Neuroimaging is increasingly being incorporated as a biomarker in the diagnostic criteria for Alzheimer’s disease to increase diagnostic certainty . The most common imaging finding of Alzheimer’s disease is cortical atrophy based on underlying neuronal loss. This atrophy predominantly affects symmetrically the parietal and temporal lobes. Late-onset Alzheimer’s disease and patients who present with APOE E4 polymorphism show, in particular, hippocampal atrophy (illustrated in Fig. 8), whereas the central region is relatively spared . A significant asymmetric atrophy of the hippocampi does not exclude the diagnosis; however, this type of atrophy is more typical for alternative causes, such as frontotemporal dementia.
Patients with an atypical Alzheimer’s disease variant, such as early-onset Alzheimer’s disease or a missing APOE E4 genotype show only mild or no hippocampal atrophy, but, instead, present with considerable posterior cortical atrophy, e.g., the precuneus region in early-onset Alzheimer’s disease (illustrated in Fig. 9) . In patients with posterior cortical atrophy, the parieto-occipital and the posterior temporal cortices are most commonly affected asymmetrically, more prominently on the right side, which results in an early visual or visuospatial impairment ahead of cognitive decline . Patients with logopenic progressive aphasia show an asymmetric more prominent left-sided atrophy of the posterior temporal cortex and the inferior parietal lobe, which results in language-related impairments .
After Alzheimer’s disease, vascular dementia is the second most frequent type of dementia in the elderly. Vascular dementia summarizes various vascular pathologies, including multiple territorial ischemic infarcts, strategic ischemic infarcts (e.g., thalamic infarcts), multiple lacunar infarcts, Binswanger’s disease, cerebral autosomal dominant arteriopathy with subcortical infarcts, and leukoencephalopathy (CADASIL) or cerebral amyloid angiopathy. The difficulty with vascular dementia is the common overlap with changes in the normal aging brain or other diseases that cause dementia. Moreover, the frequent co-existence of Alzheimer’s disease and vascular disease raise suspicion about a direct causality between these two disease entities, often exacerbating pre-existing clinical or subclinical pathologies [9, 10].
Vascular dementia presents with small- and large-vessel disease mainly leading to confluent white matter changes, lacunar infarcts, and/or postischemic cortical/subcortical cerebrovascular lesions. Based on the NINDS-AIREN criteria, the diagnostic criteria for vascular dementia are as follows: a) confluent white matter changes involving 25% of the total white matter; b) lacunar infarcts involving the frontal white matter, multiple basal ganglia, as well as both thalami; or c) large-vessel infarcts involving bilateral anterior cerebral artery territories, the paramedian thalamic territory, the inferior medial temporal lobe, the parieto-temporal or temporo-occipital association areas, the angular gyrus, and the superior frontal and parietal watershed areas in the predominant hemisphere.
The imaging features are characterized by periventricular white matter hyperintensities, which can range from punctate to confluent (Fazekas 1–3), cortical/subcortical ischemic/postischemic lesions, lacunar infarcts, enlarged Virchow-Robin spaces, and/or microbleeds (see Fig. 10) [11, 12].
A particular subgroup of vascular disorders represents cerebral amyloid angiopathy, which results from amyloid deposits in the walls of blood vessels. Cerebral amyloid angiopathy is common in Alzheimer’s disease but is also found in the absence of Alzheimer’s disease neuropathological changes .
Neuroimaging is notable for diffuse, progressive white matter hyperintensities, microbleeds located in the cortical gray-white matter junction, superficial hemosiderosis, as well as acute intraparenchymal bleeding or post-hemorrhagic parenchymal defects (illustrated in Fig. 11) [14, 15].
Another special subgroup of vascular disease is CADASIL, which is a hereditary vasculopathy based on mutations in the NOTCH3 gene [16, 17]. Patients with CADASIL show extensive white matter hyperintensities, lacunar infarctions, and hemorrhage mainly in the insulae and the anterior temporal lobes (illustrated in Fig. 12) .
Frontotemporal dementia consists of three different subtypes: the behavioral variant (40%); the progressive non-fluent aphasia (20%); and the semantic dementia (40%).
Patients who suffer from the behavioral variant of frontotemporal dementia show typically asymmetrical frontal and temporal cortical atrophy, with a gradient of the imaging findings from anterior to posterior, and a widening of the orbitofrontal sulci as one of the first signs of disease manifestation (see Fig. 13). Consequently, also the frontal horns of the lateral ventricles are widened disproportionately in the course of the disease. Atrophy also includes the insula, the anterior cingulate, the amygdala, the thalamus, and the striatum [19,20,21].
In patients with progressive non-fluent aphasia, the cortical atrophy is emphasized in the left-sided anterior perisylvian region, including the opercular and the insular regions as well as the premotor cortex, which results in effortful speech or agrammatic language production [22, 23].
In patients with semantic dementia, the cortical atrophy is particularly seen in the anterior and inferior temporal lobes (ventral and lateral regions), including the amygdala and the anterior hippocampus, again with a left-hemispheric predominance and an anterior-posterior gradient leading to an impairment of semantic performance, particularly impaired object knowledge and single-word comprehension [22, 23].
The logopenic progressive aphasia, the progressive non-fluent aphasia, and the semantic dementia can also be summarized as primary progressive aphasia.
Dementia with Lewy bodies
Patients who suffer from dementia with Lewy bodies commonly show progressive cognitive impairments, especially in executive and visuospatial functions, followed by memory. The main clinical deviation compared to Alzheimer’s disease is, among others, the additional occurrence of visual hallucinations in patients who suffer from dementia with Lewy bodies. Moreover, on MR images the medial temporal lobes are relatively preserved in contrast to patients with Alzheimer’s disease .