Optimization of high energy collimator design

Multi-hole collimators are the most important piece of instrumentation in determining the trade-off between spatial resolution and noise of nuclear medicine images. Recently, we discovered that the collimator-to-detector gap influences very much the hole-pattern artifacts and we also showed that there exist specific values of the gap that minimize the artifacts: here we call this phenomenon "penumbral masking" (PM). In this paper we studied the matter in more detail and we took into account PM in the problem of designing high-energy collimators, basically, by substituting the conventional hole-array pattern constraint with a mathematical expression of the PM effect. With this approach we found that an optimal solution to the problem always exist while, without the PM concept, for lead an optimal solution exists below 300 KeV, a suboptimal solution can be found within the range 300-320 KeV and no solution at all exist beyond 320 KeV, unless one violates one or both the penetration and the hole array pattern constraints. Thus, one is able to design artifacts-free collimators even if a somewhat lower sensitivity has to be accepted: 20% at 511 KeV with lead. Even with our approach, the weight constraint is a major problem when high-energies are implied but this does not affect the capability of reducing the hole-pattern artifacts. However, even when the theoretically optimal solution may not be feasible, this approach enlightens very much the design of high-energy collimators.