4,050 research outputs found
Towards quantitative tissue absorption imaging by combining photoacoustics and acousto-optics
We propose a strategy for quantitative photoacoustic mapping of chromophore
concentrations that can be performed purely experimentally. We exploit the
possibility of acousto-optic modulation using focused ultrasound, and the
principle that photons follow trajectories through a turbid medium in two
directions with equal probability. A theory is presented that expresses the
local absorption coefficient inside a medium in terms of noninvasively measured
quantities and experimental parameters. Proof of the validity of the theory is
given with Monte Carlo simulations.Comment: 14 pages, 5 figure
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
Infrared spectroscopy of NGC 1068: Probing the obscured ionizing AGN continuum
The ISO-SWS 2.5-45 um infrared spectroscopic observations of the nucleus of
the Seyfert 2 galaxy NGC 1068 (see companion paper) are combined with a
compilation of UV to IR narrow emission line data to determine the spectral
energy distribution (SED) of the obscured extreme-UV continuum that
photoionizes the narrow line emitting gas in the active galactic nucleus. We
search a large grid of gas cloud models and SEDs for the combination that best
reproduces the observed line fluxes and NLR geometry. Our best fit model
reproduces the observed line fluxes to better than a factor of 2 on average and
is in general agreement with the observed NLR geometry. It has two gas
components that are consistent with a clumpy distribution of dense outflowing
gas in the center and a more extended distribution of less dense and more
clumpy gas farther out that has no net outflow. The best fit SED has a deep
trough at ~4 Ryd, which is consistent with an intrinsic Big Blue Bump that is
partially absorbed by ~6x10^19 cm^-2 of neutral hydrogen interior to the NLR.Comment: 15 pp, 4 figures, ApJ accepte
Metal enrichment by radiation pressure in active galactic nucleus outflows -- theory and observations
Outflows from active galactic nuclei may be produced by absorption of
continuum radiation by UV resonance lines of abundant metal ions, as observed
in broad absorption line quasars (BALQs). The radiation pressure exerted on the
metal ions is coupled to the rest of the gas through Coulomb collisions of the
metal ions. We calculate the photon density and gas density which allow
decoupling of the metal ions from the rest of the gas. These conditions may
lead to an outflow composed mostly of the metal ions. We derive a method to
constrain the metals/H ratio of observed UV outflows, based on the Ly {\alpha}
and Si iv {\lambda}{\lambda}1394, 1403 absorption profiles. We apply this
method to an SDSS sample of BALQs to derive a handful of candidate outflows
with a higher than solar metal/H ratio. This mechanism can produce ultra fast
UV outflows, if a shield of the continuum source with a strong absorption edge
is present.Comment: 16 pages, 7 figures; accepted for publication in MNRA
Detection, numerical simulation and approximate inversion of optoacoustic signals generated in multi-layered PVA hydrogel based tissue phantoms
In this article we characterize optoacoustic signals generated from layered
tissue phantoms via short laser pulses by experimental and numerical means. In
particular, we consider the case where scattering is effectively negligible and
the absorbed energy density follows Beer-Lambert's law, i.e. is characterized
by an exponential decay within the layers and discontinuities at interfaces. We
complement experiments on samples with multiple layers, where the material
properties are known a priori, with numerical calculations for a pointlike
detector, tailored to suit our experimental setup. Experimentally, we
characterize the acoustic signal observed by a piezoelectric detector in the
acoustic far-field in backward mode and we discuss the implication of acoustic
diffraction on our measurements. We further attempt an inversion of an OA
signal in the far-field approximation.Comment: 10 pages, 6 figures, supplementary code at
https://github.com/omelchert/SONOS.gi
Passive element enriched photoacoustic computed tomography (PER PACT) for simultaneous imaging of acoustic propagation properties and light absorption\ud
We present a âhybridâ imaging approach which can image both light absorption properties and acoustic transmission properties of an object in a two-dimensional slice using a computed tomography (CT) photoacoustic imager. The ultrasound transmission measurement method uses a strong optical absorber of small cross-section placed in the path of the light illuminating the sample. This absorber, which we call a passive element acts as a source of ultrasound. The interaction of ultrasound with the sample can be measured in transmission, using the same ultrasound detector used for photoacoustics. Such measurements are made at various angles around the sample in a CT approach. Images of the ultrasound propagation parameters, attenuation and speed of sound, can be reconstructed by inversion of a measurement model. We validate the method on specially designed phantoms and biological specimens. The obtained images are quantitative in terms of the shape, size, location, and acoustic properties of the examined heterogeneitie
Eigenspectra optoacoustic tomography achieves quantitative blood oxygenation imaging deep in tissues
Light propagating in tissue attains a spectrum that varies with location due
to wavelength-dependent fluence attenuation by tissue optical properties, an
effect that causes spectral corruption. Predictions of the spectral variations
of light fluence in tissue are challenging since the spatial distribution of
optical properties in tissue cannot be resolved in high resolution or with high
accuracy by current methods. Spectral corruption has fundamentally limited the
quantification accuracy of optical and optoacoustic methods and impeded the
long sought-after goal of imaging blood oxygen saturation (sO2) deep in
tissues; a critical but still unattainable target for the assessment of
oxygenation in physiological processes and disease. We discover a new principle
underlying light fluence in tissues, which describes the wavelength dependence
of light fluence as an affine function of a few reference base spectra,
independently of the specific distribution of tissue optical properties. This
finding enables the introduction of a previously undocumented concept termed
eigenspectra Multispectral Optoacoustic Tomography (eMSOT) that can effectively
account for wavelength dependent light attenuation without explicit knowledge
of the tissue optical properties. We validate eMSOT in more than 2000
simulations and with phantom and animal measurements. We find that eMSOT can
quantitatively image tissue sO2 reaching in many occasions a better than
10-fold improved accuracy over conventional spectral optoacoustic methods.
Then, we show that eMSOT can spatially resolve sO2 in muscle and tumor;
revealing so far unattainable tissue physiology patterns. Last, we related
eMSOT readings to cancer hypoxia and found congruence between eMSOT tumor sO2
images and tissue perfusion and hypoxia maps obtained by correlative
histological analysis
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