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Angulation Errors in Parallel-Hole and Fanbeam Collimators: Computer Controlled Quality Control and Acceptance Testing Procedure

Department of Biomedical Engineering and Physics, University Hospital, Vienna
Ludwig Boltzmann Institute of Nuclear Medicine, Vienna, Austria

Correspondence: For correspondence or reprints contact: Helmar Bergmann, MD, General Hospital Vienna, Department of Biomedical Engineering and Physics, Waehringer Guertel 18-20, A-1090 Vienna, Austria.

ABSTRACT

Angulation errors in collimators of 1° or even less can seriously diminish the resolution of SPECT images. We have developed a computer-controlled quality control procedure that can be used for acceptance testing and regular routine checks. Methods: Using a marker point source and a computer-controlled x–y positioning table, we investigated 7 parallel-hole and 3 fanbeam collimators. The results are presented as collimator surface maps, which are easy to interpret visually. Results: The measurement accuracy for absolute angulation errors was better than 0.32°. Regional variations in channel tilt could be detected with an accuracy better than 0.16°. Six parallel-hole collimators were found acceptable for high-resolution SPECT imaging. For a parallel-hole collimator that had to be replaced because of nonoptimal image quality, our measurements clearly identified regions of directionally uniform angulation errors. Two fanbeam collimators showed slight concavities. Conclusion: Automation of the measurement and evaluation process make this procedure suitable for both acceptance tests and routine quality control checks. It can be applied to parallel-hole, fanbeam, converging, and diverging collimators, regardless of their individual geometry. No technical collimator specifications are needed. Our results reveal subtle mechanical deformations of collimators. They also show that for a detailed investigation, angulation error surface maps should be used to discover regional preferences in channel orientation.

Key Words: angulation • collimator • quality control • computer control