Neuroimaging in Frontotemporal dementia and Semantic dementia

Dr Anoop Varma, Consultant Neurologist, March 2007

Why do we use scans?

“Dementia” is the name given to changes in mental function and behaviour following a degenerative disease of the brain. It is now known that mental functions, such as memory, language and spatial skills are organised in separate parts of the brain and that different disease processes affect specific parts of the brain. This means that the location of a degenerative disease in the brain determines the symptoms that occur.

As you can see on the diagram, fronto-temporal dementia and semantic dementia affect areas towards the front of the brain, whereas Alzheimer’s disease affects the areas at the back.  The different disease processes are therefore associated with distinct clinical syndromes and symptoms. Frontotemporal dementia, for example, is characterised by behavioural problems and difficulties in performing complex tasks.  Patients with semantic dementia, on the other hand, have prominent difficulties in language comprehension and understanding.  In contrast, patients with Alzheimer’s disease have problems in memory, vision, and language. 

Accurate diagnosis of dementia is therefore very important and requires a clear understanding of patients’ symptoms.  The doctor needs to look at a range of different areas in order to do this.  Firstly, it is important to take a detailed clinical history from both the patient and the carer, to get a good idea of the problems that are occurring in your daily lives.  This can inform the doctor about behavioural and cognitive changes, which can then be looked at more closely using neuropsychological assessment.  It is also important to carry out a physical neurological examination, as the presence or absence of physical problems can tell the doctor more about what may be causing the mental decline.  In carrying out all of these investigations, the Neurologist’s job is to try and translate the symptoms into a meaningful ‘picture’ of the brain.  Neuroimaging can then support this process, by providing a clear and objective view of the structure and functioning of the brain.  It can help us to look for several things, such as:

  • Structural pathology (space occupying lesions and tumors)
  • White matter changes (mini-strokes)
  • Atrophy (overall brain shrinkage)
  • Regional atrophy (shrinkage of specific areas of the brain)
  • Blood flow (function)

What type of scans are there and how do they differ?

Structural scans

These scans tell us about the shape and form of the brain.  They are helpful in showing potential atrophy or ‘shrinkage’ of tissue, and also changes in structure caused by non-degenerative conditions, such as tumours or vascular disease.

Computed Tomography (CT) scans are the most basic type of scan and provide an x-ray of the skull and brain tissue.  They show bone structure very well and provide a relatively cheap and easy method of looking at basic changes in the shape of the brain.  However, they are not detailed, and may not show subtle changes that can occur in dementia.

Magnetic Resonance (MR) scans work in a different way.  The MR scanner is basically a very strong magnet, and uses radio waves and a strong magnetic field to provide clear and detailed pictures of the structure of the brain. MR scan images highlight parts of the brain that are filled with fluid, making it possible to work out which areas of the brain have become shrunken and replaced by brain fluid.

However, there are some disadvantages.  At a practical level, the scanner is extremely noisy and can be claustrophobic.  Some patients may therefore find it difficult to cope with the procedure.  Also, structural scans are unlikely to reflect very subtle changes in functioning.  Extremely large numbers of neurones need to be knocked out before a person starts to show atrophic ‘shrinkage’ of the brain.  Areas of the brain which continue to look ‘normal’ may not be functioning as they ought to.  It is therefore possible for a patient to demonstrate a marked change in cognition or behaviour, with little or no discernible change on the scan.  This is where functional neuroimaging is advantageous.

Functional scans

These scans help us to find out if the tissue that remains in the brain is actually functioning normally and whether the connections between different areas of the brain are working.

Single Photon Emission Computed Tomography (SPECT) scans are the most commonly used and provide a cheap, simple, non-invasive way of looking at the function of the brain.  The patient is injected with a chemical that emits gamma rays that are detected by the scanner. The chemical stays in the blood stream rather than being absorbed by surrounding tissues, and so is more concentrated in areas of the brain that are active and require a high level of oxygen and blood flow. The computer counts the gamma rays and uses them to form a picture. Pictures are then colour-coded so that areas of high activity appear yellow/orange and areas of the brain that are functioning poorly appear purple/blue.

Positron Emission Tomography (PET), receptor ligand, and functional MR scans are extremely detailed and allow us to look at specific ‘tracts’ or connections in the brain.  These scans are very expensive and are mainly used in research.

What can we ask of imaging?

It is NOT possible to make a diagnosis based solely on the results of scans.  Rather, imaging helps to provide support for the clinical diagnosis, which is achieved by:

  • Taking a detailed history from a carer
  • Carrying out a thorough neuropsychological assessment, dissecting specific cognitive deficits and analysing behaviour
  • Neurological examination

Neuroimaging in Frontotemporal Dementia and Semantic Dementia

Neuroimaging is simply another tool to support and check the diagnosis.  With certain clinical symptoms, one would expect a particular pattern of atrophy or dysfunction on scanning (e.g. frontal and temporal atrophy and reduced frontal lobe function in frontotemporal dementia, or temporal atrophy and reduced activity in the temporal lobes in semantic dementia).  In effect, the aim is to recognise a pattern in the results of all of the assessments – each clinical syndrome has distinct characteristics which, over many years, we have learned correspond to distinct disease pathology.

Subtypes of FTD

Even within FTD, patients may demonstrate different behavioural problems, corresponding to different patterns on imaging.  Patients who are rapid, impulsive, and can be disinhibited in their behaviour show reduced activity in the orbitofrontal regions, whereas those who are apathetic and inert show reduced activity in the dorsolateral frontal areas:

 

Semantic dementia

Conclusion

Scans can show differences between patients that correspond to their clinical presentation, helping us to work out the diagnosis.  However, they are NOT diagnostic.  It is essential to take all aspects of the clinical assessment (clinical interview, neurological examination, neuropsychological testing) into consideration.