Figure 5. Demonstrates the concept of the relative re-circulation parameter. The dotted line shows the expected changes in contrast concentration within a voxel which contains no contrast leakage. The initial peak is due to the passage of the contrast agent bolus and the second peak represents the second passage and subsequent re-circulation. The black line shows typical data observed from enhancing tumours. The elevation of the re-circulation phase data reflects slow perfusion and/or irregular flow through areas of low perfusion pressure and is equivalent to the tumour blush seen on conventional angiography. Since the expected shape of the first pass curve can be estimated by gamma variate fitting the difference between this and the actual measured value can be estimated (hatched area). This measure can then be used as an indicator of flow irregularity.