Vincent Poher

Thin wetting film lensless imaging

Lensless imaging has recently attracted a lot of attention as a compact, easy-to-use method to image or detect biological objects like cells, but failed at detecting micron size objects like bacteria that often do not scatter enough light. In order to detect single bacterium, we have developed a method based on a thin wetting film that produces a micro-lens effect. Compared with previously reported results, a large improvement in signal to noise ratio is obtained due to the presence of a micro-lens on top of each bacterium. In these conditions, standard CMOS sensors are able to detect single bacterium, e.g. E.coli, Bacillus subtilis and Bacillus thuringiensis, with a large signal to noise ratio. This paper presents our sensor optimization to enhance the SNR; improve the detection of sub-micron objects; and increase the imaging FOV, from 4.3 mm2 to 12 mm2 to 24 mm2, which allows the detection of bacteria contained in 0.5μl to 4μl to 10μl, respectively.

Laser line illumination scheme allowing the reduction of background signal and the correction of absorption heterogeneities effects for fluorescence reflectance imaging.

Abstract. Intraoperative fluorescence imaging in reflectance geometry is an attractive imaging modality as it allows to noninvasively monitor the fluorescence targeted tumors located below the tissue surface. Some drawbacks of this technique are the background fluorescence decreasing the contrast and absorption heterogeneities leading to misinterpretations concerning fluorescence concentrations. We propose a correction technique based on a laser line scanning illumination scheme. We scan the medium with the laser line and acquire, at each position of the line, both fluorescence and excitation images. We then use the finding that there is a relationship between the excitation intensity profile and the background fluorescence one to predict the amount of signal to subtract from the fluorescence images to get a better contrast. As the light absorption information is contained both in fluorescence and excitation images, this method also permits us to correct the effects of absorption heterogeneities. This technique has been validated on simulations and experimentally. Fluorescent inclusions are observed in several configurations at depths ranging from 1 mm to 1 cm. Results obtained with this technique are compared with those obtained with a classical wide-field detection scheme for contrast enhancement and with the fluorescence by an excitation ratio approach for absorption correction.

Background fluorescence reduction and absorption correction for fluorescence reflectance imaging

Intraoperative fluorescence imaging in reflectance geometry (FRI) is an attractive imaging modality as it allows to noninvasively monitor the fluorescence targeted tumors located below the tissue surface. Some drawbacks of this technique are the background fluorescence decreasing the contrast and absorption heterogeneities leading to misinterpretations concerning fluorescence concentrations. We presented a FRI technique relying on a laser line scanning instead of a uniform illumination. Here, we propose a correction technique based on this illumination scheme. We scan the medium with the laser line and acquire at each position of the line both fluorescence and excitation images. We then use the finding that there is a relationship between the excitation intensity profile and the background fluorescence one. This allows us to predict the amount of signal to subtract to the fluorescence images to get a better contrast. As the light absorption information is contained both in fluorescence and excitation images, this method also permits us to correct the effects of absorption heterogeneities, leading to a better accuracy for the detection. This technique has been validated on simulations (with a Monte-Carlo code and with the diffusion approxi- mation using NIRFAST) and experimentally with tissue-like liquid phantoms with different levels of background fluorescence. Fluorescent inclusions are observed in several configurations at depths ranging from 1 mm to 1 cm. Results obtained with this technique are compared to those obtained with a more classical wide-field detection scheme for the contrast enhancement and to the fluorescence to excitation ratio approach for the absorption correction.

Laser line scanning for fluorescence reflectance imaging: a phantom study and in vivo validation of the enhancement of contrast and resolution

Abstract. Intraoperative fluorescence imaging in reflectance geometry is an attractive imaging modality to noninvasively monitor fluorescence-targeted tumors. In some situations, this kind of imaging suffers from poor resolution due to the diffusive nature of photons in tissue. The objective of the proposed technique is to tackle this limitation. It relies on the scanning of the medium with a laser line illumination and the acquisition of images at each position of excitation. The detection scheme proposed takes advantage of the stack of images acquired to enhance the resolution and the contrast of the final image. The experimental protocol is described to fully understand why we overpass the classical limits and validate the scheme on tissue-like phantoms and in vivo with a preliminary testing. The results are compared with those obtained with a classical wide-field illumination.

Laser line scanning illumination scheme for the enhancement of contrast and resolution for fluorescence reflectance imaging

Intraoperative fluorescence imaging in reflectance geometry is an attractive imaging modality as it allows to noninvasively monitor fluorescence targeted tumors located below the tissue surface. The drawbacks of this technique are the poor resolution in the axial and lateral directions due to multiple light scattering and background fluorescence decreasing the contrast. We propose a novel fluorescence imaging method based on laser line illumination in reflectance geometry. We scan the medium with the laser line and acquire images at each position of the line. We then detect only single stripes of each image located on the excitation line or farther from it. We can also subtract the surrounding signal to the detected stripe, the optimal detection scheme depending on the depth of the object of interest. This allows us to reduce the contribution of parasite signals such as background fluorescence or excitation leaks and also enhances the resolution. These operations on the images can either be digitally done in post-processing or can directly be hardware implemented, allowing our method to be integrated in a handheld device for real-time use. This technique has been validated with tissue-like liquid phantoms with different levels of background fluorescence. Fluorescent inclusions are observed in several configurations at depths ranging from 1 mm to 1 cm. Our results are compared to those obtained with a more classical wide-field detection scheme. Finally, we propose a setup to optically implement the masking detection that will dramatically fasten the detection scheme and optimize the fluorescence light throughput of the system.

Diffuse Spectroscopy for Tissue Characterization: Application to Skin Tests Reading

In this paper, we present a low-cost optical instrument, usable in a clinical environment, to enable early skin tests reading before the onset of visual signs. Results on an ongoing clinical study are presented.