Transmission measurements determine line integrals of an absorption coefficient after taking the logarithm of incident intensity divided by transmitted intensity. Under the ideal straight-ray model, these measurements are a Radon transform; filtered backprojection reconstructs a planar slice. Angular coverage, calibration and noise control matter because differentiating measured projections amplifies high frequencies. In contrast, single-photon emission computed tomography reconstructs an emitting source through a possibly attenuating medium.
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