A variable-pitch slat stack for task-specific slit-slat collimation: A preliminary report

The slit-slat collimator is experiencing a resurgence with the trend towards SPECT systems dedicated to imaging of specific organs, such as the heart and the breast. The slit-slat, a hybrid of a parallel-hole and pinhole collimators, is optimal for imaging at medium, 10-20 cm distances. The hybrid nature of the collimation, based on the use of separate, orthogonal components, affords a unique opportunity for design of collimators that can be changed without disturbing the patient. Such collimator exchange can be invaluable when separate imaging tasks with different resolution/sensitivity needs are performed during a multi-phase study. High sensitivity imaging is an important tool for patient positioning in systems which achieve high efficiency by using their detector to image a small FOV; collimator exchange would permit the use of high resolution collimation after positioning. Beyond scout imaging used for positioning, collimator exchange may permit protocols combining a dynamic study using the high-sensitivity mode, followed by a study with a high-resolution mode, once tracer distribution reaches equilibrium. We have designed and constructed a variable slat stack prototype which permits rapid switching between hi-sens and hi-res modes. The design is based on two interpenetrating 3 mm-pitch stacks of 0.25 mm tungsten foils suspended at their edges by two sets of thin interleaved aluminum walls with narrow shelves. For the high sensitivity mode, one of the stacks can be collapsed on top of the other. In this report, we present a preliminary assessment of the prototype. Combined with our previously demonstrated interchangeable slit conveyor system, the ability to rapidly exchange collimation along the both dimensions brings an unprecedented functionality within a minimal footprint.