Scan is a safe investigation with no radiation
hazard to the person undergoing it. In fact it is safe even in pregnancy
. However, metal implants in the body or outside the body are NOT permitted( contraindicated).
Unless you have implants in your body, such as plates, screws, rods, artificial heart
valves, vascular clips, you have no reason to worry about getting inside a MRI Scanner. No, it will not affect your SMA.
Make sure you take your brace off though!