- Positron emission tomography PET scans may be performed on the High Resolution Research Tomograph (HRRT, 2-3 mm resolution) or another similar camera. Venous catheters will be used for i.v. administration of the radiotracer, venous blood sampling of AChE activity, and for the administration of glycopyrrolate and physostigmine. A radial artery catheter will be inserted by an experienced physician before the PET scan to draw arterial blood samples for metabolite analysis and for determination of the fraction of plasma radioactivity unbound to protein. At the beginning of scan, the subject's head will be immobilized and a transmission scan will be obtained for attenuation correction. PET scans will be acquired using bolus or bolus to infusion administration of up to 10 millicuries of NCFHEB or PHNO. Dynamic images of radioactivity concentration are reconstructed with corrections for attenuation, normalization, random events, scatter, and deadtime. Subject motion is corrected automatically on an event-by-event basis with the Vicra motion tracking system. Vital signs (blood pressure, pulse and respiration) are collected prior to and during each PET scan. Urine pregnancy test will be again administered on the PET scan day prior to the initiation of any imaging procedures. Smoking abstinence, when appropriate, will also be confirmed for smoking subjects prior to PET scanning.