Medium-sensitivity rotating collimator for high-performance radioisotopic imaging

The high-resolution radioisotopic detector allows the detection of small structures, but its sensitivity is poor. Increased parallel collimator sensitivity is obtained by enlarging the hole diameter. However, a hole diameter greater than detector intrinsic spatial resolution leads to artifacts on acquired images. Linear collimator motions can remove these artifacts. These motions consist in sampling the generic rectangle of the hexagonal collimator structure and theoretically lead to images without artifacts. However, such motions are difficult to perform. To this end, a circular translation motion is studied in this paper. Motion effectiveness was evaluated by an inhomogeneity index based on hexagon center loci. Several rotation radii optimizing image homogeneity were found. Moving collimator transparency was deterministically simulated using a uniform isotropic source and a line source. For every rotation radius that minimized homogeneity, uniform source images showed variations below 1%.. The smallest suitable motion radius is 0.32 times the structure diameter, which leads to a very small decrease in the field of view. A device performing such collimator motions was made for a high-resolution gamma imaging probe based on a photomultiplier position sensitive tube and tested using a low-energy medium-sensitivity collimator. The measured spatial impulse response was the same on the whole field of view for a given source-to-detector distance. An image restoration process allowed the improvement of spatial resolution from 3.2 to 2.6 mm. Moving a medium-sensitivity collimator allows us both to obtain good sensitivity with a high-resolution gamma detector and to restore high resolution in radioisotopic imaging. For thyroid diagnosis, this will increase the ability of cold nodule detection with low-energy isotopes. Furthermore, images obtained with ¹³¹I will be free of artifacts due to the septa size of medium-energy collimators