Since you're talking pixels, I assume it's a raster (bitmap) image. So select it and click "Edit Bitmap" which will import it into PhotoPaint. Now create a mask by hitting Ctrl-M (or you can use the magic wand mask...) Now go to Mask, Mask outline, Reduce, and enter 1 for the pixel value. Then go to Object, Clip Mask, Create from Mask. Now you should have your choke. Click X to exit PP and "yes" to save the new image in your Draw document.
(If this is a vector graphic, don't worry about pixels. Give it a white outline and assign a point value to it...)