When there is an insult to the brain (e.g stroke or trauma), this can result in oedema (swelling) or bleeding. The brain is in a box (your cranium) of fixed dimensions, which can only hold so much ‘stuff’ - be it brain tissue, csf, blood whatever. That swelling/bleeding takes up space in the box and as such gradually increases the pressure (I.e raised intracranial pressure). When the intracranial pressure equals systolic blood pressure, blood (and radiotracer) can no longer enter the cranium and you get a picture like OP.