i.e. what is the focal length for the pinhole camera model shown here: http://tech.snmjournals.org/content/33/1/3/F7.expansion.html
It would seem to me based on the usual camera models you find in the literature that "focal length" would be equivalent to the x-ray source to flat panel detector distance.
You are correct - but I think it gets the point across most people draw a 2d image projected onto a 1d line to work out the math as it is difficult to draw the 3D on a 2d piece of paper
The focal length is either the distance from the source to the detector plane or the distance from the source to the object isocenter.
And at this point I've convinced myself the focal length is the SID dimension (source to flat panel detector) rather than SOD (source to object isocenter).
Thanks though
Wissent calls those dimensions Gamma and Gamma_D in the pinhole model applied to CT in Figure 2: http://www.ncbi.nlm.nih.gov/pubmed/11021683
No problem with your understanding about the focal length.
However when I say 'fan-beam' geometry, it already means a 2D-to-1D implementation; its 3D-to-2D counterpart can be 'cone-beam' geometry.
Pinhole geometry is something else usually used in emission imaging instead of transmission imaging. A pinhole is a very small hole through a shielding material between the object and the detector, not a source like the focus spot in a X-ray tube. See a drawing of pinhole geometry here: https://en.wikipedia.org/wiki/File:Pinhole-camera.svg
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