Development of a blood oxygenation phantom for photoacoustic tomography combined with online pO2 detection and flow spectrometry

Photoacoustic tomography (PAT) is intrinsically sensitive to blood oxygen saturation (sO2) in vivo. However, making accurate sO2 measurements without knowledge of tissue- and instrumentation-related correction factors is extremely challenging. We have developed a low-cost flow phantom to facilitate validation of PAT systems. The phantom is composed of a flow circuit of tubing partially embedded within a tissue-mimicking material, with independent sensors providing online monitoring of the optical absorption spectrum and partial pressure of oxygen in the tube. We first test the flow phantom using two small molecule dyes that are frequently used for photoacoustic imaging: methylene blue and indocyanine green. We then demonstrate the potential of the phantom for evaluating sO2 using chemical oxygenation and deoxygenation of blood in the circuit. Using this dynamic assessment of the photoacoustic sO2 measurement in phantoms in relation to a ground truth, we explore the influence of multispectral processing and spectral coloring on accurate assessment of sO2. Future studies could exploit this low-cost dynamic flow phantom to validate fluence correction algorithms and explore additional blood parameters such as pH and also absorptive and other properties of different fluids

Coherent Imaging through Multicore Fibres with Applications in Endoscopy

Imaging through optical fibres has recently emerged as a promising method of micro-scale optical imaging within a hair-thin form factor. This has significant applications in endoscopy and may enable minimally invasive imaging deep within live tissue for improved diagnosis of disease. Multi-mode fibres (MMF) are the most common choice because of their high resolution but multicore fibres (MCF) offer a number of advantages such as widespread clinical use, ability to form approximate images without correction and an inherently sparse transmission matrix (TM) enabling simple and fast characterisation. We present a novel experimental investigation into properties of MCF important for imaging, specifically: a new method to upsample and downsample measured TMs with minimal information loss, the first experimental measurement of MCF spatial eigenmodes, a novel statistical treatment of behaviour under bending based on a wireless fading model, and an experimental observation of TM drift due to self-heating effects and discussion of how to compensate this. We next present practical techniques for imaging through MCFs, including alignment, how to parallelise TM characterisation measurements to improve speed and how to use non-interferometric phase and polarisation recovery for improved stability. Finally, we present two recent applications of MCF imaging: polarimetric imaging using a robust Bayesian inference approach, and entropic imaging for imaging early-stage tumours

Single-Step Fabrication of Multispectral Filter Arrays Using Grayscale Lithography and Metal-Insulator-Metal Geometry

© 2018 OSA. Metal-insulator-metal geometries can provide optical transmission filtering, with peak wavelength dependent on insulator thickness. Using grayscale electron beam lithography to control insulator thickness, we fabricate multispectral filter arrays, whereby dose determines wavelength

Emerging Optical Methods for Endoscopic Barrett’s Surveillance

Barrett’s oesophagus is an acquired metaplastic condition that predisposes patients to the development of oesophageal adenocarcinoma, prompting the use of surveillance regimes to detect early malignancy for endoscopic therapy with curative intent. The currently accepted surveillance regime uses white light endoscopy together with random biopsies, but suffers poor sensitivity and discards information from numerous light-tissue interactions that could be exploited to probe structural, functional and molecular changes in the tissue. Advanced optical methods are now emerging that are exquisitely sensitive to these changes and hold significant potential to improve surveillance of Barrett’s oesophagus if they can be applied endoscopically. The next decade will see some of these exciting new methods applied to Barrett’s surveillance in new device architectures for the first time, potentially leading to a longawaited improvement of the standard of care

Wide-field phase imaging for the endoscopic detection of dysplasia and early-stage esophageal cancer

© 2018 SPIE. Esophageal cancer has a 5-year survival rate below 20%, but can be curatively resected if it is detected early. At present, poor contrast for early lesions in white light imaging leads to a high miss rate in standard-of-care endoscopic surveillance. Early lesions in the esophagus, referred to as dysplasia, are characterized by an abundance of abnormal cells with enlarged nuclei. This tissue has a different refractive index profile to healthy tissue, which results in different light scattering properties and provides a source of endogenous contrast that can be exploited for advanced endoscopic imaging. For example, point measurements of such contrast can be made with scattering spectroscopy, while optical coherence tomography generates volumetric data. However, both require specialist interpretation for diagnostic decision making. We propose combining wide-field phase imaging with existing white light endoscopy in order to provide enhanced contrast for dysplasia and early-stage cancer in an image format that is familiar to endoscopists. Wide-field phase imaging in endoscopy can be achieved using coherent illumination combined with phase retrieval algorithms. Here, we present the design and simulation of a benchtop phase imaging system that is compatible with capsule endoscopy. We have undertaken preliminary optical modelling of the phase imaging setup, including aberration correction simulations and an investigation into distinguishing between different tissue phantom scattering coefficients. As our approach is based on phase retrieval rather than interferometry, it is feasible to realize a device with low-cost components for future clinical implementation

Nanodiamond preparation and surface characterization for biological applications

Nanodiamonds contain stable fluorescent emitters and hence can be used for molecular fluorescence imaging and precision sensing of electromagnetic fields. The physical properties of these emitters together with their low reported cytotoxicity make them attractive for biological imaging applications. The controlled application of nanodiamonds for cellular imaging requires detailed understanding of surface chemistry, size ranges and aggregation, as these can all influence cellular interactions. We compared these characteristics for graphitic and oxidized nanodiamonds. Oxidation is generally used for surface functionalization, and was optimized by Thermogravimetric Analysis, achieved by 445±5°C heating in air for 5 hours, then confirmed via Raman and Infrared spectroscopies. Size ranges and aggregation were assessed using Atomic Force Microscopy and Dynamic Light Scattering. Biocompatibility in breast cancer cell lines was measured using a proliferation assay. Heating at 445±5°C reduced the Raman signal of graphitic carbon (1575 cm-1) as compared to that of diamond (1332 cm-1) from 0.31±0.07 Raman intensity units to 0.07±0.04. This temperature was substantially below the onset of major mass loss (observed at 535±1°C) and therefore achieved cost efficiency, convenience and high yield. Graphitic and oxidized nanodiamonds formed aggregates in water, with a mean particle size of 192±4nm and 166±2nm at a concentration of 66μg/mL. We then applied the graphitic and oxidized nanodiamonds to cells in culture at 1μg/mL and found no significant change in the proliferation rate (-5±2% and -1±3% respectively). Nanodiamonds may therefore be suitable for development as a novel transformative tool in the life sciences

Tolerancing the alignment of large-core Optical fibers, fiber bundles and light guides using a Fourier approach

Optical fiber technology is found in a wide variety of applications to flexibly relay light between two points, enabling information transfer across long distances and allowing access to hard-to-reach areas. Large-core optical fibers and light guides find frequent use in illumination and spectroscopic applications, for example, endoscopy and high-resolution astronomical spectroscopy. Proper alignment is critical for maximizing throughput in optical fiber coupling systems; however, there currently are no formal approaches to tolerancing the alignment of a lightguide coupling system. Here, we propose a Fourier alignment sensitivity (FAS) algorithm to determine the optimal tolerances on the alignment of a light guide by computing the alignment sensitivity. The algorithm shows excellent agreement with both simulated and experimentally measured values and improves on the computation time of equivalent ray-tracing simulations by two orders of magnitude. We then apply FAS to tolerance and fabricate a coupling system, which is shown to meet specifications, thus validating FAS as a tolerancing technique. These results indicate that FAS is a flexible and rapid means to quantify the alignment sensitivity of a light guide, widely informing the design and tolerancing of coupling systems.Winston Churchill Foundation of the United States; Cancer Research UK (C14303/A17197, C47594/A16267, C47594/A21102); Seventh Framework Programme (FP7) (FP7-PEOPLE-2013-CIG-630729); Fund for Astrophysical Research (F.A.R.); National Science Foundation (NSF) (DGE-1143953)

Evaluation of illumination system uniformity for wide-field biomedical hyperspectral imaging

Hyperspectral imaging (HSI) systems collect both spatial (morphological) and spectral (chemical) information from a sample. HSI can provide sensitive analysis for biological and medical applications, for example, simultaneously measuring reflectance and fluorescence properties of a tissue, which together with structural information could improve early cancer detection and tumour characterisation. Illumination uniformity is a critical pre-condition for quantitative data extraction from an HSI system. Non-uniformity can cause glare, specular reflection and unwanted shading, which negatively impact statistical analysis procedures used to extract abundance of different chemical species. Here, we model and evaluate several illumination systems frequently used in wide-field biomedical imaging to test their potential for HSI. We use the software LightTools and FRED. The analysed systems include: a fibre ring light; a light emitting diode (LED) ring; and a diffuse scattering dome. Each system is characterised for spectral, spatial, and angular uniformity, as well as transfer efficiency. Furthermore, an approach to measure uniformity using the Kullback-Leibler divergence (KLD) is introduced. The KLD is generalisable to arbitrary illumination shapes, making it an attractive approach for characterising illumination distributions. Although the systems are quite comparable in their spatial and spectral uniformity, the most uniform angular distribution is achieved using a diffuse scattering dome, yielding a contrast of 0.503 and average deviation of 0.303 over a ±60° field of view with a 3.9% model error in the angular domain. Our results suggest that conventional illumination sources can be applied in HSI, but in the case of low light levels, bespoke illumination sources may offer improved performance.TWS is funded by the Winston Churchill Foundation of the United States. ASL is funded by the EPSRC, the George and Lillian Schiff Foundation and the Foundation Blanceflor. SEB is funded by CRUK (C14303/A17197, C47594/A16267 and C47594/A21102) and the European Union’s Seventh Framework Programme (FP7/2007-2013) under grant agreement number FP7-PEOPLE-2013-CIG- 630729. We also acknowledge support from a University of Cambridge MRC Confidence in Concept Award

Evaluation of precision in optoacoustic tomography for preclinical imaging in living subjects.

Optoacoustic Tomography (OT) is now widely used in preclinical imaging, however, precision (repeatability and reproducibility) of OT has yet to be determined. METHODS: We used a commercial small animal OT system. Measurements in stable phantoms were used to independently assess the impact of system variables on precision (using coefficient of variation, COV), including acquisition wavelength, rotational position, frame averaging. Variables due to animal handling and physiology, such as anatomical placement and anesthesia conditions were then assessed in healthy nude mice using the left kidney and spleen as reference organs. Temporal variation was assessed by repeated measurements over hours and days both in phantoms and $\textit{in vivo}$. Sensitivity to small molecule dyes was determined in phantoms and $\textit{in vivo}$; precision was assessed $\textit{in vivo}$ using IRDye800CW. RESULTS: OT COV in a stable phantom was less than 2% across all wavelengths over 30 days. The factors with greatest impact on the signal repeatability in phantoms were rotational position and user experience, both of which still resulted in a COV of less than 4%. Anatomical ROI size showed the highest variation at 12% and 18% COV in the kidney and spleen respectively, however, functional SO₂ measurements based on a standard operating procedure showed exceptional reproducibility of <4% COV. COV for repeated injections of IRDye800CW was 6.6%. Sources of variability for $\textit{in vivo}$ data included respiration rate, user experience and animal placement. CONCLUSION: Data acquired with our small animal OT system was highly repeatable and reproducible across subjects and over time. Therefore, longitudinal OT studies may be performed with high confidence when our standard operating procedure is followed.This work was funded by: the EPSRC-CRUK Cancer Imaging Centre in Cambridge and Manchester (C197/A16465); CRUK (C14303/A17197, C47594/A16267); EU-FP7-agreement FP7-PEOPLE-2013-CIG-630729; and the University of Cambridge EPSRC Impact Acceleration Account

Photoacoustic imaging using genetically encoded reporters: a review

Genetically encoded contrast in photoacoustic imaging (PAI) is complementary to the intrinsic contrast provided by endogenous absorbing chromophores such as hemoglobin. The use of reporter genes expressing absorbing proteins opens the possibility of visualizing dynamic cellular and molecular processes. This is an enticing prospect but brings with it challenges and limitations associated with generating and detecting different types of reporters. The purpose of this review is to compare existing PAI reporters and signal detection strategies, thereby offering a practical guide, particularly for the nonbiologist, to choosing the most appropriate reporter for maximum sensitivity in the biological and technological system of interest.J.B. and S.E.B. are supported by the EPSRCCRUK Cancer Imaging Centre in Cambridge and Manchester (No. C197/A16465); CRUK (Nos. C14303/A17197 and C47594/A16267); and the European Union’s Seventh Framework Programme (No. FP7/2007-2013) under Grant Agreement No. FP7-PEOPLE-2013-CIG-630729. J.Y. is partly supported by Duke MEDx Basic Research Grant. J.L. acknowledges the support of ERC Starting Grant No. 281356