Detection of X-ray beam start and stop

Abstract

A radiographic energy detecting pixel generates charges in a photosensor in response to photon impacts. A switch electrically connected to the photosensor selectively transmits collected charges to a data line. A sensing circuit electrically connected to the photosensor detects a rate of accumulation of the charges in the photosensor.

Claims

1. A radiographic energy detecting pixel circuit comprising: a photosensor for generating charges in response to photons impacting the photosensor; a data line; a controllable switching device electrically connected to the photosensor and to the data line for selectively transmitting charges from the photosensor to the data line; a sensing circuit electrically connected to the photosensor to measure a rate of charge generation in the photosensor, wherein the photosensor comprises a photodiode, the switching device comprises a TFT, one terminal of the photosensor comprises a cathode, the TFT and the sensing circuit are electrically connected to the cathode, the cathode is further electrically connected to a drain terminal of the TFT, another terminal of the photosensor comprises an anode, and wherein the anode is electrically connected to a bias voltage supply; and a sensing capacitor electrically connected between the cathode and the sensing circuit.

2. The pixel circuit of claim 1, wherein the sensing circuit comprises a current sensing circuit or a charge sensing circuit to measure a change in charge or current over time.

3. The pixel circuit of claim 2, wherein the sensing circuit measures a change in an amount of charge generated in the photosensor.

4. The pixel circuit of claim 3, wherein the sensing circuit comprises an operational amplifier, a feedback capacitor electrically connecting an output and an input of the operational amplifier, the sensing capacitor is electrically connected to the feedback capacitor, and wherein the sensing circuit transmits a signal when a rate of charge varying over time as detected by the sensing circuit exceeds a threshold.

5. The pixel circuit of claim 3, wherein the sensing circuit transmits a signal when an amount of current generated by accumulating charges varies over time beyond a threshold.

6. A radiographic detector comprising: a substrate; a plurality of dielectric layers over the substrate; an array of photosensors formed in a device layer over the dielectric layers for generating charges in response to photons impacting the photosensors, each of the photosensors comprising an anode, a cathode, and a charge collecting layer therebetween; a data line formed over the substrate substantially parallel to and adjacent to a first side of a first portion of the photosensors; a gate line formed over the substrate, under a first one of the dielectric layers, over a second one of the dielectric layers, and substantially parallel to a second side of a second portion of the photosensors different from the first portion of the photosensors and perpendicular to the data line; a switching device formed over the gate line, the switching device for selectively electrically connecting the cathode to the data line under control of the gate line; and a sense electrode formed over the substrate and under the photosensor, the sense electrode separated from the photosensor by the first one and the second one of the dielectric layers, and wherein the sense electrode is electrically linked to the photosensor by a capacitance therebetween, and wherein the sense electrode is formed under the first one and the second one of the dielectric layers.

7. A radiographic detector comprising: a substrate; a plurality of dielectric layers over the substrate; an array of photosensors formed in a device layer over the dielectric layers for generating charges in response to photons impacting the photosensors, each of the photosensors comprising an anode, a cathode, and a charge collecting layer therebetween; a data line formed over the substrate substantially parallel to and adjacent to a first side of a first portion of the photosensors; a gate line formed over the substrate and under one of the dielectric layers substantially parallel to a second side of a second portion of the photosensors different from the first portion of the photosensors and perpendicular to the data line; a switching device formed over the gate line, the switching device for selectively electrically connecting the cathode to the data line under control of the gate line; and a sense electrode formed over the substrate and under the photosensor, the sense electrode separated from the photosensor by one or more of the dielectric layers, wherein the sense electrode is electrically linked to the photosensor by a capacitance therebetween, the gate line and the sense electrode are formed under a common dielectric layer, the sense electrode is disposed under the second portion of the photosensors different from the first portion of the photosensors, the photosensors each comprise a photodiode, one terminal of the photodiodes each comprise a cathode, another terminal of the photodiodes each comprise an anode, the capacitance links the cathode to the sense electrode, a bias voltage supply is electrically connected to the anodes, and wherein the sense electrode is electrically connected to a sensing circuit comprising a current sensing circuit or a charge sensing circuit to measure a rate of charge generation over time in the photosensor.

8. The detector of claim 7, wherein the sensing circuit transmits a signal when the rate of charge generation varies over time beyond a threshold.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) So that the manner in which the features of the invention can be understood, a detailed description of the invention may be had by reference to certain embodiments, some of which are illustrated in the accompanying drawings. It is to be noted, however, that the drawings illustrate only certain embodiments of this invention and are therefore not to be considered limiting of its scope, for the scope of the invention encompasses other equally effective embodiments. The drawings are not necessarily to scale, emphasis generally being placed upon illustrating the features of certain embodiments of the invention. In the drawings, like numerals are used to indicate like parts throughout the various views. Thus, for further understanding of the invention, reference can be made to the following detailed description, read in connection with the drawings in which:

(2) FIG. 1 illustrates an exemplary digital X-ray system in which the x-ray generator and detector are under the control of an acquisition control and image processing unit.

(3) FIG. 2 is a diagram of an exemplary pixel of a prior art detector array.

(4) FIG. 3 illustrates an exemplary pixel layout of a prior art detector array.

(5) FIG. 4 is a cross-section of the pixel of FIG. 3 along section line A-A.

(6) FIG. 5 is a diagram of an exemplary imaging array.

(7) FIG. 6 illustrates exemplary timing during signal charge sensing.

(8) FIG. 7 illustrates an exemplary charge amplifier with sampling circuit.

(9) FIG. 8 illustrates an exemplary pixel with a capacitor between the photodiode cathode and a sense electrode connected to a current sense or charge sense circuit.

(10) FIG. 9 illustrates an exemplary pixel layout having a sense electrode in gate electrode metal routed under the photodiodes parallel to the gate lines.

(11) FIG. 10 is a cross-section of the pixel of FIG. 9 along the section line A-A.

(12) FIG. 11 illustrates an exemplary imaging array with sense electrode and additional isolation dielectric.

(13) FIG. 12 illustrates an exemplary imaging array with circuitry for pixel electrode voltage sensing and beam detection.

(14) FIG. 13 illustrates an exemplary pixel cross section with sense electrode formed adjacent to the photodiode.

(15) FIG. 14 illustrates an imaging array with single sense electrode.

(16) FIG. 15 illustrates an imaging array with sense electrodes grouped into regions of interest within the array.

(17) FIG. 16 illustrates a sense electrode formed between the array substrate and an electrical ground plane.

(18) FIG. 17 illustrates a single sense plane pixel configuration.

(19) FIG. 18 illustrates a multiple sense plane pixel configuration.

DETAILED DESCRIPTION OF THE INVENTION

(20) This application claims priority to U.S. Patent Application Ser. No. 62/092,395 filed Dec. 16, 2014, in the name of Tredwell, and entitled DETECTION OF X-RAY BEAM START AND STOP.

(21) In an exemplary operation of a digital x-ray detector, the photodiode cathode is reset to the reference voltage V.sub.REF by turning on the row-select TFT switch in the pixel, after which the voltage is allowed to float when the TFT switch is turned off. During this period a small amount of charge accumulates on the photodiode due to thermal generation of carriers, or dark current. The increase in charge results in a decrease in cathode voltage given by:
dV.sub.CATH/dt=I.sub.DARK/C.sub.PD
where dV.sub.CATH/dt is the rate of change of pixel electrode (photodiode cathode) voltage per unit time dt, I.sub.DARK is the photodiode dark current and C.sub.PD is the photodiode capacitance.

(22) If the prior art array of FIG. 5 is exposed using X-rays, the scintillator converts the X-rays to photons, usually between 400 nm and 700 nm in wavelength. A portion of these photons are absorbed in the photodiode. The absorption of photons from the exposure in the photodiode results in charge Q.sub.PD on the photodiode in which the increase in charge is stored on the photodiode dQ.sub.PD with respect to time t. The rate of change of dQ.sub.PD is given by integrating the photon flux ?.sub.PD(?) and the photodiode quantum efficiency ?.sub.PD(?) times the photodiode area A.sub.PD over wavelength ?:
I.sub.?(t)=dQ.sub.PD/dt=??.sub.PD(?)?.sub.PD(?,t)?A.sub.PDd?

(23) The wavelength dependence of the photon flux ?.sub.PD(?) is determined by the emission properties of the scintillator which converts X-rays into visible light photons and is fixed for a given scintillator composition. The magnitude of the flux ?.sub.PD(?,t) with time depends on a large number of factors, including the X-ray generator characteristics, the exposure time and the exposure value requested by the operator, the absorption in X-ray filters and the patient, and the conversion efficiency in photons per X-ray of the scintillator. Exposure times can vary from 10 ms in a pediatric exam in which patient movement is a concern to almost a second for low output generators for exams in which patient movement is not a major concern. Absorption in the patient can also vary by a factor of 1,000 or more from thick areas of dense bone to thin areas of soft tissue. Since the photodiode cathode is floating during exposure, the photodiode voltage, and thereby the voltage on the floating cathode, decreases by
dV.sub.CATH/dt=(I.sub.?(t)+I.sub.DARK)/C.sub.PD

(24) FIG. 8 shows one embodiment of a pixel 800 in a detector array similar to FIG. 2, but in which a sense electrode 830 is coupled to the cathode of the photodiode 802 through a sense capacitor C.sub.S 832 in the pixel 800. The sense electrode is attached to a current sense or charge sense circuit 834. Examples of sense circuits include a charge amplifier or a trans-impedance amplifier, both of which hold the voltage of the sense circuit to a reference voltage V.sub.REF and which sense the change in charge during a sampling time T.sub.S (charge amplifier) or current flow as a function of time (trans-impedance amplifier).

(25) One embodiment of an imaging array with a sense electrode, as shown in FIG. 8, is shown in the top view of FIG. 9 and in the cross section view of FIG. 10. In this exemplary embodiment no additional metal layers are required for fabrication of the array. As shown in FIGS. 9-10, a sense electrode 830 is patterned in the same metal layer as the gate line 208 and parallel to the gate line 208 between the substrate 1032 and the gate dielectric 1034. As shown in FIG. 10, the sense electrode is positioned under the photodiode cathode 203. The dielectric layers between the sense electrode and the cathode include the gate dielectric 1034 and TFT passivation dielectric 1036. The sense electrode capacitance would be given by
C.sub.S=??.sub.oL.sub.SEW.sub.SE/(t.sub.GD+t.sub.PD)

(26) Where ? is the dielectric constant of the gate dielectric 1034 and TFT passivation dielectrics 1036, ?.sub.o is the free-space dielectric constant, L.sub.SE and W.sub.SE are the lengths and widths of the sense electrode under the cathode, respectively, and t.sub.GD+t.sub.PD are the thicknesses of gate dielectric 1034 and passivation dielectrics 1036, respectively. For an exemplary pixel having dimensions of about 139 ?m?139 ?m, with L.sub.SE=4 ?m, W.sub.SE=100 ?m, t.sub.GD and t.sub.PD each about 400 nm of silicon nitride, the sense electrode capacitance per pixel would be about C.sub.S=30 fF.

(27) Additional pixel architectures with sense electrodes are possible. In one embodiment, shown in FIG. 11, a sense electrode 830 is shown in an orientation similar to that of FIG. 10 but with an additional isolation dielectric layer 1140 between the sense electrode 830 and the cathode 203. The additional dielectric layer 1140 can be used to reduce the sense capacitance value and also reduce capacitive loading of the data line 910.

(28) FIG. 12 shows a 2?3 pixel region of an imaging array 1200 utilizing the sense electrode 830 and sense capacitor 832 in each pixel 800, as described above, wherein some components in the figure are not enumerated for ease of illustration but are described above. In comparison to the prior art array 500 of FIG. 5, the array 1200 of FIG. 12 comprises sense capacitors 832 attached between the photodiode cathode and sense electrodes 830. The sense electrodes 830 are routed to a sense circuit 834. FIG. 12 shows sense capacitors 832 connected to every pixel 800 in the array 1200, although in some circumstances it may be desirable to sample (sense) a portion of the pixels (photodiodes). In one embodiment, a sparse matrix of sense capacitors could be utilized. The sense circuit 834 shown in FIG. 12 comprises a charge amplifier 1208, although other sense circuits may be used. The current induced on the sense electrode I.sub.SE(t) from a single pixel 800 is
I.sub.SE(t)=(C.sub.CE/C.sub.PD)?(I.sub.DARK,i+I.sub.?,.Math.(t))

(29) The total current induced on the sense electrode I.sub.SE(t) is the sum ? over the columns i and rows j of the current I.sub.DARK,i+I.sub.?,.Math.(t) induced by the pixels adjacent to the dataline in each row:
I.sub.SE(t)=(C.sub.S/C.sub.PD)???(I.sub.DARK,i,j+I.sub.?,i,j(t))

(30) The choice of the value of C.sub.S (sense capacitance) is a trade-off between having adequate current on the sense electrode 830 to detect the start and completion of X-ray exposure with good signal-to-noise ratio while minimizing the parasitic loading of the cathode. For example, if the sense capacitor 832 is 1% of the photodiode capacitance, then the current on the sense electrode would be 1%???(I.sub.DARK,i,j+I.sub.?,i,j(t)).

(31) At the start of exposure I.sub.?,.Math.(t) will increase from zero to an approximately steady state value during the firing of the generator and at the end of exposure I.sub.?,.Math.(t) will decrease from its approximately steady-state value to zero. It can be seen that the start and stop of exposure could be determined if the charge induced on the sense electrode 830 by capacitive coupling of the cathodes of the photodiodes and that the charge vs. time can be monitored by the charge amplifier 508 used in the sense circuit 834. The charge amplifier 508 will sense a charge given by the integral over the sampling window of the charge amplifier 508:
Q.sub.S=(C.sub.S/C.sub.PD)???I.sub.DARK,i,j+I.sub.?,i,j(t)dt

(32) We consider an embodiment in which the sense circuit 834 is operated with timing similar to the read-out circuit 506, described above. In this case the charge on the sense electrode 830 is sampled for a portion of a line time (see FIG. 6). Radiographic exposures can have a wide range in entrance exposure to the detector and in the exposure time. Table 1, below, shows two exemplary cases. The low exposure case (first column) is for a hand or neo-natal X-ray in which the beam is collimated. The minimum current (2nd last column) on the sense line 1238, determined for the example above, is 1.77 nA. If the sense line current was sampled with a charge amplifier 508 with a 30 ?s sampling time, the minimum charge (last column) would be about 100,000 electrons.

(33) TABLE-US-00001 TABLE 1 Sense line current and charge on ROIC feedback capacitor in 30 us sampling time Exposure Sense line Exposure Time current Charge in 30 us Exposure Area Exposure Min Max Min Max Min Max Condition Example Beam cm2 mR ms ms nA nA 1 KElec 1 KElec Low Hand* RQA3 100 0.01 2 10 1.77 8.84 100 500 High Chest RQA7 1,505 2.5 25 200 94.3 2,194 17,700 411,000

(34) In the embodiment of FIGS. 8-10 the sense electrode 830 is patterned in the same metal layer as is used for the row select 208 (gate metal). This places limitations on the routing of the sense electrode 830, since the sense electrode 830 cannot connect to the row select lines (gate lines). In this embodiment the sense electrodes 830 would comprise a single trace of metal under each row routed parallel to the row select electrode (gate metal) 208. Since the resistance of the sense line 830 is proportional to the length times the width, the sense line 830 would have high resistance as compared to a grid or sheet. In addition, since each sense electrode 830 must cross each of the data lines 910, the capacitance of the datalines would increase by the number of rows times the overlap capacitance between sense electrode 830 and dataline 910 (approximately 8 fF per pixel times about 3,072 rows or about 25 pF). A typical value for the total data line capacitance is about 120 pF, so the increase is about 20%. Since the thermal noise of the dataline 910 is proportional to its capacitance and is typically the largest noise source in a passive-pixel array, the increase in noise may be undesirable.

(35) A second pixel embodiment that addresses these limitations is shown in FIG. 11, which is similar in most respects to the embodiment as described above in FIG. 10. In this embodiment the sense electrode 830 is fabricated in a separate layer of conductor positioned between the substrate 1032 and the remaining conductive traces in the array and separated by dielectric layers. This allows greater choice of routing geometry, including a grid geometry, a single sheet, or a geometry which allows separate regions of interest to form separate regions for sensing. The array of FIG. 11 also allows a greater total dielectric thickness between sense electrode 830 and the dataline 910, reducing the impact of the sense electrode 830 on dataline capacitance and thereby on detector noise. Finally, the array of FIG. 11 allows independent optimization of capacitance C.sub.PE-SE between pixel electrode 203 (photodiode cathode) and sense electrode 830 by design choice of the thickness of the isolation dielectric 1140 and/or the width of the sense electrode 830.

(36) A third embodiment that also addresses these limitations is shown in FIG. 13. In this embodiment a sense electrode 1330 is fabricated adjacent to the photodiode 1302 in the pixel 1300. One terminal of the sense electrode 1330 is connected to a sense capacitor which, in turn, is connected to the pixel electrode 1303 and the second terminal of the sense electrode 1330 is connected to a sense-line, as described above in the example of FIG. 12, which is routed to the perimeter of the panels to connect the capacitors to a sense amplifier and beam detection subsystem (such as 834 of FIG. 12).

(37) Several options are possible for routing of the sense electrodes in the embodiments of FIGS. 10, 11, 13. The sense electrodes may be connected together to form a single sense plane 1400 and connected to a single sense amplifier and beam detection system 700 (FIG. 7), as illustrated in FIG. 14. The sense electrodes 1430 may be divided so that one portion (column) of the array is routed to one sense amplifier and beam detection system, as illustrated in FIG. 12. The sense electrodes may be further divided to form regions of interest (ROI) 1502 within the array, as illustrated in FIG. 15. Groupings into regions of interest 1502 may improve signal to noise for beam detection in many X-ray modalities. For example, in a chest X-ray, the perimeter of the array outside the skin line would receive an unobstructed dose of the X-ray beam and would have high signal; a ROI 1502 in that region would have high signal but, because of the smaller capacitance, it would have faster response time and lower noise. In another example, an X-ray of an extremity, such as a hand, would likely be limited to exposure just in the center of the array where the hand was positioned. In such a case, having a ROI 1502 in the central region of the array may have higher signal-to-noise ratio than having a single sense electrode covering the entire array.

(38) Sense Plane Located Below Detector Substrate

(39) In one embodiment, an array with beam sensing 1600 is illustrated in FIG. 16. In this embodiment one or more sense electrodes 1602 are positioned between the imaging array substrate 1604 and an electrical ground plane 1606. The imaging array substrate 1604 is a dielectric, such as glass or plastic. Glass substrates for TFT fabrication are typically 500 ?m to 1 mm thick. Imaging arrays have also been fabricated on plastic substrates or transferred to plastic substrates. These are typically 10 ?m to 100 ?m thick. As compared to the sense electrode of FIG. 10, the sense plane 1602 of FIG. 16 has two disadvantages: (1) because of the greater thickness of the dielectric 1610 between the pixel electrode 1608 and the sense plane 1602, the capacitance between pixel electrode 1608 and sense plane 1602 in FIG. 16 is lower than that of FIG. 10 and (2) because the sense plane 1602 of FIG. 16 underlies both the pixel electrodes 1608 and the gate lines 1612, it is subject to feed-through charge induced by the transition of gate lines 1612 from one voltage to another during array readout.

(40) The sense plane embodiment offers the advantage of not requiring additional circuits to be added to the prior-art detector array. The sense electrodes 1602 may be fabricated using thin-film metal layers, such as deposited or coated thin films of Indium Tin oxide (ITO) or Indium Gallium Zinc Oxide (IGZO). Alternatively, they may be metal foil. A second dielectric 1614 may be positioned between the sense plane and a ground plane 1606. In a radiographic detector, the ground plane 1606 is typically the mechanical plate upon which the detector is mounted. The thickness of the second dielectric 1614 is a compromise between the capacitance between the sense electrode 1602 and ground 1606, which impacts signal-to-noise ratio and the physical thickness of the assembly of FIG. 16.

(41) Various configurations of the sense plane can be chosen based on a variety of considerations, including optimization of response time and signal-to-noise ratio. In one embodiment, a configuration in which a single sense plane underlies the entire detector array is illustrated in FIG. 17. In this embodiment, a current or charge amplifier 1702 may be connected between the sense electrode 1704 and electrical ground 1706 (or any bias supply connected to system ground). A configuration in which multiple sense planes 1801, each underlying a separate region of the imaging array 1802, is shown in FIG. 18. In this case the current or charge induced on each electrode 1830 is monitored with a charge or current amplifier 1802 for that electrode (or alternatively, a single amplifier multiplexed among the electrodes). Other configurations of the multiple electrode embodiment could be designed based on the use cases anticipated for the detector.

(42) As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method, or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.), or an embodiment combining software and hardware aspects that may all generally be referred to herein as a service, circuit, circuitry, module, and/or processing system. Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon.

(43) Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any tangible medium that can contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device.

(44) Program code and/or executable instructions embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing.

(45) Computer program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the C programming language or similar programming languages. The program code may execute entirely on the user's computer (device), partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider).

(46) Computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, such as an image processor or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

(47) The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions and acts specified herein.

(48) This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.