Evaluation of a neck-shield for use during neurological studies with a whole-body PET scanner

It is well known that the shielding against out-of-field scattered radiation is not very effective on whole body PET scanners operated with their slice-defining septa retracted. Additional shielding can be employed during neurological studies since the diameter of the head is much less than the field of view required for imaging the human torso. The authors have designed, built and tested (on the CTI HR+PET scanner) a new lead-plate shielding system which mounts on the scanner's removable head-rest. They call this device the "NeuroShield". The outer diameter of the 9 mm thick lead plate is 550 mm which has a "U" shaped hole 220 mm in diameter. It is positioned just above the subject's shoulders. A moulded plastic coupling piece, produced by stereolithography, was designed to accommodate the complex shape of the headrest under the subject's neck, and provide a flat surface for mounting the lead plate. The authors' testing on human subjects involved comparing the prompt and random count rates and dead time during bolus-water activation studies. Scans on different subjects, (matched for age and sex) with no additional shielding, with the CTI Neurological Insert, and with the NeuroShield were compared. The random counts were reduced by a factor of 1.56 when using the permanent CTI Neurological Insert, and 1.44 using the NeuroShield. The NeuroShield is easily removed to allow whole body scanning. It can remain in place as the couch extends through the scanning tunnel in the gantry. Its main application will be at PET centres doing a mixture of whole body and neurological studies, where the CTI Neurological Insert cannot be installed permanently.