SIMULATION STUDY OF THE INFLUENCE OF COLL IMATOR DEFECTS ON THE UNIFORMITY OF SCINTIGRAPHIC IMAGES

The mechanical tolerances in building collimators far scintillation ca meras are studied. A simulation method has been used to quantify the e ffects of defects in hole inclination and hole diameter on the uniform ity of planar and tomographic images. The calculation takes into accou nt the geometry of the hexagonal hole collimator, the camera intrinsic resolution, the object size, the pixel size, the effect of low-pass f iltering, as well as the type, size and position of the defect. For in stance, a 0.03 mm diameter defect on several holes located in the cent ral region of a very high resolution collimator can result in a 12% un iformity artefact in tomographic imaging of an 18 cm diameter cylinder , using a 3.45 mm resolution camera, 4.5 mm pixel size, and Hamming fi ltering with a Nyquist frequency cut-off. A 0.17 degrees inclination d efect of a few holes can result in the same uniformity artefact. These results show that the building of a collimator has to be very precise .