WO2016091760A1

WO2016091760A1 - Detector and detection method

Info

Publication number: WO2016091760A1
Application number: PCT/EP2015/078711
Authority: WO
Inventors: Roland Proksa
Original assignee: Koninklijke Philips N.V.
Priority date: 2014-12-10
Filing date: 2015-12-04
Publication date: 2016-06-16

Abstract

The present invention relates to a detector and a method for detecting incident radiation. For improved detection the detector comprises a plurality of detector pixels (230) each generating pixel signals in response to incident x-ray radiation, a comparator (243) per pixel connected with a pixel (230) and adapted for generating a comparator signal when the pixel signal is greater than a threshold, a first counting device (247) per pixel connected with the comparator and adapted for generating a count signal by counting comparator signals, and a suppressing device (246) per pixel connected with at least one neighboring comparator of a neighboring pixel and adapted for suppressing the count of a comparator signal when at least one neighboring comparator signal is greater than the threshold.

Description

Detector and detection method

FIELD OF THE INVENTION

The present invention relates to a detector and a method for detecting incident radiation. BACKGROUND OF THE INVENTION

Photon-counting based spectral CT systems for examining an object, e.g. a patient or a material, such as a tire or cast part, require detectors, which can deal with the high count rate generated in today's energy- integrating CT systems.

The pixel size of a photon counting detector is a major design parameter with impact on two important imaging parameters. The incident X-ray flux per unit area is distributed over a plurality of pixels. Since most rate limiting factors are channel dependent, smaller pixels enable higher rate capabilities per detector area. Small pixels tend to suffer more from some adverse pixel neighborhood effects degrading the spectral fidelity. These effects include for example pulse sharing, induced pulses and k-escape photons. The pixel size choice requires a trade-off between the support of high flux scan protocols and spectral separation capabilities.

It is known to improve the spectral performance by charge summing as for example implemented in the CERN MEDIPIX-3 detector. The adverse neighbor effects generate pulses in two or even more neighbor pixels. Charge summing sums these shared contributions and assigns the resulting pulse to the pixel with the strongest contribution. Such method can be phrased as the winner takes it all. The drawbacks of this method are higher complexity of the electronics and a reduction of the rate capabilities.

Another important aspect is that the requirements of high count rates and spectral fidelity are not independent. Most cases with high flux e.g. outside the patient or in peripheral patient areas have relaxed requirements on the spectral performance. Pulse pileup effects will in most cases degrade the spectral performance in these cases.

US 8,378,310 B2 discloses a photon counting silicon x-ray detector in which an output of a shaping filter is reset to zero after the signal output reaches a certain threshold in order to reduce the pile-up problem during high- flux imaging. US 2011/0210235 Al discloses an array of pixels for the detection of a flash of electromagnetic radiation or a cloud of impinging high energy particles. Each pixel in t he array comprises a radiation receptor for converting the electromagnetic radiation or impinging high energy particles into a radiation signal, and a converter for converting the radiation signal into pulses. The array further comprises a circuit for comparing one or more of the criteria pulse amplitude, pulse arrival time, time lo convert a pulse in a digital signal, pulse duration lime, pulse rise and fall time or integral of pulse over time for pulses coinciding on pixels in a predetermined neighborhood. The array also comprises a circuit for suppressing those pulses that are compared negatively versus the corresponding pulses in another pixel of the neighborhood for the same one or more criteria.

WO 2007/058600 Al discloses an X-ray apparatus for acquisition of images containing spectral information. The apparatus comprises: an X-ray source, a multi-slit collimator, a set of line detectors, each of said line detectors being a linear array of photon counting channels, each of said channels comprising a photon conversion channel element being operatively arranged to convert each photon to an electric pulse, a plurality of pulse counters. The counters are operatively arranged to count said pulses in a plurality of different ranges of pulse strength, said pulse having a strength depending on the energy of said photon, and that said apparatus further comprises an arrangement for energy subtracting operation.

DE 10 2011 077 397 Al discloses an X-ray imaging device provided with an X-ray radiation source, an X-ray detector and multiple detector units for detecting X-ray quanta. An evaluation device is designed to optionally assign measured values to X-ray quanta under using a coincidence circuit or under use of statistical weights relating to the development or evaluation of measured values. SUMMARY OF THE INVENTION

It is an object of the present invention to improve the detection of incident radiation.

In a first aspect of the present invention a detector for detecting incident radiation is presented that includes a plurality of detector pixels each generating pixel signals in response to incident x-ray radiation; a comparator per pixel connected with a pixel and adapted for generating a comparator signal when the pixel signal is greater than a threshold; a first counting device per pixel connected with the comparator and adapted for generating a count signal by counting comparator signals; and a suppressing device per pixel connected with at least one neighboring comparator of a neighboring pixel and adapted for suppressing the count of a comparator signal when at least one neighboring comparator signal is greater than the threshold.

In a further aspect of the present invention a method for detecting incident radiation is presented that includes generating pixel signals in response to incident x-ray radiation; generating a comparator signal when the pixel signal is greater than a threshold; generating a count signal by counting comparator signals; and suppressing the count of a comparator signal when at least one neighboring comparator signal is greater than the threshold.

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

embodiments as the claimed detector and as defined in the dependent claims.

The present invention is based on the idea that a counter per pixel counts only if none of the direct neighbor pixel signals has exceeded the same threshold as the pixel signal. The neighbor pixel effects subdivide the pulse charge into two or more smaller charges. The proposed detector and method is capable of rejecting the smaller component and will therefore reduce the undesired low energy tail.

An OR gate may be provided for receiving a pixel signal from at least two neighboring pixels and to provide its output to the suppressing device. The OR gate allows for simple implementation and quick and reliable function.

Four neighboring comparators may be connected with the suppressing device and the four pixels of the four neighboring comparators may each be arranged by an angle of 90° around the pixel. Such implementation offers good coverage of the neighboring pixels. For example, pixels located north, east, south and west to the pixel can be chosen.

Eight neighboring comparators may be connected with the suppressing device thereby providing even better coverage. For example, pixels located north-east, south-east, south-west and north-west to the pixel can be additionally chosen.

A control device may be adapted to disable the suppressing device for a high flux modus. In order to enable quick successive readout of the sensor pixel the suppressing device can be disabled. The disabling may be implemented statically or dynamically. The control device may be a distinct device or may be integrated into an existing calculation unit.

The detector may comprise at least one output for providing the comparator signal to the at least one neighboring pixel. Such output allows interaction between the pixels and enhances the quality of estimation of the distribution of incident radiation over the pixels. Preferably, each pixel is connected to all neighboring pixels of the pixel via one or more outputs. Depending on the direction of information or signal flow the output can be or function as an input as well.

The detector may comprise a second counting device per detector pixel connected with the comparator and adapted for generating a count signal by counting comparator signals, wherein the second counting device is not connected with the

suppressing device. Now, two counting devices, such as pulse counters, are provided per pixel. While the second counting device is used as a traditional counting device, the first counting device is used as a gated counting device. The mentioned neighbor pixel effects subdivide the pulse charge into two (or more) smaller charges. The proposed detector can reject the smaller component and therefore reduces the low energy tail. This approach may be characterized as "only the winner counts". The gating or suppressing device allows a count of the pixel by the first counting device only when the largest amount of the pulse charge is located at the pixel. Thus, only one pixel having the largest amount of pulse charge can count. First counting devices of surrounding pixels with a smaller amount of pulse charge do not count. The provision of two different counting devices provides enhanced analysis of the incident radiation as two distinct counting signals are created. The second counter counts every time when radiation impinges wherein the first counter counts only when the signals of all neighboring comparators are smaller than the threshold or the signal of the pixel.

The detector may comprise an image reconstruction device connected with the first and second counting device and adapted to generate an output signal per pixel. Such an image reconstruction device can use the two counting results in different ways and improves versatility of the detector.

The image reconstruction device may be adapted to provide the count signal of the second counting device as the output signal for a high flux modus. By bypassing the suppression device and the evaluation of the neighbor pixels high speed operation of the image reconstruction device can be achieved rendering it suitable for high flux mode operation.

The image reconstruction device may be adapted to provide only one counting signal as the output signal. Such implementation reduces data bandwidth which may be crucial for certain applications. The choice of the counter can be driven dynamically by the individual reading per channel or per counting device.

The image reconstruction device may be adapted to provide a combination of the count signals of the first and second counting device as the output signal. Both results may be considered and combined based on a blending mechanism or based on the statistical significance. For a blending mechanism the second counting device is considered for high flux while the first counting device is considered for low flux. Depending e.g. on the application the percentages or amounts of the two counting signals are chosen.

For symmetry reasons, the number of pulses with losses to a neighbor pixel may be about the same as the number of additional pulses from them. The pulse tailing will therefore roughly be reduced to 50% with a tendency of rejecting more low power pulses. The control device or the image reconstruction device may control the outputs to neighbor pixels or the use of neighbor signals accordingly.

Image reconstruction is typically done by first correcting the raw counts for system or detector imperfections and to convert the data into line integrals of base materials or physical absorption effects before these projection data are than reconstruction to images. A set of base materials may be for example soft tissue and bone. Physical effects may be photo-electric absorption and Compton scattering. Considering one material projection M, M_Recon = b Mci + (1-b) Mc₂ may be used for the image reconstruction based on a blending with factor b of the materials derived from the two counters {Mci with suppression and Mc₂ w/o suppression). The blending factor b may either be a constant, selected based on the imaging protocol parameter (such as X-Ray flux, patient size, etc.) or it can be set differently for each individual detector reading. A large number of blending function can be used. The general idea is to have small values of b for high flux readings and vice versa.

In case a maximum likelihood approach is used for the material decomposition, such as described in E. Roessl, R. Proksa; "K-edge imaging in x-ray computed tomography using multi-bin photon counting detectors"; Phys. Med. Biol. 52 (2007) 4679-4696 the likelihood function (P(), equation 7, page 4683 in this paper) should contain the product of the individual likelihoods for each counter. These individual likelihood terms are calculated using the different forward models of the two channels (Sj(E), equation 4, page 4683 in this paper). This method provides a statistically blending using the likelihood functions.

In another embodiment said comparator per pixel is adapted for checking the pixel signal against one of several thresholds, i.e. the detector pixels have two or more thresholds and related counters for multi-bin energy separation. Then, in a preferred embodiment said suppressing device is adapted to suppress the count of a comparator signal when in case of coincidence at least one neighboring comparator signal has exceeded the lowest threshold earlier. In another embodiment said suppressing device is then adapted to suppress the count of a comparator signal when in case of coincidence at least one neighboring comparator signal has exceeded a higher threshold. 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 imaging device according to the present invention,

Fig. 2 shows an embodiment of a detector array according to the present invention,

Fig. 3 shows an embodiment of pixels of a detector array according to the present invention,

Fig. 4 shows a first embodiment of a counting channel of a detector array according to the present invention,

Fig. 5 shows a second embodiment of a counting channel of a detector array according to the present invention and

Fig. 6 shows an example of a state machine used in another embodiment of a detector array according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows an embodiment of an imaging device x-ray according to the present invention, by way of example a CT (Computed Tomography) scanner 10 designed as a C-arm x-ray apparatus. The CT scanner 10 (which may also look differently, e.g. in the form of a tube having a ring-shaped support) includes a support 12 and a table 14 for supporting a patient 16. The support 12 includes an x-ray source 20 that projects a beam of x- rays, such as a fan beam or a cone beam, towards a x-ray detector 24 on an opposite side of the support 12 while a portion of the patient 16 is positioned between the x-ray source 20 and the x-ray detector 24.

The x-ray source 20 may be configured to deliver radiation at a plurality of energy levels, and the x-ray detector 24 may be configured to generate image data in response to radiation at different energy levels. The x-ray source 20 may include a collimator 21 for adjusting a shape of the x-ray beam. The collimator 21 may include one or more filters (not shown) for creating radiation with certain prescribed characteristics. The x-ray detector 24 has a plurality of sensor elements (221; see Fig. 2) configured for sensing a x-ray passing through the patient 16. Each sensor element generates an electrical signal representative of an intensity of the x-ray beam as it passes through the patient 16. The support 12 may be configured to rotate about the patient 16. In another embodiment, the support 12 may be configured to rotate about the patient 16 while they are standing (or sitting) in an upright position. The positioning of the support 12 and patient 16 are not limited to the examples illustrated herein, and the support 12 may have other configurations (e.g., positions or orientations of an axis of rotation), depending on a position and orientation of a body part for which imaging is desired.

In the illustrated embodiment, the CT scanner 10 also includes a control, a processor 40, a monitor 50 for displaying data, and an input device 52, such as a keyboard or a mouse, for inputting data. The processor 40 is coupled to a control 30. The rotation of the support 12 and the operation of the x-ray source 20 are controlled by the control 30, which provides power and timing signals to the x-ray source 20 and controls a rotational speed and position of the support 12 based on signals received from the processor 40. The control 30 also controls an operation of the x-ray detector 24. For example, the control 30 can control a timing of when image signal/data are read out from the x-ray detector 24, and/or a manner (e.g., by rows or columns) in which image signal/data are read out from the x-ray detector 24. Although the control 30 is shown as a separate component from the support 12 and the processor 40, in alternative embodiments the control 30 can be a part of the support 12 or the processor 40. The processor 40, the control 30 or the detector 24 may further comprise a reconstruction unit for reconstructing one or more images from the detected x-ray radiation.

During a scan to acquire x-ray projection data (i.e., CT image data), the x-ray source 20 projects a beam of x-rays towards the x-ray detector 24 on an opposite side of the support 12, while the support 12 rotates about the patient 16. In one embodiment, the support 12 makes a 360 degree rotation around the patient 16 during image data acquisition.

Alternatively, if a full cone detector is used, the CT scanner 10 may acquire data while the support 12 rotates 180 degrees plus the fan beam angle. Other angles of rotation may also be used, depending on the particular system being employed.

The x-ray detector 24 generates image signals/data in response to radiation impinging thereon. Additional sets of image data (in particular projection data) for different support angles can be generated as the support 12 rotates about the patient. After a desired amount of image data (e.g., sufficient for reconstruction of volumetric image) have been generated, the image data can be stored in a computer readable medium for later processing, e.g. on a hard disk.

The x-ray detector 24 can be constructed in various ways. Fig. 2 shows an exemplary x-ray detector 24 comprising an imager (also called pixel array, sensor array or sensor unit) 200 that includes an optional x-ray conversion layer 210 made from a direct conversion material, such as Cadmium Telluride (CdTe), and a charge detector array 220 coupled to the x-ray direct conversion layer 210. The x-ray conversion layer 210 generates charge packages in response to x-ray radiation, and the charge detector array 220, which includes a plurality of detector elements 221, is configured to generate count signals in response to the charge packages from the x-ray conversion layer 210. In such a direct conversion pixel only a single layer is provided capable of generating electrical pixel signals in response to the incident radiation like for example x-rays or gamma rays.

The imager 200 may have a curvilinear surface (e.g., a partial circular arc). Such surface configuration is beneficial in that each of the pixels 230 of the imager 200 is located substantially the same distance from the x-ray source 20 assembly. The imager 200 can alternatively have a rectilinear surface or a surface having other profiles. Each pixel 230 (or imaging element) may have a cross sectional dimension that is approximately 200 microns or more, and more preferably, approximately 400 microns or more, although pixels having other dimensions may also be used. Preferred pixel size can be determined by a prescribed spatial resolution. Pixels 230 having 200 to 400 microns in cross sectional dimension are good for general anatomy imaging, while other cross sectional dimensions may be preferred for specific body parts.

In one embodiment, the image data are sampled from the pixels 230 one line at a time. Alternatively, the image data from a plurality of lines of the pixels 230 can be sampled simultaneously. Such arrangement reduces the time it takes to readout signals from all lines of pixels 230 in the imager 200. This in turn, improves a frame rate (i.e., number of frames that can be generated by the imager 200 per second) of the imager 200.

During use, radiation impinges on the x-ray detector 24, which then generates image signals/data in response to the radiation. For instance, radiation of a certain energy level impinges on the x-ray detector 24, which then generates image signals/data in response to the radiation of the certain energy level. After the image signals/data are read out from the photo detector array 220, radiation of the same or different energy level is directed to the detector assembly 24. The assembly 24 then generates image signals/data in response to this radiation.

The x-ray detector 24 according to the present invention further comprises a counting channel 240 per sensor element 221 or per pixel 230 for obtaining a count signal with energy information based on a pixel signal 201 or on counting photons or charge pulses generated in response to the incident x-ray radiation. Such measurement may be executed during a measurement interval. The counting channel 240 will be explained in conjunction with figures 4 and 5 in more detail. Optionally, an integrating channel may be provided per sensor element for obtaining an integration signal representing the total energy of radiation detected since a beginning of a measurement interval.

Fig. 3 shows a top view of an exemplary imager 200 having a plurality of pixels. In the following, reference is made to a pixel 230 and its counting channel 240 (not depicted in Fig. 3). The pixel 230 is surrounded by neighbor pixels 231 to 238. Neighbor pixel 231 is north of pixel 230, pixel 232 is south of pixel 230, pixel 233 is west of pixel 230 and pixel 234 is east of pixel 230. Pixel 235 is located north-east of pixel 230, pixel 236 is north-west of pixel 230, pixel 237 is south-east of pixel 230 and pixel 238 is south-west of pixel 230. Each of the neighbor pixels 231 to 238 comprises its own counting channel and its comparator signals are provided to pixel 230 and to the other neighboring pixels.

Alternatively, only a few neighbor pixels like for example pixels 231, 232, 233 and 234 are connected to pixel 230 or provide their comparator signals to pixel 230. It is also possible that pixels 235, 236, 237 and 238 are connected to pixel 230 or provide their comparator signals to pixel 230. Further, only one, two or three neighbor pixels are connected to pixel 230 or provide their comparator signals to pixel 230.

Fig. 4 depicts an exemplary counting channel 240 of pixel 230. Identical or slightly different counting channels may be arranged at the other pixels of the imager 200. An output of pixel 230 is connected with an analog front end 241 (or CSA (charge sensitive amplifier) and a pulse shaper) for converting the analog signal into a digital pixel signal 242. The pixel signal 242 is provided to an input of a comparator 243. A threshold signal 244 is provided to a further input of the comparator 243. The comparator 243 provides a comparator signal 244 at its output when the pixel signal 242 exceeds the threshold signal 244. The value or amplitude of the threshold signal 244 may be adapted according to different set-ups of the detector or the imaging device such as image mode, energy level and the like.

The comparator signal 245 is provided to a gating or suppression device 246 and to one or more outputs 253. The comparator signal 245 is provided via the output 253 to counting channels 240 of neighbor pixels.

A logical OR gate 248 or another suitable electronic circuit includes in this embodiment four inputs for receiving comparator signals of neighbor pixels 231, 232, 233 and 234. As described above, a different number or signals from different neighbor pixels may be received by OR gate 248. The output of the OR gate 248 is provided to the suppressing device 246. The output of the OR gate 248 is a logical 1 or a high value when at least one of the inputs is high or logical 1. This is the case when the comparator of a neighboring pixel outputs a high signal, i.e. when the charge of neighbor pixel is above the threshold signal 244. The threshold signal 244 is the same for the pixels or at least the same for a group of neighboring pixels.

The suppression device 246 blocks or gates the comparator signal 245 on its way to a counting device 247 such as a pulse counter. The suppression device 246 blocks the comparator signal 245 when the output from the OR gate 248 is high, i.e. when at least one neighboring pixel has received a charge above the threshold signal 244. The suppression device 246 lets pass the comparator signal 245 to the counting device 247, when the output of the OR gate 248 is low, i.e. when none of the neighboring pixels has received a charge above the threshold signal 244. In such configuration, the counting device 247 counts only when the largest partial charge was received by the pixel 230. The neighboring pixels receive the comparator signal 245 of pixel 230 via the output 253. This ensures, that neighbor pixels with a partial charge lower than the threshold signal 244 do not count. Thus, pulse tailing is reduced.

A control device 249 controls or disables the suppression device 246. In other words, the suppression device 246 can be activated and deactivated by the control device 249. Using the control device 249 the behavior of the counting channel can be adapted. With a disabled suppression device 246 the counting device 247 or the counting channel works in an usual mode in which all charge incidents are counted (as they have passed the comparator 342). With an activated suppression device 246 the counting device 247 or the counting channel is in a special mode in which only the largest partial charge incident is counted.

Fig. 5 shows a further embodiment of the counting channel 240. The embodiment of Fig. 5 corresponds in part with the embodiment shown in Fig. 4. In addition to the embodiment of Fig. 4, a second counting device 250 is provided. The second counting device 250 is arranged in parallel to the (first) counting device 247 behind the comparator 243. Accordingly, the second counting device 250 receives the comparator signal 245 at its input. The second counting device 250 is directly connected with the comparator 243, i.e. no suppression device is arranged between these two elements. Accordingly, the second counting device 250 counts all charge incidents (as they have passed the comparator 342) of the pixel 230.

An image reconstruction device 251 is connected with the first and second counting devices 247 and 250 and is adapted to generate an output signal 252 per pixel. Outputs of the first and second counting devices 247 and 250 are connected to inputs of the image reconstruction device 251. The image reconstruction device 251 evaluates or analyses the counting signals of the first and second counting devices 247 and 250 and creates based on the counting signals and optionally on further inputs like device or image settings the output signal 252.

The output signal 252 may include one counting signal, both counting signals or a blend or combination of both counting signals. If for example data bandwidth is an issue only one signal or a combined signal may be outputted by the image reconstruction device 251. If, on the other hand, maximal information is of interest both counting signals are outputted by the image reconstruction device 251.

It may be preferred that the image reconstruction device 251 receives information regarding the image or the images to be produced by the imaging device. Such information may include sharpness, focus, resolution, x-ray tube settings etc. Based on such information and/or on the counting signals the image reconstruction device 251 processes the counting signals and reconstructs or calculates the output signal 252 indicative of an image. Such foregoing provides images which fulfill certain predefined requirements. By providing two separate individual counting signals to the image reconstruction device 251 the informational basis is enhanced so that images of improved quality can be provided.

In still another embodiment the detector pixels have two or more thresholds and related counters for multi-bin energy separation. The count gating logic of the pixels can then be adapted accordingly. The processing logic (in particular the first counting device and the suppressing device) of the digital comparator signals ensures that in case of coincidence (of charge packages received in several neighbor pixel originated from the same photon) only the pixel with the strongest signal enables counting. For instance, in one embodiment all related thresholds of neighbor pixels have the pulse gating logic as described above for a single threshold detector. Hence, among neighbor pixels it is checked at which of the pixels the highest threshold is exceeded, which is the pixel for which this event is counted.

In another embodiment only the lowest threshold decides which pixel counts the passage of higher thresholds during coincidence. In other words, whenever the signal passes through a threshold, the attached counter of this pixel, but not of the neighbor pixels, counts this event.

In a particular implementation a controller (e.g. included in or representing the suppressing device) realized by a finite state machine may be used as shown in Fig. 6.

Without any pulses, the controller is in state "standby". If at least one neighbor pixel has passed the lowest threshold, the controller enters state "inhibit" (path PI) until the neighbor signals are deasserted (path P2; no neighbor pixels are above the lowest threshold). If the lowest threshold is passed by the pixel signal in state "standby", the controller enters the state "counting" (path P3) until the lowest threshold signal is deasserted (path P4; pixel signal is below the lowest threshold). Counting for all thresholds is only enabled if the controller is in state "counting". Similar to the single threshold case, gating may be statically or dynamically disabled or a pair of counters with and without gating can be used.

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 measures cannot be used to advantage.

Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. Detector for detecting incident radiation, comprising:

a plurality of detector pixels (230) each generating pixel signals in response to incident x-ray radiation;

a comparator (243) per pixel connected with a pixel (230) and adapted for generating a comparator signal when the pixel signal is greater than a threshold;

a first counting device (247) per pixel connected with the comparator and adapted for generating a count signal by counting comparator signals; and

a suppressing device (246) per pixel connected with at least one neighboring comparator of a neighboring pixel and adapted for suppressing the count of a comparator signal when at least one neighboring comparator signal is greater than the threshold.

2. Detector as claimed in claim 1, further comprising a second counting device (250) per detector pixel connected with the comparator and adapted for generating a count signal by counting comparator signals, wherein the second counting device is not connected with the suppressing device.

3. Detector as claimed in claim 2, further comprising an image reconstruction device (251) connected with the first and second counting device and adapted to generate an output signal per pixel.

4. Detector as claimed in claim 2, wherein the image reconstruction device (251) is adapted to provide the count signal of the second counting device as the output signal for a high flux modus.

5. Detector as claimed in claim 2, wherein the image reconstruction device (251) is adapted to provide only one counting signal as the output signal.

6. Detector as claimed in claim 2, wherein the image reconstruction device (251) is adapted to provide a combination of the count signals of the first and second counting device as the output signal.

7. Detector as claimed in claim 1, wherein an OR gate (248) is provided for receiving a pixel signal from at least two neighboring pixel and to provide its output to the suppressing device.

8. Detector as claimed in claim 1, wherein four neighboring comparators are connected with the suppressing device and wherein the four pixels of the four neighboring comparators each are arranged by an angle of 90° around the pixel.

9. Detector as claimed in claim 1, wherein eight neighboring comparators are connected with the suppressing device.

10. Detector as claimed in claim 1, further comprising a control device (249) adapted to disable the suppressing device for a high flux modus.

11. Detector as claimed in claim 1 , further comprising at least one output (253) for providing the comparator signal to the at least one neighboring pixel.

12. Detector as claimed in claim 1, wherein

said comparator (243) per pixel is adapted for checking the pixel signal against one of several thresholds,

said suppressing device (246) is adapted to suppress the count of a comparator signal when in case of coincidence at least one neighboring comparator signal has exceeded the lowest threshold earlier.

13. Detector as claimed in claim 1, wherein

said suppressing device (246) is adapted to suppress the count of a comparator signal when in case of coincidence at least one neighboring comparator signal has exceeded a higher threshold.

14. Imaging device comprising an x-ray source (20) for emitting radiation and a detector (200) as claimed in claim 1.

15. Method for detecting incident radiation, comprising:

generating pixel signals (242) in response to incident x-ray radiation;

generating a comparator signal (245) when the pixel signal is greater than a threshold;

generating a count signal by counting comparator signals; and suppressing the count of a comparator signal when at least one neighboring comparator signal is greater than the threshold.