Digital Image Stabilization (2009)

Computerized Tomography (CT)Nowadays CT is not considered a standard technique for diagnosing AD, least of all in early stages.Its main usefulness is in differential diagnosis since it is less expensive, faster and more widely available than MRI.CT allows ruling out some clinical situations which lead to www.intechopen.comThe Clinical Spectrum of Alzheimer's Disease -The Charge Toward Comprehensive Diagnostic and Therapeutic Strategies 148 behavioural alterations but are not AD, e.g., normotensive hydrocephalus, intra and extraaxial bleeding, etc. Neuroimaging technique Finding CT Tissue atrophy.MRI Tissue atrophy.It is more specific about grey matter.fMRI Changes in blood oxygenation level (BOLD signal) representing synaptic activity.DTI Connectivity and organization in white matter tracts.Spectroscopy Chemical content of the brain, such as NAA/Cr ratio or others.SPECT Changes in cerebral perfusion.PET Changes in glucose metabolism.PET-amyloid Measures accumulation of -amyloid, an AD-specific protein.MEG Measures magnetic fields and, indirectly, yields rather precise information about brain electrical activity CT = computerized tomography; MRI = magnetic resonance imaging; fMRI = functional magnetic resonance imaging; DTI = diffusion tensor imaging; SPECT = single-photon emission computed tomography; PET = positron emission tomography; MEG = magnetoencephalography; BOLD = blood oxygen level-dependent; NAA = N-acetyl aspartate; Cr = Creatinine; AD = Alzheimer's disease Table 1.Typical findings of different brain imaging methods used in AD and MCI diagnosis 1.2 Magnetic Resonance Imaging (MRI) 1.2.1 Structural MRI This technique provides the best possible spatial resolution.Given the fact the most robust neuroimaging finding in AD is the atrophy of mesial temporal structures, structural MRI is the most widely used approach.Hippocampal atrophy is another well known finding, but atrophy within other brain areas has also been described in entorrhinal cortex, amygdala, basal ganglia (nucleus basalis of Meynert), thalamus and bilateral parietal cortex.It is relevant to mention here that in terms of early diagnosis some consider the entorrhinal cortex as the earliest area affected by AD.Other researchers have stated that the nucleus basalis of Meynert is a key structure in AD early diagnosis (Herholz et al, 2004; Grothe et al, 2010).1.2.2Functional MRI (fMRI) fMRI provides signal intensities images associated with a relative cerebral blood flow during cognitive tasks.Resting and activation functional MRI studies have showed a lesser coordinated activity in the hippocampus, inferior parietal lobes -both bilaterally-and cingulate cortex in patients with AD.This neuroimaging technique is nowadays used to monitor AD patients' treatment.Combining genetic risk with functional MRI memory task paradigms has shown to be a very accurate pre-symptomatic predictor of cognitive decline.1.3 Diffusion Tensor Imaging (DTI) DTI allows visualizing neuronal connectivity or, more precisely, local fiber orientation and white matter tracts integrity.Patients with AD show decreased fiber density in temporal white matter, probably related to the medial temporal grey matter atrophy, as well as in the splenium of the corpus callosum.www.intechopen.comCurrently Available Neuroimaging Approaches in Alzheimer Disease (AD) Early Diagnosis 149In mild cognitive impairment (MCI), however, this decreased fiber density has been located in the anterior part of the corpus callosum. SpectroscopyIt provides information on tissue substrate or metabolite concentrations.Specifically the Nacetyl aspartate (NAA) has been used as a biomarker of neuronal death, and it has been demonstrated that in patients with MCI and AD is significantly reduced compared to healthy controls. Nuclear medicine brain imaging techniques 1.5.1 Single-photon emission computed tomography (SPECT)It measures cerebral flow by detecting a single-photon emitting tracer after its intravenous injection and brain uptake.It has a very low spatial resolution and its diagnostic accuracy is lower than PET.Nevertheless, in clinical practice, it can be useful for differentiating AD from other dementias such as frontotemporal dementia. Positron emission tomography (PET)Recently a new technique using PET with Pittsburg compound B (PIB; PIB-PET) has emerged.PIB is a tracer which allows marking β-amyloid plaques.This technique has been highly efficacious to detect β-amyloid in vivo and, therefore, could be rather useful to detect AD in its early phases. Magnetoencephalography (MEG)MEG is a technique which measures brain magnetic fields both during basal state and during cognitive activation.Its time resolution is measured in milliseconds.It is precisely this very high temporal resolution which allows evaluating small changes in brain processing during cognitive stimulation.These changes may help to differentiate between AD, MCI and normality as well as providing some clues about differences in information processing in these same conditions Different studies carried out with MEG show a decrease in magnetic fields in people with AD in temporal mesial areas during memory tasks.People suffering from AD exhibit increased slow frequencies in temporoparietal areas in basal evaluation.