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The editorial introduces the JBO Special Section on Selected Topics in Biophotonics: Translating Novel Photonics Technology into Clinical Applications.
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Interstitial fiber-based spectroscopy is gaining interest for real-time in vivo optical biopsies, endoscopic interventions, and local monitoring of therapy. Different from other photonics approaches, time-domain diffuse optical spectroscopy (TD-DOS) can probe the tissue at a few cm distance from the fiber tip and disentangle absorption from the scattering properties. Nevertheless, the signal detected at a short distance from the source is strongly dominated by the photons arriving early at the detector, thus hampering the possibility of resolving late photons, which are rich in information about depth and absorption.
Aim
To fully benefit from the null-distance approach, a detector with an extremely high dynamic range is required to effectively collect the late photons; the goal of our paper is to test its feasibility to perform TD-DOS measurements at null source–detector separations (NSDS).
Approach
In particular, we demonstrate the use of a superconducting nanowire single photon detector (SNSPD) to perform TD-DOS at almost NSDS ( ≈ 150 μm ) by exploiting the high dynamic range and temporal resolution of the SNSPD to extract late arriving, deep-traveling photons from the burst of early photons.
Results
This approach was demonstrated both on Monte Carlo simulations and on phantom measurements, achieving an accuracy in the retrieval of the water spectrum of better than 15%, spanning almost two decades of absorption change in the 700- to 1100-nm range. Additionally, we show that, for interstitial measurements at null source–detector distance, the scattering coefficient has a negligible effect on late photons, easing the retrieval of the absorption coefficient.
Conclusions
Utilizing the SNSPD, broadband TD-DOS measurements were performed to successfully retrieve the absorption spectra of the liquid phantoms. Although the SNSPD has certain drawbacks for use in a clinical system, it is an emerging field with research progressing rapidly, and this makes the SNSPD a viable option and a good solution for future research in needle guided time-domain interstitial fiber spectroscopy.
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Endoscopic optical coherence tomography (OCT) is of growing interest for in vivo diagnostics of the tympanic membrane (TM) and the middle ear but generally lacks a tissue-specific contrast.
Aim
To assess the collagen fiber layer within the in vivo TM, an endoscopic imaging method utilizing the polarization changes induced by the birefringent connective tissue was developed.
Approach
An endoscopic swept-source OCT setup was redesigned and extended by a polarization-diverse balanced detection unit. Polarization-sensitive OCT (PS-OCT) data were visualized by a differential Stokes-based processing and the derived local retardation. The left and right ears of a healthy volunteer were examined.
Results
Distinct retardation signals in the annulus region of the TM and near the umbo revealed the layered structure of the TM. Due to the TM’s conical shape and orientation in the ear canal, high incident angles onto the TM’s surface, and low thicknesses compared to the axial resolution limit of the system, other regions of the TM were more difficult to evaluate.
Conclusions
The use of endoscopic PS-OCT is feasible to differentiate birefringent and nonbirefringent tissue of the human TM in vivo. Further investigations on healthy as well as pathologically altered TMs are required to validate the diagnostic potential of this technique.
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TOPICS: Brain, Ischemia, Simulation of CCA and DLA aggregates, Windows, Speckle, 3D image reconstruction, Sampling rates, Tissues, Reconstruction algorithms, Neuroimaging
Low-frequency oscillations (LFOs) (<0.1 Hz) with respect to cerebral blood flow (CBF) have shown promise as an indicator of altered neurologic activity in the abnormal brain. Portable optical instruments have evolved to offer a noninvasive alternative for continuous CBF monitoring at the bedside compared with many large neuroimaging modalities. However, their utilization for acquiring LFOs of CBF has only been studied to a limited extent.
Aim
We aim to optimize an innovative speckle contrast diffuse correlation tomography (scDCT) system for the detection of LFOs within CBF variations.
Approach
The scDCT was optimized to achieve a higher sampling rate and a faster image reconstruction using a moving window 3D reconstruction algorithm with parallel computation. Power spectral density (PSD) analysis was performed to investigate altered LFOs during transient global cerebral ischemia in neonatal piglets.
Results
Transient global cerebral ischemia resulted in reductions in both CBF and PSD compared with their baseline values.
Conclusions
Spontaneous LFOs, combined with CBF, provide a more comprehensive assay with the potential to clarify pathological mechanisms involved in brain injury. These results support scDCT’s inclusion and application in the growing area of LFO analysis and demonstrate its inherent advantage for neurological studies in preclinical and clinical settings, such as neonatal intensive care units.
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Photoacoustic imaging (PAI) enables the visualization of optical contrast with ultrasonic imaging. It is a field of intense research, with great promise for clinical application. Understanding the principles of PAI is important for engineering research and image interpretation.
Aim
In this tutorial review, we lay out the imaging physics, instrumentation requirements, standardization, and some practical examples for (junior) researchers, who have an interest in developing PAI systems and applications for clinical translation or applying PAI in clinical research.
Approach
We discuss PAI principles and implementation in a shared context, emphasizing technical solutions that are amenable to broad clinical deployment, considering factors such as robustness, mobility, and cost in addition to image quality and quantification.
Results
Photoacoustics, capitalizing on endogenous contrast or administered contrast agents that are approved for human use, yields highly informative images in clinical settings, which can support diagnosis and interventions in the future.
Conclusion
PAI offers unique image contrast that has been demonstrated in a broad set of clinical scenarios. The transition of PAI from a “nice-to-have” to a “need-to-have” modality will require dedicated clinical studies that evaluate therapeutic decision-making based on PAI and consideration of the actual value for patients and clinicians, compared with the associated cost.
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High grade serous ovarian cancer is the most deadly gynecological cancer, and it is now believed that most cases originate in the fallopian tubes (FTs). Early detection of ovarian cancer could double the 5-year survival rate compared with late-stage diagnosis. Autofluorescence imaging can detect serous-origin precancerous and cancerous lesions in ex vivo FT and ovaries with good sensitivity and specificity. Multispectral fluorescence imaging (MFI) can differentiate healthy, benign, and malignant ovarian and FT tissues. Optical coherence tomography (OCT) reveals subsurface microstructural information and can distinguish normal and cancerous structure in ovaries and FTs.
Aim
We developed an FT endoscope, the falloposcope, as a method for detecting ovarian cancer with MFI and OCT. The falloposcope clinical prototype was tested in a pilot study with 12 volunteers to date to evaluate the safety and feasibility of FT imaging prior to standard of care salpingectomy in normal-risk volunteers. In this manuscript, we describe the multiple modifications made to the falloposcope to enhance robustness, usability, and image quality based on lessons learned in the clinical setting.
Approach
The ∼0.8 mm diameter falloposcope was introduced via a minimally invasive approach through a commercially available hysteroscope and introducing a catheter. A navigation video, MFI, and OCT of human FTs were obtained. Feedback from stakeholders on image quality and procedural difficulty was obtained.
Results
The falloposcope successfully obtained images in vivo. Considerable feedback was obtained, motivating iterative improvements, including accommodating the operating room environment, modifying the hysteroscope accessories, decreasing endoscope fragility and fiber breaks, optimizing software, improving fiber bundle images, decreasing gradient-index lens stray light, optimizing the proximal imaging system, and improving the illumination.
Conclusions
The initial clinical prototype falloposcope was able to image the FTs, and iterative prototyping has increased its robustness, functionality, and ease of use for future trials.
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Wavelength selection from a large diffuse reflectance spectroscopy (DRS) dataset enables removal of spectral multicollinearity and thus leads to improved understanding of the feature domain. Feature selection (FS) frameworks are essential to discover the optimal wavelengths for tissue differentiation in DRS-based measurements, which can facilitate the development of compact multispectral optical systems with suitable illumination wavelengths for clinical translation.
Aim
The aim was to develop an FS methodology to determine wavelengths with optimal discriminative power for orthopedic applications, while providing the frameworks for adaptation to other clinical scenarios.
Approach
An ensemble framework for FS was developed, validated, and compared with frameworks incorporating conventional algorithms, including principal component analysis (PCA), linear discriminant analysis (LDA), and backward interval partial least squares (biPLS).
Results
Via the one-versus-rest binary classification approach, a feature subset of 10 wavelengths was selected from each framework yielding comparable balanced accuracy scores (PCA: 94.8 ± 3.47 % , LDA: 98.2 ± 2.02 % , biPLS: 95.8 ± 3.04 % , and ensemble: 95.8 ± 3.16 % ) to those of using all features (100%) for cortical bone versus the rest class labels. One hundred percent balanced accuracy scores were generated for bone cement versus the rest. Different feature subsets achieving similar outcomes could be identified due to spectral multicollinearity.
Conclusions
Wavelength selection frameworks provide a means to explore domain knowledge and discover important contributors to classification in spectroscopy. The ensemble framework generated a model with improved interpretability and preserved physical interpretation, which serves as the basis to determine illumination wavelengths in optical instrumentation design.
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Medical imaging is very commonly described as synonymous with radiological imaging, yet optical imaging devices are widely distributed throughout many medical specialties. This delocalized nature of the technology reduces visibility and dominance as a cohesive medical technology sector.
Aim
Indicators of impact of medical optical systems were examined and compared to the radiology technology sector.
Approach
This study included a summary of (i) physician users, (ii) global technology valuations, and (iii) NIH funding levels. Analysis focused on comparing optical and radiological technology, comparing costs, funding, and finding differences, while tabulating strengths, weaknesses, opportunities, and threats to the field of optical imaging.
Results
The 2023 global technology revenue valuation of biomedical optical tools is $128 billion USD/year while that of radiological tools is $48 billion USD/year. A direct comparison of US NIH funding in radiology shows $8.5 billion/year, whereas optical devices are nearer to $3.6 billion USD/year. R&D investment in applications, such as endoscopy, laparoscopy, and pulse oximetry, is far below those of radiological tools when normalized by valuation.
Conclusions
The medical optical device industry is highly fragmented but has become the largest single technology sector in medicine today. When contrasted to radiology, it appears underfunded for research, where point-of-care tools such as surgery, endoscopy, laparoscopy, ophthalmology, pulse oximetry, and monitoring have more potential for development through research investment.
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The database for multipulse retinal damage thresholds for the laser safety standard (IEC 60825-1:2014) is confined, especially for elongated irradiation profiles. To ensure eye safety, retinal damage thresholds (ED50 values) need to be determined.
Aim
This study aims to examine nanosecond multipulse scenarios.
Approach
To determine ED50 values in ex vivo measurements, an optical laser setup is presented. Porcine explant tissue is irradiated with rectangular top-hat profiles. Thermal simulations are carried out on a validated computer model and retinal injury thresholds are obtained.
Results
The measurements resulted in ED50 values from 8.46 to 42.72 μJ with a slope from 1.15 to 1.4. A thermal damage in the measurements can be excluded due to the level value in combination with a different type of declining behavior for increasing pulses compared to the simulations. A dependence with increasing elongation or area of the retinal image emerges in the simulations but could not be confirmed in the measurements due to the influencing factors (biological variability, focusing, and measuring procedure).
Conclusions
Using slit apertures for beam shaping, variable rectangular spot geometries are realized without changing elements in the setup. For further evaluation of the behavior of elongated irradiation profiles, additional measurements to improve the measurement accuracy are necessary.
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Speckle contrast analysis is the basis of laser speckle imaging (LSI), a simple, inexpensive, noninvasive technique used in various fields of medicine and engineering. A common application of LSI is the measurement of tissue blood flow. Accurate measurement of speckle contrast is essential to correctly measure blood flow. Variables, such as speckle grain size and camera pixel size, affect the speckle pattern and thus the speckle contrast.
Aim
We studied the effects of spatial correlation among adjacent camera pixels on the resulting speckle contrast values.
Approach
We derived a model that accounts for the potential correlation of intensity values in the common experimental situation where the speckle grain size is larger than the camera pixel size. In vitro phantom experiments were performed to test the model.
Results
Our spatial correlation model predicts that speckle contrast first increases, then decreases as the speckle grain size increases relative to the pixel size. This decreasing trend opposes what is observed with a standard speckle contrast model that does not consider spatial correlation. Experimental data are in good agreement with the predictions of our spatial correlation model.
Conclusions
We present a spatial correlation model that provides a more accurate measurement of speckle contrast, which should lead to improved accuracy in tissue blood flow measurements. The associated correlation factors only need to be calculated once, and open-source software is provided to assist with the calculation.
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Articular cartilage exhibits a zonal architecture, comprising three distinct zones: superficial, middle, and deep. Collagen fibers, being the main solid constituent of articular cartilage, exhibit unique angular and size distribution in articular cartilage zones. There is a gap in knowledge on how the unique properties of collagen fibers across articular cartilage zones affect the scattering properties of the tissue.
Aim
This study hypothesizes that the structural properties of articular cartilage zones affect its scattering parameters. We provide scattering coefficient and scattering anisotropy factor of articular cartilage zones in the spectral band of 400 to 1400 nm. We enumerate the differences and similarities of the scattering properties of articular cartilage zones and provide reasoning for these observations.
Approach
We utilized collimated transmittance and integrating sphere measurements to estimate the scattering coefficients of bovine articular cartilage zones and bulk tissue. We used the relationship between the scattering coefficients to estimate the scattering anisotropy factor. Polarized light microscopy was applied to estimate the depth-wise angular distribution of collagen fibers in bovine articular cartilage.
Results
We report that the Rayleigh scatterers contribution to the scattering coefficients, the intensity of the light scattered by the Rayleigh and Mie scatterers, and the angular distribution of collagen fibers across tissue depth are the key parameters that affect the scattering properties of articular cartilage zones and bulk tissue. Our results indicate that in the short visible region, the superficial and middle zones of articular cartilage affect the scattering properties of the tissue, whereas in the far visible and near-infrared regions, the articular cartilage deep zone determines articular cartilage scattering properties.
Conclusion
This study provides scattering properties of articular cartilage zones. Such findings support future research to utilize optical simulation to estimate the penetration depth, depth-origin, and pathlength of light in articular cartilage for optical diagnosis of the tissue.
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Non-invasive optical measurements of deep tissue (e.g., muscle) need to take into account confounding contributions from baseline and dynamic optical properties of superficial tissue (adipose tissue).
Aim
Discriminate superficial and deep tissue hemodynamics using data collected with frequency-domain (FD) near-infrared spectroscopy (NIRS) in a dual-slope (DS) configuration.
Approach
Experimental data were collected in vivo on the forearm of three human subjects during a 3-min arterial occlusion or 1-min venous occlusion. Theoretical data were generated using diffusion theory for two-layered media with varying values of the reduced scattering coefficient (μs′) (range: 0.5 to 1.1 mm − 1) and absorption coefficient (μa) (range: 0.005 − 0.015 mm − 1) of the two layers, and top layer thickness (range: 2 to 8 mm). Data were analyzed using diffusion theory for a homogeneous semi-infinite medium.
Results
Experimental data in vivo were consistent with simulated data for a two-layered medium with a larger μs′ in the top layer, comparable absorption changes in the top and bottom layers during venous occlusion, and smaller absorption changes in the top vs. bottom layers during arterial occlusion.
Conclusions
The dataset generated by DS FD-NIRS may allow for discrimination of superficial and deep absorption changes in two-layered media, thus lending itself to individual measurements of hemodynamics in adipose and muscle tissue.
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Ultrasonic transducers facilitate noninvasive biomedical imaging and therapeutic applications. Optoacoustic generation using nanoplasmonic structures provides a technical solution for highly efficient broadband ultrasonic transducer. However, bulky and high-cost nanosecond lasers as conventional excitation sources hinder a compact configuration of transducer.
Aim
Here, we report a plasmon-enhanced optoacoustic transducer (PEAT) for broadband ultrasound generation, featuring an overdriven pulsed laser diode (LD) and an Ecoflex thin film. The PEAT module consists of an LD, a collimating lens, a focusing lens, and an Ecoflex-coated 3D nanoplasmonic substrate (NPS).
Approach
The LD is overdriven above its nominal current and precisely modulated to achieve nanosecond pulsed beam with high optical peak power. The focused laser beam is injected on the NPS with high-density electromagnetic hotspots, which allows for the efficient plasmonic photothermal effect. The thermal expansion of Ecoflex finally generates broadband ultrasound.
Results
The overdriven pulsed LD achieves a maximum optical peak power of 40 W, exceeding the average optical power of 3 W. The 22μm thick Ecoflex-coated NPS exhibits an eightfold optoacoustic enhancement with a fractional −6dB bandwidth higher than 160% and a peak frequency of 2.5 MHz. In addition, the optoacoustic amplitude is precisely controlled by the optical peak power or the laser pulse width. The PEAT-integrated microfluidic chip clearly demonstrates acoustic atomization by generating aerosol droplets at the air–liquid interface.
Conclusions
Plasmon-enhanced optoacoustic generation using PEAT can provide an approach for compact and on-demand biomedical applications, such as ultrasound imaging and lab-on-a-chip technologies.
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Post-burn scars and scar contractures present significant challenges in burn injury management, necessitating accurate evaluation of the wound healing process to prevent or minimize complications. Non-invasive and accurate assessment of burn scar vascularity can offer valuable insights for evaluations of wound healing. Optical coherence tomography (OCT) and OCT angiography (OCTA) are promising imaging techniques that may enhance patient-centered care and satisfaction by providing detailed analyses of the healing process.
Aim
Our study investigates the capabilities of OCT and OCTA for acquiring information on blood vessels in burn scars and evaluates the feasibility of utilizing this information to assess burn scars.
Approach
Healthy skin and neighboring scar data from nine burn patients were obtained using OCT and processed with speckle decorrelation, Doppler OCT, and an enhanced technique based on joint spectral and time domain OCT. These methods facilitated the assessment of vascular structure and blood flow velocity in both healthy skin and scar tissues. Analyzing these parameters allowed for objective comparisons between normal skin and burn scars.
Results
Our study found that blood vessel distribution in burn scars significantly differs from that in healthy skin. Burn scars exhibit increased vascularization, featuring less uniformity and lacking the intricate branching network found in healthy tissue. Specifically, the density of the vessels in burn scars is 67% higher than in healthy tissue, while axial flow velocity in burn scar vessels is 25% faster than in healthy tissue.
Conclusions
Our research demonstrates the feasibility of OCT and OCTA as burn scar assessment tools. By implementing these technologies, we can distinguish between scar and healthy tissue based on its vascular structure, providing evidence of their practicality in evaluating burn scar severity and progression.
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Multispectral imaging (MSI) is an approach for real-time, quantitative, and non-invasive tissue perfusion measurements. Current laparoscopic systems based on mosaic sensors or filter wheels lack high spatial resolution or acceptable frame rates.
Aim
To develop a laparoscopic system for MSI-based color video and tissue perfusion imaging during gastrointestinal surgery without compromising spatial or temporal resolution.
Approach
The system was built with 14 switchable light-emitting diodes in the visible and near-infrared spectral range, a 4K image sensor, and a 10 mm laparoscope. Illumination patterns were created for tissue oxygenation and hemoglobin content monitoring. The system was calibrated to a clinically approved laparoscopic hyperspectral system using linear regression models and evaluated in an occlusion study with 36 volunteers.
Results
The root mean squared errors between the MSI and reference system were 0.073 for hemoglobin content, 0.039 for oxygenation in deeper tissue layers, and 0.093 for superficial oxygenation. The spatial resolution at a working distance of 45 mm was 156 μm. The effective frame rate was 20 fps.
Conclusions
High-resolution perfusion monitoring was successfully achieved. Hardware optimizations will increase the frame rate. Parameter optimizations through alternative illumination patterns, regression, or assumed tissue models are planned. Intraoperative measurements must confirm the suitability during surgery.
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Postoperative surgical wound infection is a serious problem around the globe, including in countries with advanced healthcare systems, and a method for early detection of infection is urgently required.
Aim
We explore spatial frequency domain imaging (SFDI) for distinguishing changes in surgical wound healing based on the tissue scattering properties and surgical wound width measurements.
Approach
A comprehensive numerical method is developed by applying a three-dimensional Monte Carlo simulation to a vertical heterogeneous wound model. The Monte Carlo simulation results are validated using resin phantom imaging experiments.
Results
We report on the SFDI lateral resolution with varying reduced scattering value and wound width and discuss the partial volume effect at the sharp vertical boundaries present in a surgical incision. The detection sensitivity of this method is dependent on spatial frequency, wound reduced scattering coefficient, and wound width.
Conclusions
We provide guidelines for future SFDI instrument design and explanation for the expected error in SFDI measurements.
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The assessment of biomarkers in the eye is rapidly gaining traction for the screening, diagnosis, and monitoring of ocular and neurological diseases. Targeted ocular spectroscopy is a technology that enables concurrent imaging of the eye fundus and analysis of high-quality spectra from a targeted region within the imaged area. This provides structural, compositional, and functional information of specific regions of the eye fundus from a non-invasive approach to ocular biomarker detection.
Aim
The aim of our study was to demonstrate the multimodal functionality and validation of targeted ocular spectroscopy. This was done in vitro, using a reference target and a model eye, and in vivo.
Approach
Images and spectra from different regions of a reference target and a model eye were acquired and analyzed to validate the system. Targeted ocular fluorescence spectroscopy was also demonstrated with the same model. Subsequently, in vivo imaging and diffuse reflectance spectra were acquired to assess blood oxygen saturation in the optic nerve head and the parafovea of healthy subjects.
Results
Tests conducted with the reference target showed accurate spectral analysis within specific areas of the imaging space. In the model eye, distinct spectral signatures were observed for the optic disc, blood vessels, the retina, and the macula, consistent with the variations in tissue composition and functions between these regions. An ocular oximetry algorithm was applied to in vivo spectra from the optic nerve head and parafovea of healthy patients, showing significant differences in blood oxygen saturation. Finally, targeted fluorescence spectral analysis was performed in vitro.
Conclusions
Diffuse reflectance and fluorescence spectroscopy in specific regions of the eye fundus open the door to a whole new range of monitoring and diagnostic capabilities, from assessment of oxygenation in glaucoma and diabetic retinopathy to photo-oxidation and photodegradation in age-related macular degeneration.
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Although multilayer analytical models have been proposed to enhance brain sensitivity of diffuse correlation spectroscopy (DCS) measurements of cerebral blood flow, the traditional homogeneous model remains dominant in clinical applications. Rigorous in vivo comparison of these analytical models is lacking.
Aim
We compare the performance of different analytical models to estimate a cerebral blood flow index (CBFi) with DCS in adults.
Approach
Resting-state data were obtained on a cohort of 20 adult patients with subarachnoid hemorrhage. Data at 1 and 2.5 cm source-detector separations were analyzed with the homogenous, two-layer, and three-layer models to estimate scalp blood flow index and CBFi. The performance of each model was quantified via fitting convergence, fit stability, brain-to-scalp flow ratio (BSR), and correlation with transcranial Doppler ultrasound (TCD) measurements of cerebral blood flow velocity in the middle cerebral artery (MCA).
Results
The homogeneous model has the highest pass rate (100%), lowest coefficient of variation (CV) at rest (median [IQR] at 1 Hz of 0.18 [0.13, 0.22]), and most significant correlation with MCA blood flow velocities (Rs = 0.59, p = 0.010) compared with both the two- and three-layer models. The multilayer model pass rate was significantly correlated with extracerebral layer thicknesses. Discarding datasets with non-physiological BSRs increased the correlation between DCS measured CBFi and TCD measured MCA velocities for all models.
Conclusions
We found that the homogeneous model has the highest pass rate, lowest CV at rest, and most significant correlation with MCA blood flow velocities. Results from the multilayer models should be taken with caution because they suffer from lower pass rates and higher coefficients of variation at rest and can converge to non-physiological values for CBFi. Future work is needed to validate these models in vivo, and novel approaches are merited to improve the performance of the multimodel models.
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Label-free, two-photon excited fluorescence (TPEF) imaging captures morphological and functional metabolic tissue changes and enables enhanced understanding of numerous diseases. However, noise and other artifacts present in these images severely complicate the extraction of biologically useful information.
Aim
We aim to employ deep neural architectures in the synthesis of a multiscale denoising algorithm optimized for restoring metrics of metabolic activity from low-signal-to-noise ratio (SNR), TPEF images.
Approach
TPEF images of reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) and flavoproteins (FAD) from freshly excised human cervical tissues are used to assess the impact of various denoising models, preprocessing methods, and data on metrics of image quality and the recovery of six metrics of metabolic function from the images relative to ground truth images.
Results
Optimized recovery of the redox ratio and mitochondrial organization is achieved using a novel algorithm based on deep denoising in the wavelet transform domain. This algorithm also leads to significant improvements in peak-SNR (PSNR) and structural similarity index measure (SSIM) for all images. Interestingly, other models yield even higher PSNR and SSIM improvements, but they are not optimal for recovery of metabolic function metrics.
Conclusions
Denoising algorithms can recover diagnostically useful information from low SNR label-free TPEF images and will be useful for the clinical translation of such imaging.
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Rapid 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.
Aim
We aim to design and fabricate flexible, multi-layered hydrogel-based optical standards and evaluate advanced optical imaging techniques at depth.
Approach
Standards 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.
Results
These 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 1240μm, respectively. Once fabricated, the phantoms were found to be stable for many months.
Conclusions
The 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.
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Measuring hemodynamic function is crucial for health assessment. Optical signals provide relative hemoglobin concentration changes, but absolute measurements require costly, bulky technology. Speckleplethysmography (SPG) uses coherent light to detect speckle fluctuations. Combining SPG with multispectral measurements may provide important physiological information on blood flow and absolute hemoglobin concentration.
Aim
To develop an affordable optical technology to measure tissue absorption, scattering, hemoglobin concentrations, tissue oxygen saturation (StO2), and blood flow.
Approach
We integrated reflectance spectroscopy and laser speckle imaging to create coherent spatial imaging (CSI). CSI was validated against spatial frequency domain imaging (SFDI) using phantom-based measurements. In vivo arterial and venous occlusion experiments compared CSI with diffuse optical spectroscopy/diffuse correlation spectroscopy (DOS/DCS) measurements.
Results
CSI and SFDI agreed on tissue absorption and scattering in phantom tests. CSI and DOS/DCS showed similar trends and agreement in arterial occlusion results. During venous occlusion, both uncorrected and corrected blood flow decreased with increasing pressure, with an ∼200% difference in overall blood flow decrease between the methods. CSI and DOS/DCS data showed expected hemoglobin concentrations, StO2, and blood flow trends.
Conclusions
CSI provides affordable and comprehensive hemodynamic information. It can potentially detect dysfunction and improve measurements, such as blood pressure, SpO2, and metabolism.
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