Optical and acoustic characterization of freeze-thawed polyvinyl alcohol gels

\u3cp\u3ePreclinical validation of non-invasive photoacoustic imaging of carotid artery atherosclerosis requires vessel phantoms that imitate optical, acoustic and mechanical properties of vascular tissue. Polyvinyl alcohol (PVA) phantoms that are widely used as ultrasound phantoms due to their elastic properties are also promising for photoacoustics. This study contributes to the field by quantifying the optical and acoustic properties of PVA gel, and aims at the characterization of realistic phantoms for future studies. In this study, we investigated the relation between acoustic scatterers and optical absorbers to quantify optical and acoustic properties of the PVA phantoms. Four different concentrations of orgasol acoustic scatterers, and varying concentration of Indian ink and molecular dye absorbers were added to a 15 wt% PVA solution. Samples were subjected to 1 to 5 freeze-thaw cycles and were examined after each cycle to quantify the effect on the optical and the acoustic properties. Optical attenuation was measured between 400 nm and 990 nm using a plate reader. Additionally, pulse-echo plane wave ultrasound was used for acoustic characterization. Changing the concentration of orgasol between 0.5 wt% and 4 wt% increased the mean optical attenuation of PVA by 35% after the first freeze thaw cycle. Likewise, each freeze-thaw cycle increased the optical attenuation due to scattering of light by the microstructure of PVA. The absorbance of pure PVA increased 40% between the first and second cycle and 3% between the fourth and fifth cycle. While the orgasol concentration and the freeze-thaw cycles altered the acoustic speed and attenuation, the ink and the dye inclusions did not significantly affect the acoustic properties of PVA.\u3c/p\u3

Hemorrhages detection in atherosclerotic plaques using ultrasound and photoacoustic, phantom study

\u3cp\u3eRupture of an atherosclerotic plaque in the carotid artery is one of the main causes of stroke and stroke-induced death. Currently, to prevent this risk, endarterectomy is performed based on the stenosis grade assessed with Duplex ultrasound (US). However, plaque composition, e.g. presence of lipids and hemorrhages, is a more important factor of rupture risk than stenosis. The optical absorption of hemorrhages (composed of coagulated, deoxygenated blood) is different from that of oxygenated blood and allows their distinction using different wavelengths. In this study, photoacoustic (PA) imaging is used to detect intraplaque hemorrhages using contrast in optical absorption in combination with the resolution of US.\u3c/p\u3

Visualization of vasculature using a hand-held photoacoustic probe: phantom and in vivo validation

\u3cp\u3eAssessment of microvasculature and tissue perfusion can provide diagnostic information on local or systemic diseases. Photoacoustic (PA) imaging has strong clinical potential because of its sensitivity to hemoglobin. We used a hand-held PA probe with integrated diode lasers and examined its feasibility and validity in the detection of increasing blood volume and (sub) dermal vascularization. Blood volume detection was tested in custom-made perfusion phantoms. Results showed that an increase of blood volume in a physiological range of 1.3% to 5.4% could be detected. The results were validated with power Doppler sonography. Using a motorized scanning setup, areas of the skin were imaged at relatively short scanning times (<10 s/cm2) with PA. Three-dimensional visualization of these structures was achieved by combining the consecutively acquired cross-sectional images. Images revealed the epidermis and submillimeter vasculature up to depth of 5 mm. The geometries of imaged vasculature were validated with segmentation of the vasculature in high-frequency ultrasound imaging. This study proves the feasibility of PA imaging in its current implementation for the detection of perfusion-related parameters in skin and subdermal tissue and underlines its potential as a diagnostic tool in vascular or dermal pathologies.\u3c/p\u3

Unmixing multi-spectral photoacoustic sources in human carotid plaques using non-negative independent component analysis

\u3cp\u3eMulti-spectral photoacoustic imaging (MSPAI) is promising for morphology assessment of carotid plaques; however, obtaining unique spectral characteristics of chromophores is cumbersome. We used MSPAI and non-negative independent component analysis (ICA) to unmix distinct signal sources in human carotid plaques blindly. The feasibility of the method was demonstrated on a plaque phantom with hemorrhage and cholesterol inclusions, and plaque endarterectomy samples ex vivo. Furthermore, the results were verified with histology using Masson's trichrome staining. Results showed that ICA could separate recent hemorrhages from old hemorrhages. Additionally, the signatures of cholesterol inclusion were also captured for the phantom experiment. Artifacts were successfully removed from signal sources. Histologic examinations showed high resemblance with the unmixed components and confirmed the morphologic distinction between recent and mature hemorrhages. In future pre-clinical studies, unmixing could be used for morphology assessment of intact human plaque samples.\u3c/p\u3

Characterization of human carotid plaques using multi-wavelength photoacoustic imaging

\u3cp\u3eRecently, multi-spectral photoacoustic (PA) imaging has been explored to aid in the diagnosis of atherosclerosis in carotid arteries. Using multiple wavelengths, PA has the potential to reveal vital morphological information in plaques, such as intraplaque hemorrhages, lipid pools, and the fibrous cap. In this study, we used multispectral PA and plane wave ultrasound (US) hybrid-imaging to reveal the composition of human plaques ex-vivo.\u3c/p\u3

Investigation on the effect of spatial compounding on photoacoustic images of carotid plaques in the in vivo available rotational range

\u3cp\u3ePhotoacoustic imaging (PAI) is a promising imaging modality due to its high optical specificity. However, the low signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of in vivo PA images are major challenges that prevent PAI from finding its place in clinics. This paper investigates the merit of spatial compounding of PA images in arterial phantoms and the achievable improvements of SNR, when in vivo conditions are mimicked. The analysis of the compounding technique was performed on a polyvinyl alcohol vessel phantom with black threads embedded in its wall. The in vivo conditions were mimicked by limiting the rotation range in ±30°, adding turbid surrounding medium, and filling the lumen with porcine blood. Finally, the performance of the technique was evaluated in ex vivo human carotid plaque samples. Results showed that spatial compounding elevates the SNR by 5-10 dB and CNR by 1-5 dB, depending on the location of the absorbers. This paper elucidates prospective in vivo PA characterization of carotid plaques by proposing a method to enhance PA image quality.\u3c/p\u3