One method that has shown promise is neuroimaging — that is, getting a picture of the brain using techniques known as positron emission tomography (PET) and single photon emission computed tomography (SPECT). In clinical trials evaluating people newly diagnosed with PD that also included one of these imaging techniques, ten to 15 percent of these individuals had scans without evidence of a dopaminergic deficit — meaning they did not have PD at all and there was some other cause of their tremor and slowness. These individuals did not respond to levodopa therapy. Some PET techniques have been helpful in differentiating PD from atypical parkinsonisms, such as multiple system atrophy and progressive supranuclear palsy. Such tools could be helpful in both diagnosing PD and treating it earlier in its progression.