Collimator hole angulation was measured at 16 locations in three collimators used in SPECT. The parallel hole collimator measured -0.1 +/- -0.1 s.d. degrees in the X and -0.2 +/- 0.1 s.d. degrees in the Y direction. One 30 degree nominal slant hole collimator revealed a 0.1 +/- 0.3 s.d. degrees angulation in X and 26.5 +/- 1.1 s.d. degrees angulation in Y, which was unacceptable. The replacement collimator measured 0.0 +/- 0.1 s.d. degrees in X and 29.6 +/- 0.2 s.d. degrees in Y. Determination of collimator hole angulation is recommended as an acceptance test for SPECT systems. The net angle of tilt with respect to the orthogonal resulting from a particular collimator and camera head tilt was determined from summed projection images over 360 degrees of a point source placed off the axis of rotation. These measurements were sensitive within a 0.5 degree tilt angle. The method is suggested as a routine quality control procedure to optimize camera head tilt to a particular collimator. It may also reveal unexpected mechanical misalignments in the camera gantry.
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