Revealing Heartbeats Across the Face

Insights from dynamic speckle imaging

It should be noted that the speckle referred to here is generated by laser illumination, unlike the radar speckle observed in terrestrial remote sensing. Interestingly, the physical principles underlying these phenomena remain the same, despite operating on vastly different spatial and temporal scales. This shared foundation in wave interference underscores the universality of the speckle effect across scientific disciplines.

Dynamic speckle imaging has emerged as a powerful tool for probing microvascular and physiological dynamics in biological tissues. In recent years, significant progress has been made in this domain, driven by advances in image quality optimization, deeper insights into the underlying physical models, and a growing emphasis on standardizing measurements across different systems. A major focus has been the identification of parameters that can serve as reliable references between devices with varying designs and dimensions, paving the way for reproducibility and comparability in speckle-based studies.

These advancements have not only enhanced the sensitivity and reliability of speckle imaging but have also expanded its applications, from fundamental physiological research to clinical diagnostics.

Recently, we unveiled a fascinating capability: the detection of the subtle rhythm of heartbeats across the entire face.
The ability to capture spatial and temporal dynamics in such detail is an exciting step forward in this field.
I used temporal speckle contrast analysis in a recent experiment to explore how cardiac dynamics manifest themselves in the microvascularization on the skin’s surface. The results reveal not only the power of this imaging technique but also intriguing spatial variations in the timing of these physiological signals.

Experiment and Observations

In this analysis, we captured a 10-second video sequence of 800 frames (80 Hz). The integration time T is 12 ms. We calculated temporal speckle contrast using a sliding window of 50 frames. Then, the normalized VMAI index is computed following [2]:

The experiment was repeated 10 times for each of 3 individuals, using two different imaging devices. This approach allowed us to validate a key aspect of our research: establishing a common calibration standard across devices with various dimensions, as highlighted in our earlier work [2].

To minimize artifacts due to parasitic movements, we ensured that the face remained as still as possible by using customized support to stabilize it during imaging.

I focused on three regions of interest on the face: two hypervascularized areas (nose and lip) and a less vascularized area on the cheek.

A remarkable observation emerged: the cardiac cycle is evident in all regions, regardless of vascularization, in all acquisitions. However, the timing of peak contrast values varies slightly between zones, revealing spatial asynchrony in microvascular dynamics. This variability adds a layer of complexity to our understanding of how blood flow is regulated across the skin.

Key Insights

Several key findings stand out from this study:

Detecting Cardiac Pulsations Everywhere: The ability to capture cardiac dynamics across the face underscores the sensitivity and precision of our imaging system.

Temporal and Spatial Variability: The non-simultaneity of maxima across regions suggests that microcirculation is not perfectly synchronized, even within small facial areas.

Non-Ergodicity of Speckle Signals: As the sliding window size decreases, the measured temporal contrast index converges toward values similar to those obtained from spatial contrast. Conversely, when temporal estimation includes cardiac variations, it highlights the non-stationary nature of the signal.

Broader Implications

This experiment demonstrates the remarkable capability of dynamic speckle imaging to uncover physiological dynamics in unprecedented detail. The spatial variability in microvascular dynamics could be a biomarker for regional physiological or pathological conditions. Moreover, the temporal analysis and non-ergodicity provide a better understanding of the biological processes underlying these dynamics.

References

This work builds on our prior research into dynamic speckle imaging for skin microvascularization. Relevant studies include:

• [1] Colin, E., Plyer, A., Golzio, M., Meyer, N., Favre, G., & Orlik, X. (2022). Imaging of the skin microvascularization using spatially depolarized dynamic speckle. Journal of Biomedical Optics, 27(4), 046003–046003.
• [2] Orlik, X., Colin, E., & Plyer, A. (2024, June). Standardizing Laser Speckle Orthogonal Contrast Imaging: Achieving Reproducible Measurements across Instruments. In Photonics (Vol. 11, №7, p. 585). MDPI.