Patent Description:
Photon counting detectors capable of discriminating photon energies, such as X-ray photon energies, have been developed in past decades for various applications e.g. in medical imaging and material science. Photon counting detectors are operated in a pulse mode based on single event, meaning that theoretically each interaction occurred within the detection material can be processed and registered individually.

Photon counting data acquisition modules have been developed to convert an energy of photons to a count signal indicative of the number of photons having an energy above a threshold. For example, <CIT> describes a photon counting data acquisition module in form of a read-out Application Specific Integrated Circuit (ASIC) for an X-ray detector with pixels being clustered using an anti-charge-sharing grid.

In photon counting, two or more nearly simultaneously incident photons may be regarded as a single event with a higher energy, resulting in not only dead time losses, but also a distortion of the recorded pulse height spectrum. Due to pile-up effect and dead time loss, the linearity between the detected photon rate and the incident photon rate gradually lacks with increased photon fluxes. If the dead time extends by the following event arrived within its dead time, ambiguity may exist between observed count rate and incident count rate. This count rate performance is also referred to as paralyzable.

<CIT> describes a photon rate counter including a radiation detector and signal conditioning circuitry. The photon rate counter produces a count value indicative of a total number of photons received by a detector. One or more photon counters produce count values indicative of photons having varying energy characteristics. The counters disregard pileup pulses.

<CIT> relates to a detector for detecting ionizing radiation, comprising a directly converting semiconductor layer for producing charge carriers in response to incident ionizing radiation and a plurality of electrodes corresponding to pixels for registering the charge carriers and generate a signal corresponding to registered charge carriers, wherein an electrode of the plurality of electrodes is structured to two-dimensionally intertwine with at least two adjacent electrodes to register the charge carriers by said electrode and by at least one adjacent electrode.

There may be a need to provide a non-paralyzable photon-counting data acquisition device.

The object of the present invention is solved by the subject-matter of the independent claims, wherein further embodiments are incorporated in the dependent claims. It should be noted that the following described aspects of the invention apply also for the photon-counting data acquisition module, the pixelated photon-counting detector, the method for photon counting, as well as the computer program element and the computer readable medium.

A first aspect of the present invention relates to a photon-counting data acquisition module according to claim <NUM>.

Since the discriminator/counter pair is allowed to evaluate signals only in discrete time intervals, the system is inherently non-paralyzable in the sense that the observed count rate increases monotonically with the input count rate. In an example, the discriminator may be time-discrete discriminator, which can be triggered, by a detection of a maximum of a pulse, to compare the pulse with the at least one signal threshold. Alternatively, the discriminator could be time continued but it is only allowed to increment the associated counter when enabled. In other words, the discriminator operates normally, and the time-discrete component is accomplished by digitally enabling/disabling the counting function. In particular, if a local maximum is detected (e.g. during pile-up), the next local maximum will again be detected, and the discriminator is evaluated. Thus, incrementing the counters in high pile-up situations is governed by the detection of local maxima. There may be some randomization of the output count rate, and issues of the multi-energy case may be circumvented. A non-paralyzable count rate behavior may be considered advantageous, since there is no ambiguity between observed count rate and incident count rate as in the paralyzable case.

The photon-counting data acquisition module may be connected to various photodetectors including, but not limited to, photomultiplier, geiger counter, single-photon avalanche diode, superconducting nanowire single-photon detector, transition edge sensor, charge-coupled device or scintillation counter. A various spectral range may be covered, from near-infrared and ultraviolet wavelengths to high-energy regions, such as X-ray and gamma ray, for different applications such as chemical analysis, medical imaging and laser measurement. The photon-counting data acquisition module may be part of, or include an ASIC, an electronic circuit, a processor and/or memory that execute one or more software or firmware programs, a combinational logical circuit, and/or other suitable components that provide the described functionality. For example, the photon-counting data acquisition module may be a read-out ASIC for a photon-counting Computed Tomography (CT) detector.

The pulse maximum identifier may comprise a zero-crossing threshold preceded by a differentiator. A possible implementation of the pulse maximum identifier is illustrated in <FIG>.

The discriminator may include one or more comparators. Each comparator compares the amplitude of the pulse with one or more predetermined energy thresholds that correspond to one or more different energy ranges. This may be triggered by a detection of a maximum of a pulse in the at least one received train of pulses. Alternatively, the counter may be enabled in response to the detection of a maximum of a pulse. Thus, the input of each comparator is only evaluated when input signal in form of a train of pulses has reached a local maximum. The benefit may be the resulting non-paralyzable count rate behavior. The comparators may respectively produce output signals indicative of whether the energy of a detected photon event is above or below a threshold. A counter counts, for each energy range, a number of pulses that falls within the energy range based on the comparator output signals.

According to an embodiment of the present invention, each data acquisition channel is adapted for being connected to a pixel of a pixelated photon-counting detector to receive a train of pulses indicative of an energy of photons incident on the pixel, or to a cluster of sub-pixels of a pixelated photon-counting detector to receive a plurality of trains of pulses, each indicative of an energy of photons incident on a respective sub-pixel of the cluster.

The pixelated photon-counting detector may be a detector for e.g. X-ray, gamma ray, or fluorescence imaging. In an example, the photon-counting data acquisition module may be connected to a pixelated detector without sub-pixelation, such as Philips proprietary ChromAIX2, which has pixels of a pitch of about <NUM>. In another example, the photon-counting data acquisition module may be connected to a pixelated detector with sub-pixelation. In this case, each data acquisition channel may be linked to an analog front-end for dealing with sub-pixels and implement charge-sharing corrections, as will be explained hereafter and particularly with respect to the exemplary embodiment in <FIG>.

According to the first aspect of the present invention, the at least one train of pulses received from the signal input unit comprises a first train of pulses and a second train of pulses. The data acquisition channel further comprises a selection logic. The pulse maximum identifier is configured to identify maxima of a first pulse in the first train of pulses and a second pulse in the second train of pulses. The selection logic is configured to determine whether the maxima of the first pulse and the second pulse are within a coincidence window and to enable the discriminator to evaluate the first pulse in the first train of pulses or the second pulse in the second train of pulses directly if the first and second pulses are not within the coincidence window, or a sum of the first and second pulses if the first and second pulses are within the coincidence window.

The at least one train of pulses may further comprise a third train of pulses, a fourth train of pulses, etc. In other words, the following discussion is also scalable to a large number of trains of pulses. The selection logic may determine whether a given event, i.e. a pulse indicative of an energy incident on one or more sub-pixels, is confined to a single sub-pixel or charge shared across one or more neighbors. The input to the thresholds, i.e. the energy discriminator, is dependent on the decision. That is, for every single photon, a decision is made to feedthrough the shaper directly to the discriminator or to take the signal from a shaper summing mode which is continuously available.

This may allow charge sharing compensation, while avoiding a degradation of count rate performance and spectral performance. This may be beneficial for smaller pixels, which on the one hand relax the stringent requirement of the photon counting channel, while on the other hand may be severely impaired by the need to correct for charge sharing. The use of on-the-fly coincidence detection and a time-discrete discriminator circuit or a time-discrete counter may facilitate the path for using smaller pixels, while preserving both count-rate and spectral performance.

According to an embodiment of the present invention, the selection logic comprises i) a coincidence detector, ii) a switch control, and iii) a threshold sampling control. The coincidence detector is configured to evaluate a state of an output of the pulse maximum identifier and to determine whether the first pulse in the first train of pulses, the second pulse in the second train of pulses, or both the first and second pulses are within the coincidence window. The switch control is configured to determine, based on the evaluation and determination of the coincidence detector, whether to feedthrough the first pulse, the second pulse, or a sum of the first and second pulses to an input of the discriminator. The threshold sampling control is configured to evaluate a state of an output of the pulse maximum identifier and to trigger the discriminator to perform comparison based on the evaluated state.

The coincidence detector may represent a logic response to multiple inputs. Examples of the coincidence detector include, but not limited to, a truth table, a combinatorial logic, and a state machine. This will be explained hereafter and particularly with respect to <FIG>.

According to an embodiment of the present invention, the data acquisition channel further comprises an adder configured to add the first and second train of pulses.

The adder may be an analogue adder. This may be done with an amplifier, e.g. in the voltage domain or in the current domain. In an example, the signals are continuously summed into a single node. In another example, e.g. for a large number of pixels or sub-pixels, it may be advantageous to only add pixels or sub-pixels which are in fact processing a shaper pulse.

According to an embodiment of the present invention, the data acquisition channel further comprises a multiplexer adapted for forwarding the first train of pulses, the second train of pulses, and a sum of the first and second trains of pulses to the discriminator in a time-multiplexed manner.

In other words, the threshold system, i.e. the energy discriminator, may be time-multiplexed across all e.g. sub-pixels. Thus, sub-pixels within a cluster share a set of energy thresholds, i.e. the energy discriminator.

According to an embodiment of the present invention, the discriminator/counter pair further comprises a charge-sharing counter adapted for being triggered by a detection of the maxima of the first pulse and the second pulse within a coincidence window to increase a value.

The charge-sharing counter may be beneficial in determining pile-up corrections.

A second aspect of the present invention relates to a sub-pixelated photon-counting detector according to claim <NUM>.

As mentioned above, the pixelated photon-counting detector may exhibit non-paralyzable count rate behavior, which may be advantageous, since there is no ambiguity between observed count rate and incident count rate as in the paralyzable case.

According to an embodiment of the present invention, the pixel is a cluster of sub-pixels. Each data acquisition channel is configured to receive a plurality of pulse trains, each indicative of an energy of photons incident on a respective sub-pixel of the cluster.

As mentioned above, this may facilitate the path for using smaller pixels to relax the stringent requirement of the photon-counting channel, while preserving both count-rate and spectral performance.

According to an embodiment of the present invention, the pixelated photon-counting detector is at least one of an X-ray detector, a gamma ray detector, and a fluorescence detector.

A third aspect of the present invention relates to a method for photon counting according to claim <NUM>.

According to an embodiment of the present invention, the at least one received train of pulses are indicative of an energy of photons incident on a respective pixel or a respective cluster of sub-pixels of a pixelated photon-counting detector.

A fourth aspect of the present invention relates to a computer program element according to claim <NUM>.

A fifth aspect of the present invention relates to a computer readable medium according to claim <NUM>.

<FIG> shows a photon-counting data acquisition module <NUM>, in this embodiment, as part of a pixelated photon-counting detector <NUM> according to some embodiments of the present disclosure. The photon-counting data acquisition module may be part of, or include an ASIC, an electronic circuit, a processor and/or memory that execute one or more software or firmware programs, a combinational logical circuit, and/or other suitable components that provide the described functionality.

The photon-counting data acquisition module <NUM> comprises a signal input unit <NUM>, one or, in this embodiment, more data acquisition channels 14a, 14b, 14c, also collectively referred to herein as data acquisition channels <NUM>, and a signal output unit <NUM>. The signal input unit <NUM> may comprise one or more signal inputs (not shown), each being adapted for receiving a train of pulses. For simplicity only three data acquisition channels 14a, 14b, 14c are shown in <FIG>. The following discussion is also scalable to a large number of data acquisition channels. Each data acquisition channel <NUM> is adapted for converting at least one train of pulses <NUM>, such as two trains of pulses 18a, 18b illustrated in <FIG>, received from the signal input unit <NUM> to a counter signal. The signal output unit <NUM> is adapted for outputting the counter signal. Each data acquisition channel <NUM> comprises a pulse maximum identifier 20a, 20b, 20c, also collectively referred to herein as pulse maximum identifiers <NUM>, and a discriminator/counter pair 22a, 22b, 22c comprising a discriminator <NUM> (see <FIG>) and a counter <NUM> (see <FIG>), also collectively referred to herein as discriminator/counter pairs <NUM>.

Optionally, each data acquisition channel <NUM> may further comprise an analogue preprocessing chain 19a, 19b, 19c, also collectively referred to herein as analogue preprocessing chains <NUM>. The analogue preprocessing chain <NUM> is configured to amplify and filter the at least one train of pulses <NUM>. The analogue preprocessing chain <NUM> may comprise one or more charge-sensing amplifiers (CSA) and one or more pulse Shapers.

The pulse maximum identifier <NUM> is configured to identify a maximum of a pulse <NUM>, such as the pulse 24a, 24b illustrated in <FIG>, in the at least one received train of pulses <NUM>. The pulse maximum identifier <NUM> may comprise a zero-crossing threshold preceded by a differentiator. A possible implementation is illustrated in <FIG>.

The discriminator <NUM> is configured to be triggered, by a detection of a maximum of a pulse <NUM> in the at least one received train of pulses <NUM>, to compare the pulse <NUM> with at least one signal threshold to generate the counter signal. As illustrated in <FIG>, the discriminator <NUM> may comprise one or more comparators <NUM>, each comparator <NUM> comparing the amplitude of the pulse <NUM> with one or more predetermined energy thresholds that correspond to one or more different energy ranges.

A counter <NUM> counts, for each energy range, a number of pulses that falls within the energy range based on the comparator output signals. The counter <NUM> may be enabled in response to a detection of a maximum of a pulse <NUM> to generate the counter signal; that is, the discriminator itself could be time continues but it is only allowed to increment the associated counters when enabled.

The pixelated photon-counting detector <NUM> comprises an array of pixels 110a, 110b, 110c, also collectively referred to as pixels <NUM>. For simplicity, only three pixels are illustrated in <FIG>. Each data acquisition channel 14a, 14b, 14c may be adapted for being connected to a corresponding pixel 110a, 110b, 110c of the pixelated photon-counting detector <NUM> to receive a train of pulses <NUM> indicative of an energy of photons incident on the respective pixel 110a, 110b, 110c of the pixelated photon-counting detector <NUM>.

For example, the pixelated photon-counting detector <NUM> may be a semiconductor based photon-counting detector, which comprises two core components: semiconductor material, such as Si, CdTe or CZT, with two electrodes, and photon-counting module <NUM> in form of read-out ASICs. When an incident X-ray photon interacts within the semiconductor material, electrical charges i.e. electron-hole pairs, with an amount proportional to the deposited energy of the incident photon are produced and drifted towards the monolithic and pixelated electrodes separately under the influence of the externally applied electrical field. During the drifting process of electron-hole pairs, a transient current is generated and then processed by each connected data acquisition channel <NUM> through one optional analogue preprocessing chain <NUM>, including one or more charge-sensitive preamplifier (not shown) and one or more pulse shapers as illustrated in <FIG>, and the pulse maximum identifier <NUM> and the discriminator <NUM> with multiple pairs of voltage pulse height comparator and digital counter as illustrated in <FIG>.

In some applications, techniques of sub-pixelation is implemented. For example, <CIT> describes clustering pixels using an anti-scatter grid. <FIG> shows a conceptual <NUM>×<NUM> clustering of sub-pixels <NUM>N,M representing a pixel <NUM>, where N and M are positive integers. As illustrated in <FIG>, the pulse shape may be adapted in a shaper (e.g., filter) of the optional analogue preprocessing chain <NUM> at the output of each sub-pixel <NUM>N,M. Each data acquisition channel <NUM> is adapted for being connected to a cluster of sub-pixels <NUM> N,M of the pixelated photon-counting detector <NUM> to receive a plurality of trains of pulses <NUM>, each indicative of an energy of photons incident on a respective sub-pixel <NUM>N,M of the cluster.

In some situations, it is noted that the spectral performance of the photon counting data acquisition module may be limited by a so-called charge-sharing effect, where charge, which is caused by a single photon, is shared between neighboring sub-pixels. Charge sharing effect is almost unavoidable in photon counting detector because the radiation semiconductor is electrically, rather than physically, pixelated. Charge sharing effect is more pronounced for small pixels, especially less than <NUM>.

<FIG> shows a data acquisition channel <NUM> adapted for providing charge sharing compensation according to some embodiments of the present disclosure. For simplicity only two sub-pixels are illustrated, i.e. sub-pixel <NUM><NUM>,<NUM> and sub-pixel <NUM><NUM>,<NUM> representing a <NUM>×<NUM> clustering of sub-pixels. It is noted that the discussion and the model described hereafter is scalable to a large number of sub-pixels. No limitations other than noise and the digital complexity apply.

The at least one train of pulses <NUM> received from the signal input unit thus comprises a first train of pulses 18a and a second train of pulses 18b as illustrated in <FIG>, each received from a respective sub-pixel <NUM><NUM>,<NUM> , <NUM><NUM>,<NUM>. The data acquisition channel <NUM> further comprises a selection logic <NUM>.

According to the invention, the pulse maximum identifier <NUM> is configured to identify maxima of a first pulse 24a in the first train of pulses 18a and a second pulse 24b in the second train of pulses 18b, as will be explained hereafter and particularly with respect to the exemplary embodiments in <FIG>. The first and second trains of pulses may be shaper outputs of the sub-pixels <NUM><NUM>,<NUM>, <NUM><NUM>,<NUM>. The pulse maximum identifier <NUM> may comprise a zero-crossing threshold preceded by a differentiator (or delay line summing). Its output goes high when a pulse has reached its maximum and it stays high for a pre-defined time. It may serve as coincidence logic. It is based on the fact that charge sharing across sub-pixels of a single event is instantaneous in nature. That is, both signal output units of the sub-pixels <NUM><NUM>,<NUM> and <NUM><NUM>,<NUM> must necessarily exhibit a maximum at the same time. The aforementioned delay may establish a coincidence window. The minimum window is set by the uncertainty in finding the maximum due to noise and circuits tolerances. A possible implementation of the pulse maximum identifier <NUM> (although other solutions may apply) is illustrated in <FIG>. Turning back to <FIG>, the selection logic <NUM> is configured to determine whether the maxima of the first pulse 24a and the second pulse 24b are within a coincidence window and to enable the discriminator to evaluate the first pulse 24a in the first train of pulses 18a or the second pulse 24b in the second train of pulses 18b directly if the maxima of the first and second pulses are not within the coincidence window, or a sum of the first 24a and second pulses 24b if the maxima of the first and second pulses are within the coincidence window. This will be explained hereafter and particularly with respect to the exemplary embodiments in <FIG>. In other words, the selection logic is configured to detect charge sharing events based on on-the fly coincidence detection and sum signals from sub-pixels of one cluster in the event of charge sharing. If no charge sharing event is detected, the signals from the sub-pixels are transmitted directly to the discriminator. This may allow charge sharing compensation based on on-the-fly coincidence detection and a time-discrete discriminator and thus allow an accurate decision of when to evaluate a shaper signal against energy thresholds.

An example of the selection logic <NUM> is illustrated in <FIG>, although other solutions may apply. In this example, the selection logic <NUM> comprises a coincidence detector <NUM>, a switch control <NUM>, and a threshold sampling control <NUM>.

The coincidence detector <NUM> is configured to evaluate a state of an output of the pulse maximum identifier <NUM> and to determine whether the maxima of the first pulse 24a in the first train of pulses 18a, the second pulse 18b in the second train of pulses 18b, or both the maxima of the first and second pulses are within the coincidence window. For example, it may indicate how many of the sub-pixels are active at the same time within the coincidence window. For example, a truth table may be used to decode which sub-pixel is active, for example, [<NUM><NUM>] → none, [<NUM><NUM>] → <NUM><NUM>,<NUM>, [<NUM><NUM>] → <NUM><NUM>,<NUM>, [<NUM><NUM>] → both.

The switch control <NUM> is configured to determine, based on the coincidence detector <NUM>, whether to feedthrough the first pulse, the second pulse, or a sum of the first and second pulses to an input of the discriminator/counter pair <NUM>. In other words, it takes decision on which signal is to be connected to the discriminator, i.e. which sub-pixel or if the sum of both.

The threshold sampling control <NUM> is configured to evaluate a state of an output of the pulse maximum identifier <NUM> and to trigger the discriminator <NUM> to perform comparison based on the evaluated state. In other words, the threshold sampling control <NUM> monitors the state of the shaper pulses and it enables the discriminator to evaluate a result accordingly. The output may remain high for a short time, e.g. <NUM> ns, or <NUM> ns, or <NUM> ns , depending on the count-rate performance and the noise requirements of the channel. This may work in the digital domain, i.e. the threshold counters may be allowed to increment or not.

To sum the signal, the data acquisition channel <NUM> may further comprise an adder <NUM> configured to add the first and second train of pulses. The adder <NUM> may work in an analogue summing mode. The adder <NUM> may comprise a high bandwidth amplifier with an input differential pair with N inverting input branches, where N is the number of sub-pixels of a cluster. Other implementation may consist on adding current branches of the shaper circuits.

In addition, the data acquisition channel <NUM> may further comprise a multiplexer <NUM>, e.g. an analogue multiplexer, adapted for forwarding the first train of pulses, the second train of pulses, and a sum of the first and second trains of pulses to the discriminator in a time-multiplexed manner. In other words, the sub-pixels are clustered in a way that they share a set of energy thresholds. This is not energy staggering, the threshold system is time-multiplexed across all sub-pixels within a cluster. The input of the threshold system (every discriminator) is dependent on the decision on whether a given event is confined to a signal sub-pixel or charge shared across one or more neighbors. That is, for every signal photon a decision is made to feedthrough the shaper directly to the discriminator or to take the signal from a shaper summing mode which is continuously available. The multiplexer <NUM> may take the result from the switch control <NUM> and accordingly connect the required signal to the discriminator <NUM>. For overall pile-up requirements, it may be beneficial to have a high-speed multiplexer. Propagation delays may be added to all signals if required.

The discriminator/counter pair <NUM> may comprise at least one threshold <NUM> and a charge-sharing counter <NUM>. The at least one threshold <NUM> may be conventional discriminators and counters. The counters are, however, only allowed to increment their values at given time interval, i.e. when the decision of which signal needs to be evaluated is available. A simple enable bit in the digital counters may suffice to enforce this functionality. <FIG> shows an exemplary implementation. The charge-sharing counter <NUM> is adapted for being triggered by a detection of the maxima of the first pulse and the second pulse within a coincidence window to increase a value. In other words, the control signals commandeering the decisions can also be used to output a count value representative of how many events have been treated as charge shared. This may also be proven valuable in determining pile-up corrections.

To illustrate the working principle of the photon-counting data acquisition module <NUM>, <FIG> illustrate the simulation results of a single event that is not shared across two sub-pixels. For simulation purposes both sub-pixels <NUM><NUM>,<NUM> and <NUM><NUM>,<NUM> as illustrated in <FIG> provide the same train of pulses, however, with a time delay with respect to each other. In this case, the time delay is set to only <NUM> ns. The simulation shows cases with the capability of distinguishing pulses that are considered single events in a very short time interval, allowing accommodating very high counting rates.

As illustrated in <FIG>, both sub-pixels <NUM><NUM>,<NUM> and <NUM><NUM>,<NUM> are processing two unrelated single events. The first pulse 24a indicative an energy of photons incident on the sub-pixel <NUM><NUM>,<NUM> and the second pulse 24b indicative of an energy of photons incident on the sub-pixel <NUM><NUM>,<NUM> are only <NUM> ns apart, both having <NUM> keV energy (<NUM> in the simulation). The switch control <NUM> indicates that the first sub-pixel <NUM><NUM>,<NUM> is active with an output of "<NUM>" and shortly after that a second sub-pixel <NUM><NUM>,<NUM> is also active with an output "<NUM>". Since the maxima is not located within a predefined coincidence window of <NUM> ns, the switch control does not indicate the presence of charge sharing with an output of "<NUM>".

In <FIG>, the multiplexer <NUM> directs the output of the sub-pixel <NUM><NUM>,<NUM> to the input of the discriminator <NUM> in a first instance. Shortly afterwards the output of the sub-pixel <NUM><NUM>,<NUM> is directed towards the discriminator <NUM>. The change in the selection is not observable in <FIG> due to both signals having very similar values.

In <FIG>, the at least one threshold <NUM> of the discriminator <NUM> is accordingly enabled twice, once for each pulse, such that the sub-pixel <NUM><NUM>,<NUM> is evaluated and immediately thereafter the sub-pixel <NUM><NUM>,<NUM> is assessed.

In <FIG>, the lowest threshold, which is set to 50keV, registers two counts, adequately representing the arrival of two independent signal events on two sub-pixels <NUM><NUM>,<NUM> and <NUM><NUM>,<NUM>.

<FIG> illustrate the simulation results of a single event that is shared across two sub-pixels. In this case, both sub-pixels <NUM><NUM>,<NUM> and <NUM><NUM>,<NUM> again have the same train of pulses, however, with no delay. This serves the purpose to simulate the correct attribution of the charge sharing compensation mechanism. The output the sub-pixel <NUM><NUM>,<NUM> is also scaled by <NUM>%, which entails that a single <NUM> keV event is split into two smaller events and the sub-pixel <NUM><NUM>,<NUM> receives the remaining <NUM> keV.

In <FIG>, as mentioned above, the sub-pixel <NUM><NUM>,<NUM> exhibits a <NUM> keV event, while the sub-pixel <NUM><NUM>,<NUM> exhibits <NUM> keV. Since both maxima are found to occur within the coincidence window of <NUM> ns, the switch control <NUM> indicates with "<NUM>" that both signals have to be regarded as a single charge shared event.

Accordingly, as shown in <FIG>, the multiplexer <NUM> directs the addition of both signals to the discriminator input, showing a total energy of <NUM> keV (<NUM> in the simulation).

In <FIG>, the at least one threshold <NUM> of the discriminator <NUM> is accordingly enabled once, such that the sum of both signals is evaluated.

In <FIG>, since the total energy is <NUM> keV, both <NUM> keV and <NUM> keV thresholds are incremented only once, representing the arrival of a single event that is registered on both sub-pixels <NUM><NUM>,<NUM> and <NUM><NUM>,<NUM>.

<FIG> illustrate the simulation results of multiple events with a mixture of single events and charge shared events. As illustrated in <FIG>, in this simulation, each sub-pixel <NUM><NUM>,<NUM> and <NUM><NUM>,<NUM> has a unique train of pulses 18a, 18b, monochromatic at <NUM> keV. A third independent train of pulses generated and split across both sub-pixels, mimicking charge sharing events in both trains of pulses 18a, 18b. In this simulation, the sub-pixel <NUM><NUM>,<NUM> receives <NUM>% of the charge, i.e. <NUM> keV and the sub-pixel and <NUM><NUM>,<NUM> <NUM>%, i.e. <NUM> keV. All events have <NUM> keV. Therefore, if the model behaves as expected, the <NUM> keV threshold should count as many events as there were in the third independent pulse trains combined (if no pile-up). In particular, the first train of pulses 18a has a total of nineteen events in <NUM>. The second train of pulses 18b has a total of fourteen events. The third train of pulses, mimicking charge sharing events, has a total of seven single events, shared across both sub-pixels <NUM><NUM>,<NUM> and <NUM><NUM>,<NUM>, i.e. seven smaller events each.

As can be seen in <FIG>, the <NUM> keV threshold <NUM> adequately identifies <NUM> events.

As can be seen in <FIG>, the charge-sharing counter <NUM> also identifies correctly the seven events that are shared across the sub-pixels. The position of the second cursor shows how despite having occurred a total of seven shaper pulses, only six were counted, including five single events and one charge share event split across pixels.

<FIG> shows a count-rate simulation using a polychromatic tube spectrum illustrating a two sub-pixel energy spectrum <NUM>, an output of the model <NUM>, and a ground truth <NUM>. To facilitate the interpretation, a low rate of <NUM> Mcps has been used to ensure that there is no pile-up. The benefit of using the model may become more obvious with the resulting registered spectrum falling on top of the ground truth <NUM>. In this simulation it is intended to show the resulting observed count-rate as a function of subsequently increasing incident count rate. In view of execution time, this simulation has been restricted to <NUM> Monte Carlo realizations for each incident count rate point. The tube spectrum has been used for this simulation.

<FIG> shows the results of this simulation for a <NUM> keV threshold. Four traces are shown:.

The pixelated photon-counting detector <NUM> may be at least one of an X-ray detector, a gamma ray detector and a fluorescence detector. A possible implementation of the pixelated photon-counting detector <NUM> may be in an imaging system <NUM> such as a computed tomography CT scanner schematically illustrated in <FIG>. The imaging system <NUM> includes a gantry <NUM>, which is capable of rotation about a rotational axis R, which extends parallel to a z direction. A radiation source <NUM>, which in this embodiment is an x-ray tube, is mounted on the gantry <NUM> and is provided with a collimator <NUM>, which forms a conical radiation beam <NUM> from the radiation generated by the radiation source <NUM>. The radiation traverses the object being, in this embodiment, a human patient within a cylindrical examination zone <NUM> and hence the patient. The radiation beam <NUM> is incident on a photon-counting detector <NUM>, which is mounted on the gantry <NUM>. The pixelated photon-counting detector <NUM> may have a one or two-dimensional array of pixels <NUM>, which are connected to a photon-counting data acquisition module <NUM> to count individual photons incident on the pixels <NUM>. Since the discriminators are allowed to evaluate signals only in discrete time intervals, the system is inherently non-paralyzable in the sense that the observed count rate increases moronically with the input count rate. In some embodiments, each pixel <NUM> may be a cluster of sub-pixels <NUM>, the photon-counting data acquisition module <NUM> may be configured to count individual photons incident on the sub-pixels <NUM> of a cluster and to provide charge sharing compensation.

The imaging system <NUM> comprises two motors <NUM>, <NUM>. The gantry <NUM> is driven at a preferably constant but adjustable angular speed by the motor <NUM>. The motor <NUM> is provided for displacing the patient, who is arrange on a patient table, in the examination zone <NUM> parallel to the direction of the rotational axis R or the z axis. The motors <NUM>, <NUM> are controlled by a control unit <NUM>, for instance, such that the radiation source <NUM> and the patient within the examination zone <NUM> move relative to each other along a helical trajectory. However, it is also possible that the radiation source <NUM> and the patient move relatively to each other along another trajectory. For instance, in an embodiment, the radiation source <NUM> may move around the patient along a circular trajectory.

The imaging system <NUM> may further comprise an input unit <NUM> like a keyboard, a computer mouse, a touch pad, etc., and a display <NUM>. The input unit <NUM> may be adapted to allow a user to input a clustering input defining a desired clustering of the pixels. The photon counting data acquisition module <NUM> of the pixelated photon-counting detector <NUM> may be adapted to consider the desired clustering defined by the clustering input while determining the charge-sharing-photon count for a macro pixel. The input unit <NUM> may be adapted to allow for changes in software and/or hardware configurations, in order to amend the clustering, especially the size of the clusters.

<FIG> shows a flow diagram of a method <NUM> for photon counting. In step <NUM>, a maximum of a pulse in at least one received train of pulses is identified with a pulse maximum identifier. In step <NUM>, a discriminator is triggered by a detection of a maximum of a pulse in the at least one received train of pulses, to compare the pulse with at least one signal threshold. Alternatively, in step <NUM>, a counter is enabled, in response to a detection of a maximum of a pulse in the at least one received train of pulses, to generate the counter signal.

Optionally, the at least one received train of pulses are indicative of an energy of photons incident on a respective pixel or a respective cluster of sub-pixels of a pixelated photon-counting detector.

The method <NUM> may comprise further steps. In an option, the at least one received train of pulses comprises a first train of pulses and a second train of pulses. The method further comprises identifying maxima of a first pulse in the first train of pulses and a second pulse in the second train of pulses, determining whether the maxima of the first pulse and the second pulse are within a coincidence window, and evaluating the first pulse in the first train of pulses, the second pulse in the second train of pulses, or a sum of the first pulse and the second pulse based on the determination results.

The computer program element might therefore be stored on a computer unit, which might also be part of an embodiment of the present invention. This computing unit may be adapted to perform or induce a performing of the steps of the method described above. Moreover, it may be adapted to operate the components of the above described apparatus. The computing unit can be adapted to operate automatically and/or to execute the orders of a user. The data processor may thus be equipped to carry out the method of the invention.

Claim 1:
A photon-counting data acquisition module (<NUM>), comprising:
- a signal input unit (<NUM>);
- one or more data acquisition channels (<NUM>, 14a, 14b, 14c), each channel adapted for converting at least a first train of pulses (18a) from a first sub-pixel (<NUM><NUM>,<NUM>) and a second train of pulses (18b) from a second sub-pixel (<NUM><NUM>,<NUM>) received from the signal input unit to a counter signal; and
- a signal output unit (<NUM>) adapted for outputting the counter signal;
wherein each data acquisition channel comprises a pulse maximum identifier (<NUM>, 20a, 20b, 20c) and a discriminator/counter pair (<NUM>, 22a, 22b, 22c) comprising a discriminator (<NUM>) and a counter (<NUM>);
wherein the pulse maximum identifier is configured to identify a maximum of a first pulse (24a) in the first received train of pulses and a maximum of a second pulse (24b) in the second received train of pulses;
wherein the discriminator is configured to be triggered, by a detection of a maximum of a pulse in the at least first and second received trains of pulses, to compare the pulse with at least one signal threshold to generate the counter signal, or wherein the counter is configured to be enabled in response to a detection of a maximum of a pulse in the at least first and second received trains of pulses to generate the counter signal;
characterized in that the data acquisition channel further comprises:
- a selection logic (<NUM>) configured to determine whether the maxima of the first pulse and the second pulse are within a coincidence window and to enable the discriminator to evaluate the first pulse in the first train of pulses or the second pulse in the second train of pulses directly if the maxima of the first and second pulses are not within the coincidence window, or a sum of the first and second pulses if the maxima of the first and second pulses are within the coincidence window.