Photoplethysmography (PPG) is the primary optical technique used for pulse oximetry and heart rate variability (HRV) measurements. The accuracy of PPG is limited by vasoconstriction, motion artifact, and increased melanin concentration. Recent work demonstrates the promise of PPG for the early detection of injured individuals at risk for hemorrhage. However, the inability to distinguish blood loss from other forms of blood volume loss (e.g., dehydration) limits its performance. A new pulsatile blood flow measurement based on laser speckle imaging, speckleplethysmography (SPG), can overcome multiple limitations associated with PPG. We developed an eight-layer tissue model to simulate both PPG and SPG signals via Monte Carlo methods. The simulations match our in vivo results measured with a two-camera set up for simultaneous imaging of PPG and SPG. Both in silico and in vivo data suggest SPG has a much larger signal-to-noise ratio than PPG, which may prove beneficial for noncontact, wide-field optical monitoring of cardiovascular health. We also compared SPG and PPG to ECG for estimation of HRV during an orthostatic challenge performed by 17 subjects. We found that SPG estimations of HRV are highly correlated to ECG HRV for both time and frequency domain parameters and provide increased accuracy over PPG estimations of HRV. The results suggest SPG measurements are a viable alternative for HRV estimation when ECG measurements are impractical.Lastly, we developed a novel, multi-wavelength coherent spatial imaging (CSI) prototype for comparisons to diffuse optical spectroscopy and diffuse correlation spectroscopy (DOS/DCS). CSI corrects relative measurements of blood flow and oxygenation with optical properties to create absolute measurements. CSI is significantly less expensive (~$200 off-the-shelf) and more portable than DOS/DCS (~$60,000 off-the-shelf). The small probe form factor enables convenient placement of the probe onto the human finger for continuous monitoring similar to current pulse oximeters. The preliminary in vitro phantom results from CSI match gold standard scattering and absorption results (R> 0.95 for all three wavelengths). The absolute measurements of SPG, oxygenation, and hemoglobin concentration are similar to the DOS/DCS results during an in vivo arterial occlusion. These preliminary results suggest CSI may be useful for continuous critical care monitoring.

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Noncontact photoplethysmography (PPG) is limited by a poor signal-to-noise ratio (SNR). A solution to this limitation is the use of alternate sources of optical contrast to generate a complementary pulsatile waveform. One such source is laser speckle contrast, which is modulated in biological tissues by the flow rate of red blood cells. Averaging a region of interest from a speckle contrast image over time allows for the calculation of a speckleplethysmogram (SPG). Similar to PPG, SPG enables monitoring of heart rate and respiratory rate. A gap in the knowledge base exists as to the precise spatiotemporal relationship between PPG and SPG signals. We have developed an eight-layer tissue model to simulate both PPG and SPG signals in a reflectance geometry via Monte Carlo methods. We modeled PPG by compression of the upper and lower blood nets due to expansion of the larger arterial layer below. The in silico PPG peak-to-peak amplitude percent was greater at 532 nm than at 860 nm (5.6% vs. 3.0%, respectively), which matches trends from the literature. We modeled SPG by changing flow speeds of red blood cells in both the capillaries and arterioles over the cardiac cycle. The in silico SPG peak-to-peak amplitude percent was 24% at 532 nm and 40% at 860 nm. In silico results are similar to in vivo results measured with a two-camera set up for simultaneous imaging of PPG and SPG. Both in silico and in vivo data suggest SPG has a much larger SNR than PPG, which may prove beneficial for noncontact, wide-field optical monitoring of cardiovascular health.

Laser speckle imaging (LSI) is a wide-field optical technique that enables superficial blood flow quantification. LSI is normally performed in a mounted configuration to decrease the likelihood of motion artifact. However, mounted LSI systems are cumbersome and difficult to transport quickly in a clinical setting for which portability is essential in providing bedside patient care. To address this issue, we created a handheld LSI device using scientific grade components. To account for motion artifact of the LSI device used in a handheld setup, we incorporated a fiducial marker (FM) into our imaging protocol and determined the difference between highest and lowest speckle contrast values for the FM within each data set (Kbest and Kworst). The difference between Kbest and Kworst in mounted and handheld setups was 8% and 52%, respectively, thereby reinforcing the need for motion artifact quantification. When using a threshold FM speckle contrast value (KFM) to identify a subset of images with an acceptable level of motion artifact, mounted and handheld LSI measurements of speckle contrast of a flow region (KFLOW) in in vitro flow phantom experiments differed by 8%. Without the use of the FM, mounted and handheld KFLOW values differed by 20%. To further validate our handheld LSI device, we compared mounted and handheld data from an in vivo porcine burn model of superficial and full thickness burns. The speckle contrast within the burn region (KBURN) of the mounted and handheld LSI data differed by <4  %   when accounting for motion artifact using the FM, which is less than the speckle contrast difference between superficial and full thickness burns. Collectively, our results suggest the potential of handheld LSI with an FM as a suitable alternative to mounted LSI, especially in challenging clinical settings with space limitations such as the intensive care unit.