Conceptual design of high resolution and quantitative SPECT system for imaging a selected small ROI of human brain

We designed a concept of high resolution and quantitative SPECT for imaging a selected small region-of-interest (ROI) of human brain. This system is aimed at achieving high resolution less than 1 mm and being applied for imaging neurons and evaluating drug delivery system. Pinhole or cone-beam collimators are useful for high-resolution imaging of small ROI. However, when the ROI is smaller than the object, the projection data are truncated by radioisotope outside ROI. In the reconstructed image, the truncation causes the artifact and the overestimation of voxel value, which deceases quantitative accuracy of physiological functions. We are introducing the new truncation compensated 3D-OSEM (TC-3DOSEM) reconstruction method. The truncated data can be successfully reconstructed within ROI by fulfilling the condition that ROI contains a priori knowledge. In addition to small field-of-view (FOV) detector, we are introducing the parallel-hole collimator attached large FOV detector covering the entire brain, to acquire the non-truncated data and provide the priori knowledge in small ROI, even if the resolution of the detector is low. For imaging with high resolution, we are using LaBr₃(Ce) scintillator with optically coupled to position-sensitive photomultiplier tube (H8500, Hamamatsu, Japan) as the detector. And also, for proof of our concept, we performed preliminary experiment using pinhole SPECT and brain phantom. The reconstruction ROI contained the region outside the brain, that is, zero count as the priori knowledge. The truncated data were reconstructed by TC-3DOSEM. The reconstructed image without artifact and overestimation was obtained with high resolution. This preliminary experiment suggested feasibility of high resolution and quantitative SPECT for imaging a selected small ROI of human brain.