New Path and Remaining Issues in Clinical Applications of Dynamic Chest Radiography

Dynamic chest radiography (DCR) is a functional radiographic imaging technique that uses a flat panel detector (FPD). Sequential images (similar to movies) are obtained by exposing the patient to pulsed X-rays over several seconds (Figure).¹ Recent technical advances, including the development of the FPD with an expansive field of view, improved sensitivity of the X-ray detector, and advancements in computer analysis and image processing have enabled the performance of DCR. This innovative technique, based on conventional X-ray technology, can be performed in a general X-ray examination room. Notably, the radiation dose incurred is lower than the combined upper limits of the posteroanterior and lateral chest X-ray scans as recommended by the International Atomic Energy Agency.¹ The scan duration typically ranges from 7 to 15 s, depending on the clinical purpose and scan parameters. Since its first introduction in the 2000s,^2,3 numerous basic and translational studies of DCR have been conducted. DCR during breathing maneuvers has been used to assess the diaphragmatic or thoracic structural motion in respiratory conditions such as chronic obstructive pulmonary disease.^4,5 Additionally, temporal variations in pixel values in the lungs have been analyzed to evaluate air filling during breathing. Such analysis, when compared with ventilation scintigraphy, has demonstrated a reasonable correlation.⁶ In contrast, when performing DCR while holding the breath, a series of sequential images without motion is acquired. However, detailed assessment reveals slight movement of the heart border due to cardiac pumping. Moreover, subtle interval changes in pixel values in the heart and pulmonary vessels occur because of the variations in blood volume by cardiac pumping (although not visually detectable). Therefore, DCR can be used to assess pulmonary perfusion by analyzing these changes, with several studies substantiating its clinical potential.^7,8

Figure.

Dynamic chest radiography system. Pulsed X-rays are continuously emitted (≈15 times/s) from a conventional X-ray system with a pulsed X-ray generator and detected by a flat panel detector to produce a chest X-ray movie. The moving images are analyzed using the X-ray imaging workstation. (Adapted from Hiraiwa H, et al with permission.¹⁰)

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Previous studies of DCR have focused on lung information and applied it to pulmonary vascular diseases. In this issue of the Journal, Okamoto et al⁹ focus on the pixel changes in the heart. They assessed the interval changes of pixel values in the heart and transverse diameter of the cardiothoracic ratio (CTR) during 1 cardiac cycle in sequential DCR images. The study aimed to evaluate the potential of DCR in identifying left ventricular (LV) dysfunction based on the LV ejection fraction (EF) measured by echocardiography using data from 61 patients and 10 healthy volunteers. They found that the group with reduced LVEF had significantly lower %∆pixel values in the heart and %∆transverse diameter of CTR than the group with preserved LVEF. Additionally, %∆pixel values in the heart showed a higher area under the receiver-operating characteristic curve (AUC) than the %∆transverse diameter of CTR. They also observed that regions of interest (ROIs) at 3 different points in the heart had similar and high AUC values that identified low LVEF. Previously, only Hiraiwa et al have reported a study of DCR focusing on the %∆pixel value in the heart.¹⁰ Given the 3-dimensional structure of the human heart, the addition of orthogonal (anteroposterior) directional information by analyzing pixel value changes to the plane analysis of chest radiography introduces a novel concept of great interest. This perspective could positively influence future research on DCR. The studies by Okamoto et al⁹ and Hiraiwa et al¹⁰ outline a new path for the clinical utility of DCR. Although DCR is less accessible than echocardiography for LV assessment, additional values simultaneously obtained from chest radiography and lung perfusion analysis may provide promising information during clinical checkups. In my opinion, this current study suggests further potential for the clinical utility of DCR. Precise differentiation between the left and right ventricles on chest radiography remains unfeasible. Therefore, in this study, ROIs were placed in the heart, and the relationship between %∆pixel values and LVEF was assessed. From different angles, right ventricular dysfunction is also expected to be associated with %∆pixel values. Considering that DCR can evaluate pulmonary perfusion, its application to the assessment of right heart failure, such as pulmonary hypertension, may be promising. Further investigations in this field are desirable.

There are 2 important considerations when interpreting the results of this study. First, gravity strongly influences the distribution of pulmonary blood flow. Therefore, it can affect the right heart function, sequentially affecting LV function. In this study, DCR was performed in a seated position, whereas echocardiography was conducted in a supine or left-lateral position. This difference may have negatively influenced the results. Second, this study revealed that the “touching-the-edge” ROI had similar AUC value to intracardiac ROIs (10-mm medial and 5-mm medial). However, during the systolic phase, the “touching-the-edge” ROI encompasses not only the heart but also the lung, because the ROI is placed at the touching-the-edge position in the diastolic phase. Because the lung exhibits significantly higher X-ray translucency than the heart, it is recommended to consistently place the ROI within the heart throughout the entire cardiac cycle.

Concerns remain regarding the clinical use of DCR due to its novelty. Clinically, scan parameters vary and are adjusted based on the body mass index (BMI) of the patient, as chest wall thickness significantly influences pixel values in sequential images. The optimal categorization of BMI and scan parameters should be further investigated to standardize the image quality and pixel-based analysis results. Additionally, DCR is used to assess the detection of LV dysfunction as well as structural chest abnormalities and lung perfusion. Further studies should focus on exploring optimal scan conditions that balance clinical utility and low radiation dose because image quality and radiation dose have a trade-off relationship.

In summary, the study by Okamoto et al is an interesting exploration of the utility of DCR for the detection of LV systolic dysfunction. Their study offers new insights and serves as a guide for future research using DCR. In the forthcoming era characterized by a potential “heart failure pandemic”, this simple and easily accessible technology could be important in cardiovascular healthcare. Although DCR faces certain challenges that need to be overcome for broader use in clinical medicine, considering its potential for further clinical applications, research on this novel and promising technology is highly desirable.

Acknowledgment

This work was supported by a research grant from the Japan Society for the Promotion of Science (JSPS) KAKENHI (JP23K07111).

Conflicts of Interest

The author received a research grant from Konica Minolta, Inc.

References

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