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 radiatio n is not very effective on whole-body positron emission tomography (PET) sc anners operated with their slice-defining septa retracted. Additional shiel ding 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 tor so. We have designed, built, and tested (on the CTI HR+PET scanner) a new l ead-plate shielding system that mounts on the scanner's removable headrest. We call this device the "NeuroShield." The outer diameter of the 9-mm-thic k lead plate is 550 mm, which has a "U"-shaped hole 220 min. in diameter. I t is positioned just above the subject's shoulders. A molded plastic coupli ng piece, produced by stereo-lithography, was designed to accommodate the c omplex shape of the headrest under the subject's neck and provide a flat su rface for mounting the lead plate. Our testing on human subjects involved c omparing 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 th e 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 Ne uroShield. 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 centers doing a mixture of whole-body and neurological studies, where the CTI neurological insert cann ot be installed permanently.