Angulation errors in parallel-hole and fanbeam collimators: computer controlled quality control and acceptance testing procedure.

UNLABELLED

Angulation errors in collimators of 1 degrees 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 degrees. Regional variations in channel tilt could be detected with an accuracy better than 0.16 degrees. 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.