A variable focus collimator device and method for functional neurological screening. The variable focus collimator uses sets of corresponding holes in two plates of gamma shielding material, arranged so that only gamma rays emitted from a defined focal region detected. If a suitable radio-labeled tracer has been administered to the patient, this may be used to obtain and compare basal and stimulated data from a small number of regions in the brain to diagnose the presence or absence of dementias. The regions of the brain observer are typically about a cm cubed in volume, and their activity is monitored using small doses (