The investigation of human cerebral function has been one of the most challenging subjects in the last century. Imaging studies that use positron emission tomography (PET) with [0-15]water have been central for the assessment of neurophysiological activity. Accurate and quantitative estimation of regional cerebral blood flow (rCBF) for mapping of brain function requires measurement of the post-injection arterial time course of [0-15]water. The overall purpose of this work is to design and build a small inexpensive tomograph that can be placed around a subject's neck or wrist to noninvasively measure the arterial radioactivity input function. Monte Carlo simulations and experimental investigations have been performed to assist in defining the design characteristics of the detectors to be used for this system.