When injected into your blood radioisotopes will illuminate your blood vessels under an X-ray. This is what allows doctors to see blockages and deformities in the blood vessels. Because tumors are typically highly vascular, meaning they contain a lot of blood vessels, I expect that radioisotopes can help brain surgeons see tumors in the brain just as they could to see a blockage in the heart from the lack of radioisotope in one area of the heart.
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