Tomography means to take a sliced view. Tomos means slice in greek. The idea behind tomography is to take boundary measurements (usually called projections) by sending a probing input (can be X-ray, light, electrical impulses, ultrasound ...) also from the boundary. Using these measurements and appropriate modeling of the related physical phenomena, one can get to know about the interior (this forms the Reconstructed Image). Hence it is non-invasive way of imaging interior distribution of the parameter of interest. The term CT (computed tomography) is also used in place of tomography, it is quite appropriate as this kind of imaging involves some mathematical processing (termed as Reconstruction) to generate the Image.
Xray CT was the first technique to be developed and commercialized, here the contrast is due to the attenuation of the Xrays by the sample.
In Optical Tomography, Electromagnetic radiation in the visible-NIR range is used as the probe. The light in this range is non-ionizing and hence preferred to X-rays. The challenge to get good reconstruction is due to phenomena of multiple scattering of light inside the tissue.
There are many variants of optical tomographic imaging depending on which regime of scattering and also on if it is standalone or coupled physics (hybrid) imaging kind. Depending on the amount of scattering there are three regimes that a medium can be characterized into a). no scattering b) mesoscopic/weak scattering and c) high scattering(diffusive). In all these regimes there can be cases of differences in refractive index also causing in-homogeneous behavior.
In no scattering medium, the refraction in-homogeneity problem can be addressed using iterative techniques, such as Algebraic reconstruction technique.
Multimodal imaging refers to the term to describe systems developed to be able to image using two different physical principles but can be used in succession to image the sample and obtain more details such as better functional information, better resolution ....
Hybrid imaging modalities include combination of more than one imaging principle through coupled physics phenomena.