How much earlier than X-ray can ultrasound prove fracture healing?

If you get a fracture of one of the metatarsal bones in your foot, you are not going to be able to run until it heals. 

But as soon as it's healed, you will want to start running again. 
 
You may have recently seen the episode that I did where I was talking about how ultrasound may show signs of healing sooner than X-rays.
 
How much earlier can ultrasound show healing of a fracture as compared to X-ray? 
 
Well, that's what we're talking about today on the Doc On The Run Podcast.