Quantifying tissue properties and absolute hemodynamics using coherent spatial imaging

Abstract

Significance: Measuring hemodynamic function is crucial for health assessment. Optical signals provide relative hemoglobin concentration changes, but absolute measurements require costly, bulky technology. Speckleplethysmography (SPG) uses coherent light to detect speckle fluctuations. Combining SPG with multispectral measurements may provide important physiological information on blood flow and absolute hemoglobin concentration.

Aim: To develop an affordable optical technology to measure tissue absorption, scattering, hemoglobin concentrations, tissue oxygen saturation (${\mathrm{StO}}_2$), and blood flow.

Approach: We integrated reflectance spectroscopy and laser speckle imaging to create coherent spatial imaging (CSI). CSI was validated against spatial frequency domain imaging (SFDI) using phantom-based measurements. In vivo arterial and venous occlusion experiments compared CSI with diffuse optical spectroscopy/diffuse correlation spectroscopy (DOS/DCS) measurements.

Results: CSI and SFDI agreed on tissue absorption and scattering in phantom tests. CSI and DOS/DCS showed similar trends and agreement in arterial occlusion results. During venous occlusion, both uncorrected and corrected blood flow decreased with increasing pressure, with an $\sim 200\%$ difference in overall blood flow decrease between the methods. CSI and DOS/DCS data showed expected hemoglobin concentrations, ${\mathrm{StO}}_2$, and blood flow trends.

Conclusions: CSI provides affordable and comprehensive hemodynamic information. It can potentially detect dysfunction and improve measurements, such as blood pressure, $S_p{O}_2$, and metabolism.

Keywords: blood flow; diffuse optics; hemoglobin concentrations; optical properties.

Fig. 1

CSI device and processing overview. (a) A custom PCB was developed to control the synchronization between light sources and the camera. The custom PCB was connected to a sensor probe that contains the light sources (LEDs and VCSEL) on a PCB and a camera. The sensor probe was attached to the measured area. The reflected light from the LEDs and VCSEL was collected at source–detector separations of 3 to 10 mm. (b) Processing overview from raw images to absolute quantifiable metrics, including optical properties, blood flow, oxy- and deoxy-hemoglobin concentrations, and tissue oxygenation.

Fig. 2

In vitro validation of CSI-measured optical properties compared to SFDI using 18 phantoms over 1 week. (a) Comparison between CSI- and SFDI-measured ${\mu }_a$ ($R=0.986$, $p<0.0001$). (b) Comparison between CSI- and SFDI-measured ${\mu }_s^{\prime }$ ($R=0.847$, $p<0.0001$). Standard deviation error bars are shown. (c) COV of ${\mu }_a$ over 1 week. (d) COV of ${\mu }_s^{\prime }$ over 1 week.

Fig. 3

Representative in vivo validation from one subject of the CSI sensor compared to DOS/DCS during an arterial occlusion. (a) DOS/DCS arterial occlusion. (b) CSI arterial occlusion. An arterial occlusion protocol with a 2-min baseline period, followed by 2 min of full occlusion at 220 mmHg, and then 2 min of recovery. Abbreviations: oxy-hemoglobin concentration, $[{\mathrm{HbO}}_2]$; deoxy-hemoglobin concentration, [Hb]; total hemoglobin concentration, [HbT]; tissue oxygen saturation, ${\mathrm{StO}}_2$; blood flow index, BFI; and Brownian diffusion coefficient, $D_b$.

Fig. 4

Representative comparison of uncorrected and optical property-corrected blood flow from one subject using the CSI sensor during venous occlusion. The venous occlusion protocol consisted of a 2 min baseline period, followed by a stepwise venous occlusion at 20, 40, 60, 80, and 100 mmHg, where each occlusion pressure lasted 1 min. After the 100 mmHg pressure, the pressure was released for a 1 min recovery period. Uncorrected blood flow is in red and corrected blood flow is in blue. Insets show pulsatile blood flow waveform at 0 and 40 mmHg.

Fig. 5

Representative in vivo validation from one subject of the CSI sensor compared with DOS/DCS during a venous occlusion. (a) DOS/DCS venous occlusion. (b) CSI venous occlusion. The venous occlusion protocol consisted of a 2 min baseline period, followed by a stepwise venous occlusion at 20, 40, 60, 80, and 100 mmHg, where each occlusion pressure lasted 1 min. After the 100 mmHg pressure, the pressure was released for a 1 min recovery period. Abbreviations: oxy-hemoglobin concentration, $[{\mathrm{HbO}}_2]$; deoxy-hemoglobin concentration, [Hb]; total hemoglobin concentration, [HbT]; tissue oxygen saturation, ${\mathrm{StO}}_2$; blood flow index, BFI; and Brownian diffusion coefficient, $D_b$.