Slow healing fracture despite clear x-rays and MRI?

I've had some nagging foot issues beginning in September:
9/10/11-Foot stomped on in cuboid area
11/2/11-Foot still swollen, chiropractic adjustment of foot, first set of X-rays (normal)
               11/8/11- Saw ortho specialist, set of weight-bearing X-rays done, all clear, advised 30 days of rest and  supportive shoes
1/20/12-Foot still swollen over cuboid, continued periodic pain in lateral arch, requested MRI
2/1/12-MRI normal, suggested that I build back up to normal activity
    I went to the PT two days ago, who did some ultrasound, e-stim and suggested some strengthening exercises as my left (injured) leg is considerably weaker than the other.  The ultrasound caused pain on the lateral side of the foot closer to the toe, but could not be reproduced.  Since the treatment the foot was more sore over the cuboid area.  I went back today, and the ultrasound pinged a spot in the swelling over the cuboid.  The therapist turned up the intensity to confirm the spot and I literally jumped—it was a very painful, electrical shock feeling.  With poking and prodding, the tenderness could not be reproduced.
     I am worried about the pain during ultrasound.  The PT said that such pain occurs at the site of a fracture.  She agreed the lack of point tenderness in that area was odd, in addition to having 3 imaging tests done over a period of 3 months that were all normal, but was also curious about the lingering swelling.  I asked for the MRI because I had the impression it was a gold-standard diagnostic test.  PT suggested a possible stint in a boot if things don’t progress in the next few weeks.
     I’m in a rural area.  In your opinion, is it worth requesting additional weight-bearing X-rays to check out the cuboid area again now that the ultrasound caused some pain there?  My brief Google search indicates there can be false positives when diagnosing fractures with therapeutic ultrasound, but the PT seemed pretty sure of her information.

[Michael L Gross, MD]

A false positive ultrasound for fracture ia much more likely than a false negative MRI, especially when it comes to a fracture. In addition your X-rays were also negative, which although not as sensitive as an MRI, is also fairly meaningful.
More likely the problem is either within the soft tissues, or in one of the small joints of the mid foot, rather than in the bones themselves.
I think a trial of immobilization in a boot might be helpful. I would suggest wearing the boot full time, including sleep, remove it only for bathing, for a period of three weeks.  If there is no improvement, I suggest you see a orthopedist who specializes in foot and ankle.  If there is not one locally, you can probably find one at the nearest teaching hospital, or medical school.