Multimodal Intravascular Photoacoustic and Ultrasound Imaging

Heavy water coupling gel for short-wave infrared photoacoustic imaging

Significance

Changes in lipid, water, and collagen (LWC) content in tissue are associated with numerous medical abnormalities (cancer, atherosclerosis, and Alzheimer's disease). Standard imaging modalities are limited in resolution, specificity, and/or penetration for quantifying these changes. Short-wave infrared (SWIR) photoacoustic imaging (PAI) has the potential to overcome these challenges by exploiting the unique optical absorption properties of LWC > 1000    nm .

Aim

This study's aim is to harness SWIR PAI for mapping LWC changes in tissue. The focus lies in devising a reflection-mode PAI technique that surmounts current limitations related to SWIR light delivery.

Approach

To enhance light delivery for reflection-mode SWIR PAI, we designed a deuterium oxide (D 2 O , "heavy water") gelatin (HWG) interface for opto-acoustic coupling, intended to significantly improve light transmission above 1200 nm.

Results

HWG permits light delivery > 1    mJ up to 1850 nm, which was not possible with water-based coupling (> 1    mJ light delivery up to 1350 nm). PAI using the HWG interface and the Visualsonics Vevo LAZR-X reveals a signal increase up to 24 dB at 1720 nm in lipid-rich regions.

Conclusions

By overcoming barriers related to light penetration, the HWG coupling interface enables accurate quantification/monitoring of biomarkers like LWC using reflection-mode PAI. This technological stride offers potential for tracking changes in chronic diseases (in vivo) and evaluating their responses to therapeutic interventions.

Dual-wavelength UV-visible metalens for multispectral photoacoustic microscopy: A simulation study

Photoacoustic microscopy is advancing with research on utilizing ultraviolet and visible light. Dual-wavelength approaches are sought for observing DNA/RNA- and vascular-related disorders. However, the availability of high numerical aperture lenses covering both ultraviolet and visible wavelengths is severely limited due to challenges such as chromatic aberration in the optics. Herein, we present a groundbreaking proposal as a pioneering simulation study for incorporating multilayer metalenses into ultraviolet-visible photoacoustic microscopy. The proposed metalens has a thickness of 1.4 µm and high numerical aperture of 0.8. By arranging cylindrical hafnium oxide nanopillars, we design an achromatic transmissive lens for 266 and 532 nm wavelengths. The metalens achieves a diffraction-limited focal spot, surpassing commercially available objective lenses. Through three-dimensional photoacoustic simulation, we demonstrate high-resolution imaging with superior endogenous contrast of targets with ultraviolet and visible optical absorption bands. This metalens will open new possibilities for downsized multispectral photoacoustic microscopy in clinical and preclinical applications.

Miniature intravascular photoacoustic endoscopy with coaxial excitation and detection

Recent research pointed out that the degree of inflammation in the adventitia could correlate with the severity of atherosclerotic plaques. Intravascular photoacoustic endoscopy can provide the information of arterial morphology and plaque composition, and even detecting the inflammation. However, most reported work used a noncoaxial configuration for the photoacoustic catheter design, which formed a limited light-sound overlap area for imaging so as to miss the adventitia information. Here we developed a novel 0.9 mm-diameter intravascular photoacoustic catheter with coaxial excitation and detection to resolve the aforementioned issue. A miniature hollow ultrasound transducer with a 0.18 mm-diameter orifice in the center was successfully fabricated. To show the significance and merits of our design, phantom and ex vivo imaging experiments were conducted on both coaxial and noncoaxial catheters for comparison. The results demonstrated that the coaxial catheter exhibited much better photoacoustic/ultrasound imaging performance from the intima to the adventitia.

Intravascular Imaging of Atherosclerosis by Using Engineered Nanoparticles

Atherosclerosis is a leading cause of morbidity and mortality, and high-risk atherosclerotic plaques can result in myocardial infarction, stroke, and/or sudden death. Various imaging and sensing techniques (e.g., ultrasound, optical coherence tomography, fluorescence, photoacoustic) have been developed for scanning inside blood vessels to provide accurate detection of high-risk atherosclerotic plaques. Nanoparticles have been utilized in intravascular imaging to enable targeted detection of high-risk plaques, to enhance image contrast, and in some applications to also provide therapeutic functions of atherosclerosis. In this paper, we review the recent progress on developing nanoparticles for intravascular imaging of atherosclerosis. We discuss the basic nanoparticle design principles, imaging modalities and instrumentations, and common targets for atherosclerosis. The review is concluded and highlighted with discussions on challenges and opportunities for bringing nanoparticles into in vivo (pre)clinical intravascular applications.

A high-sensitivity high-resolution intravascular photoacoustic catheter through mode cleaning in a graded-index fiber

Intravascular photoacoustic imaging has been developed to evaluate the possibility of plaque rupture in atherosclerosis by high spatial resolution imaging of lipid. However, the detection sensitivity and spatial resolution are compromised by the poor focusing caused by a multimode fiber. In this work, we report an intravascular photoacoustic catheter with mode self-cleaning in a graded-index fiber to improve the beam quality and the sensitivity for lipid detection. Compared with the higher-order modes in a step-index multimode fiber, the lower-order modes generated by the self-cleaning effect in the graded-index fiber greatly enhanced the photoacoustic spatial resolution and detection sensitivity. The dominant ringing artifact caused by laser absorption of the ultrasound transducer was further reduced by using stripe suppression. A lipid plaque mimicking phantom was imaged for evaluation. Lipid particles with a small diameter of 75.7 µm were clearly observed.

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.

Intravascular molecular imaging: translating pathophysiology of atherosclerosis into human disease conditions

Progression of atherosclerotic plaque in coronary arteries is characterized by complex cellular and non-cellular molecular interactions. Within recent years, atherosclerosis has been recognized as inflammation-driven disease condition, where progressive stages are characterized by morphological changes in plaque composition but also relevant molecular processes resulting in increased plaque vulnerability. While existing intravascular imaging modalities are able to resolve key morphological features during plaque progression, they lack capability to characterize the molecular profile of advanced atherosclerotic plaque. Because hybrid imaging modalities may provide incremental information related to plaque biology, they are expected to provide synergistic effects in detecting high risk patients and lesions. The aim of this article is to review existing literature on intravascular molecular imaging approaches, and to provide clinically oriented proposals of their application. In addition, we assembled an overview of future developments in this field geared towards detection of patients at risk for cardiovascular events.

Intravascular polarization-sensitive optical coherence tomography based on polarization mode delay

Intravascular polarization-sensitive optical coherence tomography (IV-PSOCT) provides depth-resolved tissue birefringence which can be used to evaluate the mechanical stability of a plaque. In our previous study, we reported a new strategy to construct polarization-sensitive optical coherence tomography in a microscope platform. Here, we demonstrated that this technology can be implemented in an endoscope platform, which has many clinical applications. A conventional intravascular OCT system can be modified for IV-PSOCT by introducing a 12-m polarization-maintaining fiber-based imaging probe. Its two polarization modes separately produce OCT images of polarization detection channels spatially distinguished by an image separation of 2.7 mm. We experimentally validated our IV-PSOCT with chicken tendon, chicken breast, and coronary artery as the image samples. We found that the birefringent properties can be successfully visualized by our IV-PSOCT.

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.

A Novel Distal Micromotor-Based Side-Looking Intravascular Ultrasound Transducer

Cardiovascular disease has become one of the leading causes of death in China, accounting for 45.5% of all deaths in rural areas and 43.16% in urban areas. Hence, its early diagnosis is important. With the development of intravascular imaging technology, the intravascular ultrasound (IVUS) is widely used. The available commercial mechanical rotary side-looking IVUS (SL-IVUS) transducers are driven by external motors that use long flexible shafts to transmit the rotation. However, when the transducer passes through a long-curved blood vessel, it easily causes the nonuniform rotation distortion (NURD) of the image. A catheter which contains a distal motor and sodium chloride (NaCl) solution is presented in this study as an attempt to solve such issues. The NaCl solution is used to connect the transducer and micromotor so that the motor can directly drive the transducer to rotate and acquire the information of the blood vessel. The results showed that the center frequency and -6-dB fraction bandwidth of the single element were 47 MHz and 98%, respectively. The SL-IVUS catheter consists of a distal motor, with speed stability and high resolution, and has the potential to diagnose cardiovascular disease. This novel structure can decrease the dimension at the top of the catheter and reduce the risks of clinical diagnosis.

Recent Technical Progression in Photoacoustic Imaging—Towards Using Contrast Agents and Multimodal Techniques

For combining optical and ultrasonic imaging methodologies, photoacoustic imaging (PAI) is the most important and successful hybrid technique, which has greatly contributed to biomedical research and applications. Its theoretical background is based on the photoacoustic effect, whereby a modulated or pulsed light is emitted into tissue, which selectively absorbs the optical energy of the light at optical wavelengths. This energy produces a fast thermal expansion in the illuminated tissue, generating pressure waves (or photoacoustic waves) that can be detected by ultrasonic transducers. Research has shown that optical absorption spectroscopy offers high optical sensitivity and contrast for ingredient determination, for example, while ultrasound has demonstrated good spatial resolution in biomedical imaging. Photoacoustic imaging combines these advantages, i.e., high contrast through optical absorption and high spatial resolution due to the low scattering of ultrasound in tissue. In this review, we focus on advances made in PAI in the last five years and present categories and key devices used in PAI techniques. In particular, we highlight the continuously increasing imaging depth achieved by PAI, particularly when using exogenous reagents. Finally, we discuss the potential of combining PAI with other imaging techniques.

Progress of clinical translation of handheld and semi-handheld photoacoustic imaging

Photoacoustic imaging (PAI), featuring rich contrast, high spatial/temporal resolution and deep penetration, is one of the fastest-growing biomedical imaging technology over the last decade. To date, numbers of handheld and semi-handheld photoacoustic imaging devices have been reported with corresponding potential clinical applications. Here, we summarize emerged handheld and semi-handheld systems in terms of photoacoustic computed tomography (PACT), optoacoustic mesoscopy (OAMes), and photoacoustic microscopy (PAM). We will discuss each modality in three aspects: laser delivery, scanning protocol, and acoustic detection. Besides new technical developments, we also review the associated clinical studies, and the advantages/disadvantages of these new techniques. In the end, we propose the challenges and perspectives of miniaturized PAI in the future.

Effectiveness of synthetic aperture focusing and coherence factor weighting for intravascular ultrasound imaging

Synthetic aperture focusing (SAF) and coherence factor weighting (CFW) have been used to improve the lateral resolution of ultrasound images. Although the two methods are effective for array-based ultrasound imaging, many researchers have also employed the methods for single-element-based imaging including intravascular ultrasound (IVUS) imaging. For single-element-based imaging, CFW is generally calculated from the scanlines obtained by SAF and applied to the scanline obtained after coherent summation of the SAF delayed scanlines, which is called a SAF-CFW method. In the paper, a theoretical model was derived to explore the effectiveness of SAF and CFW for single-element-based imaging, and the model was used to explain that SAF is not effective for IVUS imaging in terms of enhancing the spatial resolution, although it has the advantage of improving a contrast-to-noise ratio (CNR). This means that the SAF-CFW method is not optimal for improving the spatial resolution of IVUS imaging. In contrast, it was found in simulations and experiments that applying CFW to the target scanline itself is beneficial for the spatial resolution rather than a coherent summed scanline for IVUS SAF imaging, but CNR was not as good as SAF and SAF-CFW. As a result of both simulation and experimentation, it could be concluded that focused IVUS transducers without the application of those methods may be more advantageous to improve the spatial and contrast resolution simultaneously, considering the system complexity in the implementation of such imaging methods.

In vivo longitudinal visualization of the brain neuroinflammatory response at the cellular level in LysM-GFP mice induced by 3-nitropropionic acid

Blood-brain barrier (BBB) dysfunction is related to the development of neuroinflammation in the central nervous system (CNS). Neuroinflammation has been implicated as one of the key factors in the pathogenesis of neurodegenerative diseases such as Alzheimer's disease, Huntington's disease, and Parkinson's disease. Despite its importance, the impacts and underlying cellular mechanisms of chronic BBB impairment in neurodegenerative diseases are poorly understood. In this work, we performed a longitudinal intravital brain imaging of mouse model with neuroinflammation induced by 3-nitropropionic acid (3-NP). For this, we obtained a transgenic LysM-GFP mouse expressing the green fluorescence protein (GFP) in a subset of leukocytes. By using intravenously injected fluorescence blood tracers, we longitudinally observed in vivo dynamic cellular behaviors and the BBB integrity through a 30-day neuroinflammatory state. Vascular leakages in the cerebral cortex reflecting BBB impairment were observed at two weeks, which persisted to the third week, followed by a severe inflammatory response with massive leukocytes infiltration at day 30. These descriptions can help in the development of novel approaches to treat neurodegenerative conditions.

Versatile Single-Element Ultrasound Imaging Platform using a Water-Proofed MEMS Scanner for Animals and Humans

Single-element transducer based ultrasound (US) imaging offers a compact and affordable solution for high-frequency preclinical and clinical imaging because of its low cost, low complexity, and high spatial resolution compared to array-based US imaging. To achieve B-mode imaging, conventional approaches adapt mechanical linear or sector scanning methods. However, due to its low scanning speed, mechanical linear scanning cannot achieve acceptable temporal resolution for real-time imaging, and the sector scanning method requires specialized low-load transducers that are small and lightweight. Here, we present a novel single-element US imaging system based on an acoustic mirror scanning method. Instead of physically moving the US transducer, the acoustic path is quickly steered by a water-proofed microelectromechanical (MEMS) scanner, achieving real-time imaging. Taking advantage of the low-cost and compact MEMS scanner, we implemented both a tabletop system for in vivo small animal imaging and a handheld system for in vivo human imaging. Notably, in combination with mechanical raster scanning, we could acquire the volumetric US images in live animals. This versatile US imaging system can be potentially used for various preclinical and clinical applications, including echocardiography, ophthalmic imaging, and ultrasound-guided catheterization.

Multimodal intravascular imaging technology for characterization of atherosclerosis

Early detection of vulnerable plaques is the critical step in the prevention of acute coronary events. Morphology, composition, and mechanical property of a coronary artery have been demonstrated to be the key characteristics for the identification of vulnerable plaques. Several intravascular multimodal imaging technologies providing co-registered simultaneous images have been developed and applied in clinical studies to improve the characterization of atherosclerosis. In this paper, the authors review the present system and probe designs of representative intravascular multimodal techniques. In addition, the scientific innovations, potential limitations, and future directions of these technologies are also discussed.

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.

Photoacoustic imaging for guidance of interventions in cardiovascular medicine

Imaging guidance is paramount to procedural success in minimally invasive interventions. Catheter-based therapies are the standard of care in the treatment of many cardiac disorders, including coronary artery disease, structural heart disease and electrophysiological conditions. Many of these diseases are caused by, or effect, a change in vasculature or cardiac tissue composition, which can potentially be detected by photoacoustic imaging. This review summarizes the state of the art in photoacoustic imaging approaches that have been proposed for intervention guidance in cardiovascular care. All of these techniques are currently in the preclinical phase. We will conclude with an outlook towards clinical applications.

An image reconstruction method for endoscopic photoacoustic tomography in tissues with heterogeneous sound speed

The idealized assumption of a constant speed of sound (SOS) in acoustically inhomogeneous biological tissues usually results in blurred details, acoustic distortion and artifacts in in vivo endoscopic photoacoustic tomographic (EPAT) images. In this paper, we propose an image reconstruction method to improve EPAT imaging for luminal structures with the variable SOS. In our method, an optimal SOS providing the maximal local focusing of a measuring location within the imaging region is firstly determined. The deviation in the ultrasonic propagation time caused by the variable SOS is then compensated. The grayscale images of the optical absorption distribution on the cross-sections of the luminal structures are finally reconstructed with a filtered back-projection (FBP) algorithm based on the corrected propagation time. Any prior knowledge of the SOS distribution in the imaged tissues is not required. The results of numerical simulation experiments demonstrated that the proposed method can effectively improve the image quality by reducing the misalignment of tissues, acoustic distortion and artifacts caused by the variable SOS.

All-optical dual photoacoustic and optical coherence tomography intravascular probe

Intravascular imaging in percutaneous coronary interventions can be an invaluable tool in the treatment of coronary artery disease. It is of significant interest to provide molecular imaging contrast that is complementary to structural contrast provided by optical coherence tomography (OCT) and intravascular ultrasound imaging (IVUS). In this study, we developed a dual-modality intravascular imaging probe comprising a commercial OCT catheter and a high sensitivity fiber optic ultrasound sensor, to provide both photoacoustic (PA) and OCT imaging. With PA imaging, the lateral resolution varied from 18 μm to 40 μm; the axial resolution was consistently in the vicinity of 45 μm. We demonstrated the clinical potential of the probe with 2-D circumferential PA and OCT imaging, and with multispectral PA imaging.