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FIG 1. A, Schematic representation of two capillaries containing contrast agent. The healthy capillary with an intact BBB (left) is not permeable to the contrast agent, which remains intravascular. A diseased capillary with a disrupted BBB may become permeable to the agent, leading to accumulation of contrast agent outside the vessel (right).
B, Because the contrast agent causes relaxation rate enhancement to water in its environment, initial tissue relaxation rate enhancement reflects the fraction of the tissue containing blood vessels (since the contrast agent is, at least initially, confined to the intravascular compartment). The ratio of initial enhancement in tissue to enhancement in a region of 100% blood (eg, the sagittal sinus) will then yield the fBV. Over time, if the contrast agent leaks out of the vessel into the extravascular space of the tissue, the relaxation rate will rise progressively. The rate of increase in relaxation rate is proportional to the permeability of the capillary wall to the contrast agent.
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