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Phantom tissue puts OCT to the test

BioPhotonics
Sep 2010
Marie Freebody, marie.freebody@photonics.com

PERTH, Australia – Researchers at the University of Western Australia have come up with a material that mimics human tissue for optical coherence tomography applications. The researchers believe that their material, known as a phantom, has great potential in the diagnostic imaging of cancer.

While phantoms are commonly used to benchmark the performance of existing imaging systems as well as to validate new imaging systems and novel imaging techniques, no phantom yet exists to effectively test the rigors of OCT.


An OCT image of a fibrin phantom with two layers, each with a different optical attenuation coefficient. Image dimensions are 1.7 x 5 mm. Images courtesy of the University of Western Australia.


“Currently used materials each suffer from drawbacks,” said Dr. Brendan Kennedy, who heads up the research. “For this reason, no phantom has become universally accepted in the community. Our aim was to provide researchers for the first time with a repeatable, time-efficient, long-lasting, homogeneously scattering, biocompatible and transparent phantom matrix.”

When designing a phantom for OCT experiments, one of the most important properties to get right is the way the material scatters light. A particular requirement is homogeneous optical scattering, which is crucial in studies of OCT speckle and for measuring the optical scattering coefficient.


Two fibrin phantoms located in the OCT system. The variation in turbidity between the phantoms is due to different concentrations of Intralipid.


Kennedy and colleagues opted for a fibrin-based phantom. Fibrin is a naturally occurring protein in humans that provides structural support for blood clots but, more importantly, it simulates the optical and viscoelastic properties of tissue and provides an easy-to-work-with scaffold in which to incorporate organic and/or inorganic optical scattering materials.

By introducing different organic or inorganic materials, the team can control the optical and viscoelastic properties of the phantom. In their work, described in an online edition of the Journal of Biomedical Optics published on May 10, 2010, the team claims that the fabrication time of the fibrin-based phantom is markedly shorter than for many common phantoms and that its lifetime is longer than that of other biocompatible phantoms.


A scanning electron microscope image of a fibrin phantom shows the fibrous structure of the material.


Since publication, the team has been exploring the full potential of the phantom and found it to be suitable for use in medical applications of OCT where soft tissue is imaged. For example, Kennedy and colleagues have developed a miniaturized OCT probe that is encased within a 23-gauge needle, allowing them to acquire OCT images deep within soft tissue.

But the potential doesn’t stop there. In the university’s Optical + Biomedical Engineering Laboratory, the team is developing a technique to image the viscoelastic properties of tissue using OCT. “This technique exploits the fact that pathological tissue is often stiffer than surrounding tissue and is known as optical coherence elastography. We believe it has great potential in the diagnostic imaging of cancer,” Kennedy concluded.


A member of the Optical + Biomedical Engineering Laboratory team places fibrin phantoms in the OCT system.


Since the viscoelasticity of fibrin can be changed by varying the concentration of certain components during the fabrication process, and optical scattering can be introduced using Intralipid, work is under way to characterize the effects and to help determine the full range of tissues that can be modeled with fibrin phantoms.


GLOSSARY
scattering
Change of the spatial distribution of a beam of radiation when it interacts with a surface or a heterogeneous medium, in which process there is no change of wavelength of the radiation.
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