Abstract
Photoacoustic fluctuation imaging, which exploits randomness in photoacoustic generation, provides enhanced images in terms of resolution and visibility, as compared to conventional photoacoustic images. While a few experimental demonstrations of photoacoustic fluctuation imaging have been reported, it has to date not been described theoretically. In the first part of this work, we propose a theory relevant to fluctuations induced either by random illumination patterns or by random distributions of absorbing particles. The theoretical predictions are validated by Monte Carlo finite-difference time-domain simulations of photoacoustic generation in random particle media. We provide a physical insight into why visibility artefacts are absent from second-order fluctuation images. In the second part, we demonstrate experimentally that harnessing randomness induced by the flow of red blood cells produce photoacoustic fluctuation images free of visibility artefacts. As a first proof of concept, we obtain two-dimensional images of blood vessel phantoms. Photoacoustic fluctuation imaging is finally applied in vivo to obtain 3D images of the vascularization in a chicken embryo.
Highlights
Photoacoustic (PA) imaging is a widely-spread biomedical imaging modality that makes use of ultrasound waves resulting from light absorption [1]
Visibility artefacts occur with resonant ultrasound detectors, which filter out low-frequency components of PA waves emitted by large absorbers
This is a well-known result from the Super-resolution Optical Fluctuation Imaging (SOFI) approach initially proposed for super-resolution imaging relying on blinking fluorophores
Summary
Photoacoustic (PA) imaging is a widely-spread biomedical imaging modality that makes use of ultrasound waves resulting from light absorption [1]. Limited-view artefacts occur when strongly anisotropic absorbing structures (such as blood vessels) coherently emit PA waves in some preferential directions, and these waves can not be detected due to the finite aperture and/or directivity of the ultrasound detectors In such a case, some parts of the absorbing structure are not visible on the reconstructed PA image. An approach based on multiple-speckle illumination was proposed by Gateau et al [16] In this experimental work, a random intensity distribution of speckle patterns, which changed from pulse to pulse, induced fluctuations in each pixel of the corresponding series of PA images. Our theoretical results provide a quantitative comparison between fluctuations from multiple-speckle illumination and fluctuations from random distributions of absorbing particles. We discuss the dependence of the amplitude of the fluctuation image on characteristic statistical properties (mean, variance, characteristic size) of the random process inducing fluctuations (illumination patterns or distribution of absorbing particles). In the context of photoacoustic imaging, an illustration of the differences between real-valued only images and complexmodulus images may be found in a previous publication from our group [20]
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