Miniature all-optical flexible forward-viewing photoacoustic endoscopy probe for surgical guidance

Perspectives on endoscopic functional photoacoustic microscopy

Endoscopy, enabling high-resolution imaging of deep tissues and internal organs, plays an important role in basic research and clinical practice. Recent advances in photoacoustic microscopy (PAM), demonstrating excellent capabilities in high-resolution functional imaging, have sparked significant interest in its integration into the field of endoscopy. However, there are challenges in achieving functional PAM in the endoscopic setting. This Perspective article discusses current progress in the development of endoscopic PAM and the challenges related to functional measurements. Then, it points out potential directions to advance endoscopic PAM for functional imaging by leveraging fiber optics, microfabrication, optical engineering, and computational approaches. Finally, it highlights emerging opportunities for functional endoscopic PAM in basic and translational biomedicine.

Miniature, multi-dichroic instrument for measuring the concentration of multiple fluorophores

Identification of tumour margins during resection of the brain is critical for improving the post-operative outcomes. Due to the highly infiltrative nature of glioblastoma multiforme (GBM) and limited intraoperative visualization of the tumour margin, incomplete surgical resection has been observed to occur in up to 80 % of GBM cases, leading to nearly universal tumour recurrence and overall poor prognosis of 14.6 months median survival. This research presents a miniaturized, SiPMT-based optical system for simultaneous measurement of powerful DRS and weak auto-fluorescence for brain tumour detection. The miniaturisation of the optical elements confined the spatial separation of eight select wavelengths into footprint measuring 1.5 × 2 × 16 mm. The small footprint enables this technology to be integrated with existing surgical guidance instruments in the operating room. It's dynamic ability to subtract any background illumination and measure signal intensities across a broad range from pW to mWs make this design much more suitable for clinical environments as compared to spectrometer-based systems with limited dynamic ranges and high integration times. Measurements using optical tissue phantoms containing mixed fluorophores demonstrate correlation coefficients between the fitted response and actual concentration using PLS regression being 0.95, 0.87 and 0.97 for NADH, FAD and PpIX , respectively. These promising results indicate that our proposed miniaturized instrument could serve as an effective alternative in operating rooms, assisting surgeons in identifying brain tumours to achieving positive surgical outcomes for patients.

Dual-modality rigid endoscope for photoacoustic imaging and white light videoscopy

Significance

There has been significant interest in the development of miniature photoacoustic imaging probes for a variety of clinical uses, including the in situ assessment of tumors and minimally invasive surgical guidance. Most of the previously implemented probes are either side viewing or operate in the optical-resolution microscopy mode in which the imaging depth is limited to ∼ 1    mm . We describe a forward-viewing photoacoustic probe that operates in tomography mode and simultaneously provides white light video images.

Aim

We aim to develop a dual-modality endoscope capable of performing high-resolution PAT imaging and traditional white light videoscopy simultaneously in the forward-viewing configuration.

Approach

We used a Fabry-Pérot ultrasound sensor that operates in the 1500 to 1600 nm wavelength range and is transparent in the visible and near infrared region (580 to 1250 nm). The FP sensor was optically scanned using a miniature MEMs mirror located at the proximal end of the endoscope, resulting in a system that is sufficiently compact (10 mm outer diameter) and lightweight for practical endoscopic use.

Results

The imaging performance of the endoscope is evaluated, and dual-mode imaging capability is demonstrated using phantoms and abdominal organs of an ex vivo mouse including spleen, liver, and kidney.

Conclusions

The proposed endoscope design offers several advantages including the high acoustic sensitivity and wide detection bandwidth of the FP sensor, dual-mode imaging capability, compact footprint, and an all-optical distal end for improved safety. The dual-mode imaging capability also offers the advantage of correlating the tissue surface morphology with the underlying vascular anatomy. Potential applications include the guidance of laparoscopic surgery and other interventional procedures.

Optical ultrasound sensors for photoacoustic imaging: a review

Significance

Photoacoustic (PA) imaging is an emerging biomedical imaging modality that can map optical absorption contrast in biological tissues by detecting ultrasound signal. Piezoelectric transducers are commonly used in PA imaging to detect the ultrasound signals. However, piezoelectric transducers suffer from low sensitivity when the dimensions are reduced and are easily influenced by electromagnetic interference. To avoid these limitations, various optical ultrasound sensors have been developed and shown their great potential in PA imaging.

Aim

Our study aims to summarize recent progress in optical ultrasound sensor technologies and their applications in PA imaging.

Approach

The commonly used optical ultrasound sensing techniques and their applications in PA systems are reviewed. The technical advances of different optical ultrasound sensors are summarized.

Results

Optical ultrasound sensors can provide wide bandwidth and improved sensitivity with miniatured size, which enables their applications in PA imaging.

Conclusions

The optical ultrasound sensors are promising transducers in PA imaging to provide higher-resolution images and can be used in new applications with their unique advantages.

Scaling down the dimensions of a Fabry-Perot polymer film acoustic sensor for photoacoustic endoscopy

Significance

A Fabry-Perot (FP) polymer film sensor can be used to detect acoustic waves in a photoacoustic endoscope (PAE) if the dimensions can be adequately scaled down in size. Current FP sensors have limitations in size, sensitivity, and array configurability.

Aim

We aim to characterize and demonstrate the imaging performance of a miniature FP sensor to evaluate the effects of reduced size and finite dimensions.

Approach

A transfer matrix model was developed to characterize the frequency response of a multilayer miniature FP sensor. An analytical model was derived to describe the effects of a substrate with finite thickness. Finite-element analysis was performed to characterize the temporal response of a sensor with finite dimensions. Miniature 2 × 2    mm 2 FP sensors were designed and fabricated using gold films as reflective mirrors on either side of a parylene C film deposited on a glass wafer. A single-wavelength laser was used to interrogate the sensor using illumination delivered by fiber subprobes. Imaging phantoms were used to verify FP sensor performance, and in vivo images of blood vessels were collected from a live mouse.

Results

The finite thickness substrate of the FP sensor resulted in echoes in the time domain signal that could be removed by back filtering. The substrate acted as a filter in the frequency domain. The finite lateral sensor dimensions produced side waves that could be eliminated by surface averaging using an interrogation beam with adequate diameter. The fabricated FP sensor produced a noise-equivalent pressure = 0.76 kPa, bandwidth of 16.6 MHz, a spectral full-width at-half-maximum = 0.2886 nm, and quality factor Q = 2694 . Photoacoustic images were collected from phantoms and blood vessels in a live mouse.

Conclusions

A miniature wafer-based FP sensor design has been demonstrated with scaled down form factor for future use in PAE.

Spatially resolved readout of a Fabry-Perot ultrasound sensor interrogated through a multimode optical fiber using wavefront shaping

The spatially resolved interrogation of a Fabry-Perot ultrasound sensor using a laser beam focused through a multimode fiber is demonstrated. To scan the beam across the sensor as required to read it out, optical wavefront shaping was employed to compensate for the scrambling of light in the fiber. By providing a means to map ultrasound through inexpensive, lightweight fibers, this could lead to new ultrasonic and photoacoustic imaging systems, such as endoscopes and flexible handheld probes.

Fabrication of a translational photoacoustic needle sensing probe for interstitial photoacoustic spectral analysis

In our previous study, we demonstrated the feasibility of using an all-optical interstitial photoacoustic (PA) needle sensing probe for quantitative study of tissue architectures with PA spectral analysis (PASA). In this work, we integrated the optical components into an 18 G steel needle sheath for clinical translation. The dimensions of the needle probe are identical to those of a core biopsy probe and are fully compatible with standard procedures such as prostate biopsy. To our knowledge, this is the first interstitial PA probe that can acquire signals with sufficient temporal length for statistics-based PASA. We treated the inner surface of the steel needle sheath and successfully suppressed the vibrational PA signals generated at the surface. Purposed at boosting the measurement sensitivity and extending sensing volume, we upgraded the Fabry-Pérot hydrophone with a plano-concave structure. The performance of the translational needle PA sensing probe was examined with phantoms containing microspheres. The trend of the linear spectral slopes shows negatively correlated to the microsphere dimensions while the midband-fits are positively correlated to microsphere diameters and concentrations. The PASA quantifications show the ability to differentiate microspheres with varied dimensions.

Recent Advances in Contrast-Enhanced Photoacoustic Imaging: Overcoming the Physical and Practical Challenges

For decades now, photoacoustic imaging (PAI) has been investigated to realize its potential as a niche biomedical imaging modality. Despite its highly desirable optical contrast and ultrasonic spatiotemporal resolution, PAI is challenged by such physical limitations as a low signal-to-noise ratio (SNR), diminished image contrast due to strong optical attenuation, and a lower-bound on spatial resolution in deep tissue. In addition, contrast-enhanced PAI has faced practical limitations such as insufficient cell-specific targeting due to low delivery efficiency and difficulties in developing clinically translatable agents. Identifying these limitations is essential to the continuing expansion of the field, and substantial advances in developing contrast-enhancing agents, complemented by high-performance image acquisition systems, have synergistically dealt with the challenges of conventional PAI. This review covers the past four years of research on pushing the physical and practical challenges of PAI in terms of SNR/contrast, spatial resolution, targeted delivery, and clinical application. Promising strategies for dealing with each challenge are reviewed in detail, and future research directions for next generation contrast-enhanced PAI are discussed.

Optical-resolution functional gastrointestinal photoacoustic endoscopy based on optical heterodyne detection of ultrasound

Photoacoustic endoscopy shows promise in the detection of gastrointestinal cancer, inflammation, and other lesions. High-resolution endoscopic imaging of the hemodynamic response necessitates a small-sized, high-sensitivity ultrasound sensor. Here, we utilize a laser ultrasound sensor to develop a miniaturized, optical-resolution photoacoustic endoscope. The sensor can boost the acoustic response by a gain factor of ω_o/Ω (the frequency ratio of the signal light and measured ultrasound) by measuring the acoustically induced optical phase change. As a result, we achieve a noise-equivalent pressure density (NEPD) below 1.5 mPa·Hz^-1/2 over the measured range of 5 to 25 MHz. The heterodyne phase detection using dual-frequency laser beams of the sensor can offer resistance to thermal drift and vibrational perturbations. The endoscope is used to in vivo image a rat rectum and visualize the oxygen saturation changes during acute inflammation, which can hardly be observed with other imaging modalities.

Miniaturized fiber optic ultrasound sensor with multiplexing for photoacoustic imaging

A miniaturized ultrasound sensor based on optical fiber is designed and realized for multichannel parallel ultrasound detection and photoacoustic imaging. The fiber optic sensor is composed of a polymer coating, a reflective mirror and a single-mode optical fiber, with only 125 µm in diameter. By integrating the coherent demodulation technology and multiplexing technology, which using a relatively cheap fixed wavelength laser, hundreds of sensors could work simultaneously. Meanwhile, highly sensitive ultrasound detection has been demonstrated with the noise equivalent pressure as low as 0.46 kPa and the sensor exhibits a nearly omnidirectional directivity. Furthermore, a photoacoustic imaging system based on three sensors working in parallel is demonstrated. High lateral resolutions of 165-217 µm and axial resolutions of 112-131 µm over a depth range of larger than 5 mm are obtained. A three-dimensional phantom imaging experiment is also demonstrated. Benefited from parallel detection, the imaging speed is three times faster than that of a single sensor. The miniaturized fiber optic ultrasound sensor probe provides a competitive alternative for mechanically scanning-free endoscopic imaging, which is beneficial from small size, omnidirectional directivity and parallel detection capability.

Progress of photoacoustic imaging combined with targeted photoacoustic contrast agents in tumor molecular imaging

Molecular imaging visualizes, characterizes, and measures biological processes at the molecular and cellular level. In oncology, molecular imaging is an important technology to guide integrated and precise diagnosis and treatment. Photoacoustic imaging is mainly divided into three categories: photoacoustic microscopy, photoacoustic tomography and photoacoustic endoscopy. Different from traditional imaging technology, which uses the physical properties of tissues to detect and identify diseases, photoacoustic imaging uses the photoacoustic effect to obtain the internal information of tissues. During imaging, lasers excite either endogenous or exogenous photoacoustic contrast agents, which then send out ultrasonic waves. Currently, photoacoustic imaging in conjunction with targeted photoacoustic contrast agents is frequently employed in the research of tumor molecular imaging. In this study, we will examine the latest advancements in photoacoustic imaging technology and targeted photoacoustic contrast agents, as well as the developments in tumor molecular imaging research.

Ultrathin, high-speed, all-optical photoacoustic endomicroscopy probe for guiding minimally invasive surgery

Photoacoustic (PA) endoscopy has shown significant potential for clinical diagnosis and surgical guidance. Multimode fibres (MMFs) are becoming increasingly attractive for the development of miniature endoscopy probes owing to their ultrathin size, low cost and diffraction-limited spatial resolution enabled by wavefront shaping. However, current MMF-based PA endomicroscopy probes are either limited by a bulky ultrasound detector or a low imaging speed that hindered their usability. In this work, we report the development of a highly miniaturised and high-speed PA endomicroscopy probe that is integrated within the cannula of a 20 gauge medical needle. This probe comprises a MMF for delivering the PA excitation light and a single-mode optical fibre with a plano-concave microresonator for ultrasound detection. Wavefront shaping with a digital micromirror device enabled rapid raster-scanning of a focused light spot at the distal end of the MMF for tissue interrogation. High-resolution PA imaging of mouse red blood cells covering an area 100 µm in diameter was achieved with the needle probe at ∼3 frames per second. Mosaicing imaging was performed after fibre characterisation by translating the needle probe to enlarge the field-of-view in real-time. The developed ultrathin PA endomicroscopy probe is promising for guiding minimally invasive surgery by providing functional, molecular and microstructural information of tissue in real-time.

Looking deep inside tissue with photoacoustic molecular probes: a review

Significance

Deep tissue noninvasive high-resolution imaging with light is challenging due to the high degree of light absorption and scattering in biological tissue. Photoacoustic imaging (PAI) can overcome some of the challenges of pure optical or ultrasound imaging to provide high-resolution deep tissue imaging. However, label-free PAI signals from light absorbing chromophores within the tissue are nonspecific. The use of exogeneous contrast agents (probes) not only enhances the imaging contrast (and imaging depth) but also increases the specificity of PAI by binding only to targeted molecules and often providing signals distinct from the background.

Aim

We aim to review the current development and future progression of photoacoustic molecular probes/contrast agents.

Approach

First, PAI and the need for using contrast agents are briefly introduced. Then, the recent development of contrast agents in terms of materials used to construct them is discussed. Then, various probes are discussed based on targeting mechanisms, in vivo molecular imaging applications, multimodal uses, and use in theranostic applications.

Results

Material combinations are being used to develop highly specific contrast agents. In addition to passive accumulation, probes utilizing activation mechanisms show promise for greater controllability. Several probes also enable concurrent multimodal use with fluorescence, ultrasound, Raman, magnetic resonance imaging, and computed tomography. Finally, targeted probes are also shown to aid localized and molecularly specific photo-induced therapy.

Conclusions

The development of contrast agents provides a promising prospect for increased contrast, higher imaging depth, and molecularly specific information. Of note are agents that allow for controlled activation, explore other optical windows, and enable multimodal use to overcome some of the shortcomings of label-free PAI.

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.

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.

Photoacoustic imaging for monitoring of stroke diseases: A review

Stroke is the leading cause of death and disability after ischemic heart disease. However, there is lacking a non-invasive long-time monitoring technique for stroke diagnosis and therapy. The photoacoustic imaging approach reconstructs images of an object based on the energy excitation by optical absorption and its conversion to acoustic waves, due to corresponding thermoelastic expansion, which has optical resolution and acoustic propagation. This emerging functional imaging method is a non-invasive technique. Due to its precision, this method is particularly attractive for stroke monitoring purpose. In this paper, we review the achievements of this technology and its applications on stroke, as well as the development status in both animal and human applications. Also, various photoacoustic systems and multi-modality photoacoustic imaging are introduced as for potential clinical applications. Finally, the challenges of photoacoustic imaging for monitoring stroke are discussed.

Perspective on fast-evolving photoacoustic tomography

SIGNIFICANCE

Acoustically detecting the rich optical absorption contrast in biological tissues, photoacoustic tomography (PAT) seamlessly bridges the functional and molecular sensitivity of optical excitation with the deep penetration and high scalability of ultrasound detection. As a result of continuous technological innovations and commercial development, PAT has been playing an increasingly important role in life sciences and patient care, including functional brain imaging, smart drug delivery, early cancer diagnosis, and interventional therapy guidance.

AIM

Built on our 2016 tutorial article that focused on the principles and implementations of PAT, this perspective aims to provide an update on the exciting technical advances in PAT.

APPROACH

This perspective focuses on the recent PAT innovations in volumetric deep-tissue imaging, high-speed wide-field microscopic imaging, high-sensitivity optical ultrasound detection, and machine-learning enhanced image reconstruction and data processing. Representative applications are introduced to demonstrate these enabling technical breakthroughs in biomedical research.

CONCLUSIONS

We conclude the perspective by discussing the future development of PAT technologies.

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.