The present invention discloses a method and apparatus for measuring AIF and VOF on brain perfusion CT images. The AIF and VOF are used to calculate hemodynamic parameters. In this invention, bone voxels and neighboring voxels are removed first from the perfusion images and thus only brain voxels are included in the AIF and VOF measurement procedures; moreover, the selection criteria, such as large area under the concentration-time curve, early arrival of contrast agents, and narrow effective width, are used to select appropriate arterial and venous voxels for the AIF and VOF measurements.