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Fig 2. Pictorial view of photon emission and determination of LOR (drawings are not to scale). A, Parallel-hole collimator SPECT. At each point, photons are emitted isotropically. Only those emitted along lines parallel to the channels of the collimator reach the detector. Thus, the direction of the channels of the collimator identifies the LOR. B, In PET, LORs are identified by connecting the location of 2 photons arriving in temporal coincidence at opposing detectors. C, In CT, LORs are determined by the location of detection and the known location of the x-ray beam spot. D, In MR imaging, all spins subject to the same magnetic field have the same precession frequency: Their signal intensity is summed along the lines seeing the same field, just as photons originating along the same line in A, B, or C contribute to the same LOR. Just as in other 3D radiologic techniques, the LOR, or the total signal intensity coming from a line traversing the patient, is the basic piece of information acquired in SPECT from which 3D reconstruction starts.
