High Resolution 125I Pinhole SPECT Imaging of the Mouse Thyroid With the MediSPECT Small Animal CdTe Scanner

The first in vivo tomographic I-125 imaging of the mouse thyroid carried out with the new MediSPECT small animal SPECT scanner is presented. The scanner is based on a fine pitch CdTe semiconductor pixel detector (14 x 14 mm(2), 256 x 256 square pixel with a 55 mu m side) and equipped with a set of high resolution collimators. The collimation and detection units of the scanner are mounted on a gantry, rotating around a horizontal axis, along which is placed the small animal housing. In an in vivo test, the mouse was injected with a (NaI)-I-125 solution having a total activity of 31.8 MBq. The planar projections for SPECT reconstruction were acquired with a 300 mu m pinhole (magnification 1.47 and field of view of 9.6 x 9.6 mm(2)). The projections were captured in a step-and-shoot fashion and were processed with an Ordered Subsets-Expectation Maximization reconstruction algorithm in order to obtain the SPECT images. Several I-125 imaging tests have been made by using phantoms to assess the detector spatial resolution. The measured spatial resolution with a 300 mu m pinhole is about 0.5 mm in planar imaging and better than 1 mm in tomographic imaging.