 The purpose of this study is to determine the value of Arterial Spin Labeling ASL MRI, a measure of blood flow to the brain, in Mild Cognitive Impairment MCI and compare it to existing measures. In particular, the investigators will compare ASL MRI to Positron Emission Tomography PETCT, which measures brain metabolism reflecting how well cells in a patients brain are functioning. In addition, the investigators will assess the relationship of these measures to specific protein levels associated with Alzheimers Disease in the patients cerebrospinal fluid the fluid that surrounds the brain and spinal cord obtained by lumbar puncture. By comparing the information that is available from these procedures to the patients performance on cognitive tests, the investigators hope to learn which procedures most accurately reflect and assist in determination of the potential causes of cognitive difficulties that arise with MCI, and thus, which are most useful in the clinical setting. In particular, PET scans have been found to be very useful in diagnosis of MCI and Alzheimers Disease, but the investigators want to find out if they can get the same, or better, information from an ASL MRI scan, which is less expensive and easier to acquire. 