Advanced magnetic resonance imaging techniques in the diagnostics of Alzheimer's disease: how much imaging is enough?

Abstract

Alzheimer’s disease (AD) is the most common progressive neurodegenerative disorder characterized by gradual deterioration in cognition, function, and behavior. The number of patients with dementia is increasing in the Western world. Neuroimaging plays an important role in the study and clinical diagnosis of AD. It is of utmost importance to exclude potentially treatable conditions such as brain tumors, hematomas, and hydrocephalus. According to a meta-analysis potentially reversible causes were seen in 9% and actually reversed in only 0.6% of dementia cases (0.29% partially, 0.31% fully) (1). Techniques that are widely used in clinical practice are still largely based on structural magnetic resonance (MR) imaging and volumetry. Recently Wattjes et al. (2) concluded that although MR imaging should be the preferred imaging modality due its lack of ionizing radiation and higher contrast resolution, 64-detector row computed tomography (CT) is a suitable and accurate imaging method with which to evaluate global cortical atrophy, medial temporal lobe atrophy, and white matter changes in a memory clinic setting, and it can be considered a nearly equivalent alternative to MR imaging in patients who cannot undergo MR examination. However, it should be noted that measures of brain atrophy typically reflect the relatively late stages of neuronal loss and may not be ideal for identifying early neuropathological changes. There is increasing evidence showing that the pathological process associated with AD may begin years or decades before diagnosis. But: do the advanced techniques help in assessing therapeutic effects or improve patient outcome? Early detection of AD risk would enable preventive or more effective treatment of the patients, resulting in a time delay in symptom onset that may decrease the prevalence of the disease (3). Therefore, in addition to the structural MR imaging procedures a spectrum of novel MR imaging tools is being adopted for the scientific study of pathological processes underlying the development of AD, including blood oxygenation level-dependent (BOLD) functional MRI studies of brain activation and functional connectivity, perfusion measurements with arterial spin labeling (ASL), MR spectroscopy (MRS), and diffusion tensor imaging (DTI). Functional MRI on BOLD contrast allows noninvasive in vivo assessment of hemodynamic responses to external stimuli. ASL is a non-invasive MR imaging technique for the measurement of regional cerebral blood flow by labeling of the arterial water and using it as an endogenous tracer. Although challenging (4), hippocampal MRS combined with quantitative measurements of hippocampal atrophy may improve the early diagnosis of AD. DTI can be used to determine the orientation of fiber tracts and the neural network connections between different brain regions. Fractional anisotropy has become an imaging marker commonly used to study microstructural white matter abnormalities in various pathological states. There is growing interest in using DTI for AD studies (5, 6). In this issue of Acta Radiologica a comprehensive state of the art review by Dr Li Tie-Qiang (3) discusses the use of these advanced imaging techniques mainly in research. The review allows you to get an excellent overview of the advanced MR imaging techniques and the article is highly recommended reading.