Usually the gamma-emitting tracer used in functional brain imaging is Tc-HMPAO (hexamethylpropylene amine oxime, exametazime). The similar Tc-EC tracer may also be used. These molecules are preferentially distributed to regions of high brain blood flow, and act to assess brain metabolism regionally, in an attempt to diagnose and differentiate the different causal pathologies of dementia. When used with the 3-D SPECT technique, they compete with brain FDG-PET scans and fMRI brain scans as techniques to map the regional metabolic rate of brain tissue.