Showing flow (not for the faint of heart I think)

I have an image taken of fluid flow through an irregular volume. Both the source and sink show as local maxima because the fluid in densest there. Therefore, when I display the gradients, the gradients around one of the local maxima is in the wrong direction. I have to know where in the image the flow changes from going in to going out without tracing every flow line but by dividing the image into two regions out and in. If I take a good sampling of the image and do a 3-d spline fit then I can find the minimum of the fit and use that as a border between inflow and outflow. Yes? thoughts? programming suggestions?

[Image Analyst]

Is the image a 2D image or a 3D/volumetric (like from MRI) image? Do the image pixels/voxels represent the density of the fluid or the gradient? Or do you have an image for each?