We demonstrated a transceiver system for underwater three-dimensional imaging, based on a 64 × 32 macro-pixel direct time-of-flight (dToF) SPAD detector array fabricated using complementary metal-oxide-semiconductor (CMOS) technology. The sensor featured integrated multi-event time-to-digital converters (METDC) per macro-pixel, and operated in a time-gated mode which allowed the improved rejection of the backscattering from the signal return. The performance of the system was assessed in a controlled underwater environment, utilizing a moving target placed in a water tank at a stand-off distance of 1.45 m in water. The system demonstrated rapid 3D imaging by using short acquisition times of 1 ms, which corresponds to a frame rate of 1000 fps. Depth and intensity profiles were obtained at attenuation levels equivalent up to 5.5 attenuation lengths between the transceiver and the target, and using average optical power of up to 32 mW.
Medical imaging is advancing rapidly through the development of novel laser sources and non-linear imaging methodologies. These developments are boosting deep tissue imaging allowing researchers to study diseases deep in the body enabling early diagnosis and better treatment. To help with the testing and optimization of these imaging systems and to aid in this process of deep tissue imaging, it's important to have robust, stable and reproducible standards and phantoms. Herein we present the design and fabrication of robust, multi-layered, hydrogel-based standards. The hydrogel used is a double network hydrogel consisting of two interpenetrating networks agarose and polyacrylamide. Thin layers of tough double network hydrogels are stacked to form multilayered depth standards having modality specific signaling markers embedded in between. Standard design and assembly ensured long term stability and easy transport. These proved useful in-depth imaging studies, utilizing multiple imaging modalities, including one photon fluorescence (1PEF), two photon fluorescence (2PEF), coherent anti-Stokes Raman imaging (CARS) and second harmonic generation imaging (SHG).
SignificanceSkin color affects light penetration leading to differences in its absorption and scattering properties. COVID-19 highlighted the importance of understanding of the interaction of light with different skin types, e.g., pulse oximetry (PO) unreliably determined oxygen saturation levels in people from Black and ethnic minority backgrounds. Furthermore, with increased use of other medical wearables using light to provide disease information and photodynamic therapies to treat skin cancers, a thorough understanding of the effect skin color has on light is important for reducing healthcare disparities.AimThe aim of this work is to perform a thorough review on the effect of skin color on optical properties and the implication of variation on optical medical technologies.ApproachPublished in vivo optical coefficients associated with different skin colors were collated and their effects on optical penetration depth and transport mean free path (TMFP) assessed.ResultsVariation among reported values is significant. We show that absorption coefficients for dark skin are ∼6% to 74% greater than for light skin in the 400 to 1000 nm spectrum. Beyond 600 nm, the TMFP for light skin is greater than for dark skin. Maximum transmission for all skin types was beyond 940 nm in this spectrum. There are significant losses of light with increasing skin depth; in this spectrum, depending upon Fitzpatrick skin type (FST), on average 14% to 18% of light is lost by a depth of 0.1 mm compared with 90% to 97% of the remaining light being lost by a depth of 1.93 mm.ConclusionsCurrent published data suggest that at wavelengths beyond 940 nm light transmission is greatest for all FSTs. Data beyond 1000 nm are minimal and further study is required. It is possible that the amount of light transmitted through skin for all skin colors will converge with increasing wavelength enabling optical medical technologies to become independent of skin color.
SignificanceRapid advances in medical imaging technology, particularly the development of optical systems with non-linear imaging modalities, are boosting deep tissue imaging. The development of reliable standards and phantoms is critical for validation and optimization of these cutting-edge imaging techniques.AimWe aim to design and fabricate flexible, multi-layered hydrogel-based optical standards and evaluate advanced optical imaging techniques at depth.ApproachStandards were made using a robust double-network hydrogel matrix consisting of agarose and polyacrylamide. The materials generated ranged from single layers to more complex constructs consisting of up to seven layers, with modality-specific markers embedded between the layers.ResultsThese standards proved useful in the determination of the axial scaling factor for light microscopy and allowed for depth evaluation for different imaging modalities (conventional one-photon excitation fluorescence imaging, two-photon excitation fluorescence imaging, second harmonic generation imaging, and coherent anti-Stokes Raman scattering) achieving actual depths of 1550, 1550, 1240, and 1240 μm, respectively. Once fabricated, the phantoms were found to be stable for many months.ConclusionsThe ability to image at depth, the phantom’s robustness and flexible layered structure, and the ready incorporation of “optical markers” make these ideal depth standards for the validation of a variety of imaging modalities.
KEYWORDS: Raman spectroscopy, Single photon avalanche diodes, Calcite, Fluorescence, Single walled carbon nanotubes, Signal to noise ratio, CMOS sensors, Medical research, Imaging spectroscopy, Diamond
Time-resolved Raman and fluorescence lifetime spectroscopy imaging yields new research insights with great potential in applications including biomedical diagnostics, carbon materials, and battery development. Single Photon Avalanche Diode (SPAD) arrays are ideal for such applications and we present to our knowledge the first time-resolved Raman images obtained with such sensors. Utilizing motorized and confocal scanning configurations we obtain near shot-noise limited performance, room temperature operation, millisecond spectral acquisition times, and simultaneous acquisition and discrimination of Raman and fluorescence with high spectral resolution and range. Detailed images and spectra from samples including calcite, diamond, and single-wall carbon nanotubes demonstrate the possibility of high-resolution time-resolved Raman and fluorescence imaging.
Raman spectroscopy is a hugely informative tool with a plethora of applications from biomedicine to analytical chemistry. Potentially, the technique could improve liver transplantation success rates through investigating Raman signals associated with metabolic changes prior to transplant rejection. However, studying biological systems is challenging since background fluorescence dominates the weak Raman signal. Thus, there is a need to improve signal-to-noise and Raman-tofluorescence ratios and drive down spectral acquisition times. Pulsed lasers combined with time-resolving single photon avalanche diode (SPAD) detection systems have been shown to enhance Raman and fluorescence discrimination. We report significant advances in time-correlated single photon counting (TCSPC) Raman spectroscopy using a laser exhibiting up to 200 W peak power and 40 MHz repetition rates in combination with a 512 spectral channel, 16.5 gigaevent/s throughput SPAD histogramming line sensor. Using a diamond sample, we report 0.4 MHz Raman count rates, millisecond spectral acquisition times, and signal-to-noise ratios of over 200. We demonstrate simultaneous, singleexposure acquisition of Raman and fluorescence signals in sesame oil. Time-based Raman-fluorescence discrimination techniques are subject to fluorescence signal tail influences from previous pulses, and data obtained with laser periods of 25 ns and 50 ns are presented. We achieved optimised Raman-to-fluorescence ratios through adjustment of histogram bin positions in 63 ps increments. Achieving high count rates while discriminating fluorescence from Raman signals unlocks the potential of combined Raman/fluorescence lifetime spectroscopy for biomedical imaging applications.
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