Many images show the object of interest surrounded by a region of air or other material that should be assigned a different material than regions of the same intensity within the object. The program can search for such an edge material and assign it a unique MCNP material. Two algorithms are available for setting the edge material. The edge algorithm starts from pixels at the edge of algorithm that are at the minimum (or maximum) intensity. The crop boundary algorithm uses the dominant material at the crop region boundaries. Which algorithm to use is set on the MCNP Options dialog. The material to use for the edge pixels is set from the Boundaries dialog. The search can either take place automatically when the image is loaded (as specified on the MCNP Options dialog) or be invoked from the Image context menu. The edge pixels are colored with the color specified on the MCNP Options dialog; this color is independent of the colors derived from the partitions. This figure shows an image with the edge material colored pink.
The edge algorithm first counts the number of pixels in the two rows and two columns at the edge of the image. If the number of pixels with intensities below the contrast window or at the minimum value is at least N% of the total number of pixels on the edges, the algorithm then searches for all pixels with intensities at or below the minimum that form a connected region with such pixels on the edges of the image. The crop boundary algorithm finds the material occuring in the largest number of pixels in the crop boundary. If the number of pixels with this material is at least N% of the number of pixels at the boundary, the algorithm searches for all pixels with the same material that form a connect region with such pixels on the crop boundary. The threshold N is set on the MCNP Options dialog. If the preponderance of edge pixels is at or above the maximum intensity, the algorithm operates on high intensity pixels rather than low intensity pixels. The program issues a warning, but proceeds the analysis, if the number of pixels at or below the minimum (or at or above the maximum) is less than 90% of the total number of edge pixels.
The edge algorithm cannot be undone. The DICOM file must be reloaded (without option to apply the algorithm upon loading in effect) to recover the original pixel values. Contrast changes do not affect edge material pixels. The edge region cannot be extended if new contrast settings result in more pixels at or below the minimum intensity (or at or above the maximum). If contrast changes are desired, they should be made before applying the edge algorithm.
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