Deep thrombosis characterization using photoacoustic imaging with intravascular light delivery

Detecting Oxygenator Thrombosis in ECMO: A Review of Current Techniques and an Exploration of Future Directions

Extracorporeal membrane oxygenation (ECMO) is a life-support technique used to treat cardiac and pulmonary failure, including severe cases of COVID-19 (coronavirus disease 2019) involving acute respiratory distress syndrome. Blood clot formation in the circuit is one of the most common complications in ECMO, having potentially harmful and even fatal consequences. It is therefore essential to regularly monitor for clots within the circuit and take appropriate measures to prevent or treat them. A review of the various methods used by hospital units for detecting blood clots is presented. The benefits and limitations of each method are discussed, specifically concerning detecting blood clots in the oxygenator, as it is concluded that this is the most critical and challenging ECMO component to assess. We investigate the feasibility of solutions proposed in the surrounding literature and explore two areas that hold promise for future research: the analysis of small-scale pressure fluctuations in the circuit, and real-time imaging of the oxygenator. It is concluded that the current methods of detecting blood clots cannot reliably predict clot volume, and their inability to predict clot location puts patients at risk of thromboembolism. It is posited that a more in-depth analysis of pressure readings using machine learning could better provide this information, and that purpose-built imaging could allow for accurate, real-time clotting analysis in ECMO components.

Miniaturized Stacked Transducer for Intravascular Sonothrombolysis With Internal-Illumination Photoacoustic Imaging Guidance and Clot Characterization

Thromboembolism in blood vessels can lead to stroke or heart attack and even sudden death unless brought under control. Sonothrombolysis enhanced by ultrasound contrast agents has shown promising outcome on effective treatment of thromboembolism. Intravascular sonothrombolysis was also reported recently with a potential for effective and safe treatment of deep thrombosis. Despite the promising treatment results, the treatment efficiency for clinical application may not be optimized due to the lack of imaging guidance and clot characterization during the thrombolysis procedure. In this paper, a miniaturized transducer was designed to have an 8-layer PZT-5A stacked with an aperture size of 1.4 × 1.4 mm2 and assembled in a customized two-lumen 10-Fr catheter for intravascular sonothrombolysis. The treatment process was monitored with internal-illumination photoacoustic tomography (II-PAT), a hybrid imaging modality that combines the rich contrast of optical absorption and the deep penetration of ultrasound detection. With intravascular light delivery using a thin optical fiber integrated with the intravascular catheter, II-PAT overcomes the penetration depth limited by strong optical attenuation of tissue. In-vitro PAT-guided sonothrombolysis experiments were carried out with synthetic blood clots embedded in tissue phantom. Clot position, shape, stiffness, and oxygenation level can be estimated by II-PAT at clinically relevant depth of ten centimeters. Our findings have demonstrated the feasibility of the proposed PAT-guided intravascular sonothrombolysis with real-time feedback during the treatment process.

Multiscale photoacoustic tomography using reversibly switchable thermochromics

SIGNIFICANCE

Based on acoustic detection of optical absorption, photoacoustic tomography (PAT) allows functional and molecular imaging beyond the optical diffusion limit with high spatial resolution. However, multispectral functional and molecular PAT is often limited by decreased spectroscopic accuracy and reduced detection sensitivity in deep tissues, mainly due to wavelength-dependent optical attenuation and inaccurate acoustic inversion.

AIM

Previous work has demonstrated that reversible color-shifting can drastically improve the detection sensitivity of PAT by suppressing nonswitching background signals. We aim to develop a new color switching-based PAT method using reversibly switchable thermochromics (ReST).

APPROACH

We developed a family of ReST with excellent water dispersion, biostability, and temperature-controlled color changes by surface modification of commercial thermochromic microcapsules with the hydrophilic polysaccharide alginate.

RESULTS

The optical absorbance of the ReST was switched on and off repeatedly by modulating the surrounding temperature, allowing differential photoacoustic detection that effectively suppressed the nonswitching background signal and substantially improved image contrast and detection sensitivity. We demonstrate reversible thermal-switching imaging of ReST in vitro and in vivo using three PAT modes at different length scales.

CONCLUSIONS

ReST-enabled PAT is a promising technology for high-sensitivity deep tissue imaging of molecular activity in temperature-related biomedical applications, such as cancer thermotherapy.

Detection of clot formation & lysis In-Vitro using high frequency photoacoustic imaging & frequency analysis

Clotting is a physiological process that prevents blood loss after injury. An imbalance in clotting factors can lead to lethal consequences such as exsanguination or inappropriate thrombosis. Clinical methods to monitor clotting and fibrinolysis typically measure the viscoelasticity of whole blood or optical density of plasma over time. Though these methods provide insights into clotting and fibrinolysis, they require milliliters of blood which can worsen anemia or only provide partial information. To overcome these limitations, a high-frequency photoacoustic (HFPA) imaging system was developed to detect clotting and lysis in blood. Clotting was initiated in vitro in reconstituted blood using thrombin and lysed with urokinase plasminogen activator. Frequency spectra measured using HFPA signals (10-40 MHz) between non-clotted blood and clotted blood differed markedly, allowing tracking of clot initiation and lysis in volumes of blood as low as 25 µL/test. HFPA imaging shows potential as a point-of-care examination of coagulation and fibrinolysis.

Motion Compensation for 3D Multispectral Handheld Photoacoustic Imaging

Three-dimensional (3D) handheld photoacoustic (PA) and ultrasound (US) imaging performed using mechanical scanning are more useful than conventional 2D PA/US imaging for obtaining local volumetric information and reducing operator dependence. In particular, 3D multispectral PA imaging can capture vital functional information, such as hemoglobin concentrations and hemoglobin oxygen saturation (sO₂), of epidermal, hemorrhagic, ischemic, and cancerous diseases. However, the accuracy of PA morphology and physiological parameters is hampered by motion artifacts during image acquisition. The aim of this paper is to apply appropriate correction to remove the effect of such motion artifacts. We propose a new motion compensation method that corrects PA images in both axial and lateral directions based on structural US information. 3D PA/US imaging experiments are performed on a tissue-mimicking phantom and a human wrist to verify the effects of the proposed motion compensation mechanism and the consequent spectral unmixing results. The structural motions and sO₂ values are confirmed to be successfully corrected by comparing the motion-compensated images with the original images. The proposed method is expected to be useful in various clinical PA imaging applications (e.g., breast cancer, thyroid cancer, and carotid artery disease) that are susceptible to motion contamination during multispectral PA image analysis.

Photoacoustic imaging for microcirculation

Microcirculation facilitates the blood-tissue exchange of nutrients and regulates blood perfusion. It is, therefore, essential in maintaining tissue health. Aberrations in microcirculation are potentially indicative of underlying cardiovascular and metabolic pathologies. Thus, quantitative information about it is of great clinical relevance. Photoacoustic imaging (PAI) is a capable technique that relies on the generation of imaging contrast via the absorption of light and can image at micron-scale resolution. PAI is especially desirable to map microvasculature as hemoglobin strongly absorbs light and can generate a photoacoustic signal. This paper reviews the current state of the art for imaging microvascular networks using photoacoustic imaging. We further describe how quantitative information about blood dynamics such as the total hemoglobin concentration, oxygen saturation, and blood flow rate is obtained using PAI. We also discuss its importance in understanding key pathophysiological processes in neurovascular, cardiovascular, ophthalmic, and cancer research fields. We then discuss the current challenges and limitations of PAI and the approaches that can help overcome these limitations. Finally, we provide the reader with an overview of future trends in the field of PAI for imaging microcirculation.