Investigating ultrasound–light interaction in scattering media

Significance: Ultrasound-assisted optical imaging techniques, such as ultrasound-modulated optical tomography, allow for imaging deep inside scattering media. In these modalities, a fraction of the photons passing through the ultrasound beam is modulated. The efficiency by which the photons are converted is typically referred to as the ultrasound modulation’s “tagging efficiency.” Interestingly, this efficiency has been defined in varied and discrepant fashion throughout the scientific literature. Aim: The aim of this study is the ultrasound tagging efficiency in a manner consistent with its definition and experimentally verify the contributive (or noncontributive) relationship between the mechanisms involved in the ultrasound optical modulation process. Approach: We adopt a general description of the tagging efficiency as the fraction of photons traversing an ultrasound beam that is frequency shifted (inclusion of all frequency-shifted components). We then systematically studied the impact of ultrasound pressure and frequency on the tagging efficiency through a balanced detection measurement system that measured the power of each order of the ultrasound tagged light, as well as the power of the unmodulated light component. Results: Through our experiments, we showed that the tagging efficiency can reach 70% in a scattering phantom with a scattering anisotropy of 0.9 and a scattering coefficient of 4  mm⁻¹ for a 1-MHz ultrasound with a relatively low (and biomedically acceptable) peak pressure of 0.47 MPa. Furthermore, we experimentally confirmed that the two ultrasound-induced light modulation mechanisms, particle displacement and refractive index change, act in opposition to each other. Conclusion: Tagging efficiency was quantified via simulation and experiments. These findings reveal avenues of investigation that may help improve ultrasound-assisted optical imaging techniques

Imaging moving targets through scattering media

Imaging in turbid media such as biological tissue is challenging primarily due to light scattering, which degrades resolution and limits the depths at which we can reliably image objects. There are two main approaches for realizing non-destructive optical imaging through scattering tissue: gated approaches, which serve to distinguish and reject the multiply scattered photons; and non-gated approaches, which detect both the unscattered and scattered light contributions, and leverage the information from the scattering process in order to image the object1. In terms of non-gated approaches, both wavefront shaping (WFS) and speckle-correlation-based imaging (SCI) techniques can achieve high-resolution imaging of objects hidden within scattering media1,2. WFS techniques exploit the principles of time-reversal to undo the effects of scattering, whereas SCI methods exploit the angular correlations inherent within the scattering process to reconstruct the hidden object. In contrast with WFS approaches, SCI methods do not need long acquisition times or the presence of a guide star2. However, SCI methods are currently limited to imaging sparsely tagged objects in a dark-field scenario, and are strongly impacted by noise from other sources.2 In this work, we establish a technique that allows SCI to image obscured objects in a bright-field scenario.3 Our technique leverages the temporal correlations inherent in the scattering process to distinguish the object signal from the remaining, undesired ‘background’ light contributions. By using a deterministic phase modulator to generate a spatially incoherent light source, the background light contribution is kept constant between different acquisitions and can subsequently be subtracted out. As long as the object moves between acquisitions, the signal from the object can be isolated. The object can be reconstructed from this signal with high fidelity. Using this technique, we experimentally demonstrate successful reconstruction of moving objects hidden behind and between optically translucent materials. Due to the ability to effectively isolate the object signal, our work is not limited to imaging objects in the dark-field case, but also works in bright-field scenarios, with non-emitting objects. This ability opens up many potential applications for imaging in scattering media, such as through turbulent atmosphere or biological tissue, and makes this work relevant to the technical session on ‘Biophotonics in scattering tissue.’ References 1 R. Horstmeyer et al, “Guidestar-assisted wavefront-shaping methods for focusing light into biological tissue,” Nat. Photon. 9, 563-571 (2015). 2O. Katz et al, “Non-invasive single-shot imaging through scattering layers and around corners via speckle correlations,” Nat. Photon. 8, 784-790 (2014). 3M.Cua et al, “Imaging moving targets through scattering media,” O.E. 25(4), 3935-3945 (2017) Please click Additional Files below to see the full abstract

Exploiting Speckle to Image Deeper in Scattering Media

Optical methods for imaging and focusing are advantageous in many scenarios as optics can provide exquisite spatial resolution, has multiple sources of contrast, and does not impart ionizing radiation. However, optical scattering remains a fundamental challenge which limits the depth at which we can perform imaging with good spatial resolution. This challenge motivated our investigations into methods that could make use of the scattered light in order to extend the depth of imaging through or within scattering media. In particular, we focus on answering: (1) Can one 'unscramble' the scattered light in order to recover information about the otherwise hidden object?; and (2) Can we preferentially detect the more forward scattered photons in an efficient manner in order to allow deeper penetration with modest resolution? These two questions are explored in the first two projects of the thesis: 1. The development of an imaging system that detects the scattered light and exploits correlations within the scattering process to enable imaging through scattering media at diffraction-limited resolution. 2. The introduction of a novel method, termed Speckle-Resolved Optical Coherence Tomography, that sensitively and preferentially detects the more forward scattered photons in a coherent, speckle-resolved fashion to allow deeper imaging at moderate resolution. Optical methods offer the benefit of visualizing samples that would otherwise appear transparent. Using light, one is able to visualize and measure the thickness of transparent films and coatings in a non-contact manner. The third project in my thesis focuses on using light to non-destructively visualize and characterize the evenness of the silicone oil layer that typically coats the inner surface of prefilled syringes. Characterizing the evenness of this silicone oil layer is important as it impacts the functionality of the prefilled syringe and may correlate with particle formation, which is undesirable as the number of particles in a syringe is regulated due to potential health concerns. </p

Investigating ultrasound–light interaction in scattering media

Significance: Ultrasound-assisted optical imaging techniques, such as ultrasound-modulated optical tomography, allow for imaging deep inside scattering media. In these modalities, a fraction of the photons passing through the ultrasound beam is modulated. The efficiency by which the photons are converted is typically referred to as the ultrasound modulation’s “tagging efficiency.” Interestingly, this efficiency has been defined in varied and discrepant fashion throughout the scientific literature. Aim: The aim of this study is the ultrasound tagging efficiency in a manner consistent with its definition and experimentally verify the contributive (or noncontributive) relationship between the mechanisms involved in the ultrasound optical modulation process. Approach: We adopt a general description of the tagging efficiency as the fraction of photons traversing an ultrasound beam that is frequency shifted (inclusion of all frequency-shifted components). We then systematically studied the impact of ultrasound pressure and frequency on the tagging efficiency through a balanced detection measurement system that measured the power of each order of the ultrasound tagged light, as well as the power of the unmodulated light component. Results: Through our experiments, we showed that the tagging efficiency can reach 70% in a scattering phantom with a scattering anisotropy of 0.9 and a scattering coefficient of 4  mm⁻¹ for a 1-MHz ultrasound with a relatively low (and biomedically acceptable) peak pressure of 0.47 MPa. Furthermore, we experimentally confirmed that the two ultrasound-induced light modulation mechanisms, particle displacement and refractive index change, act in opposition to each other. Conclusion: Tagging efficiency was quantified via simulation and experiments. These findings reveal avenues of investigation that may help improve ultrasound-assisted optical imaging techniques

Functional Assessment of Cardiac Responses of Adult Zebrafish (Danio rerio) to Acute and Chronic Temperature Change Using High-Resolution Echocardiography

The zebrafish (Danio rerio) is an important organism as a model for understanding vertebrate cardiovascular development. However, little is known about adult ZF cardiac function and how contractile function changes to cope with fluctuations in ambient temperature. The goals of this study were to: 1) determine if high resolution echocardiography (HRE) in the presence of reduced cardiodepressant anesthetics could be used to accurately investigate the structural and functional properties of the ZF heart and 2) if the effect of ambient temperature changes both acutely and chronically could be determined non-invasively using HRE in vivo. Heart rate (HR) appears to be the critical factor in modifying cardiac output (CO) with ambient temperature fluctuation as it increases from 78 ± 5.9 bpm at 18°C to 162 ± 9.7 bpm at 28°C regardless of acclimation state (cold acclimated CA– 18°C; warm acclimated WA– 28°C). Stroke volume (SV) is highest when the ambient temperature matches the acclimation temperature, though this difference did not constitute a significant effect (CA 1.17 ± 0.15 μL at 18°C vs 1.06 ± 0.14 μl at 28°C; WA 1.10 ± 0.13 μL at 18°C vs 1.12 ± 0.12 μl at 28°C). The isovolumetric contraction time (IVCT) was significantly shorter in CA fish at 18°C. The CA group showed improved systolic function at 18°C in comparison to the WA group with significant increases in both ejection fraction and fractional shortening and decreases in IVCT. The decreased early peak (E) velocity and early peak velocity / atrial peak velocity (E/A) ratio in the CA group are likely associated with increased reliance on atrial contraction for ventricular filling

Development of Optical Coherence Tomography for Quantitative Analysis of Cardiac Morphology

Transgenic mouse models have been instrumental in the elucidation of the molecular mechanism behind many cardiac diseases such as Marfan syndrome. However, the small size of the murine heart has hampered the characterization of its cardiac morphology. In this project, we describe the development of a murine cardiac imaging modality using optical coherence tomography (OCT). After fixation and optical clearing, the hearts were imaged from multiple perspectives. These data sets were then corrected for refraction and registered together to yield a single volume of the whole heart. From this OCT volume, we then applied techniques from computational anatomy to quantify morphological parameters such as wall thickness, luminal volume, and wall masses. Using this pipeline, we performed a preliminary study comparing the cardiac morphology of a mice model of Marfan syndrome with their wild-type counterparts

Single-shot surface 3-D imaging by optical coherence factor (Conference Presentation)

We report a single-shot surface three-dimensional (3-D) imaging method that uses optical coherence as a contrast mechanism to acquire the vertical (z-direction) information of an object. The illumination of the imaging system comes from a light source with the optical coherence length similar to the depth of field (DoF) of the optical system. Holographic recording is used to retrieve the coherence visibility factor, which is then converted to z-direction information. In the experiment, we compare the imaging results of our method to conventional incoherent imaging results, showing that this contrast mechanism is able to provide additional information. We also validate our 3D imaging results by using axial scanning full-field optical coherence tomography