Bowtie and flat filters - xcist/documentation GitHub Wiki
Bowtie filters are specified by the thickness (in cm) of Al, graphite, Cu, and Ti, as a function of alpha (the fan-angle of a detector column relative to the center channels).
cfg.bowtie_filename = 'BowtieFilename';
specifies the file that defines the bowtie.
Bowtie definition files are text files located in the directory bowtie. Each file contains one row for each alpha, and each row contains 5 numbers: alpha, Al thickness (cm), graphite thickness (cm), Cu thickness (cm), and Ti thickness (mm). VCTlarge.txt and VCTsmall.txt represent approximations of the GE LightSpeed bowtie filters. If no bowtie is needed, specify none.txt.
cfg.callback_bowtie = 'Bowtie';
specifies the function that gets the bowtie definition from BowtieFilename and converts into the format desired by CatSim. GetBowtie does not support variations in z such as the Heel effect.
cfg.enable_dynamic_bowtie is a flag that enables dynamic bowties.
For most accurate results, the “VCT” bowties should be used. The older bowtie files (bodybow.txt, headbow.txt, and none.txt) have been moved to the obsolete/bowtie directory. If you would like to use these files, (e.g. for compatibility with older experiments) you will need to revise your configuration file to point to the obsolete/bowtie directory. Please do not move them into another bowtie directory in your local copy of CatSim, because then if you inadvertently svn commit these back into the CatSim code repository, they will be in the wrong place.
Note: The bowtie file does not specify a “solid model” of the bowtie – it specifies the path length through the different materials of a bowtie. The only bowtie files implemented are for the LightSpeed VCT. For the 750HD, the bowtie is the same physical design, but it’s located at a slightly different position with respect to the source focal spot. Therefore, the bowtie files for the 750HD would have to be different, but they have not been updated to reflect this change.
Flat filters can also be placed between the source and the subject. cfg.flat_filters can specify a series of materials and thicknesses to define flat pre-patient filters. The material of the filter has to correspond to material files in materials/. See the Materials section of the Wiki. For example:
cfg.callback_filtration = 'Filter_Flat';
cfg.flat_filters = {'al' 1.5 'cu' 0.075};
specifies a filter stack of 1.5 mm aluminum and 75 microns of copper.
Note: CatSim applies a cosine factor to compute the path length through these filters across alpha (in-plane angle). Because the filters are flat, there is no dependence on the filter position with respect to the source focal spot. CatSim does not apply a cosine factor across gamma (angle in the cone beam (z) direction), unless the spectrum is also defined as a function of cone angle, in which case a cosine factor in the gamma direction is also applied.