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Laser speckle imaging based on photothermally driven convection

UC Irvine Previously Published Works (2016)

Laser speckle imaging (LSI) is an interferometric technique that provides information about the relative speed of moving scatterers in a sample. Photothermal LSI overcomes limitations in depth resolution faced by conventional LSI by incorporating an excitation pulse to target absorption by hemoglobin within the vascular network. Here we present results from experiments designed to determine the mechanism by which photothermal LSI decreases speckle contrast. We measured the impact of mechanical properties on speckle contrast, as well as the spatiotemporal temperature dynamics and bulk convective motion occurring during photothermal LSI. Our collective data strongly support the hypothesis that photothermal LSI achieves a transient reduction in speckle contrast due to bulk motion associated with thermally driven convection. The ability of photothermal LSI to image structures below a scattering medium may have important preclinical and clinical applications.

Cover page: Laser speckle imaging based on photothermally driven convection

Creative Commons 'BY' version 4.0 license

Thesis
Peer Reviewed

Laser speckle imaging of blood flow beneath static scattering media

Regan, Caitlin Anderson
Advisor(s): Choi, Bernard

UC Irvine Electronic Theses and Dissertations (2017)

Laser speckle imaging (LSI) is a wide-field optical imaging technique that provides information about the movement of scattering particles in biological samples. LSI is used to create maps of relative blood flow and perfusion in samples such as the skin, brain, teeth, gingiva, and other biological tissues. The presence of static, or non-moving, optical scatterers affects the ability of LSI to provide true quantitative and spatially resolved measurements of blood flow. With in vitro experiments using tissue-simulating phantoms, we determined that temporal analysis of raw speckle image sequences improved the quantitative accuracy of LSI to measure flow beneath a static scattering layer. We then applied the temporal algorithm to assess the potential of LSI to monitor oral health. We designed and tested two generations of miniature LSI devices to measure flow in the pulpal chamber of teeth and in the gingiva. Our preliminary clinical pilot data indicated that speckle contrast may correlate with gingival health.

To improve visualization of subsurface blood vessels, we developed a technique called photothermal LSI. We applied a short pulse of laser energy to selectively perturb the motion of red blood cells, increasing the signal from vasculature relative to the surroundings. To study the spectral and depth dependence of laser speckle contrast, we developed a Monte Carlo model of light and momentum transport to simulate speckle contrast. With an increase in the thickness of the overlying static-scattering layer, we observed a quadratic decrease in the quantity of dynamically scattered light collected by the detector. We next applied the model to study multi-exposure speckle imaging (MESI), a method that purportedly improves quantitative accuracy of subsurface blood flow measurements. We unexpectedly determined that MESI faced similar depth limitations as conventional LSI, findings that were supported by in vitro experimental data. Finally, we used the model to study the effects of epidermal melanin absorption on LSI, and demonstrated that speckle contrast is less sensitive to varying melanin content than reflectance. We then proposed a two-wavelength measurement protocol that may enable melanin-independent LSI measurements of blood flow in patients with varying skin types. In conclusion, through in vitro and in silico experiments, we were able to further the understanding of the depth dependent origins of laser speckle contrast as well as the inherent limitations of this technology.

Cover page: Laser speckle imaging of blood flow beneath static scattering media

Article
Peer Reviewed

Momentum transfer Monte Carlo for the simulation of laser speckle imaging and its application in the skin.

UC Irvine Previously Published Works (2017)

Due to its simplicity and low cost, laser speckle imaging (LSI) has achieved widespread use in biomedical applications. However, interpretation of the blood-flow maps remains ambiguous, as LSI enables only limited visualization of vasculature below scattering layers such as the epidermis and skull. Here, we describe a computational model that enables flexible in-silico study of the impact of these factors on LSI measurements. The model uses Monte Carlo methods to simulate light and momentum transport in a heterogeneous tissue geometry. The virtual detectors of the model track several important characteristics of light. This model enables study of LSI aspects that may be difficult or unwieldy to address in an experimental setting, and enables detailed study of the fundamental origins of speckle contrast modulation in tissue-specific geometries. We applied the model to an in-depth exploration of the spectral dependence of speckle contrast signal in the skin, the effects of epidermal melanin content on LSI, and the depth-dependent origins of our signal. We found that LSI of transmitted light allows for a more homogeneous integration of the signal from the entire bulk of the tissue, whereas epi-illumination measurements of contrast are limited to a fraction of the light penetration depth. We quantified the spectral depth dependence of our contrast signal in the skin, and did not observe a statistically significant effect of epidermal melanin on speckle contrast. Finally, we corroborated these simulated results with experimental LSI measurements of flow beneath a thin absorbing layer. The results of this study suggest the use of LSI in the clinic to monitor perfusion in patients with different skin types, or inhomogeneous epidermal melanin distributions.

Cover page: Momentum transfer Monte Carlo for the simulation of laser speckle imaging and its application in the skin.

Article
Peer Reviewed

Photothermal laser speckle imaging.

UC Irvine Previously Published Works (2014)

The analysis of speckle contrast in a time-integrated speckle pattern enables visualization of superficial blood flow in exposed vasculature, a method we call laser speckle imaging (LSI). With current methods, LSI does not enable visualization of subsurface or small vasculature, because of optical scattering by stationary structures. In this work we propose a new technique called photothermal LSI to improve the visualization of blood vessels. A 595 nm laser pulse was used to excite blood in both in vitro and in vivo samples. The high absorption coefficient of blood at this wavelength results in efficient conversion of optical energy to thermal energy, resulting in an increase in the local temperature and hence increased scatterer motion, and thus a transient decrease in speckle contrast. As a result, we found that photothermal LSI was able to visualize blood vessels that were hidden when imaged with a conventional LSI system.

Cover page: Photothermal laser speckle imaging.

Article
Peer Reviewed

Spatial versus temporal laser speckle contrast analyses in the presence of static optical scatterers

UC Irvine Previously Published Works (2014)

Previously published data demonstrate that the temporal processing algorithm for laser speckle contrast imaging (LSCI) can improve the visibility of deep blood vessels and is less susceptible to static speckle artifacts when compared with the spatial algorithm. To the best of our knowledge, the extent to which the temporal algorithm can accurately predict the speckle contrast associated with flow in deep blood vessels has not been quantified. Here, we employed two phantom systems and imaging setups (epi-illumination and transillumination) to study the contrast predicted by the spatial and temporal algorithms in subsurface capillary tubes as a function of the camera exposure time and the actual flow speed. Our data with both imaging setups suggest that the contrast predicted by the temporal algorithm, and therefore the relative flow speed, is nearly independent of the degree of static optical scattering that contributes to the overall measured speckle pattern. Collectively, these results strongly suggest the potential of temporal LSCI at a single-exposure time to assess accurately the changes in blood flow even in the presence of substantial static optical scattering.

Cover page: Spatial versus temporal laser speckle contrast analyses in the presence of static optical scatterers

Article
Peer Reviewed

Spatial versus temporal laser speckle contrast analyses in the presence of static optical scatterers

UC Irvine Previously Published Works (2014)

Previously published data demonstrate that the temporal processing algorithm for laser speckle contrast imaging (LSCI) can improve the visibility of deep blood vessels and is less susceptible to static speckle artifacts when compared with the spatial algorithm. To the best of our knowledge, the extent to which the temporal algorithm can accurately predict the speckle contrast associated with flow in deep blood vessels has not been quantified. Here, we employed two phantom systems and imaging setups (epi-illumination and transillumination) to study the contrast predicted by the spatial and temporal algorithms in subsurface capillary tubes as a function of the camera exposure time and the actual flow speed. Our data with both imaging setups suggest that the contrast predicted by the temporal algorithm, and therefore the relative flow speed, is nearly independent of the degree of static optical scattering that contributes to the overall measured speckle pattern. Collectively, these results strongly suggest the potential of temporal LSCI at a single-exposure time to assess accurately the changes in blood flow even in the presence of substantial static optical scattering.

Article
Peer Reviewed

Fiber-based laser speckle imaging for the detection of pulsatile flow

ICTS Publications (2015)

Article
Peer Reviewed

Design and evaluation of a miniature laser speckle imaging device to assess gingival health

UC Irvine Previously Published Works (2016)

Current methods used to assess gingivitis are qualitative and subjective. We hypothesized that gingival perfusion measurements could provide a quantitative metric of disease severity. We constructed a compact laser speckle imaging (LSI) system that could be mounted in custom-made oral molds. Rigid fixation of the LSI system in the oral cavity enabled measurement of blood flow in the gingiva. In vitro validation performed in controlled flow phantoms demonstrated that the compact LSI system had comparable accuracy and linearity compared to a conventional bench-top LSI setup. In vivo validation demonstrated that the compact LSI system was capable of measuring expected blood flow dynamics during a standard postocclusive reactive hyperemia and that the compact LSI system could be used to measure gingival blood flow repeatedly without significant variation in measured blood flow values (p<0.05). Finally, compact LSI system measurements were collected from the interdental papilla of nine subjects and compared to a clinical assessment of gingival bleeding on probing. A statistically significant correlation (?=0.53; p<0.005) was found between these variables, indicating that quantitative gingival perfusion measurements performed using our system may aid in the diagnosis and prognosis of periodontal disease.

Cover page: Design and evaluation of a miniature laser speckle imaging device to assess gingival health

Article
Peer Reviewed

Fiber‐based laser speckle imaging for the detection of pulsatile flow

UC Irvine Previously Published Works (2015)

Background and objective

In endodontics, a major diagnostic challenge is the accurate assessment of pulp status. In this study, we designed and characterized a fiber-based laser speckle imaging system to study pulsatile blood flow in the tooth.

Study design/materials and methods

To take transilluminated laser speckle images of the teeth, we built a custom fiber-based probe. To assess our ability to detect changes in pulsatile flow, we performed in vitro and preliminary in vivo tests on tissue-simulating phantoms and human teeth. We imaged flow of intralipid in a glass microchannel at simulated heart rates ranging from 40 beats/minute (bpm) to 120 bpm (0.67-2.00 Hz). We also collected in vivo data from the upper front incisors of healthy subjects. From the measured raw speckle data, we calculated temporal speckle contrast versus time. With frequency-domain analysis, we identified the frequency components of the contrast waveforms.

Results

With our approach, we observed in vitro the presence of pulsatile flow at different simulated heart rates. We characterized simulated heart rate with an accuracy of and >98%. In the in vivo proof-of-principle experiment, we measured heart rates of 69, 90, and 57 bpm, which agreed with measurements of subject heart rate taken with a wearable, commercial pulse oximeter.

Conclusions

We designed, built, and tested the performance of a dental imaging probe. Data from in vitro and in -vivo tests strongly suggest that this probe can detect the presence of pulsatile flow. LSI may enable endodontists to noninvasively assess pulpal vitality via direct measurement of blood flow.