-that completely surrounds the phantom. Let $p_l$ be the width (also height)
-of this square. The diameter of this boundary box, $p_d$ is then
-\latexonly{$$p_d = \sqrt{2}(p_l)$$}
-\latexignore{sqrt(2) * $p_l$.}
-This relationship can be seen in figure 1 with the parallel geometry.
-
-\subsubsection{Focal Length \& Field of View}
-The two important variables is the focal-length-ratio $f_lr$.
-This is used along with $p_d$ to
-define the focal length according to
-\latexonly{$$f_l = f_lr p_d$$}
-\latexignore{\\$f_l$ = $f_lr$ x $p_d$\\}
-where
-we consider the case of a first generation parallel beam CT scanner.
-
-\subsubsection{Parallel Geometry}\label{geometryparallel}\index{Concepts,Scanner,Geometries,Parallel}
+that completely surrounds the phantom. Let
+\latexonly{$P_l$}\latexignore{\emph{Pl}}
+be the width and height of this square. The diameter of this boundary box,
+\latexonly{$P_d$,}\latexignore{\emph{Pd},}
+is then
+\latexignore{\\$$\emph{Pl x sqrt(2)}$$\\}
+\latexonly{$$P_d = P_l \sqrt{2}$$}
+CT scanners actually collect projections around a circle rather than a
+square. The diameter of this circle is also the diameter of the boundary
+square
+\latexonly{$P_d$.}\latexignore{\emph{Pd}.}
+These relationships are diagrammed in figure 1.
+
+\subsubsection{View Diameter}
+The \emph{view diameter} is the area that is being processed during scanning of phantoms as
+well as during rasterization of phantoms. By default, the \emph{view diameter}
+is set equal to the \emph{phantom diameter}. It may be useful, especially for
+experimental reasons, to process an area larger (and maybe even smaller) than
+the phantom. Thus, during rasterization or during projections, \ctsim\ will
+ask for a \emph{view diameter ratio},
+\latexonly{$V_{dR}$.}\latexignore{\emph{VdR}.}
+The \emph{view diameter} is then set as
+\latexonly{$$V_d = P_d V_{dR}$$}\latexignore{\\$$\emph{Vd = Pd x VdR}$$}
+
+By using a
+\latexonly{$V_{dR}$}\latexignore{\emph{VdR}}
+less than 1, \ctsim\ will allow
+for a \emph{view diameter} less than
+\emph{phantom diameter}.
+This will lead to significant artifacts. Physically, this would
+be impossible and is analagous to inserting an object into the CT
+scanner that is larger than the scanner itself!
+
+\subsubsection{Scan Diameter}
+By default, the entire \emph{view diameter} is scanned. For experimental
+purposes, it may be desirable to scan an area either larger or smaller than
+the \emph{view diameter}. Thus, the concept of \emph{scan diameter}
+\latexonly{$S_{dR}$}\latexignore{\emph{SdR}}
+is born. The scan diameter
+\latexonly{$S_d$}\latexignore{\emph{Sd}}
+is defined as
+\latexonly{$$S_d = V_d S_{dR}$$}\latexignore{\\$$\emph{Sd = Vd x SdR}$$\\}
+By default and for all ordinary scanning, the \emph{scan diameter ratio} is to \texttt{1}. If the \emph{scan diameter ratio} is less than \texttt{1}, you
+can plan on significant artifacts.
+
+\subsubsection{Focal Length}
+The \emph{focal length},
+\latexonly{$F_l$,}\latexignore{\emph{Fl},}
+is the distance of the X-ray source to the center of
+the phantom. The focal length is set as a ratio,
+\latexonly{$F_{lR}$,}\latexignore{\emph{FlR},}
+of the view radius. Focal length is
+calculated as
+\latexonly{$$F_l = F_{lR} (V_d / 2)$$}\latexignore{\\$$\emph{Fl = FlR x (Vd / 2)}$$}
+
+For parallel geometry scanning, the focal length doesn't matter. However,
+divergent geometry scanning (equilinear and equiangular), the \emph{focal
+length ratio} should be set at \texttt{2} or more to avoid artifacts.
+
+\subsection{Parallel Geometry}\label{geometryparallel}\index{Concepts,Scanner,Geometries,Parallel}