Normal structural changes
During routine physiological aging, the brain loses volume. This manifests as enlarged ventricles (including the third ventricle) and deepened sulci, which is especially noticeable in the frontal and parietal regions. Mild atrophy of the internal structures of the temporal lobe is also characteristic.
The Virchow-Robin spaces (perivascular spaces) become wider. On CT, they can be seen in the basal ganglia near the anterior commissure, and on MRI, in the white matter of the centrum semiovale and the mesencephalon (midbrain).
Vascular bed changes are also observed: arteries (such as the basilar artery) elongate and become tortuous. The walls of the internal carotid and vertebral arteries may thicken and become calcified. MRI also frequently reveals punctate ischemic lesions in the white matter, lacunae, and microhemorrhages.
Tissue deposits
Aging is accompanied by the accumulation of various substances within brain structures, which is clearly visualized using diagnostic imaging methods.
- Iron: accumulates in the dentate nucleus (dentate nucleus), putamen (putamen), and globus pallidus (globus pallidus). Detected on MRI.
- Calcium: deposited in the globus pallidus, pineal gland, cerebral falx (cerebral falx), and choroid plexuses (especially near the foramen of Luschka). Bone transformation is sometimes possible. These changes are clearly visible on CT.
Neuroimaging and diagnostic patterns
CT, MRI, and molecular imaging (PET, SPECT) are used to assess age-related and pathological changes.
Functional methods make it possible to detect disease at the physiological level before structural atrophy appears. For example, frontotemporal lobar degeneration shows a decrease in metabolism, while in Alzheimer's disease (AD), amyloid-PET reveals pathological tracer accumulation in the cortex (normally, it binds only to the white matter).
Atrophy patterns in various diseases:
- Alzheimer's disease: hippocampus, medial temporal lobe, precuneus (precuneus).
- Frontotemporal lobar degeneration: temporal pole and frontal lobe.
- Progressive supranuclear palsy: midbrain (mesencephalon).
- Multiple system atrophy: pons (pons).
Alzheimer's disease: mechanism and assessment
AD is based on a pathological cascade: the accumulation of amyloid and tau protein triggers the formation of plaques and neurofibrillary tangles, leading to neuronal death. The process starts in the medial temporal lobe (hippocampus and entorhinal cortex) or posterior cingulate cortex, spreading further.
The main clinical sign is episodic memory impairment, although language and visuospatial deficits may come to the forefront in younger patients. Diagnosis requires confirmation of impairment in two different cognitive domains.
MRI (coronal slices) is used to assess the degree of atrophy in AD. To avoid complex volumetric analysis, the MTA visual rating scale is used. It takes into account three criteria (scored from 0 to 4):
- The width of the temporal horn.
- The width of the choroidal fissure.
- The height of the hippocampus.
Briefly: answers to topical questions
Which cognitive domains can be impaired in the early stages of Alzheimer's disease?
In the early stages of Alzheimer's disease, memory is primarily impaired, and visuospatial functions and speech may also be affected.
- Episodic memory — impairments in this area are the primary symptom of the disease.
- Visuospatial functions — impairments may predominate in younger patients.
- Language functions — speech problems may predominate in younger patients.
Establishing a clinical diagnosis requires the identification of impairments in at least two separate cognitive domains.
Which radiopharmaceuticals are used in PET for the differential diagnosis of dementias?
For the differential diagnosis of dementias during PET, amyloid and dopaminergic tracers, as well as agents for assessing metabolism, are used.
- [11C]-PIB — used in amyloid PET to detect Alzheimer's disease, which exhibits extensive tracer accumulation in the cerebral cortex.
- Dopaminergic tracers — used for the specific diagnosis of dementia with Lewy bodies.
- Fluorodeoxyglucose (FDG) — used in PET (FDG-PET) to detect metabolic decline in the early stages of frontotemporal lobar degeneration without visible atrophy.
The isotopes most commonly used for positron emission tomography are 18F or 11C.
Class and exam questions
What is the difference in amyloid imaging between normal aging and Alzheimer's disease?
During amyloid PET in healthy individuals, the tracer binds only to the white matter. In Alzheimer's disease, extensive tracer accumulation is observed in the cerebral cortex.
How is hippocampal atrophy assessed on MRI?
The visual MTA scale on coronal slices is used. The score from 0 to 4 is based on hippocampal height, temporal horn width, and choroidal fissure width.
How many cognitive domains must be impaired to diagnose primary dementia?
For a clinical diagnosis, it is necessary to confirm the presence of impairments in at least two separate cognitive domains.
What else to study on this topic
- Genetic and cardiovascular risk factors for Alzheimer's disease (including the role of APOE4)
- Molecular imaging with dopaminergic tracers in dementia with Lewy bodies
- Predicting the progression of mild cognitive impairment (MCI) to Alzheimer's disease
- Assessing cerebrovascular lesions (infarcts, lacunae) using CT and MRI protocols
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