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Background: the analysis of FDG PET scans

FDG PET scan

It is possible to visualize the regional glucose uptake in the brain by use of a PET scan (Positron Emission Tomography). In a human body several nutritious substances are used, but the brain only uses glucose. In the brain glucose is oxidized by oxygen and this provides the biological energy which enable the tissues to perform their functions. The brain uses 25 – 30% of the whole body energy – this is more than the heart needs. Therefore disturbances in brain function are immediately reflected by changed glucose consumption locally, although often only in a subtle way.

Glucose, like many other physiological substances, can be made radioactive and as such can be a tracer to determine glucose consumption in the brain. A PET scan is able to visualize the uptake of this type of tracer in the human body. As a substance these tracers are physiological compounds. They are administered in real trace amounts which means that the concentration of the tracer is so low that it does not influence the metabolism itself. Also the radiation dose is very low since the time that the radioactive label takes to decay is only a few hours. The technical substance which will be administered to measure brain glucose metabolism is FDG (Fluoro-DeoxyGlucose), a form of glucose. This substance will, like glucose, be taken up in the several regions of the brain in relation to the degree of the local glucose demand, but will only go through the first step of glucose breakdown. This specific situation allows to measure precisely how much glucose at a certain region of the brain is needed.

Furthermore the radioactive labels in this type of investigations are also physiological atoms like carbon, oxygen, fluor and others. The specific way this type of radioactive atom decays is called positron emission and thus a PET scan is built to register this phenomenon. The time a person lies with his upper body in the PET scan is about 40 minutes. The scan is performed on an outpatient base and the patient can go home afterwards without any restrictions.

Analysis

By using a complex mathematical method it is possible to create a metabolic pattern for each neurodegenerative disease. This pattern shows which parts of the brain are (most) diseased, as defined by a lower metabolism in a specific part of the brain.

This mathematical method has originally been developed in New York (United States), by the research group of Eidelberg. In Groningen we have adopted this method in collaboration with that group. To date we have implemented different patterns for 4 neurodegenerative diseases: Parkinson’s Disease (PD), Multiple System Atrophy (MSA), Progressive Supranuclear Palsy (PSP) and Alzheimer’s Disease (AD).

By using this method it is possible to compare a particular patient’s scan with the patterns we developed, and to study how a patient is scoring on these patterns. A higher score means a higher resemblance between the patient’s scan and that particular disease pattern, and consequently a higher probability for a patient to have that disease. However, at this point in time it is still necessary to follow the patient clinically to confirm the diagnosis. Further patterns for other neurodegenerative diseases will follow.