1. A system for visualizing a time-variant parameter at a plurality of positions in a biological structure, the system comprising: a determination unit configured to determine a value of the time-variant parameter at a plurality of positions in the biological structure, the structure extending in mutually perpendicular X, Y and Z directions; determine positions of a first and a second boundary extending in the Z direction, and positions of a first volume extending in Z disposed between the first and second boundaries; determine positions of a second volume generated by extending the first volume in the X-direction to both the first and the second XY boundary; determine a first visualization parameter for a plurality of time intervals from the time-variant parameters at a plurality of positions in the first volume; determine a second visualization parameter from the time-variant parameters at a plurality of positions in the second volume, and a visualization display configured to display a first representation showing the first visualization parameter at the plurality of time intervals, and display a second representation showing the first and second visualization parameters.