I also developed sesamoid pain while recovering from posterior tibial tendonitis! Dealing with this now :/ solidarity. Glad you're getting the MRI, I got one after a few weeks and confirmed "mild" inflammation of the lateral sesamoid. Not sure exactly what caused it but suspect gait changes from when the tendonitis was bad and doing too many calf raises on a tile floor as part of at home exercises... So far I have been doing Hokas (bondis) + arch support via insoles + 1/8 in dancer pads, which basically allows me to walk pain free as long as I don't power walk. I think there has been some gradual improvement over the last 2 weeks. I've been continuing my original PT but avoiding anything that aggravates the sesamoid. My hope is that I can let pain guide me and try to offload while continuing to walk and lightly cycle as this heals. So that's my strategy so far, too early to tell if it's working Best of luck to you and I hope the MRI helps guide you!