What Causes Elevated Gastrin Levels?

My 16 yr. old son has been recommended to have an octreoscan done. He had a perforated ulcer of the duodenum several months ago that was surgicaly repaired. There have been no symptoms since the surgery and healing was confirmed by endoscopy. He recently had a blood test which showed elevated gastrin levels. He has not been on any medications for several months and was not taking any when the blood test was done. I was wondering what exactly are they looking for in this scan.