Measurement and Imaging Capability of a Novel Medical Imaging Method, Ultrasound Optical Tomography, Based on Ultrasound and Laser Light in Human Tissue

Optical tissue imaging can provide biomolecular contrast that is not readily reachable with other imaging modalities. The investigators focus on developing a technique for instantaneous measurements of blood oxygenation which have significant influence in many areas like tumour detection, myocardial infarction, or stroke. However, the technique can in addition to tissue oxygenation also in general measure the optical absorption and scattering properties of tissue, and this is what the investigators will develop and further capitalize on in this project.

In general, the spatial resolution of tissue diagnostics using optical techniques is limited to a few cm due to the strong scattering properties of tissue. Using quantum designed filter structures with large acceptance angle that are many orders of magnitude narrower than any other large acceptance angle filters, this technique can image deeper into tissue with significantly better contrast-to-noise than other optical techniques.

By analyzing the frequency shifted light only, a spatial resolution equal to the ultrasound focus can be obtained. This technique is called Ultrasound Optical Tomography (UOT). A critical factor in UOT is the ability to discriminate between the light frequency-shifted by the ultrasound and the much stronger non-frequency-shifted light.

The absorption of ultrasound in tissue increases with increasing ultrasound frequency, and therefore the ultrasound frequency preferably should be just a few MHz. The small frequency shift is why previous attempts to develop UOT techniques have had limited success: conventional filtering techniques are either not narrow enough to suppress only the carrier or has a very small acceptance angle - a serious drawback as light exiting tissue propagates in all directions.

The Quantum Information Group at the Division of Atomic Physics, Department of Physics at Lund University, has world leading experience in creating narrowband (1 MHz) filters with high (~50 dB) suppression as well as a large acceptance angle. These so-called Slow Light Filters are created in inorganic crystals doped by rare-earth ions, where laser light is used to semi-permanently transfer ions to non-absorbing states. The filters are created by "burning" a spectral hole in the absorption profile of the crystal at the frequency fS so that it transmits this frequency but absorbs the unwanted background light at the frequency fL. An added and important effect of these filters is that light propagating at the transmitted frequency, fS, will be slowed down by several orders of magnitude. In this way, any remaining background from the carrier frequency after the filter can be suppressed using time gating.