A common approach in Photoacoustic Imaging (PAI) is to use a linear or curved piezoelectric transducer array, which provides flexibility and versatility during image acquisition. However, these PAI systems often have limited Field-of-View (FOV), resolution, and contrast, resulting in low quality images. In this study, a multi-transducer approach is proposed to improve FOV, resolution, and contrast, with the goal of facilitating human carotid plaque imaging. A prototype consisting of multiple Capacitive Micromachined Ultrasonic Transducers (CMUTs) on a flexible array with shared channels was developed and evaluated using simulated and ex-vivo human carotid plaque samples. In numerical simulations, the results are evaluated based on input ground truth parameters. For ex-vivo plaque samples, results for multi-transducer are evaluated and compared to the images acquired with single transducer. All the results demonstrate that the proposed approach improves contrast, FOV, and most notably, it allows resolving the structural information in the medium where more than 25% improvement in gCNR values is achieved in both simulations and experiments compared to the PA images obtained with single transducer.
KEYWORDS: Acoustics, Data modeling, In vitro testing, Speckle, Tissue optics, Tissues, Photoacoustic spectroscopy, Data acquisition, Point spread functions, Computer simulations
Significance: Physics-based simulations of photoacoustic (PA) signals are used to validate new methods, to characterize PA setups and to generate training datasets for machine learning. However, a thoroughly validated PA simulation toolchain that can simulate realistic images is still lacking.Aim: A quantitative toolchain was developed to model PA image acquisition in complex tissues, by simulating both the optical fluence and the acoustic wave propagation.Approach: Sampling techniques were developed to decrease artifacts in acoustic simulations. The performance of the simulations was analyzed by measuring the point spread function (PSF) and using a rotatable three-channel phantom, filled with cholesterol, a human carotid plaque sample, and porcine blood. Ex vivo human plaque samples were simulated to validate the methods in more complex tissues.Results: The sampling techniques could enhance the quality of the simulated PA images effectively. The resolution and intensity of the PSF in the turbid medium matched the experimental data well. Overall, the appearance, signal-to-noise ratio and speckle of the images could be simulated accurately.Conclusions: A PA toolchain was developed and validated, and the results indicate a great potential of PA simulations in more complex and heterogeneous media.
Photoacoustic imaging (PAI) has a great potential to assess vulnerable plaques in the carotid artery. However, in vivo, PAI suffers from low angular coverage, limited field of view (FOV), and lateral resolution especially when imaging a few centimeters deep in the tissue. To improve these shortcomings, here, we propose to image with multiple capacitive micromachined ultrasound transducers on a flexible substrate with orientation sensors to improve the image quality independent of the patient anatomy. We tested the multi-perspective PAI on a phantom and the experimental results demonstrate improvement in FOV, angular coverage, and resolution, strongly increasing the diagnostic capability of the PAI system.
The assessment of tissue composition using photoacoustic imaging (PAI) is a promising approach. However, the signal-to-noise ratio in PAI systems are quite low in comparison to ultrasound imaging’s especially at few centimeters depth. Multi-perspective photoacoustic imaging (MP-PAI) using multiple capacitive micromachined ultrasound transducers (CMUTs) on a flexible substrate provide a cost-efficient way to improve field of view (FOV), resolution, and angular coverage. In this work, an encoding scheme based on Hadamard codes is proposed to improve the SNR in MP-PAI. The concept is validated in an experiment using a carotid plaque tissue sample demonstrating the increase in SNR imaging with three CMUTs.
KEYWORDS: In vivo imaging, Signal detection, Image quality, Blood, Image filtering, Tissues, Digital filtering, Photoacoustic imaging, Data acquisition, Blood circulation
Significance: Intraplaque hemorrhage (IPH) is an important indicator of plaque vulnerability. Early detection could aid the prevention of stroke.
Aim: We aim to detect IPH with single wavelength PA imaging in vivo and to improve image quality.
Approach: We developed a singular value decomposition (SVD)-based filter to detect the nonstationary and stationary components in ultrasound data. A PA mask was created to detect stationary (IPH) sources. The method was tested ex vivo using phantoms and in vivo in patients.
Results: The flow and IPH channels were successfully separated in the phantom data. We can also detect the PA signals from IPH and reject signals from the carotid lumen in vivo. Generalized contrast-to-noise ratio improved in both ex vivo and in vivo in US imaging.
Conclusions: SVD-based filtering can successfully detect IPH using a single laser wavelength, opening up opportunities for more economical and cost-effective laser sources.
Capacitive micromachined ultrasound transducers (CMUTs) are gaining more interest for photoacoustic imaging due to its low-cost, high bandwidth and flexibility in size and shape. However, the beampattern of CMUTs usually exhibits wide mainlobes and high sidelobes, degrading the imaging resolution and contrast. We propose to improve the beampattern with optimization of apodization by imposing conditions on the angular distribution of the signal energy received from a PA point source to minimize the sidelobes magnitude and maintain the mainlobe energy. Both simulations and experiments results demonstrate that the proposed apodization suppress the sidelobes up to 30dB when compared with Uniform weights.
Assessment of morphology and composition of plaques is paramount to characterize their vulnerability. Spectroscopic photoacoustic imaging (sPAI) can image different components, but unmixing accuracy is subject to a proper wavelength selection. In this study, we analyzed the spectral response of plaque tissue in a broad spectral range and proposed a new wavelength selection method based on endmember determination. The method was validated in human plaque samples and phantoms. Results show that our method improves spectral unmixing, and it is possible to characterize plaque composition using at least as many wavelengths as constituents of interest.
Rupture of carotid plaques triggers stroke. Current diagnosis of stroke is based on lumen stenosis, resulting in sever overtreatment. Photoacoustic (PA) imaging can provide comprehensive and patient-specific assessment of plaque vulnerability, and prevent overtreatment. However, no in vivo PA imaging of carotid plaque is available due to low SNR. Here, we present a fast PA/US imaging system and motion corrected averaging algorithm to increase PA SNR. The imaging system and algorithm are verified ex vivo, and in vivo on patients during carotid endarterectomy (intra-operatively). The results may accelerate the clinical translation of PA imaging of carotid plaques.
The rupture of a vulnerable carotid plaque featuring a lipid-rich necrotic core and intra-plaque haemorrhages is the major cause of stroke. Photoacoustic imaging (PAI) is a promising technique for assessing plaque vulnerability in the carotid artery due to its ability to assess the chemical composition in addition to its anatomy. However, assessment of chemical composition is usually based on the absorption differences of chromophores between multiple wavelengths, which heavily increase the complexity and cost of the imaging system. In this study, a new method based on single-wavelength PAI to detect intra-plaque haemorrhages, an important indicator of plaque vulnerability, is developed. The method uses wall filtering based on singular value decomposition. To test the method, a carotid plaque phantom mimicking intra-plaque haemorrhages, lumen and vasa vasorum is designed and imaged at 808nm in vitro. The phantom experiment shows wall filtering using singular value decomposition to be a viable method capable of discriminating signals originating from the lumen, vasa vasorum and intraplaque haemorrhages, allowing for the detection of intra-plaque haemorrhages with single wavelength PAI. This enables new opportunities for PAI of vulnerable carotid plaques with more cost effective and diverse laser sources.
KEYWORDS: Signal attenuation, Arteries, In vivo imaging, Imaging systems, Transducers, Near field optics, Photoacoustic spectroscopy, Intravascular ultrasound, Photoacoustic imaging, Coronary catheterization
Intravascular photoacoustic/ultrasound imaging (IVPA/US) can image the structure and composition of atherosclerotic lesions identifying lipid-rich plaques ex vivo and in vivo. In the literature, multiple IVPA/US catheter designs were presented and validated both in ex-vivo models and preclinical in-vivo situations. Since the catheter is a critical component of the imaging system, we discuss here a catheter design oriented to imaging plaque in a realistic and translatable setting. We present a catheter optimized for light delivery, manageable flush parameters and robustness with reduced mechanical damage risks at the laser/catheter joint interface. We also show capability of imaging within sheath and in water medium.
Absorption of nanosecond laser pulses induces rapid thermo-elastic deformation in tissue, i.e. a sub-micrometer scale displacement happens within a couple of microseconds. In this study, we initially investigate the depth-resolved deformation using a 1.5 MHz phase-sensitive optical coherence tomography (OCT) system. Functional images can be reconstructed based on the detected deformation, which enables a new imaging modality called thermo-elastic deformation imaging (TDI). Our results show that the associated displacement is related to the optical absorption of the short laser pulses. The TDI images can provide tissue type information in addition to the conventional OCT images.
Intravascular Photoacoustic (IVPA) imaging is a promising new technology to assess lipid content of coronary atherosclerotic plaque, an important determinant of the risk associated with the plaque triggering a heart attack. Clinical translation of IVPA imaging requires real-time image acquisition, which has been a technological challenge. In this work, we demonstrate a high-speed, dual-wavelength IVPA imaging system at 1.7 µm wavelength, operating with a flexible catheter of 1.2 mm outer diameter (including outer sheath). The catheter was custom designed and fabricated, and used a 40 MHz transducer for intravascular ultrasound (IVUS) and IVPA imaging. The optical excitation is provided by a dual OPO system, pumped by CW diode-pumped Q-switched Nd:YAG lasers, with a repetition rate of 5 kHz. Each OPO can be tuned to a custom wavelength between 1690 and 1750 nm; two wavelengths only are needed to discriminate between plaque lipids and adipose tissue. The pulse energy is about 80 µJ. We tested the imaging performance of the presented system in a polyvinyl-alcohol (PVA) vessel mimicking phantom and human coronary arteries ex vivo. IVPA identified lipid deposits inside atherosclerotic plaque, while IVUS showed tissue structure. We demonstrated IVPA imaging at a speed of 20 frames per second, with 250 A-scans per frame. This is significantly faster than previous IVPA imaging systems, and will enable the translation of IVPA imaging into clinical practice.
The natural history of atherosclerosis is marked by changes in the lipid biochemistry in the diseased arterial wall. As lesions become more vulnerable, different cholesterol species accumulate in the plaque. Understanding unstable atherosclerosis as a pharmacological and interventional therapeutic target requires chemically specific imaging of disease foci. In this study, we aim to image atherosclerotic plaque lipids and other vessel wall constituents with spectroscopic intravascular photoacoustics (sIVPA). sIVPA imaging can identify lipids in human coronary atherosclerotic plaque by relying on contrast in the near-infrared absorption spectra of the arterial wall components. Using reference spectra acquired on pure compounds, we analyzed sIVPA data from human coronary plaques ex vivo, to image plaque composition in terms of cholesterol and cholesterol ester content. In addition, we visualized the deeper lying connective tissue layers of the adventitia, as well as the fatty acid containing adipose cells in the peri-adventitial tissue. We performed simultaneous coregistered IVUS imaging to obtain complementary morphological information. Results were corroborated by histopathology. sIVPA imaging can distinguish the most prevalent lipid components of human atherosclerotic plaques and also visualize the connective tissue layers of the adventitia and the fatty acid containing adipose cells in the peri-adventitial tissue.
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