Brain Advance Access originally published online on July 13, 2005
Brain 2005 128(11):2626-2644; doi:10.1093/brain/awh589
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Measurement of basal forebrain atrophy in Alzheimer's disease using MRI
1 Department of Psychiatry, Alzheimer Memorial Center and Geriatric Psychiatry Branch, Dementia and Neuroimaging Section, Ludwig-Maximilian University, 2 Clinical Radiology, Ludwig-Maximilian University, Grosshadern, Munich and 3 Morphological Brain Research Unit, University Würzburg, Germany
Correspondence to: Stefan J. Teipel, MD, Department of Psychiatry, Alzheimer Memorial Center and Geriatric Psychiatry Branch, Dementia and Neuroimaging Section, Ludwig-Maximilian University, Nussbaumstrasse7, 80336 Munich, Germany E-mail: stefan.teipel{at}med.uni-muenchen.de
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Alzheimer's disease is characterized by the degeneration and loss of cholinergic neurones in the nucleus basalis Meynert, located within the substantia innominata at the ventral surface of the basal forebrain. An in vivo measure of morphological changes in the nucleus basalis Meynert would be of high relevance to better understand the structural correlate of cholinergic dysfunction in Alzheimer's disease. In this study, we applied a newly developed automated technique of image regression analysis, implemented through code written in Matlab 5.3 (MathWorks, Natick, MA), to the analysis of proton density weighted structural MRI of the basal forebrain from 13 patients with Alzheimer's disease (mean age = 77.5 years, SD = 4.4 years, 8 women) and 12 healthy elderly subjects (mean age = 62.3 years, SD = 5.6 years, 6 women). This technique allows searching a large portion of the substantia innominata for signal changes. We used corresponding MRI and histological sections of a post mortem brain to map the locations of basal forebrain cholinergic nuclei into the MRI standard space. Additionally, we used voxel-based morphometry, implemented in SPM2 (Wellcome Department of Imaging Neuroscience, London, UK) to determine correlations between signal changes in the substantia innominata and cortical grey matter atrophy in the patients with Alzheimer's disease. When matching the locations of signal reductions in the in vivo MRI to the template of basal nuclei based on the postmortem brain, signal intensity was decreased in areas corresponding to anterior lateral and anterior medial nucleus basalis Meynert and increased in the third ventricle, the transverse fissure and the optic tract in patients with Alzheimer's disease compared with controls. The reduction of the signal intensity in an area corresponding to the anterior lateral nucleus basalis Meynert was significantly correlated with reduced grey matter concentration in the bilateral prefrontal cortex, inferior parietal lobule and cingulate gyrus. Our findings suggest that signal changes occur in patients with Alzheimer's disease in the substantia innominata which may be related to the loss or degeneration of cholinergic neurones and correspond to regional cortical grey matter atrophy. If replicated in an independent sample, our technique may be useful to detect degeneration of basal forebrain cholinergic neurones in vivo.
Key Words: Alzheimer's disease; neocortex; neurodegeneration; nucleus basalis Meynert
Abbreviations: MNI = Montreal Neurological Institute stereotaxic coordinate; ROI = region of interest; SPM = Statistical Parametric Mapping
Received January 27, 2005. Revised June 10, 2005. Accepted June 13, 2005.
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Introduction |
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Neuropathological data indicate a specific vulnerability of the cholinergic system, including both cortical neurones receiving cholinergic projections and cholinergic neurones projecting to the cortex, in Alzheimer's disease. Post mortem examinations show severe neuronal loss in the basal forebrain cholinergic nuclei (Vogels et al., 1990
; Lehericy et al., 1993; Cullen and Halliday, 1998) and a decrease of choline acetyltransferase and cholinesterase activity in the cerebral cortex in Alzheimer's disease (Ruberg et al., 1990). Furthermore, anticholinergic drugs induce memory dysfunction both in animal models (Harder et al., 1998; Schildein et al., 2002) and humans (Atri et al., 2004). Experimental lesions of cholinergic nuclei impair memory function in non-human primates (Muir et al., 1993). In vivo studies using PET markers of nicotinic receptor binding (Nordberg, 1993), choline acetyltransferase activity and cholinesterase activity (Herholz et al., 2004) demonstrated reduction of cortical cholinergic function in Alzheimer's disease. Together, these findings suggest that cholinergic dysfunction plays an important role in the cognitive impairment associated with Alzheimer's disease. Consistently, clinical studies suggest that enhancement of cholinergic function using (acetyl-)cholinesterase inhibitors attenuates cognitive decline in Alzheimer's disease (Birks and Melzer, 2000). The main source of cholinergic projections to the cerebral cortex are the magnocellular neurones of the nucleus basalis Meynert in the substantia innominata which separates the globus pallidus from the ventral surface of the basal forebrain (Fig. 1).
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The question is still unresolved whether degeneration of basal forebrain cholinergic neurones is primary or secondary to cortical neurodegeneration in Alzheimer's disease (Mesulam, 2004
), which is at least partially attributable to the fact that neuropathological evidence is mainly confined to advanced stages of disease and does not allow for longitudinal examinations. Therefore, an in vivo measure of morphological changes in the area of the nucleus basalis Meynert would be of high relevance to better understand the structural correlate of cholinergic dysfunction in Alzheimer's disease. A first approach to establish in vivo morphometry of the sites of cholinergic projections was demonstrated by Hanyu et al. (2002) measuring the thickness of the substantia innominata at its smallest extent in a coronal section through the anterior commissure. The conclusiveness of the data, however, was limited because the 2D measure of thickness was not accurately located within the substantia innominata and provided only a very rough estimate of atrophy.
In the present study, we used a new image regression-based analysis with a proton weighted MRI sequence to search the substantia innominata for signal differences between patients with Alzheimer's disease and healthy elderly subjects. The technique is virtually fully automated and thus, almost independent from subjective judgements, rendering it a highly reliable and fast-to-apply analysis tool. It allows to examine a large area of the substantia innominata and to anatomically locate signal changes within the volume of interest. The location and dimensions of the volume of interest were determined on the basis of morphometric data of the nucleus basalis Meynert previously published (Halliday et al., 1993) and the distribution of large neurones of the nucleus basalis Meynert localized within histological sections of a single brain (Fig. 1A). We investigated the differences of signal intensity in the substantia innominata between patients with Alzheimer's disease and healthy controls and matched the locations of signal differences to the locations of cholinergic nuclei obtained from a post-mortem brain and transferred into the MRI space using post mortem MRI data. In addition, we investigated whether signal changes in the substantia innominata were correlated with changes in regional cortical grey matter volume in patients with Alzheimer's disease.
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Post mortem data
To determine borders of nucleus basalis Meynert for the subsequent region of interest (ROI) analysis of in vivo data, one post mortem brain was processed both using MRI and histological sectioning to determine the regional distribution of the large (cholinergic) nucleus basalis Meynert neurones. The brain was from a 76-year-old woman who had died of myocardial infarction with a relapsing course. Clinical history revealed type II diabetes mellitus 2 years and a partial gastrectomia 6 years prior to death. No neuropsychological testing had been performed prior to death. However, according to her neighbours, the patient had managed her household alone until her sudden death. The diabetes had regularly been medicated by conventional oral antidiabetics.
Fresh brain weight was 1275 g. From the unstained 440 µm thick serial sections through the brain small tissue segments (
12 mm x 8 mm) from the basal forebrain, the rostral parts of the globus pallidus, the rostral parahippocampal gyrus and the neighbouring fusiform gyrus, hippocampus at the level of the lateral geniculate body and the dentate nucleus of the left cerebellar hemisphere were cut out and embedded in paraffin. From these paraffin embedded tissue segments about 20–30 sections of 12.5 µm thickness were made. These sections were stained with routine neuropathological stains including HE, Gallyas' impregnation for the demonstration of Alzheimer fibrillary tangles and the Campbell–Switzer–Martin (Campbell et al., 1987) stain for the demonstration of amyloid plaques. No amyloid depositions were found in the entorhinal region and in the cortex of the adjacent fusiform gyrus. A maximum of three tangle-bearing cells were found in the superficial layers of the entorhinal and transentorhinal cortex, whereas the majority of the pre-alpha cells (between 80 and 130 cells per visual field) were free from tangles. These findings most probably indicate age-related increased tangles and make it unlikely that the patient had even stage I of Alzheimer's disease according to Braak and Braak (1991).
Post mortem MRI
The complete brain was immersion-fixed in 10% formalin (1 part commercial 40% aqueous formaldehyde, 9 parts tap water) for 8 months. The formalin fixed brain was investigated with an MPRAGE sequence (TR 1570 ms, TE 3.93 ms, isotropic voxel with 0.9 mm3) using a 1.5-T Magnetom Sonata Maestro Class MRI scanner (Siemens Medical Solutions, Erlangen, Germany). The scan was in axial orientation parallel to the anterior–posterior commissure line. The data were reconstructed in the orthogonal coronal plane for comparison with the histological sections.
After immersion in formalin, brain weight and brain volume (determined by displacement of formalin) were monitored in regular intervals. There was a slight 3.5% increase (3.5%) in volume during the first 14 days in formalin. After this time, the total brain volume decreased to reach its final volume that was 0.3% higher than the initial volume of the unfixed brain after 2 months in formalin. The brain was left in formalin for 35 months and its volume remained unchanged during this period. The volume of the left formalin-fixated hemisphere determined by displacement of formalin 35 months after death was 461 cm3, and the volume of the same left hemisphere as determined by MRI volumetry was 475 cm3. Dehydration in 95% ethanol decreased the volume of the left hemisphere to 382 cm3 (392 cm3 as determined by MRI).
Staining of brain sections
The detailed procedure of fixation, dehydration, celloidin mounting and gallocyanin staining of the hemisphere has previously been described (Heinsen et al., 2000). In brief, the formalin-fixed brain was divided mediosagittally, the left hemisphere was dehydrated in graded series of ethanol solutions (70, 80, 96%) for 1 week per stage and soaked in 8% celloidin. The 8% celloidin was concentrated to a 16% solution by vacuum evaporation and afterwards hardened by chloroform vapours. This hardened celloidin block mantling the complete left hemisphere was sectioned on a sliding microtome at a thickness of 440 µm yielding a total of 342 slices. After rinsing in water, every second slice was stained with gallocyanin, a Nissl stain modified by Einarson (1932), according to the following procedure: the 440 µm thick celloidin sections were rehydrated from 70% alcohol to distilled water. Afterwards, they were placed overnight in performic acid (1 part of 100% formic acid to 3 parts of 30% H2O2) to enhance the selectivity of the gallocyanin stain (Merker, 1983), thoroughly rinsed in tap water and then stained in a gallocyanin solution [1.5 g gallocyanin dissolved in 5% potassium chromium(III)sulphate dodecylhydrate] for 3 h, dehydrated in alcohol and xylene, and finally coverslipped in Permount® (Heinsen and Heinsen, 1991). Figure 2 shows the serial 440 µm thick gallocyanin stained sections (every second section) through the basal forebrain region.
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Distribution of large cholinergic neurones in MNI space
The basalis Meynert nuclei were identified on the digitalized cryo-gallocyanin stained sections of the left hemisphere (Fig. 3). The location of the nuclei then was manually transferred to the corresponding slices of the post mortem MRI sequence in native space using manual drawing of ROI. Using an affine transformation of the post mortem MRI scan into Montreal Neurological Institute stereotaxic coordinate (MNI) standard space with FLIRT (FMRIB Software Library, Release 3.2, University of Oxford), we derived a 12-parameter transformation matrix from post mortem space to MNI space. This matrix then was applied to the regions of interest, resulting in an approximate distribution of basalis Meynert nuclei in MNI space. These regions then were projected onto a PD sequence in MNI space to generate a template brain for the localization of signal changes compared with nucleus basalis Meynert morphology.
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Subjects
Thirteen subjects with the clinical diagnosis of probable Alzheimer's disease according to NINCDS-ADRDA criteria, (McKhann et al., 1984
) and 12 healthy elderly control subjects underwent MRI scans. Gender distributions were similar: 8 women and 5 men in the Alzheimer's disease group and 6 women and 6 men in the comparison group (Pearson 
2-test: 2 = 0.37, df = 1, P = 0.56). The mean ages differed between the Alzheimer's disease and control groups: 77.5 years (SD = 4.4) and 62.3 years (SD = 5.6) (two-tailed t-test: t = 7.6, df = 23, P < 0.001).
Cognitive impairment in the patients with Alzheimer's disease was assessed using the Mini Mental State Examination (Folstein et al., 1975). Mean Mini Mental State Examination scores were different between groups, with 23.1 (SD = 4.1) in the Alzheimer's disease group and 29.4 (SD = 0.7) in the control group (Mann–Whitney U = 11, P < 0.001).
Medical co-morbidity in patients and controls was excluded by history, physical and neurological examination, psychiatric evaluation, chest X-ray, ECG, EEG and laboratory tests. Cranial MRI examinations were used to exclude cerebrovascular disease (operationalized as no subcortical white matter hyperintensities exceeding 10 mm in diameter or 3 in number on T2-weighted scans) and intracerebral neoplasias. In contrast, the extent of atrophy on MRI scans was not a part of the diagnostic decision process.
All subjects signed consent forms to undergo MRI and neuropsychological assessment for clinical investigation and research. The protocol was approved by the Ethical Review Board of the Medical Faculty of the Ludwig-Maximilian University, Munich, Germany.
MRI
MRI examinations were performed on a 1.5 T Siemens Magnetom Vision MRI scanner (Siemens, Erlangen, Germany). All subjects were investigated with a volumetric T1 weighted sagittally oriented MRI sequence (TR = 11.6 ms, TE = 4.9 ms, spatial resolution = 0.94 mm by 0.94 mm by 1.2 mm), subsequently named 3D sequence. Additionally all subjects underwent an 11 slices proton weighted coronally oriented inversion recovery sequence (TR/TE 3000/30 ms, inversion time 150 ms, spatial resolution = 0.7 x 0.7 x 3 mm), subsequently named PD sequence. This turbo-spin-echo sequence with seven spin-echoes was oriented perpendicular to the anterior–posterior commissure line based on a prior scout sequence and started 9 mm anterior to the anterior commissure.
Data processing for image regression analysis
The analysis was done using Statistical Parametric Mapping [(SPM2) Wellcome Department of Imaging Neuroscience, London; available at http://www.fil.ion.ucl.ac.uk/spm] (Friston et al., 1995a, b) and a code written in Matlab 5.3 (MathWorks, Natick, MA). The MRI scans were processed in five subsequent steps.
Manual preprocessing
The 3D and PD scans were manually reoriented, with the interhemispheric gap parallel to the vertical axis of the field of view and the anterior–posterior commissure line parallel to the horizontal axis. The origin was manually assigned to the anterior commissure. The reorientation matrix (6-parameter rigid body transformation) was stored.
Normalization of 3D sequence
Each 3D scan was normalized to the standard MNI T1 MRI template (Evans et al., 1993) using a set of non-linear basis functions (Ashburner et al., 1997; Ashburner and Friston, 2000). Spatial normalization used residual sum of squared differences as the matching criterion and included affine transformations and linear combination of smooth basis functions modelling global non-linear shape differences. The normalization parameters were stored.
Normalization of PD sequence
Each PD scan was co-registered to the corresponding 3D scan using a 12-parameter affine transformation with mutual information cost function. The affine transformation matrix was stored. The non-linear normalization parameters of the corresponding 3D scan were then combined with the affine transformation matrix and applied to the PD scan in native space, resulting in a PD scan non-linearly normalized in MNI space (Evans et al., 1993).
Signal inhomogeneity correction
Normalized PD scans were subjected to non-uniformity correction to remove smoothly varying modulations of image intensities related to inhomogeneities in the magnetic field. The method is implemented in SPM2 and minimizes the entropy of the probability distribution of log-transformed intensities (Ashburner, 2002).
Definition of search volumes
The substantia innominata has no clear anatomical borders at its anterior, posterior and lateral extents. Figure 1A indicates the lateral extension of large nucleus basalis Meynert neurones within the substantia innominata in the brain of an elderly woman. This lateral extension agrees with the lateral distance from midline below 25 mm in a previous morphometric study (Halliday et al., 1993). In the same study the nucleus basalis Meynert extended 4 mm anterior and 10 mm posterior to the posterior edge of the anterior commissure (Halliday et al., 1993). Therefore, for each scan two square ROI were defined, one for the left and the other for the right hemisphere, based on the location of the anterior commissure, which forms the boundary of the superior part of the end of the anterior third of the substantia innominata. The ROI extended 25 mm lateral from the midline, 13 mm ventral from the superior edge of the anterior commissure at the midline, and 3 mm anterior and 9 mm posterior from the middle of the anterior commissure, covering a volume of 3900 mm3 in each hemisphere (Fig. 4). The ROI was automatically determined after three coordinates were manually defined by one rater (S.S.) blinded to subject's identity: the middle of the anterior commissure in the left–right direction (x-coordinate), the middle of the anterior commissure in the anterior–posterior direction (y-coordinate), and the superior border of the anterior commissure in cranial–ventral direction (z-coordinate). All ROI defined such were smoothed with a 4 mm full width at half maximum isotropic Gaussian kernel. The smoothed ROI were the volumes of interest for subsequent statistical analyses.
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Reliability of coordinates
Starting coordinates (x, y and z) for the ROI were determined in all 25 subjects by two independent raters (S.S. and S.J.T.) to estimate interrater reliability.
Data processing for cortical grey matter maps
The processing of the 3D sequences followed a previously described protocol (Teipel et al., 2004). In brief, the 3D scans in standard space were smoothed (12 mm full width at half maximum isotropic Gaussian kernel) and averaged to obtain a group-specific T1 template. All structural MRI in native space were then normalized to this template. The normalized MRI were segmented into CSF, grey matter and white matter compartments using the SPM2 priors (Ashburner and Friston, 1997). Next, CSF, grey matter and white matter images were smoothed with an 8 mm kernel and averaged to obtain group-specific CSF, grey matter and white matter priors for later segmentation of native MRI scans. In addition, grey matter images were smoothed with a 12 mm kernel and averaged to obtain a group specific grey matter template.
The original MRI scans were segmented using the group-specific T1 template and grey matter, white matter and CSF priors. This segmentation step involves an affine transformation of each scan to the template with a subsequent back-projection into native space. Next, an automated brain extraction procedure that incorporated a segmentation step was used to remove non-brain tissue (Good et al., 2001). The extracted grey matter images were then normalized to the group-specific grey matter template. The normalization parameters were then applied to the original structural images in native space thereby reducing any contribution from non-brain voxels and affording optimal spatial normalization of grey matter. These normalized images were resliced to a final voxel size of 1.0 mm3 and segmented into grey and white matter, and CSF partitions. Finally, all normalized, segmented images were smoothed with a 12 mm full width at half maximum isotropic Gaussian kernel.
Statistical analysis
Image regression analysis
For statistical analysis, we employed the general linear model on a voxel basis. Prior to regression analysis, ROI were proportionally scaled to the global mean and thresholded at 20% of global intensity. Proportional scaling to the global mean allows detection of voxels with a relative decrease or a relative increase of signal intensity compared with global signal intensity. We considered significant effects in the negative and positive directions. Owing to the relatively small search volume and the a priori specified hypothesis on the location of effects within this volume, results were thresholded at an uncorrected P-level < 0.01, and an extent threshold of five contiguous voxels was applied. Independent multiple regression models were calculated for the effect of diagnosis (Alzheimer's disease versus controls) and for the effect of diagnosis (Alzheimer's disease versus controls) controlled for age.
Signal intensity in lateral substantia innominata and cortical grey matter
We regressed voxel based cortical grey matter signal on signal intensity in the substantia innominata in the patients with Alzheimer's disease. To obtain substantia innominata signal for each patient, we selected voxels that had peak T-values in the age-controlled group comparison (Alzheimer's disease < controls) and were located in the lateral substantia innominata. Talairach and Tournoux coordinates were x = 18, y = 5 and z = –9 for the right, and x = –21, y = 5 and z = –10 for the left hemisphere (Table 2). Prior to regression analysis, cortical grey matter maps were proportionally scaled to the global mean and thresholded at 40% of global intensity to reduce the influence of any remaining non-brain tissue. We considered significant effects in the positive direction. Results were thresholded at a P level < 0.005, uncorrected for multiple comparisons, and an extent threshold of 50 contiguous voxels was applied.
Localization
Localization of peak effects was based on the coordinates from the MNI template. We used a non-linear algorithm provided by Matthew Brett (MRC Cognition and Brain Sciences Unit, Cambridge, UK) (Brett et al., 2002), to transform MNI into Talairach coordinates. Peak effects then were identified from the Talairach and Tournoux atlas (Talairach and Tournoux, 1988) based on these coordinates.
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Comparison of signal intensity uncorrected for age
In the right hemisphere (Fig. 5A) a large cluster of reduced signal intensity in patients with Alzheimer's disease compared with controls was observed at the level of the anterior commissure in the lateral substantia innominata extending into the medial substantia innominata, globus pallidus and putamen (Table 1). When matching the locations of signal reductions to the template of basal nuclei based on the post mortem brain, signal was reduced in posterior Ch2, Ch3, Ch 4am and Ch 4al (slices Talairach–Tournoux y-coordinates 4 to –2), according to Mesulam's nomenclature (Mesulam et al., 1983
). Signal was not reduced in areas corresponding to Ch 4i (y-coordinates –3 to –5). Areas of Ch 4p and anterior Ch 2 were not covered by the MRI sequence. The opposite contrasts showed increased signal intensity in patients with Alzheimer's disease compared with controls in the third ventricle and a small cluster ventral to the medial substantia innominata projecting onto the optic tract and the transverse fissure (Table 1, Fig. 5B).
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5. (B) A selected section is projected onto the corresponding MPRGE scan to show the anatomical detail. The lateral cluster of increased signal is located at the transverse fissure and the optic tract.
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In the left hemisphere (Table 1, Fig. 6), signal intensities were significantly reduced at the anterior commisure level and posterior to the level of the anterior commissure in the medial and lateral substantia innominata, the medial globus pallidus and the head of the caudate nucleus. Compared with the post mortem template, signal was reduced in Ch 4al and Ch 4am and Ch 3. Signal was not reduced in areas corresponding to Ch 4i. The opposite contrast showed increased signal in the third ventricle (Table 1).
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Comparison of signal intensity corrected for age
In the right hemisphere (Fig. 7A) there was one large cluster of reduced signal intensity in the lateral substantia innominata, extending into the medial globus pallidus (Table 2). With reference to the post mortem-based template, signal was reduced in Ch 4al and Ch 4am. Signal was not reduced in Ch 4i. The opposite contrast showed increased signal ventral to the medial substantia innominata, projecting onto the optic tract and the transverse fissure (Table 2, Fig. 7B).
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In the left hemisphere, at P < 0.01 there were no clusters of significant signal change. At a more liberal threshold of P < 0.05, there was a small cluster of reduced signal in the lateral substantia innominata and in the lateral globus pallidus (Table 2, Fig. 8A). Compared with the post mortem template, the small cluster in the lateral substantia innominata corresponds to Ch 4al. Signal was not reduced in areas corresponding to Ch 3, Ch 4am and Ch 4i. The opposite contrasts showed increased signal in patients with Alzheimer's disease compared with controls in the third ventricle, transverse fissure, optic tract and hypothalamus (Table 2, Fig. 8B).
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Signal reduction in substantia innominata and cortical grey matter
Results of the correlations between signal intensity in the right substantia innominata and cortical grey matter signal are summarized in Table 3 and illustrated in Fig. 9. In decreasing order of statistical significance, clusters of relatively accelerated reduction of grey matter volume with reduced substantia innominata signal intensity were located in right predominant prefrontal cortex, right predominant insula cortex, right predominant inferior parietal lobule and precuneus, cingulate gyrus and a region posterior to the gyri orbitales, close to the anterior right lateral substantia innominata.
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Results of the correlations between signal intensity in the left substantia innominata and cortical grey matter signal are summarized in Table 4 and illustrated in Fig. 10. In decreasing order of statistical significance, clusters of relatively accelerated reduction of grey matter volume with reduced substantia innominata signal intensity were located bilaterally in prefrontal cortex, lateral temporal lobes, and posterior and anterior cingulate gyri. In addition, there was a significant cluster in the right cerebellar hemisphere.
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Reliability
The intraclass correlation coefficient for the interrater reliability to determine the ROI starting coordinates was 1.00 for the x-, 0.98 for the y- and 0.98 for the z-coordinates. Mean difference between raters was 0.1 mm (SD 0.5 mm) for the x-, 0.3 mm (SD 0.5 mm) for the y-, and 0.4 mm (SD 0.6 mm) for the z-coordinates, with differences ranging between 0 and 1 mm.
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Discussion |
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In the present study we used a new MRI-based acquisition and analysis technique to investigate signal changes in the basal forebrain in patients with Alzheimer's disease compared with controls. Signal was decreased in the anterior lateral and medial substantia innominata and increased in the third ventricle and the optic tract in patients with Alzheimer's disease. The signal intensity in the anterior lateral substantia innominata was significantly correlated with grey matter volume in the bilateral prefrontal cortex, inferior parietal lobule and cingulate gyrus. We suggest that the signal reductions in the substantia innominata may at least partly result from loss or degeneration of cholinergic neurones in the nucleus basalis Meynert.
The basal forebrain is still widely unexplored in neuroimaging. The only structural MRI study on atrophic changes in the basal forebrain in Alzheimer's disease so far measured the thickness of the narrowest portion of the substantia innominata at the level of the anterior commissure (Hanyu et al., 2002). Since the narrowest portion of the substantia innominata in most cases is ventral to the ventral segment of the globus pallidus, the approach used by Hanyu et al. (2002) restricted the analysis to the medial aspects of the substantia innominata. In the present study, we used a virtually fully automated approach to search a large portion of the basal forebrain for signal changes, including its lateral parts. The only rater interaction was the selection of the starting coordinate of the search volume, which had a very high interrater reliability.
Neuropathological evidence suggests loss of cholinergic neurones of the nucleus basalis Meynert in Alzheimer's disease (Arendt et al., 1985). In moderate to severe stages of Alzheimer's disease, the reduction in the number of nucleus basalis neurones amounts to 35–75% compared with controls (Arendt et al., 1985). In mild stages, the overall neurone loss is 22–60% (Cullen and Halliday, 1998). Changes may occur very early in the disease, since cytoskeletal abnormalities in the nucleus basalis have been observed parallel to the very early stages of cortical neurofibrillary changes (transentorhinal stage according to Braak's staging) (Sassin et al., 2000). The cholinergic nuclei of the basal forebrain can be divided into subpopulations adapting Mesulam's nomenclature (Mesulam et al., 1983). Cell group Ch 1 corresponds to the nucleus septi medialis, Ch 2 to the vertical limb of the nucleus of the diagonal band of Broca. Both groups are located medially to the nucleus accumbens septi (corresponding in horizontal sections approximately to Talairach coordinate y = 8 to 4). Ch 3 designates cells associated with the horizontal limb of the diagonal band nucleus (corresponding approximately to Talairach coordinate y = 2 to 4).The Ch 4 group contains the nucleus basalis neurones within the substantia innominata and is the most extensive cholinergic cell group. The Ch 4 group can be divided into anterior (Ch 4a), intermediate (Ch 4i) and posterior (Ch 4p) subpopulations. The anterior subpopulation can be divided into medial (Ch 4am) and lateral (Ch 4al) cell groups. Ch 4a extends from the posterior end of the tuberculum olfactorium (corresponding approximately to Talairach coordinate y = 4) to the posterior end of the anterior commissure (corresponding approximately to Talairach coordinate y = 0). Ch 4i starts at the anterior tip of the thalamus (corresponding approximately to Talairach coordinate y = 0) and extends to the posterior end of the ansa peduncularis (corresponding approximately to Talairach coordinate y = –6); Ch 4p extends from the posterior end of the ansa peduncularis to the level of the mammillary bodies (corresponding approximately to Talairach coordinate y = –10). There is no mapping of these structures in the Talairach system so far, such that the levels of the horizontal sections in the Talairach coordinate system (Talairach and Tournoux, 1988) are only rough estimates. Based on the Talairach coordinates and the mapping of the histological defined borders of cholinergic nuclei from one brain in MRI space, our search volume covers the locations of posterior Ch 2, Ch 3, Ch 4a and Ch 4i. Locations Ch 1, anterior Ch 2 and Ch 4p were not contained in the search volume used.
When we matched the locations of statistically significant decrease in signal intensity to the locations of basal forebrain cholinergic nuclei in a post-mortem brain (transformed in MRI standard space), a signal decrease in posterior Ch2, Ch 3, Ch 4al and Ch 4am in both hemispheres was observed before age correction. After age correction, signal reductions remained significant in the Ch 4al region of the right hemisphere. Only at a more liberal level of statistical significance, signal changes in Ch 4al were significant in the left hemisphere.
At present, the underlying neuropathological substrate of signal changes as determined by MRI remains speculative. The correspondence between the locations of signal decline and the borders of cholinergic nuclei determined histochemically, however, suggests that signal decline at least partially reflects neuronal changes in cholinergic nuclei, e.g. loss or shrinkage of neurones. This hypothesis is further supported by the significant correlations between signal intensity in the peak location of bilateral Ch 4al and cortical grey matter loss in bilateral superior parietal lobes, precuneus and cingulate gyrus and prefrontal cortex. These correlations indicate no causal relationship between cortical and subcortical atrophy, but suggest that both changes are related either directly or through a common factor of severity of disease. A range of studies suggests a corticotopic organization of cholinergic cell groups in the brains of non-human primates. A similar organization has been suggested in humans as well (Mesulam and Geula, 1988). In the human brain there are two major pathways from Ch 4 neurones to the cortex (Selden et al., 1998): the medial pathway supplies the medial frontal cortex and cingulum, the lateral pathway projects to the dorsal and lateral prefrontal cortices, and the temporal, parietal and occipital neocortices. The Ch 3 cell group provides the major cholinergic innervation of the olfactory bulb (Mesulam et al., 1983) which is early and severely affected by Alzheimer's disease (Christen-Zaech et al., 2003). The cholinergic innervation of the hippocampus formation arises predominantly from Ch 1 and 2 cell groups (Alonso et al., 1996). Signal increase was mainly located in the third ventricle and at the ventral surface of the basal forebrain corresponding to the locations of the optic tract and the transverse fissure. Part of this signal increase may reflect widening of CSF spaces as a consequence of local atrophy.
A potential confound of signal changes in proximity to and within the basal ganglia is tissue iron storage, which leads to susceptibility artefacts. These artefacts are a result of local alterations in the magnetic and radiofrequency fields when paramagnetic or ferromagnetic substances are located within the examined structures. Increased iron storage has been described with ageing in the globus pallidus and putamen (Hallgren and Sourander, 1958; Bartzokis et al., 1997) and associated with Alzheimer's disease in the caudate and putamen, but not the globus pallidus (Griffiths and Crossman, 1993; Bartzokis et al., 2000). Although large portions of the globus pallidus and smaller portions of the putamen were included in the ROI, the main effects of signal changes between patients with Alzheimer's disease and controls were found ventral to these nuclei, making it unlikely that these signal changes reflect only differences in iron content. Moreover, the turbo-spin-echo pulse sequence used in our set-up for quantification of substantia innominata signal intensities includes seven spin echoes pulses. These sequences are generally considered to be not sensitive for susceptibility artefacts owing to iron deposits, because the spin-echo pulse sequence rephases static field inhomogeneities, in contrast to gradient-echo sequences or echo planar imaging sequences.
It has to be considered that between-group effects might be related to age difference between patients with Alzheimer's disease and controls. A range of studies showed that the total number of nucleus basalis Meynert neurones in the ninth decade was 20–30% below that in newborns (Mann et al., 1984; McGeer et al., 1984; Lowes-Hummel et al., 1989). If reduction of neurone numbers is approximately linear with age, these data suggest that reduction would be 5% within 15 years of age (30% within 6 x 15 years). Consistently, within a more narrow age range in adults several studies could not detect significant reductions of neurone numbers (Chui et al., 1984; Gilmor et al., 1999). This makes it unlikely that the age difference between groups is the major cause for the signal reduction in the substantia innominata in our subjects. This is further supported by the replication of the peak findings in the lateral substantia innominata after linear effects of age had been taken into account. Since age and diagnosis are highly collinear, the linear model gives a conservative estimate of age adjusted effects of diagnosis.
In summary, we present a new method to search a large portion of the substantia innominata for signal changes using in vivo MRI. We found signal decrease in proton density MRI in the area of the substantia innominata in patients with Alzheimer's disease compared with healthy elderly subjects. The degree of signal decline in the lateral anterior portion of the substantia innominata was related to the degree of grey matter atrophy in bilateral prefrontal, inferior temporal, superior and inferior parietal association cortices and cingulate gyrus. We hypothesize that the observed signal changes may be related to the loss and/or degeneration of cholinergic neurones in the basal forebrain, especially in the nucleus basalis Meynert. If our findings can be validated in an independent sample, MRI of the basal forebrain could be used as an in vivo marker for the structural integrity of basal forebrain cholinergic nuclei and might be used to determine the time course of cholinergic neuronal degeneration and its temporal relationship to cortical atrophy in Alzheimer's disease.
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Acknowledgements |
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A part of the presented material originates from the doctoral thesis of S.S. (LMU Munich, manuscript in preparation). Part of this work was supported by grants of the Medical Faculty of the Ludwig-Maximilian University (Munich, Germany) to S.J.T., of the Hirnliga e. V. (Nürmbrecht, Germany) to S.J.T. and H.H., of Eisai (Frankfurt, Germany) and Pfizer (Karlsruhe, Germany) to H.H. and S.J.T. and by the German Competency Network on Dementias (Kompetenznetz Demenzen) funded by the Bundesministerium für Bildung und Forschung (BMBF), Germany.
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