Brain disorder. Coloured axial (horizontal) SPECT (single photon emission computed tomography) scan of the brain of a Mesulams syndrome patient. This syndrome is a progressive aphasia (speech loss). The most active parts of the brain are white. Activity decreases through red, yellow and green, to black (inactive). The dark area (lower right) is causing the aphasia. ECD (ethyl cysteinate dimer) and the radioactive tracer technetium-99m, injected into the brains blood supply, were taken up by more active tissue. The tracer is detected by its emission of gamma ray photons. The image was constructed using computed tomography methods.