Figure 4. (a) Time course data from a region of interest in an enhancing meningioma following injection of contrast agent. Images were acquired using a T2* weighted acquisition protocol. Note the initial dip in signal intensity as a contrast agent enters blood vessels within the voxel followed by a rapid enhancement effect as contrast leaks into surrounding tissues. The preliminary drop is due largely to T2* (susceptibility) effects in the image whereas the rise is due to residual T1 weighted (relaxivity) effect. (b) Signal change from the same region of interest from a second contrast injection performed several minutes after the first. The pre-enhancement with the initial dose of contrast has saturated the T1 sensitivity of the tissues and only the susceptibility effects are now seen. Note the significant signal drop due to intravascular contrast and the failure of the signal to return to the baseline during the re-circulation phase. (c) A calculated regional relative cerebral blood volume (rCBV) map of a large glioma. Note the high CBV in the large feeding vessels and draining veins. Central areas of poor perfusion are seen as negative values. (d) The relationship between median rCBV and tumour grade in a series of gliomas. Note the clear grade specific relationship of CBV calculated in this way.