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PCA and brain imaging

Professor Sebastian Crutch and Dr Tim Shakespeare describe the effect PCA has on the brain and some of the techniques used to image the brain.
In typical Alzheimer’s disease, it’s classically known as what’s called a temporal parietal disease. That means the temporal lobes of the brain here, which are particularly involved in laying down new memories and the parietal lobes, which are involved in the spatial elements of those memories and other skills are typically the areas of the brain which, when you look on a brain scan, you see most reduction in volume. In posterior cortical atrophy, it’s more so-called occipital lobe, right at the very back of the brain, so most visual part of the brain, and these parietal lobes which are most atrophied or shrunken down when you look at the volume of tissue. That’s where people are losing their brain cells.
And so it’s a fundamentally, that difference between the temporal focus in typical Alzheimer’s disease And the posterior occipital focus in posterior cortical atrophy, which leads to these very different presentations, that fact that the typical Alzheimer’s disease you have difficulties with memory and orientation, whereas in PCA, you get these difficulties in perceiving what and where things are. And that’s because the brain is very specialised. There are different channels for processing different types of information. Different bits of the brain are very good at processing different types of information. And that’s, again, why we see, even within posterior cortical atrophy, we see a bit of a difference.
Some people, where the atrophy is more towards the top surface, the so-called dorsal pathways of the brain, have particular profound difficulty in seeing where things are. And other people, who have a more inferior or ventral loss of tissue in those pathways, they’re the ones who have greater difficulty, relatively, with seeing what things are, as opposed to where things are. So you can get all this sort of fine tuning, which may relate to the shape or the fact that all of our brains are different shapes, initially, or the fact that the different diseases are spreading and expressing themselves in different ways between different individuals. Brain imaging is really important to help diagnose all forms of dementia, including posterior cortical atrophy.
One thing that’s really important to do is to rule out other conditions. So you can check that people don’t have a stroke or tumour that could be causing their symptoms. Now, as brain imaging has become more advanced and more accurate, you can see if there are changes in the back of the brain and you can spot that shrinkage, if it’s happening. Sometimes that can be done just from a single scan, to see if areas of the brain are kind of thinner than they should be. But sometimes you need two scans over perhaps a year’s time period, to see if there’s difference between and if there’s shrinkage happening over time.
There are a number of new, interesting brain imaging techniques that are helping us to see, not only where there’s shrinkage in the brain, but, in particular, what the pathology is, what the disease is that’s underlying the symptoms that people experience. So, for example, there’s a form of imaging that can show us where those amyloid plaques, one of the key components of Alzheimer’s disease, are forming in the brain. And you can see if those are forming, it suggests that a person does have Alzheimer’s disease, rather than one of the other causes of dementia. The standard brain scans that we do tell us a lot about the grey matter of the brain.
That’s the part that contains the cell bodies of the neurons. And it’s a layer all the way around the outside of the brain. Relatively new techniques are allowing us to study the white matter of the brain. And that’s a bit like the wiring that connects all the different parts of the brain together. And by using those techniques, we can understand more about how the disease affects the whole brain, as a kind of network together, and how those crucial connections might have a role in understanding the symptoms that people have and how the disease progresses over time.
As well as studying the structure of the brain, so that might be the thickness of the grey matter, looking for that shrinkage, it might be the wiring that connects different areas, we can also study the function of the brain. And that’s how it’s using energy. And there’s a particular type of scan that can measure the metabolism of the brain, to see if there are particular areas that aren’t using as much energy as you’d expect them to be. And so, in posterior cortical atrophy, often we find the areas at the back of the brain have low metabolism. They’re not using as much energy as you expect.
And that can help us spot that there’s something going there, at the back of the brain.

Professor Sebastian Crutch and Dr Tim Shakespeare describe the effect PCA has on the brain and some of the techniques used to image the brain.

Would you like to understand how brain scans are interpreted?
A project called ‘Seeing Dementia’ allows you to see the effect dementia can have on the brain, learn one way in which brain scans can be used to help diagnose dementia, and try yourself to see what it is like to perform visual assessments of the scans. The project aims to compare expert’s ratings with non-expert ratings from members of the public to see how variable people are in using the visual rating scale. Seeing Dementia focuses on one particular part of the brain – the hippocampus – which is an area that is particularly affected in typical memory-led Alzheimer’s disease. If you’d like to give it a try, visit the Seeing Dementia website. You’ll need to sign up for an account in order to take part.

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The Many Faces of Dementia

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