Positron emission tomography (PET) allows the quantitative measurement of 
regional cerebral flow (rCBF) in humans in quantitative terms. Gross changes in 
rCBF are due to variation in vessel diameter. Changes of rCBF also reflect 
synaptic activity (inhibition and excitation). Therefore, PET was used to 
monitor changes in blood flow during the aura and headache phase of a migraine 
attack and to investigate focal areas of increased or decreased blood flow, 
e.g., in the brain stem and midbrain. Hemispheric rCBF was unchanged in 
spontaneous migraine attacks without aura. This was true for the headache side 
as well as for the nonheadache side. Sumatriptan had no effects on cerebral 
blood flow. Regional cerebral blood flow was increased in midline brain stem 
structures during the headache phase, but also when the headache had been 
treated with sumatriptan. This persisting increased activity might reflect 
activity of a presumed migraine center in the brain stem. These changes are 
specific for migraine attacks and are not seen during attacks of cluster 
headache. Positron emission tomography measurements in the early phase of a 
migraine attack in a single subject showed flow reductions in the occipital 
cortex spreading forwards; an observation which would be compatible with the 
existence of spreading depression in humans. Our attempts to study the aura 
phase with PET have, to date, been unsuccessful.