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Imaging within Scattering Media using Two-Photon Excitation

Püls, Jeremias LU (2016) In Lund Reports on Combustion Physics, LRCP-197 FYSM30 20161
Department of Physics
Combustion Physics
Abstract
Imaging in scattering media is used in different fields such as medicine for the diagnosis of tissues or in combustion to study spray injection systems. The problem with imaging in scattering media is the blurring induced by multiply scattered photons. There exists techniques to reduce multiple scattering detection, based on
various filtering approaches.

Two-photon excitation fluorescence imaging, which is the approach proposed in this thesis, fundamentally varies from the other, existing concepts: Instead of filtering the light before detecting it, two-photon excitation fluorescence induces less multiple scattering outside the focal point of interest as scattered photons do not carry enough energy to induce fluorescence.
In this... (More)
Imaging in scattering media is used in different fields such as medicine for the diagnosis of tissues or in combustion to study spray injection systems. The problem with imaging in scattering media is the blurring induced by multiply scattered photons. There exists techniques to reduce multiple scattering detection, based on
various filtering approaches.

Two-photon excitation fluorescence imaging, which is the approach proposed in this thesis, fundamentally varies from the other, existing concepts: Instead of filtering the light before detecting it, two-photon excitation fluorescence induces less multiple scattering outside the focal point of interest as scattered photons do not carry enough energy to induce fluorescence.
In this master project a suitable fluorescence dye, Fluorescein, was chosen to compare one-photon and two-photon excitation fluorescence imaging in a cuvette. The dependence of the signal on the focusing length and dye concentration was tested. The maximum signal of one-photon excitation fluorescence was independent of the focusing lens and was located at entrance of the cuvette; whereas the maximum signal of two-photon excitation was at the focal point.
Finally, the intensity and the image contrast were measured with and without particles. A contrast improvement was observed with two-photon excitation fluorescence in comparison with one-photon when no scattering particles were added.
Nonetheless, the contrast of 1PE was higher than the contrast of 2PE fluorescence after the scattering particles were added into the cuvette.
Since the signal depends quadratically on the laser intensity, a laser with a higher pulse power is believed to improve the contrast significantly. As a conclusion, such a laser system is needed in order to further test two-photon excitation fluorescence
imaging in turbid media. (Less)
Popular Abstract
Imagine in scattering media is like taking a picture in a foggy environment. Yes it works, but it is impossible to image objects that are far away or to recognize detailed structures. The following describes a novel technique used to improve the visibility in foggy environments, named two-photon excitation.

Two-photon excitation fluorescence tries to improve imaging in scattering media, by using a fundamentally different approach than existing techniques. Instead of blocking light that induce the blurring effect, such as conventional techniques, it tries to induce less of it. Fluorescence, the absorption and reabsorption of light in a different colour, normally needs one photon, a light particle. Obviously, two-photon excitation... (More)
Imagine in scattering media is like taking a picture in a foggy environment. Yes it works, but it is impossible to image objects that are far away or to recognize detailed structures. The following describes a novel technique used to improve the visibility in foggy environments, named two-photon excitation.

Two-photon excitation fluorescence tries to improve imaging in scattering media, by using a fundamentally different approach than existing techniques. Instead of blocking light that induce the blurring effect, such as conventional techniques, it tries to induce less of it. Fluorescence, the absorption and reabsorption of light in a different colour, normally needs one photon, a light particle. Obviously, two-photon excitation fluorescence needs two such photons at the same time to induce fluorescence.

When light scatters in the foggy environment, it still carries enough energy to induce fluorescence, which is unwanted. This is different in the two-photon case, since it is unlikely that two photons scatters to the same place at the same time. The picture shows the different technique. Only at the place where lots of photons are, the focal point, a strong fluorescence signal is visible. For the one-photon case, the laser induces fluorescence everywhere along the laser beam until it dies off. In conclusion, the signal of two-photon excitation fluorescence is more controllable than for the one-photon excitation fluorescence. (Less)
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author
Püls, Jeremias LU
supervisor
organization
course
FYSM30 20161
year
type
H2 - Master's Degree (Two Years)
subject
keywords
two-photon, two-photon excitation, fluorescence, turbid media, scattering media, imaging
publication/series
Lund Reports on Combustion Physics, LRCP-197
report number
ISNR LUTFD2/TFC-197-SE
language
English
id
8885974
date added to LUP
2016-08-08 16:14:37
date last changed
2016-08-08 16:14:37
@misc{8885974,
  abstract     = {Imaging in scattering media is used in different fields such as medicine for the diagnosis of tissues or in combustion to study spray injection systems. The problem with imaging in scattering media is the blurring induced by multiply scattered photons. There exists techniques to reduce multiple scattering detection, based on
various filtering approaches.

Two-photon excitation fluorescence imaging, which is the approach proposed in this thesis, fundamentally varies from the other, existing concepts: Instead of filtering the light before detecting it, two-photon excitation fluorescence induces less multiple scattering outside the focal point of interest as scattered photons do not carry enough energy to induce fluorescence.
In this master project a suitable fluorescence dye, Fluorescein, was chosen to compare one-photon and two-photon excitation fluorescence imaging in a cuvette. The dependence of the signal on the focusing length and dye concentration was tested. The maximum signal of one-photon excitation fluorescence was independent of the focusing lens and was located at entrance of the cuvette; whereas the maximum signal of two-photon excitation was at the focal point.
Finally, the intensity and the image contrast were measured with and without particles. A contrast improvement was observed with two-photon excitation fluorescence in comparison with one-photon when no scattering particles were added.
Nonetheless, the contrast of 1PE was higher than the contrast of 2PE fluorescence after the scattering particles were added into the cuvette.
Since the signal depends quadratically on the laser intensity, a laser with a higher pulse power is believed to improve the contrast significantly. As a conclusion, such a laser system is needed in order to further test two-photon excitation fluorescence
imaging in turbid media.},
  author       = {Püls, Jeremias},
  keyword      = {two-photon,two-photon excitation,fluorescence,turbid media,scattering media,imaging},
  language     = {eng},
  note         = {Student Paper},
  series       = {Lund Reports on Combustion Physics, LRCP-197},
  title        = {Imaging within Scattering Media using Two-Photon Excitation},
  year         = {2016},
}