Measurement of collimator hole angulation and camera head tilt for slant and parallel hole collimators used in SPECT.

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.