I'm confused by your terms a little bit. If by lower GI you mean a barium enema or a colonoscopy (lower endoscopy), then it will not show a ulcer. If by upper GI, you mean an upper GI series, I believe it will not always show an ulcer. If by upper GI, you mean an upper endoscopy, then yes it will show an ulcer.
You need an Endoscopy so they can view your stomach, which will show them if there is an ulcer. Take care.
I know an Upper GI will show one I think a Lower GI series will to
Welcome to the gastroenterology community! I don't believe a CT scan can show an ulcer. If you are worried about an ulcer you should talk to a GI doctor about getting an endoscopy. What symptoms are you having?