WO2008146230A2

WO2008146230A2 - Photon counting with detection of local maxima

Info

Publication number: WO2008146230A2
Application number: PCT/IB2008/052058
Authority: WO
Inventors: Roger Steadman Booker; Cristoph Herrmann; Christian Baeumer; Guenter Zeitler
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2007-05-29
Filing date: 2008-05-26
Publication date: 2008-12-04
Also published as: WO2008146230A3

Abstract

A photon counting apparatus includes a radiation detector (100) and signal conditioning circuitry (102). A photon rate counter (106) produces a count value indicative of a total number of photons received by the detector (100). One or more photon counters (1101+N) produce count values indicative of photons having varying energy characteristics. The counters (1101+n) disregard pileup pulses.

Description

Photon counting with detection of local maxima

FIELD OF THE INVENTION

The following relates to photon counting radiation detectors. While it finds particular application to spectral and other computed tomography (CT) systems, it also relates to other applications in which pulse counting radiation detectors can be affected by pulse pileup.

BACKGROUND OF THE INVENTION

CT scanners provide useful information about the internal characteristics of an object under examination. In medical imaging, for example, CT scanners provide medical professionals with valuable information regarding the physiology of patients. CT scanners have also been used in security inspection to examine the contents of items such as baggage, and in industrial applications such as non-destructive inspection and testing. While CT scanners have proven to be beneficial in these and other applications, the development of CT scanners having spectral capabilities promises to provide still additional benefits, such as an improved ability to provide information on the material composition of an object under examination.

Spectral information can be obtained using photon counting detectors. According to such an approach, pulses generated by a detector pixel in response to incoming photons have been processed by pulse shaping circuitry that integrates and amplifies the current produced by the pixel to generate voltage pulses, the heights of which are proportional to the energy of the detected photons. The signal chain has also included a number of discriminators, each having a threshold. For each discriminator, a counter has been used to count the number of pulses that exceed the discriminator's threshold.

Unfortunately, however, these circuits are typically paralyzable at the count rates typically encountered in CT applications, and thus may not provide an output indicative of the true photon count rate. While non-paralyzable detector circuitry can also be implemented, such circuitry tends to be relatively complex, especially in the case of energy resolving detectors. Moreover, such circuits may not accurately characterize the energy of the detected radiation. Aspects of the present application address these matters and others.

SUMMARY OF THE INVENTION

In accordance with one aspect, a photon counting apparatus includes first circuitry that produces a pulse train in response to photons received by a radiation sensitive detector, a first photon counter that counts pulses of the pulse train, a local maximum detector that detects local maxima of the pulse train, a second photon counter that counts detected local maxima, and a pileup detector. The first photon counter is operatively connected to the pileup detector so as to disregard pileup pulses. According to another aspect, a method includes producing a pulse train indicative of radiation received by a radiation sensitive detector, counting the pulses of the pulse train, detecting local maxima of the pulse train, counting the detected local maxima, and detecting pulse pileups in the pulse train. The step of counting the pulses includes disregarding pileup pulses. Still further aspects of the present invention will be appreciated to those of ordinary skill in the art upon reading and understand the following detailed description.

BRIEF DESCRIPTION OF THE FIGURES

The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention.

Figure 1 depicts a CT scanner. Figure 2 depicts a detector transfer function. Figure 3 depicts a detector channel.

Figures 4A, 4B, and 4C depict electrical signals. Figure 5 depicts a method.

DETAILED DESCRIPTION OF THE EMBODIMENTS With reference to FIGUREl, a CT scanner 10 includes a rotating gantry 18 that rotates about an examination region 14. The gantry 18 supports an x-ray source 12 such as an x-ray tube. The gantry 18 also supports an x-ray sensitive detector 20 that subtends an arc on the opposite side of the examination region 14. X-rays produced by the x-ray source 12 traverse the examination region 14 and are detected by the detector 20. An object support 16 such as a couch supports a patient or other object in the examination region 14. The support 16 is preferably movable in coordination with a scan in order to provide a helical, axial, circle and line, or other desired scanning trajectory. Accordingly, the scanner 10 generates projection data indicative of the radiation attenuation along a plurality of projections or rays through the object.

The detector 20 includes a plurality of detector elements 100 disposed in an arcuate array extending in the transverse and longitudinal directions. The detector elements 100 produce electrical pulses or signals indicative of detected photons. Examples of suitable detector elements include direct conversion detectors (e.g. , cadmium zinc telluride (CZT) based detectors) and scintillator-based detectors that include a scintillator in optical communication with a photosensor. Relatively faster detectors suitable for use in photon counting at the count rates likely to be encountered during operation of the CT scanner 10 are preferred.

An amplifϊer/shaper circuit 102 receives the signal produced by a detector element 100 and produces a train of pulses having amplitudes that are proportional or otherwise a function of the energies of the photons detected by the detector element 100.

A local maximum detector 104 detects local maxima in the pulse train produced by the amplifϊer/shaper 102. A pileup detector 108 detects pulse pileups.

The signals from the amplifϊer/shaper 102 and the pileup detector 108 are also received by one or more photon counters 110i__n. In the case of a channel having a single photon counter HO₁, the photon counter 11Oi is configured to count those pulses indicative of photons having an energy that exceeds an energy minimum or floor. In the case of a channel having a two or more such photon counters 1101 __n, the additional counters are preferably configured to count pulses indicative of photons having increasingly higher energies and hence provide photon spectral information. In either case, the counters 110 are operatively connected to the pileup detector 108 so as to disregard pileup pulses.

A photon rate counter 106 counts the local maxima detected by the local maximum detector 104. In one embodiment, the first photon rate counter 106 counts the local maxima detected by the local maximum detector 104 and hence produces a count value indicative of the total number of photons received by the detector element 100. In another, the first photon rate counter 106 may be triggered by the pileup detector 108 and hence produce a count value indicative of the number of pileup pulses disregarded by the counters 110. The photon rate counter 106 should not be paralyzable over the photon count rate expected to be encountered during the operation of the scanner 10, although the photon counters 110i__n may be paralyzable. Turning briefly to FIGURE2, the transfer functions of an ideal photon counter 200, an example non-ideal, non-paralyzable counter 204, and an example paralyzable counter 206 are presented in graphical form, where the x-axis represents the true count rate and the y-axis represents the count rate observed by the detector channel. In the case of the ideal detector 202, the observed count rate equals the true count rate. In the case the non-paralyzable detector 204, the observed count rate may deviate from the true count rate, with the slope of the transfer function typically decreasing as the true count rate increases. In the case of a paralyzable detector, the slope of the transfer function becomes zero and/or negative in the range of count rates at which the detector is expected to operate. As a consequence, the observed count rate does not uniquely identify the actual count rate. Returning to FIGUREl, a corrector 112 uses the count information from the photon rate counter 106 to correct the values from the photon counters 110i__n. In one implementation, the corrector 112 uses the count information from the first photon rate counter 106 to uniquely identify the actual count rate encountered by the photon counters 11O₁-J₁. Other suitable correction techniques may also be contemplated.

While only a representative detector channel 101 is illustrated in FIGUREl for clarity of explanation, it will be appreciated that the scanner 10 includes a plurality of similarly configured detector channels 101. Those of ordinary skill in the art will also appreciate that the various counters are ordinarily reset so as to provide data indicative of projection data acquired over each of a plurality of reading periods.

A reconstructor 22 reconstructs the projection data from the various detector channels 101 to generate volumetric data indicative of the interior characteristics of the object. In the case of a system having spectral capabilities, the data from the various energy ranges may be processed (before reconstruction, after reconstruction, or both) to provide information about the material composition of the object under examination.

A controller 28 coordinates the x-ray source 12 parameters such as tube voltage and current, movement of the object support 16, operation of the data acquisition system 23, and/or other operating parameters as necessary to carry out a desired scan protocol.

A general purpose computer serves an operator console 44. The console 44 includes a human-readable output device such as a monitor or display and an input device such as a keyboard and/or mouse. Software resident on the console allows the operator to control the operation of the scanner by establishing desired scan protocols, initiating and terminating scans, viewing and otherwise manipulating the volumetric image data, and otherwise interacting with the scanner 10.

Turning now to FIGURES 3 and 4, an example detector channel will now be described in further detail. As illustrated, the channel includes a CZT based radiation detector 302. The detector 302 produces output current pulses in response to detected photons, with the total charge being proportional to or otherwise a function of the photon energy. FIGURE4A depicts a simulated pulse train produced by the detector 302 in response to photons received at an incoming count rate of 10 million counts per second (Mcps) according to a Poissonian distribution, with pileup neglected.

A charge sensitive amplifier (CSA) and signal shaper circuit 304 integrates and amplifies the current produced by the detector to form output voltage pulses, the amplitude of which is proportional to or otherwise a function of the charge pulses produced by the detector 302 and hence the energy of the detected photons. A pulse train 404 produced by the circuit 304 in response to the pulse train 402 is depicted in FIGURE4B. As can be seen, the pulses of the pulse train 404 are characterized by rapid rise time, a peak amplitude that is a function of the energy of the received photon, and a somewhat slower fall time. Depending on the time period between the receipt of successive photons, a pulse produced in response to a first photon may not decay to the baseline level before a subsequent photon is received. Such a situation may lead to inaccuracies in the apparent energy of the photons. This can be seen by comparing the heights of the various pulses in FIGURE4B with the heights of the corresponding pulses in FIGURE4A. The inaccuracy is especially apparent with respect to pulses 408, 410, the energy of which is substantially over-represented. A differentiator 306 differentiates the voltage pulses to generate a differentiator output signal of the form: Equation 1 dV

Output oc dt

A signal 406 produced by the differentiator 306 in response to the pulse train 404 is shown in FIGURE4C. As can be seen, differentiator output signal 406 increases rapidly in response to a received photon and crosses a zero or baseline level as the signal 404 reaches its peak.

A baseline crossing detector 307 detects positive-to-negative baseline crossings of the differentiator signal 406 and hence the local maxima of the pulse train 404. It will be understood that positive-to-negative refers to the baseline crossings that indicate the local maxima of the pulse train 404, and not necessarily the actual electrical polarity or level of the differentiator signal 406. Note that the baseline crossings are substantially independent of the height of their corresponding local maxima. Detection of the baseline crossing can be complicated by noise in the vicinity of the baseline. Thus, the baseline crossing detector may include a discriminator that compares the differentiator signal 406 to a threshold that is offset from (e.g., slightly below) the baseline. Noise effects can be further reduced by introducing hysteresis. One technique for implementing hysteresis is to limit the speed of the discriminator so that, once the discriminator has been activated, it takes some time until the discriminator can be again be activated by a subsequent falling edge. Other techniques for implementing hysteresis are also contemplated.

The output of the baseline crossing detector 307 is used to increment a counter 308. Provided that pulse pileups are not caused by speed limitations of the detector 302, the output of the counter provides an output indicative of the total number of photons received by the detector element 100 during a reading period.

Note that the baseline crossing detector 307 may be omitted. In one such embodiment, a discriminator may be used to compare the slope of the signal 404 to a threshold value, again indicating the presence of a local maximum. According to still another embodiment, the counter 308 is implemented as an edge triggered counter 308 and is triggered by the rising or falling edge of the signal 404, as desired.

The detector channel also includes one or more discriminators 310i__n and counters 312_1-n. Each discriminator 310 compares the pulse train 404 against a threshold value. In the case of a detector channel having only a single discriminator 31O₁, the threshold value is selected to detect those pulses indicative of photons having energies greater than a minimum value. In the case of a detector channel having multiple discriminators 310i__n, the threshold values are preferably selected to detect those pulses indicative of photons having increasingly higher energies and hence provide photon spectral information. The counters 312i__n are triggered by the discriminators 310i__n, thus providing an indication of the number of photons and their respective energies.

The pileup detector 314 detects the presence of pulse pileups in the pulse train 404, for example by determining if successive local maxima are detected within a relatively short time window. Such a situation is depicted generally in FIGURE4B, where subsequent or pileup pulses 408, 410 are detected shortly after pulses 412, 414. Depending on factors such as the decay time of the pulses, the thresholds established for the discriminators 3101 __n, and the energy of and time between successive pulses, the pileup pulses 408, 410 may be missed by one or more of the counters 312_1-n. Also as illustrated in FIGURE4B, the height of the pulses 408, 410 may also be substantially over-represented, potentially leading to errors in the measured energy of the photons.

The counters 312i__n are operatively connected to the pileup detector 314 so as to disregard the pileup pulses. In one embodiment, the counters 312 are decremented or otherwise adjusted to account for counted pileup pulses. In another, triggering of the counters 312 is delayed for a period of time sufficient to ensure that the detected pulse is not a pileup pulse.

Operation will now be described in relation to FIGURE5.

Received radiation is detected at 502.

A pulse train is produced at 504.

Local maxima of the pulse train are detected at step 506. The true counts are estimated at 508, for example by counting the local maxima. In one technique, the total number of maxima are counted. In another, the maxima indicative of pulse pileups are counted.

Pulse pileups are detected at 510.

The various pulses of the pulse train are counted at 510, with pileup pulses being disregarded. The pileup pulses may be disregarded, for example, by decrementing or otherwise adjusting the count value(s) to account for counted pileup pulses or by counting only non-pileup pulses in the first instance. Note that separate counts may be obtained for each of a plurality of energy ranges or bins.

The desired count corrections are applied at 516. An image of the object is reconstructed at 518. In the case of a spectral system, data from the various energy ranges or bins may be processed to provide material composition information.

The image data is displayed in human readable form at 520.

Other alternatives are contemplated. For example, fourth generation or other CT scanner configurations may be implemented. The x-ray source 12 and detector 20 may also remain stationary while the object support is rotated or otherwise moved, especially in the case of inanimate objects. While the foregoing has focused on application to a CT scanner, the disclosed techniques can also be used to detect ionizing and other radiation in applications other than CT. The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

CLAIMS:

1. A photon counting apparatus comprising: first circuitry (102) that produces a pulse train in response to photons received by a radiation sensitive detector; a first photon counter (HOi) that counts pulses of the pulse train; a local maximum detector (104) that detects local maxima of the pulse train; a second photon counter (106) that counts detected local maxima; a pileup detector (108), wherein the first photon counter is operatively connected to the pileup detector so as to disregard pileup pulses.

2. The photon counting apparatus of claim 1 wherein the local maximum detector

(104) includes a differentiator (304) that produces a signal indicative of the time derivative of the pulse train.

3. The photon counting apparatus of claim 1 wherein the local maximum detector (104) detects baseline crossings of the time derivative of the pulse train.

4. The apparatus of claim 1 wherein the first photon counter (HOi) counts photons having an energy greater than a minimum energy and the second photon counter (106) counts a total number of received photons.

5. The apparatus of claim 1 wherein the first photon counter (HOi) is paralyzable and the second photon counter (106) is non-paralyzable.

6. The photon counting apparatus of claim 1 wherein the pileup detector (108) identifies detected local maxima indicative of pulse pileups.

7. The photon counting apparatus of claim 1 wherein the second photon counter (106) counts the identified maxima.

8. The photon counting apparatus of claim 1 including at least a third photon counter (1 IO₃), wherein the first photon counter (HOi) counts photons having an energy greater than a first minimum energy, the third photon counter (1 IO₃) counts photons having an energy greater than a second minimum energy that is greater than the first minimum energy, and wherein the pileup detector (108) causes the first and third photon counters (1 IO3) to disregard pileup pulses.

9. The photon counting apparatus of claim 1 wherein the amplitude of the pulses produced by the first circuitry (102) is a function of the energy of the received photons and the first photon counter (HOi) includes a discriminator (31O₁) and a counter (312i).

10. The photon counting apparatus of claim 1 including a corrector (112) that uses a count value from the second photon counter (106) to correct a count value from the first photon counter (11O₁).

11. The photon counting apparatus of claim 1 including the radiation sensitive detector (100, 302).

12. A computed tomography apparatus (10) comprising the photon counting apparatus of claim 11.

13. A method comprising : producing a pulse train (404) indicative of radiation received by a radiation sensitive detector (100, 302); counting the pulses of the pulse train (402); detecting local maxima of the pulse train (402); counting the detected local maxima; detecting pulse pileups in the pulse train, wherein the step of counting includes disregarding pileup pulses (408, 410).