Investigation of phase conjugation for medical imaging

Abstract

With the development of extremely precise spectral filters based on spectral hole burning and slow light effects, the idea of their implementation in an Ultrasound Optical Tomography (UOT) medical imaging technique arose. Simulations have shown that the resulting Contrast-to-Noise (CNR) ratio is fairly good at depths up to 5 cm but decreases dramatically when probing deeper in tissue. One of the possible enhancements of this technique is to include a phase conjugator after the filter to reverse and amplify ultrasound-tagged photons and send them back to the area of interest. This project tries to investigate the possibilities of using rare-earth-ion-doped crystals as potential phase-conjugating elements in the UOT and is specifically aimed at studying the properties of an optical phase conjugation process in these media.
Experimental work was done on two crystals doped with Praseodymium and the phase-conjugated signal was studied as a function of different parameters with results discussed and presented in the respective section. In particular, efficient phase conjugation was observed in a 6-mm thick Pr:YSO crystal with reflectivity values reaching up to 124 \%. Further outlook and possible applications are given at the end of the thesis.

Popular Abstract

Despite being extremely useful in all areas of modern medicine, laser still faces a challenging obstacle when used as a diagnostics and imaging tool. Its radiation is heavily affected by tissue, i.e. light can not get deep enough in the medium without being weakened and distorted. The reason for this attenuation and distortion is two processes that happen when light propagates inside the body - absorption (the light energy is absorbed by molecules and atoms in the medium) and scattering (light randomly changes the direction of propagation due to interaction with atoms and molecules).

While little can be done to reduce the light absorption in tissue, in principle, there are certain ways that can remove the effect of scattering. One of those tricks is a process of Optical Phase Conjugation. It is an effect where one combines three laser beams in a medium such that they create a fourth beam that is the exact copy of one of the input beams but with reversed direction of propagation. This is sometimes viewed as a ``magic mirror'' that reflects the light in such a way that it travels backwards in time. Optical phase conjugation is a highly promising technique with a broad range of applications in modern physics.

This very useful phenomenon can, in theory, be implemented in a medical imaging technique called Ultrasound-mediated Optical Tomography (UOT). In this technique, laser light interacts with a sound wave that is sent into the area that the scientist wants to observe. Ultrasound is generally much more resistant to attenuating properties of tissue and propagates in media like skin and muscles more or less freely (there are some exceptions, though, e.g. - bones). At the same time light and sound can interact with each other, and when this happens, the former gets a wavelength shift (colour change). We can refer to this light as being ``tagged'' by the ultrasound. By detecting this particular colour-shifted signal, it is possible to get an idea of what is happening in the place where the light and sound interacted with each other. By moving the ultrasound pulse to different spots, people can ``map'' area that they want to observe and thus get a full image of the region of interest.

Although UOT shows a great deal of promise as a future imaging method, there is a significant amount of engineering obstacles that scientists have to overcome in order to push this technique forward into the medical industry. One of these barriers is the detection of this ultrasound-tagged light. Only a small fraction of incoming light particles is lucky enough to reach the ultrasound focus and get the colour shift, so the total signal from those will be much weaker compared to the intense background light that did not reach the ultrasound. This light not having interacted with the ultrasound should be filtered out. It can be done by using very precise filters which are transparent only in narrow `wavelength window' constructed for the tagged photons while being opaque to the unshifted background light. Still, even after filtering, it may be hard to get good contrast and resolution of the target area.

That is where optical phase conjugation comes into play: the general idea is to send back and amplify the tagged light. Phase conjugation removes the randomness of the scattering process out of the equation since the conjugated (reversed) wave is intimately connected to the original wave and will be scattered straight back to the ultrasound focus. The light will be collected on the side of the input and in theory is able to carry more power compared to the signal before the phase conjugation.

The next question is what material should people use for this special mirror? Extensive studies on the phase conjugation in different materials were conducted in recent years, and all of the studied materials have their own advantages and drawbacks, e.g some are effective but slow, while the others are fast but ineffective.
An obvious idea that comes to mind is to use the same material for phase conjugation as for the light filtering. Filters described above are based on inorganic crystals doped with rare-earth elements, so in principle, it would be convenient to use them for the phase conjugation as well. The primary goal of this master project is to investigate that possibility and study the properties of the phase conjugation in these structures.

Over the course of last year, experiments were done on two samples of different thickness. Studies on a 6-mm thick sample showed some promising results in terms of efficiency of this process but raised a lot of questions on the nature of the optical phase conjugation effect in these crystals. I have tried to give some answers to those questions in the report. This project is ultimately only a tiny part of a bigger collaboration of people working on the implementation of the UOT, and I hope that the findings of this project will give a push to further research on this issue and will help to achieve the end goal.

• author: Sytcevich, Ivan
• supervisor: Stefan Kröll; Qian Li
• organization: Department of Physics; Atomic Physics
• course: FYSM60 20171
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Phase conjugation, Ultrasound Optical Tomography, Rare-Earth-Doped Crystals.
• language: English
• id: 8908840
• date added to LUP: 2017-06-13 09:25:58
• date last changed: 2017-06-13 09:25:58