The single photon emission microscope (SPEM) is an instrument developed to obtain
high spatial resolution single photon emission computed tomography (SPECT) images of
small structures inside the mouse brain. SPEM consists of two independent imaging
devices, which combine a multipinhole collimator, a high-resolution, thallium-doped
cesium iodide [CsI(Tl)] columnar scintillator, a demagnifying/intensifier tube, and
an electron-multiplying charge-coupling device (CCD). Collimators have 300- and
450-µm diameter pinholes on tungsten slabs, in hexagonal arrays of 19 and 7 holes.
Projection data are acquired in a photon-counting strategy, where CCD frames are
stored at 50 frames per second, with a radius of rotation of 35 mm and magnification
factor of one. The image reconstruction software tool is based on the maximum
likelihood algorithm. Our aim was to evaluate the spatial resolution and sensitivity
attainable with the seven-pinhole imaging device, together with the linearity for
quantification on the tomographic images, and to test the instrument in obtaining
tomographic images of different mouse organs. A spatial resolution better than 500 µm
and a sensitivity of 21.6 counts·s-1·MBq-1 were reached, as
well as a correlation coefficient between activity and intensity better than 0.99,
when imaging 99mTc sources. Images of the thyroid, heart, lungs, and bones
of mice were registered using 99mTc-labeled radiopharmaceuticals in times
appropriate for routine preclinical experimentation of <1 h per projection data
set. Detailed experimental protocols and images of the aforementioned organs are
shown. We plan to extend the instrument's field of view to fix larger animals and to
combine data from both detectors to reduce the acquisition time or applied
activity.