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}
As mentioned above, the focal length is not used in this simple
the \emph{scan diameter ratio}. If values of less than \texttt{1} are
used for these two variables, significant distortions will occur.
+
\subsection{Divergent Geometries}\label{geometrydivergent}\index{Concepts,Scanner,Geometries,Divergent}
\subsubsection{Overview}
Next consider the case of equilinear (second generation) and equiangular
\caption{Equilinear and equiangular geometries.}
\end{figure}
+
\subsubsection{Fan Beam Angle}
For these divergent beam geometries, the \emph{fan beam angle} needs
to be calculated. For real-world CT scanners, this is fixed at the
\caption{Calculation of $\alpha$}
\end{figure}
+
Empiric testing with \ctsim\ shows that for very large \emph{fan beam angles},
greater than approximately
-\latexonly{$120^{\circ}$,}\latexignore{120 degrees,}
+\latexonly{$120^\circ$,}\latexignore{120 degrees,}
there are significant artifacts. The primary way to manage the
\emph{fan beam angle} is by varying the \emph{focal length} since the
\emph{scan diameter} by the size of the phantom.
\end{figure}
An example of the this geometry is in figure 2.5.
+
\subsubsection{Examples of Geometry Settings}
Consider increasing the focal length ratio to two leaving the
field of view ratio as 1, as in Figure 4. Now the detectors array is
Backprojection is the process of ``smearing'' the filtered projections over
the reconstructing image. Various levels of interpolation can be specified.
In general, the trade-off is between quality and execution time.
-
\parskip=10pt%
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%\backgroundcolour{255;255;255}\textcolour{0;0;0}% Has an effect in HTML only
-\winhelpignore{\title{Manual for CTSim 3.0}%
+\winhelpignore{\title{CTSim 3.0 User Manual}%
\author{Kevin Rosenberg, M.D.}%
\date{January 2001}%
}%
-\winhelponly{\title{Manual for CTSim 3.0}%
+\winhelponly{\title{CTSim 3.0 User Manual}%
\author{by Kevin Rosenberg, M.D.\\$$\image{3cm;3cm}{logo-huge.bmp}$$}%
}%
\makeindex%