Nonistropic point spread function as a result of collimator design and manufacturing defects.

A point spread function (PSF) which was of an unusual form was observed when using a low energy collimator and a 99mTc point source. A lack of radial symmetry and the detection of events over 20 cm away from the center of the source were noted. The major quantitative effect of this was a variation in the resolution of the system with direction. The consequences of this are likely to be of importance especially for single photon emission computed tomographic (SPECT) imaging. The effect was produced by a combination of faults in the design and manufacture of the low energy collimators. It demonstrates the importance of making quantitative measurements of collimator performance in the quality control assessment of gamma camera systems. For SPECT imaging the replacement of lead foil collimators with cast collimators should be considered.