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Optical projection tomography is a form of tomography involving optical microscopy. The OPT technique is sometimes referred to as Optical Computed Tomography (optical-CT) and Optical Emission Computed Tomography (optical-ECT) in the literature, to address the fact that the technique bears similarity to X-ray computed tomography (CT) and single-photon emission computed tomography (SPECT).(Further information: Optical sectioning) Fluorescence optical projection tomography visualises the distribution of dyes in the specimen.