That's great that your migraines are down to once a week now. Otherwise, I would recommend that you take a daily preventative migraine medication. If it in fact, they are only once a week, get a prescription for an abortive pain pill to treat it.
Usually CT scans and MRIs don't show anything. If they do show something, it means that you have something far more serious like a brain tumor or multiple sclerosis. So you are fortunate. That doesn't mean that you don't need treatment for your migraines. Good luck.