Regular articleA comprehensive study of gray matter loss in patients with Alzheimer’s disease using optimized voxel-based morphometry
Introduction
Alzheimer’s disease (AD) will become one of the major public health problems for the Western World in the coming decades. Aging of the population will lead to more and more people developing AD. In the long run, caring for AD patients will be a major financial backlash for the health care system Gutterman et al 1999, Trabucchi 1999. In order to provide care for these patients though, it is important to diagnose this disease and differentiate it from other dementia subtypes.
Medial temporal lobe (MTL) atrophy, as seen on MRI scans of AD patients, is a sensitive marker of AD, even in mild forms of the disease de Leon et al 1993, Fox et al 1996, Scheltens et al 1992. There are various ways to estimate brain atrophy on MRI. Visual inspection for the presence of atrophy (Scheltens et al., 1997) may be adequate in a routine clinical setting, but is not enough when quantitative measures are needed, for example, to estimate rate of tissue loss during a clinical trial. To assess such changes in a single brain structure, the region-of-interest (ROI) analysis technique may be employed (Pruessner et al., 2000), in which an experienced operator outlines the structure in question in a series of contiguous sections on a computer screen. ROI analysis constitutes, up to now, the gold standard in brain atrophy measurements (Jack et al., 1992), but there are major shortcomings such as observer/operator dependency and bias in brain structure and anatomical region boundary selection (Pruessner et al., 2000). In order to overcome these shortcomings, automated techniques have been developed Freeborough and Fox 1997, Freeborough and Fox 1998, Frisoni et al 1996, Smith et al 2001, Thompson et al 2000, the goal of which is to automatically analyze whole-brain structural MRI scans, avoiding a priori selection of regions and eliminating observer variability.
Voxel-based morphometry (VBM) is one such method developed for automated unbiased analysis of structural MRI scans. It has been used in a variety of settings and diseases (Ashburner and Friston, 2000), including VBM analysis of brain tissue loss in AD Baron et al 2001, Rombouts et al 2000. However, the initial implementation of VBM has led to debate about the limitations of affine and low-order registration algorithms Ashburner and Friston 2001, Bookstein 2001. It has been argued that imperfect registration of MRI scans to a common template can lead to false estimates of atrophy (Bookstein, 2001). Moreover, tissue classification errors during automated segmentation of brain tissue classes will produce an artificial thin rim of periventricular gray matter. In the current analysis, we tackled these two issues and applied dementia-specific VBM to study patterns of atrophy in AD compared to elderly controls, attempting to visualize similar atrophy patterns on the MRI analysis to the ones described neuropathologically Braak et al 1999, Braak and Braak 1991.
Section snippets
Subjects
Twenty-five patients with AD and twenty-five healthy controls of comparable age, recruited among spouses and friends of the patients, were included in the study. Written informed consent was obtained for all subjects. Approval of performance of MRI scans by local ethics committee was granted. All dementia subjects underwent a neuropsychological test battery. Cognitive function was measured using the Cambridge Mental Disorders of the Elderly Examination (CAMCOG) (Roth et al., 1986), which
Display of results
For qualitative analysis, mean gray matter images for each group were created and visually inspected (Fig. 5). These images did not demonstrate any collateral “spreading” of gray matter structures such as the caudate nucleus and medial thalamus. For the visualization of quantitative results areas of significant change were displayed in the coronal plane and overlaid on the group specific template (Fig. 3). We avoid the “glass brain” display, because we do not find it informative enough when
Discussion
It is well known that Alzheimer’s disease is characterized by cerebral atrophy. A recent review describes the current hypotheses about atrophy patterns in AD (Smith, 2002). In the very early stages only the entorhinal cortex is affected. Later, atrophy spreads to the hippocampus proper and the MTL. Projection neurons from the MTL die, disrupting their neurotrophic support to pyramidal cells in the association areas of the cortex. Based on projections from the MTL, the cortex in the
Conclusion
The optimized VBM we describe here allows a comprehensive in vivo analysis of patterns of atrophy in AD, and the results are in perfect agreement with histopathological findings. In addition to hippocampal and medial temporal lobe involvement, diffuse cortical atrophy with sparing of sensorimotor cortex, occipital poles, and cerebellum was noted. Furthermore, insula, caudate head, medial thalamus, and cingulum also appeared to be involved.
Acknowledgements
G. Karas is the recipient of Grant 2001-014 from Stichting Alzheimer Nederland and S.A.R.B. Rombouts is the recipient of Grant H00.17 from Hersenstichting Nederland. Additional funds were received from the Stichting Alzheimer and Neuropsychiatry Foundation, Amsterdam.
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