4D OCT reveals details of embryonic heartbeats

11 Aug 2020

Imaging platform allows new study of cardiac biomechanics.

The pumping dynamics of a mammalian embryonic heart are still not clearly understood, especially during the developmental stage where the organ is still a valveless tube.

In particular it is currently not clear how and at what developmental stage specific regions of the heart tube start contributing to the local and overall flow dynamics, and how the functions of different cardiac regions are integrated in the valveless heart tube, according to a project at the Stevens Institute of Technology and Baylor College of Medicine.

The project tackled these questions by successfully imaging the action of the embryonic organ in a mouse in new detail, through post-synchronizing sequences of OCT images to reveal the dynamics of the structure over a period of time - effectively "4D OCT." The results were reported in Journal of Biomedical Optics.

Although OCT has been employed for biomechanical analysis of embryonic hearts in the past, including the use of Doppler OCT to visualize the shear stresses induced in embryonic hearts by the pumping of blood, this is the first detailed study of the pumping process itself, according to the project. A technique for assessing cardiac pumping over embryonic development as the heart tube remodels could ultimately reveal functional changes during early cardiogenesis.

"The innovative method offers a new way of studying developmental cardiac biomechanics," commented Amy Oldenburg of the Optical Coherence Imaging Laboratory at University of North Carolina. "Analysis of the 4D OCT images allowed researchers to relate blood flow, flow resistance, and pressure gradients induced by heart wall movements."

In trials on a 9-day mouse embryo, 4D ODT was used to obtain structural and Doppler hemodynamic imaging of the beating heart, allowing the detailed pumping action to assessed via volumetric blood flow rate, flow resistance within the heart tube, and pressure gradient induced by heart wall movements. The results revealed new connections between the temporal profiles of pressure gradient and volumetric blood flow rate, and could potentially resolve a number of controversies in cardiac developmental biology.

New opportunities to study biomechanics

The study used OCT imaging with a B-scan rate of 100 Hz, applying post-acquisition synchronization to reconstruct the entire embryonic heart in 4D at the same equivalent 100 Hz frequency. This provided sufficient resolvability to capture the fast cardiac activities of interest, although a higher sampling rate is likely to improve the accuracy further and will be the subject of future work by the researchers.

The mechanism that pumps blood within the embryonic heart tube has traditionally been thought to be wavelike peristaltic contractions, but 4D OCT revealed a more detailed operation in which localized heart tube pumping in the ventricles functions through a combination of suction and pushing mechanisms.

The results also demonstrated the potentially rich data provided by this approach and its feasibility for investigating the functional relationship between blood flow and heart wall dynamics within different regions of the embryonic mammalian heart - a possibility not currently accessible by other methods, according to the project.

This could indicate that an OCT-based functional approach can be not only a useful tool to characterize cardiac pumping in the tubular embryonic heart, but also bring new opportunities to study biomechanics in normal and defected cardiogenesis.

"By building temporal profiles of critical parameters in the heart tube, evaluating their relevance, and performing causal statistical analysis, the complexity of the mechanical aspect of a beating embryonic heart can be well resolved and analyzed systematically," commented the project in its paper. "As biomechanical factors are increasingly recognized for their essential roles in stimulating and regulating the heart development, we hope this approach could inspire new ideas and innovative designs in imaging and measurement techniques to assess the embryonic cardiac biomechanics."