[Rtk-users] Physical interpretation of the input and output images when doing a forward projection

[MORIN Estelle]

Dear RTK users,

I would like to do projections of volumes stored in 3D images. However, I do not understand what the voxel values of my 3D images should represent physically speaking. Should they represent attenuation coefficients, densities or something else? Should they be normalized?

On the wiki page "RTK/Scripts/ForwardProjection", it is highlighted that the image used for the projection is in Hounsfield units. However, computing a projection on the image given in example gives me an output image with very high negative values which I can't interpret. Moreover, as the output values are so big and negative, I cannot compute the attenuated image with the formula: attenuatedImage = I0 * exp(-projection). Indeed, this results in an image with a lot of pixel values equal to infinity.

I also looked at the file "rtkSheppLoganPhantom.cxx" and its header in order to understand what should be my input image. It seems that the Shepp Logan Phantom is composed of 10 ellipsoids whose "densities" are included in the interval [-1,1]. Moreover, the sum of those densities is equal to one. Does this mean that the coefficients of my 3D images should be normalized between -1 et 1? What is the physical meaning of this?

I thank you in advance for your answer,
Best wishes,

Estelle Morin
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