Your doc is generalizing quite a bit, there's definitely a range... our 10yo has had one strata which didn't change much and a 3-4 codmans (5 revisions overall) which have changed a lot during MRIs - either way she is super sensitive to the pressure settings (a key reason she has codmans now) and has a short window before becoming quite ill. Also has had ~30 head CTs and more xrays (shunt series plus setting checks) - that radiation adds up

here's a link to more info, note some valves can be locked to not change in <=3T MRIs, 3T magnetic resonance imaging testing of externally programmable shunt valves

for that updated information.