Neural Activation MRI

When neurons are active in the brain, there is an increase in blood flow and blood volume local to that region of activity. MRI can be used to detect the change in blood flow directly. The idea is that when fresh blood flows into the slice of the brain that is being imaged, it will have a different "spin history'' (i.e., it will not have recently been hit by an orientation-flipping RF-pulse) and will thus have a greater degree of alignment with the main magnet. When another RF pulse is applied, the fresh blood will have a greater concentration of aligned protons to flip and will thus yield a greater NMR signal. The imaging of this signal happens on a timescale that is rapid with respect to the blood flow, so the change is detected. This phenomenon is the basis for one kind of imaging in fMRI. It is largely sensitive to changes in arterial blood flow (where flow is the fastest).

There is a second, and more commonly used, process that yields an fMRI signal. The neural activity that elicited the local increase in blood flow and blood volume surprisingly does not elicit a correspondingly great increase in oxygen utilization. That is, although the neural activity leads to a small increase in oxygen utilization, it is dwarfed by the increase in blood flow. Thus, there is an increase in oxygenated hemoglobin in the venous portion of the circulatory system near the site of neural activity (as well as downstream from that site). The combination of increased oxygenated hemoglobin and increased blood flow results in a decrease in the instantaneous concentration of deoxygenated hemoglobin on the venous side of the capillaries. Deoxygenated hemoglobin (unlike oxygenated hemoglobin) is a strongly paramagnetic biological molecule, and it distorts the magnetic field locally. Thus, a decrease in the local concentration of deoxyhemoglo-bin leads to a more uniform magnetic field locally and to a longer time period during which the orientations of precessing protons stay in phase. Thus, the NMR signal in a region of decreased deoxyhemoglobin concentration increases relative to its normal (neuron-ally resting) state. This phenomenon is called the blood oxygen level dependent (BOLD) effect. It is the major source of contrast in most fMRI experiments.

Breaking Bulimia

Breaking Bulimia

We have all been there: turning to the refrigerator if feeling lonely or bored or indulging in seconds or thirds if strained. But if you suffer from bulimia, the from time to time urge to overeat is more like an obsession.

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