-describe the object being scanned: rectangles, triangles, ellipses,
-sectors and segments. With these the standard phantoms used in the CT
-literature (the Herman and the Shepp-Logan) can be constructed. In fact
+describe the object being scanned. A phantom is composed a one or more
+phantom elements. These elements are simple geometric shapes,
+specifically, rectangles, triangles, ellipses, sectors and segments.
+With these the standard phantoms used in the CT literature (the Herman
+and the Shepp-Logan) can be constructed. In fact