Acoustic Scattering Mediated Single Detector Optoacoustic Tomography.

Optoacoustic image formation is conventionally based upon ultrasound time-of-flight readings from multiple detection positions. Herein, we exploit acoustic scattering to physically encode the position of optical absorbers in the acquired signals, thus reducing the amount of data required to reconstruct an image from a single waveform. This concept is experimentally tested by including a random distribution of scatterers between the sample and an ultrasound detector array. Ultrasound transmission through a randomized scattering medium was calibrated by raster scanning a light-absorbing microparticle across a Cartesian grid. Image reconstruction from a single time-resolved signal was then enabled with a regularized model-based iterative algorithm relying on the calibration signals. The signal compression efficiency is facilitated by the relatively short acquisition time window needed to capture the entire scattered wave field. The demonstrated feasibility to form an image using a single recorded optoacoustic waveform paves a way to the development of faster and affordable optoacoustic imaging systems.

Calcium Sensor for Photoacoustic Imaging.

We introduce a selective and cell-permeable calcium sensor for photoacoustics (CaSPA), a versatile imaging technique that allows for fast volumetric mapping of photoabsorbing molecules with deep tissue penetration. To optimize for Ca2+-dependent photoacoustic signal changes, we synthesized a selective metallochromic sensor with high extinction coefficient, low quantum yield, and high photobleaching resistance. Micromolar concentrations of Ca2+ lead to a robust blueshift of the absorbance of CaSPA, which translated into an accompanying decrease of the peak photoacoustic signal. The acetoxymethyl esterified sensor variant was readily taken up by cells without toxic effects and thus allowed us for the first time to perform live imaging of Ca2+ fluxes in genetically unmodified cells and heart organoids as well as in zebrafish larval brain via combined fluorescence and photoacoustic imaging.

Dynamic particle enhancement in limited-view optoacoustic tomography.

Limited-view artifacts are commonly present in optoacoustic tomography images, mainly due to practical geometrical and physical constraints imposed by the imaging systems. Herein, a new approach called dynamic particle-enhanced optoacoustic tomography (DPOT) is proposed for improving image contrast and visibility of optoacoustic images under limited-view scenarios. The method is based on a nonlinear combination of a temporal sequence of tomographic reconstructions representing sparsely distributed moving particles. We demonstrate experimental performance by dynamically imaging the flow of suspended microspheres in three dimensions, which shows promise for DPOT applicability in angiographic imaging in living organisms.

Functional optoacoustic neuro-tomography of calcium fluxes in adult zebrafish brain in vivo.

Genetically-encoded calcium indicators (GECIs) have revolutionized neuroimaging by enabling mapping of the activity of entire neuronal populations in vivo. Visualization of these powerful activity sensors has to date been limited to depth-restricted microscopic studies due to intense light scattering in the brain. We demonstrate, for the first time, in vivo real-time volumetric optoacoustic monitoring of calcium transients in adult transgenic zebrafish expressing the GCaMP5G calcium indicator. Fast changes in optoacoustic traces associated with GCaMP5G activity were detectable in the presence of other strongly absorbing endogenous chromophores, such as hemoglobin. The new functional optoacoustic neuroimaging method can visualize neural activity at penetration depths and spatio-temporal resolution scales not covered with the existing neuroimaging techniques.

Three-dimensional optoacoustic monitoring of lesion formation in real time during radiofrequency catheter ablation.

INTRODUCTION: Due to lack of reliable imaging contrast from catheter radiofrequency ablation (RFA) lesions, the vast majority of current procedures rely on indirect indicators of ablation activity, resulting in a significant number of arrhythmia reoccurrences after RFA procedures and the need for repeat surgeries. The objective of this work is to develop an accurate method for on-the-fly assessment of the durability and size of lesions formed during RFA procedures. METHOD AND RESULTS: Radiofrequency catheter ablation on freshly excised porcine ventricular myocardial tissue was optoacoustically monitored by means of pulsed-laser illumination in the near-infrared spectrum. Lesion formation during ablation was captured at a rate of 10 Hz with a 256-detector optoacoustic imaging probe. Postablated samples were imaged using multispectral excitation in the wavelength range 740-860 nm to determine the lesion contrast spectrum. Tomographic reconstruction was performed to generate 3-dimensional images of the lesions, which were compared to photographs depicting the final ablated tissue samples. Video-rate 3-dimensional tomographic reconstructions depict formation of the lesion with high contrast and spatial resolution. The size and geometry of the lesion was shown to be in excellent agreement with the histological examinations. The wavelength dependence of the lesion contrast shows a contrast peak near 780 nm. CONCLUSION: Deep-tissue 3-dimensional monitoring of RFA lesion generation in real time was demonstrated for the first time in this work. The results suggest the potential of optoacoustic monitoring for providing critical feedback on lesion position and size during radiofrequency catheter ablation, improving safety and efficacy of these treatments.

Influence of the absorber dimensions on wavefront shaping based on volumetric optoacoustic feedback.

The recently demonstrated control over light distribution through turbid media based on real-time three-dimensional optoacoustic feedback has offered promising prospects to interferometrically focus light within scattering objects. Nevertheless, the focusing capacity of the feedback-based approach is strongly conditioned by the number of optical modes (speckle grains) enclosed in the volume that can be resolved with the optoacoustic imaging system. In this Letter, we experimentally tested the light intensity enhancement achieved with optoacoustic feedback measurements from different sizes of absorbing microparticles. The importance of the obtained results is discussed in the context of potential signal enhancement at deep locations within a scattering medium where the effective speckle grain sizes approach the minimum values dictated by optical diffraction.

Shaping volumetric light distribution through turbid media using real-time three-dimensional opto-acoustic feedback.

Focusing light through turbid media represents a highly fascinating challenge in modern biophotonics. The unique capability of opto-acoustics for high-resolution imaging of light absorption contrast in deep tissues can provide a natural and efficient feedback to control light delivery in a scattering medium. While the basic feasibility of using opto-acoustic readings as a feedback mechanism for wavefront shaping has been recently reported, the suggested approaches may require long acquisition times, making them challenging to be translated into realistic tissue environments. In an attempt to significantly accelerate dynamic wavefront shaping capabilities, we present here a feedback-based approach using real-time three-dimensional opto-acoustic imaging assisted with genetic-algorithm-based optimization. The new technique offers robust performance in the presence of noisy measurements and can simultaneously control the scattered wave field in an entire volumetric region.

Light fluence normalization in turbid tissues via temporally unmixed multispectral optoacoustic tomography.

Discerning the accurate distribution of chromophores and biomarkers by means of optoacoustic imaging is commonly challenged by the highly heterogeneous excitation light patterns resulting from strong spatial variations of tissue scattering and absorption. Here we used the light-fluence dependent switching kinetics of reversibly switchable fluorescent proteins (RSFPs), in combination with real-time acquisition of volumetric multi-spectral optoacoustic data to correct for the light fluence distribution deep in scattering media. The new approach allows for dynamic fluence correction in time-resolved imaging, e.g., of moving organs, and can be extended to work with a large palette of available synthetic and genetically encoded photochromic substances for multiplexed wavelength-specific fluence normalization.

Portable spherical array probe for volumetric real-time optoacoustic imaging at centimeter-scale depths.

We report on a novel hand-held imaging probe for real-time optoacoustic visualization of deep tissues in three dimensions. The system incorporates an annular two-dimensional array of ultrasonic sensors densely distributed on a spherical surface. Simultaneous recording and processing of time-resolved data from all the channels enables acquisition of entire volumetric data sets for each illumination laser pulse. The proposed solution utilizes a transparent membrane in order to allow efficient coupling of optoacoustically generated waves to the ultrasonic detectors while avoiding direct contact of the imaged object with the coupling medium. The hand-held approach further allows convenient handling of both pre-clinical experiments as well as clinical measurements in human subjects. Here we demonstrate an imaging speed of 10 volumetric frames per second with spatial resolution down to 200 micrometers in the imaged region while also achieving imaging depth of more than 1.5 cm in living tissues without signal averaging.

Phase and group velocity measurement of ultrasonic guided wavetrains in plates by pulsed TV holography.

A method for the measurement of the phase and group velocities of guided acoustic waves is presented. For this purpose, a unique capability of a self-developed double-pulsed TV holography system to create movies of the propagation of guided acoustic wavetrains is used. Thereby, the experimental visualization of the evolution of Lamb and Rayleigh wavetrains in aluminum plates is shown, including several movies that illustrate different dispersive behaviors. Each movie shows the propagation of a narrowband wavetrain with a central frequency in the order of 1 MHz that consists of a carrier moving with the phase velocity, modulated by an envelope moving with the group velocity. Each snapshot is a two-dimensional map of the instantaneous out-of-plane displacement field of the surface points of the plate, with amplitudes in the order of several nanometers, acquired with the double-pulsed TV holography system. Then, by repeating the acquisition with successively increasing delays between the generation and detection instants, a sequence of maps is obtained from which a movie is composed. The phase and group velocities of the waves are accurately measured from the movies, yielding values that match well with the reference values for the same plates obtained from independent measurements.

High-Throughput Platform for Optoacoustic Probing of Genetically Encoded Calcium Ion Indicators.

Functional optoacoustic (OA) imaging assisted with genetically encoded calcium ion indicators (GECIs) holds promise for imaging large-scale neuronal activity at depths and spatiotemporal resolutions not attainable with existing optical microscopic techniques. However, currently available GECIs optimized for fluorescence (FL) imaging lack sufficient contrast for OA imaging and respond at wavelengths having limited penetration into the mammalian brain. Here we present an imaging platform capable of rapid assessment and cross-validation between OA and FL responses of sensor proteins expressed in Escherichia coli colonies. The screening system features optimized pulsed light excitation combined with ultrasensitive ultrasound detection to mitigate photobleaching while further allowing the dynamic characterization of calcium ion responses with millisecond precision. Targeted probing of up to six individual colonies per second in both calcium-loaded and calcium-unloaded states was possible with the system. The new platform greatly facilitates optimization of absorption-based labels, thus setting the stage for directed evolution of OA GECIs.

Optoacoustic micro-tomography at 100 volumes per second.

Optical microscopy remains a fundamental tool for modern biological discovery owing to its excellent spatial resolution and versatile contrast in visualizing cellular and sub-cellular structures. Yet, the time domain is paramount for the observation of biological dynamics in living systems. Commonly, acquisition of microscopy data involves scanning of a spherically- or cylindrically-focused light beam across the imaged volume, which significantly limits temporal resolution in 3D. Additional complications arise from intense light scattering of biological tissues, further restraining the effective penetration depth and field of view of optical microscopy techniques. To overcome these limitations, we devised a fast optoacoustic micro-tomography (OMT) approach based on simultaneous acquisition of 3D image data with a high-density hemispherical ultrasound array having effective detection bandwidth beyond 25 MHz. We demonstrate fast three-dimensional imaging of freely-swimming zebrafish larvae, achieving 3D imaging speed of 100 volumes per second with isotropic spatial resolution approaching the dimensions of large cells across a field of view exceeding 50mm3. As opposed to other microscopy techniques based on optical contrast, OMT resolves optical absorption acoustically using unfocused light excitation. Thus, no penetration barriers are imposed by light scattering in deep tissues, suggesting it as a powerful approach for multi-scale functional and molecular imaging applications.

Spiral volumetric optoacoustic tomography visualizes multi-scale dynamics in mice.

Imaging dynamics at different temporal and spatial scales is essential for understanding the biological complexity of living organisms, disease state and progression. Optoacoustic imaging has been shown to offer exclusive applicability across multiple scales with excellent optical contrast and high resolution in deep-tissue observations. Yet, efficient visualization of multi-scale dynamics remained difficult with state-of-the-art systems due to inefficient trade-offs between image acquisition time and effective field of view. Herein, we introduce the spiral volumetric optoacoustic tomography technique that provides spectrally enriched high-resolution contrast across multiple spatiotemporal scales. In vivo experiments in mice demonstrate a wide range of dynamic imaging capabilities, from three-dimensional high-frame-rate visualization of moving organs and contrast agent kinetics in selected areas to whole-body longitudinal studies with unprecedented image quality. The newly introduced paradigm shift in imaging of multi-scale dynamics adds to the multifarious advantages provided by the optoacoustic technology for structural, functional and molecular imaging.

Volumetric hand-held optoacoustic angiography as a tool for real-time screening of dense breast.

Existing mammographic screening solutions are generally associated with several major drawbacks, such as exposure to ionizing radiation or insufficient sensitivity in younger populations with radiographically-dense breast. Even when combined with ultrasound or magnetic resonance imaging, X-Ray mammography may still attain unspecific or false positive results. Thus, development of new breast imaging tools represents a timely medical challenge. We report on a new approach to high-resolution functional and anatomical breast angiography using volumetric hand-held optoacoustic tomography, which employs light intensities safe for human use. Experiments in young healthy volunteers with fibroglandular-dominated dense breasts revealed the feasibility of rendering three-dimensional images representing vascular anatomy and functional blood oxygenation parameters at video rate. Sufficient contrast was achieved at depths beyond 2 cm within dense breasts without compromising the real-time imaging performance. The suggested solution may thus find applicability as a standalone or supplemental screening tool for early detection and follow-up of carcinomas in radiographically-dense breasts. Volumetric handheld optoacoustic tomography scanner uses safe pulses of near-infrared light to render three-dimensional images of deep vascular anatomy, blood oxygenation and breast parenchyma at video rate.

Improving Optoacoustic Image Quality via Geometric Pixel Super-Resolution Approach.

High fidelity optoacoustic (photoacoustic) tomography requires dense spatial sampling of optoacoustic signals using point acoustic detectors. However, in practice, spatial resolution of the images is often limited by limited sampling either due to coarse multi-element arrays or time in raster scan measurements. Herein, we investigate a method that integrates information from multiple optoacoustic images acquired at sub-diffraction steps into one high resolution image by means of an iterative registration algorithm. Experimental validations performed in target phantoms and ex vivo tissue samples confirm that the suggested approach renders significant improvements in terms of optoacoustic image resolution and quality without introducing significant alterations into the signal acquisition hardware or inversion algorithms.

Functional optoacoustic neuro-tomography for scalable whole-brain monitoring of calcium indicators.

Non-invasive observation of spatiotemporal activity of large neural populations distributed over entire brains is a longstanding goal of neuroscience. We developed a volumetric multispectral optoacoustic tomography platform for imaging neural activation deep in scattering brains. It can record 100 volumetric frames per second across scalable fields of view ranging between 50 and 1000 mm3 with respective spatial resolution of 35-200 µm. Experiments performed in immobilized and freely swimming larvae and in adult zebrafish brains expressing the genetically encoded calcium indicator GCaMP5G demonstrate, for the first time, the fundamental ability to directly track neural dynamics using optoacoustics while overcoming the longstanding penetration barrier of optical imaging in scattering brains. The newly developed platform thus offers unprecedented capabilities for functional whole-brain observations of fast calcium dynamics; in combination with optoacoustics' well-established capacity for resolving vascular hemodynamics, it could open new vistas in the study of neural activity and neurovascular coupling in health and disease.

Extending biological imaging to the fifth dimension: evolution of volumetric small animal multispectral optoacoustic tomography.

Despite the ancient discovery of the basic physical phenomenon underlying optoacoustic imaging and tomography [1], the lack of suitable laser sources, ultrasound detection technology, data acquisition, and processing capacities has long hindered the realization of efficient imaging devices. In fact, the first high-quality images from living animals were obtained about a decade ago (Figure 1), which was followed by an exponential growth of technical developments in instrumentation, algorithms, and biomedical applications surrounding this fascinating field. The ability of optoacoustics to probe optical contrast along a wide domain of penetration scales while maintaining excellent spatiotemporal resolution representative of ultrasound imaging, as shown in Figure 2, is unparalleled among the other optical imaging modalities [2], [3].

Effects of small variations of speed of sound in optoacoustic tomographic imaging.

PURPOSE: Speed of sound difference in the imaged object and surrounding coupling medium may reduce the resolution and overall quality of optoacoustic tomographic reconstructions obtained by assuming a uniform acoustic medium. In this work, the authors investigate the effects of acoustic heterogeneities and discuss potential benefits of accounting for those during the reconstruction procedure. METHODS: The time shift of optoacoustic signals in an acoustically heterogeneous medium is studied theoretically by comparing different continuous and discrete wave propagation models. A modification of filtered back-projection reconstruction is subsequently implemented by considering a straight acoustic rays model for ultrasound propagation. The results obtained with this reconstruction procedure are compared numerically and experimentally to those obtained assuming a heuristically fitted uniform speed of sound in both full-view and limited-view optoacoustic tomography scenarios. RESULTS: The theoretical analysis showcases that the errors in the time-of-flight of the signals predicted by considering the straight acoustic rays model tend to be generally small. When using this model for reconstructing simulated data, the resulting images accurately represent the theoretical ones. On the other hand, significant deviations in the location of the absorbing structures are found when using a uniform speed of sound assumption. The experimental results obtained with tissue-mimicking phantoms and a mouse postmortem are found to be consistent with the numerical simulations. CONCLUSIONS: Accurate analysis of effects of small speed of sound variations demonstrates that accounting for differences in the speed of sound allows improving optoacoustic reconstruction results in realistic imaging scenarios involving acoustic heterogeneities in tissues and surrounding media.

Volumetric optoacoustic imaging with multi-bandwidth deconvolution.

Optoacoustic (photoacoustic) imaging based on cylindrically focused 1-D transducer arrays comes with powerful characteristics in visualizing optical contrast. Parallel reading of multiple detectors arranged around a tissue cross section enables capturing data for generating images of this plane within micro-seconds. Dedicated small animals scanners and handheld systems using 1-D cylindrically focused ultrasound transducer arrays have demonstrated real-time cross-sectional imaging and high in-plane resolution. Yet, the resolution achieved along the axis perpendicular to the focal plane, i.e., the elevation resolution, is determined by the focusing capacities of the detector and is typically lower than the in-plane resolution. Herein, we investigated whether deconvolution of the sensitivity field of the transducer could lead to tangible image improvements. We showcase the findings on experimental measurements from phantoms and animals and discuss the features and the limitations of the approach in improving resolution along the elevation dimension.

Estimation of optoacoustic contrast agent concentration with self-calibration blind logarithmic unmixing.

Chromophore quantification in optoacoustic tomography is challenging due to signal contributions from strongly absorbing background tissue chromophores and the depth-dependent light attenuation. Herein we present a procedure capable of correcting for wavelength-dependent light fluence variations using a logarithmic representation of the images taken at different wavelengths assisted with a blind unmixing approach. It is shown that the serial expansion of the logarithm of an optoacoustic image contains a term representing the ratio between absorption of the probe of interest and other background components. Under assumptions of tissue-like background absorption variations, this term can be readily isolated with an unmixing algorithm, attaining quantitative maps of photo-absorbing agent distribution.