Optical imaging methods use visible or near-infrared light, usually generated by a laser. The main advantage is that visible light is easily and cheaply generated (as opposed to e.g. X-rays), and coherent light is available. The main challenge is the scattering property of tissue, which often limits the imaging method to structures close to the surface. Three primary optical imaging methods are known:
We focus on optical transillumination tomography to reconstruct the cross-section of thin or weakly scattering samples. A spatial filter serves to eliminate scattered photons. A sketch of the first device is shown below:
Laser light is focused on the sample and collected by a photodiode after passing through a spatial filter. The sample can be translated and rotated to obtain projections at different angles. Such a collection of projections is often called sinogram, and it presents the total light absorption (and scattering) along a straight path as a function of the translational position and the angle.
A simple mathematical operation, called filtered backprojection, can now be performed on the sinogram to obtain the reconstructed cross-section of the object.
Shown below is the optical tomography scanner we built. In the center, illuminated by red laser light, is the sample cuvette with the actual sample. On top of the cuvette, a step motor can be found that is responsible for rotating the sample. The entire sample assembly is monted on top of a translational stage. Half covered behind the sample, the laser can be seen. The spatial filter is in the front.