure by DXR; (2) scheduled for conventional chest radiograph; (3) PFT results within normal limits, namely percent predicted vital capacity (%VC) > 80% and forced expiratory volume percent in one second divided by forced vital capacity (FEV1%) > 70%; (4) ability to follow instructions for forced or tidal breathing; (5) never smokers; (6) no status of pregnancy, potential pregnancy or lactation; (7) no past medical history of respiratory diseases.
The inclusion criteria were the same as those of previous studies [10, 12].
Image acquisition
Posteroanterior view of chest DXR examination in standing position was performed with a prototype x-ray system (Konica Minolta Inc., Tokyo, Japan) composed of a flat panel detector (PaxScan 4030CB, Varian Medical Systems Inc., Salt Lake City, UT, USA) and a pulsed x-ray generator (DHF-155HII) with cineradiography option (Hitachi Medical Corporation, Tokyo, Japan). During the examination, COPD patients took several tidal breaths and one forced breath separately, while normal subjects took several tidal breaths, followed by one forced breath. Conditions for DXR examination were the same as the previous reports [9,10,11,12,13]: tube voltage 100 kVp; tube current 50 mA; pulse duration of pulsed x-ray 1.6 ms; source-to-image distance 2 m; additional filter 0.5 mm Al plus 0.1 mm Cu. The additional filter was used to filter out soft x-rays. The exposure time was approximately 10–15 s. The pixel size was 388 × 388 μm, the matrix size was 1024 × 768, and the overall image area was 40 × 30 cm. The dynamic image data, captured at 7.5–15 frames/s, were synchronized with the pulsed x-ray, which prevented excessive radiation exposure to the subjects. The entrance surface dose was approximately 0.3–1.0 mGy. Each DXR image was processed with bone suppression and converted into video with 5 frames/s before application of the optical flow method.
Image conversion to video using OFM
Total variation regularization and robust L1 norm, TV-L1, optical flow estimation were adopted for motion analysis [22]. The rectangle-shaped region of interest (ROI) including the unilateral lung field in the forced inspiratory phase was located on both sides of the video. The size of the ROI of either side was usually different. Then, a larger ROI was set symmetrically with the same horizontal level as a substitute for a smaller ROI. Bilateral lung fields within the ROI were subdivided into 4-cm2 pixels to assess lung motion vector using OFM. In each pixel, the subtle interval difference in density between temporally successive DXR images with bone suppression was converted into motion vector using OFM implemented by OpenCV [23]. The obtained motion vectors were multiplied by five to represent motion/s. Only vertical motion vectors of all the pixels were superimposed on DXR images. The longest lung motion vector onto VF-DXR images in tidal inspiration, tidal expiration, forced inspiration, and forced expiration were extracted and defined as lung motion velocity (LMV). Downward direction of vectors was defined as positive direction.
Data analysis
Demographic data among normal, COPD mild, and COPD severe groups were tested using one-way analysis of variance on ranks. The difference in sex ratio among the three groups was compared using Fisher’s exact test. Pulmonary function data such as tidal volume (TV), vital capacity (VC), %VC, FEV1, FEV1%, and %FEV1 were assessed with one-way analysis of variance on ranks. The association of LMV with demographic data or pulmonary function was analyzed with Spearman’s rank correlation coefficient. Scatter plots of %FEV1 versus LMV in each phase or side were visualized. The difference in LMV in both lungs in tidal and forced inspiratory and expiratory phases among three groups was analyzed with one-way analysis of variance on ranks. Post-hoc multiple comparisons were made using the Holm-Bonferroni method. A two-group comparison of LMV between normal subjects and COPD patients were also performed using the Mann-Whitney U test.
Statistical assessment was performed using R 4.0.3 (R Foundation for Statistical Computing, Vienna, Austria) with the EZR graphical user interface (Saitama Medical Center, Jichi Medical University, Saitama, Japan) [24, 25]. More precisely, EZR is a modified version of R commander designed to add statistical functions frequently used in biostatistics. In each test, a two-sided p value of less than 0.05 was considered statistically significant.