Optimization of 15O-water activation studies in PET/CT

Objectives This work aims at reducing the noise in 15O-water brain PET activation studies performed in a modern PET/CT camera operated in 3D. Optimization of imaging conditions requires maximizing the administered activity while maintaining acceptable camera dead time. Typically, in such studies, the Neuroshield^TM is employed to limit the detection of radioactivity outside the PET FOV.

Methods This shield was designed for PET cameras with smaller patient port (56 cm vs 70 cm for Biograph) and does not offer shielding to the detector blocks located in the upper portion of the PET scanner. We have thus modified the Neuroshield^TM by the addition of an additional piece of lead covering the upper portion of the shield and evaluated the imaging performance in terms of counting rate ability and NEC measurements.

Results With the standard Neuroshield^TM, we observe that an injection of 17 mCi yields an average singles counts rate of ~120kcps, while detector blocks located in the upper portion of the detector rings reach singles counting rate exceeding 400 kcps. With the modified neuroshield, we have shown that block counting rate can be maintained around 120 kcps which effectively allows to double the amount of activity that can be injected in the patient while maintaining singles counting rate within acceptable limits (<300 kcps). A point source phantom experiment confirmed this measurement and further indicated that the NeuroShield offers protection for activity located between 6 and 30 cm away from the PET FOV.

Conclusions The proposed modification to the Neuroshield allows to double the injected activity in 15O-water PET brain activation studies which contributes at reducing the noise.