The drill is this. You need to numerous wrap coils of thin wire around your foot, a half inch apart. Then coils at 90 degrees. This provides a pattern. Take the x-ray. Then use a permanant marker to trace the wire lines and remove them. This will provide a guide for the surgeon. He will have the x-ray with the wire shown and see the lines on your foot. If you know exactly where the glass is, place the wire in parallel lines closer together. Mind that x-rays provide a distorted image of the foot, I am surprised an x-ray shows the glass but an MRI does not.
Alternatively, take a three inch wide by three inch piece of adhesive tape. Lay down ten wires one quarter inch apart. Then another ten at ninety degrees. Press this in an ink stamp pad.. Put over the area where the glass seems to be. Take your x-ray. Remove the adhesive tape. Reinforce the lines with a sharpie permanant marker. This will provide the exact location of the shard to the surgeon.