Miniaturized phased-array ultrasound and photoacoustic endoscopic imaging system

Thin flexible photoacoustic endoscopic probe with a distal-driven micro-step motor for pump-probe-based high-specific molecular imaging

Conventional photoacoustic endoscopy (PAE) is mostly for structural imaging, and its molecular imaging ability is quite limited. In this work, we address this issue and present the development of a flexible acoustic-resolution-based photoacoustic endoscopic (AR-PAE) probe with an outer diameter of 8 mm. This probe is driven by a micro-step motor at the distal end, enabling flexible and precise angular step control to synchronize with the optical parametric oscillator (OPO) lasers. This probe retains the high spatial resolution, high penetration depth, and spectroscopic imaging ability of conventional AR-PAE. Moreover, it is capable for background-free high-specific photoacoustic molecular imaging with a novel pump-probe detection technique, as demonstrated by the distribution visualizing of the FDA approved contrast agent methylene blue (MB) in an ex-vivo pig ileum. This proposed method represents an important technical advancement in multimodal PAE, and can potentially make considerable contributions across various biomedical fields.

Tailored photoacoustic apertures with superimposed optical holograms

A new method of generating potentially arbitrary photoacoustic wavefronts with optical holograms is presented. This method uses nanosecond laser pulses at 1064 nm that are split into four time-delayed components by means of a configurable multipass optical delay apparatus, which serves to map the pulses onto phase-delayed regions of a given acoustic wavefront. A single spatial light modulator generates separate holograms for each component, which are imaged onto a photoacoustic transducer comprised of a thermoelastic polymer. As a proof of concept of the broader arbitrary wavefront construction technique, the spatially- and temporally-modulated holograms in this study produce a phased array effect that enables beam steering of the resulting acoustic pulse. For a first experimental demonstration of the method, as verified by simulation, the acoustic beam is steered in four directions by around 5 degrees.

Laparoscopic Photoacoustic Imaging System Based on Side-Illumination Diffusing Fibers

OBJECTIVE

To develop a flexible miniaturized photoacoustic (PA) imaging probe for detecting anatomical structures during laparoscopic surgery. The proposed probe aimed to facilitate intraoperative detection of blood vessels and nerve bundles embedded in tissue not directly visible to the operating physician to preserve these delicate and vital structures.

METHODS

We modified a commercially available ultrasound laparoscopic probe by incorporating custom-fabricated side-illumination diffusing fibers that illuminate the probe's field of view. The probe geometry, including the position and orientation of the fibers and the emission angle, was determined using computational models of light propagation in the simulation and subsequently validated through experimental studies.

RESULTS

In wire phantom studies within an optical scattering medium, the probe achieved an imaging resolution of 0.43 ±0.09 mm and a signal-to-noise ratio of 31.2±1.84 dB. We also conducted an ex vivo study using a rat model, demonstrating the successful detection of blood vessels and nerves.

CONCLUSION

Our results indicate the viability of a side-illumination diffusing fiber PA imaging system for guidance during laparoscopic surgery.

SIGNIFICANCE

The potential clinical translation of this technology could enhance the preservation of critical vascular structures and nerves, thereby minimizing post-operative complications.

Novel oviduct endoscope combining optical coherence tomography with intratubal ultrasonography for fallopian tube exploration: An in vivo rabbit pilot study

BACKGROUND AND STUDY AIM

Currently, several limitations exist in the examination of the oviduct. In this study, the usefulness and feasibility of a novel ultrafine dual-modality oviduct endoscopy device for in vivo assessment of the oviduct were evaluated.

METHODS

Five Japanese white rabbits were selected to undergo oviduct probing using a combination of optical coherence tomography (OCT) and intratubal ultrasonography. The feasibility of the procedure was evaluated through 152 pairs of clear, clinically interpretable images obtained using spiral scanning via the pull-back method. OCT images were compared with the oviduct histopathology sections.

RESULTS

Visualization of the oviduct using both OCT and ultrasound revealed a differentiated three-layer tissue; however, ultrasound showed a poorer clarity than OCT. By comparing OCT images with the histological morphology of the oviduct, the inner low-reflective layer of the oviduct corresponds to the mucosal layer, the middle high-reflective layer corresponds to the fibrous muscle layer, and the outer low-reflective layer corresponds to the connective tissue layer. Postoperatively, the general condition of the animals was good.

CONCLUSION

This study demonstrated the feasibility and potential clinical value of the novel ultrafine dual-modality oviduct endoscope. Dual-modality imaging of OCT and intratubal ultrasonography can provide clearer microstructure of the oviduct wall.

Niche preclinical and clinical applications of photoacoustic imaging with endogenous contrast

In the past decade, photoacoustic (PA) imaging has attracted a great deal of popularity as an emergent diagnostic technology owing to its successful demonstration in both preclinical and clinical arenas by various academic and industrial research groups. Such steady growth of PA imaging can mainly be attributed to its salient features, including being non-ionizing, cost-effective, easily deployable, and having sufficient axial, lateral, and temporal resolutions for resolving various tissue characteristics and assessing the therapeutic efficacy. In addition, PA imaging can easily be integrated with the ultrasound imaging systems, the combination of which confers the ability to co-register and cross-reference various features in the structural, functional, and molecular imaging regimes. PA imaging relies on either an endogenous source of contrast (e.g., hemoglobin) or those of an exogenous nature such as nano-sized tunable optical absorbers or dyes that may boost imaging contrast beyond that provided by the endogenous sources. In this review, we discuss the applications of PA imaging with endogenous contrast as they pertain to clinically relevant niches, including tissue characterization, cancer diagnostics/therapies (termed as theranostics), cardiovascular applications, and surgical applications. We believe that PA imaging's role as a facile indicator of several disease-relevant states will continue to expand and evolve as it is adopted by an increasing number of research laboratories and clinics worldwide.

Integrated Ultrasound and Photoacoustic-Guided Laser Ablation Theranostic Endoscopic System

Advancements in ablation techniques have paved the way towards the development of safer and more effective clinical procedures for treating various maladies such as atrial fibrillation (AF). AF is characterized by rapid, chaotic atrial activation and is commonly treated using radiofrequency applicators or laser ablation catheters. However, the lack of thermal lesion formation and temperature monitoring capabilities in these devices prevents them from measuring the treatment outcome directly. In addition, poor differentiation between healthy and ablated tissues leads to incomplete ablation, which reduces safety and causes complications in patients. Hence, a novel photoacoustic (PA)-guided laser ablation theranostic device was developed around a traditional phased-array endoscope. The proposed technology provides lesion formation, tissue distinguishing, and temperature monitoring capabilities. Our results have validated the lesion monitoring capability of the proposed technology through PA correlation maps. The tissue distinguishing capability of the theranostic device was verified by the measurable differences in the PA signal between pre-and post-ablated mice myocardial tissue. The increase in the PA signal with temperature variations caused by the ablation laser confirmed the ability of the proposed device to provide temperature feedback.

Diffuse reflection spectroscopy at the fingertip: design and performance of a compact side-firing probe for tissue discrimination during colorectal cancer surgery

Optical technologies are widely used for tissue sensing purposes. However, maneuvering conventional probe designs with flat-tipped fibers in narrow spaces can be challenging, for instance during pelvic colorectal cancer surgery. In this study, a compact side-firing fiber probe was developed for tissue discrimination during colorectal cancer surgery using diffuse reflectance spectroscopy. The optical behavior was compared to flat-tipped fibers using both Monte Carlo simulations and experimental phantom measurements. The tissue classification performance was examined using freshly excised colorectal cancer specimens. Using the developed probe and classification algorithm, an accuracy of 0.92 was achieved for discriminating tumor tissue from healthy tissue.

Piezoelectric Micromachined Ultrasound Transducer Technology: Recent Advances and Applications

The objective of this article is to review the recent advancement in piezoelectric micromachined ultrasound transducer (PMUT) technology and the associated piezoelectric materials, device fabrication and characterization, as well as applications. PMUT has been an active research topic since the late 1990s because of the ultrasound application needs of low cost large 2D arrays, and the promising progresses on piezoelectric thin films, semiconductors, and micro/nano-electromechanical system technology. However, the industrial and medical applications of PMUTs have not been very significant until the recent success of PMUT based fingerprint sensing, which inspired growing interests in PMUT research and development. In this paper, recent advances of piezoelectric materials for PMUTs are reviewed first by analyzing the material properties and their suitability for PMUTs. PMUT structures and the associated micromachining processes are next reviewed with a focus on the complementary metal oxide semiconductor compatibility. PMUT prototypes and their applications over the last decade are then summarized to show the development trend of PMUTs. Finally, the prospective future of PMUTs is discussed as well as the challenges on piezoelectric materials, micro/nanofabrication and device integration.

Advances in Imaging of Inflammation, Fibrosis, and Cancer in the Gastrointestinal Tract

Gastrointestinal disease is prevalent and broad, manifesting itself in a variety of ways, including inflammation, fibrosis, infection, and cancer. However, historically, diagnostic technologies have exhibited limitations, especially with regard to diagnostic uncertainty. Despite development of newly emerging technologies such as optoacoustic imaging, many recent advancements have focused on improving upon pre-existing modalities such as ultrasound, computed tomography, magnetic resonance imaging, and endoscopy. These advancements include utilization of machine learning models, biomarkers, new technological applications such as diffusion weighted imaging, and new techniques such as transrectal ultrasound. This review discusses assessment of disease processes using imaging strategies for the detection and monitoring of inflammation, fibrosis, and cancer in the context of gastrointestinal disease. Specifically, we include ulcerative colitis, Crohn's disease, diverticulitis, celiac disease, graft vs. host disease, intestinal fibrosis, colorectal stricture, gastric cancer, and colorectal cancer. We address some of the most recent and promising advancements for improvement of gastrointestinal imaging, including unique discussions of such advancements with regard to imaging of fibrosis and differentiation between similar disease processes.

Thin ceramic PZT dual- and multi-frequency pMUT arrays for photoacoustic imaging

Miniaturized ultrasonic transducer arrays with multiple frequencies are key components in endoscopic photoacoustic imaging (PAI) systems to achieve high spatial resolution and large imaging depth for biomedical applications. In this article, we report on the development of ceramic thin-film PZT-based dual- and multi-frequency piezoelectric micromachined ultrasonic transducer (pMUT) arrays and the demonstration of their PAI applications. With chips sized 3.5 mm in length or 10 mm in diameter, square and ring-shaped pMUT arrays incorporating as many as 2520 pMUT elements and multiple frequencies ranging from 1 MHz to 8 MHz were developed for endoscopic PAI applications. Thin ceramic PZT with a thickness of 9 μm was obtained by wafer bonding and chemical mechanical polishing (CMP) techniques and employed as the piezoelectric layer of the pMUT arrays, whose piezoelectric constant d ₃₁ was measured to be as high as 140 pm/V. Benefiting from this high piezoelectric constant, the fabricated pMUT arrays exhibited high electromechanical coupling coefficients and large vibration displacements. In addition to electrical, mechanical, and acoustic characterization, PAI experiments with pencil leads embedded into an agar phantom were conducted with the fabricated dual- and multi-frequency pMUT arrays. Photoacoustic signals were successfully detected by pMUT elements with different frequencies and used to reconstruct single and fused photoacoustic images, which clearly demonstrated the advantages of using dual- and multi-frequency pMUT arrays to provide comprehensive photoacoustic images with high spatial resolution and large signal-to-noise ratio simultaneously.

3D Ultrasound-Guided Photoacoustic Imaging to Monitor the Effects of Suboptimal Tyrosine Kinase Inhibitor Therapy in Pancreatic Tumors

Pancreatic cancer is a disease with an incredibly poor survival rate. As only about 20% of patients are eligible for surgical resection, neoadjuvant treatments that can relieve symptoms and shrink tumors for surgical resection become critical. Many forms of treatments rely on increased vulnerability of cancerous cells, but tumors or regions within the tumors that may be hypoxic could be drug resistant. Particularly for neoadjuvant therapies such as the tyrosine kinase inhibitors utilized to shrink tumors, it is critical to monitor changes in vascular function and hypoxia to predict treatment efficacy. Current clinical imaging modalities used to obtain structural and functional information regarding hypoxia or oxygen saturation (StO₂) do not provide sufficient depth penetration or require the use of exogenous contrast agents. Recently, ultrasound-guided photoacoustic imaging (US-PAI) has garnered significant popularity, as it can noninvasively provide multiparametric information on tumor vasculature and function without the need for contrast agents. Here, we built upon existing literature on US-PAI and demonstrate the importance of changes in StO₂ values to predict treatment response, particularly tumor growth rate, when the outcomes are suboptimal. Specifically, we image xenograft mouse models of pancreatic adenocarcinoma treated with suboptimal doses of a tyrosine kinase inhibitor cabozantinib. We utilize the US-PAI data to develop a multivariate regression model that demonstrates that a therapy-induced reduction in tumor growth rate can be predicted with 100% positive predictive power and a moderate (58.33%) negative predictive power when a combination of pretreatment tumor volume and changes in StO₂ values pretreatment and immediately posttreatment was employed. Overall, our study indicates that US-PAI has the potential to provide label-free surrogate imaging biomarkers that can predict tumor growth rate in suboptimal therapy.

Circuits on miniaturized ultrasound imaging system-on-a-chip: a review

Trends of medical system move from a traditional in-person visit to virtual healthcare increases demands on point-of-care devices. Because ultrasound (US) is non-invasive, the demands highlight US imaging among other imaging modalities. Thanks to the development of US transducer technology, miniaturized US with application-specific integrated circuits (ASIC) have been researched. For example, applications that require small aperture sizes such as intravascular US (IVUS) and intra-cardiac echocardiography (ICE) require integration of system-on-a-chip (SoC) on the transducer. This paper reviews circuit techniques on the transmitter (TX) and receiver (RX) of the US imaging system. As TX circuits, pulser, T/RX switch, TX beamformer, and power management circuits are discussed. State-of-the-art transducer modeling, pre-amplifier, time-gain compensation, RX beamformer, quadrature sampler, and output driver are introduced as RX circuits.

The emerging role of photoacoustic imaging in clinical oncology

Clinical oncology can benefit substantially from imaging technologies that reveal physiological characteristics with multiscale observations. Complementing conventional imaging modalities, photoacoustic imaging (PAI) offers rapid imaging (for example, cross-sectional imaging in real time or whole-breast scanning in 10-15 s), scalably high levels of spatial resolution, safe operation and adaptable configurations. Most importantly, this novel imaging modality provides informative optical contrast that reveals details on anatomical, functional, molecular and histological features. In this Review, we describe the current state of development of PAI and the emerging roles of this technology in cancer screening, diagnosis and therapy. We comment on the performance of cutting-edge photoacoustic platforms, and discuss their clinical applications and utility in various clinical studies. Notably, the clinical translation of PAI is accelerating in the areas of macroscopic and mesoscopic imaging for patients with breast or skin cancers, as well as in microscopic imaging for histopathology. We also highlight the potential of future developments in technological capabilities and their clinical implications, which we anticipate will lead to PAI becoming a desirable and widely used imaging modality in oncological research and practice.

Advances in Endoscopic Photoacoustic Imaging

Photoacoustic (PA) imaging is able to provide extremely high molecular contrast while maintaining the superior imaging depth of ultrasound (US) imaging. Conventional microscopic PA imaging has limited access to deeper tissue due to strong light scattering and attenuation. Endoscopic PA technology enables direct delivery of excitation light into the interior of a hollow organ or cavity of the body for functional and molecular PA imaging of target tissue. Various endoscopic PA probes have been developed for different applications, including the intravascular imaging of lipids in atherosclerotic plaque and endoscopic imaging of colon cancer. In this paper, the authors review representative probe configurations and corresponding preclinical applications. In addition, the potential challenges and future directions of endoscopic PA imaging are discussed.

Endocavity ultrasound and photoacoustic system for fetal and maternal imaging: design, implementation, and ex-vivo validation

Purpose: Transvaginal ultrasound (TVUS) is a widely used real-time and non-invasive imaging technique for fetal and maternal care. It can provide structural and functional measurements about the fetal brain, such as blood vessel diameter and blood flow. However, it lacks certain biochemical estimations, such as hemoglobin oxygen saturation ( SO 2 ), which limits its ability to indicate a fetus at risk of birth asphyxia. Photoacoustic (PA) imaging has been steadily growing in recognition as a complement to ultrasound (US). Studies have shown PA imaging is capable of providing such biochemical estimations as SO 2 at relatively high penetration depth (up to 30 mm). Approach: In this study, we have designed and developed a multi-modal (US, PA, and Doppler) endocavity imaging system (ECUSPA) around a commercialized TVUS probe (Philips ATL C9-5). Results: The integrated system was evaluated through a set of in-vitro, ex-vivo, and in-vivo studies. Imaging of excised sheep brain tissue demonstrated the system's utility and penetration depth in transfontanelle imaging conditions. The accuracy of using the spectroscopic PA imaging (sPA) method to estimate SO 2 was validated by comparing sPA oximetry results with the gold standard measurements indicated by a blood gas analyzer. The ability of US and Doppler to measure moving blood volume was evaluated in-vivo. Spectral unmixing capabilities were tested using fluorophores within sheep brains. Conclusion: The developed system is a high resolution (about 200    μ m at 30 mm depth), real-time (at 30 Hz), and quantitative ( SO 2 estimation error < 10 % ) imaging tool with a total diameter less than 30 mm, making it suitable for intrapartum applications such as fetal and maternal diagnostics.

Efficacy of High Temporal Frequency Photoacoustic Guidance of Laser Ablation Procedures

Inaccurate placement of the ablation catheter and the inability to monitor the real-time temperature within the tissue of interest such as veins curbs the treatment efficacy of laser ablation procedures during thermal therapies. Our previous studies have validated the efficacy of photoacoustic (PA) imaging during endovenous laser ablation (EVLA) procedures. However, the PA-guided therapies suffer from low temporal resolution, due to the low pulse repetition rates of pulsed lasers, which could cause a problem during fast catheter motion and rapid temperature changes. Herein, to enhance the accuracy and sensitivity for tracking the ablation catheter tip and temperature monitoring, we proposed to develop a high frame rate (500 Hz), combined ultrasound (US), and PA-guided ablation system. The proposed PA-guided ablation system was evaluated in a set of ex vivo tissue studies. The developed system provides a 2 ms temporal resolution for tracking and monitoring the ablation catheter tip's location and temperature, which is 50 times higher temporal resolution compared to the previously proposed 10 Hz system. The proposed system also provided more accurate feedback about the temperature variations during rapid temperature increments of 10°C per 250 ms. The co-registered US and PA images have an imaging resolution of about 200 μm and a field of view of 45 × 40 mm². Tracking the ablation catheter tip in an excised tissue layer shows higher accuracy during a relatively fast catheter motion (0.5-3 mm/s). The fast US/PA-guided ablation system will potentially enhance the outcome of ablation procedures by providing location and temperature feedback.

Photoacoustic-guided surgery from head to toe [Invited]

Photoacoustic imaging-the combination of optics and acoustics to visualize differences in optical absorption - has recently demonstrated strong viability as a promising method to provide critical guidance of multiple surgeries and procedures. Benefits include its potential to assist with tumor resection, identify hemorrhaged and ablated tissue, visualize metal implants (e.g., needle tips, tool tips, brachytherapy seeds), track catheter tips, and avoid accidental injury to critical subsurface anatomy (e.g., major vessels and nerves hidden by tissue during surgery). These benefits are significant because they reduce surgical error, associated surgery-related complications (e.g., cancer recurrence, paralysis, excessive bleeding), and accidental patient death in the operating room. This invited review covers multiple aspects of the use of photoacoustic imaging to guide both surgical and related non-surgical interventions. Applicable organ systems span structures within the head to contents of the toes, with an eye toward surgical and interventional translation for the benefit of patients and for use in operating rooms and interventional suites worldwide. We additionally include a critical discussion of complete systems and tools needed to maximize the success of surgical and interventional applications of photoacoustic-based technology, spanning light delivery, acoustic detection, and robotic methods. Multiple enabling hardware and software integration components are also discussed, concluding with a summary and future outlook based on the current state of technological developments, recent achievements, and possible new directions.

Spectroscopic photoacoustic imaging of cervical tissue composition in excised human samples

Objective

Cervical remodeling is an important component in determining the pathway of parturition; therefore, assessing changes in cervical tissue composition may provide information about the cervix’s status beyond the measurement of cervical length. Photoacoustic imaging is a non-invasive ultrasound-based technology that captures acoustic signals emitted by tissue components in response to laser pulses. This optical information allows for the determination of the collagen-to-water ratio (CWR). The purpose of this study was to compare the CWR evaluated by using spectroscopic photoacoustic (sPA) imaging in cervical samples obtained from pregnant and non-pregnant women.

Methods

This cross-sectional study comprised cervical biopsies obtained at the time of hysterectomy (n = 8) and at the scheduled cesarean delivery in pregnant women at term who were not in labor (n = 8). The cervical CWR was analyzed using a fiber-optic light-delivery system integrated to an ultrasound probe. The photoacoustic signals were acquired within the range of wavelengths that cover the peak absorption of collagen and water. Differences in the CWR between cervical samples from pregnant and non-pregnant women were analyzed. Hematoxylin and eosin and Sirius Red stains were used to compare the collagen content of cervical samples in these two groups.

Results

Eight cervix samples were obtained after hysterectomy, four from women ≤41 years of age and four from women ≥43 years of age; all cervical samples (n = 8) from pregnant women were obtained after 37 weeks of gestation at the time of cesarean section. The average CWR in cervical tissue samples from pregnant women was 18.7% (SD 7.5%), while in samples from non-pregnant women, it was 55.0% (SD 20.3%). There was a significantly higher CWR in the non-pregnant group compared to the pregnant group with a p-value <0.001. A subgroup analysis that compared the CWR in cervical samples from pregnant women and non-pregnant women ≤41 years of age (mean 46.3%, SD 23.1%) also showed a significantly higher CWR (p <0.01). Lower collagen content in the pregnancy group was confirmed by histological analysis, which revealed the loss of tissue composition, increased water content, and collagen degradation.

Conclusion

The proposed bimodal ultrasound and sPA imaging system can provide information on the biochemical composition of cervical tissue in pregnant and non-pregnant women. Photoacoustic imaging showed a higher collagen content in cervical samples from non-pregnant women as compared to those from pregnant women, which matched with the histological analysis. This novel imaging method envisions a new potential for a sensitive diagnostic tool in the evaluation of cervical tissue composition.

Dual-Illumination Ultrasound/ Photoacoustic System for Cervical Cancer imaging

Early stage cancer detection technologies can provide functional information and potentially decrease the mortality rate caused by cervical cancer. In our previous work, a miniaturized ultrasound and photoacoustic endoscopic system has been developed to image the cervical tissue through the cervical canal to fulfills the need for a safe, low-cost, and high-resolution functional diagnostic system. However, the miniaturized size of endoscope and American National Standards Institute safety limits cause constraints of using high-intensity illumination during imaging. In addition, the strong light scattering of tissues limits the light penetration depth. Fortunately, the cervix anatomy allows for the delivery of additional light from the ectocervix by using an external illumination system. Here we propose a dual, co-planar illumination system, which can provide adequate illumination to the cervical tissue via combined internal and external light delivery strategies. Therefore, an increase in the area of light-tissue interaction allows us to raise the laser light energy while keeping fluence under safety limits. Thus, a reliable PA imaging can be obtained for the whole cervical tissue thickness. The system performance was tested using a Monte Carlo simulation, and laser-light fluence was calculated and compared at different depths within a simulated cervical-tissue model. The results indicated a higher and more uniform fluence in the Monte Carlo simulations. In addition, the photoacoustic imaging of the proposed system was evaluated by two cervical tissue-mimicking phantoms with human blood and graphite rods as inclusions inside it. In accordance with the simulations, the phantom study revealed a more reliable photoacoustic signal for the entire depth of the phantoms with an improved contrast to noise ratio and signal to noise ratio, and a higher coverage ratio of the imaging field of view. In summary, the dual-mode illumination system can provide more realistic information of inclusions within the tissue while considering safety limits, which can lead to more accuracy in biomarker detection for cervical cancer diagnostics.

Photoacoustic imaging with fiber optic technology: A review

Photoacoustic imaging (PAI) has achieved remarkable growth in the past few decades since it takes advantage of both optical and ultrasound (US) imaging. In order to better promote the wide clinical applications of PAI, many miniaturized and portable PAI systems have recently been proposed. Most of these systems utilize fiber optic technologies. Here, we overview the fiber optic technologies used in PAI. This paper discusses three different fiber optic technologies: fiber optic light transmission, fiber optic US transmission, and fiber optic US detection. These fiber optic technologies are analyzed in different PAI modalities including photoacoustic microscopy (PAM), photoacoustic computed tomography (PACT), and minimally invasive photoacoustic imaging (MIPAI).

Evaluation of Reconstruction Methodology for Helical Scan Guided Photoacoustic Endoscopy

Photoacoustic endoscopy (PAE), combining both advantages of optical contrast and acoustic resolution, can visualize the chemical-specific optical information of tissues inside human-body. Recently, its corresponding reconstruction methods have been extensively researched. However, most of them are limited on cylindrical scan trajectories, rather than a helical scan which is more clinically practical. On this note, this article proposes a methodology of imaging reconstruction and evaluation for helical scan guided PAE. Different from traditional reconstruction method, synthetic aperture focusing technique (SAFT), our method reconstructs image using wavefield extrapolation which significantly improves computational efficiency and even takes only 0.25 seconds for 3-D reconstructions. In addition, the proposed evaluation methodology can estimate the resolutions and deviations of reconstructed images in advance, and then can be used to optimize the PAE scan parameters. Groups of simulations as well as ex-vivo experiments with different scan parameters are provided to fully demonstrate the performance of the proposed techniques. The quantitatively measured angular resolutions and deviations agree well with our theoretical derivation results D√{r_s² +h²} / [1.25(r_s r_d +h²)] (rad) and -h l / (r_s r_d +h²) (rad), respectively D,r_d, r_s,h and l represent transducer diameter, radius of scan trajectory, radius of source position, unit helical pitch and the distance from targets to helical scan plane, respectively). This theoretical result also suits for circular and cylindrical scan in case of h = 0 .

A Ceramic PZT-based PMUT Array for Endoscopic Photoacoustic Imaging

In this paper, we present the design, fabrication, and characterization of a compact 4 × 4 piezoelectric micromachined ultrasonic transducer (pMUT) array and its application to photoacoustic imaging. The uniqueness of this pMUT array is the integration of a 4 μm-thick ceramic PZT, having significantly higher piezoelectric coefficient and lower stress than sol-gel or sputtered PZT. The fabricated pMUT array has a small chip size of only 1.8 × 1.6 mm² with each pMUT element having a diameter of 210 μm. The fabricated device was characterized with electrical impedance measurement and acoustic sensing test. Photoacoustic imaging has also been successfully demonstrated on an agar phantom with a pencil lead embedded using the fabricated pMUT array.

A Ceramic PZT-based PMUT Array for Endoscopic Photoacoustic Imaging

In this paper, we present the design, fabrication, and characterization of a compact 4 × 4 piezoelectric micromachined ultrasonic transducer (pMUT) array and its application to photoacoustic imaging. The uniqueness of this pMUT array is the integration of a 4 μm-thick ceramic PZT, having significantly higher piezoelectric coefficient and lower stress than sol-gel or sputtered PZT. The fabricated pMUT array has a small chip size of only 1.8 × 1.6 mm² with each pMUT element having a diameter of 210 μm. The fabricated device was characterized with electrical impedance measurement and acoustic sensing test. Photoacoustic imaging has also been successfully demonstrated on an agar phantom with a pencil lead embedded using the fabricated pMUT array.

Photoacoustic image improvement based on a combination of sparse coding and filtering

SIGNIFICANCE

Photoacoustic imaging (PAI) has been greatly developed in a broad range of diagnostic applications. The efficiency of light to sound conversion in PAI is limited by the ubiquitous noise arising from the tissue background, leading to a low signal-to-noise ratio (SNR), and thus a poor quality of images. Frame averaging has been widely used to reduce the noise; however, it compromises the temporal resolution of PAI.

AIM

We propose an approach for photoacoustic (PA) signal denoising based on a combination of low-pass filtering and sparse coding (LPFSC).

APPROACH

LPFSC method is based on the fact that PA signal can be modeled as the sum of low frequency and sparse components, which allows for the reduction of noise levels using a hybrid alternating direction method of multipliers in an optimization process.

RESULTS

LPFSC method was evaluated using in-silico and experimental phantoms. The results show a 26% improvement in the peak SNR of PA signal compared to the averaging method for in-silico data. On average, LPFSC method offers a 63% improvement in the image contrast-to-noise ratio and a 33% improvement in the structural similarity index compared to the averaging method for objects located at three different depths, ranging from 10 to 20 mm, in a porcine tissue phantom.

CONCLUSIONS

The proposed method is an effective tool for PA signal denoising, whereas it ultimately improves the quality of reconstructed images, especially at higher depths, without limiting the image acquisition speed.

Photoacoustic Imaging to Track Magnetic-manipulated Micro-Robots in Deep Tissue

The next generation of intelligent robotic systems has been envisioned as micro-scale mobile and externally controllable robots. Visualization of such small size microrobots to track their motion in nontransparent medium such as human tissue remains a major challenge, limiting translation into clinical applications. Herein, we present a novel, non-invasive, real-time imaging method by integrating ultrasound (US) and photoacoustic (PA) imaging modalities for tracking and detecting the motion of a single microrobot in deep biological tissue. We developed and evaluated a prototyped PA-guided magnetic microrobot tracking system. The microrobots are fabricated using photoresist mixed with nickel (Ni) particles. The microrobot motion was controlled using an externally applied magnetic field. Our experimental results evaluated the capabilities of PA imaging in visualizing and tracking microrobots in opaque tissue and tissue-mimicking phantoms. The results also demonstrate the ability of PA imaging in detecting a microrobot with the sizes less than the minimum detectable size by US imaging (down to 50 µm). The spectroscopic PA imaging studies determined an optimal wavelength (700 nm) for imaging microrobots with embedded Ni particles in oxygenated (fresh) human blood. In addition, we examined the ability of PA imaging to detect the microrobots through a nontransparent tissue mimic and at a depth of 25 mm, where conventional optical methods are unable to be used in tracking the objects. These initial results demonstrate the feasibility of an integrated US and PA imaging method to push the boundaries of microrobot applications into translational applications.

Miniature probe for dual-modality photoacoustic microscopy and white-light microscopy for image guidance: A prototype toward an endoscope

In this study, a novel photoacoustic microscopy (PAM) probe integrating white-light microscopy (WLM) modality that provides guidance for PAM imaging and complementary information is implemented. One single core of an imaging fiber bundle is employed to deliver a pulsed laser for photoacoustic excitation for PAM mode, which provides high resolution with deep penetration. Meanwhile, for WLM mode, the imaging fiber bundle is used to transmit two-dimensional superficial images. Lateral resolution of 7.2 μm for PAM is achieved. Since miniature components are used, the probe diameter is only 1.7 mm. Imaging of phantom and animals in vivo is conducted to show the imaging capability of the probe. The probe has several advantages by introducing the WLM mode, such as being able to conveniently identify regions of interest and align the focus for PAM mode. The prototype of an endoscope shows potential to facilitate clinical photoacoustic endoscopic applications.

The role of noninvasive diagnostic imaging in monitoring pregnancy and detecting patients at risk for preterm birth: a review of quantitative approaches

Preterm birth (PTB) is the leading cause of neonatal morbidity and mortality worldwide. The ability to predict patients at risk for preterm birth remains a major health challenge. The currently available clinical diagnostics such as cervical length and fetal fibronectin may detect only up to 30% of patients who eventually experience a spontaneous preterm birth. This paper reviews ongoing efforts to improve the ability to conduct a risk assessment for preterm birth. In particular, this work focuses on quantitative methods of imaging using ultrasound-based techniques, magnetic resonance imaging, and optical imaging modalities. While ultrasound imaging is the major modality for preterm birth risk assessment, a summary of efforts to adopt other imaging modalities is also discussed to identify the technical and diagnostic limits associated with adopting them in clinical settings. We conclude the review by proposing a new approach using combined photoacoustic, ultrasound, and elastography as a potential means to better assess cervical tissue remodeling, and thus improve the detection of patients at-risk of PTB.

Fan-shaped scanning approach for miniaturized photoacoustic tomography

We describe a novel scanning approach for miniaturized photoacoustic tomography (PAT), based on fan-shaped scanning of a single transducer at one or two discrete positions. This approach is tested and evaluated using several phantom and animal experiments. The results obtained show that this new scanning approach provides high image quality in the configuration of miniaturized handheld or endoscopic PAT with improved effective field of view and penetration depth.