Underwater Hyperspectral Imaging Technology and Its Applications for Detecting and Mapping the Seafloor: A Review.

Common methods of ocean remote sensing and seafloor surveying are mainly carried out by airborne and spaceborne hyperspectral imagers. However, the water column hinders the propagation of sunlight to deeper areas, thus limiting the scope of observation. As an emerging technology, underwater hyperspectral imaging (UHI) is an extension of hyperspectral imaging technology in air conditions, and is undergoing rapid development for applications in shallow and deep-sea environments. It is a close-range, high-resolution approach for detecting and mapping the seafloor. In this paper, we focus on the concepts of UHI technology, covering imaging systems and the correction methods of eliminating the water column's influence. The current applications of UHI, such as deep-sea mineral exploration, benthic habitat mapping, and underwater archaeology, are highlighted to show the potential of this technology. This review can provide an introduction and overview for those working in the field and offer a reference for those searching for literature on UHI technology.

OCT-based angiography of human dermal microvascular reactions to local stimuli: Implications for increasing capillary blood collection volumes.

OBJECTIVES: To measure and compare microvascular responses within the skin of the upper arm to local stimuli, such as heating or rubbing, through the use of optical coherence tomography angiography (OCTA), and to investigate its impact on blood volume collection. MATERIALS AND METHODS: With the use of heat packs or rubbing, local stimulation was applied to the skin of either the left or right upper arm. Data from the stimulated sites were obtained using OCTA comparing pre- and post-stimulation microvascular parameters, such as vessel density, mean vessel diameter, and mean avascular pore size. Additionally, blood was collected using a newly designed collection device and volume was recorded to evaluate the effect of the skin stimulation. RESULTS: Nineteen subjects were recruited for local stimulation study (including rubbing and heating) and 21 subjects for blood drawn study. Of these subjects, 14 agreed to participate in both studies. OCTA was successful in monitoring and measuring minute changes in the microvasculature of the stimulated skin. Compared to baseline, significant changes after local heating and rubbing were respectively found in vessel density (16% [P = 0.0004] and 33% [P < 0.0001] increase), mean vessel diameter (14% and 11% increase) and mean avascular pore size (5% [P = 0.0068] and 8% [P = 0.0005] decrease) after stimulations. A gradual recovery was recorded for each parameter, with no difference being measured after 30 minutes. Blood collection volumes significantly increased after stimulations of heating (48% increase; P = 0.049) and rubbing (78% increase; P = 0.048). Significant correlations were found between blood volume and microvascular parameters except mean avascular pore size under the heating condition. CONCLUSIONS: OCTA can provide important information regarding microvascular adaptations to local stimuli. With that, both heating and rubbing of the skin have positive effects on blood collection capacity, with rubbing having the most significant effect. Lasers Surg. Med. 50:908-916, 2018. © 2018 Wiley Periodicals, Inc.

Optical coherence tomography angiography monitors human cutaneous wound healing over time.

BACKGROUND: In vivo imaging of the complex cascade of events known to be pivotal elements in the healing of cutaneous wounds is a difficult but essential task. Current techniques are highly invasive, or lack the level of vascular and structural detail required for accurate evaluation, monitoring and treatment. We aimed to use an advanced optical coherence tomography (OCT)-based angiography (OCTA) technique for the non-invasive, high resolution imaging of cutaneous wound healing. METHODS: We used a clinical prototype OCTA to image, identify and track key vascular and structural adaptations known to occur throughout the healing process. Specific vascular parameters, such as diameter and density, were measured to aid our interpretations under a spatiotemporal framework. RESULTS: We identified multiple distinct, yet overlapping stages, hemostasis, inflammation, proliferation, and remodeling, and demonstrated the detailed vascularization and anatomical attributes underlying the multifactorial processes of dermatologic wound healing. CONCLUSIONS: OCTA provides an opportunity to both qualitatively and quantitatively assess the vascular response to acute cutaneous damage and in the future, may help to ascertain wound severity and possible healing outcomes; thus, enabling more effective treatment options.

Optical coherence tomography angiography of normal skin and inflammatory dermatologic conditions.

BACKGROUND: In clinical dermatology, the identification of subsurface vascular and structural features known to be associated with numerous cutaneous pathologies remains challenging without the use of invasive diagnostic tools. OBJECTIVE: To present an advanced optical coherence tomography angiography (OCTA) method to directly visualize capillary-level vascular and structural features within skin in vivo. METHODS: An advanced OCTA system with a 1310 nm wavelength was used to image the microvascular and structural features of various skin conditions. Subjects were enrolled and OCTA imaging was performed with a field of view of approximately 10 × 10 mm. Skin blood flow was identified using an optical microangiography (OMAG) algorithm. Depth-resolved microvascular networks and structural features were derived from segmented volume scans, representing tissue slabs of 0-132, 132-330, and 330-924 µm, measured from the surface of the skin. RESULTS: Subjects with both healthy and pathological conditions, such as benign skin lesions, psoriasis, chronic graft-versus-host-disease (cGvHD), and scleroderma, were OCTA scanned. Our OCTA results detailed variations in vascularization and local anatomical characteristics, for example, depth-dependent vascular, and structural alterations in psoriatic skin, alongside their resolve over time; vascular density changes and distribution irregularities, together with corresponding structural depositions in the skin of cGvHD patients; and vascular abnormalities in the nail folds of a patient with scleroderma. CONCLUSION: OCTA can image capillary blood flow and structural features within skin in vivo, which has the potential to provide new insights into the pathophysiology, as well as dynamic changes of skin diseases, valuable for diagnoses, and non-invasive monitoring of disease progression and treatment. Lasers Surg. Med. 50:183-193, 2018. © 2018 Wiley Periodicals, Inc.

The effect of age on the response of retinal capillary filling to changes in intraocular pressure measured by optical coherence tomography angiography.

PURPOSE: To compare the effect of elevated intraocular pressure (IOP) on retinal capillary filling in elderly vs adult rats using optical coherence tomography angiography (OCTA). METHODS: The IOP of elderly (24-month-old, N=12) and adult (6-8month-old, N=10) Brown Norway rats was elevated in 10mmHg increments from 10 to 100mmHg. At each IOP level, 3D OCT data were captured using an optical microangiography (OMAG) scanning protocol and then post-processed to obtain both structural and vascular images. Mean arterial blood pressure (MAP), respiratory rate, pulse and blood oxygen saturation were monitored non-invasively throughout each experiment. Ocular perfusion pressure (OPP) was calculated as the difference between MAP for each animal and IOP at each level. The capillary filling index (CFI), defined as the ratio of area occupied by functional capillary vessels to the total scan area but excluding relatively large vessels of >30µm, was calculated at each IOP level and analyzed using the OCTA angiograms. Relative CFI vs IOP was plotted for the group means. CFI vs OPP was plotted for every animal in each group and data from all animals were combined in a CFI vs OPP scatter plot comparing the two groups. RESULTS: The MAP in adult animals was 108±5mmHg (mean±SD), whereas this value in the elderly was 99±5mmHg. All other physiologic parameters for both age groups were uniform and stable. In elderly animals, significant reduction of the CFI was first noted at IOP 40mmHg, as opposed to 60mmHg in adult animals. Individual assessment of CFI as a function of OPP for adult animals revealed a consistent plateau until OPP reached between 40 and 60mmHg. Elderly individuals demonstrated greater variability, with many showing a beginning of gradual deterioration of CFI at an OPP as high as 80mmHg. Overall comparison of CFI vs OPP between the two groups was not statistically significant. CONCLUSIONS: Compared to adults, some, but not all, elderly animals demonstrate a more rapid deterioration of CFI vs OPP. This suggests a reduced autoregulatory capacity that may contribute to increased glaucoma susceptibility in some older individuals. This variability must be considered when studying the relationship between IOP, ocular perfusion and glaucoma in elderly animal models.

Long ranging swept-source optical coherence tomography-based angiography outperforms its spectral-domain counterpart in imaging human skin microcirculations.

There is an increasing demand for imaging tools in clinical dermatology that can perform in vivo wide-field morphological and functional examination from surface to deep tissue regions at various skin sites of the human body. The conventional spectral-domain optical coherence tomography-based angiography (SD-OCTA) system is difficult to meet these requirements due to its fundamental limitations of the sensitivity roll-off, imaging range as well as imaging speed. To mitigate these issues, we demonstrate a swept-source OCTA (SS-OCTA) system by employing a swept source based on a vertical cavity surface-emitting laser. A series of comparisons between SS-OCTA and SD-OCTA are conducted. Benefiting from the high system sensitivity, long imaging range, and superior roll-off performance, the SS-OCTA system is demonstrated with better performance in imaging human skin than the SD-OCTA system. We show that the SS-OCTA permits remarkable deep visualization of both structure and vasculature (up to ∼2 mm penetration) with wide field of view capability (up to 18×18 mm2), enabling a more comprehensive assessment of the morphological features as well as functional blood vessel networks from the superficial epidermal to deep dermal layers. It is expected that the advantages of the SS-OCTA system will provide a ground for clinical translation, benefiting the existing dermatological practice.

Robust numerical phase stabilization for long-range swept-source optical coherence tomography.

A novel phase stabilization technique is demonstrated with significant improvement in the phase stability of a micro-electromechanical (MEMS) vertical cavity surface-emitting laser (VCSEL) based swept-source optical coherence tomography (SS-OCT) system. Without any requirements of hardware modifications, the new fully numerical phase stabilization technique features high tolerance to acquisition jitter, and significantly reduced budget in computational effort. We demonstrate that when measured with biological tissue, this technique enables a phase sensitivity of 89 mrad in highly scattering tissue, with image ranging distance of up to 12.5 mm at A-line scan rate of 100.3 kHz. We further compare the performances delivered by the phase-stabilization approach with conventional numerical approach for accuracy and computational efficiency. Imaging result of complex signal-based optical coherence tomography angiography (OCTA) and Doppler OCTA indicate that the proposed phase stabilization technique is robust, and efficient in improving the image contrast-to-noise ratio and extending OCTA depth range. The proposed technique can be universally applied to improve phase-stability in generic SS-OCT with different scale of scan rates without a need for special treatment.

Single frequency MOPA based on Nd:YAG single crystal fiber and rods.

We demonstrate a single frequency 1064 nm master oscillator power amplifier (MOPA) system operating in macro-micro pulse scheme. The repetition rate for the macro pulses was 300 Hz with pulse duration of 300 µs. Micro pulses operated at 25 kHz. The master laser was a single-longitudinal-mode electro-optically Q-switched Nd:YAG laser with an output power of 250 mW and pulse duration of 33 ns. Three stages of power amplifiers based on Nd:YAG single crystal fiber and rods were designed. The final output power reached 31.3 W with pulse duration of 30 ns and linewidth of less than 130 MHz. Micro pulse energy of 13.9 mJ was obtained with a peak power of up to 464 kW. The beam quality factors (M2) were measured to be 1.56 and 1.76 in horizontal and vertical directions, respectively.

High power broadband LiF:F(2)(-) color center laser.

A high power LiF:F(2)(-) color center laser is demonstrated with broadband emission. The excitation source is a quasi-continuous wave diode side-pumped acousto-optically Q-switched Nd:YAG laser. Under an incident 1064-nm laser power of 25.4 W, the highest output power of up to 4.7 W is obtained with a macro pulse repetition rate of 400 Hz and a micro pulse repetition rate of 50 kHz. The broadband emission is centered at 1142 nm with a bandwidth of 13 nm.

Single-frequency CaWO4 Raman amplifier at 1178 nm.

Single-frequency crystalline Raman amplifications at 1178 nm were demonstrated. The seeding laser was generated from a single-frequency continuous wave fiber Raman amplifier. Three stages of Raman amplifications from CaWO4 single crystals were realized with a pulsed 1064 nm Nd:YAG laser as the pumping source. The final output pulse energy at 1178 nm was 26.7 mJ, and the pulse width was 2.9 ns, corresponding to a peak power of 5.2 MW. The overall Raman amplification ratio was up to 4.6×10(6). The linewidth was less than 500 MHz.

Four-wavelength laser based on intracavity BaWO4 Raman conversions of a dual-wavelength Q-switched Nd:YLF laser.

By using diode-end-pumped acousto-optically Q-switched intracavity Raman laser configurations, we demonstrate a four-wavelength laser emitting at 1047.0, 1053.0, 1159.4 and 1166.8 nm. Two Nd:YLiF4 crystals are employed to generate 1047.0-nm and 1053.0-nm laser radiations. These two lasers are then frequency converted by a BaWO4 Raman crystal to generate 1159.4-nm and 1166.8-nm first-Stokes waves. With pulse synchronization realized, we obtain the maximum output powers of 427, 418, 423 and 332 mW for 1047.0-nm, 1053.0-nm, 1159.4-nm and 1166.8-nm lasers, respectively. The total optical-to-optical conversion efficiency is 15.1%.

Synchronized dual tunable wavelength Q-switched Nd:Glass laser.

A flash-lamp pumped electro-optically Q-switched dual tunable wavelength Nd:Glass laser is demonstrated. An Nd:Glass rod was selected as the laser medium and a DKDP crystal was employed as the Q-switch. A cubic polarizer was placed in the cavity to divide the randomly polarized radiation into two orthogonally polarized beams. The two beams were tuned by two Littman-gratings and output independently. For Q-switched operation, the tuning range was 1050.1 - 1061.6 nm and 1050.5 - 1060.2 nm for the horizontal and vertical polarizations, respectively. When the horizontally polarized output was fixed at 1059.1 nm, the synchronized vertically polarized one can be tuned from 1051.5 to 1057.5 nm.