Had to go and find the letter! It was an Isotope Scan, info below from NACC:
a test which uses a tiny amount of radioactivity injected into the bloodstream to produce pictures of the body by using a special gamma camera that picks up radioactivity. It involves taking a blood sample and then attaching an isotope (a radioactive substance) to white or red blood
cells. This is then re-injected into the bloodstream. In IBD, red cell scans are
sometimes used to look for the cause of bleeding; white cell scans are used to look for abscesses or inflammation of the intestine. Different isotopes are used for scanning different parts of the body, such as the liver, kidneys and bones...
(It was cool to watch on the screen....bits of my bowel started changing colour as the cells travelled towards it!)