I don't know about harrington rods since they are 'old surgical metals' but newer materials being use are non-ferrous so they won't go flying to attach to the magnet or distort the images as much as ferrous materials. If all metals were attracted to the the MRI magnets I'd have had locker keys and my glasses flying across the room... or some kid would have their face plastered to the MRI borrow because of their orthodontic braces
lots of people get scanned with 'metal' in their bodies. I've seen scans with joint replacement hardwares in place which are bigger chunks of metal than spinal instrumentation can be. Certain metal hardware are meant to become anchored and part of the body spine hard ware included. People with pace makers or aneurysm clips won't be allowed even near a MRI since the magnet is always on, and a clip moving would be potentially lethal...and pacemaker's signal getting disturbed isn't good.
Some place are just too lazy or don't have the right scan protocol that counters the artifacts to do scans on people with metal in them. Things have to be done differently than doing a standard spine protocol if someone has rods vs. a rodless spine.
I've only seen one or two places out of 7(?) places I've had scans done at that specifically asked if one had harrington rods. So there maybe some places that won't scan people with harrington rods but will scan those with more recent rods.
I've had my entire body scanned and there have been more artifacts from minor movement (like..eyelids/breathing) than my mouth full of cavity fillings.
2003 - T45, L???
2005 - T50, L31
bunch of measurements between...
2011 - T60, L32
2013 - T65, L?
Posterior Fusion T3 - L3