11. Practical Illustrations Sodium Spectroscopy and Imaging: Clinical Applications

A valid distinction between cytotoxic and vasogenic edema is based mainly on the differences in blood–brain barmier permeability. Vasogenic edema fluid develops in association with a variety of pathologic conditions such as brain tumors, brain abscesses hypertension or areas of infarction. In cases of brain tumor edema fluid leaks from the tumor vessels and spreads into the surrounding white matter. Similarly, cryogenic injury of the cerebral cortex causes a transient breakdown of the blood–brain barrier in the zone bordering the necrotic region allowing edema fluid to spread into the white matter. The regions in which the blood–brain barrier is defective and those in which the edema accumulates do not necessarily correspond. MRI has a proved to be a valuable and sensitive method by which to detect vasogenic edema. Multinuclear MR imaging (proton and sodium) permits in vivo assessment of the relative distributions of water and sodium within the brain. One can define the sodium signal associated with edema fluid as well as investigate the relaxation characteristics of extra-cellular sodium, for properties that may be specific to sodium in the extra-cellular compartment. A model of vasogenic edema can thus be developed. Previously models have relied on local injury to the brain by either cold or chemical insult. The resulting region of necrotic brain has a defective blood–brain barrier and vasogenic edema forms adjacent to the injured tissue. One can avoid a mixture of necrotic and edematous brain tissue by the use of a non traumatic mode of vasogenic edema in mongrel dogs.