ACTIVE PIXEL INTRAORAL RADIOLOGICAL IMAGE SENSOR AND ASSOCIATED IMAGE ACQUISITION METHOD

Abstract

The invention relates to an active pixel dental radiography image sensor with integrated X-ray onset detection, which uses the pixels of the array to perform X-ray flash onset detection by detecting the current produced by all the photodiodes in the array. A switching circuit MUX1 thus enables, in a first phase of X-ray flash onset detection, the connection of a common NC node corresponding to the drain of a photodiode initializer transistor M1 to the input of a current-voltage conversion detection circuit DTX1. This circuit provides an output signal for X-ray flash onset detection when the current produced by all the photodiodes in the array exceeds a predetermined threshold. The switching circuit MUX1 is then controlled to connect the common NC node of the pixels to a photodiode reset voltage source, VRS.In one variant, certain pixels of the array can be used for X-ray flash end detection according to the same principle, by injecting the current produced by the photodiodes of these pixels, collected at a different common connection node, electrically isolated from the first. The flash end signal corresponds to a variation of this current that falls below a predetermined threshold. The active pixels can be of the three-transistor or more type, particularly five-transistor or more. Figure 2 for the abbreviation.

Description

Title of the invention: ACTIVE PIXEL INTRAORAL RADIOLOGICAL IMAGE SENSOR AND ASSOCIATED IMAGE CAPTURE METHOD FIELD OF INVENTION

[0001] The invention relates to the field of medical imaging, and more specifically to an intraoral dental radiological image sensor with active pixels, using CMOS technology. STATE OF THE ART

[0002] The structure of these active pixels is based on a photosensitive element (photodiode, photogrid) associated with transistors that control the different phases of image acquisition: an initialization phase for the photosensitive elements, followed by a charge integration phase, and then a pixel readout phase. For this readout, for each pixel in a row of pixels in the matrix, a voltage level corresponding to the amount of charge accumulated in the pixel during the integration phase is applied to a column conductor by means of a voltage follower transistor associated with a pixel readout node.The reading is then performed by a column-foot reading circuit, which actually samples two voltage levels: the voltage level corresponding to the amount of charge accumulated in a pixel reading node and a voltage level corresponding to a reset level of the reading node, subtracting one from the other. This improves the signal-to-noise ratio (double sampling and subtraction of correlated noise).

[0003] A dental radiographic image acquisition typically proceeds as follows: the detector is placed in the patient's mouth and positioned behind the anatomical area to be observed; the X-ray source is positioned and activated to emit a flash of X-rays towards the active face of the detector, through the biological tissues and materials of the anatomical area to be radiographed. A sequencing circuit for the detector triggers the image acquisition upon detection of the onset of the X-ray flash on the active face of the detector. The X-rays arriving at the active face of the detector are converted into an electrical signal by the pixels, either directly or after conversion to visible light by a scintillator, depending on the type of photodiode. After reading the pixels, an image of the radiographed anatomical area can be displayed on a computer screen, for example.

[0004] Detecting the onset of the X-ray flash is one of the measures necessary to reduce the radiation doses applied to patients. In the field of medical imaging, the recommendations of standards bodies in The radiation dose received by patients and practitioners is indeed very restrictive. At the X-ray source level, for each image acquisition, the intensity and duration of the X-ray flashes must be adjusted according to the patient's morphology and age, as well as the anatomical area to be radiographed, to ensure the lowest possible exposure dose without unnecessary loss, thus obtaining a good quality image in a single acquisition. At the image sensor level, the charge integration phase must be synchronized as closely as possible with the actual exposure of the active surface to the useful radiation, so that the signal-to-noise ratio is optimal with respect to the radiation intensity and duration. If the integration phase starts too early, charges corresponding to the dark current accumulate. If it starts too late, useful signal is lost.In general, it is also planned to detect the end of the X-ray flash. This improves the signal-to-noise ratio (no integration of the dark current) and also triggers the readout phase more quickly, thus improving the speed and efficiency of image acquisition, compared to a solution in which the integration time is preset to a fixed value, greater than the duration of the X-ray flash (source setting data).

[0005] The sensor must therefore incorporate a detection circuit whose function is to optimally synchronize the start of the integration phase with the onset of the X-ray flash on the active surface of the sensor. The detection circuit generally uses photodetector arrays that frame the pixel matrix, horizontally and / or vertically. These photodetector arrays are thus positioned directly behind absorbing anatomical areas (teeth, gums), which compromises or delays the detection of the X-ray flash by the sensor.From this perspective, an optimal detection zone on the active face of the sensor could be the free zone (i.e., without absorbing obstacles) between a patient's upper and lower jaws. This corresponds to a detection band located in the very center of the sensor's pixel matrix, at the expense of image capture pixels. This necessitates interpolation and filtering calculations to reconstruct the corresponding image data, since what practitioners want to visualize is the complete image of the radiographed area. This also means a loss of image quality, as interpolation introduces inaccuracies.

[0006] It is also known to use pixels dispersed in the matrix as reference pixels for detection, as described by example in US patent 6,404,854. During detection, these reference pixels are read individually, non-destructively (no reset between readings), for comparison to a threshold, and this is done continuously until an image capture decision is made when enough reference pixels have given a signal greater than a A certain threshold. One drawback of this solution is that it requires specific pixel sequencing. But above all, this solution is imperfect in making X-ray detection independent of sensor positioning: depending on the sensor's position, a majority of the reference pixels may end up behind a highly absorbing area, which delays the detection process.

[0007] Furthermore, to further reduce the X-ray dose required for radiological imaging, it is necessary to detect a very low signal level, marking the very beginning of exposure of the photosensitive area to radiation, in order to trigger the integration phase without losing useful information (or as little as possible). In other words, the goal is for the detection signal to be emitted very close to the actual arrival of the X-rays on the active surface of the sensor. For these reasons, it is more advantageous to perform direct detection of the current supplied by all the photodetectors using a capacitive transimpedance amplifier, because the amplifier's high gain allows the detection threshold to be lowered. This means that the arrival of the X-ray flash can be determined earlier, and therefore integration can begin earlier.Such amplifiers are well known and commonly used for reading pixels from infrared image sensors. Also, in the field of medical radiology, patent application WO2017 / 121728 describes a circuit for detecting the appearance of an X-ray flash which is based on such an amplifier, for reading the current supplied by an array of detection photodiodes placed on either side of the pixel array.

[0008] There is therefore a strong need to find a technical solution to enable effective detection of the appearance of X-rays, regardless of the positioning of the sensor, without sacrificing or losing image data accuracy, and which is simple to implement in existing topologies, i.e. without requiring a complete redesign and without encroaching on the active image capture area. Summary of the invention

[0009] The invention relates to a dental radiological image sensor with active pixels, with integrated detection of the appearance of X-rays, which allows efficient detection of the appearance of X-rays, independent of the positioning of the sensor in the patient's mouth, and which maximizes the active area (matrix) relative to the size of the sensor.

[0010] More specifically, the invention thus relates to an intraoral radiological image sensor in MOS technology comprising - a matrix of photosensitive pixels arranged in rows and columns, each pixel comprising a photodiode and transistors, including an initialization transistor (Ml) of photodiode connected between a photodiode node of the pixel and a first common connection node of the pixels; - a sequencing circuit providing control signals for pixel transistors to drive an image capture sequence during exposure to a ray flash, comprising a global initialization phase for the pixel photodiodes, a charge integration phase during an integration period and a pixel readout phase. According to the invention, the sensor comprises a first coupling switch driven by a first logic signal to connect said first connection node to a signal input of a first current detection circuit or to a photodiode initialization voltage source, depending on whether said first logic signal is respectively in a first logic state or a second logic state, and the sensor sequencing circuit being configured to drive a detection phase by said first detection circuit of the start of exposure to an X-ray flash to trigger the image acquisition sequence, comprising the operations of: - a) turning on the photodiode initialization transistors in all pixels simultaneously and - b) establish the first logic signal in said first logic state, having the effect of injecting into the signal input of the first detection circuit a current collected from said first connection node originating from the photodiodes of the pixels, then, - c) when a logic output signal from the first detection circuit switches from a first logic state to a second logic state, corresponding to the detection of an input current level greater than a predetermined threshold, establish the first logic signal in said second logic state which has the effect of coupling said initialization transistors which are always in the conducting state, to said initialization voltage source, thus activating the global initialization phase of the image acquisition sequence, to initialize the photodiodes before said integration phase.

[0011] In an improvement, the initializing transistors of a set of pixels in the matrix are connected not to said first connection node, but to a second connection node which is electrically isolated from said first connection node, and the sensor includes a second current detection circuit of the same type as said first detection circuit and driven by the same clock signal, with a signal input which is coupled to said second connection node, and the initializing transistors of the pixels of said set are in the conducting state at least until an end-of-exposure detection to X-ray radiation by the second detection circuit corresponding to an input current, collected at said second connection node, which falls below a predetermined threshold, and - upon detection of the end of X-ray flash exposure, stop the phase integration of the image capture sequence to initiate the playback phase.

[0012] Advantageously, a second coupling circuit is provided to connect the second connection node to the signal input of the second detection circuit or to the initialization voltage source according to a second logic control signal which is parameterized in the sensor to configure the pixels of said assembly into end-of-exposure detection pixels or image capture pixels.

[0013] According to one aspect of the invention, the gain of the capacitive transimpedance amplifier and / or the voltage threshold value of the comparator being sensor adjustment parameters set in said first detection circuit and in said second detection circuit to detect respectively a start of flash and an end of X-ray flash.

[0014] According to another aspect of the invention, the sequencing circuit is configured to trigger another image-taking sequence applying the same integration time and using the same pixels for image-taking as a previous image-taking sequence carried out during X-ray flash exposure and having provided first image data, allowing dark noise to be measured and subtracted from said first image data.

[0015] The sensor according to the invention applies in particular to active pixels with three or more transistors, in particular to five or more transistors.

[0016] According to one embodiment of the invention, a current sensing circuit of the sensor includes a capacitive transimpedance amplifier comprising a non-inverting input connected to a common mode voltage and an inverting input which forms the signal input which is coupled to a common connection node of the pixels, said amplifier being driven by a clock signal to periodically produce at the signal output, a voltage ramp as a function of the level of current injected at the signal input, which is applied to a comparator for comparison to a threshold voltage value.

[0017] The invention also relates to a corresponding radiological imaging method. Brief description of the drawings

[0018] The invention will be better understood and other advantages will become apparent upon reading the detailed description of embodiments given by way of example, and illustrated by the accompanying drawings which represent:

[0019] [fig. 1] Figure 1, a schematic diagram of the principle of an X flash start detection according to the invention adapted to an image sensor based on an active pixel structure of the three-transistor (3T) type;

[0020] [fig.2] Figure 2, a block diagram of such a pixel matrix image sensor 3T including a device for detecting the start of exposure to an X-ray flash according to the invention;

[0021] [fig.3] Figure 3, a schematic diagram of an amplifier detection device capacitive transimpedance;

[0022] [fig.4] Figure 4, a timing diagram of the control signals of a phase of detection of the occurrence of an X-ray flash and of an image acquisition sequence in an image sensor with an X-ray start detection circuit according to the invention;

[0023] [fig.5] Figure 5, a timing diagram of the signals of the start-of-detection circuit flash X, and also those of a detection circuit of the same type, used according to the invention to detect an end of flash X;

[0024] [fig.6] Figure 6, a functional diagram of the principle of the invention applied to a active pixel of the five-transistor (5T) type at least;

[0025] [fig.7] Figure 7, a block diagram of a 3T pixel sensor according to a rea ​​mode application of the invention allowing detection of the beginning and end of X-ray flashes;

[0026] [fig.8] Figure 8, a block diagram of a 5T pixel sensor, corresponding to this mode of the realization of figure 7; and

[0027] [fig.9] Figure 9, a timing diagram of the control signals for a socket radiological image according to the invention including a start and end detection of X flash to respectively trigger and stop the integration phase of a radiological image acquisition sequence. DETAILED DESCRIPTION

[0028] It should be noted that the figures are simple diagrams illustrating the invention and are not to scale. Only the functional elements or signals necessary for understanding the invention are shown. For the sake of clarity and simplicity, the same notations or references have been used throughout the description and in the figures to designate the same or similar elements.

[0029] Furthermore, in the description that follows, the terms "coupled" or "connected" when used indicate a direct or indirect electrical connection; and the term connected indicates a direct electrical connection.

[0030] The invention relates to a radiological image sensor, and more particularly an intraoral dental image sensor, in CMOS technology, using active pixels.

[0031] An active pixel is a pixel as illustrated in the example in Figure 1, which comprises a photosensitive element, generally a PH photodiode, and several transistors (MOS) all of the same type used as a switch (conducting ON or closed OFF state) or as a voltage follower, to control the different phases of the image acquisition sequence. CMOS technology itself (combination of N and P transistors) relates more specifically to the electronics surrounding the pixels (control, readout, interface electronics, etc.).

[0032] The invention is more particularly illustrated in the context of an image sensor made in a P-type doped semiconductor substrate, which is brought to a ground reference potential, generally a zero potential.

[0033] The PH photodiode is preferably of the "pinned" type, that is to say that the PN junction is formed by an N-type doped region in a weakly doped substrate (P) and this N region (cathode) is surmounted by a highly doped P-type surface layer (P++), brought to the mass reference potential of the substrate: this fixes the potential of the N-well (N region) of the photodiode to a potential Vpin (resting voltage of the photodiode) which depends only on the concentrations of the N and P dopants.

[0034] The transistors of the active pixel are of the N-type. They are controlled to the ON (closed or conducting) state by applying a positive potential VON to their gate g, which generally corresponds to the positive supply voltage Vdd of the sensor's electronic circuitry. The level of this voltage Vdd, for example 3 volts (Figure 4), depends on the CMOS technology used. They are controlled to the OFF (open or blocked) state by applying a potential to their gate g, which generally corresponds to the substrate's ground reference potential (zero potential), or to a more negative potential.

[0035] The logic control signals of the sensor can take two logic values ​​"0" or "1" corresponding respectively to the ground reference potential of the substrate and to Vdd.

[0036] The figures and explanations reflect these conventions. A person skilled in the art can easily transpose all of this to active pixels with P-type MOS transistors.

[0037] For dental radiography, active pixels with three control transistors (3T) are generally used, which prioritizes pixel storage capacity. These pixels only allow for the acquisition of so-called "sliding shutter window" images, because the end of the integration period is shifted from line to line, coinciding with the sequential reading of the pixels by scanning the lines. As for the beginning of the integration period, in the context of dental radiography, it is the same for all pixels, as it is synchronized with the detection of the appearance of the X-ray flash (or more simply "X-ray flash") on the active surface of the sensor. The effective integration time of the lines therefore increases with the direction of the line scanning. The difference is small compared to the total integration time, but this induces a line effect in the image, which can be mitigated by image processing. This is well known.

[0038] The invention will be explained more particularly for a sensor using such a 3T pixel matrix. But it will then be seen that it can easily be applied to structures with more transistors, which notably have the advantage of allowing instantaneous image capture ("snapshot"), with an integration start and end that are identical for all pixels, and more especially to 5T structures, that is to say, comprising (at least) 5 transistors. It should be noted that in the context of the invention, the The sensor is sensitive only to X-rays (by design): it is therefore blind to visible light. During the integration period, it only integrates charges that correspond to the received X-ray radiation.

[0039] Figures 1 and 2 illustrate the principle of X-ray flash start detection according to the invention in an image sensor based on an M-PIX matrix of 3T active pixels, which in this example comprises N rows and P columns of pixels. Figure 1 shows this from the perspective of a pixel in the matrix, and Figure 2 shows the implementation of the invention in the sensor, which allows the use of all the pixels in the matrix, first to perform the flash start detection and then to perform the image acquisition sequence upon detection of this flash start.

[0040] The pixel Py (Figure 1) located in the matrix on the row of pixels R; of rank i and on the column of pixels Colj of rank j, comprises: - a PH photodiode (preferably of the "pinned" type), connected between a KN node; of photodiode (cathode) and the ground reference voltage; - a photodiode initialization transistor Ml connected to the photodiode node KN i which receives on its gate g, an initialization signal RSkn; from the row of pixels of rank i to control the evacuation of charges (electrons) accumulated in the photodiode to a reference voltage source VRS (also called in the following the photodiode initialization voltage); - a transistor M2 mounted as a voltage follower, whose gate is connected to the (capacitive) node KN of the photodiode, which converts the charges accumulated in the photodiode PH into a voltage level that is transferred to the column conductor Colj when the pixel is selected for reading; and - a pixel selection transistor M3, connected in series between transistor M2 and a column conductor Colj connected to a reading circuit RD, and which receives on its gate g, a Sel; signal for reading the row of pixels R;.

[0041] A note on the notation: we denote KN;, the photodiode node of the pixels of line i of row rank. This makes it easy to associate this node with the photodiode reset signal of each line, denoted RSkn;.

[0042] The RD reading circuit makes it possible to obtain for each of the pixels of the column concerned, a numerical value which represents the quantity of charges integrated by the pixel.

[0043] As illustrated in Figure 2, there are therefore two control signals per row (row) R of pixels: the RSkn signal for photodiode initialization and the Sel signal for read selection, applied respectively to the gates of transistor M1 and transistor M3 for all pixels in row i. These signals are controlled by a DM sequencing circuit of the sensor to drive a dental radiographic imaging sequence, conventionally comprising a succession of phases following (see figure 4): - a global initialization phase 301 RSG of the photodiodes, i.e. applied simultaneously to all pixels of the matrix, activated by commanding all transistors Ml at the same time to the ON state: all signals RSkn; (i= 1 to N) are activated (brought to Von). -a 302 integration phase which starts at the same time for all pixels, controlled by disabling all RSKN signals; (return to VOFF). - a sequential readout phase 303, by scanning the pixel lines, which begins after an integration time d1NT (time elapsed since the start of the integration phase). In this phase, the Sel signals are activated (brought to VON) one after the other (one at a time), for a readout time dr. The readout circuit RD is generally configured to perform two samplings: a sampling of the signal level (SHS) corresponding to the amount of charge integrated by the photodiode since the start of the integration phase, and a sampling of a reference level, after RS ​​reset of the photodiode by activating the initialization signal RSKNide on the currently selected line being read.

[0044] In practice, on the electrical level, the drain electrode d of the initialization transistor Ml of each of the pixels is connected to a common NC connection node of all the pixels, and which is usually connected to a reference voltage source VRS, used to reset the photodiodes (discharge all the charges accumulated in the potential well of each photodiode).

[0045] Topologically, as shown in Figure 2, for each pixel, this common connection node NC corresponds to a connection point, represented by a black dot, to a conductive grid GRS (shown in thick line) implemented on a conductive topological layer of the sensor chip. The term grid is to be understood as a 2D periodic mesh, with a matrix pitch. The GRS grid is electrically connected appropriately to the reference voltage source VRS, enabling the initialization of the photodiodes in the global initialization phase RSG and in the selective initialization phases (RS1, RS1+, RS1+) during the sequential readout of the pixels.

[0046] According to the invention, and as illustrated in Figures 1 and 2, the common connection node NC (or the GRS grid) is not directly connected to the initialization voltage source VRS, but is connected to this source VRS via a switching circuit depending on the logic state "0" or "1" of a control logic signal PROB1. This switching circuit MUX1 comprises two connection paths for the NC node: a first path XI is connected to the source VRS; a second path X2 is connected to the signal input IN-DTX1 of a current-to-voltage converter DTX1 current sensing circuit for the sensor. The function of this DTX1 circuit is to provide at output a 0UT-DTX1 signal which signals the detection of the start of exposure of the sensor to the X flash, when the current I which is produced by all the photodiodes (pixels) of the matrix and collected at the common connection node NC, via the initialization transistors Ml all activated in the conducting state and the MUX1 circuit, exceeds a predetermined threshold.

[0047] More specifically, and as illustrated in the timing diagram in Figure 4, the DM sequencing circuit controls a phase 200 of X flash start detection (after exiting a STANDBY 100 standby phase) by the following succession of steps: - by placing the PROB1 signal in a first logic state, in this example the high state ("1") to select channel X2 of the MUX1 switch: the common NC connection node of all pixels is then connected to the current input IN-DTX1; and - by simultaneously activating all the RSKN photodiode initialization signals, allowing the M1 transistor of each pixel to be used as a charge transfer gate between the KN photodiode node (cathode) of the pixel's photodiode and the common connection node NC. In this way, all the charges from all the photodiodes in the array are collected at the NC node, producing the photocurrent I injected into the IN-DTX1 input of the DTX1 circuit. This is what makes the detection of the X-ray flash according to the invention efficient and precise, because it takes into account the current contribution of all the pixels in the sensor's photosensitive array: the pixel array is then used as a vast charge sink for detecting the onset of X-ray flash exposure. Thus, regardless of the sensor's position in the patient's mouth, there will always be pixels in the array for which the X-rays arrive with little or no attenuation.

[0048] When the current detection circuit DTX1 detects that the current exceeds a predetermined threshold, the logic signal at output OUT-DTX1 changes state: in the example it changes from the high logic state "1" to the low logic state "0" and this is what is detected by the sequencing circuit DM, which then triggers a 300 image capture sequence: - by placing the PROB1 signal in the other logic state, in the example the low state ("0") to select the XI channel of the MUX1 switch: the common connection node Ne of all pixels is then connected to the initialization voltage VRS; - by keeping the initialization transistors Ml active, the time of the global initialization phase 301 RSG.

[0049] The other phases of the image acquisition sequence (integration 302, sequential reading 303) driven by the DM sequencing circuit then follow each other in the usual manner. That is to say, in the invention, the pixels of the matrix are first used as pixels for detecting the start of exposure to the X flash, and then as image acquisition pixels.

[0050] Figure 3 illustrates an embodiment of a current detection circuit DTX1 for implementing detection phase 200, comprising a capacitive transimpedance amplifier CTIA1 and a voltage comparator COMP1. It is known that a capacitive transimpedance amplifier can be used to integrate the current supplied by detection photodiodes arranged at the edge of a pixel matrix and to detect the arrival of an X-ray flash, as described, for example, in the aforementioned application WO2017 / 121728. These capacitive transimpedance amplifiers are also used for reading the pixels of infrared sensors, as described, for example, in application EP 1399746.

[0051] The CTIA1 capacitive transimpedance amplifier ensures periodic integration of the input current I, at a clock frequency <e>ctia, and it is followed by the voltage comparator COMP1 to compare the voltage level of the ctiai output signal Vout to a programmable voltage threshold Vthi. The voltage comparator is configured to output a logic signal OUT-DTX1 that toggles from one initial logic state ("0" typically) to another ("1") when the Vout-Ctiai signal level exceeds the threshold VthL

[0052] The CTIA1 amplifier in Figure 3 represents a possible embodiment based on a basic configuration of a capacitive transimpedance amplifier, with a feedback loop comprising an initialization switch T1 placed in parallel with an integration capacitor Cf. However, the invention is not limited to this embodiment and covers other prior art implementations.

[0053] The non-inverting input (+) of the CTIA1 amplifier is connected to a common-mode voltage Vcmi, and the inverting input (-) is the signal input IN-DTX1 through which the current I to be measured is injected. The switch Tl is periodically closed (ON) by an initialization phase control signal. <e>ctia, to discharge the integration capacitor Cf and bring the output voltage Vout ctiai and the input IN-DTX1 to the level of the common-mode voltage Vcm, which serves as the reference point for the periodic integration of the input current I. This integration begins when the switch Tl returns to the open (OFF) state and the entire current I injected at the input is integrated across the capacitor Cf, with a gain that depends on the ratio between the feedback loop capacitance (Cf in the example) and the capacitance Cp on the signal input, which represents the parasitic (equivalent) capacitance of the photodiodes.

[0054] The output Vout ctiai is a signal which is a voltage ramp, as illustrated in figure 5.

[0055] The comparator includes, for example, an analog stage for comparing the voltage Vout-Ctiai to a configurable threshold value Vthi, and a signal conditioning stage that provides a logic signal OUT-DTX1 at the output. This signal switches, typically from a low state to a high state (Figure 5), when the voltage Vout-Ctiai crosses the threshold. above the Vthi threshold. It is this switchover that is detected by the DM sequencing circuit to synchronize (trigger) the image acquisition sequence on the arrival of the X-ray flash.

[0056] The DM sequencing circuit then switches the PROBl signal to the other logic state ("0" in the example), to select the first input channel XI of the MUX1 circuit: the common connection node NC of each pixel is then connected to the initialization voltage source VRS of the photodiodes. Since the transistors Ml are in conducting mode (RSKN; to VON), they are maintained in this mode (Figure 4). This triggers the global reset phase RSG of the pixels, which is the first phase of the image capture sequence (LFX) during X-ray flash exposure. The DM sequencing circuit then drives the integration phase, which starts simultaneously for all pixels, followed by the sequential readout phase, in the usual manner.

[0057] Figure 4 illustrates a complete sequence of X-ray flash start detection and image acquisition, after the practitioner has positioned the sensor behind the chosen anatomical area, with the active face facing the X-ray source, and then triggered the source, for example from a control computer or directly by actuation of the source. This actuation has the effect of waking the sensor from standby mode, by an activation signal transmitted by the computer and / or the source via a wireless or wired (USB) interface.

[0058] The sensor exits standby mode and enters an initialization phase 101, allowing the establishment of the supply voltage VDD and reference voltage VRS. The CTIA1 amplifier initializes ( <bctia, figure 5) : la tension en sortie Vout ctiai et en entrée IN-DTX1 au niveau de tension de mode commun Vcmi de l'amplificateur, qui est à un niveau supérieur à la tension de repos Vpin des photodiodes. Dans l'exemple Vpin est à 0,8 volt et Vcmi est à 1 volt. Comme illustré sur la figure 4, ceci a aussi pour effet de monter le potentiel des nœuds de connexion communs NC (la grille GRS) et les nœuds KN; des photodiodes (cathode) à ce même niveau de mode commun Vcmi.

[0059] The DM sequencing circuit then drives the X flash detection phase 200 as follows: - it places all the RSKN gate control lines of the Ml initializer transistors at VON; and - activates the PROB1 signal in the logic state ("1") which selects the second channel X2 of the MUX1 circuit, having the effect of connecting all the common NC connection nodes of the pixel transistors Ml (and therefore the conductive gate GRS) to the IN-DTX1 input of the current detection circuit DTX1. The SEL signals; selection in reading pixel lines all remain in the inactivated state (low state) throughout the detection phase 200.

[0060] When the X-ray flash reaches the active face of the sensor, the photodiodes generate charges, producing a current I at the IN-DTX1 input representing the contribution of all the photodiodes in the array. This current is integrated by the CTIA1 amplifier, producing a voltage ramp at the output. When the ramp voltage Vout CTIA1 exceeds the threshold Vthl, which is set to 1.5 volts in this example, the OUT-DTX1 output switches to a high (logic) state (arrow no. 1, Figure 4). This switching (rising edge detection and / or high logic state) of the OUT-DTX1 signal signals the detection of the start of the X-ray flash to the DM sequencing circuit. In a non-limiting practical implementation, the comparator C0MP1 includes a latch-type output stage activated by a sampling clock. <e>S / h at the same frequency, but out of phase, as the clock <bctia (figure 5).

[0061] When the DM circuit detects the switching of the OUT-DTX1 signal indicating the detection of the start of exposure, it can then control the successive phases of the 300(I-FX) image capture sequence during flash exposure, as already explained above: - The PROB1 signal changes its logic state (arrow #2). In this example, it goes to "0", selecting the other channel XI of the MUX1 circuit: the NC nodes (the GRS grid, Figure 2) are then all electrically connected to the reference voltage source VRS. The IN-DTX1 input is decoupled from the NC node and no longer receives any current. The output voltage Vout-Ctiai returns to (and remains at) the common-mode voltage level Vcmi and switches the OUT-DTX1 output of comparator COMP1 back to the low state (Figures 4 and 5). -The RSKN signals; photodiode initialization control signals in the active state (VDD) are maintained in the active state (transistors M1 conducting) allowing the overall initialization phase RSG 301 of the image acquisition sequence to be carried out: the photodiodes discharge their charge, and the capacitive nodes KN; are brought (arrow #3) to a voltage level (2 volts in the example) that corresponds to the reference voltage VRS (3 volts) minus the threshold voltage of transistor ML

[0062] The DM sequencing circuit then switches all RSKN signals to an inactive state (Figure 4), thus blocking all M1 transistors. This marks the beginning of the pixel integration phase 302: the photodiodes begin to integrate charges through electrical photoconversion and accumulate these charges (photodiode parasitic capacitance). The potential of the KN nodes decreases as a function of the accumulated charges (dependent on the illumination) until a minimum Vsat is reached, which corresponds to the saturation of the pixel or the readout chain.

[0063] In the example illustrated in Figure 4, the sequential reading phase 303 begins after an integration time d1NT preset to a value greater than the emission time dX of the X-ray source used. At the end of the time d1NT, counted from the beginning of the integration phase (by a counter in the sequencing circuit DM) for example), the DM sequencing circuit activates the 303 sequential read phase, allowing the reading of the pixels of each of the N lines of the matrix, line by line.

[0064] Let us take the row of pixels R; : the SEL; selection signal of this row is activated to switch to the conducting (ON) state the selection transistor M3 of each of the pixels of this row, for a reading time dr: for each pixel of the row, the voltage level supplied by the transistor M2, which corresponds to the quantity of charges accumulated at this time in the photodiode (capacitive reading node KN;), is reported to the column conductor Colj of the pixel (via M3) to be sampled (SHS) by a respective reading circuit RD; then the photodiode of each of the pixels of the row is reset by activating the RSKN; reset signal of this row only (we see that the RSKNi+i signal of the next row Ri+i remains inactivated at this time) and a new voltage level which is a reset level is reported to the column conductor Colj, and sampled (SHR) by the reading circuit.The difference between the signal level and the reset level represents the image data provided by the pixel. The same reading sequence is repeated for each row of the matrix successively.

[0065] The DM sequencing circuit is generally configured to drive, following a first 300 LFX radiological image acquisition sequence, a second 300 LOBS dark noise image acquisition sequence, applying the same integration time d1NT, to measure a dark current level when the sensor is no longer exposed to X-ray radiation. The LOBS dark noise image data are then subtracted point by point from the data of the first image. A higher quality radiological image is obtained.

[0066] Since the integration time d1NT is fixed (meaning that there is no end-of-X-ray flash detection in the sensor), the second sequence 300(Lobs) may only be performed episodically, to account for possible variations, particularly in temperature, and not systematically, at each radiological image acquisition. Between two refreshes, the dark noise image data is stored, for example in a memory circuit associated with the DM driver circuit, and subtracted from the obtained radiological image data.

[0067] The invention has been explained for a 3T pixel sensor, but it can be extended to active pixels using more than three transistors, allowing the same integration time to be applied to all pixels, by the presence of a readout node separated from the photodiode node by at least one transfer transistor. This reduces the effects of dark noise in the image.

[0068] The invention applies in particular to so-called 5T pixels (understood as comprising at least 5 transistors), which, like 3T pixels, include a transistor Ml of initialization specific to the photodiode, as illustrated in Figure 6. The invention is thus implemented in the same way, with the current detection circuit DTX1 and the MUX1 circuit allowing the common connection node NC to be connected first to the signal input IN-DTX1 of the DTX1 circuit during a detection phase of the start of exposure to an X flash, and then to the reference voltage source Vrs allowing the photodiodes to be initialized before an integration phase of an image acquisition sequence triggered by this detection

[0069] Compared to pixel 3T in Figures 1 and 2, pixel 5T (Figure 6) further comprises a capacitive readout node SN (typically a floating-point scattering node) which is separated from the photodiode node KN by a transfer transistor gate, denoted M4, driven by a transfer control signal TF. This signal is activated to transfer the charges integrated by the photodiode into the readout node. A global transfer phase, controlled by transistor M4 in all pixels simultaneously, can thus be driven, marking the end of the current integration phase in all pixels before the sequential readout phase. Pixel 5T also includes a readout node initialization transistor M5: this transistor M5 is activated during the pixel readout phase by a control signal denoted RSSNi, before the sampling SHR of a corresponding reset level, which is subtracted from the signal level (SHS).

[0070] In a variant of the invention, applicable to pixels with three or more transistors, the same current detection principle for the start of X-ray flash exposure is used to also detect the end of X-ray flash exposure, which then determines the end of the integration period. This makes it possible to adjust the pixel integration time as closely as possible to the actual dX duration of the flash, thereby reducing the level of integrated dark current.

[0071] More specifically, according to the invention, this end-of-exposure detection according to the invention is achieved by a detection circuit DTX2 similar to the DTX1 circuit, but based on a current 1' produced by a set of pixels in the matrix configured as detection pixels. As illustrated in Figures 7 (3T pixels) and 8 (5T pixels), these pixels are coupled to a different common connection node NCE, electrically isolated from the NC node, and the sensor includes a second detection circuit DTX2 to receive the current 1' photogenerated by the pixels coupled to the NCE node. This DTX2 circuit includes a capacitive transimpedance amplifier CTIA2 and a comparator COMP2 with a configurable voltage threshold Vth2. The two detectors can operate at the same operating frequency ( <Fctia). Les deux amplificateurs CTIA1 et CTIA2 ont en principe la même tension de mode commun (Vcmi=V cm2)*

[0072] Topologically, in this variant, we can then have two conducting grids isolated from each other, one which interconnects the NC nodes and the other which interconnects the NCE nodes.

[0073] As shown by the timing diagrams in Figure 5, in the initial exposure detection phase, the two detection circuits DTX1 and DTX2 operate similarly, one incorporating a current I from the matrix pixels used for image acquisition and the other incorporating a current I' from matrix pixels configured as detection pixels (this will be explained later). In both circuits, the amplifier produces a voltage ramp at the output, and the comparator outputs OUT-DTX1 and OUT-DTX2 switch to "1" in the example as soon as the ramp voltage crosses above the comparison threshold (Vthi, Vth2).

[0074] The DTX1 X-ray start detection circuit is then decoupled from the NC node (by the action of the PROB1 signal): the CTIA1 amplifier of the DTX1 detection circuit no longer receives input current: the output voltage Vout ctiai remains at the level of the common-mode voltage Vcmi, and at the output of comparator COMP1, the OUT-DTX1 signal switches back to the "0" state in the example. The pixels of the matrix coupled to the NC node are then used to perform an image acquisition sequence; the sequencing circuit, as seen previously, first drives the global reset phase RSG of the photodiodes, then activates the integration phase (Figure 9).

[0075] As long as the sensor is exposed to the X flash, the DTX2 detection circuit continues to receive a current 1' photogenerated by the pixels of the matrix coupled to the NCE node: the CTIA2 amplifier continues to produce a periodic voltage ramp at the output, which exceeds the threshold Vth2. The OUT-DTX2 output of the COMP2 comparator therefore remains unchanged, at "1" in the example.

[0076] The end of the X flash exposure results in a current 1' that hardly increases anymore: the slight increase is due to the dark current in the photodiodes. The ramp voltage then falls below the threshold Vth2: the OUT-DTX2 signal switches back to the "0" state in this example. This switching is detected by the DM sequencing circuit and used to stop the integration phase of the current image acquisition sequence.

[0077] The number of pixels in the matrix used for end-of-exposure detection can be less than 1 in 2000, relative to the total number of pixels. In practice, this is taken into account by adjusting the gains of the CTIA1 and CTIA2 amplifiers and / or by using different comparison thresholds (Vthi, Vth2) in the COMP1 and COMP2 comparators. The operating frequency ( <f>CTIA, fbs / u) of the second detector (CTIA2, COMP2): the detection speed of the end of exposure may be slightly less good because the patient is no longer irradiated.

[0078] The number of pixels in the matrix used for end-of-exposure detection may be less than the total number of pixels, for example, by a ratio of 1 to 2000. The gains of the CTIA1 and CTIA2 amplifiers can also be adjusted, and / or use different comparison thresholds (Vthi, Vth2). The operating frequency of the second detector (CTIA2, C0MP2) can also be reduced. The detection speed for the end of exposure may be slightly lower because the patient is no longer irradiated.

[0079] In practice, the detection pixels, coupled to the NCE nodes, can be pixels from a few columns and / or rows of pixels in the matrix, and / or pixels scattered throughout the matrix. In the example illustrated in Figures 7 and 8, the pixels used for X-ray flash end detection are the pixels in the first columns (set E1) and the last columns (set E2) of the matrix. For simplicity of representation, sets E1 and E2 are successive columns on each edge of the matrix (successive rows could have been used). But in practice, it is preferable to choose rows and / or columns interlaced with "normal" image capture pixel columns: for example, every other column or every fourth column from the first 20 and last 20 columns of a matrix containing several hundred pixels. Or, pixels scattered throughout the entire matrix can be chosen.Indeed, for the pixels used for end-of-flash detection (X), the signal is lost during readout. Image information must be reconstructed for each end-of-flash detection pixel by interpolation from neighboring pixels. Therefore, it is more efficient to space these detection pixels apart to minimize interpolation errors.

[0080] The common NCE connection node of the pixels used in end-of-flash detection is connected to the IN-DTX2 input of the DTX2 circuit, while the matrix pixels coupled to the NC node are used in a current image capture sequence and this common NC connection node is connected to the voltage reference source (VRS).

[0081] During the current image acquisition sequence, the Ml transistors of the pixels coupled to the NCE node must be kept in the conducting state, to allow the collection and injection of the current from the photodiodes of these pixels into the DTX2 circuit, while the Ml transistors of the pixels coupled to the NC node and performing the current image acquisition are deactivated at the end of the global initialization phase RSG, to allow the integration of charges.

[0082] For pixels coupled to the NCE node, a separate control signal for the transistors M1, denoted RSKN_D, is therefore provided, which is the same for all these pixels. This RSKN_D signal is activated as soon as the sensor wakes up from standby and remains active at least until the end of exposure is detected.

[0083] The end-of-exposure signal OUT-DTX2 is used by the DM sequencing circuit to complete the integration phase in the current image acquisition sequence: it is this OUT-DTX2 signal which thus sets the effective integration time, dIXTV, in the current radiological image acquisition sequence LFX.

[0084] As illustrated in Figures 7 and 8, it is advantageous to provide that the end-of-exposure detection is an optional function in the sensor, by providing a multiplexer MUX2 controlled by a PROB2 control logic signal, to couple the NCE node either to the IN-DTX2 input of the DTX2 circuit (channel X'2), and the associated pixels are then used as X end-of-flash detection pixels; or to the VRS voltage reference source (channel X'1), and the associated pixels are then used as image capture pixels only. The logic state of the PROB2 signal is then set by the operator, typically in a sensor parameter register, to activate or deactivate the X flash end-of-exposure detection function. For example, as illustrated, a parameter register contains a DTX-stop bit to be set, for example to 0 to deactivate the end-of-exposure detection option, and to 1 to activate it, which sets the PROB2 logic signal.

[0085] In this case, the transistors M1 of the pixels coupled to the NCE node must be controlled appropriately, depending on whether they are used for end-of-exposure detection or as image capture pixels. For example, as illustrated in the figures, a logic circuit such as a MUX3 multiplexer, driven by the same PROB2 signal, can be used, such as - when the detection option is disabled, PROB2 is at 0 and the transistor Ml of each pixel coupled to the NCE node is controlled by the RSKN control signal; corresponding to the pixel row in the matrix. - when the detection option is enabled, PROB2 is at 1, the Ml transistors of the pixels coupled to the NCE node are all controlled by the RSKN_D control signal.

[0086] Figures 7 and 8 show that the end-of-exposure detection principle according to the invention, which has just been explained, applies in the same way to a 3T or 5T pixel sensor.

[0087] Figure 9 shows more specifically the timing diagram of the start and end-of-exposure detection signals for the X flash, and the associated control signals in the case where end-of-exposure detection is implemented (PROB2="1" in the example). In the example, the LFX and Lobs image acquisition sequences 400 include a global charge transfer phase 403 to the readout nodes SN of the pixels, before the sequential readout phase 404, corresponding to a 5T pixel image sensor as illustrated in Figure 8. With a 3T pixel sensor as illustrated in Figure 7, this global transfer phase 403 does not exist, and the end-of-exposure detection triggers the sequential readout phase 404.

[0088] For the image acquisition sequence 400 (LFX or Lobs) driven by the DM sequencing circuit, after detection of the start of exposure (arrow #5), the global initialization phase 401 RSG activates all RSKN signals (i = 1 to N) simultaneously to initialize the KN photodiode nodes of the pixels coupled to the NC node. RSKN_D to initialize the KN nodes; photodiode of the pixels coupled to the NCE node; and all RSSN signals; simultaneously to initialize the SNide read nodes of all the pixels in the matrix (whether coupled to the NC or NCE node).

[0089] The end-of-exposure detection (arrow 6) triggers phase 403 of global transfer (TFG) of charges from the photodiode KN nodes to the readout SN nodes. This transfer is applied by the M4 transistors of the 5T pixels simultaneously to all image-capture pixels (TF signals). This determines the integration time, which is the same for all pixels: the same start, corresponding to the end of the RSG phase, when the RSKN signals are simultaneously deactivated; and the same end, corresponding to the end of the TFG phase, when the TF signals are simultaneously deactivated. Phase 404 of pixel readout begins. For the row of rank i selected for readout, this phase includes sampling (SHS) of the signal level of the row's pixels; resetting the SN readout node by activating the RSSN signal of the row before sampling (SHR) of the corresponding reference level.Note that the resulting data stream contains the data from the pixels coupled to the NCE node that were used for end-of-exposure detection: the data obtained (read) for these pixels will not be used in practice, but replaced by data calculated by interpolation. This does not change the sequence itself. It is at the image processing level that this is taken into account.

[0090] It should be noted that the effective integration time no longer has a fixed value determined (set) in advance: it is a value d1NTV defined from the two detection signals OUT-DTX1 and OUT-DTX2. It is therefore necessary to measure it if the same integration time is to be applied in the Lobs image acquisition sequence of the dark noise. The sequencing circuit then includes a counter to measure the effective exposure time d1NTV, between the end of the global reset phase RSG of the photodiode nodes KN; and the detection of the end of the flash (OUT-DTX2) which triggers the readout phase (pixels 3T) or the global transfer phase TFG (pixels 5T).

[0091] Since the duration of 1NTV can potentially vary with each new exposure to a flash X, dark current image capture is systematic.

[0092] In practice, the DM sequencing circuit applies filters to the OUT-DTX1 and OUT-DTX2 detection signals, making it possible to eliminate, i.e., to ignore, parasitic pulses that would be induced, in particular, during channel switching in the MUX1 and MUX2 switching circuits, by the potential variations induced on the NC or NCE connection nodes and on the KNi photodiode nodes. Also, the DM circuit can thus filter to ignore the OUT-DTX1 signal during the image capture sequences 300, and filter to ignore the OUT-DTX2 signal during the 200 start-of-detection phases flash exposure. These various filtering measures ("anti-glitch" filters) are standard measures implemented to avoid false detections.

[0093] The invention just explained makes it possible to improve the quality of radiological images, in particular dental, and at a lower cost, because they use the pixels of the matrix and current detection circuits known to those skilled in the art.< / f> < / e> < / e> < / e>

Claims

Claims

1. Intraoral radiological image sensor in MOS technology comprising -a matrix (M-PIX) of photosensitive pixels arranged in rows and columns, each pixel (Pÿ) comprising a photodiode (PH) and transistors, including a photodiode initialization transistor (Ml) connected between a photodiode node (KNi) of the pixel and a first connection node (NC) common to the pixels; - a sequencing circuit (DM) providing control signals for the pixel transistors to control a sequence (300) of image capture during exposure to an X-ray flash (FX), comprising a global initialization phase (301) of the pixel photodiodes, a charge integration phase (302) during an integration period and a pixel reading phase (303); the sensor being characterized in that it comprises a first coupling switch (MUX1) controlled by a first logic signal (PROB1) for connecting said first connection node (Ne) to a signal input (IN-DTX1) of a first current detection circuit (DTX1) or to a photodiode initialization voltage source (VRS) depending on whether said first logic signal is respectively in a first logic state or a second logic state, -and the sequencing circuit (DM) of the sensor being configured to control a phase (200) of detection by said first detection circuit (DTX1) of a start of exposure to an X-ray flash to trigger the image capture sequence (300), comprising the operations of: -a) controlling the photodiode initialization transistors (Ml) in all pixels simultaneously to the on state and - b) establishing the first logic signal (PROB1) in said first logic state, having the effect of injecting into the signal input (IN-DTX1) of the first detection circuit, a current (I) collected at said first connection node (NC) coming from the photodiodes of the pixels, then, -c) when an output logic signal (OUT-DTX1) of the first detection circuit switches from a first logic state to a second logic state, corresponding to the detection of an input current level greater than a predetermined threshold, establishing the first logic signal (PROB1) in said second logic state having the effect of coupling said transistors initialization (Ml) which are always in the on state, to said initialization voltage source (VRS), thus activating the global initialization phase (301) of the image capture sequence, to initialize the photodiodes before said integration phase.

2. Image sensor according to claim 1, wherein the first detection circuit (DTX1) comprises a capacitive transimpedance amplifier (CTIA1) comprising a non-inverting input connected to a common mode voltage (Vcml) and an inverting input which forms the signal input (IN-DTX1) which is coupled to said first connection node (NC), said amplifier being driven by a clock signal (decria) to periodically produce at the signal output (Vout ctiai), a voltage ramp depending on the level of the current injected at the signal input, which is applied to a comparator (COMP1) for comparison with a voltage threshold value (Vthi).

3. An image sensor according to claim 1 or 2, wherein each of the pixels comprises said photodiode initialization transistor (Ml), a voltage follower transistor (M2) having a gate connected to the photodiode connection node (KNi) and a read select transistor (M3) connected in series between said voltage follower transistor (M2) and a respective column conductor (Colj) connected to a pixel read circuit (RD).

4. An image sensor according to any one of claims 1 to 2, wherein each of the pixels comprises said initialization transistor (Ml), a charge transfer transistor (M4) connected in series between said photodiode connection node (KNi) and a pixel readout node (SNi), an initialization transistor (M5) of the readout node, a voltage follower transistor (M2) having a gate connected to said readout node (SNi), and a readout selection transistor (M3) connected in series between said voltage follower transistor (M2) and a respective column conductor (Colj) connected to a pixel readout circuit (RD).

5. Image sensor according to any one of claims 1 to 4, in which the initialization transistors of a set (E) of pixels of the matrix are connected not to said first connection node (NC), but to a second connection node (NCE) which is electrically isolated from said first connection node, the sensor further comprising a second current detection circuit (DTX2) of the same type as said first detection circuit and driven by the same clock signal ( <e>ctia), a signal input (IN-DTX2) of which is coupled to said second connection node (NCE), the sequencing circuit (DM) of the sensor being configured to, in said phase (200) of detecting the start of exposure: - control to the on state the initialization transistors (Ml) of all the pixels of the matrix including the pixels of said assembly coupled to the second connection node;and - upon detection of the switching of an output signal (OUT-DTX1) of the first detection circuit (DTX1): - maintaining in the on state the initialization transistors (Ml) of the pixels of said assembly, at least until detecting a switching of the output signal (OUT-DTX2) of the second detection circuit corresponding to a current injected at the input, collected at said second connection node (NCE) which passes below a predetermined threshold, and - upon detection of the switching of said output signal (OUT-DTX2) of the second detection circuit indicating a detection of the end of exposure to the X-ray flash, stopping the integration phase of the image capture sequence (300) to initiate the reading phase.;

6. Image sensor according to claim 5, comprising a second coupling circuit (MUX2) making it possible to connect the second connection node (NCE) to the signal input (IN-DTX2) of the second detection circuit or to the initialization voltage source (VRS) controlled by a second control logic signal (PROB2) which is parameterized in the sensor to configure the pixels of said set in a configurable way as end-of-exposure detection pixels or as image-taking pixels.

7. An image sensor according to claim 5 or 6 in combination with claim 2, wherein the gain of the capacitive transimpedance amplifier and / or the voltage threshold value of the comparator are sensor adjustment parameters set in said first detection circuit and in said second detection circuit to detect a flash start and an X-ray flash end respectively.

8. An image sensor according to any one of claims 1 to 7, wherein the sequencing circuit (DM) is configured to trigger another image capture sequence (300(I-obs)) applying the same integration time (d1NT, d1NTV) and using the same pixels for image capture as a previous image capture sequence (300(I-obs)) carried out during exposure to the X-ray flash and having provided first image data, making it possible to measure and subtract noise of darkness to said first image data.

9. Method for taking a radiological image by an intraoral radiological image sensor in MOS technology, said sensor comprising a matrix (M-PIX) of photosensitive pixels arranged in rows and columns, each pixel (P;j) comprising a photodiode (PH) and transistors, including a photodiode initialization transistor (Ml) connected between a photodiode node (KN;) of the pixel and a first connection node (NC) common to the pixels, characterized in that it consists in a first phase (200) of coupling said first connection node (NC) to a signal input (IN-DTX1) of a current detection circuit (DTX1) provided in the sensor to detect when the current injected at the signal input (IN-DTX1) exceeds a predetermined threshold, corresponding to a detection of the start of exposure to an X-ray flash, said detection triggering an image capture sequence (300) with the action of coupling said first connection node (NC) to a voltage source (VRS) allowing a global initialization of the photodiodes (301), before an integration phase (302) for an integration duration then a pixel reading phase (303).;

10. A method of capturing a radiological image according to claim 9, allowing control of the integration time in a current image capture sequence by an end-of-exposure detection signal provided by another current detection circuit (DTX2) of the sensor which has a signal input (IN-DTX2) coupled to a second connection node (NCE) which is electrically isolated from said first connection node, and which is connected to the initialization transistors (Ml) of a set (E) of pixels of the matrix, said other current detection circuit (DTX2) having a signal output configured to activate the end-of-exposure detection signal (OUT-DTX2) when the current (!') from the photodiodes of the pixels of said set falls below a predetermined threshold.

11. A method of taking a radiological image according to claim 10, in which the control of the integration time by an end of exposure detection signal is activated or deactivated by a second configurable control logic signal (PROB2) applied to a second coupling circuit (MUX2) making it possible to connect the second connection node (NCE) to the signal input (IN-DTX2) of the second detection circuit or to the initialization voltage source (VRS).

12. A method of taking a radiological image according to one of claims 8 to 11, in which the current detection circuit(s) provided in the sensor are of the capacitive transimpedance amplifier and comparator type.< / e>

Images