In-vivo hemodynamic imaging of acute prenatal ethanol exposure in fetal brain by photoacoustic tomography

Spatiotemporal image reconstruction to enable high-frame-rate dynamic photoacoustic tomography with rotating-gantry volumetric imagers

Significance

Dynamic photoacoustic computed tomography (PACT) is a valuable imaging technique for monitoring physiological processes. However, current dynamic PACT imaging techniques are often limited to two-dimensional spatial imaging. Although volumetric PACT imagers are commercially available, these systems typically employ a rotating measurement gantry in which the tomographic data are sequentially acquired as opposed to being acquired simultaneously at all views. Because the dynamic object varies during the data-acquisition process, the sequential data-acquisition process poses substantial challenges to image reconstruction associated with data incompleteness. The proposed image reconstruction method is highly significant in that it will address these challenges and enable volumetric dynamic PACT imaging with existing preclinical imagers.

Aim

The aim of this study is to develop a spatiotemporal image reconstruction (STIR) method for dynamic PACT that can be applied to commercially available volumetric PACT imagers that employ a sequential scanning strategy. The proposed reconstruction method aims to overcome the challenges caused by the limited number of tomographic measurements acquired per frame.

Approach

A low-rank matrix estimation-based STIR (LRME-STIR) method is proposed to enable dynamic volumetric PACT. The LRME-STIR method leverages the spatiotemporal redundancies in the dynamic object to accurately reconstruct a four-dimensional (4D) spatiotemporal image.

Results

The conducted numerical studies substantiate the LRME-STIR method's efficacy in reconstructing 4D dynamic images from tomographic measurements acquired with a rotating measurement gantry. The experimental study demonstrates the method's ability to faithfully recover the flow of a contrast agent with a frame rate of 10 frames per second, even when only a single tomographic measurement per frame is available.

Conclusions

The proposed LRME-STIR method offers a promising solution to the challenges faced by enabling 4D dynamic imaging using commercially available volumetric PACT imagers. By enabling accurate STIRs, this method has the potential to significantly advance preclinical research and facilitate the monitoring of critical physiological biomarkers.

Application of photoacoustic computed tomography in biomedical imaging: A literature review

Photoacoustic computed tomography (PACT) is a hybrid imaging modality that combines optical excitation and acoustic detection techniques. It obtains high-resolution deep-tissue images based on the deep penetration of light, the anisotropy of light absorption in objects, and the photoacoustic effect. Hence, PACT shows great potential in biomedical sample imaging. Recently, due to its advantages of high sensitivity to optical absorption and wide scalability of spatial resolution with the desired imaging depth, PACT has received increasing attention in preclinical and clinical practice. To date, there has been a proliferation of PACT systems designed for specific biomedical imaging applications, from small animals to human organs, from ex vivo to in vivo real-time imaging, and from simple structural imaging to functional and molecular imaging with external contrast agents. Therefore, it is of great importance to summarize the previous applications of PACT systems in biomedical imaging and clinical practice. In this review, we searched for studies related to PACT imaging of biomedical tissues and samples over the past two decades; divided the studies into two categories, PACT imaging of preclinical animals and PACT imaging of human organs and body parts; and discussed the significance of the studies. Finally, we pointed out the future directions of PACT in biomedical applications. With the development of exogenous contrast agents and advances of imaging technique, in the future, PACT will enable biomedical imaging from organs to whole bodies, from superficial vasculature to internal organs, from anatomy to functions, and will play an increasingly important role in biomedical research and clinical practice.

High-resolution imaging in studies of alcohol effect on prenatal development

Fetal alcohol syndrome represents the leading known preventable cause of mental retardation. FAS is on the most severe side of fetal alcohol spectrum disorders that stem from the deleterious effects of prenatal alcohol exposure. Affecting as many as 1 to 5 out of 100 children, FASD most often results in brain abnormalities that extend to structure, function, and cerebral hemodynamics. The present review provides an analysis of high-resolution imaging techniques that are used in animals and human subjects to characterize PAE-driven changes in the developing brain. Variants of magnetic resonance imaging such as magnetic resonance microscopy, magnetic resonance spectroscopy, diffusion tensor imaging, along with positron emission tomography, single-photon emission computed tomography, and photoacoustic imaging, are modalities that are used to study the influence of PAE on brain structure and function. This review briefly describes the aforementioned imaging modalities, the main findings that were obtained using each modality, and touches upon the advantages/disadvantages of each imaging approach.

Monitoring neonatal brain hemorrhage progression by photoacoustic tomography

Neonatal brain hemorrhage (NBH) is the most common neurological disorder in neonates and its clinical interventions are very limited. Understanding the pathology of NBH by non-invasive in-vivo characterization of standardized animal models is essential for developing potential treatments. Currently, there is no suitable tool to provide non-invasive, non-ionizing dynamic imaging of neonatal mouse models with high resolution, high contrast, and deep imaging depth. In this study, we implemented a fast 3D photoacoustic tomography (PAT) system suitable for imaging neonatal mouse brains with good image quality and demonstrated its feasibility in non-invasive monitoring of the dynamic process of NBH in the whole neonatal mouse brain. The results present a high resolution and sensitivity for NBH detection. Both morphological and hemodynamic changes of the hematoma were accurately obtained. Our results demonstrated the potential of PAT as a powerful tool for the preclinical study of neonatal brain hemorrhage.

Path analysis of the impact of prenatal alcohol on adult vascular function

BACKGROUND

The vascular system may be particularly vulnerable to prenatal alcohol exposure (PAE). Alterations in angiogenesis and epigenetic changes to vascular development have been implicated as a probable mechanism for this vulnerability.

METHODS

We assessed the long-term impact of prenatal alcohol exposure (PAE) on adult vascular health using a prospective cohort first identified while in utero. Participants with no PAE (n = 37, mean age = 36.7 [SD = 1.6] years) were compared to participants with PAE (n = 51, mean age = 36.3 [SD = 1.7] years). Their vascular health was assessed by arterial blood pressure (BP) and peripheral arterial tonometry, which yields an index of endothelial function (reactive hyperemia index) and a measure of arterial stiffness (augmentation index). Blood samples were collected to assess cholesterol levels and insulin resistance (glucose, hemoglobin A1C, and insulin). Path analysis was used to examine the direct and indirect effects of PAE on vascular health after adjusting for other known physical outcomes.

RESULTS

Participants with a history of PAE weighed less, trended towards being shorter, had smaller body mass, and had more alcohol-related dysmorphic features than those without PAE. Path analysis suggested that the impact of PAE on BP was through its indirect relationships with height, body mass index, and dysmorphic features and resulted in protective effects relative to the Contrast group who were disproportionately overweight. PAE was also found to have a direct negative effect on endothelial function. An index of total alcohol-related dysmorphic features was negatively had both a direct effect on arterial stiffness and an indirect effect on endothelial function.

CONCLUSIONS

Prenatal alcohol exposures' impact on vascular function is not independent of other common physical and environmental factors but endothelial function and arterial stiffness seemed most compromised after controlling for these other factors. Level of alcohol-related dysmorphic features seems to be predictive of more adverse effects than endothelial function and vascular stiffness.

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.

An Excitation-Reception Collinear Probe for Ultrasonic, Photoacoustic, and Thermoacoustic Tri-Modal Volumetric Imaging

Imaging systems that integrate multiple modalities can reveal complementary anatomic and functional information as they exploit different contrast mechanisms, which have shown great application potential and advantages in preclinical studies. A portable and easy-to-use imaging probe will be more conducive to transfer to clinical practice. Here, we present a tri-modal ultrasonic (US), photoacoustic (PA), and thermoacoustic (TA) imaging system with an excitation-reception collinear probe. The acoustic field, light field, and electric field of the probe were designed to be coaxial, realizing homogeneous illumination and high-sensitivity detection at the same detection position. US images can provide detailed information about structures, PA images can delineate the morphology of blood vessels in tissues, and TA images can reveal dielectric properties of the tissues. Moreover, phantoms and in vivo human finger experiments were performed by the tri-modal imaging system to demonstrate its performance. The results show that the tri-modal imaging system with the proposed probe has the ability to detect small breast tumors with a radius of only 2.5 mm and visualize the anatomical structure of the finger in three dimensions. Our work confirms that the tri-modal imaging system equipped with a collinear probe can be applied to a variety of different scenarios, which lays a solid foundation for the application of the tri-modality system in clinical trials.

Tissue clearing and imaging methods for cardiovascular development

Tissue imaging in 3D using visible light is limited and various clearing techniques were developed to increase imaging depth, but none provides universal solution for all tissues at all developmental stages. In this review, we focus on different tissue clearing methods for 3D imaging of heart and vasculature, based on chemical composition (solvent-based, simple immersion, hyperhydration, and hydrogel embedding techniques). We discuss in detail compatibility of various tissue clearing techniques with visualization methods: fluorescence preservation, immunohistochemistry, nuclear staining, and fluorescent dyes vascular perfusion. We also discuss myocardium visualization using autofluorescence, tissue shrinking, and expansion. Then we overview imaging methods used to study cardiovascular system and live imaging. We discuss heart and vessels segmentation methods and image analysis. The review covers the whole process of cardiovascular system 3D imaging, starting from tissue clearing and its compatibility with various visualization methods to the types of imaging methods and resulting image analysis.