Digital detector array with dynamic noise adaptation

Abstract

The invention relates to a detector array for detecting electromagnetic radiation with dynamic noise adaptation, comprising an array of pixels that are sensitive to electromagnetic radiation and output an electrical signal, the level of which depends on the intensity of the electromagnetic radiation received, the pixels being addressed in groups of at least one row, the detector being configured to: - operate at at least two different image-acquisition frequencies; - determine, for all the frequencies, the maximum available readout time (LPML) for one of the groups of at least one row; and - use, for each of the frequencies, the entire maximum available time (LPML) for reading out the group.

Description

DESCRIPTION Title of the invention: Digital matrix detector with dynamic noise adaptation

[0001] The invention relates to a digital matrix detector with dynamic noise adaptation.

[0002] The invention lies in the technical field of medical imaging, and more specifically in that of digital detectors, particularly in the medical sector. The invention is not limited to medical imaging; it can also be implemented in the field of non-destructive testing. More particularly, the invention relates to a digital detector and the integration of electrical signals from pixels via the columns of the pixel matrix in order to convert them into a digital signal. It can be applied to any type of imager, including X-ray, gamma, visible, and infrared imagers. The invention is described here in the field of X-ray medical imaging, by way of example, and without loss of applicability to other imaging fields of all types of electromagnetic radiation.

[0003] In a matrix detector, a pixel represents the detector's basic sensing element. Each pixel converts the electromagnetic radiation to which it is exposed into an electrical signal. The electrical signals from the different pixels are collected during a readout phase of the matrix and then digitized so that they can be processed and stored to form an image. The pixels comprise an area sensitive to electromagnetic radiation and deliver the electrical signals.

[0004] A digital matrix detector comprises row conductors, each connecting the pixels of the same row, and column conductors, each connecting the pixels of the same column. The row conductors are connected to driver modules, usually located on an edge of the matrix, which control the individual pixels. The column conductors are connected to conversion circuits, also usually located on an edge of the matrix, which can be called the "column foot." The conversion circuits read the pixel matrix. It is understood that the terms "rows" and "columns" are purely conventional and can be reversed.

[0005] Each pixel typically includes a component sensitive to electromagnetic radiation, such as a photodetector, which can be, for example, a photodiode, a photoresistor, or a phototransistor. Large photosensitive arrays exist, containing several million pixels arranged in rows and columns. Each pixel also includes an electronic circuit comprising at least one readout switch. The electronic circuit may also include other switches, capacitors, and resistors, downstream of which the readout switch is located. The combination of the sensitive component and the electronic circuit generates electrical signals during an image acquisition phase and collects them during a subsequent readout phase. The electronic circuit may allow for the reset of the signal collected in each pixel after it has been read.The role of the read switch is to transfer or copy, into a column conductor, the signals collected by the electronic circuit based on the information received from the sensing component. This transfer occurs when the read switch receives the instruction from a line conductor during the read phase. The read switch is connected, at one of its terminals, to the sensing component, either directly or indirectly, and at the other terminal, to the conversion circuit via one of the column conductors. This other terminal of the read switch corresponds to the pixel output.

[0006] During the read phase, a read instruction is sent to all actuators in the same row of the matrix via a row conductor. Each pixel in this row is read by transferring its electrical information—charge, voltage, current, frequency, etc.—to the column conductor to which it is associated.

[0007] For an image frame, pixel lines can be selected sequentially, one after the other following a scanning direction of the matrix lines, for a line selection time corresponding to a fraction of the frame time, allowing the application of appropriate signals, for example voltages, to the pixels of the line considered.

[0008] A matrix detector according to the invention, for example based on photodiodes or phototransistors sensitive to light radiation in the visible band, For X-ray or gamma radiography, it is generally associated with a scintillator, for example made of cesium iodide (Csl), which converts X-ray or gamma photons into visible photons to which photodetectors are sensitive. The combination of photodetectors and a scintillator ensures what is called indirect conversion of X-ray or gamma photons.

[0009] Alternatively, the invention can be implemented in a so-called direct conversion detector in which X-ray or gamma photons are directly converted into electrical charges in a conversion layer associated with a matrix. Each pixel of this matrix may include an associated storage capacitor, and, as in indirect conversion detectors, an electronic switch. The storage capacitor is itself associated with an electrode in contact with the conversion layer to locally receive the electrical charges resulting from the conversion of the received X-ray photons.

[0010] An example of a matrix detector in which the invention can be implemented is given, for example, in document EP 3 900 323 A1. This is a patent application filed in the name of the applicant.

[0011] To date, several materials have been successfully tested for use in conversion coatings. These include cadmium telluride (CdTe), amorphous selenium (a:Se), thallium bromide (TIBr), perovskites, and lead oxides (PbO).

[0012] A key specification of a digital detector is its electronic noise. In the case of an X-ray detector, the greater the noise, the higher the X-ray dose required to limit the effects of the electronic noise. Traditionally, a detector operating mode is set with a constant line reading time, optimized for the highest possible speed.

[0013] Electronic noise is generated at various points in the detector's imaging chain, particularly in the panel that converts visible photons into electrons. The noise generated by this panel is related to various capacitors: photodiodes, thin-film transistors (TFTs), columns, and parasitic capacitances. One way to reduce noise is to have the shortest possible readout column. This can be achieved by dividing the column in half and reading each half-column. independently. This column cut-off necessitates doubling the number of readout circuits, which increases the detector's cost. Furthermore, while column noise is reduced, the noise due to the increased number of readout circuits is conversely amplified.

[0014] It is also possible to control noise by adjusting the image capture time depending on the application. For example, in a mode where still images are captured, the image capture time can be increased compared to a mode where a video sequence is recorded. It is even possible to define intermediate modes between still and video modes.

[0015] However, for the reading phase, it is customary to implement only one timing diagram associated with the detector's reading circuit.

[0016] The traditional solution is to have a single line integration time for a given mode. This time is fixed regardless of the desired image readout speed, that is, regardless of the time between two consecutive image capture phases. When the readout phase for all detector lines is complete, there is a delay until the next image to be read. To handle the fastest speeds, the line readout time must therefore be as short as possible, which leads to an increase in noise in the readout circuits.

[0017] One solution has been to create additional specific modes to reduce noise. But this multiplies the modes and adds operating points; there is no continuous noise optimization.

[0018] One aim of the invention is to automatically optimize electronic noise to meet all current and future system requirements.

[0019] According to one aspect of the invention, a matrix detector for the detection of electromagnetic radiation with dynamic noise adaptation is proposed, comprising a matrix of pixels sensitive to electromagnetic radiation and delivering an electrical signal whose level depends on the intensity of the received electromagnetic radiation, the pixels being addressed by groups of at least one line, the detector being configured to: - operate at least two different image acquisition frequencies, - determine, for all frequencies, the maximum available time for the reading one of the groups of at least one line, - use, for each of the frequencies, the entire maximum available time to read the grouping.

[0020] Utilizing the full available read time to read a line or group of lines in the matrix automatically optimizes the electronic noise of the read circuits. A read circuit may include an integration circuit on which, during the read phase, the charges from a selected pixel are accumulated. By adjusting the duration for which the charges are accumulated, the bandwidth of the integration circuit is modified. More precisely, the longer the integration time, the lower the bandwidth of the integration circuit and the lower the electronic noise of the integration circuit. The invention makes it possible to reduce the electronic noise of the read circuits as soon as the image read speed is reduced. The integration time is adjusted according to the speed.

[0021] In one embodiment, the detector is configured to link an image capture phase and a pixel matrix reading phase, the maximum available time being the time separating the end of a first image capture phase from the beginning of a second successive image capture phase, from which is subtracted the time of an initialization or reset sequence of the pixels of the matrix, this difference being divided by the number of groups of at least one line.

[0022] In one embodiment, the detector is configured to use the entire maximum available time by means of a time loop adjustable in number of iterations, inserted into a single adjustable timing diagram.

[0023] According to one embodiment, the adjustable time loop includes at least one iteration.

[0024] In one embodiment, the detector is configured to include a dynamic noise-adaptive radiation detection on / off device.

[0025] According to one embodiment, the radiation is X-rays, gamma rays, visible radiation, or infrared radiation.

[0026] According to another aspect of the invention, an X-ray imaging system is also proposed, equipped with a detector according to the invention and an electromagnetic radiation source, comprising several operating modes having respective frequencies of successive image recordings of the radiation received by the detector, and characteristics of the emission of the electromagnetic radiation source.

[0027] The invention will be better understood upon examination of some embodiments described by way of non-limiting examples and illustrated by the accompanying drawings, in which:

[0028] Figures 1a and 1b schematically illustrate the operation of a semiconductor detector for X-ray detection, according to the state of the art;

[0029] Figures 2a and 2b schematically illustrate the operation of a semiconductor detector for X-ray detection, according to one aspect of the invention;

[0030] Figures 3a and 3b schematically illustrate the operation of a semiconductor detector for X-ray detection, according to one aspect of the invention; and

[0031] Figure 4 schematically illustrates the operation of a semiconductor detector for X-ray detection, according to one aspect of the invention; In all figures, elements with identical references are similar.

[0032] Figures 1a and 1b schematically illustrate the operation of a semiconductor detector for X-ray detection, according to the state of the art.

[0033] An X-ray imaging system, comprising a detector and an X-ray source, requests the detector to perform an X-ray window (xrw) of x ms in an operating mode m. The X-ray imaging system then sends a frame request (fr) to indicate the start of image acquisition. The detector then initiates an image acquisition phase by executing a timing diagram. In this timing diagram, an xrw (X-ray window) is performed, during which X-rays are emitted. This is followed by a pixel array readout phase lasting a pixel array readout time (LP). In the case of Figure 1a, the next frame request (fr) then follows, indicating the start of the next image acquisition. This case corresponds to a high image acquisition frequency, corresponding to the minimum reading time of an image by the detector, or "frame rate" in English.

[0034] In the case of Figure 1b, corresponding to a low image acquisition frequency, corresponding to an image read time greater than the minimum image read time by the detector, there is a waiting time DA until the next image request fr, to indicate the start of the next image capture.

[0035] The invention relates to a semiconductor detector with dynamic noise adaptation, particularly for X-ray detection. The detector is equipped with a sensor comprising a matrix of photosensitive pixels and delivering an electrical signal whose level depends on the intensity of the received radiation, the pixels being arranged in rows, the detector being configured, on demand, with a frequency f for recording successive images of the received radiation, of an emission characteristic of an X-ray source among pulsed or continuous, to determine the maximum reading time LPML of a row of the matrix, and to use the entirety of this maximum reading time to read a row of the matrix.

[0036] Figures 2a and 2b schematically illustrate the operation of a semiconductor detector for X-ray detection, according to one aspect of the invention. Whether at a high image acquisition frequency, as illustrated in Figure 2a, or at a low image acquisition frequency, as illustrated in Figure 2b, there is no longer a waiting time DA until the next image request fr, beyond a certain image acquisition frequency, because the detector is configured to determine the maximum read time LPM of an image and use it in its entirety, and consequently configured to determine the maximum read time LPML of a row of the matrix and use it in its entirety. In practice, the invention can be implemented for any image acquisition frequency lower than a maximum frequency shown in Figures 1a and 2a.Below a minimum image acquisition frequency, increasing the readout time no longer provides a significant reduction in the electronic noise of the readout circuits. Consequently, automatic adaptation of the readout time is performed within a given image acquisition frequency band. The low and high acquisition frequencies shown in Figures 2a and 2b, for example, form the limits of the frequency band within which the invention is implemented. Below this frequency... With a low image acquisition frequency, a fixed reading time can be maintained, and a waiting time can be maintained until the next image capture.

[0037] Figures 2a and 3b schematically illustrate the operation of a semiconductor detector for X-ray detection, according to one aspect of the invention, for the respective cases of high image acquisition frequency and low image acquisition frequency. They specify the maximum readout time (LPM) of an image, for the cases shown in Figures 2a and 2b.

[0038] In both of these cases, a SIR sequence for initializing or resetting the pixels of the matrix ("reset" in English) is represented, with a fixed duration independent of the number of rows. Then, each row is read using the entire maximum LPML reading time possible for reading the row or group of rows, as determined by the detector.

[0039] Each possible maximum LPML read time for reading the line includes a first fixed sub-duration (SD1), a second variable sub-duration (SD2), and a third fixed sub-duration (SD3).

[0040] During the sub-duration SD2, for each pixel in a line or group of lines, the read switch remains open to allow the reading of the electronic signals accumulated in each pixel by the associated read circuits. The sub-durations SD1 and SD3 can correspond, in particular, to dead times between the reading of consecutive lines or groups of lines. The invention implements a specific timing diagram for controlling the lines of the pixel matrix. In this timing diagram, the duration for which the read switches are open is adjusted according to the image acquisition frequency. Additionally, it is possible to control other components, particularly switches belonging to the read circuits.

[0041] Figure 4 represents a fusion of figures 2b and 3b.

[0042] An example of an application to explain the invention could be the following.

[0043] The X-ray imaging system sends a command to the detector to specify the mode it wants to use: - Mode number (generally indicates the pixel grouping and whether the mode is pulsed or continuous) - The mode gain (changes the integration capacity of the reading circuit) - The viewing area (or "windowing" in English), the number of rows ni and columns ne that we want in the output image. For the example, we consider a full-field window: ni x ne. - The X window (xrw). - Selection or not to use the "Dynamic Noise Adaptation" in English according to the invention. - The image acquisition frequency (or speed) that it intends to maintain f, expressed in fps (fps for images per second or "frames per second" in English).

[0044] The detector calculates the maximum reading time (LPM) it has to read the pixel matrix: 1 / f - xrw. In a register, the detector knows that it has a fixed drive time of dfp ms and a minimum line read time of dll ms. Therefore, it knows that the fastest it can read the pixel matrix is ​​LPMmax = dfp + dll x ni (in ms).

[0045] He therefore knows that he has a time to distribute of tr = LPM - LPMmax (in ms) over ni lines, which means tir = tr / ni (in ms) to add.

[0046] The detector is configured to use the entire maximum time to read a line of the matrix, by means of a time loop adjustable in number of iterations, is inserted into a single adjustable timing diagram.

[0047] To vary the line time, there is, for example, a loop that allows for variable time SD2. By default, the loop is executed or iterated at least once. Each additional iteration of the loop P adds a fixed time step pf to the integration of a line. To add a time step to a line time, the loop tlr / pf must therefore be iterated a few times.

[0048] Thus, with a single timing diagram, it is possible to manage several time lines.

[0049] It can be advantageous to read several neighboring pixels of the detector simultaneously. This simultaneous reading degrades spatial resolution but increases reading speed. Such pixel grouping is known as "binning." For the same detector, two modes may be offered to a user: a first mode where the reading is performed without Grouping is one mode, and a second mode groups contiguous pixels. In grouping mode, for pixels belonging to two contiguous rows, grouping consists of simultaneously closing the read switches of the pixels in question. The read time then relates to the grouping of the two contiguous rows. It is, of course, possible to group pixels belonging to more than two contiguous rows.

Claims

DEMANDS 1. Matrix detector for the detection of electromagnetic radiation with dynamic noise adaptation, comprising an array of pixels sensitive to electromagnetic radiation and delivering an electrical signal whose level depends on the intensity of the received electromagnetic radiation, the pixels being addressed by groups of at least one line, the detector being configured to: - operate at least two different image acquisition frequencies, - determine, for all frequencies, the maximum available reading time (LPML) of one of the groups of at least one line, - use, for each of the frequencies, the entire maximum available time (LPML) to read the grouping.

2. Matrix detector according to claim 1, configured to link an image capture phase (xrw) and a readout phase (LPM) of the pixel matrix, the maximum available time (LPML) being the time separating the end of a first image capture phase (xrw) from the beginning of a second successive image capture phase (xrw) from which is subtracted the time of a sequence (SIR) of initialization or reset of the pixels of the matrix, this difference being divided by the number of groups of at least one line.

3. Matrix detector according to claim 1, configured to use the entire maximum available time (LPML), by means of a time loop adjustable in number of iterations inserted in a single adjustable timing diagram.

4. Detector according to claim 2, wherein the adjustable time loop comprises at least one iteration.

5. Detector according to any one of the preceding claims, configured to include a radiation detection activation / deactivation device with dynamic noise adaptation.

6. Detector according to any one of the preceding claims, wherein the radiation is X-ray, gamma, visible, or infrared radiation.

7. Electromagnetic radiation imaging system, equipped with a detector according to any one of the preceding claims and a source of electromagnetic radiation, comprising several operating modes having respective frequencies of successive image recordings of the radiation received by the detector, and characteristics of the emission from the electromagnetic radiation source.

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