Figure 1.
(A) The collimation process is illustrated. Gamma photons released by the radiopharmaceutical travel away from the site of emission in all directions. Those that impact the collimator at an angle cannot pass through. Gamma photons that reach the collimator in alignment with the channels of the collimator can pass through and reach the detector (a sodium iodide crystal in this illustration). The limit to resolution is the size of the photomultipliers, distance from the structure of interest, and the resolution of the collimator. (B) Detection of gamma photons by sodium iodide crystal detectors is a multi-step process. The gamma photon (red star −1) strikes the sodium iodide crystal (orange starburst) and a burst of visible light photons (yellow stars) is released (2). The light photons radiate in all directions (3). Some of those visible light photons will impact on photodetectors (purple cylinders −4). Photodetectors closer to the site of gamma photon impact will receive more visible light photons, while those further away from the site will receive less (4). Those photodetectors that receive more visible light photons will release a greater electrical current, while the detectors further away will release a small electrical current (5). This is the basis of the Anger circuitry for geometric localization. (C) Cadmium-zinc-telluride (CZT) detectors are solid-state and directly convert gamma photons into electrons at room temperature. When a gamma photon (1) strikes a CZT detector (2), pairs of electrons and holes are generated (red and blue balls, respectively). These create an electrical potential across the detector which is proportional to the energy of the impinging radiation. The resolution is limited by the size of the CZT detector—currently about 2.4 mm on a side.