Mohammed T. Abou-Saleh

Principles and Practice of Geriatric Psychiatry.Editors: Professor John R. M. Copeland, Dr Mohammed T. Abou-Saleh and Professor Dan G. BlazerCopyright & 2002 John Wiley & Sons LtdPrint ISBN 0-471-98197-4 Online ISBN 0-470-84641-065Functional Magnetic Resonance Imaging (fMRI)K. Ranga R. KrishnanDuke University Medical Center, Durham, MC, USAFunctional MRI (fMRI) is a new technique that has allowedthe non-invasive assessment of physiological function of thebrain in humans. The phenomena of nuclear magneticresonance has been utilized to accomplish this. The firstattempt at producing a functional image of the human brainutilized an invasive technique of evaluating blood flow changesin the brain, assessed by using a contrast agent. The noninvasivemethod was introduced by Ogawa, and is based on ablood oxygenation level-dependent contrast (BOLD) Thiscontrast originates from the homogeneity induced by deoxyhaemoglobinin red blood cells. Signal intensities in MRIs aretherefore acquired in a manner sensitive to this BOLD contrast,which is effected with regional deoxyhaemoglobin content. Therationale underlying the changes in deoxyhaemoglobin is thatregional blood flow increases, while the oxygen consumptionrate is not altered significantly, resulting in a lower deoxyhaemoglobincontent per unit volume of brain tissue. The signalintensity in the image acquired sensitive to BOLD thereforeincreases in active regions relative to those regions which arenot active.Another technique that has been developed, called EPISTARand FAIR, utilizes tagging of blood spins to measure cerebralblood flow changes. In a recent study, we have demonstratedthat, using EPISTAR, there is a reduction in blood flow in theleft frontal lobe in depressed patients. BOLD as well as FAIRcan be developed and utilized in regular MRI 1.5 teslascanners. However, many particularly exciting findings areemerging from high-field MRI, 4 tesla and greater. BOLDresponse is greater in a higher magnetic field, which producesan improved contrast.APPLICATION OF fMRIfMRI has been mostly utilized with the BOLD type of measure toevaluate brain regions involved in a task performance. Unlikepositron emission tomography (PET), fMRI allows a single trialdesign and a single subject design and measures images that canbe acquired in tenths of milliseconds, which is still much slowerthan the temporal response of neurons but similar to the responseof the vascular system. In the design paradigms in general, thecontrast is measured before and during a task and this can bedone repeatedly to obtain an average within a subject on/off task.It can also be used to track the evolution of a particular task overtime, that is the temporal evolution of the signal relative to theexecution of this task can be obtained and averaged followingrepeated executions of the same task. It also allows evaluation oflearning, errors, habituation, etc. The technique has had limitedapplicability so far to disease conditions, except as it relates to theevaluation of working memory tasks and other components ofmemory in patients with mild cognitive impairment, as a preludeto assessing whether they are likely to develop dementia over time.Clearly, this technique offers a significant possibility for understandingmany of the neuropsychological functions that arealtered in aging and acquiring the ability to assess their changesrelative to particular diseases of aging.REFERENCEUgurbil K, Hu X, Chen W et al. Functional mapping in the human brainusing high magnetic fields. Phil Trans R Soc Lond 1999; 354: 1195–213.Principles and Practice of Geriatric Psychiatry, 2nd edn. Edited by J. R. M. Copeland, M. T. Abou-Saleh and D. G. Blazer&2002 John Wiley & Sons, Ltd