Aner Lev

In vivo demonstration of the ultrasound-modulated light technique

We present, to our knowledge for the first time, results of ultrasound-modulated light signals on living tissues. In particular, we analyze, both theoretically and experimentally, the effect of speckle fluctuations on the signal. We find that two different kinds of noise compete--shot noise and speckle noise--and are present at different levels in static phantoms and ex vivo tissue samples on the one hand and in dynamic phantoms and living tissues on the other hand.

Detection of inhomogeneities with ultrasound tagging of light

Ultrasound modulated light for optical tomography is very useful, since it can provide three-dimensional data with minimal mathematical processing. Although several experimental studies have shown the potential of this method, the link between the ultrasound location and the modulated signal intensity at the detector is not yet fully understood. We derive an analytical formula relating the position of the ultrasound transducer and the optical signal at the detector. We also derive an expression for the signal-to-shot-noise ratio as a function of the transducer position. We show that in certain conditions this ratio is only slowly decreasing as a function of the light penetration depth, which makes this technique attractive for optical tomography.

Acousto-optical detection of hidden objects via speckle based imaging

Optical detection of objects hidden behind opaque screening layers is a challenging problem. We demonstrate an optically detected echographic-like method that combines collimated acoustic and laser beams. The acoustic waves cross the screening layers, and their back-reflection from the hidden objects is detected through the analysis of a dynamic laser speckle pattern created at the outer surface of the screening layer. Real-time remote detection of moving targets 15 meters away, with a few mm resolution is demonstrated using a very sensitive camera detection scheme.

Quantitative ultrasound modulated light tomography

Ultrasound modulated light tomography (UMLiT) is a very attractive method for optical imaging of turbid media. Different schemes have been developed for effectively discriminating between the non-modulated and modulated photons and locate absorbing inhomogeneities. L. Wang has shown the possibility of using chirped ultrasound as well as Radon transformed-based tomography for three-dimensional imaging, C. Boccara has demonstrated the use of ultrafast cameras in order to increase the signal to noise ratio. Our group has shown the use of pulsed ultrasound for three-dimensional localization. However, although there have been a strong effort towards imaging objects in phantoms, very few experiments have shown quantitative results probing the limits of the technique in terms of resolution and sensitivity. We will present experimental results obtained in a reflection configuration. In a first series of experiments we present results obtained on living mice and rabbits, showing two and three-dimensional representations. In a second series of experiments, we have prepared different Agar-based phantoms in which small absorption inhomogeneities have been introduced. The background effective attenuation was 0.05 cm-1. Several 5 mm diameter inclusions have been introduced in the phantom in different geometries with an effective absorption varying from twice to ten times the background. These inclusions could be detected up to a depth of 4 cm.

Ultrasonic probing of the banana photon distribution in turbid media

Probing photon density in diffusive media is very important in order to model and understand their propagation. It is possible to detect photons outside the medium, but their non-invasive detection inside it is still an unsolved problem. An elegant, semi-invasive approach to perform this task is to scan a small absorbing sphere inside the turbid medium and measure the light outside the sample when the sphere is present and when it is not. However this method requires the medium to be liquid and such a procedure cannot be performed in the case of biological tissues. Ultrasound tagging of light has been introduced initially for transillumination imaging in turbid media, and then extended to the case of reflection imaging. Here we present results showing that it is possible to map the photon density inside solid turbid media by locally tagging photons using an ultrasonic field. We experimentally retrieve the well-known banana-shaped photons distribution when the source and the detectors are in a back-scattering configuration, using a gel-based homogeneous phantom. We also present experiments where hemoglobin has been introduced inside the gel. By fitting the experimental results with the theoretical formula, we are able to quantitatively retrieve the amount of hemoglobin introduced inside the gel, not only from data obtained by scanning the ultrasound waist inside the phantom, the in put and output fibers staying fixed.

Remote imaging of concealed objects

Optical detection of objects hidden behind opaque screening layers is a challenging problem. We demonstrate an optically detected echographic-like method that combines collimated acoustic and laser beams. The acoustic waves cross the screening layers and their back-reflection from the hidden objects is detected through the analysis of a dynamic laser speckle pattern created at the outer surface of the screening layer. Real-time remote detection of moving targets 15 meters away, with a few mm resolutions is demonstrated using a very sensitive camera detection scheme.

Pulsed-ultrasound tagging of light in living tissues

Ultrasound can be used in order to locally modulate, or tag, light in a turbid medium. This tagging process is made possible due to the extreme sensitivity of laser speckle distribution to minute changes within the medium. This hybrid technique presents several advantages compared to all-optical tomographic techniques, in that the image resolution is fixed by the ultrasound focus diameter. To our best knowledge, only in vitro experiments have been performed, either on tissue-like phantoms or meat. However a strong difference exists between these sample and living tissues. In living tissues, different kind of liquids flow through the capillaries, strongly reducing the sspeckle autocorrelation time. We have performed experiments on both mice and humans, showing that the autocorrelation time is much shorter than what was previously thought. We show however that it is possible to obtain signal with acceptable signal to noise ratio down to a few cm depth. We will also discuss the origin and characteristics of the speckle noise.