Precheck! Also when I'm traveling with children they won't send me (or them) through it.

My daughter had an MRI this week and she was wearing a Puma shirt (made of yoga pant-like material) and it set off their metal detector they used to make sure she didn't have any metal on/in her. They told me a lot of athleisure wear has metal in it, including lululemon. So it makes sense that a sports bra might be made of the same material.