Segmentation of vessel structures from photoacoustic images with reliability assessment

Photoacoustic imaging enables the imaging of soft biological tissue with combined optical contrast and ultrasound resolution. One of the targets of interest is tissue vasculature. However, the photoacoustic images may not directly provide the information on, for example, vasculature structure. Therefore, the images are improved by reducing noise and artefacts, and processed for better visualisation of the target of interest. In this work, we present a new segmentation method of photoacoustic images that also straightforwardly produces assessments of its reliability. The segmentation depends on parameters which have a natural tendency to increase the reliability as the parameter values monotonically change. The reliability is assessed by counting classifications of image voxels with different parameter values. The resulting segmentation with reliability offers new ways and tools to analyse photoacoustic images and new possibilities for utilising them as anatomical priors in further computations. Our MATLAB implementation of the method is available as an open-source software package [P. Raumonen, Matlab, 2018].

Photoacoustic image formation based on sparse regularization of minimum variance beamformer

Delay-and-sum (DAS) is the most common algorithm used in photoacoustic (PA) image formation. However, this algorithm results in a reconstructed image with a wide mainlobe and high level of sidelobes. Minimum variance (MV), as an adaptive beamformer, overcomes these limitations and improves the image resolution and contrast. In this paper, a novel algorithm, named Modified-Sparse-MV (MS-MV), is proposed in which a ℓ 1-norm constraint is added to the MV minimization problem after some modifications, in order to suppress the sidelobes more efficiently, compared to MV. The added constraint can be interpreted as the sparsity of the output of the MV beamformed signals. Since the final minimization problem is convex, it can be solved efficiently using a simple iterative algorithm. The numerical results show that the proposed method, MS-MV beamformer, improves the signal-to-noise (SNR) about 19.48 dB, in average, compared to MV. Also, the experimental results, using a wire-target phantom, show that MS-MV leads to SNR improvement of about 2.64 dB in comparison with the MV.

A handheld microscope integrating photoacoustic microscopy and optical coherence tomography

The combination of optical resolution photoacoustic microscopy (ORPAM) and optical coherence tomography (OCT) is capable of providing complementary imaging contrasts. Unfortunately, the miniaturization of ORPAM remains a major challenge in the development of a handheld dual-modality imaging microscope with OCT. Here, we report the design and evaluation of an integrated ORPAM and OCT imaging probe using a two-dimensional MEMS (micro-electro-mechanical-system)-based optical scanner. This microscope, weighting 35.4 g, has an ultracompact size of 65×30×18 mm³, and an effective imaging area of 2×2 mm². The experimental lateral resolutions are 3.7 μm (ORPAM) and 5.6 μm (OCT), and the axial resolutions are measured as 120 μm (ORPAM) and 7.3 μm (OCT). Besides phantom and animal experiments, we carried out oral imaging of a healthy volunteer to show the clinical feasibility of this technique.

Performance of optoacoustic and fluorescence imaging in detecting deep-seated fluorescent agents

Fluorescent contrast agents are widely employed in biomedical research. While many studies have reported deep tissue imaging of fluorescent moieties using either fluorescence-based or absorption-based (optoacoustic) imaging systems, no systematic comparison has been performed regarding the actual performance of these imaging modalities in detecting deep-seated fluorescent agents. Herein, an integrated imager combining epi-fluorescence and volumetric optoacoustic imaging capabilities has been employed in order to evaluate image degradation with depth for several commonly-used near-infrared dyes in both modes. We performed controlled experiments in tissue-mimicking phantoms containing deeply embedded targets filled with different concentrations of Alexa Fluor 700, Alexa Fluor 750, indocyanine green (ICG) and IRDye 800CW. The results are further corroborated by multi-modal imaging of ICG through mouse tissues in vivo. It is shown that optoacoustics consistently provides better sensitivity in differentiating fluorescent targets located at depths beyond 2 mm in turbid tissues, as quantified by evaluating image contrast, signal to noise ratio and spatial resolution performance.

Linear array-based real-time photoacoustic imaging system with a compact coaxial excitation handheld probe for noninvasive sentinel lymph node mapping

We developed a linear ultrasound array-based real-time photoacoustic imaging system with a compact coaxial excitation handheld photoacoustic imaging probe for guiding sentinel lymph node (SLN) needle biopsy. Compared with previous studies, our system and probe have the following advantages: (1) the imaging probe is quite compact and user-friendly; (2) laser illumination and ultrasonic detection are achieved coaxially, enabling high signal-to-noise ratio; and (3) GPU-based image reconstruction enables real-time imaging and displaying at a frame rate of 20 Hz. With the system and probe, clear visualization of the SLN at the depth of 2 cm (~human SLN depth) was demonstrated on a living rat. A fine needle was pushed towards the SLN based on the guidance of real-time photoacoustic imaging. The proposed photoacoustic imaging system and probe was shown to have great potential to be used in clinics for guiding SLN needle biopsy, which may reduce the high morbidity rate related to the current gold standard clinical SLN biopsy procedure.

Optical resolution photoacoustic microscopy based on multimode fibers

Photoacoustic microscopy (PAM) is a multiscale imaging technique. In optical-resolution photoacoustic microscopy (OR-PAM), a single mode (SM) fiber is normally used as the source of optical excitation to be focused into a diffraction-limited spot. Recent advances in OR-PAM have improved its imaging speed using microelectromechanical systems (MEMS). Here we report for the first time the use of a multimode (MM) fiber as the optical excitation source for high resolution OR-PAM in vivo imaging. A high-speed MEMS scanner based OR-PAM system combined with the mechanical movement to provide wide area imaging was used. The use of multimode fiber for achieving tight optical focus would make the optical alignment easier and high repetition rate light delivery possible for high-speed OR-PAM imaging. A lateral resolution of 3.5 µm and axial resolution of 27 µm with ~1.5 mm imaging depth was successfully demonstrated using the system. The efficacy of multimode fibers for achieving tight focus is beneficial for developing high-resolution photoacoustic endoscopy systems and can be combined with other optical endoscopic imaging modalities as well.

Gel wax-based tissue-mimicking phantoms for multispectral photoacoustic imaging

Tissue-mimicking phantoms are widely used for the calibration, evaluation and standardisation of medical imaging systems, and for clinical training. For photoacoustic imaging, tissue-mimicking materials (TMMs) that have tuneable optical and acoustic properties, high stability, and mechanical robustness are highly desired. In this study, gel wax is introduced as a TMM that satisfies these criteria for developing photoacoustic imaging phantoms. The reduced scattering and optical absorption coefficients were independently tuned with the addition of TiO₂ and oil-based inks. The frequency-dependent acoustic attenuation obeyed a power law; for native gel wax, it varied from 0.71 dB/cm at 3 MHz to 9.93 dB/cm at 12 MHz. The chosen oil-based inks, which have different optical absorption spectra in the range of 400 to 900 nm, were found to have good photostability under pulsed illumination with photoacoustic excitation light. Optically heterogeneous phantoms that comprised of inclusions with different concentrations of carbon black and coloured inks were fabricated, and multispectral photoacoustic imaging was performed with an optical parametric oscillator and a planar Fabry-Pérot sensor. We conclude that gel wax is well suited as a TMM for multispectral photoacoustic imaging.

Large area laser scanning optical resolution photoacoustic microscopy using a fibre optic sensor

A laser scanning optical resolution photoacoustic microscopy (LS OR-PAM) system based on a stationary fibre optic sensor is described. The sensor comprises an optically resonant interferometric polymer cavity formed on the tip of a rounded single mode optical fibre. It provides low noise equivalent pressure (NEP = 68.7 Pa over a 20 MHz measurement bandwidth), a broad bandwidth that extends to 80 MHz and a near omnidirectional response. The latter is a significant advantage, as it allows large areas (>1cm²) to be imaged without the need for translational mechanical scanning offering the potential for fast image acquisition. The system provides a lateral resolution of 8 µm, an axial resolution of 21 µm, and a field of view up to 10 mm × 10 mm. To demonstrate the system, in vivo 3D structural images of the microvasculature of a mouse ear were obtained, showing single capillaries overlaying larger vessels as well as functional images revealing blood oxygen saturation.

Optoacoustic imaging at kilohertz volumetric frame rates

State-of-the-art optoacoustic tomographic imaging systems have been shown to attain three-dimensional (3D) frame rates of the order of 100 Hz. While such a high volumetric imaging speed is beyond reach for other bio-imaging modalities, it may still be insufficient to accurately monitor some faster events occurring on a millisecond scale. Increasing the 3D imaging rate is usually hampered by the limited throughput capacity of the data acquisition electronics and memory used to capture vast amounts of the generated optoacoustic (OA) data in real time. Herein, we developed a sparse signal acquisition scheme and a total-variation-based reconstruction approach in a combined space-time domain in order to achieve 3D OA imaging at kilohertz rates. By continuous monitoring of freely swimming zebrafish larvae in a 3D region, we demonstrate that the new approach enables significantly increasing the volumetric imaging rate by using a fraction of the tomographic projections without compromising the reconstructed image quality. The suggested method may benefit studies looking at ultrafast biological phenomena in 3D, such as large-scale neuronal activity, cardiac motion, or freely behaving organisms.

Multispectral photoacoustic microscopy of lipids using a pulsed supercontinuum laser

We demonstrate optical resolution photoacoustic microscopy (OR-PAM) of lipid-rich tissue between 1050-1714 nm using a pulsed supercontinuum laser based on a large-mode-area photonic crystal fiber. OR-PAM experiments of lipid-rich samples show the expected optical absorption peaks near 1210 and 1720 nm. These results show that pulsed supercontinuum lasers are promising for OR-PAM applications such as label-free histology of lipid-rich tissue and imaging small animal models of disease.

Intracerebral haemorrhage-induced injury progression assessed by cross-sectional photoacoustic tomography

In this study, we in vivo examined injury progression after intracerebral haemorrhage (ICH) induced by collagenase in mice using cross-sectional photoacoustic tomography (csPAT). csPAT displayed high resolution with high sensitivity for ICH detection. The PAT images obtained showed high correlation with conventional histologic images. Quantitative analysis of the hematoma areas detected by csPAT showed high consistency with the neurologic deficit score (NDS). By utilizing the dual-wavelength method, the development of the hemoglobin area was monitored. Our results indicated that noninvasive csPAT can be used to track the dynamic progression of post-ICH, and to evaluate therapeutic interventions in preclinical ICH models.

In vivo studies of transdermal nanoparticle delivery with microneedles using photoacoustic microscopy

Microneedle technology allows micron-sized conduits to be formed within the outermost skin layers for both localized and systemic delivery of therapeutics including nanoparticles. Histological methods are often employed for characterization, and unfortunately do not allow for the in vivo visualization of the delivery process. This study presents the utilization of optical resolution-photoacoustic microscopy to characterize the transdermal delivery of nanoparticles using microneedles. Specifically, we observe the in vivo transdermal delivery of gold nanoparticles using microneedles in mice ear and study the penetration, diffusion, and spatial distribution of the nanoparticles in the tissue. The promising results reveal that photoacoustic microscopy can be used as a potential imaging modality for the in vivo characterization of microneedles based drug delivery.

Quantitative photoacoustic tomography augmented with surface light measurements

Quantitative photoacoustic tomography is an imaging modality in which distributions of optical parameters inside tissue are estimated from photoacoustic images. This optical parameter estimation is an ill-posed problem and it needs to be approached in the framework of inverse problems. In this work, utilising surface light measurements in quantitative photoacoustic tomography is studied. Estimation of absorption and scattering is formulated as a minimisation problem utilising both internal quantitative photoacoustic data and surface light data. The image reconstruction problem is studied with two-dimensional numerical simulations in various imaging situations using the diffusion approximation as the model for light propagation. The results show that quantitative photoacoustic tomography augmented with surface light data can improve both absorption and scattering estimates when compared to the conventional quantitative photoacoustic tomography.

Photoacoustic/ultrasound dual imaging of human thyroid cancers: an initial clinical study

We reported an initial clinical study of in vivo human thyroid by a photoacoustic/ultrasound handheld probe. Our dual-modality system is based on a high-end clinical ultrasound machine. Both healthy and cancerous thyroids were imaged non-invasively, and we compared the photoacoustic imaging with color Doppler ultrasound. The results of photoacoustic thyroid imaging could reveal many blood vessels that were not sensitive for Doppler ultrasound. Our study demonstrated that photoacoustic imaging could provide important complementary information for traditional ultrasound thyroid examination, which has a great potential for clinical diagnosis.

Flow-mediated dilatation test using optoacoustic imaging: a proof-of-concept

Label-free multispectral optoacoustic tomography (MSOT) has recently shown superior performance in visualizing the morphology of human vasculature, especially of smaller vessels, compared to ultrasonography. Herein, we extend these observations towards MSOT interrogation of macrovascular endothelial function. We employed a real-time handheld MSOT scanner to assess flow-mediated dilatation (FMD), a technique used to characterize endothelial function. A data processing scheme was developed to quantify the dimensions and diameter changes of arteries in humans and determine wall distensibility parameters. By enabling high-resolution delineation of the blood-vessel wall in a cross-sectional fashion, the findings suggest MSOT as a capable alternative to ultrasonography for clinical FMD measurements.

Quantifying melanin concentration in retinal pigment epithelium using broadband photoacoustic microscopy

Melanin is the dominant light absorber in retinal pigment epithelium (RPE). The loss of RPE melanin is a sign of ocular senescence and is both a risk factor and a symptom of age-related macular degeneration (AMD). Quantifying the RPE melanin concentration provides insight into the pathological role of RPE in ocular aging and the onset and progression of AMD. The main challenge in accurate quantification of RPE melanin concentration is to distinguish this ten-micrometer-thick cell monolayer from the underlying choroid, which also contains melanin but carries different pathognomonic information. In this work, we investigated a three-dimensional photoacoustic microscopic (PAM) method with high axial resolution, empowered by broad acoustic detection bandwidth, to distinguish RPE from choroid and quantify melanin concentrations in the RPE ex vivo. We first conducted numerical simulation on photoacoustic generation in the RPE, which suggested that a PAM system with at least 100-MHz detection bandwidth provided sufficient axial resolution to distinguish the melanin in RPE from that in choroid. Based on simulation results, we integrated a transparent broadband micro-ring resonator (MRR) based detector in a homebuilt PAM system. We imaged ex vivo RPE-choroid complexes (RCCs) from both porcine and human eyes and quantified the absolute melanin concentrations in the RPE and choroid, respectively. In our study, the measured melanin concentrations were 14.7 mg/mL and 17.0 mg/mL in human and porcine RPE, and 12 mg/mL and 61 mg/mL in human and porcine choroid, respectively. This study suggests that broadband PAM is capable of quantifying the RPE melanin concentration from RCCs ex vivo.

In vivo photoacoustics and high frequency ultrasound imaging of mechanical high intensity focused ultrasound (HIFU) ablation

The thermal effect of high intensity focused ultrasound (HIFU) has been clinically exploited over a decade, while the mechanical HIFU is still largely confined to laboratory investigations. This is in part due to the lack of adequate imaging techniques to better understand the in-vivo pathological and immunological effects caused by the mechanical treatment. In this work, we explore the use of high frequency ultrasound (US) and photoacoustics (PA) as a potential tool to evaluate the effect of mechanical ablation in-vivo, e.g. boiling histotripsy. Two mice bearing a neuroblastoma tumor in the right leg were ablated using an MRI-HIFU system conceived for small animals and monitored using MRI thermometry. High frequency US and PA imaging were performed before and after the HIFU treatment. Afterwards, the tumor was resected for further assessment and evaluation of the ablated region using histopathology. High frequency US imaging revealed the presence of liquefied regions in the treated area together with fragmentized tissue which appeared with different reflecting proprieties compared to the surrounding tissue. Photoacoustic imaging on the other hand revealed the presence of deoxygenated blood within the tumor after the ablation due to the destruction of blood vessel network while color Doppler imaging confirmed the blood vessel network destruction within the tumor. The treated area and the presence of red blood cells detected by photoacoustics were further confirmed by the histopathology. This feasibility study demonstrates the potential of high frequency US and PA approach for assessing in-vivo the effect of mechanical HIFU tumor ablation.

Label-free photoacoustic imaging of the cardio-cerebrovascular development in the embryonic zebrafish

Zebrafish play an important role in biology, pharmacology, toxicology, and medicine. The cardio-cerebrovascular development of zebrafish is particularly critical to understand both brain disorders and cardiovascular diseases in human. In this paper, we applied optical resolution photoacoustic microscopy (ORPAM) to image the whole-body vasculature of the embryonic zebrafish with a special focus on the development of the cardio-cerebrovascular system. Using the intrinsic optical absorption contrast of the embryo, we successfully visualized the formation of the cardio-cerebrovascular network in high-resolution using a 10 × objective, and monitored the whole-body vascular development using a 4 × objective. In addition, we evaluated the impact of the eggshell and pigment inhibitor on the image quality. Our results suggest that ORPAM is capable of studying the cardio-cerebrovascular development of zebrafish in the embryonic stage, and thus has the potential to investigate the cardiovascular and cerebrovascular diseases of human in the future.

In vivo photoacoustic lipid imaging in mice using the second near-infrared window

Photoacoustic imaging has emerged as a promising technique to improve preclinical and clinical imaging by providing users with label-free optical contrast of tissue. Here, we present a proof-of-concept study for noninvasive in vivo murine lipid imaging using 1210 nm light to investigate differences in periaortic fat among mice of different gender, genotypes, and maturation. Acquired lipid signals suggest that adult male apoE^-/- mice have greater periaortic fat accumulation compared to adolescent males, apoE^-/- females, and wild-type mice. These results demonstrate the potential of photoacoustic tomography for studying vascular pathophysiology and improving the diagnosis of lipid-based diseases.

Real-time volumetric lipid imaging in vivo by intravascular photoacoustics at 20 frames per second

Lipid deposition can be assessed with combined intravascular photoacoustic/ultrasound (IVPA/US) imaging. To date, the clinical translation of IVPA/US imaging has been stalled by a low imaging speed and catheter complexity. In this paper, we demonstrate imaging of lipid targets in swine coronary arteries in vivo, at a clinically useful frame rate of 20 s-1. We confirmed image contrast for atherosclerotic plaque in human samples ex vivo. The system is on a mobile platform and provides real-time data visualization during acquisition. We achieved an IVPA signal-to-noise ratio of 20 dB. These data show that clinical translation of IVPA is possible in principle.

Deep tissue photoacoustic computed tomography with a fast and compact laser system

Photoacoustic computed tomography (PACT) holds great promise for biomedical imaging, but wide-spread implementation is impeded by the bulkiness of flash-lamp-pumped laser systems, which typically weigh between 50 - 200 kg, require continuous water cooling, and operate at a low repetition rate. Here, we demonstrate that compact lasers based on emerging diode technologies are well-suited for preclinical and clinical PACT. The diode-pumped laser used in this study had a miniature footprint (13 × 14 × 7 cm³), weighed only 1.6 kg, and outputted up to 80 mJ per pulse at 1064 nm. In vitro, the laser system readily provided over 4 cm PACT depth in chicken breast tissue. In vivo, in addition to high resolution, non-invasive brain imaging in living mice, the system can operate at 50 Hz, which enabled high-speed cross-sectional imaging of murine cardiac and respiratory function. The system also provided high quality, high-frame rate, and non-invasive three-dimensional mapping of arm, palm, and breast vasculature at multi centimeter depths in living human subjects, demonstrating the clinical viability of compact lasers for PACT.

Identification and removal of laser-induced noise in photoacoustic imaging using singular value decomposition

Singular value decomposition (SVD) was used to identify and remove laser-induced noise in photoacoustic images acquired with a clinical ultrasound scanner. This noise, which was prominent in the radiofrequency data acquired in parallel from multiple transducer elements, was induced by the excitation light source. It was modelled by truncating the SVD matrices so that only the first few largest singular value components were retained, and subtracted prior to image reconstruction. The dependency of the signal amplitude and the number of the largest singular value components used for noise modeling was investigated for different photoacoustic source geometries. Validation was performed with simulated data and measured noise, and with photoacoustic images acquired from the human forearm and finger in vivo using L14-5/38 and L40-8/12 linear array clinical imaging probes. The use of only one singular value component was found to be sufficient to achieve near-complete removal of laser-induced noise from reconstructed images. This method has strong potential to increase image quality for a wide range of photoacoustic imaging systems with parallel data acquisition.

Quantitative photoacoustic image reconstruction improves accuracy in deep tissue structures

Photoacoustic imaging (PAI) is emerging as a potentially powerful imaging tool with multiple applications. Image reconstruction for PAI has been relatively limited because of limited or no modeling of light delivery to deep tissues. This work demonstrates a numerical approach to quantitative photoacoustic image reconstruction that minimizes depth and spectrally derived artifacts. We present the first time-domain quantitative photoacoustic image reconstruction algorithm that models optical sources through acoustic data to create quantitative images of absorption coefficients. We demonstrate quantitative accuracy of less than 5% error in large 3 cm diameter 2D geometries with multiple targets and within 22% error in the largest size quantitative photoacoustic studies to date (6cm diameter). We extend the algorithm to spectral data, reconstructing 6 varying chromophores to within 17% of the true values. This quantitiative PA tomography method was able to improve considerably on filtered-back projection from the standpoint of image quality, absolute, and relative quantification in all our simulation geometries. We characterize the effects of time step size, initial guess, and source configuration on final accuracy. This work could help to generate accurate quantitative images from both endogenous absorbers and exogenous photoacoustic dyes in both preclinical and clinical work, thereby increasing the information content obtained especially from deep-tissue photoacoustic imaging studies.

Coherent-weighted three-dimensional image reconstruction in linear-array-based photoacoustic tomography

While the majority of photoacoustic imaging systems used custom-made transducer arrays, commercially-available linear transducer arrays hold the benefits of affordable price, handheld convenience and wide clinical recognition. They are not widely used in photoacoustic imaging primarily because of the poor elevation resolution. Here, without modifying the imaging geometry and system, we propose addressing this limitation purely through image reconstruction. Our approach is based on the integration of two advanced image reconstruction techniques: focal-line-based three-dimensional image reconstruction and coherent weighting. We first numerically validated our approach through simulation and then experimentally tested it in phantom and in vivo. Both simulation and experimental results proved that the method can significantly improve the elevation resolution (up to 4 times in our experiment) and enhance object contrast.

High power visible light emitting diodes as pulsed excitation sources for biomedical photoacoustics

The use of visible light emitting diodes (LEDs) as an alternative to Q-switched lasers conventionally used as photoacoustic excitation sources has been explored. In common with laser diodes, LEDs offer the advantages of compact size, low cost and high efficiency. However, laser diodes suitable for pulsed photoacoustic generation are typically available only at wavelengths greater than 750nm. By contrast, LEDs are readily available at visible wavelengths below 650nm where haemoglobin absorption is significantly higher, offering the prospect of increased SNR for superficial vascular imaging applications. To demonstrate feasibility, a range of low cost commercially available LEDs operating in the 420-620nm spectral range were used to generate photoacoustic signals in physiologically realistic vascular phantoms. Overdriving with 200ns pulses and operating at a low duty cycle enabled pulse energies up to 10µJ to be obtained with a 620nm LED. By operating at a high pulse repetition frequency (PRF) in order to rapidly signal average over many acquisitions, this pulse energy was sufficient to generate detectable signals in a blood filled tube immersed in an Intralipid suspension (µs' = 1mm(-1)) at a depth of 15mm using widefield illumination. In addition, a compact four-wavelength LED (460nm, 530nm, 590nm, 620nm) in conjunction with a coded excitation scheme was used to illustrate rapid multiwavelength signal acquisition for spectroscopic applications. This study demonstrates that LEDs could find application as inexpensive and compact multiwavelength photoacoustic excitation sources for imaging superficial vascular anatomy. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Photoacoustic clutter reduction by inversion of a linear scatter model using plane wave ultrasound measurements

Photoacoustic imaging aims to visualize light absorption properties of biological tissue by receiving a sound wave that is generated inside the observed object as a result of the photoacoustic effect. In clinical applications, the strong light absorption in human skin is a major problem. When high amplitude photoacoustic waves that originate from skin absorption propagate into the tissue, they are reflected back by acoustical scatterers and the reflections contribute to the received signal. The artifacts associated with these reflected waves are referred to as clutter or skin echo and limit the applicability of photoacoustic imaging for medical applications severely. This study seeks to exploit the acoustic tissue information gained by plane wave ultrasound measurements with a linear array in order to correct for reflections in the photoacoustic image. By deriving a theory for clutter waves in k-space and a matching inversion approach, photoacoustic measurements compensated for clutter are shown to be recovered.

Toward in vivo biopsy of melanoma based on photoacoustic and ultrasound dual imaging with an integrated detector

Melanoma is the most dangerous type of skin cancer with high lethal rate. Tumor thickness and tumor-associated vasculature are two key parameters for staging melanoma. Previous techniques for diagnosing melanoma have insurmountable restrictions, such as invasive, low specificity, or inaccurate depth measurement. Here we develop an integrated photoacoustic (PA) and ultrasound (US) imaging system dedicated to overcome these limitations. An integrated detector with sound-light coaxial/confocal design and flexible coupling mode is employed for the combined PA/US imaging strategy. PA imaging results enable a clear characterization of tumor angiogenesis with high resolution and high contrast. Furthermore, accurate thickness measurements of melanoma in different stages can be resolved with the simultaneously obtained PA/US image. Phantom experiments and in vivo animal experimental results demonstrate the integrated PA/US system could provide potential for noninvasive biopsy of melanoma.

High frame rate photoacoustic imaging at 7000 frames per second using clinical ultrasound system

Photoacoustic tomography, a hybrid imaging modality combining optical and ultrasound imaging, is gaining attention in the field of medical imaging. Typically, a Q-switched Nd:YAG laser is used to excite the tissue and generate photoacoustic signals. But, such photoacoustic imaging systems are difficult to translate into clinical applications owing to their high cost, bulky size often requiring an optical table to house such lasers. Moreover, the low pulse repetition rate of few tens of hertz prevents them from being used in high frame rate photoacoustic imaging. In this work, we have demonstrated up to 7000 Hz photoacoustic imaging (B-mode) and measured the flow rate of a fast moving object. We used a ~140 nanosecond pulsed laser diode as an excitation source and a clinical ultrasound imaging system to capture and display the photoacoustic images. The excitation laser is ~803 nm in wavelength with ~1.4 mJ energy per pulse. So far, the reported 2-dimensional photoacoustic B-scan imaging is only a few tens of frames per second using a clinical ultrasound system. Therefore, this is the first report on 2-dimensional photoacoustic B-scan imaging with 7000 frames per second. We have demonstrated phantom imaging to view and measure the flow rate of ink solution inside a tube. This fast photoacoustic imaging can be useful for various clinical applications including cardiac related problems, where the blood flow rate is quite high, or other dynamic studies.

Real-time GPU-accelerated processing and volumetric display for wide-field laser-scanning optical-resolution photoacoustic microscopy

Fast signal processing and real-time displays are essential for practical imaging modality in various fields of applications. However, the imaging speed in optical-resolution photoacoustic microscopy (OR-PAM), in particular, depends on factors such as the pulse repetition rate of the laser, scanning method, field of view (FOV), and signal processing time. In the past, efforts to increase acquisition speed either focused on developing new scanning methods or using lasers with higher pulse repetition rates. However, high-speed signal processing is also important for real-time volumetric display in OR-PAM. In this study, we carried out parallel signal processing using a graphics processing unit (GPU) to enable fast signal processing and wide-field real-time displays in laser-scanning OR-PAM. The average total GPU processing time for a B-mode PAM image was approximately 1.35 ms at a display speed of 480 fps when the data samples were acquired with 736 (axial) × 500 (lateral) points/B-mode-frame at a pulse repetition rate of 300 kHz. In addition, we successfully displayed maximum amplitude projection images of a mouse's ear as volumetric images with an FOV of 3 mm × 3 mm (500 × 500 pixels) at 1.02 s, corresponding to 0.98 fps.

In vivo cell characteristic extraction and identification by photoacoustic flow cytography

We present a photoacoustic flow cytography with fast cross-sectional (B-scan) imaging to precisely identify specific cells in vivo. The B-scan imaging speed of the system is up to 200 frame/s with a lateral resolution of 1.5 μm, which allows to dynamically image the flowing cells within the microvascular. The shape, size and photoacoustic intensity of the target cells are extracted from streaming images and integrated into a standard pattern to distinguish cell types. Circulating red blood cells and melanoma cells in blood vessels are simultaneously identified on melanoma-bearing mouse model. The results demonstrate that in vivo photoacoustic flow cytography can provide cells characteristics analysis and cell type's visual identification, which will be applied for noninvasively monitoring circulating tumor cells (CTCs) and analyzing hematologic diseases.

Specific imaging of atherosclerotic plaque lipids with two-wavelength intravascular photoacoustics

The lipid content in plaques is an important marker for identifying atherosclerotic lesions and disease states. Intravascular photoacoustic (IVPA) imaging can be used to visualize lipids in the artery. In this study, we further investigated lipid detection in the 1.7-µm spectral range. By exploiting the relative difference between the IVPA signal strengths at 1718 and 1734 nm, we could successfully detect and differentiate between the plaque lipids and peri-adventitial fat in human coronary arteries ex vivo. Our study demonstrates that IVPA imaging can positively identify atherosclerotic plaques using only two wavelengths, which could enable rapid data acquisition in vivo.

Optical mesoscopy without the scatter: broadband multispectral optoacoustic mesoscopy

Optical mesoscopy extends the capabilities of biological visualization beyond the limited penetration depth achieved by microscopy. However, imaging of opaque organisms or tissues larger than a few hundred micrometers requires invasive tissue sectioning or chemical treatment of the specimen for clearing photon scattering, an invasive process that is regardless limited with depth. We developed previously unreported broadband optoacoustic mesoscopy as a tomographic modality to enable imaging of optical contrast through several millimeters of tissue, without the need for chemical treatment of tissues. We show that the unique combination of three-dimensional projections over a broad 500 kHz-40 MHz frequency range combined with multi-wavelength illumination is necessary to render broadband multispectral optoacoustic mesoscopy (2B-MSOM) superior to previous optical or optoacoustic mesoscopy implementations.

Near-infrared spectroscopic photoacoustic microscopy using a multi-color fiber laser source

We demonstrate a simple multi-wavelength optical source suitable for spectroscopic optical resolution photoacoustic microscopy (OR-PAM) of lipid-rich tissue. 1064 nm laser pulses are converted to multiple wavelengths beyond 1300 nm via nonlinear optical propagation in a birefringent optical fiber. OR-PAM experiments with lipid phantoms clearly show the expected absorption peak near 1210 nm. We believe this simple multi-color technique is a promising cost-effective approach to spectroscopic OR-PAM of lipid-rich tissue.

Assessing breast tumor margin by multispectral photoacoustic tomography

An unmet need exists in high-speed and highly-sensitive intraoperative assessment of breast cancer margin during conservation surgical procedures. Here, we demonstrate a multispectral photoacoustic tomography system for breast tumor margin assessment using fat and hemoglobin as contrasts. This system provides ~3 mm tissue depth and ~125 μm axial resolution. The results agreed with the histological findings. A high sensitivity in margin assessment was accomplished, which opens a compelling way to intraoperative margin assessment.

Photoacoustic imaging of acupuncture effect in small animals

Acupuncture has been a powerful clinical tool for treating chronic diseases. However, there is currently no appropriate method to clarify the therapeutic effect of acupuncture. Here, we use photoacoustic tomography (PAT) to study the effect of acupuncture on mouse brain blood vessels. Ten healthy mice were stimulated with acupuncture needles on two acupoints. PAT images were obtained before and after acupuncture. We report that stimulation of certain acupoints resulted in changes in hemodynamics/blood flow at these points. The results demonstrate that PAT can non-invasively detect blood flow changes in mouse brain under acupuncture. This pilot study shows the potential of PAT as a visualization tool for illuminating the mechanism of acupuncture and promoting its clinical applications.

A low-cost photoacoustic microscopy system with a laser diode excitation

Photoacoustic microscopy (PAM) is capable of mapping microvasculature networks in biological tissue and has demonstrated great potential for biomedical applications. However, the clinical application of the PAM system is limited due to the use of bulky and expensive pulsed laser sources. In this paper, a low-cost optical-resolution PAM system with a pulsed laser diode excitation has been introduced. The lateral resolution of this PAM system was estimated to be 7 µm by imaging a carbon fiber. The phantoms made of polyethylene tubes filled with blood and a mouse ear were imaged to demonstrate the feasibility of this PAM system for imaging biological tissues.

Photoacoustic and ultrasound imaging using dual contrast perfluorocarbon nanodroplets triggered by laser pulses at 1064 nm

Recently, a dual photoacoustic and ultrasound contrast agent-named photoacoustic nanodroplet-has been introduced. Photoacoustic nanodroplets consist of a perfluorocarbon core, surfactant shell, and encapsulated photoabsorber. Upon pulsed laser irradiation the perfluorocarbon converts to gas, inducing a photoacoustic signal from vaporization and subsequent ultrasound contrast from the resulting gas microbubbles. In this work we synthesize nanodroplets which encapsulate gold nanorods with a peak absorption near 1064 nm. Such nanodroplets are optimal for extended photoacoustic imaging depth and contrast, safety and system cost. We characterized the nanodroplets for optical absorption, image contrast and vaporization threshold. We then imaged the particles in an ex vivo porcine tissue sample, reporting contrast enhancement in a biological environment. These 1064 nm triggerable photoacoustic nanodroplets are a robust biomedical tool to enhance image contrast at clinically relevant depths.

The potential of photoacoustic microscopy as a tool to characterize the in vivo degradation of surgical sutures

The ex vivo and in vivo imaging, and quantitative characterization of the degradation of surgical sutures (∼500 μm diameter) up to ∼1cm depth is demonstrated using a custom dark-field photo-acoustic microscope (PAM). A practical algorithm is developed to accurately measure the suture diameter during the degradation process. The results from tissue simulating phantoms and mice are compared to ex vivo measurements with an optical microscope demonstrating that PAM has a great deal of potential to characterize the degradation process of surgical sutures. The implications of this work for industrial applications are discussed.

Integrated photoacoustic, ultrasound and fluorescence platform for diagnostic medical imaging-proof of concept study with a tissue mimicking phantom

The structural and molecular heterogeneities of biological tissues demand the interrogation of the samples with multiple energy sources and provide visualization capabilities at varying spatial resolution and depth scales for obtaining complementary diagnostic information. A novel multi-modal imaging approach that uses optical and acoustic energies to perform photoacoustic, ultrasound and fluorescence imaging at multiple resolution scales from the tissue surface and depth is proposed in this paper. The system comprises of two distinct forms of hardware level integration so as to have an integrated imaging system under a single instrumentation set-up. The experimental studies show that the system is capable of mapping high resolution fluorescence signatures from the surface, optical absorption and acoustic heterogeneities along the depth (>2cm) of the tissue at multi-scale resolution (<1µm to <0.5mm).

In-Vivo functional optical-resolution photoacoustic microscopy with stimulated Raman scattering fiber-laser source

In this paper a multi-wavelength optical-resolution photoacoustic microscopy (OR-PAM) system using stimulated Raman scattering is demonstrated for both phantom and in vivo imaging. A 1-ns pulse width ytterbium-doped fiber laser is coupled into a single-mode polarization maintaining fiber. Discrete Raman-shifted wavelength peaks extending to nearly 800 nm are generated with pulse energies sufficient for OR-PAM imaging. Bandpass filters are used to select imaging wavelengths. A dual-mirror galvanometer system was used to scan the focused outputs across samples of carbon fiber networks, 200μm dye-filled tubes, and Swiss Webster mouse ears. Photoacoustic signals were collected in transmission mode and used to create maximum amplitude projection C-scan images. Double dye experiments and in vivo oxygen saturation estimation confirmed functional imaging potential.

Design and evaluation of a laboratory prototype system for 3D photoacoustic full breast tomography

Photoacoustic imaging can visualize vascularization-driven optical absorption contrast with great potential for breast cancer detection and diagnosis. State-of-the-art photoacoustic breast imaging systems are promising but are limited either by only a 2D imaging capability or by an insufficient imaging field-of-view (FOV). We present a laboratory prototype system designed for 3D photoacoustic full breast tomography, and comprehensively characterize it and evaluate its performance in imaging phantoms. The heart of the system is an ultrasound detector array specifically developed for breast imaging and optimized for high sensitivity. Each detector element has an acoustic lens to enlarge the acceptance angle of the large surface area detector elements to ensure a wide system FOV. We characterized the ultrasound detector array performance in terms of frequency response, directional sensitivity, minimum detectable pressure and inter-element electrical and mechanical cross-talk. Further we evaluated the system performance of the laboratory prototype imager using well-defined breast mimicking phantoms. The system possesses a 2 mm XY plane resolution and a 6 mm vertical resolution. A vasculature mimicking object was successfully visualized down to a depth of 40 mm in the breast phantom. Further, tumor mimicking spherical objects with 5 and 10 mm diameter at 20 mm and 40 mm depths are recovered, indicating high system sensitivity. The system has a 170 × 170 × 170 mm(3) FOV, which is well suited for full breast imaging. Various recommendations are provided for performance improvement and to guide this laboratory prototype to a clinical version in future.

Simultaneous three-dimensional photoacoustic and laser-ultrasound tomography

A tomographic setup that provides the co-registration of photoacoustic (PA) and ultrasound (US) images is presented. For pulse-echo US-tomography laser-induced broadband plane ultrasonic waves are produced by illuminating an optically absorbing target with a short near-infrared laser pulse. Part of the same pulse is frequency doubled and used for the generation of PA waves within the object of interest. The laser-generated plane waves are scattered at the imaging object and measured with the same interferometric detector that also acquires the photoacoustic signals. After collection and separation of the data image reconstruction is done using back-projection resulting in three-dimensional, co-registered PA and US images. The setup is characterized and the resolution in PA and US mode is estimated to be about 85 µm and 40 µm, respectively. Besides measurements on phantoms the performance is also tested on a biological sample.

A least-squares fixed-point iterative algorithm for multiple illumination photoacoustic tomography

The optical absorption of tissues provides important information for clinical and pre-clinical studies. The challenge in recovering optical absorption from photoacoustic images is that the measured pressure depends on absorption and local fluence. One reconstruction approach uses a fixed-point iterative technique based on minimizing the mean-squared error combined with modeling of the light source to determine optical absorption. With this technique, convergence is not guaranteed even with an accurate measure of optical scattering. In this work we demonstrate using simulations that a new multiple illumination least squares fixed-point iteration algorithm improves convergence - even with poor estimates of optical scattering.

Continuous real-time photoacoustic demodulation via field programmable gate array for dynamic imaging of zebrafish cardiac cycle

A four dimensional data set of the cardiac cycle of a zebrafish embryo was acquired using postacquisition synchronization of real time photoacoustic b-scans. Utilizing an off-axis photoacoustic microscopy (OA-PAM) setup, we have expanded upon our previous work with OA-PAM to develop a system that can sustain 100 kHz line rates while demodulating the bipolar photoacoustic signal in real-time. Real-time processing was accomplished by quadrature demodulation on a Field Programmable Gate Array (FPGA) in line with the signal digitizer. Simulated data acquisition verified the system is capable of real-time processing up to a line rate of 1 MHz. Galvanometer-scanning of the excitation laser inside the focus of the ultrasonic transducer enables real data acquisition of a 200 by 200 by 200 pixel, volumetric data set across a 2 millimeter field of view at a rate of 2.5 Hz.

A low memory cost model based reconstruction algorithm exploiting translational symmetry for photoacustic microscopy

A model based reconstruction algorithm that exploits translational symmetries for photoacoustic microscopy to drastically reduce the memory cost is presented. The memory size needed to store the model matrix is independent of the number of acquisitions at different positions. This helps us to overcome one of the main limitations of previous algorithms. Furthermore, using the algebraic reconstruction technique and building the model matrix "on the fly", we have obtained fast reconstructions of simulated and experimental data on both two- and three-dimensional grids using a traditional dark field photoacoustic microscope and a standard personal computer.

Characterization of ovarian tissue based on quantitative analysis of photoacoustic microscopy images

In this paper, human ovarian tissue with malignant and benign features was imaged ex vivo using an optical-resolution photoacoustic microscopy (OR-PAM) system. The feasibility of PAM to differentiate malignant from normal ovarian tissues was explored by comparing the PAM images morphologically. Based on the observed differences between PAM images of normal and malignant ovarian tissues in microvasculature features and distributions, seven features were quantitatively extracted from the PAM images, and a logistic model was used to classify ovaries as normal or malignant. 106 PAM images from 18 ovaries were studied. 57 images were used to train the seven-parameter logistic model, and a specificity of 92.1% and a sensitivity of 89.5% were achieved; 49 images were then tested, and a specificity of 81.3% and a sensitivity of 88.2% were achieved. These preliminary results demonstrate the feasibility of our PAM system in mapping microvasculature networks as well as characterizing the ovarian tissue, and could be extremely valuable in assisting surgeons for in vivo evaluation of ovarian tissue during minimally invasive surgery.

Photoacoustic tomography system for noninvasive real-time three-dimensional imaging of epilepsy

A real-time three-dimensional (3D) photoacoustic imaging system was developed for epilepsy imaging in small animals. The system is based on a spherical array containing 192 transducers with a 5 MHz central frequency. The signals from the 192 transducers are amplified by 16 homemade preamplifier boards with 26 dB and multiplexed into a 64 channel data acquisition system. It can record a complete set of 3D data at a frame rate of 3.3 f/s, and the spatial resolution is about 0.2 mm. Phantom experiments were conducted to demonstrate the high imaging quality and real time imaging ability of the system. Finally, we tested the system on an acute epilepsy rat model, and the induced seizure focus was successfully detected using this system.

Near-infrared light photoacoustic ophthalmoscopy

We achieved photoacoustic ophthalmoscopy (PAOM) imaging of the retina with near-infrared (NIR) light illumination. A PAOM imaging system with dual-wavelength illumination at 1064 nm and 532 nm was built. We compared in vivo imaging results of both albino and pigmented rat eyes at the two wavelengths. The results show that the bulk optical absorption of the retinal pigment epithelium (RPE) is only slightly higher than that of the retinal vessels at 532 nm while it becomes more than an order of magnitude higher than that of the retinal vessels at 1064 nm. These studies suggest that although visible light illumination is suitable for imaging both the retinal vessels and the RPE, NIR light illumination, being more comfortable to the eye, is better suited for RPE melanin related investigations and diagnoses.

Non-contact photoacoustic tomography and ultrasonography for tissue imaging

The detection of ultrasound in photoacoustic tomography (PAT) and ultrasonography (US) usually relies on ultrasonic transducers in contact with the biological tissue. This is a major drawback for important potential applications such as surgery and small animal imaging. Here we report the use of remote optical detection, as used in industrial laser-ultrasonics, to detect ultrasound in biological tissues. This strategy enables non-contact implementation of PAT and US without exceeding laser exposure safety limits. The method uses suitably shaped laser pulses and a confocal Fabry-Perot interferometer in differential configuration to reach quantum-limited sensitivity. Endogenous and exogenous inclusions exhibiting optical and acoustic contrasts were detected ex vivo in chicken breast and calf brain specimens. Inclusions down to 0.5 mm in size were detected at depths well exceeding 1 cm. The method could significantly expand the scope of applications of PAT and US in biomedical imaging.

Photoacoustic section imaging with an integrating cylindrical detector

A piezoelectric detector with a cylindrical shape is investigated for photoacoustic section imaging. Images are acquired by rotating a sample in front of the cylindrical detector. With its length exceeding the size of the imaging object, it works as an integrating sensor and therefore allows reconstructing section images with the inverse Radon transform. Prior to the reconstruction the Abel transform is applied to the measured signals to improve the accuracy of the image. A resolution of about 100 µm within a section and of 500 µm between sections is obtained. Additionally, a series of images of a zebra fish is shown.