Imagine a SPECT image of a patient with no radioactivity injected but with a number of capsules filled with radioactivity attached to his skin. These are called "fiducial markers" and can be used for image registration if their locations can be accurately determined.
Sketch image of what you think the histogram of a black and white SPECT image would look like for patient with only fiducial markers, where range of each pixel is [0, 255] or [black, white] (i.e. values in between 0 and 255 are various shades of gray).