Adjustable resolution/sensitivity slit-slat collimator for task-specific clinical SPECT imaging

Abstract

Clinical SPECTs diagnostic performance is mainly limited by the absorptive collimation which leads to the resolution/sensitivity trade-off, with the optimal balance dependent on the imaging task. Even in a single study it is desirable to have the capability to perform different tasks without disturbing the patient. An example is adaptive SPECT, applicable to cardiac systems which derive high performance from the use of a small field of view (FOV) effectively viewed by the system's detector. A fast scout scan performed with a high-sensitivity (hi-sens) large-FOVcollimator can be used to guide the placement of the heart into the appropriate FOV for imaging with a higher resolution collimator. An ideal system would provide several options for the FOV to effectively accommodate patients across the population. Another application for variable collimation is dynamic SPECT study of tracer kinetics, performed with a hi-sens collimator after a bolus injection, followed by imaging with a higher resolution collimator once the tracer distribution has stabilized. Long studied by the clinical research community for its potential to detect coronary problems at an earlier stage, dynamic SPECT has so far proven impractical due to limitations imposed by absorptive collimators, though applicable radiotracers are available. Collimator sensitivity change during a study is possible and practical for slit-slat collimators (SSC), hybrids of parallel-hole and pinhole collimators, due to the two physically separate 1D orthogonal collimators. SSC is the appropriate collimator for imaging of medium-sized organs due to the favorable distance dependence of its sensitivity. We developed an adjustable SSC for the C-SPECT cardiac system. The SSC is capable of changing the resolution configuration in both dimensions without disturbing the patient. The adjustable slat stack is based on two interpenetrating 6 mm-pitch stacks of tungsten foils of 0.2 mm thickness, to yield a 3 mm pitch for the high-resolution (hi-res) mode. For the hi-sens mode, one of the stacks is collapsed on top of the other by a pneumatic actuator, yielding a 6 mm pitch. At the same time, a precision conveyor is used to exchange the slits for those corresponding to a different resolution and FOV. The presented system's ability to rapidly change collimation along both dimensions brings an unprecedented functionality within a minimal footprint, a key step towards optimization of the most limiting aspect of SPECT imaging.