In mathematics, the Radon transform is the integral transform which takes a function f defined on the plane to a function Rf defined on the (two-dimensional) space of lines in the plane, whose value at a particular line is equal to the line integral of the function over that line. The transform was introduced in 1917 by Johann Radon, who also provided a formula for the inverse transform. Radon further included formulas for the transform in three dimensions, in which the integral is taken over planes (integrating over lines is known as the X-ray transform). It was later generalized to higher-dimensional Euclidean spaces, and more broadly in the context of integral geometry. The complex analogue of the Radon transform is known as the Penrose transform. The Radon transform is widely applicable to tomography, the creation of an image from the projection data associated with cross-sectional scans of an object.
Radon transform. Maps f on the (x,y)-domain to Rf on the (α,s)-domain.
Radon transform of the indicator function of two squares shown in the image below. Lighter regions indicate larger function values. Black indicates zero.
Original function is equal to one on the white region and zero on the dark region.
If a function represents an unknown density, then the Radon transform represents the projection data obtained as the output of a tomographic scan. Hence the inverse of the Radon transform can be used to reconstruct the original density from the projection data, and thus it forms the mathematical underpinning for tomographic reconstruction, also known as iterative reconstruction.
Horizontal projections through the shape result in an accumulated signal (middle bar). The sinogram on the right is generated by collecting many such projections as the shape rotates. Here, color is used to highlight which object is producing which part of the signal. Note how straight features, when aligned with the projection direction, result in stronger signals.
The Radon transform data is often called a sinogram because the Radon transform of an off-center point source is a sinusoid. Consequently, the Radon transform of a number of small objects appears graphically as a number of blurred sine waves with different amplitudes and phases.
Let be a function that satisfies the three regularity conditions:
the double integral , extending over the whole plane, converges;
for any arbitrary point on the plane it holds that
The Radon transform, , is a function defined on the space of straight lines by the line integral along each such line as:
Concretely, the parametrization of any straight line with respect to arc length can always be written:
where is the distance of from the origin and is the angle the normal vector to makes with the -axis. It follows that the quantities can be considered as coordinates on the space of all lines in , and the Radon transform can be expressed in these coordinates by:
More generally, in the -dimensional Euclidean space, the Radon transform of a function satisfying the regularity conditions is a function on the space of all hyperplanes in . It is defined by:
where the integral is taken with respect to the natural hypersurfacemeasure, (generalizing the term from the -dimensional case). Observe that any element of is characterized as the solution locus of an equation , where is a unit vector and . Thus the -dimensional Radon transform may be rewritten as a function on via:
It is also possible to generalize the Radon transform still further by integrating instead over -dimensional affine subspaces of . The X-ray transform is the most widely used special case of this construction, and is obtained by integrating over straight lines.
Thus the two-dimensional Fourier transform of the initial function along a line at the inclination angle is the one variable Fourier transform of the Radon transform (acquired at angle ) of that function. This fact can be used to compute both the Radon transform and its inverse. The result can be generalized into n dimensions:
The dual Radon transform is a kind of adjoint to the Radon transform. Beginning with a function g on the space , the dual Radon transform is the function on Rn defined by:
The integral here is taken over the set of all hyperplanes incident with the point , and the measure is the unique probability measure on the set invariant under rotations about the point .
Concretely, for the two-dimensional Radon transform, the dual transform is given by:
In the context of image processing, the dual transform is commonly called back-projection as it takes a function defined on each line in the plane and 'smears' or projects it back over the line to produce an image.
This is a natural rotationally invariant second-order differential operator. On , the "radial" second derivative is also rotationally invariant. The Radon transform and its dual are intertwining operators for these two differential operators in the sense that:
In analysing the solutions to the wave equation in multiple spatial dimensions, the intertwining property leads to the translational representation of Lax and Philips. In imaging and numerical analysis this is exploited to reduce multi-dimensional problems into single-dimensional ones, as a dimensional splitting method.
Intuitively, in the filtered back-projection formula, by analogy with differentiation, for which , we see that the filter performs an operation similar to a derivative. Roughly speaking, then, the filter makes objects more singular. A quantitive statement of the ill-posedness of Radon Inversion goes as follows:
where is the previously defined adjoint to the Radon Transform. Thus for , we have:
The complex exponential is thus an eigenfunction of with eigenvalue . Thus the singular values of are . Since these singular values tend to , is unbounded.
Compared with the Filtered Back-projection method, iterative reconstruction costs large computation time, limiting its practical use. However, due to the ill-posedness of Radon Inversion, the Filtered Back-projection method may be infeasible in the presence of discontinuity or noise. Iterative reconstruction methods (e.g.iterative Sparse Asymptotic Minimum Variance) could provide metal artefact reduction, noise and dose reduction for the reconstructed result that attract much research interest around the world.
For computational purposes, the power of the Laplacian is commuted with the dual transform to give:
where is the Hilbert transform with respect to the s variable. In two dimensions, the operator appears in image processing as a ramp filter. One can prove directly from the Fourier slice theorem and change of variables for integration that for a compactly supported continuous function of two variables:
Thus in an image processing context the original image can be recovered from the 'sinogram' data by applying a ramp filter (in the variable) and then back-projecting. As the filtering step can be performed efficiently (for example using digital signal processing techniques) and the back projection step is simply an accumulation of values in the pixels of the image, this results in a highly efficient, and hence widely used, algorithm.
Explicitly, the inversion formula obtained by the latter method is:
The dual transform can also be inverted by an analogous formula:
The main theorem about this transform is that this transform induces an equivalence of the categories of perverse sheaves on the projective space and its dual projective space, up to constant sheaves.
Radon, Johann (1917), "Über die Bestimmung von Funktionen durch ihre Integralwerte längs gewisser Mannigfaltigkeiten", Berichte über die Verhandlungen der Königlich-Sächsischen Akademie der Wissenschaften zu Leipzig, Mathematisch-Physische Klasse [Reports on the Proceedings of the Royal Saxonian Academy of Sciences at Leipzig, Mathematical and Physical Section], Leipzig: Teubner (69): 262–277; Translation: Radon, J.; Parks, P.C. (translator) (1986), "On the determination of functions from their integral values along certain manifolds", IEEE Transactions on Medical Imaging, 5 (4): 170–176, doi:10.1109/TMI.1986.4307775, PMID18244009, S2CID26553287.