Photon counting detector and photon counting method

Abstract

The invention relates to a photon counting detector and a method. The detector (2) comprises a scintillator (10) configured to convert incident gamma radiation into optical photons, a pixelated photodetector (11) configured to detect a flux of optical photons, and a circuit (12). The circuitry is configured to: determine, for each photodetector pixel, a photon count by accumulating a number of optical photons detected by the respective photodetector pixel during an integration period; comparing, for each photodetector pixel or group of photodetector pixels, a single photon count or multiple photon counts to a count threshold; detecting an event if one or more photon counts exceed the count threshold for each photodetector pixel or group of photodetector pixels; and temporarily adjusting the count threshold for use in the comparison in one or more subsequent integration periods based on the energy of the detected event.

Description

Technical Field

[0001] The invention relates to a photon counting detector, a corresponding photon counting method and an imaging device comprising a photon counting detector for detecting radiation, in particular gamma radiation and X-ray radiation. Background Art

[0002] Conventionally, photon counting is performed using a photomultiplier tube (PMT) detector, which includes a photosensitive photocathode and a set of "multiplier" anode terminals. The photocathode emits at least one electron in response to the impact of a photon, and (one or more) electrons then strike successive anode terminals, where each such event causes a cascade emission of electrons, thereby producing a multiplication effect that causes a measurable electrical pulse. The PMT is capable of high-speed photon counting. The PMT detector has many disadvantages, including that it is a relatively bulky discrete device with a large optical window that operates at high voltages and is prone to failure due to the vacuum tube design and high operating voltages. For example, this type of radiation detector is disclosed in US8723132B2.

[0003] Silicon photomultiplier (SiPM) devices have been developed to overcome some of these shortcomings, and SiPM devices provide photon counting detectors that are easy to integrate with silicon-based signal / data processing circuits. In some embodiments, SiPM devices use avalanche photodiodes as light sensors. When biased above its breakdown voltage, the avalanche diode breaks down in response to the collision of a single photon. Such devices are sometimes referred to as single-photon avalanche diode (SPAD) detectors. In a typical SPAD detector, the avalanche photodiode is reverse biased above its breakdown voltage and connected in series with a quenching resistor. The collision of a single photon causes the PN junction to break down in the multiplication (i.e., "avalanche") cascade of electrons flowing as a measurable current in the SPAD detector. Since this current reduces the reverse bias across the avalanche diode to a level below its breakdown voltage, the current is quenched relatively quickly due to the voltage on the resistor. Additionally or alternatively, an active quenching subcircuit including (for example) one or more diodes, resistors, and / or transistors can provide faster quenching. Some illustrative examples of SiPM detector arrays are disclosed in US9268033B2 and US10078141B2.

[0004] Digital silicon photomultipliers are used as photon counting detectors (sometimes also referred to as photon detectors) in high performance PET / CT systems. By coupling to a scintillator with time-of-flight capability, the fast triggering logic of digital silicon photomultipliers achieves temporal resolution in the range of 100ps-200ps. The temporal resolution of these systems is particularly limited by the temporal spread of the scintillation process. New detector concepts using cue photons have been proposed, which potentially provide improvements in temporal resolution significantly below 100ps.

[0005] Indirect photon counting spectroscopy CT uses fast scintillator crystals coupled to fast sampling single photon detectors to detect and measure the energy of single X-ray quanta. However, due to high X-ray flux, the detection of X-ray signals is hampered by the presence of pile-up, especially when the scintillator material exhibits a significant slow decay near the fast components, which makes it difficult to achieve short integration times, which are important for high counting rate capabilities. Summary of the invention

[0006] It is an object of the present invention to solve or at least reduce the problem of detector pixel pile-up.

[0007] In a first aspect of the present invention, a photon counting detector is provided, comprising:

[0008] a scintillator configured to convert incident gamma radiation into optical photons,

[0009] a pixelated photodetector configured to detect a flux of optical photons,

[0010] The circuit is configured as:

[0011] for each photodetector pixel, determining a photon count by accumulating the number of optical photons detected by the corresponding photodetector pixel during an integration period,

[0012] For each photodetector pixel or group of photodetector pixels, a single photon count or multiple photon counts are compared to a count threshold,

[0013] An event is detected if, for each photodetector pixel or group of photodetector pixels, one or more photon counts exceeds the count threshold; and

[0014] The counting threshold is temporarily adjusted based on the energy of the detected event for use in the comparison in one or more subsequent integration time periods.

[0015] In another aspect of the invention, a corresponding photon counting method is proposed.

[0016] In yet another aspect of the present invention, an imaging device comprising a photon counting detector as disclosed herein is proposed.

[0017] Preferred embodiments of the invention are defined in the dependent claims.

[0018] A photon counting detector (e.g., for indirect conversion photon counting spectral CT) comprises a scintillator (e.g., a fast scintillator crystal) coupled to a pixelated photodetector (e.g., a single optical photon detector such as an analog or digital silicon photomultiplier). Additional circuitry provides a sampling waveform of the optical photon flux out of the scintillator. One problem with detecting individual radiation quanta (e.g., X-ray or gamma quanta) in the sampling waveform is the reliable detection of the start of each event, especially in the presence of a pile-up of multiple pulses, which may overlap significantly when the scintillator material exhibits a slow decay component of, for example, several 10s or 100s of ns. Since part of the previously radiated signal is still present, it adds to the signal of the currently radiated photon and increases its value.

[0019] Furthermore, in order to actually detect the start of a new event, the counting threshold used by the comparator must be set higher in order to detect the transition to a new pulse. However, setting the threshold too high makes the detector blind to low energy radiation photons. Setting the threshold too low will enable the acquisition of low energies, which are important indicators in, for example, spectral CT images, but at the same time will result in "afterpulses" in the case of high energy photons, because the detector will trigger in the slow tail of the scintillation light in the case of high energy radiation. These false detections will end up in the low energy bins and will add noise or cause artifacts. The problem becomes even more severe in the presence of high pile-up and optical crosstalk between adjacent pixels.

[0020] To overcome these problems, a counting threshold is temporarily adjusted based on the energy of the detected event for comparison in one or more subsequent integration time periods, wherein a single photon count or multiple photon counts are compared to the counting threshold (also called a trigger threshold or comparator threshold) to detect an event (for each photodetector pixel or group of photodetector pixels) when one or more photon counts exceed the counting threshold.

[0021] In order to avoid too many adjustments of the counting threshold, in an embodiment, the circuit is configured to temporarily adjust the counting threshold only if the detected event exceeds a predetermined energy threshold (i.e., a minimum energy level). For example, in an embodiment, the counting threshold may be adjusted only if the detected event or average event exceeds a predetermined energy threshold by an amount or percentage below: an amount of at least 30 keV, in particular an amount in the range of 60 keV to 120 keV; or a percentage of at least 50%, in particular a percentage in the range of 100% to 200% (assuming, for example, an average energy of an average event of 60 keV).

[0022] In an embodiment, the circuit is configured to: use one of a plurality of predetermined energy thresholds, in particular one of energy-related predetermined energy thresholds; and / or adjust the count threshold based on the used predetermined energy threshold and / or the energy of the detected event. For example, for lower energies, a lower energy threshold may be applied compared to higher energies. Furthermore, for higher energies, the count threshold may be increased to a greater extent than for higher energies.

[0023] In a preferred embodiment, the circuit is configured to temporarily adjust the counting threshold, in particular temporarily increase the counting threshold, according to the amount or percentage by which the detected event exceeds a predetermined energy threshold (i.e., a minimum energy level). Therefore, according to actual conditions, the adjustment of the threshold can be controlled to avoid an adjustment that is too strong or too weak.

[0024] For example, as provided in an embodiment, the counting threshold may be temporarily increased based on the energy of the detected event by: a preset value, or a value that is higher the more the detected event exceeds the predetermined energy threshold. Thus, if a high energy event is already present, the counting threshold may be increased by a larger amount than if an intermediate or small energy event is already present, which reduces the effect of pile-up but avoids the detector being "blinded" for smaller energy events. Thus, in a practical implementation, the circuit may be configured to temporarily increase the counting threshold by: an amount of at least 20 keV, particularly an amount in the range of 30 keV to 60 keV; or a percentage of at least 20%, particularly a percentage in the range of 25% to 50%.

[0025] In another embodiment, the circuit may be configured to temporarily increase the count threshold and control the count threshold to decrease over time to a baseline count threshold after the increase. For the decrease, any function may generally be used after the increase. For example, a linear or logarithmic or hyperbolic decrease may be implemented. The baseline count threshold should generally be low enough to enable detection of low energy radiation quanta, such as gamma quanta in the range of 20keV to 30keV.

[0026] Regarding the timing of the reduction, in an embodiment, the count threshold can be controlled to decrease over time to the baseline count threshold within a time interval, wherein the time interval is in the range of a portion of the scintillator decay time or in the range of 5 to 200 ns, in particular in the range of 10 to 100 ns, or until the next detected event triggers a new temporary adjustment of the count threshold.

[0027] In another embodiment, the circuit may be configured to determine the temporary increase by multiplying a predetermined factor with one of:

[0028] said energy of said detected event,

[0029] the number of photon counts of the detected event, and

[0030] The average energy of the energy bins to which the detected events have been assigned. The predetermined factor may be set in advance, for example, by a user or a manufacturer of the detector, or may be modifiable by the user depending on the type of application, imaging modality, imaging system, etc. In an exemplary embodiment, the predetermined factor may be set in the range of 25% to 50% of the corresponding energy or energy bin.

[0031] In a simple embodiment, the circuit is configured to temporarily increase the count threshold by adding an offset to a baseline count threshold, wherein the offset is retrieved from a look-up table or calculated based on the energy of the detected event.

[0032] The circuit may also be configured to limit the temporary increase in the count threshold to a maximum count threshold or a maximum percentage or half of the maximum photon energy of the detected event. This avoids an increase that is too strong and therefore avoids the risk of failing to detect an event that is too weak. For example, the temporary increase in the count threshold may be limited to a maximum count threshold in the range of 25% to 50% of the sampled energy.

[0033] The pixelated photodetectors may preferably be silicon photomultiplier (SiPM) detectors, wherein each detector pixel comprises an array of silicon avalanche photodiodes (SPADs).

[0034] The claimed imaging device comprising a photon counting detector as disclosed herein may be, for example, a PET device, a CT device, a SPECT device, a time-of-flight (TOF) PET device or a Compton camera. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the accompanying drawings:

[0036] Figure 1 A schematic diagram of a conventional photon counting detector is shown.

[0037] FIG2 shows a graph of a typical energy histogram and corresponding count rate for a conventional photon counting detector.

[0038] Figure 3 A schematic diagram of an embodiment of a photon counting detector according to the present invention is shown.

[0039] FIG. 4 shows a graph of a typical energy histogram and corresponding count rate of a photon counting detector according to the present invention.

[0040] Figure 5 A diagram showing the scintillator input of a conventional photon counting detector, the setting of the counting threshold, and the detection signal of the detected events.

[0041] Figure 6 Graphs showing the same scintillator input, adjustable settings of the counting threshold and the detection signal of the detected events of a photon counting detector according to the present invention.

[0042] Figure 7 A diagram showing an exemplary setting of a count threshold according to the present invention. DETAILED DESCRIPTION

[0043] Figure 1 A schematic diagram of a conventional photon counting detector 1 (particularly a scintillator-based energy spectrum photon counting detector with a fixed threshold) is shown. The detector 1 includes a scintillator 10 and a pixelated photodetector 11 (e.g., a digital SiPM), the scintillator 10 converts incident gamma radiation (e.g., X-ray radiation) into optical photons, and the pixelated photodetector 11 detects the flux of optical photons and samples them, thereby providing (e.g., every 5ns) the number of detected photons. The detected optical photons are further processed and evaluated by a circuit 12 (or a corresponding unit).

[0044] In the comparison unit 13, the number of detected photons X (or the sum of several samples) is compared with a fixed threshold value A. If the value X exceeds A, an event is detected and the finite state machine 14 starts the acquisition by accumulating a predefined number of samples in the energy accumulator 15. At the end of the acquisition, the accumulator value is stored in the X-ray energy register 16 and the accumulator 15 is reset to be ready for the next event. The values ​​stored in the energy register 16 are then sorted in the energy histogram 17, where the corresponding bins are added.

[0045] FIG2 shows a graph of a typical energy histogram and corresponding count rate for a conventional photon counting detector. Figure 2AAn example of an energy histogram for a simulated monoenergetic input with a fixed energy threshold is shown in Figure 1. The corresponding count rate is Figure 2B The example in Figure 2 shows the inherent problem of using a fixed threshold for a wide range of input energies, creating spurious energy hits for high energy quanta. This effect can be reduced by a low energy threshold, but the spurious count rate remains in the energy histogram. The low threshold also makes the detection more sensitive to pile-up, i.e., to overlap of detected light pulses in time and space.

[0046] An element of the invention is a variable threshold level which is temporarily adjusted (particularly increased) based on previously detected energy and which has a time varying component which decreases over time to restore the original (baseline) threshold.

[0047] Figure 3 A schematic diagram of an embodiment of a photon counting detector 2 according to the present invention is shown. The general structure of the detector chain remains the same as in the above reference Figure 1 The main difference is that in the circuit 12', the comparison unit 13' uses a variable threshold value instead of a fixed threshold value as used by the comparison unit 13. This is implemented in this embodiment by using an offset unit 18, which provides an additional offset value O, which is added to the fixed energy threshold value A (also called baseline threshold).

[0048] Optionally, in addition, an upper limit value B is defined to prevent the threshold from exceeding a certain maximum value and making the pixel insensitive.

[0049] Assuming that the offset O is initially at 0, then Figure 1 An event is started in a similar manner as explained above. After the energy of the event has been acquired and latched in the X-ray energy register 16, the offset unit 18 calculates the threshold offset O based on a predetermined function (e.g., a strictly monotonically decreasing function f(E,T)), where E is the energy of the last event and T is the time counted from the end of the last acquisition. The goal of the function f is to provide an offset based on the energy E of the previous event and to decrease back to 0 within a time interval based on f(T).

[0050] In a simple implementation, f(E,T) scales E by a factor less than 1.0 to estimate a starting value for O. This value is stored in register O which is added to register A. Subsequently, register O may be decremented by 1 every clock sample until it reaches a value of 0, or a new value for O is set by the following event.

[0051] The advantage of this approach is that the fixed threshold A can be set to a much lower value, thereby maintaining the detection of low energy X-rays while suppressing the detection of slow tail components in high energy events.

[0052] For illustration, a simple example shall be given, assuming that the user preset threshold (i.e., value A) is set to 10 optical photons, and the signal range of interest lies between 30 and 120 photons. For example, it is also assumed that the scintillator 10 exhibits a slow decay component of tens of ns and a fast component of a few ns, and the ratio between the fast component and the slow component is 2:1. When a 120-photon event is detected, there will be about 40 photons in the slow component, resulting in a second "event" after integration of the fast component (which, if preferred, is due to a higher immunity to pile-up). The proposed method temporarily increases the threshold (i.e., A=10 photons) by a value (i.e., offset O) depending on the energy detected in the fast component (e.g., by 50% of 50% of the fast component=80 photons=40 photons) to 50 photons, which prevents re-triggering on the slow decay component.

[0053] To restore the previous threshold (ie, A=10), this additional offset will decrease by a fixed (or variable) value every clock cycle. Thus, the method temporarily increases the threshold after a high energy event and restores its original value within a given (fixed or variable) number of clocks.

[0054] In a preferred embodiment, there may be a lower energy limit for applying the correction, and there may also be an upper limit on the threshold. The offset may be selected based on the measured energy of previous X-ray photons (e.g., an integrated value scaled by a fixed factor) or to a user-selected value depending on the energy bin into which the last X-ray photon was sorted. The threshold recovery function may be linear or some other suitable function, such as hyperbolic or logarithmic.

[0055] FIG4 shows a typical energy histogram and a corresponding graph of the counting rate of a photon counting detector according to the present invention. Figure 4A As shown in , the proposed adaptive thresholding significantly improves the separation of the energy histogram (i.e., well-separated energy peaks), thereby removing false low-energy hits of high energy input. Figure 4B As shown, following the dead-time model for high rates, the count rate is estimated correctly.

[0056] Therefore, the present invention overcomes the problems of known photon counting detectors by adaptively varying the counting threshold.

[0057] Figure 5 A diagram showing the scintillator input, setting of the counting threshold, and the detection signal of a detected event for a conventional photon counting detector is shown. The signal (scintillator input) and the counting threshold are each represented by the number of detected optical photons (which is the sum of several samples; this number is converted to keV with a simple gain conversion factor). The detection signal is a logic signal.

[0058] This example shows three detected gamma pulses from scintillators with different energies. As can be seen, if the integration time is short, the fixed threshold setting can create multiple hits. In particular, fixed threshold detection can result in double sampling of high energy pulses in the middle by immediately starting a new integration when only a portion of the pulse is integrated in the period of 250ns to 280ns.

[0059] Figure 6 A diagram showing the same scintillator input, adjustable settings of the counting threshold, and the detection signal of the detected event of a photon counting detector according to the present invention is shown. As shown here, after the first event is detected, the threshold is increased by adding an offset to the baseline threshold and then slowly lowering the threshold back to its baseline value over a certain period of time (in this example, following a linear decreasing function). This prevents double sampling of high energy pulses.

[0060] In embodiments of the present invention, spatial and dynamic adjustments can be made. The present invention provides programmable thresholding for each pixel in the detection area (or for groups of adjacent pixels, group by group), wherein parameters such as minimum and maximum thresholds and gradients can be individually changed (spatially). In addition, control parameters for thresholds can be adjusted during scanning (over time) (even after each integration period (typical value is about 100 μs)) to change the behavior on the measured or expected count rate.

[0061] Figure 7 A diagram showing an exemplary setting of a count threshold according to the present invention is shown. It shows how the threshold is increased from a baseline value A to an increased value B by adding an offset O and then slowly reducing the offset back to zero so that the threshold returns to A.

[0062] For example, one or more parameters may be controlled by the user or manufacturer, e.g., based on the type of application, imaging equipment, etc.:

[0063] A baseline value A (typically, for example, in the range of 20 keV to 30 keV);

[0064] Offset value O (typically in the range of 30keV to 60keV or as a percentage of A in the range of 25% to 50%);

[0065] Function to reduce the increased value back to the offset;

[0066] The time period T from the start to the end of the increase (usually in the range of 50ns to 250ns;

[0067] a factor for determining the offset O by multiplying the factor by the energy of the detected event or the number of detected events (photon counts), or the average energy of the energy bins to which the detected events have been assigned (typically the factor is in the range of 25% to 50% of the corresponding energy or energy bin); and

[0068] An upper limit for the increasing count threshold (typically in the range of 25% to 50% of the sample energy).

[0069] The present invention overcomes the problem of false hits when sampling high energies with a low threshold combined with a short integration time. It achieves clean energy separation through adaptive thresholding and provides accurate counting rates and energy band separation. The present invention is particularly useful in systems using scintillator materials with fast and slow decay.

[0070] Although the invention has been described and illustrated in detail in the drawings and the foregoing description, such description and illustration should be considered illustrative or exemplary rather than restrictive; the invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments can be understood and implemented by those skilled in the art in practicing the claimed invention based on a study of the drawings, the disclosure, and the appended claims.

[0071] In the claims, the word "comprising" does not exclude other elements or steps, and the word "a" or "an" does not exclude a plurality. A single element or other unit may perform 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.

[0072] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A photon counting detector, comprising: a scintillator (10) configured to convert incident gamma radiation into optical photons, a pixelated photodetector (11) configured to detect a flux of optical photons, A circuit (12) is configured as follows: for each photodetector pixel, determining a photon count by accumulating the number of optical photons detected by the corresponding photodetector pixel during an integration period, For each photodetector pixel or group of photodetector pixels, a single photon count or multiple photon counts are compared to a count threshold, An event is detected if, for each photodetector pixel or group of photodetector pixels, one or more photon counts exceeds the count threshold; and The counting threshold is temporarily adjusted based on the energy of the detected event for use in the comparison in one or more subsequent integration time periods.

2. The photon counting detector according to claim 1, in, The circuit (12) is configured to temporarily adjust the counting threshold only if the detected event exceeds a predetermined energy threshold.

3. The photon counting detector according to claim 2, in, The circuit (12) is configured to temporarily adjust the counting threshold only if the detected events or average events exceed an amount or percentage below a predetermined energy threshold: an amount of at least 30keV, in particular an amount in the range of 60keV to 120keV; or a percentage of at least 50%, in particular a percentage in the range of 100% to 200%.

4. The photon counting detector according to claim 3, in, The circuit is configured as: Using one of a plurality of predetermined energy thresholds, in particular one of energy-related predetermined energy thresholds; and / or The count threshold is adjusted based on a predetermined energy threshold used and / or the energy of the detected event.

5. The photon counting detector according to claim 1, in, The circuit (12) is configured to temporarily adjust the counting threshold, in particular temporarily increase the counting threshold, according to the amount or percentage by which the detected event exceeds a predetermined energy threshold.

6. A photon counting detector according to any one of the preceding claims, in, The circuit (12) is configured to temporarily increase the counting threshold based on the energy of the detected event: a default value; or A value is set, and the more the detected events exceed the predetermined energy threshold, the higher the value is.

7. The photon counting detector according to claim 6, in, The circuit (12) is configured to temporarily increase the counting threshold by: an amount of at least 20 keV, in particular an amount in the range of 30 keV to 60 keV; or a percentage of at least 20%, in particular a percentage in the range of 25% to 50%.

8. The photon counting detector according to claim 6, in, The circuit (12) is configured to: temporarily increase the count threshold; and after the increase, control the count threshold to decrease to a baseline count threshold over time, in particular, after the increase, control the count threshold according to a predetermined function to decrease to the baseline count threshold over time.

9. The photon counting detector according to claim 7 or 8, in, The circuit (12) is configured to control the count threshold to decrease to the baseline count threshold over time within a time interval, wherein the time interval is in the range of a portion of the scintillator decay time or in the range of 5 to 200 ns, in particular in the range of 10 to 100 ns, or until a next detected event triggers a new temporary adjustment of the count threshold.

10. The photon counting detector according to any one of claims 6 to 9, in, The circuit (12) is configured to determine the temporary increase by multiplying a predetermined factor by one of: said energy of said detected event, the number of photon counts of the detected event, and The average energy of the energy bins to which the detected event has been assigned.

11. A photon counting detector according to any one of the preceding claims, in, The circuit (12) is configured to temporarily increase the count threshold by adding an offset to a baseline count threshold, wherein the offset is retrieved from a lookup table or calculated based on the energy of the detected event.

12. A photon counting detector according to any one of the preceding claims, in, The circuit (12) is configured to limit the temporary increase of the count threshold to a maximum count threshold or a maximum percentage or half of a maximum photon energy of the detected event.

13. A photon counting detector according to any one of the preceding claims, in, The circuit (12) is configured to limit the temporary increase of the count threshold to a maximum count threshold within a range of 25% to 50% of the sampled energy.

14. An imaging device comprising a photon counting detector (2) according to any of the preceding claims.

15. A photon counting method, comprising: for each photodetector pixel or group of photodetector pixels of a pixelated photodetector (11) configured to detect a flux of optical photons converted by a scintillator (10) from incident gamma radiation, determining a photon count by accumulating the number of optical photons detected by the corresponding photodetector pixel during an integration period, For each photodetector pixel or group of photodetector pixels, a single photon count or multiple photon counts are compared to a count threshold, An event is detected if, for each photodetector pixel or group of photodetector pixels, one or more photon counts exceed the count threshold, and The counting threshold is temporarily adjusted based on the energy of the detected event for use in the comparison in one or more subsequent integration time periods.

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