Calculations (in Echo):

PISA (Proximal Isovelocity Surface Area):
    Definition- As blood converges toward an orifice, doppler flow imaging reveals concentric "hempispheres" or "shells", which represent isovelocity surfaces. As the blood accelerates towards the orifice, velocity aliasing occurs and a distinct red-blue interface occurs at the boundary of the shells. It is larger in large volume jets and smaller in small volume jets. It also will change its size depending on the color Doppler scale factor. The radius that you will calculate is located opposite of the jet. PISA is just one of many ways to calculate severity of MR or certain high pressure jets through an orifice.  

    More detailed info about PISA can be found at this excellent site, they have done it better justice than I have: