WO2006080004A2

WO2006080004A2 - Method and system for x-ray radiation imaging

Info

Publication number: WO2006080004A2
Application number: PCT/IL2006/000097
Authority: WO
Inventors: Nikolai Pavlov
Original assignee: Gamma Medica-Ideas (Norway) As; Barash, David
Priority date: 2005-01-27
Filing date: 2006-01-25
Publication date: 2006-08-03
Also published as: WO2006080004A3; IL166553A0

Abstract

A method for X-ray imaging with pulsed X-ray source, X-ray detector (11) suitable for that source and synchronized detector gated current integration sampling and readout electronic circuit (13), that can be used to increase signal-to-noise ratio on X-ray images taken using same X-ray dose or to decrease X-ray dose needed to observe same signal-to-noise ratio on X-ray images Signal to noise ratio is improved since integration time for the detector current is shorter so less charge produced by leakage current of the detector is integrated.

Description

Method and system for X-ray radiation imaging

FIELD OF THE INVENTION

This invention relates to X-ray radiation and imaging and, in particular, to a method and apparatus for detecting X-rays.

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J.P.Moy, Moirans, France TRI ELL X.

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Radiation Monitoring Devices, Inc., 44 Hunt Street, Watertown, MA 02172.

[4] Development of Flat-Panel X-ray Image Sensors, Yoshihiro Izumi Osamu Teranuma

Tamotsu Sato Kazuhiro Uehara Hisao Okada,Satoshi Tokuda Toshiyuki Sato.

[5] Development and Evaluation of a Digital Radiographic System Based on CMOS Image Sensor, Ho Kyung Kim, Gyuseong Cho, Member, IEEE, Seung Wook Lee,

Young Hoon Shin, and Hyo Sung Cho, IEEE TRANSACTIONS ON NUCLEAR

SCIENCE₅ VOL. 48, NO. 3, JUNE 2001.

[6] Medical imaging with mercuric iodide direct digital radiography flat-panel X-ray detectors, H. Gilboa, A. Zuck, O. Dagan, A. Vilensky, B.N. Breen, A. Taieb, B. Reisman and H. Hermon, Real-Time Radiography, Jerusalem Technology Park,

Jerusalem 91487 Israel, .G. Zentai, L. Partain, Ginzton Technology Center, Mountain

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[7] Design of a pulsed X-ray system for fluorescent lifetime measurements with a timing accuracy ofl09ps, S. E. Derenzo, W. W. Moses, and S. C. Blankespoor,Lawrence

Berkeley Laboratory, University of California, Berkeley, CA 94720,M. Ito, and K.

Oba,Hamamatsu Photonics K.K., Hamamatsu City, Japan, IEEE Trans. Nucl. Sci. NS-

41, pp. 698-702 (1994). BACKGROUND OF THE INVENTION

Currently there are two different approaches to X-ray radiation imaging with semiconductor detectors or scintillator and photodetector combination. The first is current integration, the second is X-ray photon counting. When using the current integration approach, the current generated by X-ray photons absorbed in the detector is integrated for each detector element (pixel) over the acquisition time. Then, the accumulated charge is used to create an image whose respective pixel intensities are a function of the accumulated charge per pixel.

When using the photon counting approach, hits of X-ray photons in the detector are counted in a separate counter for each detector element (pixel) over the acquisition time. Then, values accumulated in the counters are used to create an image whose respective pixel intensities are a function of the accumulated number of photons counted per pixel.

However, there are some problems and limitations arising in either the first or the second approach, as will now be explained with reference to Fig. 1 showing graphically X- ray radiation intensity, the charge for the signal and for the noise versus time axis using a conventional approach for establishing an X-ray hit using charge integration. It is seen that charge generated by X-ray photons absorbed in the detector is integrated over the acquisition time. The integrated charge is used to create an X-ray image whose pixels have an intensity that corresponds to the integrated charge at the respective location. At the same time, noise corresponding to charge produced by leakage currents is integrated. In this case, X-ray power stays constant within the acquisition, signal charge increases linearly with time, and noise charge increases as the square root of time (random walk equation). The signal is sampled at a specified time, such that during each sampling period the total X-ray power corresponding to the integrated X-ray charge is as shown.

When using the first approach, the minimal X-ray dose required to create an image is limited by leakage current noise integrated over acquisition time. In order to achieve minimal dose to the object, image detectors having very low leakage current should be used. This represents a serious challenge for manufacture of this type of detector and limits the class of materials that can be used, hi addition, saturation of some detector elements with accumulated charge limits the maximum X-ray radiation intensity that can be acquired. When using the second approach, the minimal X-ray dose required to create an image is limited by statistical fluctuations in the number of X-ray photons detected by the detector elements (pixels). The image can be enhanced by applying some energy acceptance window for photons detected in order to suppress background from scattered X-ray photons. However, this approach represents a serious challenge for the design of high quantum efficiency detector-preamplifier-readout combination since it requires triggering on X-ray photons (having lower energy than gamma photons in SPECT/PET applications) with a very high maximum count rate.

SUMMARY OF THE INVENTION It is an object of the invention to improve the ratio of quality of the image to X-ray radiation dose and decrease the impact of X-ray detector leakage current, thereby imposing less stringent requirements on the maximum permitted leakage current.

This object is achieved in accordance with a first aspect of the invention by a method of pulsed X-ray imaging using an X-ray detector with gated synchronized current integration, the method comprising: providing repetitive X-ray pulses of controlled intensity with a pulse duration shorter than a characteristic charge collection time of the X-ray detector, detecting said X-ray pulses and producing a corresponding electrical current, and integrating the electrical current corresponding to each X-ray pulse so as to derive successive charge signals each synchronized to a respective X-ray pulse thereby achieving increased ratio of X-ray generated charge over leakage and series noise generated charge.

According to a second aspect of the invention there is provided an X-ray detection system comprising: a X-ray detector adapted to produce a corresponding electrical current upon detection of a pulse of X-ray radiation, a pulsed X-ray source providing successive X-ray pulses of controlled intensity each having a pulse duration that is shorter than a characteristic charge collection time of the X-ray detector, and a readout circuit adapted for electrical connection to the X-ray detector and to the pulsed X-ray source and being configured for integrating electrical current corresponding to each X-ray pulse so as to derive successive charge signals each synchronized to a respective X-ray pulse thereby achieving an increased ratio of X-ray generated charge over leakage and series noise generated charge.

The readout circuit may be provided as a retrofit unit for the X-ray detection system. In such case, the readout circuit may comprise: a detector input for connecting to an X-ray detector that is adapted to produce a corresponding electrical current upon detection of a pulse of X-ray radiation, and a synchronization input for connecting to a pulsed X-ray source providing successive X-ray pulses of controlled intensity each having a pulse duration that is shorter than a characteristic charge collection time of the X-ray detector; said readout circuit being configured for integrating the electrical current corresponding to each X-ray pulse so as to derive successive charge signals each synchronized to a respective X-ray pulse thereby achieving increased a ratio of X-ray generated charge over leakage and series noise generated charge.

The invention can be applied in X-ray radiation imaging when minimum X-ray radiation dose to the object (patient) is required. This is achieved by an X-ray radiation imaging method using one or several pulsed X-ray sources, an array of X-ray radiation detectors in the form of a semiconductor detector or a scintillator and photodetector combination and integrated readout electronics.

The method according to the invention may be implemented according to two different embodiments. The first embodiment is based on the first conventional approach to X-ray imaging mentioned above, while the second embodiment is based on the second conventional approach mentioned above.

In the first embodiment, the X-ray intensity during pulse of a controlled pulsed X- ray source is set to provide multiple X-ray photon hits to most of the detector elements (pixels) while still avoiding saturation of detector and preamplifier with generated charge. This is achieved at the design stage of the system by properly configuring the X-ray intensity used according to the X-ray source position, detector efficiency and expected X- ray attenuation in the object imaged. The readout circuit integrates the detector current integration obtained during each X-ray pulse, so as to accumulate the detector signal charge arising from X-ray pulses generated. The integrated current which represents the signal charge is sampled after each pulse. The synchronization is achieved by electronic timing circuitry that generates control pulses for both X-ray source control and gating of the detector current integration. The accumulated charge is sampled after each pulse generated and averaged to create an X-ray image. The total number of X-ray pulses generated is adjusted to achieve an X-ray exposure that provides an image of required signal-to-noise ratio.

Thus, according to the first embodiment of the invention, the leakage current is integrated over a significantly shorter time than required in conventional methods. As a consequence, the X-ray dose required to achieve the same signal-to-noise ratio defined by leakage current noise is lower, or, with the same X-ray dose, better signal-to-noise ratio is achieved.

In the second embodiment, the pulse amplitude of a controlled pulsed X-ray source is set to provide mostly single X-ray photon hits to most of the detector elements (pixels).

This is achieved at the design stage of the system by properly configuring the X-ray intensity used according to the X-ray source position, pulse duration, detector efficiency and expected X-ray attenuation in the object image.

The readout electronic circuit provides detector current integration for a period of time synchronized to detector response arising from X-ray pulses generated, in order to achieve maximum ratio of the X-ray generated charge to the combined leakage and series noise generated charge. The integrated current which represents the signal charge is sampled after each pulse and used to decide if a photon hit a given detector element (pixel) and, if so, to determine the energy of the X-ray photon detected. The energy information observed is then used to set an energy acceptance window for photons detected in order to improve image quality by e.g. discarding Compton scattered photons.

Thus, according to the second embodiment of the invention, there is no need for triggering circuitry as a part of the electronic readout circuit and sampling of the charge corresponding to energy of the photon detected is precisely synchronized with the photon absorption event. As a consequence, significantly higher energy resolution for the photons detected can be achieved and the design of the detector-preamplifier-readout electronics combination is simplified since no triggering circuitry is used.

By such means, the invention also potentially allows use of semiconductor detectors materials having rather high leakage current. In summary, the innovative ideas associated with the invention are:

1. A controlled pulsed X-ray source, providing repetitive X-ray pulses of controlled intensity with a pulse duration shorter than the characteristic charge collection time of the X-ray detector; 2. An X-ray detector or an array of detectors either in the form of scintillator and photodetector combination or in the form of a direct X-ray photon conversion detector adapted to detect X-ray radiation generated by the pulsed X-ray source by producing a corresponding electrical signal; 3. A readout electronic circuit that is electrically coupled to the X-ray detector for gated synchronized current integration providing sampling of those charges synchronized to X-ray pulses in order to achieve maximum ratio of X-ray generated charge to leakage and series noise generated charge.

BRIEF DESCRIPTION OF THE DRAWINGS hi order to understand the invention and to see how it may be carried out in practice, some exemplary embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1 is a graphical representation showing a conventional approach for establishing an X-ray hit using current integration; Fig. 2 is a block diagram showing the functionality of an X-ray detection system according to a first exemplary embodiment of the invention;

Fig. 3 is a block diagram showing the functionality of an X-ray detection system according to a second exemplary embodiment of the invention; and

Fig. 4 is a graphical representation showing on separate axes X-ray power, and the charge integrated for the signal and for the noise all versus time.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Fig. 2 is a block diagram showing the functionality of an X-ray detection system 10 according to a first embodiment of the invention. The X-ray detection system 10 comprises an X-ray detector 11 adapted to produce a corresponding electrical current upon detection of a pulse of X-ray radiation and a pulsed X-ray source 12 providing successive X-ray pulses of controlled intensity each having a pulse duration that is shorter than a characteristic charge collection time of the X-ray detector. The X-ray detector 11 and the pulsed X-ray source 12 are coupled to a readout circuit 13 via respective connectors 14 and 15. The readout circuit 13 comprises a current integration circuit 16 coupled to the connector 14, which thus constitutes a detector input for coupling the readout circuit 13 to the detector 11. Likewise, the readout circuit 13 comprises a synchronization circuit 17 coupled to the connector 15, which constitutes a synchronization input for coupling the readout circuit 13 to the pulsed X-ray source 12. The synchronization circuit 17 is also coupled to the current integration circuit 16 for generating control pulses for synchronizing gating of the current integration circuit to 16 each pulse generated by the X-ray source 12. A charge sampling unit 18 is coupled to the current integration circuit 16 for sampling the accumulated charge after each pulse generated so as to obtain accumulated charge samples, and an averaging unit 19 is coupled to the charge sampling unit 18 for averaging the accumulated charge samples to create an X-ray image. In a variation of this embodiment of the invention, the averaging unit 19 also performs X-ray photon energy window discrimination.

Fig. 3 is a block diagram showing the functionality of an X-ray detection system 20 according to a second embodiment of the invention. In Fig. 3, identical reference numerals identify components that are common also to the system 10 shown in Fig. 3 and no further description is necessary. Thus, the difference between the two embodiments is that in the system 20 an energy discriminating photon counting unit 21 is coupled to the charge sampling unit 18 for accumulating the number of detected X-ray photons in a predetermined energy window in order to create an X-ray image.

Fig. 4 is a graphical representation showing X-ray radiation intensity (power), the charge integrated for the signal and for the noise versus time axis using a modified approach according to the invention where an X-ray hit is established using a pulsed X-ray source and synchronized gated current integration. It corresponds to both embodiments of the invention proposed. In this case, X-ray dose is concentrated in pulses while having the same integrated dose, so signal charge increases linearly within pulse time, noise charge increase as square root of time within pulse time (random walk equation). Since total integration time is shorter, less of the noise charge is observed.

Claims

CLAIMS:

1. A method of pulsed X-ray imaging using an X-ray detector with gated synchronized current integration, the method comprising: providing repetitive X-ray pulses of controlled intensity with a pulse duration shorter than a characteristic charge collection time of the X-ray detector, detecting said X-ray pulses and producing a corresponding electrical current, and integrating the electrical current corresponding to each X-ray pulse so as to derive successive charge signals each synchronized to a respective X-ray pulse thereby achieving an increased ratio of X-ray generated charge over leakage and series noise generated charge.

2. The method according to claim 1 for use with a system that is configured so that during most X-ray pulses only a single photon interaction or no interaction per detector element occurs, said method further including: sampling the accumulated charge after each pulse generated so as to obtain charge accumulated within one X-ray pulse; using charge accumulated within one X-ray pulse to determine whether X-ray photon were absorbed by the detector, and if so,

(i) determining the energy of the absorbed X-ray photon; and

(ii) using energy information of the absorbed X-ray photon to enhance the image.

3. An X-ray detection system (10, 20) comprising: a X-ray detector (11) adapted to produce a corresponding electrical current upon detection of a pulse of X-ray radiation, a pulsed X-ray source (12) providing successive X-ray pulses of controlled intensity each having a pulse duration that is shorter than a characteristic charge collection time of the X-ray detector, and a readout circuit (13) adapted for electrical connection to the X-ray detector and to the pulsed X-ray source and being configured for integrating electrical current corresponding to each X-ray pulse so as to derive successive charge signals each synchronized to a respective X-ray pulse thereby achieving increased ratio of X-ray generated charge over leakage and series noise generated charge.

4. The X-ray detection system according to claim 3, wherein the X-ray detector comprises an array of detectors.

5. The X-ray detection system according to claim 3 or 4, wherein the or each X-ray detector is a combined scintillator and photodetector.

6. The X-ray detection system according to claim 3 or 4, wherein the or each X-ray detector is a direct X-ray photon conversion detector.

7. The X-ray detection system according to any one of claims 3 to 6, wherein the readout circuit (13) includes: a synchronization input for coupling to the pulsed X-ray source, a current integration circuit, and a synchronization circuit (17) coupled to the synchronization input and to the current integration circuit for generating control pulses for synchronizing gating of the current integration circuit to each pulse generated by the X-ray source.

8. The X-ray detection system according to any one of claims 3 to 7, wherein the readout circuit (13) further includes: a charge sampling unit (18) for sampling the accumulated charge after each pulse generated so as to obtain accumulated charge samples, and an averaging unit (19) for averaging the accumulated charge samples to create an X-ray image.

9. The X-ray detection system according to any one of claims 3 to 7, wherein the readout circuit (13) further includes: a charge sampling unit (18) coupled to the current integration circuit for sampling the accumulated charge after each pulse generated so as to obtain accumulated charge samples, and energy discriminating photon counting unit (21) coupled to the charge sampling unit for accumulating the number of detected X-ray photons in a predetermined energy window in order to create an X-ray image.

10. A readout circuit (13) for an X-ray detection system, the readout circuit comprising: a detector input for connecting to an X-ray detector that is adapted to produce a corresponding electrical current upon detection of a pulse of X-ray radiation, and a synchronization input for connecting to a pulsed X-ray source providing successive X-ray pulses of controlled intensity each having a pulse duration that is shorter than a characteristic charge collection time of the X-ray detector; said readout circuit being configured for integrating the electrical current corresponding to each X-ray pulse so as to derive successive charge signals each synchronized to a respective X-ray pulse thereby achieving increased a ratio of X-ray generated charge over leakage and series noise generated charge.

11. The readout circuit according to claim 10, including: a current integration circuit (16), and a synchronization circuit (17) coupled to the synchronization input and to the current integration circuit for generating control pulses for synchronizing gating of the current integration circuit to each pulse generated by the X-ray source.

12. The readout circuit according to claim 10 or 11 , including: a charge sampling unit (18) coupled to the current integration circuit (16) for sampling the accumulated charge after each pulse generated so as to obtain accumulated , charge samples, and an averaging unit (19) coupled to the charge sampling unit for averaging the accumulated charge samples to create an X-ray image.

13. The readout circuit according to claim 10 or 11 , including: a charge sampling unit (18) coupled to the current integration circuit for sampling the accumulated charge after each pulse generated so as to obtain accumulated charge samples, and energy discriminating photon counting unit (21) coupled to the charge sampling unit for accumulating the number of detected X-ray photons in a predetermined energy window in order to create an X-ray image.