Impacts of the murine skull on high-frequency transcranial photoacoustic brain imaging

Non-invasive photoacoustic tomography (PAT) of mouse brains with intact skulls has been a challenge due to the skull's strong acoustic attenuation, aberration, and reverberation, especially in the high-frequency range (>15 MHz). In this paper, we systematically investigated the impacts of the murine skull on the photoacoustic wave propagation and on the PAT image reconstruction. We studied the photoacoustic acoustic wave aberration due to the acoustic impedance mismatch at the skull boundaries and the mode conversion between the longitudinal wave and shear wave. The wave's reverberation within the skull was investigated for both longitudinal and shear modes. In the inverse process, we reconstructed the transcranial photoacoustic computed tomography (PACT) and photoacoustic microscopy (PAM) images of a point target enclosed by the mouse skull, showing the skull's different impacts on both modalities. Finally, we experimentally validated the simulations by imaging an in vitro mouse skull phantom using representative transcranial PAM and PACT systems. The experimental results agreed well with the simulations and confirmed the accuracy of our forward and inverse models. We expect that our results will provide better understanding of the impacts of the murine skull on transcranial photoacoustic brain imaging and pave the ways for future technical improvements.

[Review on photoacoustic tomographic image reconstruction for acoustically heterogeneous tissues]

Acoustic properties of biological tissues usually vary inhomogeneously in space. Tissues with different chemical composition often have different acoustic properties. The assumption of acoustic homogeneity may lead to blurred details, misalignment of targets and artifacts in the reconstructed photoacoustic tomography (PAT) images. This paper summarizes the main solutions to PAT imaging of acoustically heterogeneous tissues, including the variable sound speed and acoustic attenuation. The advantages and limits of the methods are discussed and the possible future development is prospected.

Redox-Activatable and Acid-Enhanced Nanotheranostics for Second Near-Infrared Photoacoustic Tomography and Combined Photothermal Tumor Therapy

Tumor phototheranostics in the second near-infrared window (NIR-II, 1000-1700 nm) holds great promise due to high spatiotemporal precision, enhanced penetration depth, and therapeutic efficacy. However, current "always-on" NIR-II phototheranostic agents remain restricted by the inherent nonspecificity from the pseudosignal readout and undesirable treatment-related side effects. To address these challenges, herein we explore an activatable and biocompatible nanotheranostics that generates diagnostic and therapeutic effects only after specific activation and enhancement by tumor microenvironmental redox and acid while keeping silent at normal tissues. Such an intelligent "turn-on" chromogenic nanotheranostics allows in vivo nearly zero-background photoacoustic tomography (PAT) and combined effective photothermal tumor therapy (PTT) both in the NIR-II range with minimal adverse effects. In light of the high sensitivity, superior penetration depth, and biocompatibility, this stimuli-activatable NIR-II photo-nanotheranostics provides broad prospects for the investigation and intervention of deep-tissue redox and acid-associated physiological and pathological events.

All-reflective ring illumination system for photoacoustic tomography

Given that breast cancer is the second leading cause of cancer-related deaths among women in the United States, it is necessary to continue improving the sensitivity and specificity of breast imaging systems that diagnose breast lesions. Photoacoustic (PA) imaging can provide functional information during in vivo studies and can augment the structural information provided by ultrasound (US) imaging. A full-ring, all-reflective, illumination system for photoacoustic tomography (PAT) coupled to a full-ring US receiver is developed and tested. The US/PA tomography system utilizes a cone mirror and conical reflectors to optimize light delivery for PAT imaging and has the potential to image objects that are placed within the ring US transducer. The conical reflector used in this system distributes the laser energy over a circular cross-sectional area, thereby reducing the overall fluence. This, in turn, allows the operator to increase the laser energy achieving better cross-sectional penetration depth. A proof-of-concept design utilizing a single cone mirror and a parabolic reflector is used for imaging cylindrical phantoms with light-absorbing objects. For the given phantoms, it has been shown that there was no restriction in imaging a given targeted cross-sectional area irrespective of vertical depth, demonstrating the potential of mirror-based, ring-illuminated PAT system. In addition, the all-reflective ring illumination method shows a uniform PA signal across the scanned cross-sectional area.

Combination of Astragali Polysaccharide and Curcumin Improves the Morphological Structure of Tumor Vessels and Induces Tumor Vascular Normalization to Inhibit the Growth of Hepatocellular Carcinoma

Normalizing the disordered tumor vasculature, rather than blocking it, is a novel method for anticancer therapy. Astragali polysaccharide (APS) and curcumin were reported to be active against carcinomas. However, the effect and mechanism of the combination of APS and curcumin on vascular normalization in hepatocellular carcinoma (HCC) was not clear. In the present study, effects of combined APS and curcumin on tumor vascular normalization were evaluated in HepG2 tumor-bearing mice. Photoacoustic tomography (PAT) was performed to observe the morphological structure of tumor vessels in vivo. The microstructure of the tumor vessels was also analyzed through scanning electron microscopy. Additionally, the expression of CD31 and NG2 was analyzed by immunohistochemical staining. Tumor vessels of HepG2 tumor-bearing mice treated with the combination were sparse with uniform growth, morphology rules, and complete vascular walls, which had fewer branches and sprouts. ECs of tumor vessels were arranged regularly and were tightly connected, tending toward normalization. The expression of CD31 was reduced while NG2 was increased significantly by the combination of APS and curcumin. The results indicated that APS and curcumin in combination showed a better effect on inhibiting tumor growth in an orthotopic nude-mouse model of HCC. More important, the combination induced normalization of tumor vascular better than APS or curcumin administration alone, improving the morphological structure of tumor vessels and promoting maturation of tumor vessels. The results of the present study provided a reasonable possibility for combination therapy of APS and curcumin in the treatment of HCC via tumor vascular normalization.

High-speed Intraoperative Assessment of Breast Tumor Margins by Multimodal Ultrasound and Photoacoustic Tomography

Conventional methods for breast tumor margins assessment need a long turnaround time, which may lead to re-operation for patients undergoing lumpectomy surgeries. Photoacoustic tomography (PAT) has been shown to visualize adipose tissue in small animals and human breast. Here, we demonstrate a customized multimodal ultrasound and PAT system for intraoperative breast tumor margins assessment using fresh lumpectomy specimens from 66 patients. The system provides the margin status of the entire excised tissue within 10 minutes. By subjective reading of three researchers, the results show 85.7% [95% confidence interval (CI), 42.0% - 99.2%] sensitivity and 84.6% (95% CI, 53.7% - 97.3%) specificity, 71.4% (95% CI, 30.3% - 94.9%) sensitivity and 92.3% (95% CI, 62.1% - 99.6%) specificity, and 100% (95% CI, 56.1% - 100%) sensitivity and 53.9% (95% CI, 26.1% - 79.6%) specificity respectively when cross-correlated with post-operational histology. Furthermore, a machine learning-based algorithm is deployed for margin assessment in the challenging ductal carcinoma in situ tissues, and achieved 85.5% (95% CI, 75.2% - 92.2%) sensitivity and 90% (95% CI, 79.9% - 95.5%) specificity. Such results present the potential of using mutlimodal ultrasound and PAT as a high-speed and accurate method for intraoperative breast tumor margins evaluation.

Spatially variant regularization based on model resolution and fidelity embedding characteristics improves photoacoustic tomography

Photoacoustic tomography tends to be an ill-conditioned problem with noisy limited data requiring imposition of regularization constraints, such as standard Tikhonov (ST) or total variation (TV), to reconstruct meaningful initial pressure rise distribution from the tomographic acoustic measurements acquired at the boundary of the tissue. However, these regularization schemes do not account for nonuniform sensitivity arising due to limited detector placement at the boundary of tissue as well as other system parameters. For the first time, two regularization schemes were developed within the Tikhonov framework to address these issues in photoacoustic imaging. The model resolution, based on spatially varying regularization, and fidelity-embedded regularization, based on orthogonality between the columns of system matrix, were introduced. These were systematically evaluated with the help of numerical and in-vivo mice data. It was shown that the performance of the proposed spatially varying regularization schemes were superior (with at least 2 dB or 1.58 times improvement in the signal-to-noise ratio) compared to ST-/TV-based regularization schemes.

Contrast-enhanced photoacoustic imaging in the second near-infrared window using semiconducting polymer nanoparticles

Photoacoustic imaging (PAI) is a fast growing deep-tissue imaging modality. However, light scattering and absorption in biological tissues limit imaging depth. Short near-infrared wavelengths (650 to 950 nm) are widely used for PAI. Using longer near-infrared wavelengths reduces scattering. We demonstrate deep-tissue contrast-enhanced in vivo photoacoustic imaging at a wavelength of 1064 nm. An ultranarrow bandgap semiconducting polymer poly (thienoisoindigo-alt-diketopyrrolopyrrole) (denoted as PIGD) is designed and demonstrated for imaging at 1064 nm. By embedding colloidal nanoparticles (NPs) of PIGD in chicken-breast tissue, an imaging depth of ∼5  cm is achieved. Intravenous injection of PIGD NPs in living rats showed brain vascular images with ∼2 times higher contrast compared with the brain vascular images without any contrast agent. Thus, PIGD NPs as an NIR-II contrast agent opens new opportunities for both preclinical and clinical imaging of deep tissues with enhanced contrast.

Recent progress in photoacoustic molecular imaging

By acoustically detecting the optical absorption contrast, photoacoustic (PA) tomography (PAT) has broken the penetration limits of traditional high-resolution optical imaging. Through spectroscopic analysis of the target's optical absorption, PAT can identify a wealth of endogenous and exogenous molecules and thus is inherently capable of molecular imaging with high sensitivity. PAT's molecular sensitivity is uniquely accompanied by non-ionizing radiation, high spatial resolution, and deep penetration in biological tissues, which other optical imaging modalities cannot achieve yet. In this concise review, we summarize the most recent technological advancements in PA molecular imaging and highlight the novel molecular probes specifically made for PAT in deep tissues. We conclude with a brief discussion of the opportunities for future advancements.

Image reconstruction with the Heaviside equation in photoacoustic tomography accounting for dispersive acoustic media

A challenging issue in photoacoustic biomedical imaging is to take into account the presence of dispersive acoustic media, since these are prone to induce amplitude attenuation and scattering of the photoacoustic frequency components. These perturbations are largely the cause for which the photoacoustic tomographic image reconstruction from projections lacks a plane-wave transport formalism. Attending this problem, we further develop an analytic formalism of the transport and its numerical implementation accounting for dispersive acoustic media. We differentiate three variations of an acoustically perturbing media. Our object of interest is a numerical description of the light absorption map of a coronal human breast image. Then, we analyze conditions for which the propagation of photoacoustic perturbations can obey the generalized Heaviside telegraph equation. In addition, we provide a study of the causality consistency of the wave propagation models. We observe transport implications due to the presence of dispersive acoustic media and derive model adjustments that include attenuation and diffusion approximations within the two-dimensional forward problem. Next, we restore the inverse problem description with the deduced perturbation components. Finally, we solve the nonlinear inverse problem with a numerical strategy for a filtered backprojection reconstruction. At a stage prior to the image reconstruction, we compensate for the effect of acoustic attenuation and diffusion to calculate the inversions of the wave perturbations located within the projections. In this way, we manage to significantly reduce reconstruction artifacts. In consequence, we prevent the use of some additional image processing of noise reduction. We demonstrate a feasible strategy on how to solve the stated nonlinear inverse problem of photoacoustic tomography accounting for dispersive acoustic media. In particular, we emphasize efforts to achieve an analytical description, and thus an algorithm is placed, for imaged sound perturbations to be cleaned from acoustic scattering in a simplified manner.

Parameterized joint reconstruction of the initial pressure and sound speed distributions for photoacoustic computed tomography

Accurate estimation of the initial pressure distribution in photoacoustic computed tomography (PACT) depends on knowledge of the sound speed distribution. However, the sound speed distribution is typically unknown. Further, the initial pressure and sound speed distributions cannot both, in general, be stably recovered from PACT measurements alone. In this work, a joint reconstruction (JR) method for the initial pressure distribution and a low-dimensional parameterized model of the sound speed distribution is proposed. By employing a priori information about the structure of the sound speed distribution, both the initial pressure and sound speed can be accurately recovered. The JR problem is solved by use of a proximal optimization method that allows constraints and non-smooth regularization functions for the initial pressure distribution. The gradients of the cost function with respect to the initial pressure and sound speed distributions are calculated by use of an adjoint state method that has the same per-iteration computational cost as calculating the gradient with respect to the initial pressure distribution alone. This approach is evaluated through 2D computer-simulation studies for a small animal imaging model and by application to experimental in vivo measurements of a mouse.

Parameterized joint reconstruction of the initial pressure and sound speed distributions for photoacoustic computed tomography

Accurate estimation of the initial pressure distribution in photoacoustic computed tomography (PACT) depends on knowledge of the sound speed distribution. However, the sound speed distribution is typically unknown. Further, the initial pressure and sound speed distributions cannot both, in general, be stably recovered from PACT measurements alone. In this work, a joint reconstruction (JR) method for the initial pressure distribution and a low-dimensional parameterized model of the sound speed distribution is proposed. By employing a priori information about the structure of the sound speed distribution, both the initial pressure and sound speed can be accurately recovered. The JR problem is solved by use of a proximal optimization method that allows constraints and non-smooth regularization functions for the initial pressure distribution. The gradients of the cost function with respect to the initial pressure and sound speed distributions are calculated by use of an adjoint state method that has the same per-iteration computational cost as calculating the gradient with respect to the initial pressure distribution alone. This approach is evaluated through 2D computer-simulation studies for a small animal imaging model and by application to experimental in vivo measurements of a mouse.

Model-Based Learning for Accelerated, Limited-View 3-D Photoacoustic Tomography

Recent advances in deep learning for tomographic reconstructions have shown great potential to create accurate and high quality images with a considerable speed up. In this paper, we present a deep neural network that is specifically designed to provide high resolution 3-D images from restricted photoacoustic measurements. The network is designed to represent an iterative scheme and incorporates gradient information of the data fit to compensate for limited view artifacts. Due to the high complexity of the photoacoustic forward operator, we separate training and computation of the gradient information. A suitable prior for the desired image structures is learned as part of the training. The resulting network is trained and tested on a set of segmented vessels from lung computed tomography scans and then applied to in-vivo photoacoustic measurement data.

Stochastic Proximal Gradient Algorithms for Multi-Source Quantitative Photoacoustic Tomography

The development of accurate and efficient image reconstruction algorithms is a central aspect of quantitative photoacoustic tomography (QPAT). In this paper, we address this issues for multi-source QPAT using the radiative transfer equation (RTE) as accurate model for light transport. The tissue parameters are jointly reconstructed from the acoustical data measured for each of the applied sources. We develop stochastic proximal gradient methods for multi-source QPAT, which are more efficient than standard proximal gradient methods in which a single iterative update has complexity proportional to the number applies sources. Additionally, we introduce a completely new formulation of QPAT as multilinear (MULL) inverse problem which avoids explicitly solving the RTE. The MULL formulation of QPAT is again addressed with stochastic proximal gradient methods. Numerical results for both approaches are presented. Besides the introduction of stochastic proximal gradient algorithms to QPAT, we consider the new MULL formulation of QPAT as main contribution of this paper.

In Vivo Photoacoustic Imaging of Brain Injury and Rehabilitation by High-Efficient Near-Infrared Dye Labeled Mesenchymal Stem Cells with Enhanced Brain Barrier Permeability

Stem cell migration and interaction with pathology are critical to understand the complexity and status of disease recovery progress. However, the dynamic visualization still remains a great challenge due to imaging technical limitation, cell labeling difficulty, or blood-brain barrier (BBB). Herein, fast photoacoustic tomography (PAT) with optical molecular probes is applied to noninvasively monitor traumatic brain injury (TBI) and its rehabilitation. The vascular distribution and TBI hemorrhage are clearly imaged, longitudinally monitored, and quantified. Bone mesenchymal stem cells (BMSCs) labeled with modified Prussian blue particles (PBPs), excellent near-infrared dyes and photoacoustic contrasts, are intravenously injected to the mice for improved observation and efficient therapy. BMSCs are demonstrated to be capable of overcoming BBB with enhanced delivery of PBPs to the brain parenchyma. Notably, the versatile BMSCs are observed by PAT to home to the damage region and repair the ruptured vasculature. Moreover, the wound treated by BMSCs exhibits much faster recovery speed than that without treatment. These findings can potentially provide a new noninvasive and high-resolution approach to image TBI, monitor recovery process, and especially trace BMSCs. This study will stimulate extensive researches on brain diseases and provide promising strategies of dye labeled BMSCs in regenerative medicine.

Perspectives, potentials and trends of ex vivo and in vivo optical molecular pathology

It is pivotal for medical applications, such as noninvasive histopathologic characterization of tissue, to realize label-free and molecule-specific representation of morphologic and biochemical composition in real-time with subcellular spatial resolution. This unmet clinical need requires new approaches for rapid and reliable real-time assessment of pathologies to complement established diagnostic tools. Photonic imaging combined with digitalization offers the potential to provide the clinician the requested information both under in vivo and ex vivo conditions. This report summarizes photonic approaches and their use in combination with image processing, machine learning and augmented virtual reality that might solve current challenges in modern medicine. Details are given for pathology, intraoperative diagnosis in head and neck cancer and endoscopic diagnosis in gastroenterology.

Quantitative photoacoustic imaging of two-photon absorption

Photoacoustic tomography (PAT) is a hybrid imaging modality where we intend to reconstruct optical properties of heterogeneous media from measured ultrasound signals generated by the photoacoustic effect. In recent years, there have been considerable interests in using PAT to image two-photon absorption, in addition to the usual single-photon absorption, inside diffusive media. We present a mathematical model for quantitative image reconstruction in two-photon photoacoustic tomography (TP-PAT). We propose a computational strategy for the reconstruction of the optical absorption coefficients and provide some numerical evidences based on synthetic photoacoustic acoustic data to demonstrate the feasibility of quantitative reconstructions in TP-PAT.

Joint reconstruction of the initial pressure and speed of sound distributions from combined photoacoustic and ultrasound tomography measurements

The initial pressure and speed of sound (SOS) distributions cannot both be stably recovered from photoacoustic computed tomography (PACT) measurements alone. Adjunct ultrasound computed tomography (USCT) measurements can be employed to estimate the SOS distribution. Under the conventional image reconstruction approach for combined PACT/USCT systems, the SOS is estimated from the USCT measurements alone and the initial pressure is estimated from the PACT measurements by use of the previously estimated SOS. This approach ignores the acoustic information in the PACT measurements and may require many USCT measurements to accurately reconstruct the SOS. In this work, a joint reconstruction method where the SOS and initial pressure distributions are simultaneously estimated from combined PACT/USCT measurements is proposed. This approach allows accurate estimation of both the initial pressure distribution and the SOS distribution while requiring few USCT measurements.

Noncontact holographic detection for photoacoustic tomography

A holographic method for high-speed, noncontact photoacoustic tomography is introduced and evaluated. Relative changes of the object's topography, induced by the impact of thermoelastic pressure waves, were determined at nanometer sensitivity without physical contact. The object's surface was illuminated with nanosecond laser pulses and imaged with a high-speed CMOS camera. From two interferograms measured before and after excitation of the acoustic wave, surface displacement was calculated and then used as the basis for a tomographic reconstruction of the initial pressure caused by optical absorption. The holographic detection scheme enables variable sampling rates of the photoacoustic signal of up to 50 MHz. The total acquisition times for complete volumes with 230 MVoxel is far below 1 s. Measurements of silicone and porcine skin tissue phantoms with embedded artificial absorbers, which served as a model for human subcutaneous vascular networks, were possible. Three-dimensional reconstructions of the absorbing structures show details with a diameter of 310  μm up to a depth of 2.5 mm. Theoretical limitations and the experimental sensitivity, as well as the potential for in vivo imaging depending on the detection repetition rate, are analyzed and discussed.

Dynamic in vivo imaging of small animal brain using pulsed laser diode-based photoacoustic tomography system

We demonstrate dynamic in vivo imaging using a low-cost portable pulsed laser diode (PLD)-based photoacoustic tomography system. The system takes advantage of an 803-nm PLD having high-repetition rate ∼7000  Hz combined with a fast-scanning single-element ultrasound transducer leading to a 5 s cross-sectional imaging. Cortical vasculature is imaged in scan time of 5 s with high signal-to-noise ratio ∼48. To examine the ability for dynamic imaging, we monitored the fast uptake and clearance process of indocyanine green in the rat brain. The system will find applications to study neurofunctional activities, characterization of pharmacokinetic, and biodistribution profiles in the development process of drugs or imaging agents.

Oxygen breathing challenge- the simplest theranostic

Multispectral optoacoustic tomography provides insights into tumor vascular oxygenation with high temporal and spatial resolution non-invasively. New work indicates that a simple oxygen breathing challenge can reveal differences in tumor, potentially as a prognostic biomarker.

A High-performance Compact Photoacoustic Tomography System for In Vivo Small-animal Brain Imaging

In vivo small-animal imaging has an important role to play in preclinical studies. Photoacoustic tomography (PAT) is an emerging hybrid imaging modality that shows great potential for both preclinical and clinical applications. Conventional optical parametric oscillator-based PAT (OPO-PAT) systems are bulky and expensive and cannot provide high-speed imaging. Recently, pulsed-laser diodes (PLDs) have been successfully demonstrated as an alternative excitation source for PAT. Pulsed-laser diode PAT (PLD-PAT) has been successfully demonstrated for high-speed imaging on photoacoustic phantoms and biological tissues. This work provides a visualized experimental protocol for in vivo brain imaging using PLD-PAT. The protocol includes the compact PLD-PAT system configuration and its description, animal preparation for brain imaging, and a typical experimental procedure for 2D cross-sectional rat brain imaging. The PLD-PAT system is compact and cost-effective and can provide high-speed, high-quality imaging. Brain images collected in vivo at various scan speeds are presented.

Second generation slit-based photoacoustic tomography system for vascular imaging in human

Slit-based photoacoustic tomography is a newly developed technique that improves the elevation numerical aperture of a linear array through acoustic diffraction. The slit, placed at the acoustic focus of a linear array, effectively forms an array of virtual detectors with high receiving angle, which subsequently improves the elevation resolution. However, due to the complex implementation, our original system could only image phantoms and sacrificed animals. In this report, the system has been significantly improved. In particular, we designed a slit holder that can be directly mounted to the transducer array for easy adjustment of slit width and simultaneous scanning of both the array and the slit. To enlarge the imaging field of view, we replaced the single circular optical fiber bundle with a bifurcated line fiber bundle which moved simultaneously with the array and the slit. The data acquisition system has also been updated to double the imaging speed. With these improvements, the new system can image a 3.8 × 4 cm² region within 40 seconds and the object only needs to be coupled through ultrasound gel. We successfully used the system to image vasculatures in the palm and forearm of human volunteers. 3D palm vascular image of human palm.

[Non-contacting photoacoustic tomography in biological samples]

In photoacoustic imaging the ultrasonic signals are usually detected by contacting transducers. For some applications, contact with the tissue should be avoided, e.g. in those of brain functional imaging. As alternatives to contacting transducers interferometric techniques can be used to acquire photoacoustic signals remotely. Here, a system for non-contact photoacoustic tomography imaging (NCPAT) has been established. This approach enables NCPAT not to exceed laser exposure safety limits. The stimulated source of NCPAT utilized a laser with center wavelength of 532 nm and output intensity of 17.5 mJ/cm ², and a laser heterodyne interferometry was used to receive the photoacoustic signals. The NCPAT was used to implement on a rotational imaging geometry for photoacoustic tomography with a real-tissue phantom. The photoacoustic imaging was obtained by applying a reconstruction algorithm to the data acquired for NCPAT. Experiments results showed that the NCPAT system with detection 15 dB bandwidth of 2.25 MHz could resolve spherical optical inclusions with dimension of 500 μm and multi-layered structure with optical contrast in strongly scattering medium. The method could expand the scope of photoacoustic and ultrasonic technology to in-vivo biomedical applications where contact is impractical.

Use of a single xenon flash lamp for photoacoustic computed tomography of multiple-centimeter-thick biological tissue ex vivo and a whole mouse body in vivo

While lasers have been commonly used as illumination sources in photoacoustic (PA) imaging, their high purchase and maintenance costs, as well as their bulkiness, have hindered the rapid clinical dissemination of PA imaging. With this in mind, we explore an alternative illumination source for PA tomography—a xenon flash lamp with high pulse energy and a microsecond pulse width. We demonstrate that, by using a single xenon flash lamp, we can image both a black latex cord placed in chicken breast tissue at a depth of up to 3.5 cm ex vivo and an entire mouse body in vivo. Our findings indicate that the xenon flash lamp, producing optical illumination that is safe for humans, can be potentially applied to human tissue imaging.