When an object is subjected to x-ray radiation some of the x-ray photons are absorbed and some pass through the object, unscattered, to impinge on an x-ray detector. This is referred to as “direct radiation”. Some x-rays are absorbed and others are scattered. The intensity of the scatter produced may exceed the magnitude of the direct radiation detected by the detector. The scattered radiation results in poor image quality by reducing contrast and increasing noise. Absorbed x-rays provide the contrast in an x-ray image. If scattered x-ray photons hit the detector random noise in the image is increased since it is not possible to identify where the scattered x-ray photons have come from.
The most widely adopted technique used to address the problem of scatter is to place an anti-scatter grid between the x-ray detector and the object under test. An anti-scatter grid comprises a series of spaced apart parallel lamellae formed of x-ray absorbing material. A large proportion of scattered x-rays engage one of the lamella and are absorbed. It is therefore predominantly the direct radiation that is detected by the x-ray detector when an anti-scatter grid is present.
The original anti-scatter grid is described in U.S. Pat. No. 1,164,987 (Bucky).
One of the problems with anti-scatter grids is that in addition to reducing the effect of scatter in the detected image, the x-ray absorbing lamellae absorb some of the direct radiation, that is those photons travelling in the path of the lamellae.
In order compensate for the photons lost in the anti scatter grid and hence the reduced image quality, it is common practice to increase the x-ray flux. However, this is disadvantageous where the x-ray imaging is of x-ray sensitive material. This is of most concern in medical imaging where the x-ray radiation dose to the patient must be increased to compensate for the presence of the anti-scatter grid.
Some attempts have been made to reduce the x-ray power used in x-ray imaging.
U.S. Pat. No. 7,551,716 instead of using an anti-scatter grid uses mathematical methods to determine approximately the scatter x-ray photons. It is asserted that by utilising mathematical methods to determine approximately the scatter x-ray photons, the x-ray dose can be reduced or the signal to noise ratio increased when compared with an x-ray apparatus using an anti-scatter grid.
An x-ray apparatus that includes a multi-absorption plate is described in the applicant's patent application published under number GB2498615. In this x-ray apparatus the x-ray energy spectrum is perturbed in many different ways. This apparatus and method provides information that allows material properties to be identified. Patent application number GB2498615 is incorporated herein by reference.
The prior art addresses problems caused by scatter radiation either by using an anti-scatter grid to reduce substantially the scatter radiation, or by applying a mathematical correction to remove an estimation of what the scatter radiation might be from an x-ray image.
The present invention addresses the issue of scatter in a new way which involves measuring both the direct and the scattered radiation. This is achieved by imposing a change on the apparatus that affects both the direct and scattered radiation.
Scatter radiation has previously been considered as impacting negatively on an x-ray image. Previous attempts at combatting the problem of scatter focus on optimising the removal of scatter radiation, so that the image is as far as possible formed by direct x-ray radiation impinging on the x-ray detector.
As well as providing for measurement of scatter radiation, the present invention provides for scattered radiation to be used in the identification of materials and thicknesses of materials and in improving the contrast to noise ratio.
By comparing a measured output signal from an x-ray detector to an independent measure of the scattered radiation it is then possible to identify the scatter radiation within the image that contains direct radiation and scatter radiation, and to remove the scatter radiation from the image. It is thereby possible to increase the contrast to noise ratio. In the context of medical applications and other applications where x-rays are used to analyse materials that may be harmed by x-rays, the present invention allows either the dose to be reduced to produce a similar standard of image, or using the same x-ray dose a better image can be generated.