Micro single photon emission computed tomography (microSPECT) is a powerful tool for quantifying the distribution of a radioactive compound in living animals. When a SPECT radioisotope decays, one or more gamma rays (i.e., photons), having particular energies, are released in a certain direction. Since high-energy photons (gamma-rays) cannot be focused using conventional lenses, "collimators" are used to restrict the angle of the emitted photons that actually reach the detector.
     Commonly used SPECT isotopes are shown in the downloadable Table 1. Some SPECT isotopes release only gamma rays, but others also release beta rays. This needs to be carefully considered when planning experiments since beta rays can damage normal tissue.
     Click here for a downloadable tables of Pure Gamma Emitters and Mixed Gamma/Beta Emitters (arranged by increasing half-life) for the Mosaic HP Minverve microSPECT/CT.
     The choice of collimator has a profound effect on the resolution and sensitivity of SPECT. The first type, a pinhole collimator is essentially a hole in a lead plate. Like a pinhole camera, only photons having an angle that pass through the hole will reach the detector, hence sensitivity is low. Resolution, however, can be quite high if a large detector is used and the image is magnified onto it by placing the pinhole close to the animal and the detector far away. The second type, a multi-pinhole collimator is often employed to improve performance. The third type of SPECT collimator is called a parallel-hole collimator and can be thought of as a plate composed of lead straws. Only photons co-linear with the holes will pass through them and reach the detector. Although parallel-hole collimators have significant advantages for human and large animal scanning, multi-pinhole collimators are the most commonly used ones for rodent studies.
     To create a 3-D image, the SPECT collimator/collimator sandwich is rotated in a circular fashion around the animal, and data are acquired at each angle of rotation. A typical SPECT acquisition will take 20-60 minutes. After acquisition, one of several possible reconstruction algorithms is used to create the final 3-D image. When combined with a microCT scanner, investigators are able to quantify the exact amount of radioactivity in any organ or tissue of interest in a living animal. The key variables to a successful experiment are the chemical form of the single photon-emitting isotope, the energy(ies) of the single photon-emitting isotope, the half-life of the radioisotope relative to the process under study (typically within 1-2 half-lives), the amount of radioactivity injected, the specific activity of the injected isotope, the spatial resolution and field-of-view of the chosen SPECT collimator, and the sensitivity of the detection system of the microSPECT scanner.

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