Functional Magnetic Resonance Imaging

The field of MR brain mapping is made possible by exploiting properties of cerebral hemodynamics subsequent to neuronal activity. With the onset of neuronal activity there is an associated change in cerebral metabolism, blood flow, blood volume and oxygen consumption [1-5]. The complex physiologic interplay of these parameters is not well understood. However, current research efforts are aimed at understanding these physiologic parameters and would certainly improve the methodology and localization of functional magnetic resonance imaging. The first functional brain images were the work of Belliveau et al. [6]. The detection of blood flow changes associated with different brain states was accomplished with the use of an exogenous contrast agent. These experiments required two different contrast injections during different mental states, flashing light and darkness. The difference in cerebral blood flow was evident by the contrast (flow) related changes. The drawback to this method was the required use of contrast and therefore, the inability to do several experiments in a single subject. A different type of MR hemodynamic-based method, which did not require exogenous contrast, was developed independently by Kwong et al. and Ogawa [7-9]. In Kwong's experiments the MR signal contrast was derived from either the T1 shortening due to changes in blood flow or the differences in oxygenation. Kwong showed in both a visual and motor task that it was possible to measure brain activation. Ogawa, on the other hand, had been investigating the MR signal properties associated with oxygenated and deoxygenated blood [10]. The focus of this work was to use blood as an endogenous contrast agent. As the fraction of oxygenated to deoxygenated blood changes so does the MR signal. This effect has been called BOLD, blood oxygenation level dependent, imaging. This is by far the method of choice in most functional imaging labs studying primary sensory and motor areas as well as higher levels of cognition such as attention, language, and memory. How is it possible to measure brain activation from blood oxygenation changes in the blood? A generally assumed model of the physiology of brain activation is given below. 1. Cortical activation occurs. There is a local demand for energy in the form of oxygen or glycogen. Figure 1-a. 2. A hemodynamic delay occurs on the order of 4-8 seconds. Figure 1-b. 3. There is an increase in the capillary perfusion (blood flow and/or volume) in the region to deliver the metabolic products. This …