Functional Magnetic Resonance Imaging

Over the past three decades, magnetic resonance imaging (MRI) has developed into several more specialized imaging modalities including functional magnetic resonance imaging (fMRI). This relatively new technology allows for simultaneous visualization of both structure and physiological function of the brain. fMRI has a variety of applications that have earned it [Page 305]a place in clinical medicine and have established it as a valuable tool for research. Among its many contributions, fMRI has added to the understanding of the brain as it relates to obesity.

fMRI provides a high resolution and noninvasive report of neuron activity in the brain by detecting blood oxygen levels. Regional increases in neural activity increase oxygen demand and subsequently causes localized increases in blood flow. As a result, oxygen concentration is altered. Detection of these changes, also known as the blood-oxygen-level dependent (BOLD) effect, is the basis of fMRI. Other related functional neuroimaging techniques include diffusion MRI, which measures diffusion of water molecules, and positron emission tomography (PET), which measures uptake of radiolabeled tracer molecules.

Because functional neuroimaging techniques are able to correlate physiological function with anatomy, they provide researchers, neuroradiologists, neurosurgeons, and radiation oncologists the ability to plan more precise treatments that will best preserve brain function. These imaging techniques are also important in the assessment of disease states such as stroke, dementia, seizure disorders, and multiple sclerosis (all disease states that affect the morbidly obese more than normal-weight individuals).

Understanding the brain's response to excessive food intake is a focus of obesity research as it is likely the major contributing factor for developing obesity. For this reason, obesity research with fMRI has targeted both the nonconscious (homeostatic) and conscious (perceptual, emotional, and cognitive) aspects of eating behavior.

The role of the hypothalamus in the nonconscious regulation of energy homeostasis is well established. fMRI has shown a profound and sustained decrease in neural activity of two distinct regions of the hypothalamus after glucose ingestion. This decrease in neural activity was significantly reduced in obese individuals when compared to lean individuals. In addition, several limbic and paralimbic structures such as the insula, hippocampus, and orbitofrontal cortex show exaggerated responses to hunger in obese individuals compared to lean individuals. Difficulty arises when trying to separate hunger under homeostatic control from hunger related to the pleasure of eating.

Functional neuroimaging helps to identify abnormal responses in various brain regions elicited by complex behaviors such as hunger and eating. More research, however, is needed to understand the relationships of these structures and their candidacy as neural risk factors for obesity. fMRI will continue to be an important imaging tool in identifying the role of the brain in obesity.