Performance characteristics of an interventional multispectral photoacoustic imaging system for guiding minimally invasive procedures

Improving needle visibility in LED-based photoacoustic imaging using deep learning with semi-synthetic datasets

Photoacoustic imaging has shown great potential for guiding minimally invasive procedures by accurate identification of critical tissue targets and invasive medical devices (such as metallic needles). The use of light emitting diodes (LEDs) as the excitation light sources accelerates its clinical translation owing to its high affordability and portability. However, needle visibility in LED-based photoacoustic imaging is compromised primarily due to its low optical fluence. In this work, we propose a deep learning framework based on U-Net to improve the visibility of clinical metallic needles with a LED-based photoacoustic and ultrasound imaging system. To address the complexity of capturing ground truth for real data and the poor realism of purely simulated data, this framework included the generation of semi-synthetic training datasets combining both simulated data to represent features from the needles and in vivo measurements for tissue background. Evaluation of the trained neural network was performed with needle insertions into blood-vessel-mimicking phantoms, pork joint tissue ex vivo and measurements on human volunteers. This deep learning-based framework substantially improved the needle visibility in photoacoustic imaging in vivo compared to conventional reconstruction by suppressing background noise and image artefacts, achieving 5.8 and 4.5 times improvements in terms of signal-to-noise ratio and the modified Hausdorff distance, respectively. Thus, the proposed framework could be helpful for reducing complications during percutaneous needle insertions by accurate identification of clinical needles in photoacoustic imaging.

Video-rate dual-modal photoacoustic and fluorescence imaging through a multimode fibre towards forward-viewing endomicroscopy

Multimode fibres (MMFs) are becoming increasingly attractive in optical endoscopy as they promise to enable unparallelled miniaturisation, spatial resolution and cost. However, high-speed imaging with wavefront shaping has been challenging. Here, we report the development of a video-rate dual-modal photoacoustic (PA) and fluorescence microscopy probe with a high-speed digital micromirror device (DMD) towards forward-viewing endomicroscopy. Optimal DMD patterns were obtained using a real-valued intensity transmission matrix algorithm to raster-scan a 1.5  μ m-diameter focused beam at the distal fibre tip for imaging. The PA imaging speed and spatial resolution were varied from  ∼ 2 to 57 frames per second and from 1.7 to 3  μ m, respectively. Further, high-fidelity PA images of carbon fibres and mouse red blood cells were acquired at unprecedented speed. The capability of dual-modal imaging was demonstrated with phantoms. We anticipate that with further miniaturisation of the ultrasound detector, this probe could be integrated into medical needles to guide minimally invasive procedures.

Review of consensus test methods in medical imaging and current practices in photoacoustic image quality assessment

SIGNIFICANCE

Photoacoustic imaging (PAI) is a powerful emerging technology with broad clinical applications, but consensus test methods are needed to standardize performance evaluation and accelerate translation.

AIM

To review consensus image quality test methods for mature imaging modalities [ultrasound, magnetic resonance imaging (MRI), x-ray CT, and x-ray mammography], identify best practices in phantom design and testing procedures, and compare against current practices in PAI phantom testing.

APPROACH

We reviewed scientific papers, international standards, clinical accreditation guidelines, and professional society recommendations describing medical image quality test methods. Observations are organized by image quality characteristics (IQCs), including spatial resolution, geometric accuracy, imaging depth, uniformity, sensitivity, low-contrast detectability, and artifacts.

RESULTS

Consensus documents typically prescribed phantom geometry and material property requirements, as well as specific data acquisition and analysis protocols to optimize test consistency and reproducibility. While these documents considered a wide array of IQCs, reported PAI phantom testing focused heavily on in-plane resolution, depth of visualization, and sensitivity. Understudied IQCs that merit further consideration include out-of-plane resolution, geometric accuracy, uniformity, low-contrast detectability, and co-registration accuracy.

CONCLUSIONS

Available medical image quality standards provide a blueprint for establishing consensus best practices for photoacoustic image quality assessment and thus hastening PAI technology advancement, translation, and clinical adoption.

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.

Extending Imaging Depth in PLD-Based Photoacoustic Imaging: Moving Beyond Averaging

Pulsed laser diodes (PLDs) promise to be an attractive alternative to solid-state laser sources in photoacoustic tomography (PAT) due to their portability, high-pulse repetition frequency (PRF), and cost effectiveness. However, due to their lower energy per pulse, which, in turn, results in lower fluence required per photoacoustic signal generation, PLD-based photoacoustic systems generally have maximum imaging depth that is lower in comparison to solid-state lasers. Averaging of multiple frames is usually employed as a common practice in high PRF PLD systems to improve the signal-to-noise ratio of the PAT images. In this work, we demonstrate that by combining the recently described approach of subpitch translation on the receive-side ultrasound transducer alongside averaging of multiple frames, it is feasible to increase the depth sensitivity in a PLD-based PAT imaging system. Here, experiments on phantom containing diluted India ink targets were performed at two different laser energy level settings, that is, 21 and [Formula: see text]. Results obtained showed that the imaging depth improves by ~38.5% from 9.1 to 12.6 mm for 21- [Formula: see text] energy level setting and by ~33.3% from 10.8 to 14.4 mm for 27- [Formula: see text] energy level setting by using λ /4-pitch translation and average of 128 frames in comparison to λ -pitch data acquired with the average of 128 frames. However, the achievable frame rate is reduced by a factor of 2 and 4 for λ /2 and λ /4 subpitch translation, respectively.

Internal-Illumination Photoacoustic Tomography Enhanced by a Graded-Scattering Fiber Diffuser

The penetration depth of photoacoustic imaging in biological tissues has been fundamentally limited by the strong optical attenuation when light is delivered externally through the tissue surface. To address this issue, we previously reported internal-illumination photoacoustic imaging using a customized radial-emission optical fiber diffuser, which, however, has complex fabrication, high cost, and non-uniform light emission. To overcome these shortcomings, we have developed a new type of low-cost fiber diffusers based on a graded-scattering method in which the optical scattering of the fiber diffuser is gradually increased as the light travels. The graded scattering can compensate for the optical attenuation and provide relatively uniform light emission along the diffuser. We performed Monte Carlo numerical simulations to optimize several key design parameters, including the number of scattering segments, scattering anisotropy factor, divergence angle of the optical fiber, and reflective index of the surrounding medium. These optimized parameters collectively result in uniform light emission along the fiber diffuser and can be flexibly adjusted to accommodate different applications. We fabricated and characterized the prototype fiber diffuser made of agarose gel and intralipid. Equipped with the new fiber diffuser, we performed thorough proof-of-concept studies on ex vivo tissue phantoms and an in vivo swine model to demonstrate the deep-imaging capability (~10 cm achieved ex vivo) of photoacoustic tomography. We believe that the internal light delivery via the optimized fiber diffuser is an effective strategy to image deep targets (e.g., kidney) in large animals or humans.

Characterization of interventional photoacoustic imaging (iPAI) capabilities in biological tissues

BACKGROUND

Interventional photoacoustic imaging (iPAI) could improve ultrasound-guided minimally invasive procedures by enabling high precision needle steering, target detection, and molecular and physiologic tissue assessment. However, iPAI capabilities including visualization field, imaging depth, and spatial resolution are not well understood in biological tissues commonly encountered in clinical practice. Therefore, the potential clinical utility of iPAI remains unclear. We aim to experimentally determine iPAI capabilities in a variety of biological tissues, to assess its potential for clinical translation.

METHODS

We constructed an iPAI system capable of simultaneous real-time ultrasound (US) and photoacoustic imaging. This system delivers light directly into tissues using optical fiber integrated into a 16-gauge needle and detects photoacoustic signals with an external linear array ultrasound probe. iPAI's geometric visualization field, maximum imaging depth, and spatial resolution were experimentally determined in fat, muscle, kidney, and liver tissues by processing photoacoustic signal intensities of reference targets placed circumferentially around the fiber tip. The maximum detection depths of blood and indocyanine green (ICG), important common endogenous and exogenous contrast agents, respectively, were estimated in each tissue type by comparing their signal intensities with the reference target signal.

RESULTS

iPAI could be performed in real-time concurrently with US and achieved a nearly spherical visualization field centering around the optical fiber tip in all tissues. Maximum imaging depths from the fiber tip were 54.1 ± 1.3, 50.0 ± 1.5, 32.7 ± 1.1, and 16.9 ± 1.3 mm in fat, muscle, kidney, and liver tissues, respectively. Calculated maximum detection depths for blood were 41.5 ± 3.0, 39.5 ± 2.1, 24.4 ± 4.0, and 8.6 ± 2.0 mm and detection depths for ICG at 0.05  mg/mL concentration were 46.6 ± 2.5, 42.6 ± 1.4, 28.2 ± 3.9, and 12.1 ± 1.5 mm in fat, muscle, kidney, and liver, respectively. Sub-100μ m axial resolution and submillimeter lateral resolution were achieved in all tissues, and resolution did not significantly vary with distance from the fiber tip.

CONCLUSIONS

Interventional photoacoustic imaging (iPAI) allows real-time visualization of a circumferential volume of tissue around an optical fiber tip, with submillimeter spatial resolution and tissue-dependent imaging depth. Our data strongly support further development of clinical iPAI systems as they could improve needle steering, target detection, and molecular and physiologic tissue assessment during minimally invasive procedures.

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).

Portable and Affordable Light Source-Based Photoacoustic Tomography

Photoacoustic imaging is a hybrid imaging modality that offers the advantages of optical (spectroscopic contrast) and ultrasound imaging (scalable spatial resolution and imaging depth). This promising modality has shown excellent potential in a wide range of preclinical and clinical imaging and sensing applications. Even though photoacoustic imaging technology has matured in research settings, its clinical translation is not happening at the expected pace. One of the main reasons for this is the requirement of bulky and expensive pulsed lasers for excitation. To accelerate the clinical translation of photoacoustic imaging and explore its potential in resource-limited settings, it is of paramount importance to develop portable and affordable light sources that can be used as the excitation light source. In this review, we focus on the following aspects: (1) the basic theory of photoacoustic imaging; (2) inexpensive light sources and different implementations; and (3) important preclinical and clinical applications, demonstrated using affordable light source-based photoacoustics. The main focus will be on laser diodes and light-emitting diodes as they have demonstrated promise in photoacoustic tomography-the key technological developments in these areas will be thoroughly reviewed. We believe that this review will be a useful opus for both the beginners and experts in the field of biomedical photoacoustic imaging.

Evaluation of Fluence Correction Algorithms in Multispectral Photoacoustic Imaging

Multispectral photoacoustic imaging (MPAI) is a promising emerging diagnostic technology, but fluence artifacts can degrade device performance. Our goal was to develop well-validated phantom-based test methods for evaluating and comparing MPAI fluence correction algorithms, including a heuristic diffusion approximation, Monte Carlo simulations, and an algorithm we developed based on novel application of the diffusion dipole model (DDM). Phantoms simulated a range of breast-mimicking optical properties and contained channels filled with chromophore solutions (ink, hemoglobin, or copper sulfate) or connected to a previously developed blood flow circuit providing tunable oxygen saturation (SO₂). The DDM algorithm achieved similar spectral recovery and SO₂ measurement accuracy to Monte Carlo-based corrections with lower computational cost, potentially providing an accurate, real-time correction approach. Algorithms were sensitive to optical property uncertainty, but error was minimized by matching phantom albedo. The developed test methods may provide a foundation for standardized assessment of MPAI fluence correction algorithm performance.

Photoacoustic imaging for surgical guidance: Principles, applications, and outlook

Minimally invasive surgeries often require complicated maneuvers and delicate hand-eye coordination and ideally would incorporate "x-ray vision" to see beyond tool tips and underneath tissues prior to making incisions. Photoacoustic imaging has the potential to offer this feature but not with ionizing x-rays. Instead, optical fibers and acoustic receivers enable photoacoustic sensing of major structures-such as blood vessels and nerves-that are otherwise hidden from view. This imaging process is initiated by transmitting laser pulses that illuminate regions of interest, causing thermal expansion and the generation of sound waves that are detectable with conventional ultrasound transducers. The recorded signals are then converted to images through the beamforming process. Photoacoustic imaging may be implemented to both target and avoid blood-rich surgical contents (and in some cases simultaneously or independently visualize optical fiber tips or metallic surgical tool tips) in order to prevent accidental injury and assist device operators during minimally invasive surgeries and interventional procedures. Novel light delivery systems, counterintuitive findings, and robotic integration methods introduced by the Photoacoustic & Ultrasonic Systems Engineering Lab are summarized in this invited Perspective, setting the foundation and rationale for the subsequent discussion of the author's views on possible future directions for this exciting frontier known as photoacoustic-guided surgery.

Towards Clinical Translation of LED-Based Photoacoustic Imaging: A Review

Photoacoustic imaging, with the capability to provide simultaneous structural, functional, and molecular information, is one of the fastest growing biomedical imaging modalities of recent times. As a hybrid modality, it not only provides greater penetration depth than the purely optical imaging techniques, but also provides optical contrast of molecular components in the living tissue. Conventionally, photoacoustic imaging systems utilize bulky and expensive class IV lasers, which is one of the key factors hindering the clinical translation of this promising modality. Use of LEDs which are portable and affordable offers a unique opportunity to accelerate the clinical translation of photoacoustics. In this paper, we first review the development history of LED as an illumination source in biomedical photoacoustic imaging. Key developments in this area, from point-source measurements to development of high-power LED arrays, are briefly discussed. Finally, we thoroughly review multiple phantom, ex-vivo, animal in-vivo, human in-vivo, and clinical pilot studies and demonstrate the unprecedented preclinical and clinical potential of LED-based photoacoustic imaging.

Photoacoustic imaging of the human placental vasculature

Minimally invasive fetal interventions require accurate imaging from inside the uterine cavity. Twin-to-twin transfusion syndrome (TTTS), a condition considered in this study, occurs from abnormal vascular anastomoses in the placenta that allow blood to flow unevenly between the fetuses. Currently, TTTS is treated fetoscopically by identifying the anastomosing vessels, and then performing laser photocoagulation. However, white light fetoscopy provides limited visibility of placental vasculature, which can lead to missed anastomoses or incomplete photocoagulation. Photoacoustic (PA) imaging is an alternative imaging method that provides contrast for hemoglobin, and in this study, two PA systems were used to visualize chorionic (fetal) superficial and subsurface vasculature in human placentas. The first system comprised an optical parametric oscillator for PA excitation and a 2D Fabry-Pérot cavity ultrasound sensor; the second, light emitting diode arrays and a 1D clinical linear-array ultrasound imaging probe. Volumetric photoacoustic images were acquired from ex vivo normal term and TTTS-treated placentas. It was shown that superficial and subsurface branching blood vessels could be visualized to depths of approximately 7 mm, and that ablated tissue yielded negative image contrast. This study demonstrated the strong potential of PA imaging to guide minimally invasive fetal therapies.

Photoacoustic image-guided interventions

Photoacoustic imaging has demonstrated its potential for diagnosis over the last few decades. In recent years, its unique imaging capabilities, such as detecting structural, functional and molecular information in deep regions with optical contrast and ultrasound resolution, have opened up many opportunities for photoacoustic imaging to be used during image-guided interventions. Numerous studies have investigated the capability of photoacoustic imaging to guide various interventions such as drug delivery, therapies, surgeries, and biopsies. These studies have demonstrated that photoacoustic imaging can guide these interventions effectively and non-invasively in real-time. In this minireview, we will elucidate the potential of photoacoustic imaging in guiding active and passive drug deliveries, photothermal therapy, and other surgeries and therapies using endogenous and exogenous contrast agents including organic, inorganic, and hybrid nanoparticles, as well as needle-based biopsy procedures. The advantages of photoacoustic imaging in guided interventions will be discussed. It will, therefore, show that photoacoustic imaging has great potential in real-time interventions due to its advantages over current imaging modalities like computed tomography, magnetic resonance imaging, and ultrasound imaging.

Three-dimensional interventional photoacoustic imaging for biopsy needle guidance with a linear array transducer

Needle placement is important for many clinical interventions, such as tissue biopsy, regional anesthesia and drug delivery. It is essential to visualize the spatial position of the needle and the target tissue during the interventions using appropriate imaging techniques. Based on the contrast of optical absorption, photoacoustic imaging is well suited for the guidance of interventional procedures. However, conventional photoacoustic imaging typically provides two-dimensional (2D) slices of the region of interest and could only visualize the needle and the target when they are within the imaging plane of the probe at the same time. This requires great alignment skill and effort. To ease this problem, we developed a 3D interventional photoacoustic imaging technique by fast scanning a linear array ultrasound probe and stitching acquired image slices. in vivo sentinel lymph node biopsy experiment shows that the technique could precisely locate a needle and a sentinel lymph node in a tissue volume while a perfusion experiment demonstrates that the technique could visualize the 3D distribution of injected methylene blue dye underneath the skin at high temporal and spatial resolution. The proposed technique provides a practical way for photoacoustic image-guided interventions.

Minimally invasive photoacoustic imaging: Current status and future perspectives

Photoacoustic imaging (PAI) is an emerging biomedical imaging modality that is based on optical absorption contrast, capable of revealing distinct spectroscopic signatures of tissue at high spatial resolution and large imaging depths. However, clinical applications of conventional non-invasive PAI systems have been restricted to examinations of tissues at depths less than a few cm due to strong light attenuation. Minimally invasive photoacoustic imaging (miPAI) has greatly extended the landscape of PAI by delivering excitation light within tissue through miniature fibre-optic probes. In the past decade, various miPAI systems have been developed with demonstrated applicability in several clinical fields. In this article, we present an overview of the current status of miPAI and our thoughts on future perspectives.

3D Ultrasonic Needle Tracking with a 1.5D Transducer Array for Guidance of Fetal Interventions

Ultrasound image guidance is widely used in minimally invasive procedures, including fetal surgery. In this context, maintaining visibility of medical devices is a significant challenge. Needles and catheters can readily deviate from the ultrasound imaging plane as they are inserted. When the medical device tips are not visible, they can damage critical structures, with potentially profound consequences including loss of pregnancy. In this study, we performed 3D ultrasonic tracking of a needle using a novel probe with a 1.5D array of transducer elements that was driven by a commercial ultrasound system. A fiber-optic hydrophone integrated into the needle received transmissions from the probe, and data from this sensor was processed to estimate the position of the hydrophone tip in the coordinate space of the probe. Golay coding was used to increase the signal-to-noise (SNR). The relative tracking accuracy was better than 0.4 mm in all dimensions, as evaluated using a water phantom. To obtain a preliminary indication of the clinical potential of 3D ultrasonic needle tracking, an intravascular needle insertion was performed in an in vivo pregnant sheep model. The SNR values ranged from 12 to 16 at depths of 20 to 31 mm and at an insertion angle of 49° relative to the probe surface normal. The results of this study demonstrate that 3D ultrasonic needle tracking with a fiber-optic hydrophone sensor and a 1.5D array is feasible in clinically realistic environments.

Three-Dimensional Ultrasonic Needle Tip Tracking with a Fiber-Optic Ultrasound Receiver

Ultrasound is frequently used for guiding minimally invasive procedures, but visualizing medical devices is often challenging with this imaging modality. When visualization is lost, the medical device can cause trauma to critical tissue structures. Here, a method to track the needle tip during ultrasound image-guided procedures is presented. This method involves the use of a fiber-optic ultrasound receiver that is affixed within the cannula of a medical needle to communicate ultrasonically with the external ultrasound probe. This custom probe comprises a central transducer element array and side element arrays. In addition to conventional two-dimensional (2D) B-mode ultrasound imaging provided by the central array, three-dimensional (3D) needle tip tracking is provided by the side arrays. For B-mode ultrasound imaging, a standard transmit-receive sequence with electronic beamforming is performed. For ultrasonic tracking, Golay-coded ultrasound transmissions from the 4 side arrays are received by the hydrophone sensor, and subsequently the received signals are decoded to identify the needle tip's spatial location with respect to the ultrasound imaging probe. As a preliminary validation of this method, insertions of the needle/hydrophone pair were performed in clinically realistic contexts. This novel ultrasound imaging/tracking method is compatible with current clinical workflow, and it provides reliable device tracking during in-plane and out-of-plane needle insertions.

Photoacoustic Source Detection and Reflection Artifact Removal Enabled by Deep Learning

Interventional applications of photoacoustic imaging typically require visualization of point-like targets, such as the small, circular, cross-sectional tips of needles, catheters, or brachytherapy seeds. When these point-like targets are imaged in the presence of highly echogenic structures, the resulting photoacoustic wave creates a reflection artifact that may appear as a true signal. We propose to use deep learning techniques to identify these types of noise artifacts for removal in experimental photoacoustic data. To achieve this goal, a convolutional neural network (CNN) was first trained to locate and classify sources and artifacts in pre-beamformed data simulated with -Wave. Simulations initially contained one source and one artifact with various medium sound speeds and 2-D target locations. Based on 3,468 test images, we achieved a 100% success rate in classifying both sources and artifacts. After adding noise to assess potential performance in more realistic imaging environments, we achieved at least 98% success rates for channel signal-to-noise ratios (SNRs) of -9dB or greater, with a severe decrease in performance below -21dB channel SNR. We then explored training with multiple sources and two types of acoustic receivers and achieved similar success with detecting point sources. Networks trained with simulated data were then transferred to experimental waterbath and phantom data with 100% and 96.67% source classification accuracy, respectively (particularly when networks were tested at depths that were included during training). The corresponding mean ± one standard deviation of the point source location error was 0.40 ± 0.22 mm and 0.38 ± 0.25 mm for waterbath and phantom experimental data, respectively, which provides some indication of the resolution limits of our new CNN-based imaging system. We finally show that the CNN-based information can be displayed in a novel artifact-free image format, enabling us to effectively remove reflection artifacts from photoacoustic images, which is not possible with traditional geometry-based beamforming.

In Vivo Imaging of Microvasculature during Anesthesia with High-Resolution Photoacoustic Microscopy

Anesthesia monitoring is extremely important in improving the quality of anesthesia and ensuring the safety of patients in operation. Photoacoustic microscopy (PAM) is proposed to in vivo image the skin microvasculature of 10 nude mice undergoing general anesthesia by using the isoflurane gas with a concentration of 3%. Benefiting from strong optical absorption of hemoglobin, PAM has good contrast and high resolution in mapping of microvasculature. A series of high quality images can clearly reveal the subtle changes of capillaries in morphology over time. Two indices, vessel intensity and vessel density, are extracted from these images to measure the microvasculature quantitatively. The imaging results show that the vessel intensity and density are increased over time. After 65 min, the vessel intensity increased 42.7 ± 8.6% and the density increased 28.6 ± 12.2%. These indices extracted from photoacoustic images accurately reflect the greater blood perfusion undergoing general anesthesia. Additionally, abnormal reductions of vessel intensity and density are also observed as overtime anesthesia. This preclinical study suggests that PAM holds potential to monitor anesthesia by imaging the skin microvasculature.

Handheld Real-Time LED-Based Photoacoustic and Ultrasound Imaging System for Accurate Visualization of Clinical Metal Needles and Superficial Vasculature to Guide Minimally Invasive Procedures

Ultrasound imaging is widely used to guide minimally invasive procedures, but the visualization of the invasive medical device and the procedure’s target is often challenging. Photoacoustic imaging has shown great promise for guiding minimally invasive procedures, but clinical translation of this technology has often been limited by bulky and expensive excitation sources. In this work, we demonstrate the feasibility of guiding minimally invasive procedures using a dual-mode photoacoustic and ultrasound imaging system with excitation from compact arrays of light-emitting diodes (LEDs) at 850 nm. Three validation experiments were performed. First, clinical metal needles inserted into biological tissue were imaged. Second, the imaging depth of the system was characterized using a blood-vessel-mimicking phantom. Third, the superficial vasculature in human volunteers was imaged. It was found that photoacoustic imaging enabled needle visualization with signal-to-noise ratios that were 1.2 to 2.2 times higher than those obtained with ultrasound imaging, over insertion angles of 26 to 51 degrees. With the blood vessel mimicking phantom, the maximum imaging depth was 38 mm. The superficial vasculature of a human middle finger and a human wrist were clearly visualized in real-time. We conclude that the LED-based system is promising for guiding minimally invasive procedures with peripheral tissue targets.

Internal-illumination photoacoustic computed tomography

We report a photoacoustic computed tomography (PACT) system using a customized optical fiber with a cylindrical diffuser to internally illuminate deep targets. The traditional external light illumination in PACT usually limits the penetration depth to a few centimeters from the tissue surface, mainly due to strong optical attenuation along the light propagation path from the outside in. By contrast, internal light illumination, with external ultrasound detection, can potentially detect much deeper targets. Different from previous internal illumination PACT implementations using forward-looking optical fibers, our internal-illumination PACT system uses a customized optical fiber with a 3-cm-long conoid needle diffuser attached to the fiber tip, which can homogeneously illuminate the surrounding space and substantially enlarge the field of view. We characterized the internal illumination distribution and PACT system performance. We performed tissue phantom and in vivo animal studies to further demonstrate the superior imaging depth using internal illumination over external illumination. We imaged a 7.5-cm-deep leaf target embedded in optically scattering medium and the beating heart of a mouse overlaid with 3.7-cm-thick chicken tissue. Our results have collectively demonstrated that the internal light illumination combined with external ultrasound detection might be a useful strategy to improve the penetration depth of PACT in imaging deep organs of large animals and humans.

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.

Through-needle all-optical ultrasound imaging in vivo: a preclinical swine study

High-frequency ultrasound imaging can provide exquisite visualizations of tissue to guide minimally invasive procedures. Here, we demonstrate that an all-optical ultrasound transducer, through which light guided by optical fibers is used to generate and receive ultrasound, is suitable for real-time invasive medical imaging in vivo. Broad-bandwidth ultrasound generation was achieved through the photoacoustic excitation of a multiwalled carbon nanotube-polydimethylsiloxane composite coating on the distal end of a 300-μm multi-mode optical fiber by a pulsed laser. The interrogation of a high-finesse Fabry-Pérot cavity on a single-mode optical fiber by a wavelength-tunable continuous-wave laser was applied for ultrasound reception. This transducer was integrated within a custom inner transseptal needle (diameter 1.08 mm; length 78 cm) that included a metallic septum to acoustically isolate the two optical fibers. The use of this needle within the beating heart of a pig provided unprecedented real-time views (50 Hz scan rate) of cardiac tissue (depth: 2.5 cm; axial resolution: 64 μm) and revealed the critical anatomical structures required to safely perform a transseptal crossing: the right and left atrial walls, the right atrial appendage, and the limbus fossae ovalis. This new paradigm will allow ultrasound imaging to be integrated into a broad range of minimally invasive devices in different clinical contexts.

Ultrasonic Needle Tracking with a Fibre-Optic Ultrasound Transmitter for Guidance of Minimally Invasive Fetal Surgery

Ultrasound imaging is widely used for guiding minimally invasive procedures, including fetal surgery. Visualisation of medical devices such as medical needles is critically important and it remains challenging in many clinical contexts. During in-plane insertions, a needle can have poor visibility at steep insertion angles and at large insertion depths. During out-of-plane insertions, the needle tip can have a similar ultrasonic appearance to the needle shaft when it intersects with the ultrasound imaging plane. When the needle tip is not accurately identified, it can damage critical structures, with potentially severe consequences, including loss of pregnancy. In this paper, we present a tracking system to directly visualise the needle tip with an ultrasonic beacon. The waves transmitted by the beacon were received by an external ultrasound imaging probe. Pairs of co-registered images were acquired in rapid succession with this probe: a photoacoustic image obtained with the system in receive-only mode, and a conventional B-mode ultrasound image. The beacon comprised a custom elastomeric nanocomposite coating at the distal end of an optical fibre, which was positioned within the lumen of a commercial 22 gauge needle. Delivery of pulsed light to the coating resulted in the photoacoustic generation of ultrasonic waves. The measured tracking accuracies in water in the axial and lateral dimensions were 0.39±0.19 mm and 1.85±0.29 mm, respectively. To obtain a preliminary indication of the clinical potential of this ultrasonic needle tracking system, needle insertions were performed in an in vivo fetal sheep model. The results demonstrate that ultrasonic needle tracking with a fibre-optic transmitter is feasible in a clinically realistic fetal surgery environment, and that it could be useful to guide minimally invasive procedures by providing accurate visualisation of the medical device tip.

Looking beyond the imaging plane: 3D needle tracking with a linear array ultrasound probe

Ultrasound is well suited for guiding many minimally invasive procedures, but its use is often precluded by the poor visibility of medical devices. When devices are not visible, they can damage critical structures, with life-threatening complications. Here, we developed the first ultrasound probe that comprises both focused and unfocused transducer elements to provide both 2D B-mode ultrasound imaging and 3D ultrasonic needle tracking. A fibre-optic hydrophone was integrated into a needle to receive Golay-coded transmissions from the probe and these data were processed to obtain tracking images of the needle tip. The measured tracking accuracy in water was better than 0.4 mm in all dimensions. To demonstrate the clinical potential of this system, insertions were performed into the spine and the uterine cavity, in swine and pregnant ovine models in vivo. In both models, the SNR ranged from 13 to 38 at depths of 22 to 38 mm, at out-of-plane distances of 1 to 15 mm, and at insertion angles of 33 to 42 degrees relative to the probe surface normal. This novel ultrasound imaging/tracking probe has strong potential to improve procedural outcomes by providing 3D needle tip locations that are co-registered to ultrasound images, while maintaining compatibility with current clinical workflow.

Photoacoustic reflection artifact reduction using photoacoustic-guided focused ultrasound: comparison between plane-wave and element-by-element synthetic backpropagation approach

Reflection artifacts caused by acoustic inhomogeneities constitute a major problem in epi-mode biomedical photoacoustic imaging. Photoacoustic transients from the skin and superficial optical absorbers traverse into the tissue and reflect off echogenic structures to generate reflection artifacts. These artifacts cause difficulties in the interpretation of images and reduce contrast and imaging depth. We recently developed a method called PAFUSion (photoacoustic-guided focused ultrasound) to circumvent the problem of reflection artifacts in photoacoustic imaging. We already demonstrated that the photoacoustic signals can be backpropagated using synthetic aperture pulse-echo data for identifying and reducing reflection artifacts in vivo. In this work, we propose an alternative variant of PAFUSion in which synthetic backpropagation of photoacoustic signals is based on multi-angled plane-wave ultrasound measurements. We implemented plane-wave and synthetic aperture PAFUSion in a handheld ultrasound/photoacoustic imaging system and demonstrate reduction of reflection artifacts in phantoms and in vivo measurements on a human finger using both approaches. Our results suggest that, while both approaches are equivalent in terms of artifact reduction efficiency, plane-wave PAFUSion requires less pulse echo acquisitions when the skin absorption is the main cause of reflection artifacts.

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.

Photoacoustic-guided focused ultrasound for accurate visualization of brachytherapy seeds with the photoacoustic needle

An important problem in minimally invasive photoacoustic (PA) imaging of brachytherapy seeds is reflection artifacts caused by the high signal from the optical fiber/needle tip reflecting off the seed. The presence of these artifacts confounds interpretation of images. In this letter, we demonstrate a recently developed concept called photoacoustic-guided focused ultrasound (PAFUSion) for the first time in the context of interstitial illumination PA imaging to identify and remove reflection artifacts. In this method, ultrasound (US) from the transducer is focused on the region of the optical fiber/needle tip identified in a first step using PA imaging. The image developed from the US diverging from the focus zone at the tip region visualizes only the reflections from seeds and other acoustic inhomogeneities, allowing identification of the reflection artifacts of the first step. These artifacts can then be removed from the PA image. Using PAFUSion, we demonstrate reduction of reflection artifacts and thereby improved interstitial PA visualization of brachytherapy seeds in phantom and <italic<ex vivo</italic< measurements on porcine tissue.

Synthetic-aperture based photoacoustic re-beamforming (SPARE) approach using beamformed ultrasound data

Photoacoustic (PA) imaging has been developed for various clinical and pre-clinical applications, and acquiring pre-beamformed channel data is necessary to reconstruct these images. However, accessing these pre-beamformed channel data requires custom hardware to enable parallel beamforming, and is available for a limited number of research ultrasound platforms. To broaden the impact of clinical PA imaging, our goal is to devise a new PA reconstruction approach that uses ultrasound post-beamformed radio frequency (RF) data rather than raw channel data, because this type of data is readily available in both clinical and research ultrasound systems. In our proposed Synthetic-aperture based photoacoustic re-beamforming (SPARE) approach, post-beamformed RF data from a clinical ultrasound scanner are considered as input data for an adaptive synthetic aperture beamforming algorithm. When receive focusing is applied prior to obtaining these data, the focal point is considered as a virtual element, and synthetic aperture beamforming is implemented assuming that the photoacoustic signals are received at the virtual element. The resolution and SNR obtained with the proposed method were compared to that obtained with conventional delay-and-sum beamforming with 99.87% and 91.56% agreement, respectively. In addition, we experimentally demonstrated feasibility with a pulsed laser diode setup. Results indicate that the post-beamformed RF data from any commercially available ultrasound platform can potentially be used to create PA images.

In vivo demonstration of reflection artifact reduction in photoacoustic imaging using synthetic aperture photoacoustic-guided focused ultrasound (PAFUSion)

Reflection artifacts caused by acoustic inhomogeneities are a critical problem in epi-mode biomedical photoacoustic imaging. High light fluence beneath the probe results in photoacoustic transients, which propagate into the tissue and reflect back from echogenic structures. These reflection artifacts cause problems in image interpretation and significantly impact the contrast and imaging depth. We recently proposed a method called PAFUSion (Photoacoustic-guided focused ultrasound) to identify such reflection artifacts in photoacoustic imaging. In its initial version, PAFUSion mimics the inward-travelling wavefield from small blood vessel-like PA sources by applying ultrasound pulses focused towards these sources, and thus provides a way to identify the resulting reflection artifacts. In this work, we demonstrate reduction of reflection artifacts in phantoms and in vivo measurements on human volunteers. In view of the spatially distributed PA sources that are found in clinical applications, we implemented an improved version of PAFUSion where photoacoustic signals are backpropagated to imitate the inward travelling wavefield and thus the reflection artifacts. The backpropagation is performed in a synthetic way based on the pulse-echo acquisitions after transmission on each single element of the transducer array. The results provide a direct confirmation that reflection artifacts are prominent in clinical epi-photoacoustic imaging, and that PAFUSion can strongly reduce these artifacts to improve deep-tissue photoacoustic imaging.

Coded excitation ultrasonic needle tracking: An in vivo study

PURPOSE

Accurate and efficient guidance of medical devices to procedural targets lies at the heart of interventional procedures. Ultrasound imaging is commonly used for device guidance, but determining the location of the device tip can be challenging. Various methods have been proposed to track medical devices during ultrasound-guided procedures, but widespread clinical adoption has remained elusive. With ultrasonic tracking, the location of a medical device is determined by ultrasonic communication between the ultrasound imaging probe and a transducer integrated into the medical device. The signal-to-noise ratio (SNR) of the transducer data is an important determinant of the depth in tissue at which tracking can be performed. In this paper, the authors present a new generation of ultrasonic tracking in which coded excitation is used to improve the SNR without spatial averaging.

METHODS

A fiber optic hydrophone was integrated into the cannula of a 20 gauge insertion needle. This transducer received transmissions from the ultrasound imaging probe, and the data were processed to obtain a tracking image of the needle tip. Excitation using Barker or Golay codes was performed to improve the SNR, and conventional bipolar excitation was performed for comparison. The performance of the coded excitation ultrasonic tracking system was evaluated in an in vivo ovine model with insertions to the brachial plexus and the uterine cavity.

RESULTS

Coded excitation significantly increased the SNRs of the tracking images, as compared with bipolar excitation. During an insertion to the brachial plexus, the SNR was increased by factors of 3.5 for Barker coding and 7.1 for Golay coding. During insertions into the uterine cavity, these factors ranged from 2.9 to 4.2 for Barker coding and 5.4 to 8.5 for Golay coding. The maximum SNR was 670, which was obtained with Golay coding during needle withdrawal from the brachial plexus. Range sidelobe artifacts were observed in tracking images obtained with Barker coded excitation, and they were visually absent with Golay coded excitation. The spatial tracking accuracy was unaffected by coded excitation.

CONCLUSIONS

Coded excitation is a viable method for improving the SNR in ultrasonic tracking without compromising spatial accuracy. This method provided SNR increases that are consistent with theoretical expectations, even in the presence of physiological motion. With the ultrasonic tracking system in this study, the SNR increases will have direct clinical implications in a broad range of interventional procedures by improving visibility of medical devices at large depths.

Multispectral optoacoustic and MRI coregistration for molecular imaging of orthotopic model of human glioblastoma

Multi-modality imaging methods are of great importance in oncologic studies for acquiring complementary information, enhancing the efficacy in tumor detection and characterization. We hereby demonstrate a hybrid non-invasive in vivo imaging approach of utilizing magnetic resonance imaging (MRI) and Multispectral Optoacoustic Tomography (MSOT) for molecular imaging of glucose uptake in an orthotopic glioblastoma in mouse. The molecular and functional information from MSOT can be overlaid on MRI anatomy via image coregistration to provide insights into probe uptake in the brain, which is verified by ex vivo fluorescence imaging and histological validation. In vivo MSOT and MRI imaging of an orthotopic glioma mouse model injected with IRDye800-2DG. Image coregistration between MSOT and MRI enables multifaceted (anatomical, functional, molecular) information from MSOT to be overlaid on MRI anatomy images to derive tumor physiological parameters such as perfusion, haemoglobin and oxygenation.

Music-of-light stethoscope: a demonstration of the photoacoustic effect

In this paper we present a system aimed at demonstrating the photoacoustic (PA) effect for educational purposes. PA imaging is a hybrid imaging modality that requires no contrast agent and has a great potential for spine and brain lesion characterisation, breast cancer and blood flow monitoring notably in the context of fetal surgery. It relies on combining light excitation with ultrasound reception. Our brief was to present and explain PA imaging in a public-friendly way suitable for a variety of ages and backgrounds.

We developed a simple, accessible demonstration unit using readily available materials. We used a modulated light emitting diode (LED) torch and an electronic stethoscope. The output of a music player was used for light modulation and the chest piece of the stethoscope covered by a black tape was used as an absorbing target and an enclosed chamber.

This demonstration unit was presented to the public at the Bloomsbury Festival On Light in October 2015. Our stall was visited by over 100 people of varying ages. Twenty families returned in-depth evaluation questionnaires, which show that our explanations of the photoacoustic effect were well understood. Their interest in biomedical engineering was increased.

Interventional multispectral photoacoustic imaging with a clinical ultrasound probe for discriminating nerves and tendons: an ex vivo pilot study

Accurate and efficient identification of nerves is an essential component of peripheral nerve blocks. While ultrasound (US) imaging is increasingly used as a guidance modality, it often provides insufficient contrast for identifying nerves from surrounding tissues such as tendons. Electrical nerve stimulators can be used in conjunction with US imaging for discriminating nerves from surrounding tissues, but they are insufficient to reliably prevent neural punctures, so that alternative methods are highly desirable. In this study, an interventional multispectral photoacoustic (PA) imaging system was used to directly compare the signal amplitudes and spectra acquired from nerves and tendons ex vivo, for the first time. The results indicate that the system can provide significantly higher image contrast for discriminating nerves and tendons than that provided by US imaging. As such, photoacoustic imaging could be valuable as an adjunct to US for guiding peripheral nerve blocks.

In-plane ultrasonic needle tracking using a fiber-optic hydrophone

PURPOSE

Accurate and efficient guidance of needles to procedural targets is critically important during percutaneous interventional procedures. Ultrasound imaging is widely used for real-time image guidance in a variety of clinical contexts, but with this modality, uncertainties about the location of the needle tip within the image plane lead to significant complications. Whilst several methods have been proposed to improve the visibility of the needle, achieving accuracy and compatibility with current clinical practice is an ongoing challenge. In this paper, the authors present a method for directly visualizing the needle tip using an integrated fiber-optic ultrasound receiver in conjunction with the imaging probe used to acquire B-mode ultrasound images.

METHODS

Needle visualization and ultrasound imaging were performed with a clinical ultrasound imaging system. A miniature fiber-optic ultrasound hydrophone was integrated into a 20 gauge injection needle tip to receive transmissions from individual transducer elements of the ultrasound imaging probe. The received signals were reconstructed to create an image of the needle tip. Ultrasound B-mode imaging was interleaved with needle tip imaging. A first set of measurements was acquired in water and tissue ex vivo with a wide range of insertion angles (15°-68°) to study the accuracy and sensitivity of the tracking method. A second set was acquired in an in vivo swine model, with needle insertions to the brachial plexus. A third set was acquired in an in vivo ovine model for fetal interventions, with insertions to different locations within the uterine cavity. Two linear ultrasound imaging probes were used: a 14-5 MHz probe for the first and second sets, and a 9-4 MHz probe for the third.

RESULTS

During insertions in tissue ex vivo and in vivo, the imaged needle tip had submillimeter axial and lateral dimensions. The signal-to-noise (SNR) of the needle tip was found to depend on the insertion angle. With the needle tip in water, the SNR of the needle tip varied with insertion angle, attaining values of 284 at 27° and 501 at 68°. In swine tissue ex vivo, the SNR decreased from 80 at 15° to 16 at 61°. In swine tissue in vivo, the SNR varied with depth, from 200 at 17.5 mm to 48 at 26 mm, with a constant insertion angle of 40°. In ovine tissue in vivo, within the uterine cavity, the SNR varied from 46.4 at 25 mm depth to 18.4 at 32 mm depth, with insertion angles in the range of 26°-65°.

CONCLUSIONS

A fiber-optic ultrasound receiver integrated into the needle cannula in combination with single-element transmissions from the imaging probe allows for direct visualization of the needle tip within the ultrasound imaging plane. Visualization of the needle tip was achieved at depths and insertion angles that are encountered during nerve blocks and fetal interventions. The method presented in this paper has strong potential to improve the safety and efficiency of ultrasound-guided needle insertions.