WO2009072044A2

WO2009072044A2 - Imaging apparatus, calibration assembly, device and method for obtaining a monochromatic flow of x-ray radiation, and methods of calibrating a detector element

Info

Publication number: WO2009072044A2
Application number: PCT/IB2008/054997
Authority: WO
Inventors: Jens-Peter Schlomka; Ewald Roessl
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2007-12-04
Filing date: 2008-11-28
Publication date: 2009-06-11
Also published as: WO2009072044A3

Abstract

An imaging apparatus based on the detection of X-ray photons, in particular for medical use, is described. The imaging apparatus comprises a radiation source (16) for generating a flow (18) of X-ray radiation, a detector (20) for detecting X-ray radiation, a control unit adapted to control the radiation source (16), to read out information from the detector (20) and to process the information into a visual image, and a device (10) for obtaining a monochromatic flow (12) of X-ray radiation. Furthermore, a device (10) is described, which comprises a positioning apparatus (34) and a crystalline element (36) mounted on the positioning apparatus (34), wherein the positioning apparatus (34) is adapted to position the crystalline element (36) relative to a flow (18) of X-ray radiation generated by a radiation source (16) such that the crystalline element (36) reflects a monochromatic flow (12) of X-ray radiation according to Bragg' s law. Finally, a corresponding calibration assembly, a method of obtaining a monochromatic flow (12) of X-ray radiation and methods of calibrating a detector element (42) of a detector (20) are described.

Description

IMAGING APPARATUS, CALIBRATION ASSEMBLY, DEVICE AND METHOD FOR

OBTAINING A MONOCHROMATIC

FLOW OF X-RAY RADIATION, AND METHODS OF CALIBRATING A DETECTOR

ELEMENT

FIELD OF THE INVENTION

The present invention relates to an imaging apparatus, a calibration assembly, a device and a method for obtaining a monochromatic flow of X-ray radiation, and methods of calibrating a detector element.

BACKGROUND OF THE INVENTION

Computer tomography (CT, also called computed tomography) has evolved into a commonly used means, when it comes to generating a three-dimensional image of the internals of an object. The three-dimensional image is created on the basis of a large number of two-dimensional X-ray images taken around a single axis of rotation. While CT is most commonly used for medical diagnosis of the human body, it has also been found applicable for non-destructive materials testing and security applications, e.g. baggage screening. Detailed information regarding the basics and the application of CT can be found in the book "Computed Tomography" by Willi A. Kalender, ISBN 3-89578-216-5. One of the key innovative aspects in future CT and X-ray imaging is the energy-resolved analysis based on the counting of photons which are transmitted by the object when being exposed to X-ray radiation. This technique is commonly referred to as "Spectral CT". Depending on the number and energy of the transmitted photons, it can be concluded through which types of material the X-ray beams have traveled. In particular, this allows identification of different parts, tissues and materials within a human body.

Concerning the energy-resolved analysis, Spectral CT may revolutionize CT by the addition of "contrast-agent-only" imaging capabilities. Such contrast agents can also be targeted and Spectral CT may thus become a molecular imaging modality. Further advantages of Spectral CT are the elimination of beam- hardening effects and quantification of tissue composition, so that Spectral CT will increase the range of CT applications. This means that there is great interest in improving Spectral CT techniques and techniques related to performing Spectral CT. As indicated above, the energy-resolved detection of photons is a prerequisite for Spectral CT. Photons in the energy range of 15 to 150 keV with a required energy resolution inferior to spectroscopic detectors (dE/E -10%) are used in Spectral CT. Generally, energy-binning is used, which means that the energy of an incoming photon is compared with several thresholds (a lower and an upper threshold define an energy bin), and the number of photons for the respective bin is incremented by one for each photon.

It is of great importance that the positions of these thresholds are set very accurately and identically for each detector element. However, these thresholds may shift during operation and it is therefore required to check and re-set thresholds during operation of such a scanner, e.g. in a calibration scan prior to each measurement, during start-up, or at given times of the day.

Furthermore, the detector response of each detector element when illuminated with monochromatic radiation is very important for processing the spectral data. Finally, knowledge of the spectrum and its changes introduced e.g. by anode roughening is also crucial for the exact data analysis.

US 2003/0215052 Al describes a calibration source for X-ray detectors and discloses a method and a device for calibrating the energy response of detectors of photons in the range of about 0.5 keV to at least 100 keV. The device makes use of the inherent property of a polarizable crystal such as a pyroelectric crystal to produce monochromatic X-rays when the crystal is heated or cooled in a partial vacuum. Specific calibration energies of X-ray emission may be selected for the user's application by selecting a coating to the pyroelectric crystal and an external foil. However, the proposed device is rather complex and requires the provision of an additional radiation source.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved imaging apparatus based on the detection of X-ray photons, in particular for medical use, which can provide calibration capabilities with a reduced complexity and/or at reduced cost. It is a further object of the present invention to provide a corresponding improved calibration assembly for calibrating an imaging apparatus and an improved device for obtaining a monochromatic flow of X-ray radiation. Finally, it is an object of the present invention to provide an improved method of obtaining a monochromatic flow of X-ray radiation and improved methods of calibrating a detector element of a detector of an imaging apparatus. According to one aspect of the invention, the object is achieved by a device for obtaining a monochromatic flow of X-ray radiation in an imaging apparatus based on the detection of X-ray photons, the device comprising: a positioning apparatus, - a crystalline element mounted on the positioning apparatus, wherein the positioning apparatus is adapted to position the crystalline element relative to a flow of X-ray radiation generated by a radiation source such that the crystalline element reflects a monochromatic flow of X-ray radiation according to Bragg 's law.

As will be described in further detail below, the present invention discloses an inventive concept for obtaining a monochromatic flow of X-ray radiation in an imaging apparatus, in particular for medical use, wherein the imaging apparatus is based on the detection of X-ray photons, in particular employing Spectral CT. In order to obtain a monochromatic flow of X-ray radiation, it is no longer necessary to provide a monochromatic radiation source. Instead, a radiation source with a broader energy range or a broader frequency band can be employed, which is typically easier to provide and less costly than a specific monochromatic radiation source. Further advantages will be described hereinafter.

The present invention makes use of Bragg's law which states that a reflection of a radiation wave having a certain frequency will only be reflected off a crystal plane when a particular angle of incidence with respect to the crystal plane is given. If a plurality of radiation waves having a plurality of different frequencies impinges on a surface of a crystal, only a narrow band of frequencies will be reflected off the crystal. By varying the angle of incidence, certain frequencies can be selected from the plurality of incident radiation waves. The frequency band or the energy range of the reflected radiation waves was found to be so small that it can be considered monochromatic in the context of calibration. Thus, the present invention does not only provide a monochromatic flow of X- ray radiation in an imaging apparatus, which is required to calibrate the imaging apparatus, but also discloses that different wavelengths or different energy levels can be achieved for the monochromatic flow of X-ray radiation. In particular, this allows easy provision of a monochromatic flow of X-ray radiation at different energy levels that are typically required when calibrating an energy-resolving (Spectral CT) detector.

The present invention has the main advantage that it is no longer required to provide an additional radiation source for the detector energy calibration process. Instead, the radiation source that is already provided in the imaging apparatus and is used during normal operation of the imaging apparatus can also be used for calibrating the detector of the imaging apparatus. This reduces the complexity of the imaging apparatus and avoids the large number of measures that have to be taken when introducing a further radiation source.

It is yet a further advantage that, since the standard radiation source of the imaging apparatus can be used during calibration, it is also possible to determine the spectrum of the radiation source. In order to achieve this, a calibration process, which will be explained in more detail hereinafter, is performed on the detector elements of the detector of the imaging apparatus. The calibration can be performed with a good accuracy, because the wavelength or the energy level that is reflected off the surface of the crystal can be well controlled. Once the calibration values have been determined for the detector elements, the radiation source can then be made to radiate directly onto the detector elements, and the output values of the detector elements, in particular the spectral response of the detector elements, can be determined. This information immediately provides the spectrum of the radiation source.

As described above, a positioning apparatus is used to position the crystalline element relative to the flow of X-ray radiation generated by the radiation source. Since the radiation pattern of the radiation source is known or can be easily determined, the position of the crystalline element required to achieve the reflection of a particular wavelength can be easily determined, and the positioning apparatus can position the crystalline element accordingly. Moreover, the positioning apparatus can be positioned in such a way that the reflected radiation waves have a desired energy level and are also directed into a desired direction. This means that it is possible to direct a monochromatic flow of X-ray radiation with a certain energy towards a certain detector element or towards a certain group of detector elements.

Therefore, the present invention discloses a novel technique for calibrating a Spectral CT imaging apparatus by using a monochromatic flow of X-radiation. It is very flexible, because the monochromatic flow can be provided at varying energy levels and can be directed towards different positions of the detector to be calibrated.

In a preferred embodiment, the device further comprises an aperture mask adapted to cause only a selected pattern of the flow of X-ray radiation to impinge on the crystalline element.

This enhances the flexibility and precision of obtaining the monochromatic flow of X-ray radiation and can also help to narrow the range of energy bands contained in the monochromatic flow. Without an aperture mask, it will typically be required to make the crystalline element very small in the direction parallel to the direction of the X-ray radiation, as there could otherwise be an undesirably large range of angles of incidence and therefore an undesirably large range of energies in the reflected waves. Using the aperture mask allows use of a crystalline element which is conveniently large and allows defining the angle of incidence by configuring the aperture mask accordingly. The aperture mask has the further advantage that it can be embodied to shield all radiation from the radiation source directed at the detector, except for those radiation waves that are particularly designated to reach the crystalline element and to be reflected off towards the detector.

In a further preferred embodiment, the selected pattern is a fan-type pattern. In general, it is possible to calibrate the detector elements of the detector individually. However, by making the selected pattern a fan-type pattern, it becomes possible to direct the monochromatic flow of X-ray radiation to a plurality of detector elements, in particular a row of detector elements or any other group of detector elements, and to perform calibration on these detector elements in parallel. This makes the overall calibration process less time-consuming. In a further preferred embodiment, the aperture mask is mounted on a displacement stage adapted to displace the aperture mask relative to the radiation source.

This embodiment allows easy setting of the energy level and the monochromatic flow direction. The radiation pattern of the radiation source is typically shaped as a cone. Therefore, when moving the aperture mask relative to the radiation source, in particular in a linear fashion, the angle of incidence can be varied, even though the radiation source and the crystalline element may remain at a fixed position, assuming a large enough crystal. It should be noted that different angles of incidence can also be achieved if the crystalline element moves with the aperture mask.

In a further preferred embodiment, the positioning apparatus comprises a translation stage for linearly translating the crystalline element and a tilting stage for tilting the crystalline element.

In this manner, great flexibility concerning setting of the energy level and the monochromatic flow direction can be achieved, because the position of the crystalline element, i.e. its location and its orientation, is highly variable. Tilting the crystalline element provides easy control with regard to the energy band that is to be reflected. (For completeness' sake, it is pointed out that - if no further measures are taken - tilting of the crystalline element will also change the location at which the monochromatic flow impinges on the detector.) By translating the crystalline element, the location at which the monochromatic flow impinges on the detector can be varied. (For completeness' sake, it is pointed out that - without further measures - the energy level received by a certain detector element will also change.)

As already explained above, providing an aperture mask further enhances the ability to direct a monochromatic flow at a certain energy level towards a certain location of the detector. Since all measures, considered individually, allow a change of the energy level received at a certain detector element and allow directing the monochromatic flow towards a certain detector element or a plurality of detector elements, the combination of these measures provides a powerful means for obtaining the different monochromatic flows at different locations of the detector in order to calibrate an energy-resolving imaging apparatus. It will be explained hereinafter how these individual measures are coordinated with one another.

In a further preferred embodiment, the device further comprises a positioning control unit adapted to position the crystalline element on the basis of a desired direction and energy level of the monochromatic flow of X-ray radiation. In this embodiment, a desired direction or a desired location on the detector is specified along with a desired energy level. The positioning control unit will process this input into settings and/or instructions that will make the positioning apparatus position the crystalline element in order to obtain the desired monochromatic flow.

In a preferred embodiment, the crystalline element is made of a single crystalline material.

A single crystal is particularly well-suited for obtaining a monochromatic flow having a very narrow energy band or frequency range. The single crystalline material is preferably Si or Ge and preferably embodied as a flat layer.

In a preferred embodiment, the device further comprises a displacement apparatus for displacing the device relative to a fixed point to which the displacement apparatus is attached.

The displacement apparatus allows keeping the device for obtaining the monochromatic flow within the imaging apparatus, even during normal operation. In order to achieve this, the displacement apparatus positions the device outside of the cone of the radiation source during normal operation. When a calibration is to be performed, the displacement apparatus moves the device into such a position that the monochromatic flow of X-ray radiation as described above can be achieved. The calibration process can therefore be performed easily, even prior to each measurement. According to another aspect of the invention, this object is achieved by an imaging apparatus based on the detection of X-ray photons, in particular for medical use, the imaging apparatus comprising: a radiation source for generating a flow of X-ray radiation, - a detector for detecting X-ray radiation, a control unit adapted to control the radiation source, to read out information from the detector and to process the information into a visual image, and a device as mentioned above.

The general functionality of such an imaging apparatus, which is in particular a computer tomograph, is well known, see, for example, the book cited in the introductory part of this application. The radiation source emits radiation in a rather broad energy band or frequency range. This radiation is directed at a target to be inspected and becomes attenuated depending on the material through which the radiation travels. In general, the degree of attenuation caused by a certain material will be larger at certain frequencies and smaller at other frequencies or energy levels. In other words, a variety of materials can be distinguished by looking at their spectral response. Analyzing the spectral response of the target in the detector provides information that can be used to obtain a slice-type image of the target. As explained above, the device according to the present invention allows inline calibration of the energy axis of a Spectral CT detector, determination of the spectral response function of the detector and also measurement of the spectrum of the radiation source.

The imaging apparatus is preferably embodied as a CT-scanner, in particular a CT-scanner employing multi-energy CT.

According to another aspect of the invention, this object is achieved by a calibration assembly for calibrating an imaging apparatus based on the detection of X-ray photons, the calibration assembly comprising: a processing unit for controlling a calibration process and determining calibration values, a storage device for storing the calibration values, and a device as mentioned above. The components for calibrating an imaging apparatus are well known and are therefore only briefly described. The overall process of calibration is controlled by a processing unit, which can be embodied separately or included in a central control unit of the imaging apparatus, e.g. being part of a computer or a computer program. The information that is obtained during calibration is used to determine correction values or calibration values. These calibration values are stored in a storage device and will be used to correct the results of subsequent measurements. As explained above, the device according to the present invention obviates the need for an additional radiation source used during calibration.

According to another aspect of the invention, this object is achieved by a method of obtaining a monochromatic flow of X-ray radiation in an imaging apparatus based on the detection of X-ray photons, the method comprising the steps of: providing a crystalline element, and obtaining a flow of X-ray radiation onto the crystalline element such that the crystalline element reflects a monochromatic flow of X-ray radiation according to Bragg 's law.

As explained hereinbefore, this method allows obtaining a monochromatic flow of X-ray radiation by using a rather simple structural arrangement. It is no longer required to provide an additional monochromatic radiation source.

According to another aspect of the invention, this object is achieved by a method of calibrating a detector element of a detector of an imaging apparatus based on the detection of X-ray photons, the method comprising the steps of: obtaining a monochromatic flow of X-ray radiation, in the manner as mentioned above, directed at the detector element, reading at least one output value of the detector element, and - determining at least one calibration value for the detector element using the at least one output value and at least one expected value.

One aspect of significant importance for calibrating an imaging apparatus, and in particular one or a plurality of detector elements of the detector of the imaging apparatus, is to obtain a monochromatic flow of X-ray radiation. This can be advantageously achieved by using the device according to the present invention, as described above. When the monochromatic flow impinges on the detector element, at least one output value of the detector is read. Typically, this will be the energy bin that corresponds to the energy level of the monochromatic flow. However, it may be advantageous to also acquire the outputs of the adjacent energy bins or to obtain the total spectral response of the detector caused by the monochromatic flow. Since the energy level of the monochromatic flow is determined by the position of the crystalline element, the desired/expected value or plurality of values that would ideally be output by the detector element are known. Therefore, at least one calibration value for the respective detector element can be determined by using the output value or values and the expected value or values. If a plurality of energy bins is used, it is advantageous to perform calibration by using monochromatic flows of different energy levels and to determine a set of calibration values for the total spectrum of the detector element.

According to another aspect of the invention, this object is achieved by a method of calibrating a detector element of a detector of an imaging apparatus based on the detection of X-ray photons, the method comprising the steps of: obtaining a first monochromatic flow of X-ray radiation at a first energy, in the manner as mentioned above, reflected towards the detector element, wherein the flow of X-ray radiation is incident on the crystalline element at a first angle of incidence, reading at least one first output value of the detector element, - varying the first angle of incidence to a second angle of incidence, obtaining a second monochromatic flow of X-ray radiation at a second energy reflected towards the detector element, reading at least one second output value of the detector element, and determining at least one calibration value for the detector element using the at least one first output value and the at least one second output value.

Several applications do not require the detector elements to correctly determine the energy level of impinging radiation on an absolute scale, but rather a relative scale, i.e. the positions of the individual energy bins with respect to one another, is sufficient. This relative alignment is sometimes referred to as calibration of the energy scale. This type of calibration process can be advantageously achieved by using the teachings of the present invention, because monochromatic flows at two energy levels can be easily achieved.

The general concept underlying this type of calibration is that the relation of different energy levels and the corresponding detector responses can be described by means of a function. Typically, an approximation of a linear relationship is sufficient. In order to calibrate this relation, specifically to determine the variables of the functional relationship, at least two measurements at two different energy levels are performed. The variables can be determined by means of the resulting output values of the detector.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention are shown in the drawings and will be explained in more detail in the description below, in which:

Fig. 1 shows an imaging apparatus based on the detection of X-ray photons; Fig. 2 shows a device for obtaining a monochromatic flow of X-ray radiation in the imaging apparatus shown in Fig. 1; Fig. 3 shows the geometric relationships that can be considered in order to provide a monochromatic flow of X-ray radiation of a certain energy level towards a certain location of the detector of the imaging apparatus shown in Fig. 1;

Fig. 4 shows a first embodiment of a method of calibrating a detector element; and

Fig. 5 shows a second embodiment of a method of calibrating a detector element.

DESCRIPTION OF EMBODIMENTS Fig. 1 is a block diagram showing a device 10 for obtaining a monochromatic flow 12 of X-ray radiation, symbolized by a single wave, in an imaging apparatus 14. The imaging apparatus 14 is based on the detection of X-ray photons and is used particularly for medical applications.

The imaging apparatus 14 comprises a radiation source 16 for generating a polychromatic flow 18 of X-ray radiation, symbolized by a plurality of waves. It has also a detector 20 for detecting X-ray radiation, and a control unit 22. The control unit 22 is adapted to control the radiation source 16, to read out information from the detector 20 and to process the information into a visual image 24. Furthermore, the imaging apparatus has a calibration assembly 26 comprising a processing unit 28 for controlling a calibration process and determining calibration values, and a storage device 30 for storing the calibration values. It should be noted that the control unit 22 and the processing unit 28 can also be embodied in one single unit, e.g. a computer.

Furthermore, the imaging apparatus 14 comprises a positioning control unit 32 adapted to position the crystalline element 36 on the basis of a desired direction and energy level of the monochromatic flow 12 of X-ray radiation. The functionality of the device, in particular in connection with the displacement stage, will now be described.

Fig. 2 (left) is a plan view and (right) a side elevation of the device 10. The device 10 comprises a positioning apparatus 34 and a crystalline element 36 which is mounted on the positioning apparatus 34. The positioning apparatus 34 is adapted to position the crystalline element 36 relative to a polychromatic flow 18 of X-ray radiation generated by the radiation source 16. The radiation flow, i.e. both the polychromatic part 18 and the monochromatic part 12, is symbolized by the sector of a circle 38. The borderline between the polychromatic flow 18 and the monochromatic flow 12 is symbolized by the short broken line 40. The radiation is of a fan-type pattern and impinges on a plurality of detector elements 42 of the detector 20.

As can be seen in the side elevation, the device comprises an aperture mask 44 which ensures that the polychromatic flow 18 coming from the radiation source 16 impinges on the crystalline element 36 only at a certain angle. The setting of this angle will be described hereinafter.

The positioning apparatus 34 comprises a translation stage 46 for linearly translating the crystalline element 36 and a tilting stage 48 for tilting the crystalline element 36. The aperture mask 44 is mounted on a displacement stage 50 which allows moving the aperture mask 44 along with the crystalline element 36 or relative to the crystalline element 36. As will be explained hereinafter, this arrangement allows directing a monochromatic flow 12 of a certain energy level towards a certain detector element 42 or a plurality of detector elements 42.

Fig. 2 also shows in its side elevation that the device 10 comprises a displacement apparatus 51 for displacing the device 10 relative to a fixed point 52 to which the displacement apparatus 51 is attached. The displacement apparatus 51 will particularly enable the device 10 including the aperture mask 44 to be moved into and out of the radiation field of the radiation source 16.

For the sake of clarity, it is pointed out that the movements invoked by the translation stage 46, the displacement stage 50 and the displacement apparatus 51 take place in a substantially horizontal direction when considering the orientation of Fig. 2. The crystalline element 36 is tilted around an axis that is perpendicular to the plane of the drawing in the side elevation of Fig. 2.

Fig. 3 shows the geometric relationships that have to be considered when a monochromatic flow of X-ray radiation at a certain energy E is desired to be directed towards a detector position z

The wavelength of radiation of a given energy E is given by

_Λ he

E [1]

with h being Planck's constant and c the speed of light.

Using Bragg' s law, the scattering angle θ can be derived. nλ = Id sin(θ ) i^-i

Here, d is the lattice spacing of the given crystal plane and n is an integer describing the order of reflection (here taken as n=l). If we assume a "symmetric" reflection (the crystal plane is parallel to the surface) the scattering angle equals the angle of incidence onto the crystal and the exit angle.

α, =θ _[3]

The following formulas can be derived from the geometry as shown in Figure

3:

SCD is the source-to-crystal distance along the beam direction and CDD is the crystal-detector-distance, h is the distance of the crystal surface from the fan-type beam and z is the distance of the detector element from the central plane. Now the angle of incidence CC₁ and the required tilt angle α_t can be defined by:

[6]

CC, - GC₉

CC, = — [7]

For a given angle of incidence OC₁, the height h can be derived from equation 6 or it can be tabulated. For small angles, this results in SCD ^■ CDD - 2OL₁ - Z - SCD h = CDD + SCD [8]

The calculations shown here can be applied to focus-centered and flat-panel detectors. For each individual column, the distances SCD and CDD will change and h and CC₁ have to be set accordingly.

The following Table gives some examples of parameters for one particular setup in which a monochromator Si(11 l)-crystal was used. Geometric parameters SCD= 100mm and CDD=600mm were used as examples. The calculations were made for the central detector row (z=0):

Table

As can be seen from the above calculations, it is possible to define a certain location z onto which a monochromatic flow of a certain energy level E is to impinge, and to calculate the required position, i.e. location and orientation, of the crystalline element 36. Due to the fan-type beam, a large number of detector elements 42 can be calibrated at the same time, as can best be seen in Fig. 2, and a short calibration period can be achieved.

Fig. 4 is a block diagram of a method of calibrating a detector element 42, which method includes a method of obtaining a monochromatic flow 12 of X-ray radiation. In a first step Sl, a crystalline element 36 is provided. Subsequently, in a second step S2, a polychromatic flow 18 of X-ray radiation is obtained on the crystalline element 36 so that the crystalline element 36 reflects a monochromatic flow 12 of X-ray radiation according to Bragg's law.

In a further step S3, at least one output value of the detector element 42 is read. The output value or values represent the response of the detector element 42 due to the monochromatic flow 12. The value or values that should ideally be provided by the detector element 42 have previously been stored as the expected value or values. Therefore, at least one calibration value for the detector element can be determined by using the measured at least one output value and the at least one expected value. Finally, it is checked in step S5 if all detector elements 42 have been calibrated. If there are still detector elements 42 to be calibrated, the method continues with step S2. Otherwise, the method terminates and the calibration process is completed.

The calibration values that are determined are stored in the storage device 30 and will be considered during subsequent measurements.

Fig. 5 discloses a second embodiment of a method of calibrating a detector element 42. Initially, a crystalline element 36 is provided in step SI l and a polychromatic flow 18 of X-ray radiation is obtained on the crystalline element 36 (step S 12) so that a crystalline element 36 reflects a monochromatic flow 12 of X-ray radiation. It should be noted that, in order to obtain this first monochromatic flow 12 of X-ray radiation, the polychromatic flow 18 of X-ray radiation is incident on the crystalline element 36 at a first angle of incidence.

In step S 13, at least one first output value of the detector element 42 is read. Subsequent to this, in step S 14, the first angle of incidence is varied to a second angle of incidence, so that in step S 15 a second monochromatic flow 12 at a second energy is reflected towards the detector element 42. It should be noted that, in order to make the monochromatic flow 12 reach the same detector element 12, the translation stage 46 of the positioning apparatus 34 or the displacement apparatus 51 will be used to compensate for the change in the angle of incidence CC₁. With the second monochromatic flow impinging on the detector element 42, at least one second output value of the detector element 42 is read in step S 16. Having the first and second output value or values that have been obtained at a first energy level and at a second energy level, at least one calibration value for the detector element can be determined (step S 17). Finally, it is checked in step Sl 8 if all detector elements 42 have been calibrated. If there are still detector elements 42 to be calibrated, the method continues with step S 12. Otherwise, the method terminates and the calibration process is completed.

The at least one calibration value based on first and second output values is preferably determined as follows. To calibrate the energy scale, a certain angle of incidence CC₁ is set. As explained above in detail, the energy that is selected from the spectrum can be calculated from the geometry parameters and from this angle CC₁. Now the detector 20 is read out in a spectroscopic fashion.

For a detector 20, which only has a small number of energy bins, this is preferably realized by changing the thresholds of the bins and performing subsequent measurements ("threshold scan"). The spectral response can then be obtained by differentiation. An alternative approach would be to have a dedicated detector mode, which allows spectroscopic measurements. This will give the spectral response directly, which is assumed to have a linear relationship in this case.

From the spectral response at Ei, the threshold position Ti with the highest intensity is picked as the first calibration value. Now a second energy E₂ is selected by changing the angle of incidence, and the measurement is repeated, the maximum intensity now giving T₂. The energy scale calibration E(T) can be obtained from the two measurements by linear inter and extrapolation:

E(T) = ciT + b p]

with

E₇ - E, a = — ^L

T - T [10] b = E₇ - aT₇

[H]

The calibration parameters are stored and used for subsequent medical or preclinical scans.

The calibration assembly 26 can also be used to map the energy response function for many incident energies by repeating the above-described measurements for many energies in the range of interest. In this case, the function E(T) can be obtained more precisely by selecting optimum parameters a and b by e.g. a least square fit of equation 9 to the experimental results.

Finally, the device can be used to measure the spectrum of the radiation source 16. This is done by integrating the detector signals over all measured energies or a range of energies and taking into account the energy dependence of the efficiency of the device 10 and of the detector 20. The latter scan can be used to cross-check the alignment by testing whether a maximum intensity is found at the energy of the characteristic emission lines of the X-ray tube anode material. In summary, the present invention discloses a novel concept of obtaining a monochromatic flow of X-ray radiation in an imaging apparatus which can be advantageously used in the process of calibrating the imaging apparatus. The presented teachings obviate the need for an additional radiation source for calibration purposes and allow inline calibration of the imaging apparatus, even between measurements.

While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustrations and descriptions are to be considered illustrative or as examples and are not restrictive; the invention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, use of the verb "comprise" and its conjugations does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The terms "left", "right", etc. are used only for easy understanding of the invention and do not limit the scope of the invention.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting their scope.

Claims

CLAIMS:

1. A device (10) for obtaining a monochromatic flow (12) of X-ray radiation in an imaging apparatus (14) based on the detection of X-ray photons, the device (10) comprising: a positioning apparatus (34), - a crystalline element (36) mounted on the positioning apparatus (34), wherein the positioning apparatus (34) is adapted to position the crystalline element (36) relative to a flow (18) of X-ray radiation generated by a radiation source (16) such that the crystalline element (36) reflects a monochromatic flow (18) of X-ray radiation according to Bragg 's law.

2. A device according to claim 1, further comprising an aperture mask (44) adapted to cause only a selected pattern of the flow (18) of X-ray radiation to impinge on the crystalline element (36).

3. A device according to claim 2, wherein the selected pattern is a fan-type pattern.

4. A device according to claim 2, wherein the aperture mask (44) is mounted on a displacement stage (50) adapted to displace the aperture mask (44) relative to the radiation source (16).

5. A device according to claim 1, wherein the positioning apparatus (34) comprises a translation stage (46) for linearly translating the crystalline element (36) and a tilting stage (48) for tilting the crystalline element (36).

6. A device according to claim 1, further comprising a positioning control unit (32) adapted to position the crystalline element (36) on the basis of a desired direction and energy level of the monochromatic flow (12) of X-ray radiation.

7. A device according to claim 1, wherein the crystalline element (36) is made of a single crystalline material.

8. A device according to claim 1, further comprising a displacement apparatus (51) for displacing the device (10) relative to a fixed point (52) to which the displacement apparatus (51) is attached.

9. An imaging apparatus (14) based on the detection of X-ray photons, in particular for medical use, the imaging apparatus (14) comprising: - a radiation source (16) for generating a flow (18) of X-ray radiation, a detector (20) for detecting X-ray radiation, a control unit (22) adapted to control the radiation source (16), to read out information from the detector (20) and to process the information into a visual image (24), and - a device (10) according to claim 1.

10. A calibration assembly (26) for calibrating an imaging apparatus (14) based on the detection of X-ray photons, the calibration assembly (26) comprising: a processing unit (28) for controlling a calibration process and determining calibration values, a storage device (30) for storing the calibration values, and a device (10) according to claim 1.

11. A method of obtaining a monochromatic flow (12) of X-ray radiation in an imaging apparatus (14) based on the detection of X-ray photons, the method comprising the steps of: providing (Sl, Sl 1) a crystalline element (36), and obtaining (S2, S 12) a flow (18) of X-ray radiation onto the crystalline element (36) such that the crystalline element (36) reflects a monochromatic flow (12) of X-ray radiation according to Bragg 's law.

12. A method of calibrating a detector element (42) of a detector (20) of an imaging apparatus (14) based on the detection of X-ray photons, the method comprising the steps of: obtaining a monochromatic flow (12) of X-ray radiation according to claim 11 , directed at the detector element (42), reading (S3) at least one output value of the detector element (42), and determining (S4) at least one calibration value for the detector element (42) using the at least one output value and at least one expected value.

13. A method of calibrating a detector element (42) of a detector (20) of an imaging apparatus (14) based on the detection of X-ray photons, the method comprising the steps of: - obtaining a first monochromatic flow (12) of X-ray radiation at a first energy

(Ei) according to claim 11, reflected towards the detector element (42), wherein the flow (18) of X-ray radiation is incident on the crystalline element (36) at a first angle of incidence (CC₁), reading (S13) at least one first output value (Ti) of the detector element (42), varying (S 14) the first angle of incidence to a second angle of incidence, - obtaining (S 15) a second monochromatic flow (12) of X-ray radiation at a second energy (E₂) reflected towards the detector element (42), reading (S 16) at least one second output value (T₂) of the detector element (42), and determining (S 17) at least one calibration value (a, b) for the detector element (42) using the at least one first output value (Ti) and the at least one second output value (T₂).