Abstract
Photoacoustic (PA) imaging offers several attractive features as a biomedical imaging modality, including excellent spatial resolution and functional information such as tissue oxygenation. A key limitation, however, is the contrast to noise ratio that can be obtained from tissue depths greater than 1-2 mm. Microbubbles coated with an optically absorbing shell have been proposed as a possible contrast agent for PA imaging, offering greater signal amplification and improved biocompatibility compared to metallic nanoparticles. A theoretical description of the dynamics of a coated microbubble subject to laser irradiation has been developed previously. The aim of this study was to test the predictions of the model. Two different types of oil-coated microbubbles were fabricated and then exposed to both pulsed and continuous wave (CW) laser irradiation. Their response was characterized using ultra high-speed imaging. Although there was considerable variability across the population, good agreement was found between the experimental results and theoretical predictions in terms of the frequency and amplitude of microbubble oscillation following pulsed excitation. Under CW irradiation, highly nonlinear behavior was observed which may be of considerable interest for developing different PA imaging techniques with greatly improved contrast enhancement.
Highlights
Over the past decade, photoacoustic (PA) imaging has emerged as a new modality combining the safety and portability of ultrasound imaging with the specificity of optical imaging and offering excellent spatial resolution.1,2 PA imaging exploits the PA effect3 whereby absorption of modulated electromagnetic radiation leads to heating followed by expansion and contraction of the absorbing material
It is important to note here that the differences observed between the theoretical resonant radius and the measured resonant radius can partly result from the addition of the imprecisions arising when simulating the bubble heating without Rayleigh-Plesset dynamics
The bubbles were first irradiated with a pulsed laser and good agreement between the measured natural frequency and the theoretical predictions was found
Summary
Photoacoustic (PA) imaging has emerged as a new modality combining the safety and portability of ultrasound imaging with the specificity of optical imaging and offering excellent spatial resolution. PA imaging exploits the PA effect whereby absorption of modulated electromagnetic radiation leads to heating followed by expansion and contraction of the absorbing material. PA imaging exploits the PA effect whereby absorption of modulated electromagnetic radiation leads to heating followed by expansion and contraction of the absorbing material This results in the generation of a pressure wave and these acoustic emissions can be detected and used to reconstruct an image. A possible alternative for contrast enhancement is to use microbubbles having a gas core of 1–2 lm in diameter, which can be transient or stabilized by a surfactant or polymer coating. These are well established as contrast agents for ultrasound imaging on account of their excellent acoustic scattering cross section and nonlinear behavior.. These are well established as contrast agents for ultrasound imaging on account of their excellent acoustic scattering cross section and nonlinear behavior. Phase change microdroplets, which turn into bubbles upon laser irradiation, have been shown to produce a tenfold increase in the amplitude of acoustic emissions compared to those produced by nanoparticles. One disadvantage, is the large amount of
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