I have heard of this before, experiencing heat is not common though and for the life of me all i can remember is it having something to do with micro metal bits in face and hair products....
okay i found this that mentions a possible cause of you experiencing heat on your head ie your hair products
"It is important that you do not wear any makeup or hairspray, as many of these products have tiny metal particles that could interfere with the scan and reduce the quality of the images. They might cause the area to heat up and, on the rare occasion, burn your skin."
https://www.insideradiology.com.au/mri/
Hope that helps......JJ