| Abstract |
Optical Computed Tomography (OPT) is an imaging technique which has a working principle similar to Computed Tomography, slightly different. OPT operates with the use of visible light(LED,laser) instead of ionizing radiation (X-rays) used in Computed Tomography. Also, in OPT the sample is rotated in constrast with CT, where the X-ray source and the detector are rotating around the patient. For the computational part, the mathematical principles used for the 3D Tomographic Reconstruction are the same. The basic principle of optical computed tomography is the creation of the 3D volume of the object based on photos from different angles(projections). The only restriction with OPT is that the sample must be light transparent.
For the purpose of this project, 2 different geometries were used, meaning 2 different optical systems for the recording of projections and the illumination of the sample: 1) Transmission geometry,which is used for absorption mea¬surements and 2) Reflection geometry which is used for fluorescence measure-ments. For the Transmission case a white light LED bulb was used and pro¬jections were recorded with the use of a camera. In this setup, two polarizers are interpolated,the first one between the sample and the bulb and the second one between the camera and the sample. The case where the two polarizers are parallel corresponds to absorption measurements.When the second polarizer is perpendicular to the direction of the first camera detects signal from the sample,which corresponds to scattering measurements. For the reflection geometry, two lasers were used: An 514nm argon-on laser and a 635nm He-Ne laser. Due to the sample that will be measured,the beam is guided with mirrors to a beam splitter(50%-50%) in order to strike the two mirrors placed left and right from the camera lens and illuminate the sample homogeneously. The reflected and transmitted beams pass through lenses of negative focal length so they can expand and illuminate the whole specimen.
The samples that were used were gummy bears, a plastic box that it's dimensios were measured with the ImageJ software and the skull of the mouse.For the case of the skull,a base was constructed to load the sample to the OPT rotational base. The skull was embedded inside a cylindrical 1% agarose gel that was loaded to the base using 2 needles. In the reflection case, the skull was inside a water bath to avoid refraction effects from the gel-air interface. Both transmission and reflection geometry were used for absorption and fluorescence measurements, with the 3D reconstructed volumes providing a 3D mapping for them. In the end, the two laser wavelengths were used to observe fluorescence from the dyes that the skull was painted with. These dyes were Alizarin Red S and Alcian Blue. With the use of the data from projections and 3D volumes, it was observed that every dye emits in different areas of the skull:Alcian Blue emits in the cartilage and Alizarin Red S emits in the bone.
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Development of a multiparametric optical computed tomography system for the study of biological samples
