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434 Illustrated Textbook of Neuroanatomy

Positron Emission Tomography In positron emission tomography (PET), radioactively la- belled, metabolically active chemicals are injected into the bloodstream. The emission of positron emitting radioactive isotopes is measured. Sensors in the PET scanner detect the radioactivity as the compound accumulates in various regions of the brain. These emission data are computer-processed to produce images of the distribution of the chemicals through- out the brain ( Fig. 33.6 ), the most commonly used PET trac- er being a labelled form of glucose (Fludeoxyglucose).

The uses of PET scanning in the functional imaging of the brain are as follows: With this method, different aspects of neurotransmitter • activity can be mapped in a specific part of the brain. PET is able to identify specific brain receptors associated with particular neurotransmitters through its ability to image radiolabelled receptor ‘ligands’. PET scan can show blood flow in various parts of the • brain. It also tells about oxygen and glucose metabolism in the • tissues of the working brain. These measurements reflect the amount of brain activity • in the various regions of the brain. PET scan helps in getting in-depth knowledge of the • functioning of the brain. PET is useful in diagnosis of early cases of dementia (Al- • zheimer’s disease), brain tumour, strokes, etc., because all these diseases cause early changes in the brain metabo- lism which are easily detected by PET scanning. Functional Magnetic Resonance Imaging Functional magnetic resonance imaging (fMRI) measures signal changes in the brain that are due to changing neu- ral activity ( Fig. 33.7 ). Increases in neural activity cause changes in the MR signal. This mechanism is referred to as the BOLD (blood-oxygen-level dependent) effect. Increased neural activity causes an increased demand for oxygen, and the vascular system actually overcompensates for this, in- creasing the amount of oxygenated haemoglobin relative to deoxygenated haemoglobin. This allows images to be gener- ated that reflect on the brain structures that are activated (and how) during the performance of different tasks. fMRI is used to reveal brain structures and processes associated

Figure 33.6 PET SCAN of brain. This is the F-18-FDG (Fludeoxyglu- cose) PET scan, showing the axial section of a normal brain. It can be ap- preciated that there is a relatively higher FDG uptake in grey matter than white matter. PET scan is used to know the metabolic activity of different areas of brain for diagnosis of primary malignant tumour, for differentiating recurrence versus post-radiotherapy inflammatory changes and also for dif- ferentiating various dementia. The brain on the colour map shows metabolic activity in increasing order as black < dark blue < green < yellow < red < pink < white. Courtesy: Dr Jay Kumar Rai, Head Nuclear Medicine, SAIMS, Indore.

Oedema

Oedema

Anterior horn of lateral ventricle

Anterior horn of lateral ventricle

Oedema

Oedema

Tumour

Tumour

Body of lateral ventricle

Body of lateral ventricle

Posterior horn

Posterior horn

Occipital lobe

Occipital lobe

(b)

(a)

Figure 33.7 MR images of the head showing the presence of tumour (glioma; marked by arrow) in the right cerebral hemisphere. (a) T1-weighted image. Note the midline shift. (b) T2-weighted image of the same patient as shown in (a). In (b) the MRI shows a lighter image of CSF (inside the ventricle), tumour and oedematous area. These structures contain water. The white matter appears dark as it contains a lot of fat due to myelination of fibres ( Continued ).

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