BASELINE SHIFT DETERMINATION FOR A PHOTON DETECTOR

Abstract

The present invention relates to determining baseline shift of an electrical signal generated by a photon detector (102) of an X-ray examination device (101). For this purpose, the photon detector comprises a processing unit (103) that is configured to determine a first crossing frequency of a first pulse height threshold by the electrical signal generated by the photon detector. The first pulse height threshold is located at a first edge of a noise peak in the pulse height spectrum of the electrical signal.

Claims

1. Processing unit for processing an electrical signal generated by a photon detector of an x-ray examination device; wherein the processing unit is configured to determine a baseline shift of the electrical signal by determining a change of a first rate of exceeding a first pulse height threshold by the electrical signal; and wherein the first pulse height threshold is a pulse height at a first edge of a noise peak in a pulse height spectrum of the electrical signal.

2. The processing unit according to claim 1, wherein the first edge is a rising edge of the noise peak.

3. The processing unit according to claim 1, wherein the processing unit is further configured to determine the baseline shift of the electrical signal by determining a change of a second rate of exceeding a second pulse height threshold of the electrical signal; wherein the second pulse height threshold is a pulse height on a second edge of the noise peak in the pulse height spectrum of the electrical signal; and wherein the second edge is a falling edge of the noise peak.

4. The processing unit according to claim 1, wherein the processing unit is configured to determine a first reference value for the first rate; and wherein the processing unit is configured to determine the change of the first rate by subtracting a currently measured rate of exceeding the first pulse height threshold by the electrical signal from the first reference value.

5. The processing unit according to claim 1, wherein the processing unit is configured to perform a threshold scan of the electrical signal for determining the pulse height spectrum of the electrical signal; and wherein the processing unit is configured to set the first reference point at an inflection point of a edge of the noise peak of the determined pulse height spectrum.

6. The processing unit according to claim 1, wherein the processing unit is configured to subtract the baseline shift from the electrical signal.

7. the processing unit according to claim 1, wherein the processing unit is configured to save the determined baseline shift for different points in time in a storage unit.

8. The processing unit according to claim 1, wherein the processing unit is configured to determine the baseline shift on the basis of a functional relation between the baseline shift and the change of the first rate.

9. A photon detector for an x-ray examination device comprising a processing unit according to claim 1.

10. An X-ray examination device comprising the photon detector according to claim 9.

11. The x-ray examination device according to claim 11, further comprising: an x-ray source; a control unit for turning off the x-ray source when the processing unit determines the baseline shift.

12. A method for determining a baseline shift in an electrical signal of a photon detector of an x-ray examination device, the method comprising the steps: determining a change of a first rate of exceeding a first pulse height threshold by the electrical signal, wherein the first pulse height threshold is a pulse height at a first edge of a noise peak in a pulse height spectrum of the electrical signal; and determining the baseline shift of the electrical signal on the basis of the determined change of the first rate.

13. The method according to claim 12, further comprising the steps: determining a change of a second rate of exceeding a second pulse height threshold by the electrical signal, wherein the second pulse height threshold is a pulse height at a second edge of the noise peak in the pulse height spectrum of the electrical signal; and determining the baseline shift of the electrical signal on the basis of the determined change of the second rate.

14. A program element, which, when it is executed by a processing unit, instructs the processing unit to carry out the method of claim 12.

15. A computer-readable medium, on which a program element is stored, which, when it is executed by a processing unit, instructs the processing unit to carry out the method of claim 12.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG. 1 shows an examination device comprising a photon detector and a processing unit according to an exemplary embodiment of the invention.

[0045] FIG. 2 shows a pulse height spectrum according to an exemplary embodiment of the invention.

[0046] FIG. 3 shows a noise peak of a pulse height spectrum according to an exemplary embodiment of the invention.

[0047] FIG. 4 shows a flow-chart of a method according to an exemplary embodiment of the invention.

[0048] FIG. 5 shows a flow-chart of a method according to an exemplary embodiment of the invention.

[0049] The figures are schematic and may not be true to scale. If, in the following description, the same reference sign are used with respect to different figures, they refer to the same or similar elements. The same or similar elements may also be provided with different reference signs.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0050] In FIG. 1, an X-ray examination device 100 according to an exemplary embodiment of the invention is shown. The X-ray examination device 100 comprises a gantry 101 which comprises an X-ray source 104 and a photon detector 102. The X-ray source 104 and the photon detector 102 can be rotated to different angular positions of the gantry 101 such that a probe 106 can be examined from different angles. The X-ray source 104 is configured to emit X-rays 105, which at least partly penetrate through the probe 106 and are detected by the photon detector 102. The photon detector 102 comprises a processing unit 103 for processing an electrical signal generated by the photon detector 102 of the X-ray examination device 100. The processing unit 103 is configured to determine a baseline shift of the electrical signal by determining a change of a first rate of exceeding a first pulse height threshold by the electrical signal. Furthermore, the first pulse height threshold is a pulse height at a first edge of a noise peak in a pulse height spectrum of the electrical signal.

[0051] The processing unit 103 can hereby actively compensate the baseline shift of the electrical signal or can store the baseline shift for different points in time on a storage unit 108 of a computer system 107 connected to the X-ray examination device. In this way, the processing unit 109 of the computer system 107 has access to the values of the baseline shift for the different points in time stored in the storage unit 108 such that the detector readout can be corrected by the software of the computer system 107 carried out by the processing unit 109.

[0052] Furthermore, the X-ray source comprises a control unit 110, for example a grid switching device or a shutter for turning on and off the X-ray source 104. In this way, the X-ray source can be switched off for a short fraction of a readout frame of the detector 102 such that the baseline shift can be determined during this time.

[0053] Although an X-ray examination device having a gantry is shown, the X-ray examination device can also have another structure.

[0054] In FIG. 2, a pulse height spectrum 200 according to an exemplary embodiment of the invention is shown. The pulse height spectrum 200 is a typical pulse height spectrum of a photon counting detector of an X-ray examination device described in the context of the invention. The x-axis 201 represents the pulse height thresholds and the y-axis 202 represents the rate of exceeding or crossing the pulse height threshold. In other words, the y-axis 202 represents the crossing frequencies for the different pulse height thresholds.

[0055] In this exemplary embodiment, the pulse height spectrum 200 is an integrated pulse height spectrum. Thus, for every pulse height threshold value, the total number of events or pulses exceeding or crossing the specific pulse height threshold value is shown. Since the electrical signal comprises a non-vanishing noise component, the pulse height spectrum 200 comprises a Gaussian noise peak 203. The signal of the detector induced by the X-rays of the X-ray source is indicated by the curve 204. The complete signal comprising the noise peak 203 and the X-ray signal 204 is shown by the curve 205. It can be assumed that the shape of the noise peak changes when the X-ray signal 204 is added to the noise peak 203.

[0056] A greater noise component in the electrical signal would increase the full-width-half-maximum of the noise peak.

[0057] In FIG. 3, a pulse height threshold 300 of the electrical signal of the photon detector according to an exemplary embodiment of the present invention is shown, when the X-ray source is turned off The X-axis 301 represents the pulse height threshold which is measured and the y-axis 302 represents the rate of exceeding or crossing said pulse height threshold. The noise peak 303 is, for example, determined during a threshold scan, which is performed when the X-ray source is turned off.

[0058] Since the X-ray source is turned off, the pulse height spectrum 300 comprises only the Gaussian noise peak 303. The noise peak comprises a first, rising edge 305 and a second, falling edge 304. The position 306 at the x-axis 301 represents the first pulse height threshold p.sup.− and the position 307 at the x-axis 301 represents the second pulse height threshold p.sup.+. The first pulse height threshold p.sub.− 306 is located at an inflection point of the first edge 305 of the noise peak 303 and the second pulse height threshold p.sup.+ 307 is located at an inflection point of the second edge 304 of the noise peak 303.

[0059] The position 308 at the y-axis 302 indicates the first reference value H for the first rate of exceeding the first pulse height threshold H(p.sup.−) 306 (first crossing frequency). Although not explicitly shown, a similar reference value for the second rate of exceeding the second pulse height threshold H(p.sup.+) 307 (second crossing frequency) can be determined, which reference value of the second pulse height threshold H(p.sup.+) can be of the same value as the reference value H of the first pulse height threshold H(p.sup.−).

[0060] If during the operation of the photon detector, the baseline shift of the photon detector changes, a different rate of exceeding the first pulse height threshold H(p.sup.−) 306 by the signal can be determined. For example, a greater rate 311 of exceeding the first pulse height threshold 306 relating to a positive change 309 of the first rate can be measured. However, also a lower rate 312 of exceeding the first pulse height threshold 306 can be measured, which represents a negative change 310 of the first rate.

[0061] Because of the steep gradient of the noise peak 303 at the edges 305 and 304, the rates of crossing the first or second pulse height threshold 306, 307 strongly depends on the baseline shift of the electric signal. In other words, when the baseline shifts over time, the shift can be detected very sensitively by measuring the changes of the first or second rate.

[0062] The frequency of noise counts may be most sensitive to changes in the baseline when one or two thresholds are placed at the points 306 and/or 307 in the pulse height spectrum 300. For the first pulse height threshold 306, any positive baseline shift will cause the frequency to decrease rapidly whereas any negative baseline shift will cause a crossing frequency of the first pulse height threshold to increase rapidly.

[0063] To first order, the instantaneous baseline shift may be approximated as:

b(t)=b.sub.0+c[H−H(p.sup.−)],

where b(t) denotes the currently determined baseline, b.sub.0 the initial value of the baseline, c the constant of proportionality between the baseline and the change of the crossing frequency, H the reference value of the first rate of exceeding the first pulse height threshold (first crossing frequency), and H(p.sup.−) the currently determined first rate of exceeding the first pulse height threshold p.sup.− (first crossing frequency). In other words, [H−H(p.sup.−)] denotes the change of the first rate (first crossing frequency). However, the functional relation may also comprise higher order terms. The values for b.sub.0 and c may be determined during calibration of the photon detector.

[0064] It can be seen, that the changes of the first rates 309 and 310 strongly depend on the changes 314 and 313 of changes in the baseline.

[0065] In FIG. 4, a flow-chart for a method for determining a baseline shift in an electrical signal of a photon detector of an X-ray examination device according to an exemplary embodiment is shown. The method comprises the step S1 of determining a change of a first rate of exceeding a first pulse height threshold by the electrical signal, wherein the first pulse height threshold is a pulse height at a first edge of a noise peak in a pulse height spectrum of the electrical signal. Furthermore, the method comprises step S2 of determining the baseline shift of the electrical signal on the basis of the determined change of the first rate.

[0066] In FIG. 5, a further flow-chart for a method for determining a baseline shift in an electrical signal of a photon detector of an X-ray examination device according to a further exemplary embodiment of the invention is shown. The method comprises step S1 of determining a change of a first rate of exceeding a first pulse height threshold by the electrical signal, wherein the first pulse height threshold is a pulse height at a first edge of a noise peak in a pulse height spectrum of the electrical signal. Furthermore, the method comprises step S3 of determining a change of a second rate of exceeding a second pulse height threshold by the electrical signal, wherein the second pulse height threshold is a pulse height at a second edge of the noise peak in the pulse height spectrum of the electrical signal. Furthermore, the method comprises step S2 of determining the baseline shift of the electrical signal on the basis of the determined change of the first rate and step S4 of determining the baseline shift of the electrical signal on the basis of the determined change of the second rate.

[0067] The invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative and/or exemplary and not restrictive. The invention is not limited to the disclosed embodiments.

[0068] Other variations to the disclosed embodiments can be understood and affected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. 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. A program element may be stored/distributed on a suitable medium, such as an optical storage medium or a solid state medium supplied together with or as a part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope of the claims.

LIST OF REFERENCE SIGNS

[0069] 100 X-ray examination device [0070] 101 gantry [0071] 102 photon detector [0072] 103 processing unit [0073] 104 X-ray source [0074] 105 X-rays [0075] 106 specimen [0076] 107 computer system [0077] 108 storage unit [0078] 109 processing unit of the computer system [0079] 110 control unit [0080] 200 pulse height spectrum [0081] 201 x-axis [0082] 202 y-axis [0083] 203 noise peak [0084] 204 x-ray signal [0085] 205 combined curve [0086] 300 pulse height spectrum [0087] 301 x-axis [0088] 302 y-axis [0089] 303 noise peak [0090] 304 falling edge [0091] 305 rising edge [0092] 306 first pulse height threshold p.sup.− [0093] 307 second pulse height threshold p.sup.+ [0094] 308 reference value for the first rate H [0095] 309 positive change of the first rate [0096] 310 negative change of the first rate [0097] 311 greater first rate H(p.sup.−) [0098] 312 smaller first rate H(p.sup.−) [0099] 313 positive change of the baseline shift [0100] 314 negative change of the baseline shift