Ask the Expert: MRI scans

MRI scans are a big part of MS diagnosis and ongoing care. We spoke to Consultant Neurologist Will Brown of Addenbrookes Hospital, Cambridge about MRI scans and what they can tell us in MS. 

The main reason to do an MRI scan in MS is to look for lesions. The MRI scans through the brain in two different dimensions. An axial scan goes horizontally from top to bottom, and a sagittal scan goes vertically. We can then move up and down and left to right and look for abnormalities in different places. 

You will hear noise as you’re scanned. This goes on for a few minutes then there’s a gap, followed by more noise, and another gap. The noisy moments are when we collect different sequences, which means we change the MRI settings to look for slightly different things. The main sequences we use in MS are called T2, FLAIR and gadolinium-enhanced, also known as contrast.  

On a standard strength NHS scanner, a T2 sequence will pick up just over 60% of lesions and a FLAIR sequence will pick up just over 70%. The lesions show up as white patches on the scan. The sequence we do after a gadolinium injection helps us to age a lesion, as it will show up lesions that are recent, around four weeks old.

There are two main uses for an MRI in diagnosis. The first is to rule out other conditions that might have similar symptoms as MS, but look different on a scan.

The second use for MRI is to get to a diagnosis as early as possible. To diagnose MS we need to see that there's a spread of lesions from different times and in different places. It used to be that people would need to have their first symptoms and then wait until new, different symptoms developed before they could be diagnosed with MS. We now know that the earlier we start treatments the better the long-term outcomes, so we want to be able to diagnose it before waiting for another set of symptoms.

We can use an MRI scan to prove that there's evidence of MS in different places in the brain, but you can also use the MRI scan to show that there's a spread in time. For example, if on one scan you find lesions that enhance as well as lesions that do not enhance with gadolinium, well that says that there are new and old lesions.  Similarly, if you repeat a scan and find new lesions, then we know there's old and new lesions, even if the patient has not noticed new symptoms.

When we look at lesions on an MRI scan, we can see that each one has a little vein in the centre, which helps us think about what's actually happening in MS. That vein leaks inflammation into the tissue around it, and therefore that's the first component of a lesion. Nerves normally have a layer of insulation called myelin that feeds and supports the nerves as well as improving their function. When inflammation happens, the nerves lose their myelin. So, the second component of a lesion is demyelination, the loss of myelin. The third component is that the nerve has then been left there exposed. So not only has the myelin come off and it's not functioning, but also with time there can be nerve loss or nerve death.

So actually a lesion is a combination of inflammation, demyelination and nerve loss. Can we see every lesion? No. Even in the highest power scanners which we don't really use in NHS practice, you can only see up to 85 percent of lesions. Sometimes there will be lesions that we just can't see on a scan. We also can only see about five percent of lesions in the grey matter in the brain, so we certainly don't see all lesions.

A lot of people say, "My symptoms are gradually worsening, but the scan is unchanged. Why is that?". The main message is that MS is more than just lesions. There are other processes going on as well.

You might hear the word atrophy being used. That means that there's shrinkage of the brain, so if you are looking at one scan alone it's hard to account for that shrinkage. There are some fancy computer ways to compare scans, but it's very difficult. What we'd see with lots of atrophy is bigger spaces between brain tissues, called sulci. The bigger gaps reflect loss of nerves. 

As we age, everyone's brains shrink at a slow rate. We know that the longer people have MS, the rate of atrophy increases, and it is greatest in people who have progressive disease.

The other thing is that even though the tissue around the lesions looks normal in the scan, in fact it's not normal. There are some new ways of actually assessing what's happening in that tissue. So, when people say their symptoms are worsening it isn't a surprise to us.

What we do know is that most of the drugs we currently use in MS work best in people who have evidence of new lesions, because they are targeting inflammation and lesions are a feature of inflammation. However, the shrinkage or atrophy, and the abnormalities in the normal appearing tissues are indirectly related to inflammation. This means that they are likely to reflect a more complex process that does not respond to these drugs. 

Although we try to encourage people to take ownership of their treatment, this is one example where it can be harmful. If you haven't seen a neurologist yet, then by definition a non-neurologist has ordered your scan. The radiologist who reports the scan can only do that on the basis of the information they've got, so a neurologist will need to talk to and examine you, and then interpret the scan in light of that. 

So, if I was a patient and I had been given a scan report that said 'in keeping with multiple sclerosis' or 'demyelination', I would take that with a pinch of salt until a neurologist has seen it too. Lesions on a scan can happen in 10 to 15 percent of people who do not have MS. As we get older that number increases, such that over the age of 50 the majority of us have those lesions even though we don't have MS. So it's really important to get the context right.

The second thing is that if there's an incidental finding on a scan, like an abnormality of the blood vessels (an aneurysm), or if someone has a benign growth, we'd want to give that in context. Seeing that you have an aneurysm or a tumour on a scan report could be devastating, and usually would cause a lot of unnecessary worry.  These could have been present since birth and are unlikely to change in the future and will cause no symptoms. So I think that's where having a neurologist can be very useful rather than trying to interpret a report alone.

One of the most important messages here is that there is a big difference between what a person with MS looks like and what their scan looks like. People can have hundreds of lesions and yet have very little in the way of symptoms, and vice versa. Also some people can have lots of atrophy and minimal symptoms whereas the reverse in seen in others.

What we know is that at a group level, if we take an average of people who have lots of lesions, they will have more symptoms and more disability than people with fewer lesions. That is the same with atrophy; the more atrophy there is the more disability there will be. However, the overlap between the groups means that you can't take one person and predict what their symptoms will be or what the future is going to hold for them.

The same is true of the location of lesions. For example, a lesion in the spinal cord or lower part of your brain is more likely to be associated with disability than a lesion higher up in the brain. But it is just based on averages, so I wouldn't pay too much attention to it as an individual. There is just too much noise and variation to make any useful predictions for a specific person.

This is a thorny issue because we don't really have good data. In the UK, there are no guidelines that specify the frequency that people with MS should have an MRI scan. In some countries and in some centres in the UK, an MRI is done annually, but it may depend on the clinician.

If a person has had one episode of symptoms, but not fulfilled the diagnostic criteria for MS, then an MRI six to 12 months later could be helpful in providing the data for a diagnosis. Otherwise it is usually a question of monitoring how people are getting on, while being practical about why we are doing the test and what we will do with that information. There should be a conversation between the person with MS and with the neurologist because there's a lot of variation. Where we have limited resources we have to think through how scanning is going to influence or improve care for a person.

If, for example a person with MS says, "I'm not going to go on treatment even if I find new lesions on a scan", we should be pragmatic about whether doing that scan is appropriate. If you're on a moderately effective treatment then a new lesion might indicate that a treatment change would be appropriate. But if you're already on the highest efficacy drug and you find a new lesion, it's challenging to think what you're going to do with that information, if there is no more powerful drug option to change to.

• Read a personal experience with an MRI scan:  What is it like to have an MRI scan?
• Find out more about the MS diagnostic process: How is MS diagnosed?
• Find out about other tests used in MS: What is it like to have a visual evoked potentials test?