How accurrate is an MRI?

I recently had 2 abnormal CT scans done. I was hospitalized for the first abnormal CT scan so they did an MRI with and w/out dye but the neurologist didnt give any explination and said that im fine & nothing showed up. I was taken back to the hospital a few days later for the same symptoms & they ran another CT scan and found the same small abnormality on my brain cord.. i finally had someone print out a copy of the report and it stated that they think its a lesion.. How accurate are MRI's for detecting this?? They also said in the report that my "lesion" is 6mm big. Im not sure if that could have affected the results.. Please someone help me as this is very frusterating for my family and myself.