TECHNICAL FIELD The invention generally relates to phase contrast x-ray systems with x-ray absorbing components, and more particularly to x-ray absorbing materials deposited in gratings used to create and detect phase shifts.

BACKGROUND

It is common practice to set up an x-ray imaging system such that an object is placed in the x-ray beam between the x-ray source and the detector. In case of a phase contrast imaging system, it is common practice to also place a number of gratings in the x-ray beam, between the source and the detector. The purpose of these gratings is to create an interference pattern such that when an object is placed in the x-ray beam, its phase-shift and scatter properties affect the interference pattern. The resulting interference pattern can then be read by a detector and, with appropriate algorithms, allows image reconstruction of the object's phase contrast and scatter properties. This apparatus is typically referred to as a grating interferometer. By capturing several images, with slight differences in the relative position of the gratings, three types of images can be captured with such a system: the absorption image ("classic" x-ray image), the phase-contrast image (which represent the refractive properties of an object) and the dark-field image (which represent the scattering properties of an object). In this way, an x-ray examination can provide information that is not attainable with classic absorption x-ray. For example, the phase contrast image can be used to distinguish between different types of liquids or different types of soft tissue; the dark field image can be used to image fine-detail structures such as the alveoli in the lungs. Figure 1 is a schematic diagram illustrating an exemplary phase contrast x-ray system 100 with three gratings; a source grating GO, a phase grating G1 and an analyzer absorption grating G2. An example of an x-ray system with such a set of gratings can be found in US2007/0183580, which relates to a focus/detector system of an x-ray apparatus for generating phase contrast recordings. The system includes a beam source equipped with a focus and a focus-side source grating GO, arranged in the beam path and generates a field of ray-wise coherent x-rays, a grating/detector arrangement having a phase grating G1 and grating lines arranged parallel to the source grating for generating an interference pattern, an analysis grating G2 and a detector having a multiplicity of detector elements arranged flat for measuring the position-dependent radiation intensity behind the phase grating. When such a system is to be used for medical examinations, the detector, as well as the G2 grating, needs to be of the same, or similar, size as the part of the body to be examined. For example, for chest x-ray the size of the detector is typically in the order of 43 x 43 cm. In order for such a system to work well, the absorbing lines of the G2 grating need to have high x-ray absorption, which means that they need to be made of a highly x-ray absorbing material. Because the width of the lines is in the micrometer range, a pre-fabricated grating structure is typically produced using lithography and 3D processing methods, whereafter absorbing material is filled in the trenches. In initial research work, the fill material of choice has been gold; however, because a substantial amount of gold is required to fill a whole grating, and gold is a very expensive material, this solution is not commercially viable for widespread use. Attempts have been made to replace gold with certain other heavy elements but there are challenges with each of these elements, such as toxicity and process-related difficulties with respect to filling a grating. SUMMARY

The present invention overcomes these and other drawbacks of the prior art. It is a general object to provide an improved absorption grating with respect to manufacturability, scalability, and cost.

As mentioned, x-ray absorbing materials are traditionally chosen among the heavy elements. These materials such as gold can typically be found in period 6 or higher in the periodic table of elements. However, when analyzing x-ray absorption properties of all elements, the inventors have found that also some lighter materials can be good x-ray absorbers in a specific energy range.

In particular, it is desirable to have an absorption grating where the absorbing lines are filled with a highly x-ray absorbing material, which is optimized for the x-ray energies in the intended application, without having to select materials among heavy elements.

According to the proposed technology, there is provided an x-ray absorbing grating and a corresponding method for manufacturing such an x-ray absorbing grating for a phase contrast x-ray system configured for operation in a specific intended x-ray spectrum. In this aspect, at least a substantial part of the x-ray spectrum is between 30 keV and 90 keV. The x-ray absorbing grating includes a grating structure and x-ray absorbing material deposited in the grating structure, and the x-ray absorbing material for the grating includes one or more elements having their characteristic absorption K-edges below, or in the lower end, of the intended x-ray spectrum. By way of example, the x-ray absorbing material for the grating includes an alloy or mixture comprising any of these elements.

In a particular example, the x-ray absorbing material for the grating includes one or more elements having a characteristic absorption K-edge at or below 30 keV.

For example, a main part of the x-ray spectrum may be in the range between 30 keV and 70 to 90 keV.

Optionally, the x-ray spectrum is a typical x-ray spectrum used for medical x- ray examinations spanning the energy range from 30 keV to 70 keV.

In another example, the x-ray spectrum begins already at 15-20 keV, but the main part of the spectrum is in the range between 30 keV and 70 to 90 keV.

Alternatively, at least part of the x-ray spectrum extends over energies of 90 keV, but the main part of the spectrum is in the range between 30 keV and 70 to 90 keV.

Preferably, the x-ray absorbing material is solid at room temperature and/or is suitable for filling into a grating by melting, plating or using other process technologies. As an example, the x-ray absorbing material may include one or more elements from period 5 of the periodic table.

In a particular example, the x-ray absorbing material deposited in the grating structure forms x-ray absorbing grating lines. For example, the grating structure may be a structure with long trenches separated by dividing walls, wherein the x-ray absorbing material is filled in the trenches. There is also provided a phase contrast x-ray system comprising an x-ray absorbing grating as discussed herein.

Basically, the invention offers one or more of the following advantages: Better manufacturability, better scalability to large size gratings and lower cost.

The invention is particularly useful in the following technical applications: Phase contrast and dark field imaging, primarily for use in medical examinations but also suitable for security, non-destructive testing and scientific applications.

Other advantages of the invention will be appreciated when reading the below detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which: FIG. 1 is an exemplary schematic diagram illustrating a phase contrast x-ray system with three gratings. FIG. 2A shows an example x-ray absorption spectrum for medical examinations and example x-ray mass attenuation curves for period 5 elements. FIG. 2B shows an example x-ray absorption spectrum for medical examinations and example x-ray mass attenuation curves for period 6 elements.

FIG. 2C shows an example x-ray absorption spectrum for medical examinations and example x-ray mass attenuation curves as a combined chart for both example period 5 and period 6 elements.

FIG. 3A is schematic diagram illustrating an example of a grating structure seen from the side, where the grating is basically a structure with long trenches separated by dividing walls.

FIG. 3B is a schematic diagram illustrating an example of a grating structure seen from above. FIG. 4 is a schematic flow diagram illustrating an example of a method for manufacturing an x-ray absorbing grating for a phase contrast x-ray system adapted for operation in a specific intended x-ray spectrum.

DETAILED DESCRIPTION

Throughout the drawings, the same reference numbers are used for similar or corresponding elements. FIG. 1 is an exemplary schematic diagram illustrating a phase contrast x-ray system 100 with three gratings; a source grating GO, a phase grating G1 and an analyzer absorption grating G2. The GO and G2 gratings are absorption gratings, which means that they are filled with x-ray absorbing material, whereas the G1 grating is a phase grating without fill material. The source grating (GO), typically placed close to the X-ray tube, is an aperture mask with transmitting slits. It creates an array of periodically repeating line sources and effectively enables the use of relatively large, X-ray sources, without compromising the coherence requirements of the arrangement formed by G1 and G2. The image contrast itself is formed via the combined effect of the two gratings G1 and G2. The second grating (G1 ) acts as a phase mask, and imprints periodic phase modulations onto the incoming wave field. Through the Talbot effect, the phase modulation is transformed into an intensity modulation in the plane of G2, forming a linear periodic interference pattern perpendicular to the optical axis and parallel to the lines of G1. The third grating (G2), with absorbing lines and the same periodicity and orientation as the interference pattern created by G1 , is placed in the detection plane, immediately in front of the detector. When one of the gratings is scanned along the transverse direction, the intensity signal in each pixel detector plane oscillates, which can be read out by the detector.

It is a good idea is to select, as absorbing material for a grating, a material that has good x-ray absorption properties, e.g. in the energy range that is typically used in medical examinations, and that also has other material properties that makes it suitable for filling into a grating structure. By way of example, it is an advantage if the absorbing material is suitable for filling into a silicon structure, which enables the use of efficient silicon wafer-processing methods. A typical x-ray spectrum used for medical x-ray examinations spans the energy range from 30 to 70 keV. This is sometimes referred to as a 70 kVp (kilovolt peak) spectrum. For some examinations with conventional x-ray, a higher peak energy can be used, such as 90 kVp or even 110 kVp. However, for phase contrast imaging it is an object to have a small spread in energy, which means that 70 or 90 kVp spectra are normally preferred; hereinafter these are examples of spectra in the intended energy range. An example 70 kVp spectrum is shown in figures 2A-C. It should though be understood that the proposed technology is not limited thereto. In medical applications, there may be a substantial amount of filtering (e.g. using aluminum) in the x-ray path to remove low-energetic x-rays. This filtering makes the x-ray spectrum "harder", and the resulting spectrum therefore normally begins at around 30 keV. In other applications, such as material analysis and dental applications, the amount of filtering can be lower, and therefore the resulting x-ray spectrum may begin already at 15-20 keV, but with the main part of the spectrum in the range between 30 and 70 to 90 keV. On the other hand, there may also be applications where at least part of the x- ray spectrum may extend over energies of 90 keV, but where the main part of the spectrum is still in the range between 30 and 70 to 90 keV.

The proposed technology is therefore particularly suitable for applications where at least a substantial part of the x-ray spectrum is between 30 and 90 keV.

It is therefore an object to identify a material with high x-ray absorption in the 30 to 90 keV energy range. In addition, the selected material should be solid at room temperature; it should be suitable for filling into a grating by melting, plating or using other process technologies. It is also a desire to identify a material which is not toxic or otherwise harmful to the environment or people handling the material.

By way of example, it is an advantage if the absorbing material is suitable for filling into a silicon structure, which enables the use of efficient silicon wafer- processing methods. Traditionally, x-ray absorbing materials are chosen among the heavy elements of the periodic table. These materials can typically be found in period 6 or higher in the periodic table of elements; examples are gold, lead and bismuth. Gold, because of its high atomic number and density, is generally considered a very good x-ray absorber, followed by lead and bismuth. However, when analyzing x-ray absorption properties of all materials, the inventors have found that also some lighter materials are good x-ray absorbers in the intended energy range.

All materials have a characteristic absorption K-edge; this represents a discontinuity in the x-ray absorption curve as shown in figures 2A-C. At energies right above the K-edge, the absorption rate is higher than at energies right below the K-edge. Typically, heavy elements have their K-edges in the range of 50 keV or higher, i.e. within or often above the intended x-ray energy range. However, a thorough analysis of all elements made by the inventors reveals that some lighter materials, which have their K-edges below, or in the lower end, of the intended x-ray spectrum, can surprisingly be quite good x-ray absorbers for the intended x-ray energy range. These materials are not typically considered good x-ray absorbers because of their relatively low atomic number and density. In particular, some materials from period 5 of the periodic table have quite good x-ray absorption properties in the 30-90 keV range. Examples include silver, indium and tin.

Fig. 2A shows an example x-ray absorption spectrum for medical examinations and x-ray mass attenuation curves for period 5 elements, and FIG. 2B shows an example x-ray absorption spectrum for medical examinations and example x-ray mass attenuation curves for period 6 elements. A combined chart is shown in FIG. 2C. It should be noted that the intended energy range of 30-90 keV is merely an example and that an absorption grating made of a period 5 element will also work outside of this range. It should also be noted that there are other applications where this technology can be applied, for example in materials analysis. It should also be noted that highly absorbing materials with suitable properties for filling in gratings can be formed by mixing two or more elements. The materials suitable for grating filling may thus include elements or materials having their K-edges below, or in the lower end, of the intended x-ray spectrum, or alloys or mixtures thereof or alloys or mixtures comprising any of these elements. Examples include elements from period 5, such as silver, indium and tin, having their K-edges below, or in the lower end, of the intended x-ray spectrum. In addition, the materials should preferably have low toxicity and be solid at room temperature.

In their research, the inventors have realized that some relatively light materials, which are not typically considered good x-ray absorbers because of their relatively low atomic number and density, may in fact be quite good x-ray absorbers in the 30-90 keV range. These may for example be relatively light materials, which have their characteristic absorption K-edges at or below 30 keV. FIG. 3A is schematic diagram illustrating an example of a grating structure seen from the side. In this example, the grating 10 is basically a structure with long trenches separated by dividing walls. FIG. 3B is a schematic diagram illustrating an example of a grating structure seen from above.

FIG. 4 is a schematic flow diagram illustrating an example of a method for manufacturing an x-ray absorbing grating for a phase contrast x-ray system adapted for operation in a specific intended x-ray spectrum. In this example, at least a substantial part of the x-ray spectrum is between 30 keV and 90 keV. Basically, the method comprises:

S1 : providing a grating structure;

S2: selecting an x-ray absorbing material that includes one or more elements having their characteristic absorption K-edges below, or in the lower end, of the intended x-ray spectrum, and S3: depositing the x-ray absorbing material in the grating structure.

By way of example, the x-ray absorbing material for the grating includes one or more elements having a characteristic absorption K-edge at or below 30 keV. For example, a main part of the x-ray spectrum may be in the range between 30 keV and 70 to 90 keV.

In a particular example, the x-ray spectrum is a typical x-ray spectrum used for medical x-ray examinations spanning the energy range from 30 keV to 70 keV. For example, the x-ray spectrum may begin already at 15-20 keV, but where the main part of the spectrum is in the range between 30 keV and 70 to 90 keV.

Optionally, at least part of the x-ray spectrum extends over energies of 90 keV, but where the main part of the spectrum is in the range between 30 keV and 70 to 90 keV. The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible.