Cannot say for sure without evaluating the studies.
It may be possible that the mass can be seen via the MRI and not via the ultrasound. Of note, is that the MRI was meant to evaluate the lower back, and not the kidney.
If there are conflicting results, you can consider a dedicated MRI or CT scan evaluating the kidney. If these scans are negative, it is unlikely that a mass was present, and the previous MRI reading may have been an artifact.
You can discuss these options with your personal physician or in conjunction with a urologist.
Followup with your personal physician is
essentialEssential balance
Essential hypertension
Essential tremor.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
http://www.straightfromthedoc.com