Resumo

RATIONALE: There is no doubt that small animals are fundamental in pre-clinical research. On the other hand, non-invasive imaging techniques are a standard in the clinical environment, for anatomic (e.g., ultrasound, magnetic resonance, computed tomography) or functional (e.g., SPECT and PET) studies. However, considering the size of small animal organs relative to human ones, imaging of those organs is a challenge. When done, non invasive, in vivo imaging allows assessing the effect of the experimental protocol in different time points along the experiment on the same animal. In that way, the animal can be used as its own control, reducing time and costs and helping to solve ethical issues. OBJECTIVE: Given the higher availability of clinical SPECT devices in Brazilian hospitals compared to PET ones, we developed a low cost, multipurpose SPECT system intended to obtain high quality tomographic images of small animal organs. The implemented device must be machine independent, being possible to adapt to different clinical gamma cameras. MATERIALS AND METHODS: The system consists of a hardware kit to upgrade the available gamma camera, based on a single or multiple pinhole collimator and shielding, in combination with a computer controlled, step motor-based stage for positioning and rotation of the target. Additionally, a software tool based on the Maximum Likelihood algorithm was developed, which appropriately produces the three-dimensional model of the target’s radiopharmaceutical distribution, from the acquired projections. Different physical phantoms (microDefrise, microHotRod and rat brain phantoms) and small animal organs (kidneys, heart, thyroid and skull) of Wistar rats (200 to 500 grams) were imaged. In all cases, images were obtained using 99mTc-labeled radiopharmaceuticals (DMSA, MIBI, Pertecnetate and DTPA; 140 keV +/- 10% energy window). RESULTS: The developed kit was tested on three different clinical gamma cameras: DST Sopha, Siemens Orbiter and Elscint. With the current configuration, it is possible to reach a tomographic spatial resolution of 1.5 mm (FWHM) with 36 projections (30 seconds/projection) and radiopharmaceutical doses equivalent or smaller than those used in clinical trials, when imaging rat organs. In the rat’s thyroid image protocol, it was possible to identify the individual lobes (1 mm in diameter, 3 mm between lobes), showing that very small nuclei can be clearly identified. By using multiple pinhole collimators (3 to 5 pinholes), time per projection (and with it the total imaging time) can be reduced by a factor 3, in this way reducing the stress on the animal under analysis. CONCLUSION: The developed system allows for an easy and inexpensive upgrade of an already available gamma camera, to obtain functional images of small animal organs and with application in neuroscience studies as diverse as e.g., experimental models of Parkinson’s and Alzheimer’s diseases, epilepsy, neuronal ischemia and autonomic innervation. Acknowledges: J. Mejia is supported by FAPESP grants 07/50339-3 and 07/58052-5. A.A. de Castro is CAPES fellow. J.P. Leite is supported by FAPESP CInAPCe (05/56447-7). The authors wish to thank the technical staff of the Nuclear Medicine Section, HC-USPRP and HB-FAMERP, for invaluable help in preparing and manipulating the targets during the tests.