Now consider dose isolines in this plane. Using pixel data as we have, where would the 0% line be? Since everything inside this line must receive 0% or greater, it must lie at the outside boundary. Similarly, the 100% line would lie at the 100% pixel boundaries. What about the 50% line? Since the pixels have only 2 values, there can be only two lines, 0% and 100%. The 50% line can't lie at the 0% boundary, since everything inside is not 50% or greater, so it must lie at the 100% boundary. In the end, EVERY line except the 0% line would lie at the 100% boundary. Clearly this is not what we expect to see. We are used to thinking in terms of a dose continuum which is created by interpolation from a set of points, not pixels. |