The time-concentration development of a contrast injection, bolus, into the blood stream is determining the contrast of many medical screening methods, like MRI. A restricted blood flow in the brain gives altered maximum concentrations and also time shifts when they occur. These things can be monitored with today's techniques but the blood pressure and flow cannot be found. By modelling a brain with nodes and connecting vessels, the bolus evaluation can be simulated. By restricting the blood flow at various depths in the vascular tree, it is possible to predict how the blood flow is affected and connect this to the measured bolus. This will show how the brain is extracting oxygen and will be very helpful in understanding various diseases or trauma in the brain. It will also be helpful for doctors in selecting and evaluating various treatments.