Simultaneous multi-head calibration for pinhole SPECT

Pinhole collimation is used as a magnifying geometry to overcome the resolution constraints of the gamma cameras used in SPECT. It is very sensitive to the mechanical position of the aperture relative to the axis of rotation. It is also sensitive to the axial position of the aperture. When one camera head is used, there is no intrinsic reference point for determining the axial mechanical shift. However, when multiple heads are used, the axial origin can he defined as the center of the first aperture. The other apertures can be measured relative to the first. A procedure has been developed for simultaneously calibrating multiple heads by using a single point source in the field of view of all heads. The gantry is rotated through 360 degrees. The centroids are fit using a maximum-likelihood method to an equation that describes the axial and transaxial geometry. The three coordinates of the point source are determined along with the mechanical and electronic shifts in the axial and transaxial directions; the axial mechanical shift of the first aperture is set to zero by definition. Another advantage of multi-head calibration is that a further constraint on the position of the point source is made. In this study, we evaluated the statistical accuracy of simultaneous calibration versus individual calibration. Simultaneous fitting with a single point source gives reduced statistical uncertainty, especially when the point source is offset axially from the apertures. We also apply the calibration information to a sub-micro disk phantom with 1.5 mm disks