I want to second Bob's assessment of the "lesion" question.
Since a lesion is an abnormality of the brain tissue or spinal cord tissue that may or may not be seen on the MRI, it is an absolute fact that you must have lesions to have MS - diagnosed or not. But, as Bob noted, not all lesions are seen on MRI.
The other kind of lesion is that spot on the MRI that represents an area of damage in the tissue. Most neurologists insist on seeing MRI spots to diagnose MS. It is the rare neuro that is confident enough in his/her own skill that will diagnose MS with an apparently negative MRI. We do have people here on the forum that have received a diagnosis of MS without MRI lesions, but they are few in number.
Just because one MRI is read as negative, it doesn't mean that a person does not have MS. MS is a clinical disease not an MRI disease. The neurologist who understands the disease will take a thorough history looking for the suggestive signs of MS attacks and relapses separated by remissions. They will do a VERY thorough neuro exam - which contains dozens of tests, looking for signs that the central nervous system (brain and spinal cord) has sustained some sort of injury.
If both of these parts are fulfilled, then the doc will do all the necessary tests (mostly blood tests) that will show whether some other known disease could be the culprit in causing the patient's symptoms and abnormalities on the exam. If all the MS mimics have been ruled out, then the diagnostic rules say that the diagnosis of MS can be made and not further evidence is "required." The rule - or Criteria - recommend that an MRI be done looking for evidence to back up the diagnosis, but lesions are NOT required.
If the patient's symptoms, problems and the neuro exam don't show enough information for the doctor to be confident in the diagnosis then the MRI can be used to provide more evidence to fill in the gaps. This is where many neuros have their problems. They are not confident and don't really believe in the process of ruling out other diseases. Some neuros actually believe that the MRI MUST show certain numbers of lesions that are in certain locations and are of certain shapes, like Bob said. This is really too bad. The MRIs of people with MS can be highly variable. Many neuro's are just not comfortable with "variability."
Other tests that show supportive evidence are the lumbar puncture and the evoked potentials. These tests are of great help when they show positive results. They are of little help with they are negative. Being negative DOES NOT mean that the person does not have MS.
The Health Pages that may help explain some of these things are:
You will likely have many more questions. That's one reason we are here!