Matched collimators for pixellated gamma camera

Recent years have witnessed the development of small field of view (FOV) nuclear medicine cameras for applications requiring either high spatial resolution and/or compact camera size. Dedicated gamma detectors with high intrinsic spatial resolutions have been built; however, collimation remains perhaps the key determinant of performance for single gamma imaging. In current pixellated gamma cameras, parallel-hole collimators are used with no attempt to match the locations of the collimator holes with the locations of the detector elements. This results in the collimator septa crossing the active portions of the detector crystals. If parallel-hole collimators whose feature sizes exactly matched those of the crystal array can be fabricated, the inactive portions of both structures can be superimposed through precise alignment, maximizing the detection efficiency of the system. We describe here a matched collimator prototype, built in a collaborative effort between the University of Virginia (UVa) and Mikro Systems Inc. (MSI, Charlottesville, VA) to be used with a dedicated breast imaging gamma camera. The camera contains a pixellated array of NaI(TI) crystals of size 1.3 mm with an inter-crystal gap of 0.2 mm. We outline the fabrication process of the prototype, and the process of its alignment with the detector elements. The 2-dimensional Fourier transform (2D-FT) of fluctuations in pixel intensities about the average was obtained for unaligned and aligned collimator settings. While the 2D-FT obtained with the collimator unaligned shows the presence of off-centered peaks, indicating Moire patterns, the absence of these peaks when the collimator is aligned, verifies the reduction of any rotational offsets between the collimator and the detector array. The change in system efficiency with varying amount of collimator-array offset was determined. A variation in efficiency of 11% was obtained experimentally compared to the theoretical 28% increase expected for the given detector geometry.