Fig. (1) Shows a healthy 3D surface rendering of SPECT information, looking at the top 45% of brain perfusion, anything below that level shows up as a hole or a dent. The holes do not mean no perfusion, they mean low perfusion, compared to a healthy dataset. Activity below the top forty-fifth percentile is 3 standard deviations below normal [10]. A healthy scan shows full, even, symmetrical perfusion.