MRI...please answer...?

im having an MRI on my left knee at 10am EST, so please answer... this will be me thrid MRI on this knee, and is being done after my knee collasped last monday... when it collasped i felt a sickening grinding/ripping sound from within my knee... after which my knee was very painful, swollen and UNSTABLE....i ended up going to the ER cause my knee felt like it was going to fall apart... i was placed in an immobolizer and told to be minimal weight bearing... anyway i went to my followup(prescheduled from first knee injury) and my ortho orderd the MRI...i have had two"bad" experiences with MRIs... where nothing really showed up... it showed a "minor" tear to my MCL(which i was told should have been healed by now... 9 months post injury)my question is... how accurate are MRIs if there is still swelling at the injury site? i know what goes on with MRIs... so im guess im only worried because what if it shows "nothing" again... my knee is still unstable and i feel as though something is wrong