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Womens Health

Women's Health - Poster Session D

Poster CC6 - Optimizing Laser Wavelengths to Minimize Skin Pigment Bias in Wearable Laser Speckle Device for Postpartum Hemorrhage

Friday, October 25, 2024
3:30 PM - 4:30 PM EST
Location: Exhibit Hall E, F & G

Introduction:  Postpartum hemorrhage (PPH) is the leading cause of maternal mortality globally, requiring timely and accurate detection for effective intervention. PPH accounts for 11% of maternal fatalities in the US and disproportionately affects black women and women of color. We previously developed a low-cost wearable device that monitors peripheral perfusion through laser speckle flow index (LSFI) monitoring to detect PPH. The laser speckle device utilizes a 785 nm laser and a Raspberry Pi Camera detector. Relative peripheral perfusion can be assessed by analyzing videos of the changing laser speckle pattern produced when laser light is scattered by flowing blood in human tissue. Many optical devices have shown skin pigmentation bias due to variations of light absorption by melanin. Melanin, the primary pigment in human skin, significantly influences light absorption across different skin tones and can affect how much light reaches blood vessels and is scattered, potentially compromising accuracy in darker skin tones. Melanin absorption exponentially decreases with increasing wavelengths beyond 330 nm, thus longer wavelengths have less absorption by melanin. This study evaluates the performance of an LSFI device with a 785 vs. 904 nm light source in a tissue mimicking flow phantoms with varying pigment levels that mimic a range of skin tones. We also compare the findings with a commercial laser doppler system, the gold standard method in microvascular perfusion. We hypothesized that the longer wavelength LSFI device would reduce LSFI differences caused by skin tone.

Materials and

Methods:  This study evaluated the performance of the LSFI device using two different laser wavelengths on three custom 3D printed resin-based tissue-mimicking flow phantoms that were designed to simulate various skin tones by adding different amounts of India ink (550 µL, 1550 µL, and 2050 µL). India ink was chosen due to its similar absorption to melanin. A syringe pump directed a blood-mimicking solution of intralipid at various flow rates (0-3 mm/s) through a tube that passed through the tissue-mimicking flow phantoms.
The device was first tested using a 785 nm laser speckle device which was placed on top of each phantom within a 3D printed holder that included enclosures for the phantom, LSFI device, and the commercial laser Doppler sensor. The camera captured laser speckle videos at each flow rate for each phantom and the captured videos were processed to produce mean LSFI values.
This testing procedure then was repeated using a 904 nm laser to evaluate the device's performance at a wavelength less absorbed by melanin. The device components and flow phantoms remained consistent across both sets of trials, with the primary variable being the laser wavelength. This approach allowed for a direct comparison of the two wavelengths' effectiveness in mitigating skin pigment bias in detecting changes in intralipid flow. These studies provide insight on how changes in peripheral perfusion caused by postpartum hemorrhage might be impacted across skin tones. The results were analyzed by calculating the percent change in LSFI between lightly and darkly pigmented phantoms.

Results, Conclusions, and Discussions: Both LSFI wavelengths showed significant differences in mean LSFI signal as a function of pigmentation, with higher levels of pigmentation leading to lower LSFI values (Figure 1a,b). This was also reflected in the large percent difference in mean LSFI values between lightly and darkly pigmented phantoms (Figure 1c,d). Unexpectedly, the 904 nm laser demonstrated a larger percent change due to pigmentation than the 785 nm (Figure 1c,d). Additionally, the laser Doppler sensor failed to provide results that scaled with changes in flow rate in the high ink-containing phantoms.
The LSFI sensor uses a silicon detector which shows reduced sensitivity at longer wavelengths like 904 nm. This likely contributes to the overall reduction in LSFI amplitude observed in the 904 nm data. The lower camera sensitivity combined with the higher pigmentation levels appears to have driven the results from the high pigmentation phantoms towards the noise floor of the camera, further increasing the difference is mean LSFI based on pigmentation. Choosing a sensor that is sensitive to infrared light would enable a fair comparison of these wavelengths in future studies. In addition, pursuing methods to create a pigmentation-related correction factor to mitigate differences in signal due to skin pigmentation could also be pursued to minimize or eliminate the pigmentation effects observed in the mean LSFI data. The laser Doppler findings suggest a potential limitation in the ability of laser Doppler to assess blood flow accurately in individuals with darker skin tones.
Further investigation is needed to determine the optimal wavelength and device configuration for minimizing skin pigment bias in LSFI wearables. The results underscore the need for further research and development to refine LSFI and other optical tools to improve their accuracy across diverse populations. Additional studies should further explore alternative laser wavelengths, sensor materials, and the implementation of correction factors to better mitigate the impact of skin pigmentation on measurement accuracy. By advancing these areas, we can enhance the reliability of blood flow measurements across diverse populations, thereby contributing to improved maternal health outcomes.

Acknowledgements (Optional):  WashU Center for Women's Health Engineering
WashU Radiology
WashU Ob/Gyn