Clinical utility of dynamic chest radiography in the oblique view to evaluate cardiac contraction: a case description

Introduction

Dynamic chest radiography (DCR) is a minimally invasive imaging modality that enables real-time, wide-field visualization of the thorax with high spatial and temporal resolution, facilitated by computer-assisted tracking of thoracic motion (1). DCR can be used to evaluate pulmonary ventilation and circulation by detecting variations in pixel values, without the use of contrast media (2). Higher pixel values correspond to increased X-ray penetration to the flat panel, whereas reduced X-ray exposure results in lower pixel values. Accordingly, changes in pixel value within a designated region directly reflect variations in moisture content. Sequential chest images can be captured using DCR at a rate of 15 frames per second during respiratory and cardiac cycles, allowing for quantitative analysis of pixel value fluctuations (2,3).

Flat-panel detector DCR has recently been integrated into routine clinical practice (3,4). This technique offers extensive information regarding lung structure and function, pulmonary ventilation, and circulation (5-7). It also provides information on the motion of the diaphragm and phrenic nerve palsy (8-10). DCR has demonstrated utility in the detection of acute pulmonary thromboembolism and in the diagnosis of chronic thromboembolic pulmonary hypertension (11-13). Furthermore, comparative studies between DCR and nuclear medicine ventilation-perfusion imaging have been conducted to evaluate pulmonary function (2,14). As a dynamic imaging modality, DCR serves as a valuable tool for the assessment of cardiovascular conditions (3,15).

A recent study, which utilized radiographic imaging in the frontal plane in both the standing (posteroanterior) and supine (anteroposterior) positions, demonstrated a significant correlation between DCR image parameters and hemodynamic measurements obtained by right heart catheterization in patients with heart failure (16). Another investigation (17) highlighted the discriminative capacity of DCR in detecting left ventricular dysfunction, suggesting that alterations in pixel values on DCR can serve as indicators of left ventricular impairment.

Oblique imaging by simple X-ray is often performed in the field of orthopedics; for example, it is used to evaluate various joint fractures in clinical practice. Moreover, this method is used in the field of cardiology; for example, it may be used to check for proper lead positioning in patients with cardiac resynchronization therapy devices (18,19). Previous studies have reported the evaluation of cardiac function and contraction using DCR in patients with heart failure and heart disease, respectively (16,17). However, these reports were based on the frontal view and did not evaluate the heart in the oblique or lateral views.

In this study, we hypothesized that DCR may be useful for capturing cardiac contraction in the oblique view, as well as in the frontal and lateral views. In this case report, we evaluated the use of DCR in the oblique view, as well as in the frontal and lateral views, to capture cardiac contraction in one healthy volunteer.

Case presentation

Study population

This study was conducted as part of a prospective observational study on DCR and was approved by the Ethics Committee of Nagoya University (date: 30 June 2023/No. 2023-0103). One healthy volunteer was selected, who was a 41-year-old male with a height of 170 cm, a body weight of 65 kg, and a body surface area of 1.75 m². All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Helsinki Declaration (as revised in 2013). The volunteer was fully informed about the risks of exposure and provided written informed consent for publication of this article and accompanying images and videos. A copy of the written consent is available for review by the editorial office of this journal.

DCR imaging procedure

The DCR examination was performed by a radiologist and a radiology technician. Sequential chest radiographs were acquired utilizing a dynamic flat-panel detector DCR system, which comprised an X-ray motion analysis workstation (Konica Minolta, Inc., Tokyo, Japan), a portable digital radiography system (AeroDR fine motion; Konica Minolta, Inc.), and a conventional X-ray system with a pulsed X-ray generator (RADSpeed Pro; Shimadzu Corporation, Kyoto, Japan) (Figure 1). Pulsed X-rays were continuously emitted at a frequency of approximately 15 frames per second by the conventional X-ray system. These X-rays were captured by the dynamic flat-panel detector to generate a chest X-ray movie. The resulting moving images were subsequently analyzed using the X-ray imaging workstation.

Figure 1 DCR system. This image was reproduced from Hiraiwa et al. (16) under CC-BY-NC-ND license. DCR, dynamic chest radiography.

The acquisition of cardiopulmonary perfusion images required a 7-second breath-hold. The incident surface dose for each dynamic chest X-ray during this period was around 0.8 mGy, with an effective dose of 0.16 mSv and a per-frame dose of 7.6 µGy. The total exposure dose was calculated using the following formula: pulse dose × 15 frames per second × imaging duration. At Nagoya University Hospital, the average exposure dose is 1.8 mGy, which is below the dose of 1.9 mGy recommended by the International Atomic Energy Agency for combined frontal and lateral chest radiographs. The pixel value range was 65,536 (16-bit), with the signal intensity directly proportional to the incident exposure detected by the flat-panel detector.

Two types of DCR image were taken, including standard imaging and PH2-MODE, which visualizes intracardiopulmonary blood flow. PH2-MODE visualizes the degree of change in the pixel value from end-diastole, which is used as the reference frame. PH2-MODE imaging uses colors to represent the amount of change in signal from the cardiac diastolic frame. Specifically, a low volume of blood in the ventricle is indicated in blue and a high volume of blood in the ventricle is indicated in red. During ventricular diastole, the signal is redder in color because there is more blood in the ventricular lumen. The signal becomes bluer from the diastolic frame to the systolic frame as blood is ejected from the ventricle. In conjunction with changes in blood volume during ventricular diastole and systole, blood volume inside the aorta and pulmonary vessels also changes, so the signal in the regions near the cardiac chambers also changes. Signal changes are measured as changes in pixel value. After removing temporal changes in the pixel value related to respiration by applying a bandpass filter, changes in the pixel value from the reference frame (end-diastole) in each phase can be analyzed (13,20) (Figure 2).

Figure 2 PH2-MODE image based on the change in pixel value from the reference frame (end-diastole) (A). The blue signal indicates low blood volume, while the red signal indicates high blood volume. A more detailed description of PH2-MODE of DCR imaging has been published previously (13) and is illustrated (B). DCR, dynamic chest radiography.

DCR imaging protocols: frontal, oblique, and lateral views

The DCR imaging conditions in the standing position are shown in Table 1. In the standing position, the healthy volunteer was imaged in the frontal, oblique, and lateral views. We used the angle board with lines drawn at 15° increments, and the volunteer accordingly changed their position in the forward, clockwise, and counterclockwise directions (Figure 3A,3B, Figure S1A-S1H).

Figure 3 Angle board in the standing position. The angle board with lines drawn in 15° increments (A). The volunteer rotated their body clockwise or counterclockwise in 15° increments in the standing position (B).

After imaging in the frontal posterior-anterior view, the volunteer first tilted their body axis 15° to the left at increments of 15° relative to the flat-panel detector, and images in the right anterior oblique (RAO) view were obtained at 15°, 30°, 45°, 60°, 75°, and 90° sequentially in the left-to-right direction. Next, the body axis was tilted to the right in 15° increments, and images in the left anterior oblique (LAO) view were taken at 15°, 30°, 45°, 60°, 75°, and 90° sequentially in the right-to-left direction (Figure 4). The body angles were checked using an angle meter to ensure that accurate angles were maintained, not only at the feet but also at the chest (Figure S1D,S1F,S1H).

Figure 4 DCR imaging protocol in the standing position. LAO, left anterior oblique; RAO, right anterior oblique; DCR, dynamic chest radiography.

The DCR imaging conditions in the supine position are shown in Table 2. In the supine position, after frontal imaging in the anterior–posterior view, the volunteer was first tilted 15° to the left toward the flat-panel detector, and then the body axis was tilted 15°, 30°, 45°, and 60° sequentially in the left posterior oblique (LPO) view. Next, the volunteer tilted their body axis 15° to the right in each direction, and images were taken in the right posterior oblique (RPO) view at 15°, 30°, 45°, and 60° sequentially (Figure 5). The volunteer used a positional immobilization cushion to maintain their position (Figure S2A-S2F). In addition, an angle meter was used to adequately check that the correct angle was maintained (Figure S2B,S2E).

Figure 5 DCR imaging protocol in the supine position. RPO, right posterior oblique; LPO, left posterior oblique; DCR, dynamic chest radiography.

Patient perspective

“I participated in this study as a healthy volunteer, and DCR imaging was completely painless, although the breath-hold was a little longer than a simple chest X-ray. The instructions during DCR imaging were clear and easy to follow.”

Results

DCR imaging and cardiac contraction

The visualization of wall motion and the identification of wall motion regions outlined below were based on the findings of DCR imaging alone. The findings were not cross-validated with echocardiography, computed tomography, or magnetic resonance imaging. Although we did perform transthoracic echocardiography in the supine position, it was not possible to directly compare the echocardiographic findings with those of DCR because the vectors and angles of the echocardiography beam did not match the vectors and angles of the DCR X-rays, and the observation range was different between the two imaging methods. Therefore, the DCR results identifying wall motion regions and delineating the left and right ventricular regions are considered speculative rather than definitive and should be interpreted as such.

In the standing position, when rotated counterclockwise in the RAO view (Video 1, 1A-1G), left ventricular motion was easier to visualize at 30° (Video 1, 1C) than in the frontal (Video 1, 1A) and 15° (Video 1, 1B) images, while apical motion was effectively visualized at 45° (Video 1, 1D) and 60° (Video 1, 1E). PH2-MODE images obtained at 90° were more useful for visualizing changes in blood flow in the left ventricular lumen than PH2-MODE images obtained at 75° (Video 1, 1F). Overall, PH2-MODE images facilitated visualization of changes in blood flow in the left ventricular lumen (Video 1, 1G). Rotation to 90° better visualized left ventricular posterior wall motion and basal motion (Video 1, 1G) than rotation to 75° (Video 1, 1F). Also in the standing position, clockwise rotation in the LAO view (Video 1, 1H-1M) better facilitated visualization of right ventricular body free wall motion at 45° (Video 1, 1J) and 60° (Video 1, 1K) than at 15° (Video 1, 1H) and 30° (Video 1, 1I). At 90°, right ventricular motion was visible (Video 1, 1M).

In the supine position, when rotated clockwise in the RPO view, basal motion was better visualized at 45° and 60° than in the frontal (Video 2, 2A) and 30° (Video 2, 2B) images (Video 2, 2C,2D). Apical motion was difficult to observe because it overlapped with the diaphragm. Also in the supine position, when rotated counterclockwise in the LPO view, the overall motion of the right ventricular main body was easier to observe at 45° and 60° than at 30° (Video 2, 2E). PH2-MODE enabled effective visualization of changes in blood flow in the lumen of the right ventricular main body (Video 2, 2F,2G).

Summarizing the observations, the frontal view clearly showed contraction and dilation of the entire heart. When imaging in the oblique view, the pulsed X-ray generator and the flat-panel detector were always kept parallel, while the subject’s body was moved to achieve oblique imaging, which mimics cineangiography during cardiac catheterization. In the standing position, the left ventricular motion and contraction could be easily visualized by rotating the angle of the body axis in the counterclockwise direction. When the body axis was rotated in the clockwise direction, it was easy to see right ventricular movement and contraction. In the supine position, right ventricular movement and contraction were easier to see when the body axis angle was rotated in the counterclockwise direction. Meanwhile, when the body axis was rotated in the clockwise direction, it was easy to see left ventricular movement and contraction. PH2-MODE imaging, which visualizes intracardiopulmonary blood flow, may also be useful for understanding cardiac contraction.

Discussion

This is the first report on the use of DCR to observe cardiac contraction in a young healthy subject in the oblique view, as well as in the lateral and frontal views, which is not possible with conventional frontal and lateral plain chest X-ray imaging. The left ventricular ejection fraction of the subject was 65%, confirming that cardiac function was good.

Previous studies have reported the evaluation of cardiac function and contraction using DCR in patients with heart failure and heart disease, respectively (16,17). However, these reports were based on the frontal view and did not evaluate the heart in the oblique or lateral views, which is the novelty of the present study. In practice, when simple chest X-ray is performed on a patient with cardiac disease, the presence or absence of an enlarged cardiac shadow, pulmonary congestion, pleural effusion, and aortic or pulmonary artery shadows is generally checked in conjunction with frontal or lateral imaging. In patients with implanted cardiac devices, oblique imaging may also be performed to confirm the position of pacemaker or cardiac resynchronization therapy device leads (18,19).

In this study, the following imaging angles were used, which are assumed to be available in real-world clinical practice: standing position: 0° (frontal), RAO 45° or 60°, 90° (lateral), LAO 45° or 60°, 90° (lateral); supine position: 0° (frontal), LPO 45° or 60°, RPO 45° or 60°. It is important to note that the subject in this study was a young, healthy volunteer. However, age-related factors, such as changes in heart position, may alter the ability of DCR to visualize cardiac dynamics in older patients, and angle adjustments may be necessary, which should be evaluated in future studies.

One difference between the standing and supine positions is that in the standing position, the heart is upright because of the effect of gravity, which may make it easier to visualize left ventricular motion and contraction. In the supine position, the heart is lying flat and spreads out horizontally (especially the right heart system), which may make the supine position better for observing the right ventricle than the left ventricle. It should be noted that the left and right ventricles cannot be completely separated for evaluation, which is one of the research limitations and a future challenge.

It is important to note that the view of the heart on DCR is not exactly the same as that on cineangiography during cardiac catheterization. In other words, the X-ray generator and flat-panel detector are in parallel under the same conditions, but in the supine position, the direction of X-ray transmission is anterior-posterior with DCR, whereas in cineangiography, it is posterior-anterior. In the present study, the X-ray generator and flat-panel detector were fixed, and the subject’s body axis was tilted. With cineangiography, the X-ray generator and flat-panel detector move together as a C-arm, while the subject’s body is fixed parallel to the ground.

DCR is characterized by its ability to observe cardiac contraction during beating, which is not possible with conventional simple chest X-ray. With DCR, it may be possible to obtain detailed information on the degree of contraction of the heart and wall motion by region by performing not only simple frontal and lateral imaging, but also oblique imaging. As this was a preliminary study in a single healthy volunteer, we plan to conduct similar studies in the future to further investigate the application of DCR imaging in the oblique view in patients with cardiac diseases.

Although we did perform transthoracic echocardiography in the supine position, which showed good left ventricular contraction and a left ventricular ejection fraction of 65%, it was not possible to compare the echocardiographic findings with the findings obtained by DCR because the vectors and angles of the echocardiography beam did not match those of the DCR X-rays, and the observation range was different between the two imaging methods. However, a previous study (17) has reported the usefulness of DCR for identifying left ventricular dysfunction. In that study, DCR was performed on 61 patients with cardiovascular disease and 10 healthy volunteers. The results showed that the rate of change in left ventricular pixel value could identify patients with reduced left ventricular ejection fraction (<50%). This indicates that DCR can indeed capture the degree of cardiac contraction in both healthy subjects and in patients with cardiac disease. In that study, the median age of the healthy volunteers was 43 years, and the median body surface area, left ventricular ejection fraction, left ventricular end-diastolic diameter (LVDd), and left ventricular end-systolic diameter (LVDs) were 1.6 m², 59%, 44 mm, and 28 mm, respectively. The healthy volunteer in the present study also underwent transthoracic echocardiography in the prone position at the same time as DCR imaging, and the results of echocardiography showed good left ventricular and right ventricular contraction. The healthy volunteer was 41 years old and had a body surface area of 1.75 m², a left ventricular ejection fraction of 65%, an LVDd of 45 mm, and an LVDs of 27 mm, similar to the healthy volunteers in the previous study. Furthermore, in the previous study, the median percent change in pixel value (%Δpixel value) of cardiac regions of interest on DCR in healthy volunteers ranged from 11.4% to 20.8% in the sitting position in the frontal view. In the present study, the %Δpixel value in the healthy volunteer was 18.8% in the standing position (frontal view) and 16.0% in the supine position (frontal view). These %Δpixel values of cardiac regions of interest on DCR show that, although there were differences in the imaging conditions and in the posture of the patients when performing DCR (sitting vs. standing or supine), the results of the present study are not inconsistent with those of the previous study (17). Therefore, although we did not directly compare the results of DCR with left ventricular ejection fraction measured by echocardiography, a previous study provided a good indication that DCR can capture the degree of cardiac contraction and identify reduced left ventricular ejection fraction. In our future research, we aim to compare DCR images with images obtained using other modalities, such as echocardiography, computed tomography, and magnetic resonance imaging, which will enable us to investigate the reliability and clinical applicability of DCR for evaluating cardiac structure, function, and dynamics.

Although DCR has demonstrated potential as an imaging modality to evaluate cardiac structure and function, its potential limitations should also be considered to obtain a more balanced view of its applicability. First, the subject in this study was a young volunteer. However, age-related factors, such as changes in heart position, may alter the ability of DCR to visualize cardiac dynamics in older patients, which should be evaluated in future studies. Moreover, the value of DCR for evaluating cardiac structure and function in those with cardiac disease should be further clarified. Second, although DCR was able to assess cardiac structure and function in the present study, further studies are needed to thoroughly validate how well the findings of DCR correlate with the current widely used measures of cardiac function, such as left ventricular ejection fraction measured by echocardiography. These insights could help to determine whether DCR has potential clinical application value. Third, motion artifacts can appear on DCR images obtained in PH2-MODE, resulting from the decrease in lung density with cardiac contraction and the increase in lung density with cardiac relaxation. The limitation of motion artifacts on DCR has been reported previously (2,13). Finally, echocardiography is a well-established assessment method, so some clinicians and researchers may find it more difficult to recognize the possible additional benefits of DCR for cardiac structure and function evaluation. Therefore, it will be important to consider how DCR would fit into the current patient care pathway.

Taken together, the findings suggest that DCR may be a valuable tool for evaluating cardiac morphology and contraction. If DCR demonstrates good reproducibility and reliability for such evaluations in large-sample studies in the future, we envision it to be particularly useful in certain applications. Importantly, DCR is not proposed as a replacement for echocardiography. Rather, it could be positioned as a tool for non-cardiologists to conduct preliminary cardiac function assessment and screening before referral to a specialized cardiologist for echocardiography examination, which is often difficult to perform by non-cardiologists. Given the high demand for cardiologists in many countries, the wait time to see a doctor can be quite long. Therefore, having a primary care physician perform DCR may help to reduce the workload of cardiologists when longitudinal monitoring of cardiac contractility is needed, bridging the time to the patient’s visit to a cardiologist for more detailed evaluation by echocardiography. DCR may also be a useful physical examination tool for use in healthy subjects for the screening and early detection of cardiac disease. Many cardiac diseases are widely known to decrease cardiac contraction, such as dilated cardiomyopathy in young patients; ischemic cardiomyopathy, which increases with age; and valvular heart disease associated with atrial fibrillation, the early detection of which can be very useful in clinical practice.

This study has some limitations that should be considered. Only a single young, healthy volunteer was included. Therefore, the utility of DCR for evaluating cardiac dynamics in older individuals should be evaluated in the future given the possibility of age-related changes in cardiac position. Given that this study was based on the DCR images of a single patient obtained in one session, the reproducibility and generalizability of the findings remain unclear. Notably, cardiac morphology and anatomy differ among individuals; therefore, angle adjustments may be needed to capture cardiac structure and function information. Therefore, the imaging angles proposed in this study may not be generalizable to all individuals. Computed tomography and magnetic resonance imaging were not performed in this case, so comparison of DCR with these imaging modalities could not be performed. Although we did perform transthoracic echocardiography in the supine position, it was not possible to directly compare the echocardiographic findings with those of DCR because they were not evaluated under the same conditions. Specifically, the vectors and angles of the echocardiography beam did not match the vectors and angles of the DCR X-rays, and the observation range was different between the two imaging methods. Finally, motion artifacts were present on the DCR images obtained in PH2-MODE. This is a well-known technical limitation of DCR (2,13), and we hope that with continued technical advances, this limitation will be resolved in the future. Future studies should compare the results of DCR imaging with those obtained using other imaging modalities to evaluate the reliability of DCR to accurately evaluate cardiac structure and function.

Conclusions

The findings of this study provide an early indication that DCR images obtained in the oblique view may provide useful information on cardiac contraction and wall motion that is difficult to obtain with conventional X-ray in the frontal and lateral views. However, these observations, as well as the reliability, reproducibility, and clinical applicability of DCR in the oblique view, need to be further evaluated in larger sample studies in the future.

Acknowledgments

The authors wish to thank Koki Furuo, RT (Radiological Technology, Department of Medical Technique, Nagoya University Hospital, Nagoya, Japan), and Noritsugu Matsutani and Ryoichi Watanabe, MS (Healthcare Business Headquarters, Konica Minolta, Inc., Tokyo, Japan), for providing advice on the technical aspects of this study.

Footnote

Funding: This study was funded by Konica Minolta, Inc., and Konica Minolta Science and Technology Foundation (Konica Minolta Imaging Science Encouragement Award to H.H.).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-24-1679/coif). H.H. received research grants from Konica Minolta, Inc., Konica Minolta Science and Technology Foundation. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted as part of a prospective observational study on DCR and was approved by the Ethics Committee of Nagoya University (date: 30 June 2023/No. 2023-0103). All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Helsinki Declaration (as revised in 2013). Written informed consent was obtained from the volunteer for publication of this article and accompanying images and videos. A copy of the written consent is available for review by the editorial office of this journal.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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