We report the assessment of cerebral blood flow (CBF) changes with a wide-field laser Doppler imager based on a CCD camera detection scheme, in vivo, in mice. The setup enables the acquisition of data in minimally invasive conditions. In contrast with conventional laser Doppler velocimeters and imagers, the Doppler signature of moving scatterers is measured in the frequency domain, by detuning a heterodyne optical detection. The quadratic mean of the measured frequency shift is used as an indicator of CBF. We observe a significant variability of this indicator in an experiment designed to induce blood flow changes.
KEYWORDS: Acousto-optics, In vivo imaging, Liquids, Ultrasonography, Speckle, Modulation, Spatial resolution, Acoustics, Signal to noise ratio, Beam propagation method
The aim of this paper is to show that we can perform acousto-optical
signal acquisition of one datapoint (or voxel of a 3D image)
in a very short time (2 - 4 ms), in order to overcome the speckle
decorrelation effect. To demonstrate this, we have performed
experiments in in dynamic scattering media such as liquids.
We will show that we can work with pulsed wave ultrasound, to reduce the sound irradiation duration in order to be compatible with safety
limits. These are significant steps towards in-vivo experiments.
Ultrasound focusing through complex media can be achieved using time-reversal techniques. These techniques make use of back-propagating ultrasonic waves generated by localized sources. Such sources generally consist of high acoustic contrasts echoing ultrasonic waves generated by an incident ultrasonic field, or directly by point-like transducers inserted at the desired focusing location. In this work, we experimentally investigate time-reversal of acoustic waves generated by photo-acoustic emission. A frequency-doubled Q-switched Nd:YAG laser was used to illuminate phantom with 5-ns laser pulses. A 128-element ultrasonic transducer array, with a center frequency of 1.5 MHz, was used to detect acoustic waves generated by optically absorbing targets suspended in water. A dedicated 32-channel electronics was used to time-reverse and re-emit the detected ultrasonic field. Gel spheres dyed with India ink (diameter approximately 1-2 mm)illuminated by the laser beam were used to generate the photo-acoustic waves. Time-reversal of the detected field was performed to focus ultrasound in the presence of highly defocusing media in front of the transducer array. We demonstrate how this allows correcting for the aberration in order to provide good quality images in the isoplanetic region surrounding the photo-acoustic source.
We present a new detection scheme for acousto-optic tomography, based on pseudo-random modulation of the utlrasound and illumination combined with hterodyne parallel speckle detection. This setup allows to perform tomographies inside several centimeter-thick scattering sample. Test experiments confirm the suitability of this method to perform tomographies inside various types of optically scattering media.
We present a new detection scheme for acousto-optic tomography,
based on pulsed-wave ultrasound and illumination combined with
heterodyne parallel speckle detection. This setup allows to
perform tomographies inside several centimeter-thick scattering
samples. Test experiments confirm the suitability of this method
to perform tomographies inside various types of optically
scattering media, including liquids.
A new concept of acousto-optical imaging is emerging based on an interferometric setup containing a photorefractive crystal as the
recombination plate, and a single detector. This wavefront-adaptive holography technique is promising since the measurements are made in real-time with a high flux collection and at a high rate, faster than the speckle decorrelation time. We present here a detailed model that describes correctly the measured signal, whether in a temporal phase or amplitude modulation of the ultrasound.
The Acousto-optical imaging technique in scattering media can be explained by using the concept of a virtual light source moving inside the medium. Its emission depends on the local optical properties of the insonified area, making it an interesting candidate for in-depth tissue (several centimeters)probing. Evidence of this statement in the restrictive case of a chirp-reduced-size virtual source is shown using a technique that records a film during one chirp modulation of the imaging system. A Fast Fourier Transform treatment correlates the Fourier spectrum of time-sampled transmitted light to the position of the source along the ultrasonic beam, revealing its millimetric size.
Although tumors can show important contrast in their optical
properties at an early stage of development, they are difficult to
image optically due the diffusive nature of biological tissues. Such tumors can also be detected by "classical" ultrasound (US) imaging, but the acoustic constrast is often weak at early stages. Acousto-optical (AO) imaging combines light and ultrasound : light carries the desired information and ultrasound provides the spatial resolution. Based on a previous work made by the group of L.V. Wang, we present AO images obtained with chirped US. This modulation of the US frequency allows to encode a spatial region of the medium in the frequency spectrum of the AO signal. We can then obtain
the optical contrast along the US path with improved resolution. The
technique was apply to the imaging of buried objects in phantoms and
to the vizualization of the "virtual source".
We use various probes to measure local temperature induced by internal or external heating of active or passive devices: Mirage detection can reveal heating of few ppb of the input power, whereas photothermal microscopy provides sub-micron spatial resolution. Temperature distribution is measured through periodic deflection or reflectivity mapping at frequencies high enough to confine heating near the source. Scanning InGaAsP/InP lasers facets, shows the weak influence of nonradiative recombination, in agreement with the high output power of these lasers before degradation. On strained-layer InGaAs quantum well lasers we obtained a drastic temperature rise, that we explain through simple model based on line heating for the laser cavity and point heating located at the facet. On a damaged laser the result, demonstrate clearly the existence of heating zones far from the facet along the laser cavity.
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