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Two-dimensional (2D) photoacoustic (PA) imaging based on array transducers provide high spatial resolution in the lateral direction by adopting receive dynamic focusing. However, the quality of PA image is often deteriorated by poor elevational resolution which is achieved by an acoustic lens. To overcome this limitation, we present a three-dimensional (3D) image reconstruction method using a commercial one-dimensional (1D) array transducer.In the method, the elevational resolution is improved by applying synthetic aperture focusing (SAF) technique along the elevational direction. For this, a commercially available 1D array transducer with an acoustic lens is modeled and appropriate synthetic focusing delay that can minimize the effect of the acoustic lens is derived by mathematical analysis.From the simulation and experiment results, it was demonstrated that the proposed method can enhance the image quality of PA imaging, i.e., elevational resolution and signal-to-noise ratio (SNR).3D PA images with improved elevational resolution were achieved using a clinical 1D array transducer.The presented method may be useful for clinical application such as detecting microcalcification, imaging of tumor vasculature and guidance of biopsy in real time.

In this paper, we present a volumetric transrectal ultrasound (TRUS) imaging under the presence of radial scanning angle disorientation (SAD) in a resource-limited diagnostic setting. Herein, we test our hypothesis that a synthetic radial aperture focusing (TRUS-rSAF) technique, in which a radial plane in target volume is reconstructed by coherent compounding of multiple transmittance/reception events, will reject a randomized SAD in a free-hand scanning setup based on external angular tracking. Based on an analytical model of the TRUS-rSAF technique, we first tested specific scenarios using a clinically available TRUS transducer under different SADs in a range of normal distributions (σ = 0.1°, 0.2°, 0.5°, 1°, 2°, and 5°). We found a benefit of the TRUS-rSAF technique for higher robustness when the SAD is contained within the radial synthetic aperture window, i.e., ±0.71° from a target scanning angle. However, no enhancement was found in spatial resolution because of the limited transmit beam field of the clinical TRUS transducer, limiting the synthetic aperture window. We further evaluated the TRUS-rSAF technique with a modified TRUS transducer for an extended synthetic aperture window to test whether higher spatial resolution and robustness to SAD can be obtained in the same evaluation setup. Widening of the synthetic aperture window (±3.54°, ± 5.91°, ± 8.27°, ± 10.63°, ± 12.99°, ± 15.35°) resulted in proportional enhancements of spatial resolution, but it also progressively built up sidelobe artifacts due to randomized synthesis with limited phase cancellations. The results suggest the need for careful calibration of the TRUS-rSAF technique to enable TRUS imaging with free-hand radial scanning and external angle tracking in resource-limited settings.

Prostate-specific membrane antigen (PSMA) is a promising diagnostic and therapeutic target for prostate cancer (PC). Poly(amidoamine) [PAMAM] dendrimers serve as versatile scaffolds for imaging agents and drug delivery that can be tailored to different sizes and compositions depending upon the application. We have developed PSMA-targeted PAMAM dendrimers for real-time detection of PC using fluorescence (FL) and photoacoustic (PA) imaging. A generation-4, ethylenediamine core, amine-terminated dendrimer was consecutively conjugated with on average 10 lysine-glutamate-urea PSMA targeting moieties and a different number of sulfo-cyanine7.5 (Cy7.5) near-infrared dyes (2, 4, 6 and 8 denoted as conjugates II, III, IV and V, respectively). The remaining terminal primary amines were capped with butane-1,2-diol functionalities. We also prepared a conjugate composed of Cy7.5-lysine-suberic acid-lysine glutamate-urea (I) and control dendrimer conjugate (VI). Among all conjugates, IV showed superior in vivo target specificity in male NOD-SCID mice bearing isogenic PSMA+ PC3 PIP and PSMA- PC3 flu xenografts and suitable physicochemical properties for FL and PA imaging. Such agents may prove useful in PC cancer detection and subsequent surgical guidance during excision of PSMA-expressing lesions.

Despite current progress achieved in the surgical technique of radical prostatectomy, post-operative complications such as erectile dysfunction and urinary incontinence persist at high incidence rates. In this paper, we present a methodology for functional intra-operative localization of the cavernous nerve (CN) network for nerve-sparing radical prostatectomy using near-infrared cyanine voltage-sensitive dye (VSD) imaging, which visualizes membrane potential variations in the CN and its branches (CNB) in real time. As a proof-of-concept experiment, we demonstrate a functioning complex nerve network in response to electrical stimulation of the CN, which was clearly differentiated from surrounding tissues in an in vivo rat prostate model. Stimulation of an erection was confirmed by correlative intracavernosal pressure (ICP) monitoring. Within 10 minutes, we performed trans-fascial staining of the CN by direct VSD administration. Our findings suggest the applicability of VSD imaging for real-time, functional imaging guidance during nerve-sparing radical prostatectomy.

ObjectiveTo demonstrate the feasibility of utilizing spectroscopic photoacoustic (sPA) imaging to detect radiofrequency (RF) ablation-induced necrosis in anin vivoenvironment where cardiac motion is present. Additionally, a framework for mapping the necrotic lesion using cardiac-gated sPA (CG-sPA) imaging was introduced to provide intraoperative monitoring of the extent of ablation-induced necrosis. The proposed framework aimed to overcome the challenging lack of intraoperative monitoring with conventional RF procedures, which are a minimally invasive therapy for heart arrhythmia, including atrial fibrillation (A-fib).MethodsThe sPA imaging method was used to differentiate ablated tissue from non-ablated tissue by measuring PA spectrum variation. The degree of necrosis, or necrotic extent (NE), was computed by dividing the quantified ablated tissue contrast by the total contrast from both ablated and non-ablated tissues. The necrotic area and extent were visualized using a continuous colormap. To compensate for tissue motion during the cardiac cycle, cardiac-gating was applied to the sPA data based on image similarity.ResultsIn vivovalidation of the concept was conducted in a swine model. The ablationinduced necrotic lesion at the surface of the beating heart was successfully depicted throughout the cardiac cycle through CG-sPA imaging.ConclusionThe results suggest that the introduced CG-sPA imaging frame is capable of detecting ablation-induced lesions on the beating heart.SignificanceThe proposed framework has great potential to be incorporated into clinical workflow to guide ablation procedures intraoperatively and improve operative outcomes.

An iterative fluence compensation and spectral unmixing algorithm for spectroscopic photoacoustic imaging (SPA) is described. The algorithm focuses on solving the optical inverse problem. It employs optical prior knowledge of tissues, leverages a Monte Carlo simulator for fluence estimation, and assumes a linear mixed model for absorption, scattering, and anisotropy. After an initial guess of the tissue composition, the algorithm sequentially estimates the light fluence, solves for tissue concentrations in spectral unmixing, and updates the optical parameters iteratively until the estimated initial pressure converges to the measurement. The algorithm was validated in simulation, where ground truth data was synthesized from an in vivo study of prostate cancer on mice model. Performance of the algorithm and the scenario without fluence compensation was compared, and the convergence and the precision of the algorithm are reported.

The realization of a parallel-beamforming (pBF) method in ultra-compact ultrasound imaging systems is challenging to achieve uncompromised beamforming accuracy in limited hardware and power resources. In this paper, we present a new hardware- and power-efficient pBF method that utilizes a shared first-in-first-out block on a post-fractional delay filtering architecture (pBF-sFIFO). For an analog-to-digital conversion (ADC) rate given, the proposed pBF-sFIFO method yielded beamforming accuracy comparable to that by an unconstrained pBF (pBF-CON) method, with up to 15% less power consumption. Otherwise, a conventional time-sharing pBF method (pBF-TS) was more vulnerable to aliasing artifacts. Even though increasing ADC rate in the pBF-TS method could recover the beamforming accuracy, exponential need in power consumption was inevitable. Therefore, the pBF-sFIFO method would be an effective solution to enable advanced imaging features in ultra-compact ultrasound imaging systems.

Photoacoustic (PA) imaging has attracted much attention as a new biomedical imaging modality due to its ultrasonic spatial resolution, optical contrast resolution, and deeper imaging depth than oth...

This study presents a system-level optimization of spectroscopic photoacoustic (PA) imaging for prostate cancer (PCa) detection in three folds. First, we present a spectral unmixing model to segregate spectral system error (SSE). We constructed two noise models (NMs) for the laser spectrotemporal fluctuation and the ultrasound system noise. We used these NMs in linear spectral unmixing to denoise and to achieve high temporal resolution. Second, we employed a simulation-aided wavelength optimization to select the most effective subset of wavelengths. NMs again were considered so that selected wavelengths were not only robust to the collinearity of optical absorbance, but also to noise. Third, we quantified the effect of frame averaging on improving spectral unmixing accuracy through theoretical analysis and numerical validation. To validate the whole framework, we performed comprehensive studies in simulation and an in vivo experiment which evaluated prostate-specific membrane antigen (PSMA) expression in PCa on a mice model. Both simulation analysis and in vivo studies confirmed that the proposed framework significantly enhances image signal-to-noise ratio (SNR) and spectral unmixing accuracy. It enabled more sensitive and faster PCa detection. Moreover, the proposed framework can be generalized to other spectroscopic PA imaging studies for noise reduction, wavelength optimization, and higher temporal resolution.

In this paper, we present a novel analytical approach to optimize radial synthetic aperture focusing framework for high-definite and high-sensitive volumetric transrectal ultrasound imaging (TRUS-rSAF). A closed-form analytical description of beam profile defines spatial resolution and grating lobe positions in the TRUS-rSAF imaging of radial plane and validated by a heuristic testing of the critical parameters. Given the theoretical foundation, we optimize the TRUS-rSAF system configuration to balance the spatial and temporal resolution, grating lobe artifacts, and signal-to-noise ratio (SNR) in radial plane with a design criterion to outperform a clinical volumetric TRUS (TRUS-REF) imaging. The results showed that the proposed analytical optimization provides significant improvements of imaging quality in radial plane even over an in-plane microconvex TRUS imaging. Therefore, our analytical approach provides a optimal framework for effective TRUS-rSAF imaging in clinics.

The multi-bounce laser microphone utilizes optical methods to detect the displacement of a gold-covered thin film diaphragm caused by ultrasound signal pressure waves. This sensitive all-optical sensing technique provides new opportunities for advanced ultrasound imaging as it is expected to achieve a higher detection signal-to-noise ratio (SNR) in a broader spectrum, as compared to conventional ultrasonic transducers. The technique does not involve signal time-averaging and the real-time enhancement in detection SNR stems from the amplification of signal strength due to multiple bouncing off the diaphragm. The system was previously developed for detecting acoustic signatures generated by explosives and were limited to lower than 10 kHz in frequency. To demonstrate its feasibility for biomedical imaging applications, preliminary experiments were conducted to show high fidelity detection of ultrasound waves with frequencies ranging from 100 kHz to in excess of 1 MHz. Experimental results are also presented in this work demonstrating the improved detection sensitivity of the multi-bounce laser microphone in detecting ultrasound signals when compared with a commercial Fabry-Perot type optical hydrophone. Furthermore, we also applied the multi-bounce laser microphone to detect photoacoustic signatures emitted by India ink when a LED bar is used as the excitation source without signal averaging.

We present a light-emitting diode (LED)-based transcranial photoacoustic measurement (LED-trPA) of oxyhemoglobin (HbO2) saturation at superior sagittal sinus (SSS) in hypoxic neonatal piglets. The optimal LED imaging wavelengths and frame averaging scheme were determined based on in vivo characterization of transcranial sensitivity. Based on the framework (690/850 nm with >20 frame averaging), graded hypoxia was successfully identified in neonatal piglets in vivo with less than 10.0 % of root mean squared error (RMSE). This preclinical study suggests the feasibility of a rapid, cost-effective, and safe LED-trPA monitoring of perinatal hypoxia-ischemia and prompt interventions for clinical use.

We hypothesize that noninvasive photoacoustic imaging can accurately measure cerebral venous oxyhemoglobin saturation (So2) in a neonatal model of hypoxia-ischemia. In neonatal piglets, which have a skull thickness comparable to that of human neonates, we compared the photoacoustic measurement of sagittal sinus So2 against that measured directly by blood sampling over a wide range of conditions. Systemic hypoxia was produced by decreasing inspired oxygen stepwise (i.e., 100, 21, 19, 17, 15, 14, 13, 12, 11, and 10%) with and without unilateral or bilateral ligation of the common carotid arteries to enhance hypoxia-ischemia. Transcranial photoacoustic sensing enabled us to detect changes in sagittal sinus O2 saturation throughout the tested range of 5–80% without physiologically relevant bias. Despite lower cortical perfusion and higher oxygen extraction in groups with carotid occlusion at equivalent inspired oxygen, photoacoustic measurements successfully provided a robust linear correlation that approached the line of identity with direct blood sample measurements. Receiver-operating characteristic analysis for discriminating So2 2 noninvasively over a wide range and support its use for early detection of neonatal hypoxia-ischemia. NEW & NOTEWORTHY We present data to validate the noninvasive photoacoustic measurement of sagittal sinus oxyhemoglobin saturation. In particular, this paper demonstrates the robustness of this methodology during a wide range of hemodynamic and physiological changes induced by the stepwise decrease of fractional inspired oxygen to produce hypoxia and by unilateral and bilateral ligation of the common carotid arteries preceding hypoxia to produce hypoxia-ischemia. This technique may be useful for diagnosing risk of neonatal hypoxic-ischemic encephalopathy.

A noninvasive monitor for concurrent evaluation of placental and fetal sagittal sinus sO 2 for both antepartum surveillance at the late 2nd and 3rd trimesters and intrapartum monitoring would be a great advantage over current methods. A PA fetal brain and placental monitor has potential value to rapidly identify the fetus at risk for developing hypoxia and ischemia of a sufficient degree that brain injury or death may develop, which may be prevented by intervention with delivery and other follow-up treatments.

Objective: Perinatal ischemic stroke is estimated to occur in 1/2300–1/5000 live births, but early differential diagnosis from global hypoxia-ischemia is often difficult. In this study, we tested the ability of a hand-held transcranial photoacoustic (PA) imaging probe to non-invasively detect a focal photothrombotic stroke (PTS) within 2 h of stroke onset in a gyrencephalic piglet brain. Approach: About 17 stroke lesions of approximately 1 cm2 area were introduced randomly in anterior or posterior cortex via the light/dye PTS technique in anesthetized neonatal piglets (n = 11). The contralateral non-ischemic region served as control tissue for discrimination contrast for the PA hemoglobin metrics: oxygen saturation, total hemoglobin (tHb), and individual quantities of oxygenated and deoxygenated hemoglobin (HbO2 and HbR). Main results: The PA-derived tissue oxygen saturation at 2 h yielded a significant separation between control and affected regions-of-interest (p < 0.0001), which were well matched with 24 h post-stroke cerebral infarction confirmed in the triphenyltetrazolium chloride-stained image. The quantity of HbO2 also displayed a significant contrast (p = 0.021), whereas tHb and HbR did not. The analysis on receiver operating characteristic curves and multivariate data analysis also agreed with the results above. Significance: This study shows that a hand-held transcranial PA neuroimaging device can detect a regional thrombotic stroke in the cerebral cortex of a neonatal piglet. In particular, we conclude that the oxygen saturation metric can be used alone to identify regional stroke lesions. The lack of change in tHb may be related to arbitrary hand-held imaging configuration and/or entrapment of red blood cells within the thrombotic stroke.

In this paper, we present a novel system-on-chip (SOC) solution for a portable ultrasound imaging system (PUS) for point-of-care applications. The PUS-SOC includes all of the signal processing modules (i.e., the transmit and dynamic receive beamformer modules, mid- and back-end processors, and color Doppler processors) as well as an efficient architecture for hardware-based imaging methods (e.g., dynamic delay calculation, multi-beamforming, and coded excitation and compression). The PUS-SOC was fabricated using a UMC 130-nm NAND process and has 16.8 GFLOPS of computing power with a total equivalent gate count of 12.1 million, which is comparable to a Pentium-4 CPU. The size and power consumption of the PUS-SOC are $27\times 27~{\rm mm}^{2}$ and 1.2 W, respectively. Based on the PUS-SOC, a prototype hand-held US imaging system was implemented. Phantom experiments demonstrated that the PUS-SOC can provide appropriate image quality for point-of-care applications with a compact PDA size ( $200\times 120\times 45~{\rm mm}^{3}$ ) and 3 hours of battery life.

Coherent manipulation of light-matter interactions is pivotal to the advancement of nanophotonics. Conventionally, the non-resonant optical Stark effect is harnessed for band engineering by intense laser pumping. However, this method is hindered by the transient Stark shifts and the high-energy laser pumping which, by itself, is precluded as a nanoscale optical source due to light diffraction. As an analog of photons in a laser, surface plasmons are uniquely positioned to coherently interact with matter through near-field coupling, thereby, providing a potential source of electric fields. Herein, the first demonstration of plasmonic Stark effect is reported and attributed to a newly uncovered energy-bending mechanism. As a complementary approach to the optical Stark effect, it is envisioned that the plasmonic Stark effect will advance fundamental understanding of coherent light-matter interactions and will also provide new opportunities for advanced optoelectronic tools, such as ultrafast all-optical switches and biological nanoprobes at lower light power levels.

The implementation of dynamic delay calculations (DDCs) is challenging for ultra-compact ultrasound imaging due to the enormous computation and power consumption requirements. Here, we present an efficient pseudo-DDC method based on optimal zone segmentation (PDC-Optimal), which significantly decreases these requirements relative to an unconstrained DDC method: reductions in flip-flops of 84.35% and in look-up tables of 94.19%, respectively. The reductions lead to an up to 94.53% lower dynamic power consumption and provide image quality comparable to the unconstrained DDC method. The proposed PDC-Optimal method also provides adaptive flexibility between beamforming accuracy and battery life using the delay error allowance, a user-definable parameter. A conventional pseudo-DDC method using uniform zone segmentation (PDC-Conv) presented substantial image degradation in the near imaging field when the same number of zone segments was used. Therefore, the PDC-Optimal method provides an efficient yet flexible DDC solution to improve the experiences for ultra-compact ultrasound imaging system users.

We present a proof-of-concept of an automatic integration of photoacoustic (PA) imaging on clinical ultrasound (US) imaging platforms. Here we tackle two critical challenges: the laser synchronization and the inaccessibility to the beamformer core embedded in commercial US imaging platform. In particular, the line trigger frequency (LTF) estimation and the asynchronous synthetic aperture inverse beamforming (ASAIB) were developed and evaluated in both k-Wave simulation and phantom experiment. The proposed method is an economical solution to enable PA imaging on a greater number of US equipment to further thrive the PA imaging research community.

Engineered, photoacoustic transduction layers are often required in systems that cannot sustain repeated optical excitation owing to material degradation that can occur as a result of optical irradiation. To produce acoustic waves with sufficient amplitude in these systems, polymer matrix nanocomposites can be used for transduction and can serve as photoacoustic transmitters. These nanocomposites can be designed to produce large acoustic generation efficiency while minimizing collateral effects on surrounding materials - this can be important in biomedical applications where tissue damage must be avoided. This work focuses on the use of nanocomposites as photoacoustic transmitters and assesses the optical, thermal and acoustic behaviors of these materials to identify critical parameters that affect their performance under different irradiation conditions.

Transcranial functional photoacoustic (fPA) voltage-sensitive dye (VSD) imaging promises to overcome current temporal and spatial limitations of current neuroimaging modalities. The technique previously distinguished global seizure activity from control neural activity in groups of rats. To validate the focal specificity of transcranial fPA neuroimaging in vivo, we now present proofs-of-concept that the results differentiate between low- and high-dose N-methyl-D-aspartate (NMDA) evoked neural activity in rat hippocampus. Concurrent quantitative EEG (qEEG) and microdialysis recorded real-time circuit dynamics and glutamate concentration change, respectively. We hypothesized that location-specific fPA VSD contrast would identify the neural dynamics in hippocampus with the correlation to NMDA evoked focal glutamate release and time-specific EEG signals. To test the hypothesis, we infused 0.3 to 3.0 mM NMDA at 2 μl/min over 60 min via an implanted microdialysis probe. The dialysate samples collected every 20 min during the infusion were analyzed for focal changes in extracellular glutamate release. Transcranial fPA VSD imaging provided NMDA-evoked VSD responses with positive correlation to extracellular glutamate concentration change at the contralateral side of the microdialysis probe. The graded response represents the all-or-none gating system of the dentate gyrus (DG) in hippocampus. Quantitative EEG (qEEG) successfully confirmed induction of focal seizure activity during NMDA infusion. We conclude that transcranial fPA VSD imaging distinguished graded DG gatekeeping functions, based on the VSD redistribution mechanism sensitive to electrophysiologic membrane potential. The results suggest the potential future use of this emerging technology in clinics and science as an innovative and significant functional neuroimaging modality.

Biomedical imaging-guided cancer therapy should have capabilities of both accurate tumor diagnosis and high therapeutic efficacy for the personalized treatment. Various biomedical imaging-guided cancer therapies are currently being investigated to overcome current limitations that include low sensitivity of diagnosis and poor drug delivery to the tumor site. Here, we report the development of a multifunctional theranostic contrast agent demonstrating high sensitive photoacoustic and ultrasound imaging and effective local delivery of anticancer drug to a tumor site. A microbubble (porphyrin-MB) was developed using phospholipid-porphyrin conjugates to enhance ultrasound and photoacoustic signal intensities simultaneously. Paclitaxel-loaded human serum albumin nanoparticles (PTX-HSA-NPs) were then conjugated onto the surface of the microbubble. The developed PTX-HSA-NPs conjugated porphyrin-MB (porphyrin-MB-NPs) provided sensitive, dual modal images of a tumor at 700 nm optimal laser wavelength for photoacoustic imaging and 5-14 MHz operating frequency for the ultrasound imaging. In addition, porphyrin-MB-NPs efficiently suppressed tumor growth by ultrasound exposure. Exposure to the focused ultrasound triggered the collapse of porphyrin-MB-NPs, resulting in the local release of PTX-HSA-NPs and enhanced penetration into the tumor site. The increased preferential accumulation and penetration of PTX-HSA-NPs suppressed tumor growth 10-fold more than without exposure to ultrasound. In conclusion, the developed porphyrin-MB-NPs establish a new paradigm in simultaneous bi-functional ultrasound/photoacoustic imaging diagnosis and locally triggered release of nanomedicine and enhanced chemotherapy efficiency.

A sensitive, non-invasive method to detect localized prostate cancer, particularly for early detection and repetitive study in patients undergoing active surveillance, remains an unmet need. Here we propose a molecular photoacoustic (PA) imaging approach by targeting the prostate-specific membrane antigen (PSMA), which is over-expressed in the vast majority of prostate cancers. We performed spectroscopic PA imaging in an experimental model of prostate cancer, namely, in immunocompromised mice bearing PSMA+ (PC3 PIP) and PSMA− (PC3 flu) tumors through administration of the known PSMA-targeted fluorescence agent, YC-27. Differences in contrast between PSMA+ and isogenic control tumors were observed upon PA imaging, with PSMA+ tumors showing higher contrast in average of 66.07-fold with five mice at the 24-hour post-injection time points. These results were corroborated using standard near-infrared fluorescence imaging with YC-27, and the squared correlation between PA and fluorescence intensities was 0.89. Spectroscopic PA imaging is a new molecular imaging modality with sufficient sensitivity for targeting PSMA in vivo, demonstrating the potential applications for other saturable targets relevant to cancer and other disorders.

Minimally-invasive monitoring of electrophysiological neural activities in real-time—that enables quantification of neural functions without a need for invasive craniotomy and the longer time constants of fMRI and PET—presents a very challenging yet significant task for neuroimaging. In this paper, we present in vivo functional PA (fPA) imaging of chemoconvulsant rat seizure model with intact scalp using a fluorescence quenching-based cyanine voltage-sensitive dye (VSD) characterized by a lipid vesicle model mimicking different levels of membrane potential variation. The framework also involves use of a near-infrared VSD delivered through the blood-brain barrier (BBB), opened by pharmacological modulation of adenosine receptor signaling. Our normalized time-frequency analysis presented in vivo VSD response in the seizure group significantly distinguishable from those of the control groups at sub-mm spatial resolution. Electroencephalogram (EEG) recording confirmed the changes of severity and frequency of brain activities, induced by chemoconvulsant seizures of the rat brain. The findings demonstrate that the near-infrared fPA VSD imaging is a promising tool for in vivo recording of brain activities through intact scalp, which would pave a way to its future translation.

Perinatal arterial ischemic stroke can result in long-term deficits in motor, cognitive, attention, and executive functions as well as persistent seizures. Rapid diagnosis is critical for newborns who suffer a focal ischemic stroke in order to distinguish prenatal vs. postnatal stroke, and to differentiate perinatal stroke from global hypoxic ischemia (HI). In this study, we present photoacoustic (PA) quantification for brain tissue oxygenation on an in vivo focal stroke model based on a newborn piglet.

A great need exists to non-invasively quantify the neurotransmitter activity in real-time to build a comprehensive functional map of a brain. In this paper, we present real-time recording of neuronal membrane potential change in vivo using transcranial photoacoustic (PA) voltage-sensitive dye (VSD) imaging on a rat in seizure.

Multi-modal interface in medicine has been of interest as it extends clinical vision and diagnostic to a higher dimension. In this paper, we develop a real-time intra-operative guidance for robot-assisted surgical operation using a dual photoacoustic (PA) and fluorescence (FL) imaging based on a common pulsed laser and commercial ultrasound array transducer.

Quantification of brain function is a significant milestone towards understanding of the underlying workings of the brain. Photoacoustic (PA) imaging is the emerging brain sensing modality by which the molecular light absorptive contrast can be non-invasively quantified from deep-lying tissue (~several cm). In this BRAIN initiative effort, we propose high-speed transcranial PA imaging using a novel, compact pulsed LED illumination system (Prexion Inc., Japan) with 200-uJ pulse energy for 75-ns duration, and pulse repetition frequency (PRF) up to 4kHz at near-infrared (NIR) wavelengths of 690-nm and 850-nm switchable in real-time. To validate the efficacy of the proposed system, preliminary ex vivo experiments were conducted with mice skull and human temporal bone, which included vessel-mimicking tubes filled with 10% Indian Ink solution and light absorptive rubber material, respectively. The results indicated that significant PA contrast, 150% signal-to-noise ratio (SNR), can be achieved through the mice skull only with 64 subsequent frame averaging. The minimal number of frames for averaging required was only 16 to generate signal above background noise, leading to 250 Hz frame rate in the strictest temporal frame separation. Furthermore, distinguishable PA contrast was achieved with human temporal bone with 64-frame averaging. Overall, the preliminary results indicate that the LED illumination system can be a cost-effective solution for high-speed PA brain imaging in preclinical and clinical applications, compared to expansive and bulky Nd:YAG laser systems commonly used in PA imaging.

Precise sentinel lymph node (SLN) identification is crucial not only for accurate diagnosis of micro-metastases at an early stage of cancer progression but also for reducing the number of SLN biopsies (SLNB) to minimize their severe side effects. Furthermore, it is desirable that an SLNB guidance should be as safe as possible in routine clinical use. Although there are currently various SLNB guidance methods for pre-operative or intra-operative assessment, none are ideal. We propose a real-time SLNB guidance method using contrast-enhanced tri-modal images (i.e., ultrasound, photoacoustic, and fluorescence) acquired by a recently developed hand-held tri-modal probe. The major advantage of tri-modal imaging is demonstrated here through an in vivo study of the technically-difficult case of nodal obstruction that frequently leads to false-negative results in patients. The results in a tumor model in rabbits and normal controls showed that tri-modal imaging is capable of clearly identifying obstructed SLNs and of indicating their metastatic involvement. Based on these findings, we propose an SLNB protocol to help surgeons take full advantage of the complementary information obtained from tri-modal imaging, including for pre-operative localization, intra-operative biopsy guidance and post-operative analysis.

Monitoring of the membrane potential is possible using voltage sensitive dyes (VSD), where fluorescence intensity changes in response to neuronal electrical activity. However, fluorescence imaging is limited by depth of penetration and high scattering losses, which leads to low sensitivity in vivo systems for external detection. In contrast, photoacoustic (PA) imaging, an emerging modality, is capable of deep tissue, noninvasive imaging by combining near infrared light excitation and ultrasound detection. In this work, we develop the theoretical concept whereby the voltage-dependent quenching of dye fluorescence leads to a reciprocal enhancement of PA intensity. Based on this concept, we synthesized a novel near infrared photoacoustic VSD (PA-VSD) whose PA intensity change is sensitive to membrane potential. In the polarized state, this cyanine-based probe enhances PA intensity while decreasing fluorescence output in a lipid vesicle membrane model. With a 3-9 μM VSD concentration, we measured a PA signal increase in the range of 5.3 % to 18.1 %, and observed a corresponding signal reduction in fluorescence emission of 30.0 % to 48.7 %. A theoretical model successfully accounts for how the experimental PA intensity change depends on fluorescence and absorbance properties of the dye. These results not only demonstrate the voltage sensing capability of the dye, but also indicate the necessity of considering both fluorescence and absorbance spectral sensitivities in order to optimize the characteristics of improved photoacoustic probes. Together, our results demonstrate photoacoustic sensing as a potential new modality for sub-second recording and external imaging of electrophysiological and neurochemical events in the brain.

This paper investigates whether photoacoustic imaging (PAI) can provide the visualization of microcalcifications in breast tissue. For this, the geometrical correlation between the 3-D PA images of breast microcalcifications within ex vivo specimens and the corresponding mammograms was ascertained. Also, the optical absorbance of the calcification compositions (i.e., hydroxyapatite and calcium oxalate) was measured and compared with the PA responses of the microcalcifications. The experimental results demonstrated that the PA images discriminated between the microcalcifications and the surrounding tissue, and their locations in PA images reasonably meshed with those of the microcalcifications appeared in the mammograms. Also, the change in PA signal amplitude along the laser wavelength agreed with the absorbance of hydroxyapatite associated with the relatively high potential of malignant cancers, but not calcium oxalate with only benign cases.

Localized, non-invasive cell stimulation has many applications. Here we show the initial proof-of-concept in vitro study of the photoacoustic cell stimulation approach, which is amenable to high-throughput screening applications. The proposed method is implemented as follows: 1) the localized excitation using the focused pulsed laser delivery on an absorptive material placed inside the well containing the cells, 2) cell stimulation by the photoacoustic pressure generated, and 3) fluorescence quantification of the membrane potential change over time. The preliminary proof-ofconcept in vitro study is conducted with primary neurons isolated from mouse cerebral cortex. The absorptive rubber media generates the photoacoustic pressure by the pulsed laser excitation. The experimental results show the feasibility of photoacoustic cell stimulation approach by indicating the significant membrane potential chance from the photoacoustically-stimulated primary neurons. Otherwise, the sham control without any photoacoustic stimulation shows minimal membrane potential change. We envisage that the proposed approach can allow broad strategies for noninvasive cell stimulation by using the photoacoustic contrasts situated at inside or outside of the body such as external absorptive materials or intravascularly-injected photoacoustic contrast particles.

Objective To investigate the feasibility of ex vivo multispectral photoacoustic (PA) imaging in differentiating cholesterol versus neoplastic polyps, and benign versus malignant polyps, of the gallbladder. Materials and methods A total of 38 surgically confirmed gallbladder polyps (24 cholesterol polyps, 4 adenomas, and 10 adenocarcinomas) from 38 patients were prospectively included in this study. The surgical specimens were set on a gel pad immersed in a saline-filled container. The PA intensities of polyps were then measured, using two separate wavelength intervals (421-647 nm and 692-917 nm). Mann-Whitney U test was performed for the comparison of normalized PA intensities between the cholesterol and neoplastic polyps, and between the benign and malignant polyps. Kruskal-Wallis test was conducted for the comparison of normalized PA intensities among the cholesterol polyps, adenomas, and adenocarcinomas. Results A significant difference was observed in the normalized PA intensities between the cholesterol and neoplastic polyps at 459 nm (median, 1.00 vs. 0.73; p = 0.032). Comparing the benign and malignant polyps, there were significant differences in the normalized PA intensities at 765 nm (median, 0.67 vs. 0.78; p = 0.013), 787 nm (median, 0.65 vs. 0.77; p = 0.034), and 853 nm (median, 0.59 vs. 0.85; p = 0.028). The comparison of the normalized PA intensities among cholesterol polyps, adenomas, and adenocarcinomas demonstrated marginally significant differences at 765 nm (median, 0.67 vs. 0.66 vs. 0.78, respectively; p = 0.049). Conclusion These preliminary results indicate that benign versus malignant gallbladder polyps might exhibit different spectral patterns on multispectral PA imaging.

The contradictory biological function of eumelanin (photoprotection vs photosensitization) has long been a topic of debate in a wide range of disciplines such as chemistry, physics and biology. For understanding full spectrum of eumelanin's photobiological aspect, revealing how eumelanin's complex structural organization dictates its photophysical properties is critical step. Here, we report a practical approach to controlling the hierarchically assembled structure of natural eumelanin, which leads to disassembly of its structure into subunits and oxidized subunits, respectively. Based on the well-characterized model system, it was possible to systematically determine how the photophysical properties of eumelanin are ruled by its hierarchical assembly organization. Particularly, our experiments reveal that the chemical oxidation of eumelanin's subunits, which leads to delamination of their stacked layer structure, is critical to significantly increase their photochemical reactivity to generate ROS under UV irradiation. This result provides clear experimental evidence that oxidative degradation of eumelanin, which might be induced by phagosomal enzymatic activity in the process of melanomagenesis, is responsible for triggering the negative photobiological role of eumelanin such as ROS source needed for development of malignant melanoma.