Radiologists always advice to remove all the electronic prosthesis, ornaments and metals for proper imaging. Coming to the point the MRI machine while on work can generate upto 110 decibel noise, which is high and can damage the hearing. Now a days sofisticated silent scanner systems are used in some places, better to go for that. As she was using the hearing aid let her continue the same, if it's a programmable hearing aid and covers the range it can be reporogrammed according to the new loss level.