Photon counting device and method

Abstract

The invention relates a photon counting device and method for counting photon interactions in a piece of converter material and addressing the issue of charge sharing. The occurrence of a charge sharing event is already detected upon the onset of the pulse, taking into consideration an onset of a pulse in a neighboring pixel within a preferably very short coincidence window. According to the invention, it is detected whether a pulse is being processed and one or more neighboring pixels are scouted to decide whether a simultaneous interaction has been registered within a very short coincidence window.

Claims

1. A photon counting device for counting photon interactions in a piece of converter material, comprising: a plurality of energy discriminators and a pulse detector, each being arranged to receive a pulse generated by a photon interacting with the piece of converter material, and a plurality of counters , each of the counters being coupled to one of the energy discriminators, for counting photon interactions with energy discrimination according to an energy level of the coupled energy discriminator, wherein the pulse detector is arranged to provide a pulse logic signal upon receiving an input of a predetermined energy level, the predetermined energy level being lower than the energy levels of the energy discriminators, wherein the photon counting device further comprises a prevention circuitry arranged to receive a neighboring logic signal indicating an input of the predetermined energy level in at least one neighboring piece of converter material, wherein the prevention circuitry is arranged to selectively prevent a pulse from being counted by any one of the plurality of counters in case the pulse logic signal and the neighboring logic signal are present together in a coincidence window of predetermined size.

2. The photon counting device according to claim 1, wherein the prevention circuitry is provided with a digital propagation delay network for determining whether the pulse logic signal and the neighboring logic signal are present together in the coincidence window, and/or wherein the pulse logic signal is provided with a monostable characteristic.

3. The photon counting device according to claim 1, comprising an output for providing the pulse logic signal to a photon counting device for counting photon interactions in a neighboring piece of converter material.

4. The photon counting device according to claim 3, comprising a pulse window delay section for delaying the providing of the pulse logic signal by a predetermined period of time.

5. The photon counting device according to claim 1, comprising a neighbor window delay section for delaying the receiving of the neighboring logic signal by the prevention circuitry by a predetermined period of time.

6. The photon counting device according to claim 1, comprising a discriminator delay section for delaying the receiving of the pulse by the plurality of energy discriminators by a predetermined period of time and/or a plurality of counter delay sections, each provided between an output of an energy discriminator and an input of a counter and providing a delay of a predetermined period of time.

7. The photon counting device according to claim 1, wherein the prevention circuitry is further arranged to stop preventing the pulse from being counted upon receiving an end signal from an energy discriminator and/or the pulse detector, said end signal indicating that the pulse has expired.

8. The photon counting device according to claim 1, comprising a pulse counter arranged to count photon interactions based on the pulse logic signal provided by the pulse detector.

9. The photon counting device according to claim 1, comprising a coincidence counter arranged to count coincidence events of the pulse logic signal and the neighboring logic signal being present together in the coincidence window.

10. The photon counting device according to claim 9, wherein the prevention circuitry is arranged to receive separate neighboring logic signals, each of the separate neighboring logic signal indicative of an input of predetermined energy level in a different neighboring piece of converter material, wherein the coincidence counter is arranged to provide separate counts of coincidence events for the different neighboring pieces of converter material.

11. The photon counting device according to claim 1, comprising an adding section arranged for selectively adding the pulse generated by the photon interacting the piece of converter material and a pulse generated by the photon interacting with the neighboring piece of converter material, and an arbitration control section arranged to control the adding section based on the pulse logic signal and the neighboring logic signal.

12. The photon counting device according to claim 11, comprising a pulse output arranged for outputting the received pulse generated by the photon interacting with the piece of converter material to a photon counting device for counting photon interactions in the neighboring piece of converter material.

13. The photon counting device according to claim 11, comprising a switch arranged for preventing the plurality of energy discriminators from receiving the pulse, wherein the arbitration control section is arranged to control such that either the adding section adds the pulse generated by the photon interacting the piece of converter material and the pulse generated by the photon interacting with the neighboring piece of converter material or the switch prevents the plurality of energy discriminators from receiving the pulse.

14. A photon counting method for counting photon interactions in a piece of converter material, comprising the steps of: receiving a pulse generated by a photon interacting with the piece of converter material, providing a pulse logic signal upon the pulse reaching a predetermined energy level, and preventing, in case a neighboring logic signal indicating an input of predetermined energy level in at least one neighboring piece of converter material and the pulse logic signal are present together in a coincidence window of predetermined size, the pulse from being counted by any one of a plurality of counters, each of the counters being coupled to one of a plurality energy discriminators, for counting photon interactions with energy discrimination according to an energy level of the coupled energy discriminator, each energy discriminators being arranged to receive the pulse.

15. A non-transitory software product for counting photon interactions, the software product comprising program code means for causing a photon counting device according to claim 1 to carry out the steps of the method as claimed in claim 14 when the software product is run on the photon counting device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following drawings:

(2) FIG. 1 shows a block diagram illustrating a photon counting device in accordance with an embodiment of the invention,

(3) FIG. 2 shows time series of two simulated pixels,

(4) FIG. 3 shows a zoom of an area marked in FIG. 2,

(5) FIG. 4 shows a comparison of count-rate figures with and without the use of the prevention circuitry of the present invention,

(6) FIG. 5 shows a block diagram illustrating a modification of the photon counting device shown in FIG. 1.

(7) FIG. 6 shows a portion of a prevention circuitry in accordance with another embodiment of the invention,

(8) FIG. 7 shows two neighboring photon counting devices including the portion of the prevention circuitry shown in FIG. 6, and

(9) FIG. 8 shows a schematic flow diagram illustrating a method for counting photons according to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(10) FIG. 1 shows a block diagram illustrating a photon counting device in accordance with an embodiment of the invention.

(11) The photon counting device 1 in this embodiment includes a direct converter 3, providing its output to a pulse shaper 5, which in turn provides the shaped pulse to a number of energy discriminators 7 (or threshold sections). For each of these threshold sections 7 a corresponding counter 9 is provided.

(12) The basic outline of the photon counting device 1 corresponds to conventional photon counting devices, with which the skilled person is well familiar. Accordingly, no additional explanation to the basic operation of these elements needs to be given.

(13) The term pixel is used here, depending on the context, as either referring to the particular piece of converter material or to the combination of such pixel with its corresponding photon counting device.

(14) As indicated above, each pixel includes a typical photon counting channel with several energy discriminators. In this embodiment, an additional threshold 11 (pulse threshold, PTH, or pulse detector) is provided with a low energy level (e.g. 5 keV, or sufficiently above the electronic noise).

(15) This additional threshold or pulse detector 11 acts for pulse detection, i.e. detecting that a pulse is being processed. The output of the pulse detector 11 has a monostable characteristic, i.e. upon a pulse being detected the output remains active over a limited time period (e.g. 1 ns).

(16) The output of the pulse detector 11 is logically compared, with a prevention circuitry 13, with the output of corresponding pulse detectors 11 of the adjacent pixels (e.g. north, south, west and east, but not restricted to this). If any (OR 17) of this logical AND functions 15 produces an active output, it serves as indication that coincidence events were registered between Pixel 1 and any neighbor pixel.

(17) In FIG. 1, the neighboring pixel or photon counting device 1 is shown without giving further details, except for indicating the pulse detector 11. Furthermore, for readability only one of the neighboring photon counting devices 1 is shown, while the photon counting device 1 in this embodiment is arranged to receive pulse detector outputs from four neighboring photon counting devices 1.

(18) The above mentioned coincidence hit within a very short time interval (e.g. 1 ns) may be resulting from a charge sharing event.

(19) To this end, the embodiment shown in FIG. 1 enables a flip-flop 19 that in turn disables the counters 9 from registering this event. The flip-flop 19 may be reset by the lower threshold returning back to an inactive state 21, even though in practice any threshold return back to an inactive state may be used for such reset. Since all pulse detector outputs across pixels have the same monostable characteristic, the coincidence window corresponds ultimately to the duration of the active state of the monostable output.

(20) In FIG. 1, the pulse detector output of all pixels is fed into the centre pixel with a given window delay 23. This delay 23 allows the centre pixel observing the past relative to its own events. This may not be required as all pixels may be allowed to decide to reject pulses on their own.

(21) Also, in FIG. 1, the outputs of all discriminators 7 go through a respective propagation delay block 25. This is provided to ensure that the decision to reject or not is taken before the lowest threshold discriminator 7 may be allowed to increase its corresponding counter 9. Any necessity to include such a discriminator delay 25 therefore depends on the shaper speed and the position of the lowest threshold with respect to the energy level of the pulse detector 11 (i.e., after the pulse detector 11 becomes active, the lowest threshold 7 may not be active within the next e.g. >1 ns).

(22) In an alternative embodiment (not shown), such an additional pulse detector is not provided as such, as the lowest threshold/discriminator may be used instead followed by a monostable stage. Put in another way, the pulse detector 11 of the present embodiment may also be coupled to a corresponding counter, such that is also functions as a discriminator similar to discriminators 7.

(23) FIG. 2a) shows time series of two simulated pixels in terms of a normalized shaper outputs 101, 103 in arbitrary units over time. FIG. 2b) shows the corresponding pulse detection triggers. The dotted rectangle 105 shows an event that was coincident in two neighboring pixels. The lower graph in FIG. 2c) shows that the event 111 was indeed caught by the coincidence detection and it will, according to some embodiments of the present invention, consequently disable the counting mechanism for that particular event.

(24) FIG. 3 shows a zoom of the area marked by the rectangle 105 in FIG. 2. The overlap (111) of the Pulse Detection signals 107, 109 (FIG. 3b) is indicative of the near-simultaneous occurrence of both events.

(25) It is noted here that, as the simulation was made to show the functionality of the detection, events are mono-energetic and no differentiation of regular events and charge-sharing events was made.

(26) FIG. 4 shows a comparison of count-rate figures with and without the use of the prevention circuitry of the present invention in terms of observed count rate (Mcps/pixel) over impinging count rate (ICR, Mcps/pixel).

(27) Due to high rates of events it is to be expected that some true events are also rejected, even though such true events do not correspond to a charge sharing event but rather take place by near simultaneous impact of two separate photons. The main impact of such undesired rejection is a loss of count-rate. FIG. 4 shows count-rate figures of a pixel with (line 113) and without (115) charge sharing rejection. As expected the registered count-rate in case of preventing the counting of charge sharing events is somewhat lower. As the rate increases the probability of falsely rejecting truly separate events increases. Beyond the maximum rate, extreme pile up makes the pulse detection more difficult and the count-rate curve therefore asymptotically follows the paralyzable model. In moderate rates (e.g. 0-10 Mcps/pixel ICR), the impact of the rejection circuit is minimal and a significant improvement of the spectrum quality may be expected by having sorted out events which were caused by charge sharing. FIG. 4 already accounts for the coincidence of four adjacent pixels onto a denominated centre pixel.

(28) FIG. 5 shows a block diagram illustrating a modification of the photon counting device shown in FIG. 1.

(29) In this modification, it is provided to register the number of events that have been rejected for every given pixel (curve 117 in FIG. 4). Such a count value can be easily attained by extending the embodiment shown in FIG. 1 to also include an additional counter 27 as shown in FIG. 5. In this embodiment a charge sharing events will be counted in two or more pixels, i.e. each pixel in coincidence will register the event. If required, measures can be taken to register the charge shared event in one pixel. The pixels involved in the event can be resolved from the active states of the AND function 15.

(30) The structure of the photon counting device shown in FIG. 5 corresponds to that shown in FIG. 1, except for the addition of the counter 27 counting the instances where two pulses where found to be present in a coincidence window.

(31) FIG. 6 shows a portion 31 of a prevention circuitry in accordance with another embodiment of the invention.

(32) The embodiment addresses a possible implementation to restore a charge sharing event in the analogue domain.

(33) Similar to the embodiments shown in FIGS. 1 and 4, a pulse detector 11 is provided, the output of which is logically compared (AND functions 15) with neighboring logic signals delayed by windows delays 23.

(34) Deviating from the embodiments shown in FIGS. 1 and 4, the OR function and the RS flip-flop are replaced by an arbitration control section 29. The purpose of this arbitration control section is to decide which pixel is to receive the full collected charge, i.e. it detects which pixels were in coincidence and selects the direction of the charge sum. This selection can be done in very simple terms, e.g. having a predefined weight where charge is always transferred from South-to-North and East-to-West.

(35) Outputs of the arbitration control section include signals A, B, C and D (explained below).

(36) This embodiment exploits the fact that the coincidence window and decision can be made very quickly (e.g. within 1-2 ns). Such a quick reaction to an incoming pulse allows selecting the shaper to be added before the transient response reached the lowest threshold.

(37) FIG. 7 shows two neighboring photon counting devices including the portion 31 of the prevention circuitry 13 shown in FIG. 6. For sake of simplicity, here only two of the neighboring photon counting devices 1 are (partially) shown.

(38) The photon counting device 1 includes, similar to that shown in FIG. 1 or 4, a direct converter 3 and a pulse shaper 5. The output of the pulse shaper 5 is coupled to an adding section 33 and also provided to neighboring photon counting devices 1 (signal X).

(39) The adding section 33 is here further coupled to the output of the pulse shaper 5 of the neighboring photon counting device 1.

(40) The signal A provided by the arbitration control section 29 (see FIG. 6) controls a switch 35, which connects or disconnects the output of the neighboring shaper 5 to an adder 37 of the adding section 33.

(41) The output (signal Z) of the adding section 33 is directed to the discriminators (not shown) of the photon counting device 1. Between the discriminators and the adding section 33 a further switch 39 is provided, which is controlled by signal B from the arbitration control section 29.

(42) Assume that the arbitration control section 29 determines that the pulses received by the shaper 5 of the two photon counting devices 1 shown in FIG. 7 are due to a charge sharing event and therefore should be counted by the upper counting device 1. Signal A is provided to connect the adder 37 not only with the shaper 5 of the upper counting device 1 but also with the shaper 5 of the lower counting device 1. Further, signal B is provided to the switch 39 such that the output of the adding section 33 indeed reaches the discriminators.

(43) Correspondingly, the adding section 33 and the switch 39 of the lower photon counting device 1 are controlled by the corresponding arbitration control section (not shown) to stop propagation of signals to the discriminators (and therefore also to the corresponding counters (not shown)).

(44) The further signals C, D are used for controlling corresponding circuitry with respect to other neighboring photon counting devices.

(45) FIG. 8 shows a schematic flow diagram illustrating a method for counting photons according to an embodiment of the invention.

(46) In a receiving step 50, a pulse generated by a photon interacting with a piece of converter material is received.

(47) The received pulse is provided to a pulse detector and as soon as the pulse reaches a predetermined energy level, in a signal providing step 52, a pulse logic signal is provided.

(48) In case a neighboring logic signal indicating an input of predetermined energy level in at least one neighboring piece of converter material and the pulse logic signal are present together in a coincidence window of predetermined size, in a prevention step 54, the pulse is prevented from being counted by any one of a plurality of counters, each of the counters being coupled to one of a plurality energy discriminators, for counting photon interactions with energy discrimination according to an energy level of the coupled energy discriminator, each energy discriminators being arranged to receive the pulse.

(49) 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.

(50) 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.

(51) The above discussion to some extend relates to energy resolving photon counting detectors for spectral CT. The present invention may however also be put to use also in other detectors for a wide range of applications, e.g. photon-counting mammography.

(52) 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.

(53) A single processor, device 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.

(54) Operations like detecting, determining an energy, providing signal, counting events or signals and preventing propagation of a signal can be implemented as program code means of a computer program and/or as dedicated hardware.

(55) A computer program may be stored and/or 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.

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