A CT Scan would not cause a hemangioma. I have an 'outpouching' in a large blood vessel above my brain stem. I have a copy of the image files. Mine is about 1 cm. The radiologist was not concerned. As in your case, watching it for symptoms and changes is about all that is suggested. They are usually asymptomatic and not ever noticed until a image is taken for other reasons. People often live their whole lives without ever knowing they are there.