WO2010070583A1

WO2010070583A1 - X-ray examination device and method

Info

Publication number: WO2010070583A1
Application number: PCT/IB2009/055761
Authority: WO
Inventors: Jens-Peter Schlomka; Ewald Roessl; Roland Proksa
Original assignee: Philips Intellectual Property and Standards GmbH; Koninklijke Philips Electronics NV
Current assignee: Philips Intellectual Property and Standards GmbH; Koninklijke Philips NV
Priority date: 2008-12-17
Filing date: 2009-12-15
Publication date: 2010-06-24
Anticipated expiration: 2011-06-17

Abstract

The present invention relates to an X-ray examination device by which the problems of high count rate in the construction of a spectral CT scanner based on photon counting can be overcome. The proposed X-ray examination device comprises: an X-ray source (2) for emitting an X-ray beam (4) of X-ray radiation while rotating around an imaging region (5), an X-ray detector (6) having a plurality of detector cells (61) for detecting X- ray radiation emitted by said X-ray source (2) and having passed through said imaging region (5), a control unit (9) for modulating the source current of said X-ray source (2) between at least two different source currents to obtain at least two detection data sets for at least two different X-ray fluxes, wherein the lowest X-ray flux is low enough to avoid overloading of the X-ray detector (6) in the direct X-ray beam, and a reconstruction unit (10) for reconstructing an X-ray image from said at least two detection data sets, wherein the pixel values of the pixels of said X-ray image are reconstructed taking into account whether or not the higher X-ray flux resulted in an overloading of the X-ray detector (6) at the respective detector cells.

Description

X-ray examination device and method

FIELD OF THE INVENTION

The present invention relates to an X-ray examination device and a corresponding method as well as a computer program. The invention relates particularly to single- or multi- layer photon counting X-ray detectors operated under conditions of ultra- high X-ray fluxes, like, e.g., medical X-ray CT, pre-clinical CT, or CT for material inspection or security applications

BACKGROUND OF THE INVENTION

There is currently one main obstacle to overcome in the realization of a spectral CT scanner based on photon counting detectors: the count rate limitations of state-of- the-art detector systems to about 5-10 million counts per second and pixel. Conventional CT systems are optimized for short scanning times and are therefore operated at very high photon flux rates of about 1 billion counts per second and pixel. Thus, there is a discrepancy between the count rate of available detectors and the count rate required for CT systems.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an X-ray examination device and a corresponding method as well as a computer program by which this discrepancy can be resolved and by which the problems of high count rate in the construction of a spectral CT scanner based on photon counting can be overcome.

In a first aspect of the present invention an X-ray examination device is presented comprising: an X-ray source for emitting an X-ray beam of X-ray radiation while rotating around an imaging region, - an X-ray detector having a plurality of detector cells for detecting X-ray radiation emitted by said X-ray source and having passed through said imaging region, a control unit for modulating the source current of said X-ray source between at least two different source currents to obtain at least two detection data sets for at least two different X-ray fluxes, wherein the lowest X-ray flux is low enough to avoid overloading of the X-ray detector in the direct X-ray beam, and a reconstruction unit for reconstructing an X-ray image from said at least two detection data sets, wherein the pixel values of the pixels of said X-ray image are reconstructed taking into account whether or not the higher X-ray flux resulted in an overloading of the X-ray detector at the respective detector cells.

In a further aspect of the present invention a corresponding method is presented.

In a still further aspect of the present invention a corresponding computer program is presented comprising program code means for causing a computer to control an X-ray examination device comprising an X-ray source for emitting an X-ray beam of X-ray radiation while rotating around an imaging region and an X-ray detector having a plurality of detector cells for detecting X-ray radiation emitted by said X-ray source and having passed through said imaging region, said computer program comprising program code means to control the X-ray examination device.

Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claim method has similar and/or identical preferred embodiments as the claimed device and as defined in the dependent claims.

The present invention is partially based on the idea to sub-divide the active detector volume into smaller detection cells, each of which is connected to a separate readout channel. This allows the reconstruction of images based, for instance, on photon counting despite the fact that certain cells in the detector structure are subject to (counter) overflow or excessive pulse pile-up during high-flux illumination. The method works due to the limited number of cells affected by the overflow and the use of the data obtained from the low flux scan.

Further, according to the present invention very fast source (tube) current modulation is applied during scanning. This can be obtained e.g. via a grid switch operated in such a way that is does not fully cut off the electron beam from the cathode of the X-ray source, but instead only reduces its flux. Several source current modulation modes are possible according to preferred embodiment, such as switching between two currents between successive views, switching between more than two currents from view to view, measuring N (N>1) views with one current and then letting the focus jump back in position ("x-deflection") and switching current for the next N views, or a combination of the latter two modes. Preferably, the detector is a photon-counting detector, but also a sensitive integrating detector can be subject to overload so that the invention makes sense for such other detectors.

According to a preferred embodiment said control unit is adapted for modulating the source current of said X-ray source such that the highest X-ray flux is adapted to the X-ray flux required for the desired type of examination, in particular such that it does not overload the X-ray detector at least along X-rays of high attenuation. Thus, the control unit takes into account the type of application, for instance the part of a patient's body to control the source current accordingly. Preferably, said control unit is adapted for modulating the source current of said X-ray source and for controlling the acquisition of detection data such that each of the at least two detection data sets are sufficient for separately reconstructing an X-ray image. This has the advantage that in any case at least two X-ray images are separately available, which could also both be displayed, but could also be processed to generate a better X-ray image. Further, it is proposed that said control unit is adapted for modulating the source current of said X-ray source and for controlling the acquisition of detection data such that the source current is switched between successive views. Thus, detection data of different detection data sets are alternately acquired, wherein the detection data of different detection data sets are almost acquired at the same, only slightly displaced projections with only a minimal time delay. Thus, movement of the object of interest, e.g. of a patient due to respiration or heart movement, does not have a large effect on the various detection data sets. It should be noted that the switching is done between at least two different currents.

According to another embodiment said control unit is adapted for modulating the source current of said X-ray source and for controlling the acquisition of detection data such that the source current is switched after a number of views, in particular after having obtained detection data from sufficient views to reconstruct an X-ray image and/or after a full rotation around the imaging region. This embodiment provides the advantage that part of (or all) data of a particular detection data set are obtained within the shortest possible time to allow separate images of the two currents with minimum object movement. Furthermore, the lower switching frequency reduces the technical complexity of the tube- generator unit.

To obtain interpolated detection data sets from the same views the reconstruction unit preferably further comprises an interpolation unit for rebinning or interpolating the detection data of at least one of said at least two detection data sets. This enables a better reconstruction of the detection data sets, particularly if in the reconstruction detection data from different detection data sets are used.

Preferably, said reconstruction unit is adapted for reconstructing separate X- ray images from said at least two detection data sets, wherein said reconstruction unit further comprises an interpolation unit for rebinning or interpolating the image data of at least one of said X-ray images to obtain X-ray images at the same sample points, and wherein said reconstruction unit is further adapted to reconstruct the final X-ray image from the X-ray images at the same sample points. As is known to the skilled person, sample points means locations (points) in the projection space having coordinates phi and t, s, and z wherein phi is the angle of rotation of the gantry, t, s are the spatial coordinates on the 2D detector and z is the position of the x-ray tube along the axis of rotation in respect to the scanned object.

According to a further embodiment said reconstruction unit is adapted for first reconstructing separate sinograms from said at least two detection data sets, wherein said reconstruction unit further comprises an interpolation unit for rebinning or interpolating the sinogram data of at least one of said sinogram to obtain sinograms at the same sample points, and wherein said reconstruction unit is further adapted to reconstruct the final X-ray image from the sinograms at the same sample points. The advantage is that the sinograms better allow to detect if there is an overload in the respective pixels of the detector than reconstructed X-ray images. To avoid the use of detection data in the reconstruction that have been obtained with an overloaded detector, it is preferred that said reconstruction unit is adapted for checking if a detection data element is obtained by an overloaded detector element and for using only detection data elements for reconstructing the X-ray image which have been obtained by not overloaded detector elements. According to another embodiment said reconstruction unit is adapted for checking if a detection data element of at least the detection data set obtained for the highest X-ray flux is obtained by an overloaded detector element and for using in the reconstruction of the pixel values of a pixel of said X-ray image the detection data elements of all detection data sets obtained with the respective detector cells, if said check is negative, and the detection data elements of only the detection data sets obtained with the respective not overloaded detector cells, if said check is positive.

In other words, preferably the two or more source currents differ strongly and are chosen such that the lowest flux is low enough so that the detector does not overload in the direct beam (e.g. 60mA)the highest flux is chosen as high as required for the application (e.g. 60OmA) intermediate fluxes are chosen in between the lowest and the highest flux. Preferably, sinograms are separately obtained from the different detection data sets, and then in these sinogram - optionally after a step of rebinning or interpolation - the above check and use of data for the reconstruction is applied.

Still further, in an embodiment said reconstruction unit is adapted for checking if a detection data element of at least the detection data set obtained for the highest X-ray is obtained by an overloaded detector element and for using in the reconstruction of the pixel values of a pixel of said X-ray image the detection data elements of all detection data sets obtained with the respective detector cells if said check is negative, and the detection data elements of only the detection data sets obtained with the respective not overloaded detector cells, weighted with a first weighting factor, if said check is positive, said first weighing factor taking into account the ratio of the source currents or the X-ray fluxes, at which the at least two detection data sets have been acquired.

Said first weighing factor is, for instance, determined as (source current at the high flux measurement) / (source current at the low flux measurement) + 1. The use of the weighting factor accounts for the lower photon flux obtained with the lower source current and ensures a continuous transition between data from overloaded and not overloaded pixels. According to still a further embodiment said reconstruction unit is adapted for weighting the detection data element with a second weighing factor and for using the weighted detection data elements for reconstructing the X-ray image. This ensures a smooth transition between the strong yes or no decision in said check, and thus provides smoother border lines in the reconstructed image.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings

Fig. 1 shows an embodiment of an examination device in accordance with the present invention,

Fig. 2 schematically illustrates the data acquisition scheme according to the present invention,

Fig. 3 shows sinograms measured with a low and a high X-ray flux, where the overloaded pixel data are marked black, Fig. 4 shows a flowchart of an embodiment of the method according to the present invention,

Fig. 5 shows a sinogram obtained in accordance with the present invention, and Fig. 6 shows images reconstructed from the sinograms shown in Figs. 3 and 5.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows a first embodiment of a medical X-ray examination apparatus according to the present invention, in particular a CT imaging system. The CT imaging system shown in Fig. 1 includes a gantry 1 which is capable of rotation about an axis of rotation R which extends parallel to the z direction. The radiation source 2, in particular a (conventional) polychromatic X-ray tube for emitting a broad energy spectrum of X-rays, is mounted on the gantry 1. The X-ray tube 2 is provided with a collimator device 3 which forms a conical radiation beam 4 from the radiation produced by the X-ray tube 2. The radiation traverses an object (not shown), such as a patient, in a region of interest in a cylindrical examination zone (imaging region) 5. After having traversed the examination zone 5, the X-ray beam 4 is incident on a X-ray detector unit 6, in this embodiment a two- dimensional photon-counting detector having a plurality of detector cells 61, which is mounted on the gantry 1 and which converts incident X-ray radiation into detection data signals.

The gantry 1 is driven at a preferably constant but adjustable angular speed by a motor 7. A further motor 8 is provided for displacing the object, e.g. the patient who is arranged on a patient table in the examination zone 5, parallel to the direction of the axis of rotation R or the z axis. These motors 7, 8 are controlled by a control unit 9, for instance such that the radiation source 2 and the examination zone 5 move relative to one another along a helical trajectory. However, it is also possible that the object or the examination zone 5 is not moved, but that only the X-ray source 2 is rotated.

The data acquired by the detector 6 are provided to an image processing unit (reconstruction unit) 10 for data processing, in particular for reconstructing an X-ray image from the detection data. The image processing unit 10 may optionally include an interpolation unit 12 for rebinning or interpolating the image data and/or sinograms of X-ray images and/or sinograms to obtain X-ray images and/or sinograms at the same sample points. The reconstructed image can finally be provided to a display 11 for displaying the image. Also the image processing device 10 is preferably controlled by the control unit 9. Fig. 2 schematically illustrates the data acquisition scheme according to the present invention applying source current modulation, i.e. modulation of the source current provided to the X-ray source 2 under control of the control unit 9. A high X-ray flux 20 and a low X-ray flux 21 during acquisition are indicated in Fig. 2, i.e. subsequent measurements are carried out with two (or more) different source currents and thus X-ray fluxes. The detector 6, however, will be overloaded in the direct beam of the high current measurement obtained when the high X-ray flux 20 is applied.

The detection data are particularly obtained such that during gantry rotation measurements with different source currents are alternately carried out, represented by the two different hatchings. It is also possible to measure subsequently one rotation with high source current and then a second rotation with low source current or switch after half a rotation. However, this scheme does not allow moving the object/patient between the subsequent rotations.

As a result, two sinograms will be obtained as shown in Fig. 3. The differences are the following: The low-flux sinogram shown in Fig. 3a is noisier, but has no overloaded pixels. The high-flux sinogram shown in Fig. 3b has detector elements, which signaled a photon-flux overload - shown in cross hatch in Fig. 3b, but less noisy signals in the high attenuation regions.

According to the present invention is now proposed to combine the detection data obtained with the two (or more) different X-ray fluxes and to reconstruct an image by taking into account whether or not the higher X-ray flux resulted in an overloading of the X- ray detector at the respective detector cells.

An embodiment of the method according to the present invention is illustrated by the flowchart shown in Fig. 4. In the first step SlO the at least two detection data sets are obtained as explained above applying two different X-ray fluxes, for instance by providing two different source currents to the X-ray source. In the next step Sl 1 at least two sinograms are obtained (one for each detection data set) in a known manner.

Subsequently, the at least two sinograms are combined. For each measured X- ray it is detected (or checked), if the high- flux measurement (detection data element obtained when a high X-ray flux was applied) produced an overload in the detector (S 12).

If no overload was produced for a particular measurement, then the at least two measurements for the same sample point are added (S 14), after a step of rebinning / interpolation (S 13) is applied to the at least two sinograms such that they represent measurements at the same sample points. If an overload was produced for a particular measurement, then - again after a step of rebinning / interpolation (S 13) is applied to the at least two sinograms - from the at least two measurements for the same sample point only the measurements are used with low X-ray flux which did not result in an overload, preferably weighted with an weighting factor /, however (S 15). In case of two detection data sets said weighting factor/ is preferably calculated from the ratio of the two source currents: current of high flux measurement current of low flux measurement

As alternative to step S 15, it is also possible according to another embodiment to replace the yes/no decision (if the detector cell was overloaded or not at a particular measurement) by a smooth transition between the scenarios. This can be achieved by using a weighting parameter of the two data sets, which depends on the measured count rate.

From the combination of the two measurements in the above described way a combined sinogram is thus obtained (S16) as depicted in Fig. 5. As can be seen from this combined sinogram there is lower noise in the central region of the sinogram compared to the sinograms shown in Fig. 3.

Finally, this combined sinogram is used for reconstruction of an image (S 17) in a known manner. Fig. 6 shows images reconstructed from the sinograms shown in Figs. 3 and 5. Fig. 6a shows an image reconstructed from the sinogram shown in Fig. 3a (derived from low-flux measurements). Fig. 6b shows an image reconstructed from the sinogram shown in Fig. 3b (derived from high-flux measurements, but assuming no detector overflow). Fig. 6c shows an image reconstructed from the combined sinogram shown in Fig. 5. As can be seen from Fig. 6c, even though the outer region is very noisy, a low noise level is maintained in the central region as desired.

Optionally, additional smoothing or filtering could be applied to the noisy sinogram, i.e. the sinogram shown in Fig. 3a obtained from low-flux measurements, thus reducing the spatial resolution in the outer parts, but maintaining a constant noise level.

A further option is that in step S 15 only the measurements are used with low X-ray flux which did not result in an overload, without weighting them with any weighting factor, i.e. measurements with high X-ray flux which resulted in an overload are completely ignored.

Various methods can be applied to modulate the X-ray tube current. This can in principle be achieved, for instance, by changing the X-ray tube filament current, using a grid switch, using two filaments with individual grid switches, and/or using two tubes. The invention applies primarily to photon counting X-ray detectors based on single layer or multiple layer (3D) structured photon-counting detectors, operated under conditions of ultra-high X-ray fluxes, like, e.g., medical X-ray CT, pre-clinical CT, or CT for material inspection or security applications. It allows reconstructing images of essentially the same quality as a detector with unlimited count rate performance.

In summary X-ray tube current modulation during acquisition is preferably used according to the present invention to provide at least two detection data sets: One set obtained with high flux to provide enough photons for imaging the highly absorbing parts of the object and a second data set obtained with low flux (e.g. with 10% of the flux) to obtain spectral data of the peripheral parts of the body or object.

The X-ray tube photon flux of the high flux data set greatly exceeds the maximum count rate of the detector in the primary beam and therefore the detector will overload in the regime of low attenuation. Data processing is required to gradually lower the weight of the data obtained from these parts of the detector. Further processing (e.g. interpolation) is used to combine the two data sets into one data set with a very high dynamic range.

The present invention enables the use of a single- or multi-layer photon counting detectors with limited counting capabilities in a high- flux application. The dose utilization is rather high since SNR reduction only occurs in the peripheral parts of the object. While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and 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, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. X-ray examination device comprising: an X-ray source (2) for emitting an X-ray beam (4) of X-ray radiation while rotating around an imaging region (5), an X-ray detector (6) having a plurality of detector cells (61) for detecting X- ray radiation emitted by said X-ray source (2) and having passed through said imaging region (5), a control unit (9) for modulating the source current of said X-ray source (2) between at least two different source currents to obtain at least two detection data sets for at least two different X-ray fluxes, wherein the lowest X-ray flux is low enough to avoid overloading of the X-ray detector (6) in the direct X-ray beam, and a reconstruction unit (10) for reconstructing an X-ray image from said at least two detection data sets, wherein the pixel values of the pixels of said X-ray image are reconstructed taking into account whether or not the higher X-ray flux resulted in an overloading of the X-ray detector (6) at the respective detector cells.

2. X-ray examination device as claimed in claim 1, wherein said control unit (9) is adapted for modulating the source current of said X-ray source (2) such that the highest X-ray flux is adapted to the X-ray flux required for the desired type of examination, in particular such that it does not overload the X-ray detector (6) at least along X-rays of high attenuation.

3. X-ray examination device as claimed in claim 1, wherein said control unit (9) is adapted for modulating the source current of said X-ray source (2) and for controlling the acquisition of detection data such that each of the at least two detection data sets are sufficient for separately reconstructing an X-ray image.

4. X-ray examination device as claimed in claim 1, wherein said control unit (9) is adapted for modulating the source current of said X-ray source (2) and for controlling the acquisition of detection data such that the source current is switched between successive views.

5. X-ray examination device as claimed in claim 1, wherein said control unit (9) is adapted for modulating the source current of said X-ray source (2) and for controlling the acquisition of detection data such that the source current is switched after a number of views, in particular after having obtained detection data from sufficient views to reconstruct an X-ray image and/or after a full rotation around the imaging region.

6. X-ray examination device as claimed in claim 1, further comprising an interpolation unit (12) for rebinning or interpolating the detection data of at least one of said at least two detection data sets to obtain interpolated detection data sets from the same views.

7. X-ray examination device as claimed in claim 1, wherein said reconstruction unit (10) is adapted for reconstructing separate X- ray images from said at least two detection data sets, wherein said reconstruction unit (10) further comprises an interpolation unit (12) for rebinning or interpolating the image data of at least one of said X-ray images to obtain X-ray images at the same sample points, and wherein said reconstruction unit (10) is further adapted to reconstruct the final X-ray image from the X-ray images at the same sample points.

8. X-ray examination device as claimed in claim 1, wherein said reconstruction unit (10) is adapted for first reconstructing separate sinograms from said at least two detection data sets, wherein said reconstruction unit (10) further comprises an interpolation unit for rebinning or interpolating the sinogram data of at least one of said sinogram to obtain sinograms at the same sample points, and wherein said reconstruction unit (10) is further adapted to reconstruct the final X-ray image from the sinograms at the same sample points.

9. X-ray examination device as claimed in claim 1, wherein said reconstruction unit (10) is adapted for checking if a detection data element is obtained by an overloaded detector element and for using only detection data elements for reconstructing the X-ray image which have been obtained by not overloaded detector elements.

10. X-ray examination device as claimed in claim 1, wherein said reconstruction unit (10) is adapted for checking if a detection data element of at least the detection data set obtained for the highest X-ray flux is obtained by an overloaded detector element and for using in the reconstruction of the pixel values of a pixel of said X-ray image the detection data elements of all detection data sets obtained with the respective detector cells, if said check is negative, and the detection data elements of only the detection data sets obtained with the respective not overloaded detector cells, if said check is positive.

11. X-ray examination device as claimed in claim 1, wherein said reconstruction unit (10) is adapted for checking if a detection data element of at least the detection data set obtained for the highest X-ray is obtained by an overloaded detector element and for using in the reconstruction of the pixel values of a pixel of said X-ray image the detection data elements of all detection data sets obtained with the respective detector cells if said check is negative, and the detection data elements of only the detection data sets obtained with the respective not overloaded detector cells, weighted with a first weighting factor, if said check is positive, said first weighing factor taking into account the ratio of the source currents or the X-ray fluxes, at which the at least two detection data sets have been acquired.

12. X-ray examination device as claimed in claim 11, wherein said first weighing factor is determined as (source current at the high flux measurement) / (source current at the low flux measurement) + 1.

13. X-ray examination device as claimed in claim 1, wherein said reconstruction unit (10) is adapted for weighting the detection data element with a second weighing factor and for using the weighted detection data elements for reconstructing the X-ray image.

14. X-ray examination method comprising the steps of: - emitting an X-ray beam of X-ray radiation by an X-ray source while rotating around an imaging region, detecting X-ray radiation emitted by said X-ray source (2) and having passed through said imaging region by an X-ray detector having a plurality of detector cells (61), modulating the source current of said X-ray source between at least two different source currents to obtain at least two detection data sets for at least two different X- ray fluxes, wherein the lowest X-ray flux is low enough to avoid overloading of the X-ray detector (6) in the direct X-ray beam, and reconstructing an X-ray image from said at least two detection data sets, wherein the pixel values of the pixels of said X-ray image are reconstructed taking into account whether or not the higher X-ray flux resulted in an overloading of the X-ray detector (6) at the respective detector cells.

15. Computer program comprising program code means for causing a computer to control an X-ray examination device comprising an X-ray source for emitting an X-ray beam of X-ray radiation while rotating around an imaging region and an X-ray detector having a plurality of detector cells for detecting X-ray radiation emitted by said X-ray source and having passed through said imaging region, said computer program comprising program code means to control the X-ray examination device to - modulate the source current of said X-ray source between at least two different source currents to obtain at least two detection data sets for at least two different X-ray fluxes, wherein the lowest X-ray flux is low enough to avoid overloading of the X-ray detector in the direct X-ray beam, and reconstruct an X-ray image from said at least two detection data sets, wherein the pixel values of the pixels of said X-ray image are reconstructed taking into account whether or not the higher X-ray flux resulted in an overloading of the X-ray detector (6) at the respective detector cells.