Why is it normal for the SheppLogan created through rtk to have particularly low contrast

 using DSLType = rtk::DrawSheppLoganFilter<OutputImageType, OutputImageType>;
    DSLType::Pointer dsl2 = DSLType::New();

Because it is the design of the Shepp Logan! See chapter 3 of Kak & Slaney, figure 3.4b, the contrast in the center is 1%. This is designed to be a difficult reconstruction task. Not to be mixed with the modified high contrast version of the Shepp Logan in Matlab, see here which has been probably developed to make the task easier. This one is described in page 201 of “Toft PA, Sørensen JA. The Radon Transform - Theory and Implementation. Kgs. Lyndgby, Denmark: Technical University of Denmark (DTU), 1996.” It can be drawn with rtk::DrawGeometricPhantomImageFilter using the following text file

//  High  contrast  Shepp  Logan  phantom,  idem  as  Matlab  
[Ellipsoid]  A=0.6900  B=0  C=0.9200  x=0     y=0  z=0        beta=0    gray=2.0
[Ellipsoid]  A=0.6624  B=0  C=0.8740  x=0     y=0  z=-0.0184  beta=0    gray=-1.6
[Ellipsoid]  A=0.1100  B=0  C=0.3100  x=0.22  y=0  z=0        beta=-18  gray=-0.4  
[Ellipsoid]  A=0.1600  B=0  C=0.4100  x=-0.22 y=0  z=0        beta=18   gray=-0.4  
[Ellipsoid]  A=0.2100  B=0  C=0.2500  x=0     y=0  z=0.35     beta=0    gray=0.2  
[Ellipsoid]  A=0.0460  B=0  C=0.0460  x=0     y=0  z=0.1      beta=0    gray=0.2  
[Ellipsoid]  A=0.0460  B=0  C=0.0460  x=0     y=0  z=-0.1     beta=0    gray=0.2  
[Ellipsoid]  A=0.0460  B=0  C=0.0230  x=-0.08 y=0  z=-0.605   beta=0    gray=0.2  
[Ellipsoid]  A=0.0230  B=0  C=0.0230  x=0     y=0  z=-0.606   beta=0    gray=0.2  
[Ellipsoid]  A=0.0230  B=0  C=0.0460  x=0.06  y=0  z=-0.605   beta=0    gray=0.2  
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thank you