Is there such a thing as an MRI to determine how deep the acne scars are in ones cheeks?

So sorry you are suffering in this way. There's really no need for MRI. An experienced, board certified cosmetic dermatologist is able to determine what kinds of scars and get a very good idea of the necessary treatments to improve them from an in-person examination that allows for manipulation of the skin. Unfortunagtely, I am not surprised by your lack of success and frustration with the fraxel laser, etc treatments, as I have been cautioning readers of my answers for years that these expensive, heavily marketed and promoted "bells and whistles" simply do not have the hard science to back up the hype. Sorry. However, there are real treatments, such as subcision and TCA CROSS that have several decades long track records for efficacy and safety in all skin types. You would be wise to make sure that you find an experienced acne scar specialist and avoid venues hawking any other supposedly "next big thing" devices they have just purchased. Hope this helps and best of luck.