The Use of MRI and PET for Clinical Diagnosis of Dementia and Investigation of Cognitive Impairment : A Consensus Report

and recommendations: Current American Academy of Neurology (AAN) guidelines for dementia diagnosis recommend imaging to identify structural brain diseases that can cause cognitive impairment. Accruing scientific evidence supports the use of imaging for the detection of early AD in individuals who have mild cognitive impairment (MCI), especially of the amnestic type (which may represent a prodromal form of AD). However, more research is needed to establish the clinical value of imaging in MCI. While much of this scientific evidence comes from quantitative analysis of structural brain MRI, qualitative estimates of medial temporal atrophy are highly correlated with quantitative measures and are much more suitable for clinical diagnosis. The clinical use of structural brain imaging confers substantial assistance in the early diagnosis of AD. In this regard, the following recommendations are offered: • Follow the American Academy of Neurology guidelines; obtain brain imaging as part of dementia evaluation when AD is suspected and expand the AAN guideline to include individuals who have amnestic MCI. • The impact of extensive white matter disease or a single lacunar infarct outside the thalamus in the presence of MCI remains unclear. Further research in imaging of a broad spectrum of individuals with MCI is needed to determine the exact utility of predicting a future “diagnosis of AD” and the particular type of MCI that is likely to be “prodromal AD.” • Coronal brain imaging, preferably perpendicular to the long axis of the hippocampus, should be included in routine MRI protocols. • Standardize imaging parameters whenever possible; consistent use of a standard imaging protocol and method of interpretation will translate into clinically meaningful results. Individual sites may need to tailor exact sequences to the type of brain imaging machine available. • The use of a widely adopted (standardized and wellvalidated) protocol is essential for the clinical interpretation of medial temporal atrophy on MRI. However, presently there is a paucity of such qualitative instruments. Two scales are in use , both of which have demonstrated (1) interand intra-rater reliability, (2) correlations with quantitative volumetric and neuropsychology and (3) prospective predictive value. The Work Group recommends the development and/or validation of uniform methods to measure, describe, or interpret the clinical significance of structural changes observed in brain images. • Prospective studies are needed to evaluate the utility of MRI for differential diagnosis and early detection. There is a potential role for MRI in identifying AD by proven associations with AD pathology; future research should focus on the use of qualitative interpretation of various types of anatomical brain images from disparate populations to assess more fully the true utility of this method for the early diagnosis of dementia. • Use of CT imaging for early diagnosis of AD should be the focus of future research for those individuals who cannot receive MRI 19, . A. Rationale for Structural Imaging in the Early Detection of Dementia The pathology of AD has a definite topographic distribution 23) in regions that can be well characterized by available neuroimaging methods. Hippocampal atrophy is an early marker of AD 24, 25, 28, 29, 30–33) that correlates with impairments in memory function. In addition, AD leads to progressive loss of brain volume throughout the cerebral cortex and other areas that is significantly greater in AD patients than agematched controls and that correlates with the rate of cognitive deterioration. Neuropathological studies also show that the pathological features of AD may be present for years before clinical symptoms are evident 43) and are often present in individuals with memory impairment who are not demented, suggesting that most individuals with MCI have early AD. Additionally, careful pathological studies find a close association between hippocampal size as imaged by MRI and the extent of AD pathology. Given that structural imaging is a relatively easy and noninvasive method to evaluate early AD and is closely associated with the pathological features of the disease, a number of studies have evaluated the prognostic significance of finding hippocampal atrophy in high-risk populations. B. The Prognostic Significance of Hippocampal Atrophy