Scanning a physical part like a gasket or similar for profiling can be problematic. Because parts have a thickness they produce shadows when you scan them in. Once in your raster to vector converter it can be difficult to tell where the part ends and the shadow begins.
Here are three suggestions to help get around the problem.
To reduce shadowing, put a black sheet of paper or card over the part in the scanner. You may also want to try spray painting the part white. When you scan, the shadows will simply become part of the black background.
If your converter has drawing tools, trace around the part manually.