Optimisation of the design of round-hole parallel collimators for ultra-compact nuclear medicine imaging

Hexagonal-hole parallel collimators are generally desirable over round-hole parallel collimators for most all medical imaging applications due to the optimum sensitivity offered by such a close-packed arrangement of apertures. However, such a sophisticated close-packed collimator arrangement is unlikely to adapt itself well to newly emerging, ultra-compact nuclear medicine gamma cameras. This is principally due to difficulties in machining and fabrication. In searching for alternative collimator designs, one must attempt, as far as possible, to preserve sensitivity, while optimising the spatial resolution achievable with the gamma camera. This work presents a general analysis of round-hole parallel collimator design for nuclear medicine imaging of 140 keV incident gamma-rays. Optimised designs are considered for a variety of source–collimator distances (z=10, 20, 30, 40 mm) which represent typical sentinel node to collimator distances for our particular medical imaging application. Resolution and sensitivity characteristics are plotted as a function of collimator thickness and hole diameter. For each value of source–collimator distance, and for each collimator thickness investigated, the trade-off between sensitivity and spatial resolution is expressed as a series of characteristic curves.