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Significance: In handheld laser speckle contrast imaging (LSCI), motion artifacts (MA) are inevitable. Suppression of MA leads to a valid and objective assessment of tissue perfusion in a wide range of medical applications including dermatology and burns. Our study shines light on the sources of these artifacts, which have not yet been explored.We propose a model based on optical Doppler effect to predict speckle contrast drop as an indication of MA. Aim: We aim to theoretically model MA when an LSCI system measuring on static scattering media is subject to translational displacements.We validate the model using both simulation and experiments. This is the crucial first step toward creating robustness against MA. Approach: Our model calculates optical Doppler shifts in order to predict intensity correlation function and contrast of the time-integrated intensity as functions of applied speed based on illumination and detection wavevectors. To validate the theoretical predictions, computer simulation of the dynamic speckles has been carried out. Then experiments are performed by both high-speed and low-framerate imaging. The employed samples for the experiments are a highly scattering matte surface and a Delrin plate of finite scattering level in which volume scattering occurs. Results: An agreement has been found between theoretical prediction, simulation, and experimental results of both intensity correlation functions and speckle contrast. Coefficients in the proposed model have been linked to the physical parameters according to the experimental setups. Conclusions: The proposed model provides a quantitative description of the influence of the types of illumination and media in the creation of MA. The accurate prediction of MA caused by translation based on Doppler shifts makes our model suitable to study the influence of rotation. Also the model can be extended for the case of dynamic media, such as live tissue.

Contains fulltext : 248292.pdf (Publisher’s version ) (Open Access) BACKGROUND: Skin microvasculature changes are crucial in psoriasis development and correlate with perfusion. The noninvasive Handheld Perfusion Imager (HAPI) examines microvascular skin perfusion in large body areas using laser speckle contrast imaging (LSCI). OBJECTIVES: To (i) assess whether increased perilesional perfusion and perfusion inhomogeneity are predictors for expansion of psoriasis lesions and (ii) assess feasibility of the HAPI system in a mounted modality. METHODS: In this interventional pilot study in adults with unstable plaque psoriasis, HAPI measurements and color photographs were performed for lesions present on one body region at week 0, 2, 4, 6 and 8. The presence of increased perilesional perfusion and perfusion inhomogeneity was determined. Clinical outcome was categorized as increased, stable or decreased lesion surface between visits. Patient feedback was collected on a 10-point scale. RESULTS: In total, 110 lesions with a median follow-up of 6 (IQR 6.0) weeks were assessed in 6 patients with unstable plaque psoriasis. Perfusion data was matched to 281 clinical outcomes after two weeks. A mixed multinomial logistic regression model revealed a predictive value of perilesional increased perfusion (OR 9.90; p

We present an acousto-optic imaging system operating in reflection-mode and utilizing a pair of compact, triggerable lasers with 532 and 1064 nm wavelength and nanosecond pulse duration. The system maps the fluence rate distribution of light transmitted through optically scattering samples. The imaging is performed using an acousto-optic probe comprising an ultrasound linear array with attached optical fiber on one side and a camera on the other. The described hardware configuration images samples with access restricted to one side only and ensures mobility of the entire setup. The major challenge of the introduced approach is mitigating the effects of laser parameter instabilities and precise synchronization of ultrasound and laser pulses. We solved this issue by developing an electronic feedback circuit and a microcontroller-based synchronization and control system triggering the ultrasound scanner. Schematics and details regarding control algorithms are introduced. The imaging performance of the system is demonstrated on examples of results obtained for solid, acoustically-homogeneous and optically scattering phantoms with and without light absorbing inclusions present. Adjusting the size and location of the region of interest within the camera sensor matrix and the number of laser pulses illuminating every frame allows for significant improvements in terms of the achievable peak signal to noise ratio. We demonstrate that the developed synchronization algorithm and system play a crucial role in ensuring imaging quality and accuracy.

Fluid flow shear stresses are strong regulators for directing the organization of vascular networks. Knowledge of structural and flow dynamics information within complex vasculature is essential for tuning the vascular organization within engineered tissues, by manipulating flows. However, reported investigations of vascular organization and their associated flow dynamics within complex vasculature over time are limited, due to limitations in the available physiological pre-clinical models, and the optical inaccessibility and aseptic nature of these models. Here, we developed laser speckle contrast imaging (LSCI) and side-stream dark field microscopy (SDF) systems to map the vascular organization, spatio-temporal blood flow fluctuations as well as erythrocytes movements within individual blood vessels of developing chick embryo, cultured within an artificial eggshell system. By combining imaging data and computational simulations, we estimated fluid flow shear stresses within multiscale vasculature of varying complexity. Furthermore, we demonstrated the LSCI compatibility with bioengineered perfusable muscle tissue constructs, fabricated via molding techniques. The presented application of LSCI and SDF on perfusable tissues enables us to study the flow perfusion effects in a non-invasive fashion. The gained knowledge can help to use fluid perfusion in order to tune and control multiscale vascular organization within engineered tissues.

Laser speckle contrast imaging (LSCI) is a non-invasive and affordable technique to visualize skin perfusion. Handheld use of the system facilitates measurements on various skin areas in a flexible manner. However, movement artefacts caused by handheld operation or test subject movements hamper its performance. In this work, we study the influence of the laser beam type in handheld-LSCI by evaluating the speckle contrast on static objects for beams with planar, spherical or scrambled wavefronts, and for movement artefacts caused by tilting or translation of wavefronts. We show that the scrambled waves made by often-used engineered diffusers lead to significantly larger movement artefacts than planar or spherical waves.

One of the remaining challenges of bringing photoacoustic imaging to clinics is the occurrence of reflection artifacts. Previously, we proposed a method using multi-wavelength excitation to identify and remove the RAs. However, this method requires at least 3 wavelengths. Here we improve the method further by reducing the required number of wavelengths to 2. We experimentally demonstrate this new method and compare it with the previous one. Results show that this new method holds great feasibility for identifying reflection artifacts in addition to preserving all advantages of the previous method.

Background Careful post-operative monitoring of free flaps is important in flap survival; immediate action increases flap salvage rate. Although various methods are available, room for improvement remains. Thermal cameras have proven their value in medicine and are nowadays readily available at low costs. The objective of this study was to evaluate the potential of an affordable infrared thermal camera and software in the detection of failing free flaps during post-operative monitoring. Methods Free myocutaneous rectus abdominis flaps were harvested in 16 female landrace pigs and replanted after several hours of storage. All flaps were assessed with indocyanine green fluorescence angiography as well as hourly clinical assessment of skin colour, turgor and capillary refill. Furthermore, thermal photographs were taken simultaneously with the FLIR One thermal camera smartphone module. These photographs were processed in MATLAB and evaluated on their additional value as an indicator for flap failure. Results Out of 16 flaps, three flaps failed due to arterial failure and one flap developed venous congestion. The mean flap temperature compared to adjacent control skin proved to be most indicative for flap failure. All unsuccessful flaps showed lower temperatures after failure compared to the uncompromised free flaps. Conclusions An affordable thermal camera module can potentially contribute to post-operative free flap monitoring. Vascular compromise in free flaps can be distinguished by investigating relative temperature differences between the flap and reference skin. Until the FLIR One camera has been extensively investigated in a human population, it should be used in conjunction with conventional monitoring techniques. Level of evidence: Level IV, diagnostic study

Pulsed lasers in photoacoustic tomography systems are expensive, which limit their use to a few clinics and small animal labs. We present a method to realize tomographic ultrasound and photoacoustic imaging using a commercial LED-based photoacoustic and ultrasound system. We present two illumination configurations using LED array units and an optimal number of angular views for tomographic reconstruction. The proposed method can be a cost-effective solution for applications demanding tomographic imaging and can be easily integrated into conventional linear array-based ultrasound systems. We present a potential application for finger joint imaging in vivo, which can be used for point-of-care rheumatoid arthritis diagnosis and monitoring.

In peripheral arterial disease the tissue microcirculation becomes impaired, ultimately resulting in chronic limb threatening ischaemia (CLTI) in 5% of cases. 1 CLTI prognosis is poor unless successful revascularisation is performed. 2 Endovascular and hybrid revascularisation procedures are performed under fluoroscopic and angiographic guidance to visualise the per-operative impact on the macrocirculation. Unfortunately, both lack the ability to visualise the impact on microcirculation, while restoring the microcirculation plays a pivotal role in CLTI. 3 By introducing additional per-operative assessment of the microcirculation, the impact of revascularisation on the microcirculation can be assessed in real time. This allows the surgical team to extend or adjust the revascularisation procedure in a timely manner, improving (prediction of) the clinical outcome.

Significance: Performance of an acousto-optic imaging system is limited by light fluence rate and acoustic pressure field distributions characteristics. In optically scattering media, the former determines the achievable contrast, whereas the latter the imaging resolution. The system parameters can be shaped by changing relative positions of ultrasound (US) transducer array and optodes. However, in the case of many potential clinical applications, optimization possibilities in this regard are limited, as a sample is accessible from one side only and using a water tank for coupling is not feasible. Aim: We investigate the possibilities of improving performance of an acousto-optic imaging system operating in reflection mode geometry with linear US array in direct contact with a sample using plane wave instead of focused US pulses. Approach: Differences in acoustic pressure field distributions for various transducer excitation patterns were determined numerically and experimentally. Acousto-optic images of phantoms with and without optically absorbing inclusions were acquired by measuring laser speckle contrast decrease due to the light modulation by plane wave and focused US pulses with different apodization patterns. Results: The residual acoustic pressure field components occupy relatively large volume and contribute to light modulation. Using nonsteered plane wave US pulses instead of focused ones allows one to mitigate their influence. It also allows one to obtain clear two-dimensional reconstructions of light fluence rate maps by shifting transducer apodization along the lateral direction. Conclusions: Using nonsteered plane wave US pulses allows one to achieve better imaging performance than with focused pulses in the assumed system geometry.

Diagnosis of peripheral artery disease in people with diabetes and a foot ulcer using current non-invasive blood pressure measurements is challenging. Laser speckle contrast imaging (LSCI) is a promising non-invasive technique to measure cutaneous microcirculation. This study investigated the association between microcirculation (measured with both LSCI and non-invasive blood pressure measurement) and healing of diabetic foot ulcers 12 and 26 weeks after measurement. We included sixty-one patients with a diabetic foot ulcer in this prospective, single-center, observational cohort-study. LSCI scans of the foot, ulcer, and ulcer edge were conducted, during baseline and post-occlusion hyperemia. Non-invasive blood pressure measurement included arm, foot, and toe pressures and associated indices. Healing was defined as complete re-epithelialization and scored at 12 and 26 weeks. We found no significant difference between patients with healed or non-healed foot ulcers for both types of measurements (p = 0.135–0.989). ROC curves demonstrated moderate sensitivity (range of 0.636–0.971) and specificity (range of 0.464–0.889), for LSCI and non-invasive blood pressure measurements. Therefore, no association between diabetic foot ulcer healing and LSCI-measured microcirculation or non-invasive blood pressure measurements was found. The healing tendency of diabetic foot ulcers is difficult to predict based on single measurements using current blood pressure measurements or LSCI.

Oxygen saturation imaging has potential in several preclinical and clinical applications. Dual-wavelength LED array-based photoacoustic oxygen saturation imaging can be an affordable solution in this case. For the translation of this technology, there is a need to improve its accuracy and validate it against ground truth methods. We propose a fluence compensated oxygen saturation imaging method, utilizing structural information from the ultrasound image, and prior knowledge of the optical properties of the tissue with a Monte-Carlo based light propagation model for the dual-wavelength LED array configuration. We then validate the proposed method with oximeter measurements in tissue-mimicking phantoms. Further, we demonstrate in vivo imaging on small animal and a human subject. We conclude that the proposed oxygen saturation imaging can be used to image tissue at a depth of 6&ndash, 8 mm in both preclinical and clinical applications.

Significance: Practical implementation of acousto-optic imaging (AOI) encounters difficulties that prevent it from rapid adoption in clinical use. In many practical medical applications, the region of interest may be accessed only from one side, and using a water tank for coupling is not feasible. The solution might be to use reflection-mode imaging with an electronically scanned ultrasound (US) focus. Such an approach, however, entails considerable challenges. Aim: The possibilities of detecting and localizing light-absorbing inclusions inside turbid media by combining reflection-mode AOI conducted using a one-dimensional US array with electronic scanning of the US focus are investigated experimentally and signal processing algorithms that could be used for this purpose are introduced. Approach: We determine the speckle contrast decrease due to the acousto-optic effect as a function of the US focal point coordinates. Different signal postprocessing techniques are investigated. Results: A significant decrease in the determined speckle contrast difference values is observed due to the presence of light-absorbing inclusions. However, local minima occur in the plots only under specific conditions. Subtracting individual distributions and determining symmetry deviations allow for localizing the inclusions. Conclusions: Detection and localization of optically distinct regions are possible using the introduced approach. Signal postprocessing is required in a general case.

Functional performance of handheld laser speckle contrast imaging (LSCI) is compromised by movement artefacts. Here we quantify the movements of a handheld LSCI system employing electromagnetic (EM) tracking and measure the applied translational, tilt and on-surface laser beam speeds. By observing speckle contrast on static objects, the magnitudes of translation and tilt of wavefronts are explored for various scattering levels of the objects. We conclude that for tissue mimicking static phantoms, on-surface speeds play a dominant role to wavefront tilt speed in creation of movement artefacts. The ratio depends on the optical properties of the phantom. Furthermore, with the same applied speed, the drop in the speckle contrast increases with decreasing reduced scattering coefficient, and hence the related movement artefact increases.

Tomographic photoacoustic and ultrasound imaging is essential for isotropic spatial resolution and to obtain a full view of the target tissue. However, tomographic systems with pulsed laser sources and custom made transducer arrays are expensive. Additionally, there are other factors that limit the wide use of photoacoustic and ultrasound tomographic systems which include the size of the tomographic systems that use pulsed laser and the laser safety issues. A cost-effective, compact and safe photoacoustic and ultrasound tomographic system can find several imaging applications both in clinics and small animal labs. LED-based photoacoustic imaging has shown the potential to bring down the cost, enable faster imaging with high pulse repetition rate and is safer when compared to pulsed lasers. The conventional US system can be adopted for photoacoustic imaging by adding a light source to it. Hence, linear transducer arrays are preferred as they are cheaper and allow faster imaging. The combination of LED-based illumination and linear transducer array-based tomographic imaging can be a cost-effective alternative to current tomographic imaging, especially in point-of-care applications.

Significance. Quantifying human milk composition is important for daily nutritional management in neonatal intensive cares worldwide. Photonic solutions based on visible light can potentially aid in this analysis, as energy content of human milk depends largely on fat content, and the optical scattering properties of human milk predominantly depend on the size and concentration of fat globules. However, it is expected that human milk scattering changes upon homogenization, routinely done before analysis, which may affect fat globule size. Aim. The first aim of this study was to investigate how the most common homogenization methods (gently inverting by hand, vortexing, and sonication) affect the optical properties of human milk. The second aim was to estimate the scattering contribution of casein micelles, the second most dominant scatterers in human milk. Approach. We combined diffuse reflectance spectroscopy with spectroscopic optical coherence tomography to measure the scattering coefficient μs, reduced scattering coefficient μs′, and anisotropy g between 450 and 600 nm. Results. Sonication induced the strongest changes in μs, μs′, and g compared to the gently inverted samples (203%, 202%, and 7%, respectively, at 550 nm), but also vortexing changed μs′ with 20%. Although casein micelles only showed a modest contribution to μs and g at 550 nm (7% and 1%, respectively), their contribution to μs′ was 29%. Conclusions. The scattering properties of human milk strongly depend on the homogenization method that is employed, and gentle inversion should be the preferred method. The contribution of casein micelles was relatively small for μs and g but considerably larger for μs′.

Photoacoustic imaging has been a focus of research for clinical applications owing to its ability for deep visualization with optical absorption contrast. However, there are various technical challenges remaining for this technique to find its place in clinics. One of the challenges is the occurrence of reflection artifacts. The reflection artifacts may lead to image misinterpretation. Here we propose a new method using multiple wavelengths for identifying and removing the reflection artifacts. By imaging the sample with multiple wavelengths, the spectral response of the features in the photoacoustic image is obtained. We assume that the spectral response of the reflection artifact is better correlated with the proper image feature of its corresponding absorber than with other features in the image. Based on this, the reflection artifacts can be identified and removed. Here, we experimentally demonstrated the potential of this method for real-time identification and correction of reflection artifacts in photoacoustic images in phantoms as well as in vivo using a handheld photoacoustic imaging probe.

We evaluate a portable ultrasound and photoacoustic imaging (PAI) system for the feasibility of a point-of-care assessment of clinically evident synovitis. Inflamed and non-inflamed proximal interphalangeal joints of 10 patients were examined and compared with joints from 7 healthy volunteers. PAI scans, ultrasound power Doppler (US-PD), and clinical examination were performed. We quantified the amount of photoacoustic (PA) signal using a region of interest (ROI) drawn over the hypertrophic joint space. PAI response was increased 4 to 10 fold when comparing inflamed with contralateral non-inflamed joints and with joints from healthy volunteers (p

The non-invasive quantification of total haemoglobin concentrations [tHb] is highly desired for the assessment of haematologic disorders in vulnerable patient groups, but invasive blood sampling is still the gold standard in current clinical practice. This work demonstrates the potential of visible-light spectroscopic optical coherence tomography (sOCT) for quantifying the [tHb] in human whole blood. To accurately quantify the [tHb] from the substantial optical attenuation by blood in the visible wavelength range, we used a combination of zero-delay acquisition and focus tracking that ensures optimal system sensitivity at any depth inside the sample. Subsequently, we developed an analysis model to adequately correct for the high scattering contribution by red blood cells to the sOCT signal. We validate our method and compare it to conventional sOCT (without focus tracking and zero-delay acquisition) through ex-vivo measurements on flowing human whole blood, with [tHb] values in the clinical range of 7–23 g/dL. For our method with optimized sensitivity, the measured and expected values correlate well (Pearson correlation coefficient = 0.89, p

With human milk being the most important source of infant nutrition, the protection and support of breastfeeding are essential from a global health perspective. Nevertheless, relatively few objective methods are available to investigate human milk composition and lactation physiology when a mother experiences breastfeeding problems. We argue that optics and photonics offer promising opportunities for this purpose. Any research activity within this new application field starts with a thorough understanding on how light interacts with human milk. Therefore, the aim of this study was to investigate the full set of optical properties for human milk and the biological variability therein. Using a novel approach that combines spatially resolved diffuse reflectance spectroscopy (SR-DRS) and spectroscopic optical coherence tomography (sOCT) between 450 and 650 nm, we quantified the absorption coefficient µ(a), scattering coefficient µ(s), reduced scattering coefficient µ(s)’, anisotropy g and backscattering coefficient µ(b,NA) of mature human milk from 14 participants released at different stages during a breastfeed (foremilk, bulk milk and hindmilk). Significant correlations were found between µ(a), µ(s), µ(s)’ and µ(b,NA) and the biochemically determined fat concentration per sample (R(s) = 0.38, R(s) = 0.77, R(s) = 0.80, R(s) = 0.44 respectively). We explained the observed variations in the optical properties of human milk using Mie theory and the biological variability in both the concentration and size distribution of milk fat globules. In conclusion, we have provided a full set of optical properties for human milk, which can hopefully serve as a starting point for future biophotonic studies on human milk and the milk containing lactating breast.

OBJECTIVE: A major challenge for treating diabetic foot ulcers is estimating the severity of ischemia, as the currently used non-invasive diagnostic techniques provide relatively poor prognostic values. Laser speckle contrast imaging (LSCI) is a promising non-invasive technique to assess microcirculation. Our aim was to investigate the stability and reproducibility of LSCI for the assessment of microcirculation in the diabetic foot, the relation of LSCI results to currently used non-invasive blood pressure measurements, and the ability of LSCI to discriminate between the degrees of ischemia. APPROACH: Thirty-three participants with diabetic foot ulcers were included in this prospective, single centre, observational cohort study that was conducted in the Netherlands. They were classified as non-ischemic, ischemic or critical-ischemic based on criteria formulated in the international guidelines. Two clinicians performed LSCI scans of the foot, consisting of baseline measurements, followed by two stress tests (post-occlusion peak and elevation test). With three measurement conditions and five regions of interest of the foot per patient, a total of 15 measurements were available for analyses. MAIN RESULTS: The intra-observer agreement of LSCI was high (interclass correlation coefficient (ICC) = 0.711-0.950; p

Objective: Knowledge on the hemodynamics of the human lactating breast contributes to our understanding of lactation physiology, as well as the development and management of breastfeeding problems. The objective of this pilot study was to investigate whether laser Doppler perfusion monitoring (LDPM) can be employed to measure physiological changes in mammary cutaneous perfusion during milk extraction. Approach: We evaluated mammary cutaneous perfusion with LDPM in nine lactating women during milk extraction in both the ipsilateral (‘milk extracting’) and contralateral (‘passive’) breast. Fourier domain filtering of the LDPM signal was applied to correct for the influence of the periodic tissue movement caused by the breast pump. Main results: Cutaneous perfusion increased temporary during 23.7 ± 18.9 s by 18%–74% for all women who sensed their milk ejection reflex (n = 6) in both the ipsilateral and contralateral breast. For those women who did not sense a milk ejection reflex (n = 3), the changes in cutaneous perfusion were less outspoken (maximally 26%). Significance: This pilot study provides new insights into mammary hemodynamics and demonstrates that LDPM is a promising method for the further investigation of physiological changes in mammary cutaneous perfusion during milk ejection. Objective feedback on the occurrence and progression of milk ejection can support lactation research in general, maternal breastfeeding confidence, and may be an early indicator for the development of breastfeeding problems.

A decreased hemoglobin concentration (tHb) in blood (anemia) is associated with impaired oxygen delivery to organs, which can result in organ damage and heart failure. Currently, tHb analysis requires invasive methods (e.g. a fingerstick), which are time consuming and cause discomfort to the patient. Using optical spectroscopy, the tHb can be estimated by quantifying light absorption in blood. However, the accuracy of current noninvasive optical techniques for tHb quantification is limited by the background attenuation of skin and the unknown blood volume fraction in the total optical probing volume. Spectroscopic optical coherence tomography (sOCT) allows for quantitative measurements of the optical attenuation in a confined measurement volume, potentially enabling non-invasive estimation of the hemoglobin concentration within individual blood vessels. Although multiple studies have shown that sOCT is capable of quantifying localized oxygen saturation, quantification of the tHb has not yet been reported for physiologically relevant concentrations. With a home-built visible-light sOCT system we quantified optical attenuation in the visible wavelength range (450–600nm). Implementation of both zero-delay acquisition and focus tracking optimized system sensitivity and ensured that the measured attenuation is only affected by the attenuation of the sample itself. We validated our method ex-vivo on human whole blood from healthy volunteers (tHb within 12-18 g/dL). The hematocrit was varied to cover the entire pathophysiological range (tHb within 9-21 g/dL) by either dilution with PBS, or plasma removal. Our system quantified the tHb in whole blood throughout the entire pathophysiological range with an accuracy of 10%.

Optical absorption contrast, large imaging depth at ultrasonic resolution, and the potential of functional/quantitative imaging are the features driving the rapid development of photoacoustic (PA) imaging. For quantitative and functional PA imaging, the fluence distribution is required to transform a PA image to a map of absorption coefficients. A suitable method to estimate the fluence for in vivo applications must not require a priori knowledge of the medium optical properties, should work for various illumination conditions, and must be applicable to different PA imaging geometries. Existing methods of estimating fluence in tissue do not meet these requirements simultaneously. Here we present a method to measure the fluence distribution in tissue using acousto-optics (AO) that meets all the above requirements. We developed a PA and AO tomography system for small-animal imaging to investigate the potential and the feasibility of fluence-corrected PA imaging using our method in a single instrument. We performed experiments on phantoms, an ex vivo tissue sample, and freshly sacrificed mice. We demonstrate that the correction for spatial and spectral fluence variations in PA images establishes the direct relation between image value and optical absorption, which in turns improves the quantitative estimation of blood oxygen saturation.

A method for measuring the fluorescence quantum yield Ff of samples using a soft transparent tube with 0.58 mm inner diameter inside an integrating sphere is presented. Based on simple energy balance within the sphere, an equation for determining Ff in terms of the optical output signal is derived. In order to determine the feasibility of measuring Ff using the present integrating sphere-spectrometer setup, the Ff of Rhodamine 6G (R6G) solutions is determined. The measured Ff of R6G in water and in ethanol are 0.647±0.005 and 0.881±0.025, respectively. These values are close to those found in literature, with the measured Ff of R6G in ethanol within the range of values reported for a similar integrating sphere method. Recommendations are given for measuring the fluorescence quantum yield of low efficiency fluorophores of biomedical interest.

Reflection artifacts caused by acoustic inhomogeneities are a critical problem in epi-mode biomedical photoacoustic imaging. High light fluence beneath the probe results in photoacoustic transients, which propagate into the tissue and reflect back from echogenic structures. These reflection artifacts cause problems in image interpretation and significantly impact the contrast and imaging depth. We recently proposed a method called PAFUSion (Photoacoustic-guided focused ultrasound) to identify such reflection artifacts in photoacoustic imaging. In its initial version, PAFUSion mimics the inward-travelling wavefield from small blood vessel-like PA sources by applying ultrasound pulses focused towards these sources, and thus provides a way to identify the resulting reflection artifacts. In this work, we demonstrate reduction of reflection artifacts in phantoms and in vivo measurements on human volunteers. In view of the spatially distributed PA sources that are found in clinical applications, we implemented an improved version of PAFUSion where photoacoustic signals are backpropagated to imitate the inward travelling wavefield and thus the reflection artifacts. The backpropagation is performed in a synthetic way based on the pulse-echo acquisitions after transmission on each single element of the transducer array. The results provide a direct confirmation that reflection artifacts are prominent in clinical epi-photoacoustic imaging, and that PAFUSion can strongly reduce these artifacts to improve deep-tissue photoacoustic imaging.

Acousto-optic imaging exploits the effects of modulation of light by acoustic waves propagating in an optically scattering medium to determine its optical properties. The imaging resolution is determined by the acoustic pressure field distribution inside the sample. In the ideal case, this distribution should be limited to a relatively small, strictly defined volume within which the light modulation occurs. Practical applications of this technique impose specific requirements on transducers configuration which constitute additional limitations on shaping the acoustic pressure field characteristics. We investigate the possibilities of improving performance of an acousto-optic imaging system utilizing a linear ultrasound array by comparing results obtained using plane wave and focused pulses and different apodization patterns. Acoustic pressure field distributions were determined numerically and with hydrophone measurements. We introduce an ultrasound focus quality coefficient in order to describe and compare the confinement of the expected light modulation volume. Acousto-optic images were obtained experimentally by measuring contrast decrease of the interference patterns of a laser light transmitted through samples and recorded by a camera. The results show that for focused pulses the off-focal pressure field components have significant influence on the determined values, and that this issue can be mitigated by using plane wave imaging.

Photoacoustic tomography is a hybrid imaging technique that combines high optical tissue contrast with high ultrasound resolution. Direct reconstruction methods such as filtered backprojection, time reversal and least squares suffer from curved line artefacts and blurring, especially in case of limited angles or strong noise. In recent years, there has been great interest in regularised iterative methods. These methods employ prior knowledge on the image to provide higher quality reconstructions. However, easy comparisons between regularisers and their properties are limited, since many tomography implementations heavily rely on the specific regulariser chosen. To overcome this bottleneck, we present a modular reconstruction framework for photoacoustic tomography. It enables easy comparisons between regularisers with different properties, e.g. nonlinear, higher-order or directional. We solve the underlying minimisation problem with an efficient first-order primal-dual algorithm. Convergence rates are optimised by choosing an operator dependent preconditioning strategy. Our reconstruction methods are tested on challenging 2D synthetic and experimental data sets. They outperform direct reconstruction approaches for strong noise levels and limited angle measurements, offering immediate benefits in terms of acquisition time and quality. This work provides a basic platform for the investigation of future advanced regularisation methods in photoacoustic tomography., submitted to "Physics in Medicine and Biology". Changes from v1 to v2: regularisation with directional wavelet has been added; new experimental tests have been included

Background: The most severe diabetic foot ulcers are those related with critical ischemia, which is primarily diagnosed with non-invasive diagnostics. However, these diagnostics have several disadvantages. For example, they only provide global indications of the (macro)level of ischemia. A potential solution can be found in novel optical imaging techniques for local assessment of the microcirculation in diabetic foot ulcers. This review provides an overview of these imaging techniques (Laser Doppler Perfusion Imaging, Laser Speckle Contrast Imaging, Photoacoustic Imaging and Hyperspectral Imaging) and their applicability for the diagnostic assessment of microcirculation in diabetic foot ulcers. Method: For each technique, the following parts are described: a) their technical background; b) general clinical applications; and, c) its application for microcirculation assessment in diabetic foot ulcers. Parts a-b are based on a narrative review of the literature, part c on a systematic review that was performed in the database Scopus, covering the period from January 1, 2000 to November 31, 2017. Results: Each of these techniques has specific advantages and disadvantages for imaging microcirculation. Potential clinical use depends on measurement aims, and clinical relevance. However, none of the techniques has a strongly established clinical relevance yet: we found a limited number of publications describing clinical outcomes. Future research is needed to determine which technique is the most clinically relevant for the assessment of microcirculation in diabetic foot ulcers. Conclusion: Although promising, the currently available novel optical techniques need to be further improved technically and prospective trials are necessary to evaluate their clinical value.

The deep imaging capability and optical absorption contrast offered by photoacoustic imaging promote the use of this technology in clinical applications. By exploiting the optical absorption properties of endogenous chromophores such as hemoglobin and lipid, molecular information at a depth of a few centimeters can be unveiled. This information shows promise to reveal lesions indicating early stage of various human diseases, such as cancer and atherosclerosis. In addition, the use of exogenous contrast agents can further extend the capability of photoacoustic imaging in clinical diagnosis and treatment. In this review, the current state of the art and applications of photoacoustic molecular probes will be critically reviewed, as well as some spearheading translational efforts that have taken place over the past 5 years. Some of the most critical barriers to clinical translation of this novel technology will be discussed, and some thoughts will be given on future endeavors and pathways.

Laser-induced acoustic waves in tissue can be detected by measurement of the pressure transient or displacement at the tissue surface. Measurement of these pressure transients using piezoelectric sensors is the most commonly used method in photoacoustics. Although optical methods for detection of acoustic waves have been developed as well [1-6], piezoelectric sensors have the advantage that broadband sensors are easier to construct and have a slightly better sensitivity than optical detection systems [7]. The shape and size of the piezoelectric element determine the characteristics, such as the directivity pattern and the frequency response of the sensor. Sensors with a small element size (“point sensors”) have a very large angular aperture, and thus are able to detect acoustic signals originating from a large volume in the tissue. As a result, reconstruction algorithms are needed to obtain an image. In contrast, for sensors with a narrow angular aperture, e.g., ring-shaped or annular sensors, the time traces can be regarded as 1D depth images (amplitude (A)-scans) of photoacoustic sources inside the measurement volume. Comparable to an echographic B-scan, imaging with such a ring-shaped sensor is carried out by scanning the sensor over the tissue surface, permitting real-time reconstruction of the corresponding part of the image without the need for information on adjacent sensor positions.