# X-ray transform

In 1963, A.M. Cormack introduced a powerful diagnostic tool in radiology, computerized tomography, which is based on the mathematical properties of the X-ray transform in the Euclidean plane [a1] (cf. also Tomography). For a compactly supported continuous function $f$, its X-ray transform $X f$ is a function defined on the family of all straight lines $l$ in $\mathbf{R} ^ { 2 }$ as follows: let the unit vector $\theta$ represent the direction of $l$ and let $p$ be its signed distance to the origin, so that $l$ is represented by the pair $( \theta , p )$ (as well as $( - \theta , - p )$); then

\begin{equation*} X f ( l ) = X f ( \theta , p ) = \int _ { - \infty } ^ { \infty } f ( x + t \theta ) d t, \end{equation*}

where $x$ is an arbitrary point on the line $l$. This transform had already been considered in 1917 by J. Radon, who found its inverse with the help of its adjoint, given by the average value $F _ { x } ( q )$ of the $X f ( l)$ over the family of all lines $l$ which are at a (signed) distance $q$ from the point $x$, namely,

\begin{equation*} F _ { x } ( q ) = \frac { 1 } { 2 \pi } \int _ { S ^ { 1 } } X f ( \theta , x \cdot \theta + q ) d \theta \end{equation*}

where $x . \theta$ is the Euclidean inner product between $x$ and $\theta$. Radon then showed that the function $f$ can be recovered by the formula

\begin{equation*} f ( x ) = - \frac { 1 } { \pi } \int _ { 0 } ^ { \infty } \frac { d F _ { x } ( q ) } { q }. \end{equation*}

The generalization of the X-ray transform to Euclidean spaces of arbitrary dimension and replacing the family of all lines by the family of all affine subspaces of a fixed dimension is known as the Radon transform [a1]. For the Radon transform in the broader context of symmetric spaces, see also [a2].

Note that the adjoint of the X-ray transform can be traced back to the Buffon needle problem (1777): find the average number of times that a needle of length $l$, dropped at random on a plane, intersects one of the lines of a family of parallel lines located at a distance $D \geq l$ (cf. also Buffon problem). As explained in [a3], Chapt. 5, the solution leads to the consideration of a measure $\omega$ on the space of all lines in the plane and of $\omega$ invariance under all rigid motions. This measure induces a functional $K$ on the family of compact sets $\Omega$ by

\begin{equation*} K ( \Omega ) = \int _ { \lambda \bigcap \Omega \neq \phi } d \omega ( \lambda ), \end{equation*}

which is basically the adjoint of the X-ray transform. Thus, among the generalizations of the X-ray transform and its adjoint, one also finds basic links to integral geometry [a3], [a6], combinatorial geometry [a4], convex geometry [a5], as well as the Pompeiu problem.

How to Cite This Entry:
X-ray transform. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=X-ray_transform&oldid=52345
This article was adapted from an original article by Carlos A. Berenstein (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article