High resolution 125I pinhole SPECT imaging of the mouse thyroid with the MediSPECT small animal CdTe scanner

We present first in vivo tomographic Na¹²⁵I imaging of the mouse thyroid carried out with the new MediSPECT small animal SPECT scanner, based on a fine pitch CdTe pixel detector (14×14 mm², 256×256 square pixel of 55 μm side) and equipped with a high resolution collimators set. The MediSPECT scanner is based on a gantry, hosting the collimation and detection units, rotating around a horizontal axis along which is the small animal housing. The mouse was injected with a Na¹²⁵I solution with a total activity of 31.8 MBq. The planar projections for SPECT reconstruction were acquired with a 300 μm tungsten pinhole (magnification 1.47 and a Field of View — FoV - of 9.6×9.6 mm²). The projections have been captured in a step-and shoot fashion by rotating the detector around the animal holder. To obtain the SPECT image all the projections are elaborated with a C++ software based on ordered subsets-expectation maximization (OS-EM) reconstruction algorithm, developed by our group. Test images for assessing the MediSPECT scanner spatial resolution have been also acquired using phantoms and are also presented in this wok. The measured spatial resolution with a 300 μm pinhole is, in planar imaging, about 500 μm and, in tomographic imaging, better than 900 μm. The modular structure of MediSPECT allows for multimodal imaging: at present, a Fluorescence Reflectance Imaging head has been installed, for planar optical imaging in addition to SPECT imaging.