How interesting. I just had an MRI and decided to be proactive and had exactly the same question. I've used them before with wool but don't have any on hand and would have to go a distance to get some. I believe the first instructions I had were from a nurse who practices natural cures and has studied for years and years. She said wool flannel... and I chose to go as organic as possible - untreated fabric, *unbleached. Will try to find same again soon, but only have cotton towels now and have already started treatment with that - for now. Hope this helps. Good luck! PS: There are places online that sell natural unbleached utreated fabrics. If I find some I'll come back and give you the links. ;-) Peace L.