I have a Harrington Rod, and as "dailyhope" suggested they use the MRI machine with a lower magnet--1.5.
Whenever you have an MRI, let scheduling know that you have a Harrington Rod and need the 1.5 machine.
At the time of your MRI, confirm it is the 1.5 machine. (One time, they initially had me in the 3.0 (higher magnet) MRI machine by mistake.
Most of the time imaging centers will know this but you will always need to confirm.