Application of Diffusion Tensor Imaging in Alzheimer's Disease :Quantification of White Matter Micro-structural Changes

Alzheimer's disease (AD) is the most common cause of dementia in the elderly, accountings for 60-70% of all demented cases. It is a neuro-pathological diagnosis determined by presence of neurofibrillary tangles and senile plaques in the brain of patients with dementia. The disease frequently starts with memory impairment, but is invariably followed by a progressive global cognitive impairment. The major risk factor for Alzheimer disease is age, with prevalence doubling every 5 years after the age of 65.

Diagnosis of Alzheimer's typically involves physical and neurological exams, as medical history and mental status evaluation, laboratory investigations and it involves brain imaging (such as MRI) which could identify other causes of problems such as stroke, tumor or head trauma. By physical and neurological examination, Alzheimer's disease characterized by gradual onset and progressive decline in cognition with sparring of motor and sensory function until later stages; the average course of Alzheimer's disease is approximately a decade, with a range of 3 to 20 years duration from diagnosis to death .Memory impairment is present in the earliest stages of the disease; patients have difficulty learning new information and retaining it for more than few minutes. As the disease advances, the ability to learn increasingly compromised, more distant memories are lost. Other cognitive loses include aphasia, apraxia, disorientation and impaired judgment. Cognitive impairment affects daily life; patients have difficulty planning meals, managing finances or medication, using telephone, driving. Many capacities may remain intact until later stages including performance of self-care activities of daily living as eating, bathing. Patients evidence personality alteration, irritability, anxiety, depression. Delusion, hallucination and aggression. Laboratory Evaluation includes Biochemical markers as measurement of Cerebrospinal fluid including Tau protein, amyloid beta peptides or neural thread protein and measurement of urinary biomarkers including neural thread protein. Also testing including Apo lipoprotein E epsilon 4 allele presenilin genes, amyloid precursor gene or TREM2. Diffusion tensor imaging (DTI) is an imaging technology based on magnetic resonance diffusion weighted imaging, which can make quantitative analysis of anisotropy of water molecules in different directions, so as to observe the microstructure of tissues non-invasively. So Diffusion tensor imaging can provide information of fiber orientation, the injury of fiber, and membrane permeability which cannot be obtained from conventional MRI. Diffusion tensor imaging enables mapping of White matter microstructure changes in development, aging and neurological disorders, From the tensor, it's possible to derive the mean diffusivity (DM) and the fractional anisotropy (FA) which is the most robust measures of anisotropy which measure the degree of deviation from isotropic diffusion. ). More recently, an additional DT-MRI derived index has been proposed .This index measures the degree of similarity of orientation of neighboring voxels and its named inter-voxel coherence (C).

So Diffusion tensor imaging has therefore become a powerful technique in the study of neurodegenerative diseases in recent years.