PET in Alzheimer's disease

Alzheimer'sdisease is the most common form of dementia in the elderly, affecting around two-thirds of dementia patients aged 60-70 years. Its prevalence is age-related: in 60-64 year-olds it is found in about 1% of people, compared to 20-40% of patients in 85 year-olds. The diagnosis is established on the basis of clinical assessment as well as the results of neuropsychological tests, which indicate progressive cognitive loss.

PET scanning can be a valuable marker of the presence of Alzheimer's Disease, as it shows neuronal damage. It is noteworthy that in some people with Alzheimer's, the disease develops on a substrate of mild cognitive impairment, which becomes apparent on PET/CT, in the form of asymmetric areas of reduced glucose metabolism that combine with the clinical picture. The PET/CT scan shows areas of reduced glucose metabolism in both temporal and parietal regions, as well as in the posterior cingulate and pre-cingulate regions - a somewhat typical picture, as this type of 'pattern' of impairment is common in patients with early onset of the disease, i.e. under 60 years of age.

Amyloid in studies

In people with Alzheimer's disease, in addition to assessing cerebral glucose metabolism, the presence of amyloid β (Aβ) in the brain can be assessed. The most widely used radiopharmaceutical for amyloid assessment is Pittsburgh Compound B (PiB) labelled with carbon 11C - it has a high affinity for Aβ aggregates. Quantitative assessment of the presence of Aβ in the brain is extremely useful in the diagnosis of Alzheimer's, as its accumulation in the brain, represents the initial stage of the disease.

Dementia with Lewy Bodies (DLB) and PET/CT scanning

DLB represents the second most common dementing condition among the elderly population. It is characterised by the presence of Lewy bodies in the cortex, subcortex and brainstem. Moreover, DLB belongs to the whole spectrum of diseases with Lewy bodies, just like Parkinson's disease.

PET/CT in DLB patients shows reduced glucose consumption in both parietal lobes, the posterior temporal lobes and the posterior cingulate region. This is a pattern that is similar to that found in Alzheimer's Disease. In DLB, glucose consumption is also reduced in the occipital lobes, typically spared in Alzheimer's. However, there are cases in which the pattern of metabolic abnormalities is very similar to those found in Alzheimer's sufferers - in which case, a test to assess dopaminergic system activity helps in differentiation. PET/CT with 18F-DOPA, a radiopharmaceutical that accumulates within the presynaptic terminals of dopaminergic neurons, comes to the aid of such an assessment. In the course of DLB, decreased accumulation of 18F-DOPA is found in the caudate nucleus head and shell, and this picture is also observed in patients with Parkinson's disease and dementia.

photo: panthermedia

Rare neurodegenerative diseases

Corticobasal degeneration (CBD) typically manifests itself through asymmetric dystonia and a characteristic sensation of alien limb (alien hand syndrome). PET/CT shows an asymmetric decrease in the accumulation of radiopharmaceuticals in the sensorimotor cortex with involvement of the subcortical nuclei and thalamus on the same side. In addition, a reduction in 18F-FDG accumulation in the medial part of the cingulate cortex and asymmetric cortical atrophy in the frontal and parietal regions can also be observed.

Posteriorcortical atrophy (PCA), is also categorised as a rare neurodegenerative disease. It is also dominated by visual impairment and, it is considered a kind of Alzheimer's Disease, where the most common functional impairments, include cognitive impairment with memory and language skills. In PCA, the most common impairment is cortical visual function. PET/CT shows reduced glucose metabolism in the temporoparietal region and posterior cingulate and visual cortex. [1]