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In mathematics, the X-ray transform (also called John transform) is an integral transform introduced by Fritz John in 1938 that is one of the cornerstones of modern integral geometry. It is very closely related to the Radon transform, and coincides with it in two dimensions. In higher dimensions, the X-ray transform of a function is defined by integrating over lines rather than over hyperplanes as in the Radon transform. The X-ray transform derives its name from X-ray tomography because the X-ray transform of a function ''ƒ'' represents the scattering data of a tomographic scan through an inhomogeneous medium whose density is represented by the function ''ƒ''. Inversion of the X-ray transform is therefore of practical importance because it allows one to reconstruct an unknown density ''ƒ'' from its known scattering data. In detail, if ''ƒ'' is a compactly supported continuous function on the Euclidean space R''n'', then the X-ray transform of ''ƒ'' is the function ''Xƒ'' defined on the set of all lines in R''n'' by : where ''x''0 is an initial point on the line and θ is a unit vector giving the direction of the line ''L''. The latter integral is not regarded in the oriented sense: it is the integral with respect to the 1-dimensional Lebesgue measure on the Euclidean line ''L''. The X-ray transform satisfies an ultrahyperbolic wave equation called John's equation. The Gauss hypergeometric function can be written as an X-ray transform . ==References== *. * *Citation | last1=Helgason | first1=Sigurdur | title=Geometric analysis on symmetric spaces | publisher= location=Providence, R.I. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「X-ray transform」の詳細全文を読む スポンサード リンク
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