Recent research progress of photoacoustic endoscopy in the digestive system

Introduction Photoacoustic imaging (PAI) was developed in the 1990s, based on Alexander Graham Bell’s discovery of the photoacoustic effect.[1] The principles of PAI state that tissues exposed to brief light pulses absorb and transform energy into heat, increasing the local temperature and leading to thermal expansion and generation of ultrasonic waves. Such ultrasonic waves (i.e., photoacoustic signals) are detected using ultrasonic detectors positioned outside the tissue, with the creation of a corresponding photoacoustic image.[1] The benefits of PAI include high spatial resolution, strong image contrast, and deep imaging depth.[2] PAI can obtain information on hemoglobin, lymphatics, lipid composition, and blood oxygen metabolism.[3–7] Thus, its use has significantly improved the sensitivity and specificity of cancer diagnosis.[8–10] Recently, significant advancements have been made in photoacoustic tomography (PAT), which uses the photoacoustic effect to generate 3-dimensional (3D) and cross-sectional images.[11,12] In addition to being speckle-free,[13] PAT obtains deep, high spatial resolution images by detecting ultrasonic waves, as ultrasonic scattering is approximately 2 to 3 orders of magnitude lower than optical scattering,[12,14,15] and acoustic diffraction is 2 to 3 orders of magnitude lower than the radiofrequency equivalent. PAT comprises 3 main imaging modalities: photoacoustic endoscopy (PAE), photoacoustic computed tomography,[16,17] and photoacoustic microscopy (PAM).[18,19] Moreover, PAT can be integrated with other imaging methods such as optical coherence tomography,[20–22] confocal microscopy,[23] magnetic resonance imaging,[24] and ultrasound (US) imaging.[5] The PAE system integrates an optical fiber, transducer, and lens into the endoscope probe and performs endoscopic imaging using a miniature probe for both light delivery and acoustic detection.[25] The current summative studies of PAE did not specifically target digestive tract diseases. Hence, we aimed to focus on the development and potential use of PAE devices for digestion evaluation. Clinical demands for PAE Although visible-light endoscopy is frequently used for the visualization of the intestinal wall to diagnose various disorders, gastroenterologists have been unable to identify early epithelial dysplastic alterations that may lead to cancer development.[26] In addition to endoscopy, EUS, which was first reported in 1980, has rapidly developed in recent decades,[27–30] allowing the acquisition of high-resolution, large-penetration-depth images of the gastrointestinal system and adjacent tissues.[31,32] However, the potential of soft-tissue imaging to provide physiologically precise functional information is constrained by sonic-based picture contrast. In contrast, PAE is ideal for soft tissue imaging due to its excellent functional optical contrast and spatial resolution. Moreover, as PAE and EUS utilize the same transducer, combining the 2 modalities to achieve multimodality PAE is simple, enabling the assessment of the digestive tract structure and depth of tumor invasion, as well as the microvascular morphology and blood oxygen saturation within a few millimeters of the superficial surface of the intestinal wall. Such combined technology provides doctors with multiscale, multiparametric imaging, which aids in the early detection and treatment of diseases, in addition to providing new concepts and techniques for the high-resolution imaging of lesion depth and targeted detection of tumor neovascularization. PAE development In 2009, Yang et al. developed a unique PAE technique using a miniature imaging probe, which allowed researchers to obtain ex vivo or in situ images of the digestive system of rats[33]; this was the first report on PAE [Table 1]. Table 1 - Development of PAE Year Research Group Modalities Probe Diameter (mm) Application Additional Information 2009 Yang et al. PAE 4.2 Rat gastrointestinal tract — 2010 Yuan et al. PAIE 10 Pig colorectal wall 360° view 2012 Yang et al. PA/USE 3.8 Rat lower gastrointestinal tract and rabbit upper gastrointestinal tract 270° view 2012 Yang et al. PA/USE 2.5 Adult Sprague-Dawley rat descending colon Only suitable for lower GI tract 2014 Yang et al. PAE 3.2 New Zealand rabbit upper esophagus Flexible shaft-based 2015 Yang et al. OR-PAEM 3.8 Rat colorectum — 2018 Xiong et al. AF-PAE 9 Rabbit rectum 360° view 2018 Li et al. PA/USE 2.5 Sprague-Dawley rat colorectum 360° view 2019 Li et al. PA/USE 1.5 Sprague-Dawley rat rectal wall — 2019 Li et al. OR-PAE 5 Porcine small intestine and rabbit rectum 3D vascular networks Has a large-depth-of-field 2019 Xiong et al. PAE 3 Rabbit colorectum — 2022 Kim et al. PA/USE 3.38 Esophagogastric junction of a swine and rat colorectum — 2022 Pang et al. PA/USE — Porcine small intestine Based on the line-focused transducer 2022 Liang et al. PAE 2 Rat rectum — 2022 Xie et al. PA/USE 11 Pig rectum’s inner and outer walls 360° view 2023 Jiang et al. AR-PA/USE 8 Rabbit VX2 tumor model Centimeter-scale deep penetration and 360° view PAE, photoacoustic endoscopy; PAIE, photoacoustic imaging endoscope; PA/USE, acoustic resolution-photoacoustic/ultrasound endoscopy; OR-PAEM, optical resolution photoacoustic endoscopic microscopy; AF-PAE, autofocusing photoacoustic endoscope; OR-PAE, optical resolution photoacoustic endoscopy; AR-PA/USE, acoustic resolution-photoacoustic/ultrasound endoscopy; 3D, 3-dimensional; GI, gastrointestinal. However, this single-mode device provided limited information and in-depth knowledge of the structure and functionality of biological tissues could only be obtained using multimodal PAE. In 2012, Yang et al. presented an endoscope with simultaneous photoacoustic and ultrasonic modes, with a probe smaller than those of the first generation.[34] In a later experiment, 2 rabbit esophagi were scanned both ex vivo and in vivo by making use of this PA-US endoscope, with the successful acquisition of 3D-PA images displaying vascular and luminal anatomy.[35] In 2012, the authors developed another dual-mode endoscopic probe with a smaller outer diameter of 2.5 mm. This device was subsequently used to image the descending colon of adult rats in vivo.[36] A few years later, both Abran and colleagues’ and Kang and colleagues’ teams reported 3-modal instruments for US, PA, and fluorescence imaging.[37,38] Additionally, the slow imaging rates (<10 frames per second) of existing PA-US endoscopes limited their clinical translation; hence, a combined PA-US endoscopic system that could capture up to 50 frames per second was designed in 2019. With a 1.5-mm outer diameter, it could fit into the accessory channel of a standard endoscope, permitting the visualization of the rectal wall vasculature and anatomical architecture in Sprague-Dawley rats.[39] Spatial resolution has always been a concern in PAEs. However, the pursuit of a deeper penetration often leads to a decreased imaging resolution. Currently, acoustic resolution photoacoustic endoscopic microscopy (AR-PAEM) focuses on the received acoustic signal and laser beam at the same point for imaging; however, optical resolution photoacoustic endoscopic microscopy (OR-PAEM) uses a highly concentrated laser beam to replace the field illumination of AR-PAEM, resulting in a higher resolution.[40] Yang and colleagues’ team, for instance, developed an OR-PAEM system and demonstrated the viability of PAT-based intravital microscopy for the first time by imaging the colorectum of rats and creating a 3D depiction of its vasculature.[41] Moreover, Li et al. created a large depth-of-field optical-resolution PAE (OR-PAE) in 2019 that retained a mostly consistent transverse resolution while imaging targets at various depths, without requiring axial scanning. Its implementation demonstrated the ability to examine biological tissues at various depths and the potential of the endoscope for clinical colorectal applications.[42] On this basis, various research groups have pursued significant improvements by refining the scanning systems in various ways. The most common method involved improving the mechanical scanning system. To improve the signal acquisition time, a fast preclinical PAI endoscope with coaxial characteristics of sound, ring, and light transducer arrays was developed by Yuan et al. in 2010, which could receive PA signals from a 360° field of view (FOV).[43] In 2014, Yang et al. developed a novel catheter-based mechanical scanning PAE system using a flexible shaft; the outer diameter of the flexible catheter component was 3.2 mm, which allowed it to fit into the clinical video endoscope channel.[44] In 2016, Xiao and colleagues’ team proposed a hollow structure and epoxy lens-focusing sensor for AR-PAEM.[45] In gastrointestinal tract imaging, probes face the challenge of optical defocusing, which leads to a decrease in transverse resolution. To solve this issue, Xiong et al. developed an autofocusing PA endoscope (AF-PAE) in 2018 that achieved a focus-shifting range of 2 to 10 mm in a 360° FOV with high image contrast and transverse resolution.[46] In 2018, Li et al. designed a PAE-EUS dual-mode device with a small enclosed imaging catheter with a 360° FOV. This catheter could be inserted into a 2.8-mm-diameter conventional endoscopic instrument channel. Using such system, the authors obtained in vivo 3D PA or US images of the colorectum of a rat.[47] Moreover, Xiong et al. designed a new PAE for shape-adapting panoramic imaging in 2019. By applying a red-transparent water balloon, the probe could extract a holonomic vascular network layer and identify boundaries with improved axial resolution.[48] In the past 2 years, mechanical scanning systems have been rapidly developed. A 3.38-mm-diameter catheter-based, combined optical-resolution PA-US microprobe system was developed by Kim et al. in 2022, for example, for obtaining PA and US images of a pig’s esophagogastric junction in vivo. Additionally, hierarchically developed mesh-like capillary networks have been visualized in vivo in the colorectum of mice.[49] In the same year, Pang et al. reported an AR-PA/USE with a line-focused transducer that achieved autofocusing, thereby improving image contrast and signal-to-noise ratio (SNR) and solving the target signal difference flaw in the telephoto region of US imaging.[50] In 2022, Xie et al. designed a compact PAE/EUS probe that provided 360° structural and functional imaging, using a ring-shaped laser beam and circular array transducer to avoid the instability caused by point-by-point mechanical scanning. The imaging distance determined by phantom experiments was large enough to span the range of 12 to 30 mm, which is consistent with the complex intestinal environment, and the probe could access functional information at the inner surface and at various depths.[51] In 2023, Jiang et al. developed a low-cost AR-PA/USE system with cross-scale imaging capability, multispectral imaging, centimeter-scale deep penetration, and a 360° FOV. This system could detect tumor angiogenesis and accurately visualize the depth of tumor invasion using indocyanine green (ICG).[52] Compared with the mechanical scanning system, the optical scanning–based PAE system is more compact and has a better SNR and faster scanning speed. In 2017, Guo and colleagues, for instance, reported a high-resolution PA endomicroscopy probe that utilized microelectromechanical systems scanning; resected rectums and in vivo mouse ears were photographed to demonstrate the potential of the probe in biological and therapeutic applications.[53] Furthermore, a miniature endoscope using a novel Fabry-Pérot acoustic transducer was reported in 2018.[54] In 2019, Chen and colleagues designed a mini-PAE using a Fabry-Pérot acoustic transducer, with a 2.4-mm-diameter, high-lateral-resolution, and wide FOV.[55] In 2022, Liang and colleagues developed a 2-mm-diameter optical-resolution PAE utilizing the optical heterodyne detection of US, which could image gastrointestinal hemodynamics in vivo. This provided a spatial resolution of 7.4 μm, enabling visualization of the change in tissue oxygen saturation (spo2) during rectal inflammation and providing the related metabolic data.[56] In addition to the advances in PAE devices, contrast agents have been developed to improve the sensitivity and specificity of PAE.[57] Using PA contrast agents, tissue function and the related molecular information can be better acquired with PAE, thereby aiding in the subsequent evaluation of tissue structure, shape, physiological, and metabolic functions, as well as pathological features of digestive tract tumors. Endogenous contrast agents are molecular substances, such as the most commonly used hemoglobin. Measuring hemoglobin levels can help identify differences in abnormal blood vessels, hypoxia, and oxygenation between tissues to detect and characterize cancerous tissue[8–10,58]; hence, this can be additionally used for perfusion and inflammation imaging.[59] Other endogenous contrast agents include melanin,[61,62] lipids,[63] water,[64] and bilirubin.[65] Exogenous contrast agents have been introduced for drug delivery, cancer detection, and therapeutic diagnostics using nanotechnology for photothermal therapy[59] and include small organic molecules,[66,67] nanoparticles,[68–74] and carbon-based contrast agents[75] (Figures 1 and 2).Figure 1: The development of PAE. PAE, photoacoustic endoscopy; AR-PAEM, acoustic resolution photoacoustic endoscopic microscopy; OR-PAEM, optical resolution photoacoustic endoscopic microscopy; AF-PAE, autofocusing photoacoustic endoscope; PAIE, photoacoustic imaging endoscope; MEMS, microelectromechanical systems; FP, Fabry-Pérot; PA-USE, photoacoustic-ultrasound endoscopy; AR-PA/USE, acoustic resolution-photoacoustic/ultrasound endoscopy; PA, photoacoustic.Figure 2: Applications of photoacoustic imaging in digestive diseases. ICG, indocyanine green; US, ultrasound; EGFR, epidermal growth factor receptor; HCC, hepatocellular carcinoma; so 2, oxygen saturation; PA, photoacoustic; US, ultrasound; PAE-EUS, photoacoustic endoscopy-EUS; PAI, photoacoustic imaging.Applications in benign and malignant diseases of the digestive system Applications in benign diseases Crohn disease Traditional noninvasive imaging cannot distinguish fibrosis from inflammation in strictures. In contrast, PAI can assess the hemoglobin and collagen levels in tissues to identify intestinal stenosis. In 2016, PAI was used to detect inflammation and fibrosis in Crohn disease by simultaneously measuring the amounts of collagen and hemoglobin in animal and human tissues.[76] Furthermore, Zhu et al. utilized a PA-US dual-mode device to distinguish between inflammatory and fibrotic intestinal strictures in 2018[77] and found that the different strictures showed substantial changes in the PA signal intensities.[78] In 2019, the authors discovered that concurrent PA-US imaging could quantify the stiffness of intestinal strictures.[79] Zhu et al. designed a prototype capsule-shaped AR-PA endoscope, which discriminated fibrosis and intestinal inflammation in rabbit models in vivo; PAI was able to follow the progression of intestinal fibrosis, thereby providing crucial data for therapeutic decision-making.[80] In 2022, Zhu et al. demonstrated that a prototype endoscopic PAT and US balloon catheter could identify the molecular structure of colonic fibrotic lesions and inflammation in vivo in rabbit models[81] (Table 2). Table 2 - PAE applications in digestive diseases. Disease Year Content of Research Crohn disease 2016 Identify inflammation and fibrosis.Assess intestinal strictures in Crohn disease. 2018 Use the PA signal difference in vivo to separate inflammatory and fibrotic intestinal strictures. 2019 Quantify the stiffness of intestinal strictures. 2019 Monitor the progression of fibrosis over time. 2022 Characterize molecular content of fibrotic lesions and inflammation. Esophageal cancer 2015 Imaging of 2 intact rabbit esophagi using PAE-EUS. 2017 Accurately identify the tumors of esophagus and reflux esophagitis. Gastric cancer 2018 Observe layered tissues and tumor angiogenesis in the submucosa. 2018 Assess the feasibility of detection in gastric cancer. 2019 Identified simulated gastric tumor at a depth of 5.5 mm below the mucosa. 2020 Diagnose early stomach submucosal tumors and distinguish between gastric cancer and normal gastric mucosa. 2021 Layered US images of the stomach wall and high-resolution vascular imaging were obtained. Liver cancer 2014 Visual ICG space distribution on US images to produce liver tumor-representative fluorescence image. 2016 Use the EGFR-specific peptide to make HCC xenograft tumors visible in live mice. 2018 Investigate the effects of sorafenib on the spo 2 levels in the tissues of HCC animal models. Pancreatic cancer 2017 Use the multimodal PA-fluorescence molecular endoscope to image pancreatic cancer. 2018 Identify pancreatic tumor according to PA signal differences. Colorectal cancer 2010 Distinguish human healthy tissue and colorectal cancer tissue. 2010 Find a CT26 tumor’s location, then use PAI to guide the tumor’s ablation. 2018 Distinguish benign and malignant colorectal tissue.Identify the invasion depth and imaging characteristics of colorectal cancer. 2019 Distinguish benign and malignant colorectal tissues.Evaluate the treatment thereof. 2023 Detect colorectal cancer with ICG. PA, photoacoustic; PAE-EUS, photoacoustic endoscopy-EUS; US, ultrasound; ICG, indocyanine green; EGFR, epidermal growth factor receptor; HCC, hepatocellular carcinoma; spo2, oxygen saturation; PAI, photoacoustic imaging. Applications in malignant diseases Esophageal cancer In 2015, Yang et al. imaged 2 intact rabbit esophagi using PA-US endoscopy and the first PA images of the vasculature of the esophagus of a vertebrate; this suggested that PAE may be a useful tool for evaluating the esophageal function and structure.[35] In 2017, an in situ morphological and biomechanical analysis of reflux esophagitis and esophageal tumors in rabbit esophagi using PAE demonstrated that this technique may be able to detect early esophageal diseases in clinical settings.[82] Gastric cancer Although gastroscopy can only diagnose superficial gastric cancer, EUS can assess both the mucosa and submucosa to the tumor and invasion cannot the boundaries with the blood or PAI and may be used for the clinical of early gastric cancer providing knowledge on invasion morphological and other structural thereby the of In 2018, et al. introduced an system, which obtained PA images of the and blood the that PAI combined with endoscopy can detect tumor angiogenesis and layered tissues in the In 2019, et al. used a PA scanning imaging system to identify submucosal the simulated gastric tumor at a depth of 5.5 mm below the In a PA-US dual-mode device was used to diagnose submucosal Additionally, the system could characterize the gastric wall structure and tumor with the tumor and In Kim et al. obtained layered US and high-resolution vascular images of the stomach walls of large for the first time, as well as vascular PA images with a Such techniques are to have therapeutic applications by aiding in and in endoscopic submucosal Liver and pancreatic cancer With the development of PAI its high resolution and contrast to improve the of liver cancer and provide a for the of boundaries and In 2014, et al. used fluorescence and US imaging to the potential of PAI with ICG in improving tumor and 2016, et al. visualized hepatocellular xenograft tumors with epidermal growth factor in live using PAI, that the method may improve the detection of liver cancer and is a for HCC and can the oxygenation state of tumors. the levels in HCC sorafenib treatment is In 2018, et al. showed that using PAI as a for noninvasive of the therapeutic in HCC models in Moreover, PAI has in the of pancreatic cancer. et al. multimodal PA fluorescence molecular endoscopic imaging in a rat pancreatic cancer using probes combined with PAI have tumors and showed that the PA signal of pancreatic tumor is higher than that of the Although endoscopic US is of the modalities for pancreatic diseases, it cannot provide information on blood or oxygen endoscopy, on the other provides a more of liver and pancreatic Furthermore, PAI technology has been used to and achieve This that PAE may be used to in the Colorectal cancer Colorectal cancer is of the most imaging methods to its and therapeutic are In 2010, of the colorectal wall of a was in using preclinical PAI Additionally, this system ex vivo colorectal cancer tissues from healthy human colorectal et al. used PAI to and a CT26 tumor in the of a mouse in In 2018, and colleagues used a novel US and system to image 8 colorectal this both the colorectal cancer imaging characteristics and the of Yang et al. used a system to characterize and image human rectal and colon ex vivo in 2019. The PA signal the malignant on the was significantly that the tumor was that PAT may be able to cancerous and healthy tissues in the rectum and In 2023, Jiang et al. showed that AR-PA/USE was for colorectal cancer detection used with a contrast such as PAI or PA multimodal imaging provides high-resolution functional imaging that can be used as a novel tool for and colorectal tumors. PAE can be used for in vivo imaging to distinguish benign and malignant colorectal this method is a potential new instrument for the early of colorectal cancer. and The field of PAI has rapidly in recent In PAE and multimodality PAE are for and vascular and treatment may be a potential for Although substantial has been made in PAE, to its implementation in clinical clinical PAE studies are in the and clinical are an system and its high the of clinical Furthermore, imaging creating a small flexible probe that can the digestive endoscopic instrument channel with a 360° FOV. research the development of a PAE imaging technology suitable for the digestive which can achieve a FOV and is with digestive endoscope instrument Additionally, related to tumor growth and development can be obtained to improve the of benign and malignant tumors and their that and PAE technology to an clinical tool for the and treatment of various gastrointestinal diseases.