Significance: Intravascular photoacoustic (IVPA) imaging can identify native lipid in atherosclerotic plaques in vivo. However, the large number of laser pulses required to produce 3D images is a safety concern that has not been fully addressed.
Aim: We aim to evaluate if irradiation at wavelengths and dosages relevant to IVPA imaging causes target vessel damage.
Approach: We irradiate the carotid artery of swine at one of several energy dosages using radiation at 1064 or 1720 nm and use histological evaluation by a pathologist to identify dose-dependent damage.
Results: Media necrosis was the only dose-dependent form of injury. Damage was present at a cumulative fluence of 50 J / cm2 when using 1720 nm light. Damage was more equivocally identified at 700 J / cm2 using 1064 nm.
Conclusions: In prior work, IVPA imaging of native lipid in swine has been successfully conducted below the damage thresholds identified. This indicates that it will be possible to use IVPA imaging in a clinical setting without damaging vessel tissue. Future work should determine if irradiation causes an increase in blood thrombogenicity and confirm whether damaged tissue will heal over longer time points.
Accurate measurements of microelastic properties of soft tissues in-vivo using optical coherence elastography can be affected by motion artifacts caused by cardiac and respiratory cycles. This problem can be overcome using a multielement ultrasound transducer probe where each ultrasound transducer is capable of generating acoustic radiation force (ARF) and, therefore, creating shear waves in tissue. These shear waves, produced during the phase of cardiac and respiratory cycles when tissues are effectively stationary, are detected at the same observation point using phase-sensitive optical coherence tomography (psOCT). Given the known distance between the ultrasound transducers, the speed of shear wave propagation can be calculated by measuring the difference between arrival times of shear waves. The combined multitransducer ARF/psOCT probe has been designed and tested in phantoms and ex-vivo studies using fresh rabbit heart. The measured values of shear moduli are in good agreement with those reported in literature. Our results suggest that the developed multitransducer ARF/psOCT probe can be useful for many in-vivo applications, including quantifying the microelasticity of cardiac muscle.
Many complex diseases such as diastolic dysfunction and some types of cardiomyopathy are often characterized by an increased stiffness of heart muscles which can potentially cause heart failure. While changes of heart muscle’s geometry could be detected by various imaging methods, non-invasive measurements of stiffness of the heart muscle are desired to assess such areas of the heart tissues without invasive surgery.
A novel minimally-invasive method of stiffness assessment of heart muscle – optical coherent elastography (OCE) – is based on a combination of applied acoustic radiation force for mechanical excitation of tissue with subsequent phase-sensitive optical coherence tomography (psOCT) measurements of spatio-temporal response of tissue. A minimally invasive probe comprising a small, 2x2 mm size, low-frequency (<5MHz) ultrasound transducer and a clinically approved psOCT imaging fiber was incorporated into a single housing such that psOCT beam and acoustic excitation beam were parallel. Acoustic radiation pressure pulse was applied to initiate tissue displacement and propagation of shear waves that were detected by psOCT. Given the known offset between ultrasound and psOCT beams, the speed of shear waves was measured and shear elastic modulus of the heart tissues can be reconstructed.
The initial results demonstrate that our OCE probe can produce and measure the displacements on the order of several ten nanometers in heart tissue-mimicking phantoms. The results indicate that translate-rotate scanning of OCE probe can simultaneously image the tissue and map its shear elastic modulus.
KEYWORDS: Tissues, Image segmentation, Absorption, Data modeling, Monte Carlo methods, Tissue optics, Photoacoustic imaging, Arteries, Imaging systems, 3D modeling
Coronary heart disease (the presence of coronary atherosclerotic plaques) is a significant health problem in the industrialized world. A clinical method to accurately visualize and characterize atherosclerotic plaques is needed. Intravascular photoacoustic (IVPA) imaging is being developed to fill this role, but questions remain regarding optimal imaging wavelengths. We utilized a Monte Carlo optical model to simulate IVPA excitation in coronary tissues, identifying optimal wavelengths for plaque characterization. Near-infrared wavelengths (≤1800 nm) were simulated, and single- and dual-wavelength data were analyzed for accuracy of plaque characterization. Results indicate light penetration is best in the range of 1050 to 1370 nm, where 5% residual fluence can be achieved at clinically relevant depths of ≥2 mm in arteries. Across the arterial wall, fluence may vary by over 10-fold, confounding plaque characterization. For single-wavelength results, plaque segmentation accuracy peaked at 1210 and 1720 nm, though correlation was poor (<0.13). Dual-wavelength analysis proved promising, with 1210 nm as the most successful primary wavelength (≈1.0). Results suggest that, without flushing the luminal blood, a primary and secondary wavelength near 1210 and 1350 nm, respectively, may offer the best implementation of dual-wavelength IVPA imaging. These findings could guide the development of a cost-effective clinical system by highlighting optimal wavelengths and improving plaque characterization.
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