83 research outputs found
Overcoming the acoustic diffraction limit in photoacoustic imaging by localization of flowing absorbers
The resolution of photoacoustic imaging deep inside scattering media is
limited by the acoustic diffraction limit. In this work, taking inspiration
from super-resolution imaging techniques developed to beat the optical
diffraction limit, we demonstrate that the localization of individual optical
absorbers can provide super-resolution photoacoustic imaging well beyond the
acoustic diffraction limit. As a proof-of-principle experiment, photoacoustic
cross-sectional images of microfluidic channels were obtained with a 15 MHz
linear CMUT array while absorbing beads were flown through the channels. The
localization of individual absorbers allowed to obtain super-resolved
cross-sectional image of the channels, by reconstructing both the channel width
and position with an accuracy better than . Given the discrete
nature of endogenous absorbers such as red blood cells, or that of exogenous
particular contrast agents, localization is a promising approach to push the
current resolution limits of photoacoustic imaging
Photoacoustic generation by a gold nanosphere: From linear to nonlinear thermoelastics in the long-pulse illumination regime
We investigate theoretically the photoacoustic generation by a gold
nanosphere in water in the thermoelastic regime. Specifically, we consider the
long-pulse illumination regime, in which the time for electron-phonon
thermalisation can be neglected and photoacoustic wave generation arises solely
from the thermo-elastic stress caused by the temperature increase of the
nanosphere or its liquid environment. Photoacoustic signals are predicted
computed based on the successive resolution of a thermal diffusion problem and
a thermoelastic problem, taking into account the finite size of the gold
nanosphere and the temperature-dependence of the thermal expansion coefficient
of water. For sufficiently high illumination fluences, this temperature
dependence yields a nonlinear relationship between the photoacoustic amplitude
and the fluence. For nanosecond pulses in the linear regime, we show that more
than 90 % of the emitted photoacoustic energy is generated in water, and the
thickness of the generating layer around the particle scales close to the
square root of the pulse duration. Our results demonstrate that the
point-absorber model introduced by Calasso et al.[17] significantly
overestimates the amplitude of photoacoustic waves in the nonlinear regime. We
therefore provide quantitative estimates of a critical energy, defined as the
absorbed energy required such that the nonlinear contribution is equal to that
of the linear contribution. Our results suggest that the critical energy scales
as the volume of water over which heat diffuses during the illumination pulse.
Moreover, thermal nonlinearity is shown to be expected only for sufficiently
high ultrasound frequency. Finally, we show that the relationship between the
photoacoustic amplitude and the equilibrium temperature at sufficiently high
fluence reflects the thermal diffusion at the nanoscale around the gold
nanosphere.Comment: Published in Physical Review B, 16 pages, 14 figure
Super-resolution photoacoustic imaging via flow induced absorption fluctuations
In deep tissue photoacoustic imaging the spatial resolution is inherently
limited by the acoustic wavelength. We present an approach for surpassing the
acoustic diffraction limit by exploiting temporal fluctuations in the sample
absorption distribution, such as those induced by flowing particles. In
addition to enhanced resolution, our approach inherently provides background
reduction, and can be implemented with any conventional photoacoustic imaging
system. The considerable resolution increase is made possible by adapting
notions from super-resolution optical fluctuations imaging (SOFI) developed for
blinking fluorescent molecules, to flowing acoustic emitters. By generalizing
SOFI mathematical analysis to complex valued signals, we demonstrate
super-resolved photoacoustic images that are free from oscillations caused by
band-limited detection. The presented technique holds potential for
contrast-agent free micro-vessels imaging, as red blood cells provide a strong
endogenous source of naturally fluctuating absorption
Acousto-optical coherence tomography with a digital holographic detection scheme
Acousto-optical coherence tomography (AOCT) consists in using random phase
jumps on ultrasound and light to achieve a millimeter resolution when imaging
thick scattering media. We combined this technique with heterodyne off-axis
digital holography. Two-dimensional images of absorbing objects embedded in
scattering phantoms are obtained with a good signal-to-noise ratio. We study
the impact of the phase modulation characteristics on the amplitude of the
acousto-optic signal and on the contrast and apparent size of the absorbing
inclusion
Radiative transfer and diffusion limits for wave field correlations in locally shifted random media
The aim of this paper is to develop a mathematical framework for
opto-elastography. In opto-elastography, a mechanical perturbation of the
medium produces a decorrelation of optical speckle patterns due to the
displacements of optical scatterers. To model this, we consider two optically
random media, with the second medium obtained by shifting the first medium in
some local region. We derive the radiative transfer equation for the
cross-correlation of the wave fields in the media. Then we derive its diffusion
approximation. In both the radiative transfer and the diffusion regimes, we
relate the correlation of speckle patterns to the solutions of the radiative
transfer and the diffusion equations. We present numerical simulations based on
our model which are in agreement with recent experimental measurements
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