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Clinical Investigations |
1 Department of Radiology, National Center Hospital for Mental, Nervous, and Muscular Disorders, National Center of Neurology and Psychiatry, Tokyo, Japan
2 Department of Radiology, University of Washington Medical Center, Seattle, Washington
3 Department of Psychiatry, Showa University School of Medicine, Tokyo, Japan
4 Department of Neuropsychiatry, Tsukuba University, Ibaraki, Japan
5 Department of Geriatric Medicine, National Center Hospital for Mental, Nervous, and Muscular Disorders, National Center of Neurology and Psychiatry, Tokyo, Japan
| ABSTRACT |
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Key Words: Alzheimers disease SPECT MRI statistical parametric mapping
| INTRODUCTION |
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The aim of this study was to determine the initial abnormality and the longitudinal changes in both morphologic and functional measurements in the whole brain of the same individuals with AD. To study both changes not only in medial temporal structures but also in other brain areas, and to avoid subjectivity and adopt the principle of data-driven analysis, we applied a statistical parametric mapping (7) software program, which is a voxel-based analysis in stereotactic space.
| MATERIALS AND METHODS |
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Twenty-five healthy volunteers (16 men, 9 women; age range, 5984 y; mean age, 71.2 ± 7.3 y) with no memory impairment or cognitive disorders served as control subjects. Each underwent SPECT and MRI once. Their performance was within normal limits both on the revised Wechsler Memory Scale (11) and on the revised Wechsler Adult Intelligence Scale (12). The MMSE score was 29.5 ± 0.6. The healthy volunteers did not differ significantly from the AD patients in age or education. They had no close relatives with psychiatric or neurologic disorders. The ethics committee of the National Center of Neurology and Psychiatry approved this study for healthy volunteers, all of whom gave informed consent to participate.
All of the patients and healthy volunteers were screened by questionnaire and medical history to exclude those with medical problems potentially affecting the central nervous system, including a current or past neurologic disorder, head trauma with loss of consciousness, brain tumor, hypertension, heart disease, diabetes mellitus, Cushings disease, steroid use, alcohol or drug abuse, epilepsy, schizophrenia, major depression, and posttraumatic stress disorder. In addition, in none was asymptomatic cerebral infarction detected by T2-weighted MRI.
Brain MRI Procedure
All MRI studies were performed on a 1.0-T system (Magnetom Impact Expert; Siemens, Erlangen, Germany). A 3-dimensional volumetric acquisition of a T1-weighted gradient echo sequence produced a gapless series of thin sagittal sections using an MPRage sequence (echo time/repetition time, 4.4/11.4; flip angle, 15°; acquisition matrix, 256 x 256; 1 excitation; field of view, 31.5 cm; slice thickness, 1.23 mm).
Brain SPECT Procedure
Before SPECT was performed, an intravenous line was established in all subjects. While lying supine with eyes closed in a dimly lit, quiet room, each received an intravenous injection of 600 MBq 99mTc-ethyl cysteinate dimer. Ten minutes after this injection, brain SPECT was performed using a triple-head gamma camera (MULTISPECT3; Siemens, Hoffman Estates, IL) equipped with high-resolution fanbeam collimators. For each camera, projection data were obtained in a 128 x 128 format for 24 angles of 120° at 50 s per angle. A Shepp and Logan Hanning filter was used for SPECT image reconstruction at 0.7 cycle per centimeter. Attenuation correction was performed using Changs method.
Image Analysis
Voxel-based morphometry for an MR image was performed as described in our previous study (13). The theory and algorithm of this voxel-based morphometry were well documented by Ashburner and Friston (14). The acquired MR images were reformatted to gapless 2-mm-thick transaxial images. Images were analyzed using Statistical Parametric Mapping 99 (SPM99) (Wellcome Department of Cognitive Neurology, London, U.K.) running on MATLAB (The MathWorks, Inc., Sherborn, MA). Spatial normalization fitted each individual brain to a standard template brain (Talairach and Tournoux (15)) in 3-dimensional space, so as to correct for differences in brain size and shape and facilitate intersubject averaging. In spatial normalization, only 12-parameter affine transformation was used to avoid segmentation errors caused by partial-volume effects inherently created by warping (16). Normalized MR images were then segmented into gray matter, white matter, cerebrospinal fluid, and other compartments using a modified version of the clustering algorithm, the maximum likelihood "mixture model" algorithm (14,17). The segmentation procedure involves calculating for each voxel a Bayesian probability of belonging to each tissue class based on a priori MRI information with a nonuniformity correction (14,17). The segmented gray matter images were then subjected to an affine and nonlinear spatial normalization. The spatially normalized gray matter images were smoothed with an isotropic gaussian kernel 12 mm in full width at half maximum (FWHM) to use the partial-volume effect to create a spectrum of gray matter intensities. The gray matter intensities are equivalent to the weighted average of gray voxels located in the volume fixed by the smoothing kernel. Regional intensities can therefore be taken as equivalent to gray matter volumes (14).
Voxel-based analysis of SPECT data was performed also using SPM99. The images were spatially normalized using SPM99 to an original template for 99mTc-ethyl cysteinate dimer (18). Then, images were smoothed with a gaussian kernel 12 mm in FWHM.
Statistical Analysis
The processed images were analyzed using SPM99, which implements a general linear model. The overall mean of gray matter volume and global cerebral blood flow was treated as a confounding covariate. A proportional scaling routine was used to achieve global normalization of voxel values between scans.
We studied differences in gray matter volume and regional cerebral blood flow (rCBF) between AD patients and age-matched healthy volunteers using t statistics. The resulting sets of t values constituted statistical parametric maps (SPM(t)). The SPM(t) were transformed to the unit normal distribution (SPM(Z)) and were subjected to a threshold of P < 0.001. To correct for multiple nonindependent comparisons inherent in this analysis, the resulting foci were then characterized in terms of their spatial extent. This characterization is in terms of the probability that a region of the observed number of voxels, or more, could have occurred by chance over the entire volume analyzed. The significance of each region was estimated with a threshold of P = 0.05 using distributional approximations from the theory of gaussian fields (7). Anatomic localization was according to the atlas of Talairach and Tournoux (15) using a set of linear transformations (19).
| RESULTS |
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| DISCUSSION |
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On the other hand, our longitudinal study (6) did not find a significant decrease in hippocampal blood flow in patients with very early AD with a mean MMSE score of 26.2. In patients with mild to moderate AD, Ishii et al. (28) found that PET showed no significant rCBF decrease in spite of atrophy in the hippocampus. Ibanez et al. (29) also reported no significant decreases in glucose metabolism in the medial temporal structures of AD patients. This study showed that the medial temporal structures with significantly decreased rCBF were much narrower and more posterior than those with decreased gray matter volume. These findings suggest that medial temporal structures show functional reductions to a lesser degree than does atrophy in very mild to moderate AD. This discordance may result from a plastic response in AD. The perforant path, which arises from the entorhinal cortex and has been reported to be the first affected in AD (22), is the major cortical input to the hippocampus. The neuronal loss in the entorhinal cortex of AD patients appears to act as a stimulus in a similar manner to that of the lesion in the rat brain, where loss of a set of axons induces sprouting of the remaining afferents and replacement of the lost connections to maintain synaptic activity (30,31). This compensatory response of reinnervation in the course of AD would result in milder functional changes than morphologic changes.
In a very early stage of AD, even before a clinical diagnosis of probable AD is possible, decreases of rCBF and glucose metabolism in the posterior cingulate gyri and precunei have been reported using PET (3,4) and SPECT (5,6). In mild to moderate AD, investigations (29,32) also reported reductions of rCBF or glucose metabolism in this area, as was shown by this study. Reduced PET measures of glucose metabolism in this area persist even after partial-volume effects are taken into account; thus, the reduction is more than just an artifact resulting from an increase in cerebral fluid space (29). The observation that metabolic reduction in this area predicts cognitive decline in presymptomatic persons indicates that the pathophysiologic process begins well before even mild or questionable dementia is recognized clinically (33). PET measures of glucose hypometabolism reflect decreased synaptic activity caused either by loss or by dysfunction of synapses (34), and regional metabolic deficits observed on PET may reflect projections from dysfunctional neurons in other brain lesions. In nonhuman primates, lesions of the entorhinal cortex cause a significant and long-lasting metabolic decline in a small set of remote brain regions, especially in the inferior parietal, posterior temporal, posterior cingulate, and associative occipital cortices and in the posterior hippocampal regions (35). These results suggest that flow or metabolic reduction in the posterior cingulate gyrus and precuneus indicates the earliest functional changes in AD as a remote effect caused by neuroanatomic disconnection with the rhinal cortex.
The region containing the posterior cingulate gyrus and precuneus is known to be important in memory (36). The retrosplenial cortex receives input from the subiculum and projects to the anterior thalamus, thus providing an alternative route between the hippocampus and thalamus. Medial temporal structures involved in memory receive anterior thalamic input directly through the cingulate bundle and indirectly through a relay in the retrosplenial cortex (37). This thalamocortical portion of the circuit of Papez (38) may be important in memory, and lesions of the cingulum and retrosplenial cortex may cause memory dysfunction by disrupting this pathway. A PET study also showed activation in the precuneus during episodic memory retrieval tasks (36).
Many researchers have shown metabolic and blood flow reductions in the parietotemporal association cortex (39). This finding has been widely recognized as a diagnostic pattern for AD. The current rCBF reduction in the extensive associative parietal cortex without extensive atrophy agreed well with a previous PET study (40). In the associative temporoparietal cortex, glucose metabolism showed a significant decrease without regional tissue loss as assessed by 11C-methionine accumulation. The associative frontal cortex is reduced in certain patients, often those with advanced AD (39). Several investigators have consistently addressed spread to the frontal lobes in longitudinal changes of functional alteration in the cerebral cortex of AD (39), as was shown in this study. In frontal areas, the anterior cingulate gyrus and orbitofrontal areas were also involved. Decreased rCBF in the anterior cingulate gyrus has been reported in patients with questionable AD at baseline SPECT who converted to AD on follow-up (5). After involvement of the medial temporal structures, neuropathologic changes of neurofibrillary tangles spread to the basal forebrain and anterior cingulate before encroaching on the neocortical association areas (20,41). The posterior location of reduced rCBF, in comparison with the location of reduced gray matter volume, in the associative temporal lobes may indicate that the reduced rCBF is caused by a remote effect (35). Our findings suggesting reduction of both rCBF and gray matter volume in the thalamus and caudate nuclei in an advanced stage are consistent with neuropathologic findings (20) and previous reports using voxel-based morphometry of MRI in AD (16,26).
| CONCLUSION |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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For correspondence or reprints contact: Hiroshi Matsuda, MD, Department of Radiology, National Center Hospital for Mental, Nervous, and Muscular Disorders, National Center of Neurology and Psychiatry, 4-1-1 Ogawahigashi, Kodaira, Tokyo, 187-8551, Japan.
E-mail: matsudah{at}ncnpmusashi.gr.jp
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