Quantitative MR imaging in Alzheimer disease

Structural neuroimaging in the detection and prognosis of pre-clinical and early AD

Current research supports the strong potential of structural MRI profiles, even within cross-sectional designs, as a promising method for the discrimination of Alzheimer's Disease (AD) from normal controls and for the prediction of Mild Cognitive Impairment (MCI) progression and conversion to AD. Findings suggest that measures of structural change in mesial and lateral temporal, cingulate, parietal and midfrontal areas may facilitate the assessment of a treatment's ability to halt the progressive structural loss that accompanies clinical decline in MCI. The performance of prediction is likely to continue to improve with the incorporation of measures from other neuroimaging modalities, clinical assessments, and neuromedical biomarkers, as the regional profile of individuals at risk for progression is refined.

MRI in dementia

With cognitive disorders increasingly common, clinicians urgently need faster and more accurate tools to classify such disorders and to noninvasively monitor therapeutic interventions. In this review, we provide information on MRI techniques that enable the study of the morphology, neuronal integrity, and metabolism of dementing illnesses. In addition, we explore the usefulness of such techniques as surrogate markers of these diseases.

Early structural changes in individuals at risk of familial Alzheimer's disease: a volumetry and magnetization transfer MR imaging study

Presenilin 1 (PS1) mutation carriers provide the opportunity to asses early features of neurodegeneration in familial Alzheimer's disease (AD). Gray matter (GM) regional volume loss and decrease of magnetization transfer ratio (MTR) consistent with microstructural changes have been reported in sporadic AD. We performed a regional volumetric and MTR analysis in carriers of PS1 mutations. Six non-demented mutated PS1 carriers (5 with memory deficits) and 14 healthy subjects were examined with high resolution T1-weighted images for volumetry and with T2* weighted images for MTR. Cortical GM volume and MTR values were derived. Compared to healthy controls, the GM volume of the left temporal and inferior parietal cortex and the MTR of the temporal cortex bilaterally were significantly decreased in PS1 gene carriers. In the latter, the temporal lobe MTR showed a trend for correlation with memory and executive function scores. Early neurodegeneration in non-demented subjects at risk for familial AD may be associated with atrophy and decreased MTR in the temporal cortex.

Sodium MR imaging detection of mild Alzheimer disease: preliminary study

BACKGROUND AND PURPOSE

There is significant interest in the development of novel noninvasive techniques for the diagnosis of Alzheimer disease (AD) and tracking its progression. Because MR imaging has detected alterations in sodium levels that correlate with cell death in stroke, we hypothesized that there would be alterations of sodium levels in the brains of patients with AD, related to AD cell death.

MATERIALS AND METHODS

A total of 10 volunteers (5 with mild AD and 5 healthy control subjects) were scanned with a 20-minute sodium (23Na) MR imaging protocol on a 3T clinical scanner.

RESULTS

After normalizing the signal intensity from the medial temporal lobes corresponding to the hippocampus with the ventricular signal intensity, we were able to detect a 7.5% signal intensity increase in the brains of patients with AD (AD group, 68.25% +/- 3.4% vs control group, 60.75% +/- 2.9%; P < .01). This signal intensity enhancement inversely correlated with hippocampal volume (AD group, 3.22 +/- 0.50 cm3 vs control group, 3.91 +/- 0.45 cm3; r2 = 0.50).

CONCLUSIONS

This finding suggests that sodium imaging may be a clinically useful tool to detect the neuropathologic changes associated with AD.

Alzheimer disease: quantitative structural neuroimaging for detection and prediction of clinical and structural changes in mild cognitive impairment

PURPOSE

To use structural magnetic resonance (MR) images to identify a pattern of regional atrophy characteristic of mild Alzheimer disease (AD) and to investigate whether presence of this pattern prospectively can aid prediction of 1-year clinical decline and increased structural loss in mild cognitive impairment (MCI).

MATERIALS AND METHODS

The study was conducted with institutional review board approval and compliance with HIPAA regulations. Written informed consent was obtained from each participant. High-throughput volumetric segmentation and cortical surface reconstruction methods were applied to MR images from 84 subjects with mild AD, 175 with MCI, and 139 healthy control (HC) subjects. Stepwise linear discriminant analysis was used to identify regions that best can aid discrimination of HC subjects from subjects with AD. A classifier trained on data from HC subjects and those with AD was applied to data from subjects with MCI to determine whether presence of phenotypic AD atrophy at baseline was predictive of clinical decline and structural loss.

RESULTS

Atrophy in mesial and lateral temporal, isthmus cingulate, and orbitofrontal areas aided discrimination of HC subjects from subjects with AD, with fully cross-validated sensitivity of 83% and specificity of 93%. Subjects with MCI who had phenotypic AD atrophy showed significantly greater 1-year clinical decline and structural loss than those who did not and were more likely to have progression to probable AD (annual progression rate of 29% for subjects with MCI who had AD atrophy vs 8% for those who did not).

CONCLUSION

Semiautomated, individually specific quantitative MR imaging methods can be used to identify a pattern of regional atrophy in MCI that is predictive of clinical decline. Such information may aid in prediction of patient prognosis and increase the efficiency of clinical trials.

Characterization of white matter damage in animal models of multiple sclerosis by magnetization transfer ratio and quantitative mapping of the apparent bound proton fraction f*

Quantitative magnetization transfer magnetic resonance imaging (qMT-MRI) can be used to improve detection of white matter tissue damage in multiple sclerosis (MS) and animal models thereof. To study the correlation between MT parameters and tissue damage, the magnetization transfer ratio (MTR), the parameter f* (closely related to the bound proton fraction) and the bound proton transverse relaxation time T2B of lesions in a model of focal experimental autoimmune encephalomyelitis (EAE) were measured on a 7T animal scanner and data were compared with histological markers indicative for demyelination, axonal density, and tissue damage. A clear spatial correspondence was observed between reduced values of MTR and demyelination in this animal model. We observed two different levels of MTR and f* reduction for these lesions. One was characterized by a pronounced demyelination and the other corresponded to a more severe loss of the cellular matrix. Changes in f* were generally more pronounced than those of MTR in areas of demyelination. Moreover, a reduction of f* was already observed for tissue where MTR was virtually normal. No changes in T2B were observed for the lesions. We conclude that MTR and qMT mapping are efficient and reliable readouts for studying demyelination in animal models of MS, and that the analysis of regional f* might be even superior to the analysis of MTR values. Therefore, quantitative mapping of f* from human brains might also improve the detection of white matter damage in MS.

Molecular Imaging and Magnetic Resonance Imaging in Early Diagnosis of Alzheimer's Disease

Alzheimer's disease (AD), a progressive neurodegenerative disorder, is the most common cause of dementia in the elderly. Magnetic resonance (MR) or computed tomography (CT) imaging is recommended for routine evaluation of dementias. The development of molecular imaging agents and the new techniques of MR for AD are critically important for early diagnosis, neuropathogenesis studies and assessing treatment efficacy in AD. Neuroimaging using nuclear medicine techniques such as SPECT, PET and MR spectroscopy has the potential to characterize the biomarkers for Alzheimer's disease. The present review summarizes the results of radionuclide imaging and MR imaging in AD.

Network analysis of intrinsic functional brain connectivity in Alzheimer's disease

Functional brain networks detected in task-free (“resting-state”) functional magnetic resonance imaging (fMRI) have a small-world architecture that reflects a robust functional organization of the brain. Here, we examined whether this functional organization is disrupted in Alzheimer's disease (AD). Task-free fMRI data from 21 AD subjects and 18 age-matched controls were obtained. Wavelet analysis was applied to the fMRI data to compute frequency-dependent correlation matrices. Correlation matrices were thresholded to create 90-node undirected-graphs of functional brain networks. Small-world metrics (characteristic path length and clustering coefficient) were computed using graph analytical methods. In the low frequency interval 0.01 to 0.05 Hz, functional brain networks in controls showed small-world organization of brain activity, characterized by a high clustering coefficient and a low characteristic path length. In contrast, functional brain networks in AD showed loss of small-world properties, characterized by a significantly lower clustering coefficient (p<0.01), indicative of disrupted local connectivity. Clustering coefficients for the left and right hippocampus were significantly lower (p<0.01) in the AD group compared to the control group. Furthermore, the clustering coefficient distinguished AD participants from the controls with a sensitivity of 72% and specificity of 78%. Our study provides new evidence that there is disrupted organization of functional brain networks in AD. Small-world metrics can characterize the functional organization of the brain in AD, and our findings further suggest that these network measures may be useful as an imaging-based biomarker to distinguish AD from healthy aging.

Alzheimer's disease (AD) is a brain disorder characterized by progressive impairment of episodic memory and other cognitive domains resulting in dementia and, ultimately, death. Functional neuroimaging studies have identified brain regions that show abnormal brain function in AD. Although there is converging evidence about the identity of these regions, it is not clear how this abnormality affects the functional organization of the whole brain. In order to characterize the functional organization of the brain, our approach uses small-world measures, which have also been used to study systems such as social networks and the internet. We use graph analytical methods to compute these measures of functional connectivity brain networks, which are derived from fMRI data obtained from healthy elderly controls and AD patients. The AD patients had significantly lower regional connectivity, and showed disrupted global functional organization, when compared to healthy controls. Moreover, our results indicate that cognitive decline in Alzheimer's disease patients is associated with disrupted functional connectivity in the entire brain. Our findings further suggest that small-world measures may be useful as an imaging-based biomarker to distinguish AD from healthy aging.

Altered neuronal gene expression in brain regions differentially affected by Alzheimer's disease: a reference data set

Alzheimer's Disease (AD) is the most widespread form of dementia during the later stages of life. If improved therapeutics are not developed, the prevalence of AD will drastically increase in the coming years as the world's population ages. By identifying differences in neuronal gene expression profiles between healthy elderly persons and individuals diagnosed with AD, we may be able to better understand the molecular mechanisms that drive AD pathogenesis, including the formation of amyloid plaques and neurofibrillary tangles. In this study, we expression profiled histopathologically normal cortical neurons collected with laser capture microdissection (LCM) from six anatomically and functionally discrete postmortem brain regions in 34 AD-afflicted individuals, using Affymetrix Human Genome U133 Plus 2.0 microarrays. These regions include the entorhinal cortex, hippocampus, middle temporal gyrus, posterior cingulate cortex, superior frontal gyrus, and primary visual cortex. This study is predicated on previous parallel research on the postmortem brains of the same six regions in 14 healthy elderly individuals, for which LCM neurons were similarly processed for expression analysis. We identified significant regional differential expression in AD brains compared with control brains including expression changes of genes previously implicated in AD pathogenesis, particularly with regard to tangle and plaque formation. Pinpointing the expression of factors that may play a role in AD pathogenesis provides a foundation for future identification of new targets for improved AD therapeutics. We provide this carefully phenotyped, laser capture microdissected intraindividual brain region expression data set to the community as a public resource.

Expanded high-resolution genetic study of 109 Swedish families with Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease that affects approximately 20 million persons all over the world. There are both sporadic and familial forms of AD. We have previously reported a genome-wide linkage analysis on 71 Swedish AD families using 365 genotyped microsatellite markers. In this study, we increased the number of individuals included in the original 71 analysed families besides adding 38 new families. These 109 families were genotyped for 1100 novel microsatellite markers. The present study reports on the linkage data generated from the non-overlapping genotypes from the first genome scan and the genotypes of the present scan, which results in a total of 1289 successfully genotyped markers at an average density of 2.85 cM on 468 individuals from 109 AD families. Non-parametric linkage analysis yielded a significant multipoint LOD score in chromosome 19q13, the region harbouring the major susceptibility gene APOE, both for the whole set of families (LOD=5.0) and the APOE varepsilon4-positive subgroup made up of 63 families (LOD=5.3). Other suggestive linkage peaks that were observed in the original genome scan of 71 Swedish AD families were not detected in this extended analysis, and the previously reported linkage signals in chromosomes 9, 10 and 12 were not replicated.

Alzheimer's disease and mild cognitive impairment

As our society ages, age-related diseases assume increasing prominence as both personal and public health concerns. Disorders of cognition are particularly important in both regards, and Alzheimer's disease is by far the most common cause of dementia of aging. In 2000, the prevalence of Alzheimer's disease in the United States was estimated to be 4.5 million individuals, and this number has been projected to increase to 14 million by 2050. Although not an inevitable consequence of aging, these numbers speak to the dramatic scope of its impact. This article focuses on Alzheimer's disease and the milder degrees of cognitive impairment that may precede the clinical diagnosis of probable Alzheimer's disease, such as mild cognitive impairment.

Diffusion tensor imaging of normal-appearing white matter in mild cognitive impairment and early Alzheimer disease: preliminary evidence of axonal degeneration in the temporal lobe

BACKGROUND AND PURPOSE

Diffusion tensor imaging (DTI) is a sensitive technique for studying cerebral white matter. We used DTI to characterize microstructural white matter changes and their associations with cognitive dysfunction in Alzheimer disease (AD) and mild cognitive impairment (MCI).

MATERIALS AND METHODS

We studied elderly subjects with mild AD (n = 6), MCI (n = 11), or normal cognition (n = 8). A standardized clinical and neuropsychological evaluation was conducted on each subject. DTI images were acquired, and fractional anisotropy (FA), axial diffusivity (DA), and radial diffusivity (DR) of normal-appearing white matter (NAWM) in frontal, temporal, parietal, and occipital lobes were determined. These diffusion measurements were compared across the 3 groups, and significant differences were further examined for correlations with tests of cognitive function.

RESULTS

Compared with normal controls, AD subjects demonstrated decreased FA and increased DR in the temporal, parietal, and frontal NAWM and decreased DA in temporal NAWM. MCI subjects also showed decreased FA and decreased DA in temporal NAWM, with decreased FA and increased DR in parietal NAWM. Diffusion measurements showed no differences in occipital NAWM. Across all subjects, temporal lobe FA and DR correlated with episodic memory, frontal FA and DR correlated with executive function, and parietal DR significantly correlated with visuospatial ability.

CONCLUSIONS

We found evidence for functionally relevant microstructural changes in the NAWM of patients with AD and MCI. These changes were present in brain regions serving higher cortical functions, but not in regions serving primary functions, and are consistent with a hypothesized loss of axonal processes in the temporal lobe.

Severe Amnesic Syndrome and Collecting Behavior After Surgery for Craniopharyngioma

Significant neuropsychologic sequelae were induced by total removal of craniopharyngioma via a frontobasal interhemispheric approach. A 50-year-old right-handed man developed severe amnesic syndrome and collecting behavior after total removal of a craniopharyngioma. He had very poor results on tests of learning, recall, and recognition for both verbal and nonverbal tasks. Magnetic resonance imaging revealed damage to the bilateral mammillary bodies and fornices, and N-isopropyl-p-[23I]iodoamphetamine single photon emission computed tomography showed decreased cerebral blood flow in the bilateral frontal lobes, predominantly in the right, and regions around the third ventricle. The present case suggests that damage to the brain structures surrounding the third ventricle associated with surgery for craniopharyngioma may result in amnesic syndrome and collecting behavior. Generally, the frontobasal interhemispheric approach is the optimum choice for the removal of craniopharyngioma without significant sequelae related to the surgical method, but the risk of neuropsychologic disturbances must be kept in mind.