Digital matrix detector with dynamic noise adaptation

Abstract

A matrix detector for the detection of electromagnetic radiation with dynamic noise adaptation, 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 row, the detector being configured, from a readout frequency of at least one image of the received electromagnetic radiation, and an integration capacitance of a detector readout circuit from among a plurality of integration capacitance values, to determine the maximum read time (LPML) of a group of at least one row of the matrix, and to use the entirety of this maximum read time (LPML) to read the group of at least one row of the matrix. Figure for the abstract: Fig. 3a,

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 notably be implemented in the field of non-destructive testing. The invention relates more particularly 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 elementary sensitive element of the detector. Each pixel converts the electromagnetic radiation to which it is subjected into an electrical signal. The electrical signals from the different pixels are collected during a matrix readout phase 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, generally located on an edge of the matrix, which control the individual pixels. The column conductors are connected to conversion circuits, also generally located on an edge of the matrix, which can be called the "column foot." The conversion circuits allow the pixel matrix to be read. It is understood that the terms "rows" and "columns" are purely conventional and can be reversed.

[0005] Each pixel generally comprises a component sensitive to electromagnetic radiation, such as a photodetector, which may be, for example, a photodiode, a photoresistor, or a phototransistor. Large photosensitive arrays exist that can have several million pixels arranged in rows and columns. Each pixel further comprises a circuit An electronic circuit comprising at least one read switch. The electronic circuit may also include other switches, capacitors, and resistors, downstream of which the read switch is located. The assembly consisting of the sensing component and the electronic circuit generates electrical signals during image acquisition and collects them during playback. The electronic circuit may reset the signal collected in each pixel after a transfer for pixel readout. The role of the read switch is to transfer or copy, into a column conductor, the signals collected by the electronic circuit according to the information received from the sensing component. This transfer occurs when the read switch receives the instruction from a line conductor during the readout phase.The read switch is connected, at one of its terminals, to the sensitive 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 reading phase, a read instruction is issued to all actuators of the same row of the matrix by means of a row conductor. Each pixel of 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, the 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, based on photodiodes or phototransistors sensitive to light in the visible band, is generally combined, for X-ray or gamma radiography, with a scintillator, for example made of cesium iodide (Csl), enabling the conversion of X-ray or gamma photons into visible photons to which photodetectors are sensitive. The combination of photodetectors and a scintillator ensures an 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 which may include an associated storage capacitor, and, as in indirect conversion detectors, with 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] To date, several materials have been successfully tested for use in creating a conversion layer. These include cadmium telluride (CdTe), amorphous selenium (a:Se), thallium bromide (TIBr), perovskites, lead oxides (PbO)...

[0011] A key specification of a digital detector is its electronic noise. In the case of an X-ray detector, the greater the noise, the greater 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, adapted to the highest possible speed.

[0012] 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 the various capacitors: photodiode, thin-film transistor (TFT), column, and parasitic capacitances. One way to reduce noise is to have the shortest possible readout column. To achieve this, it is possible to cut the column in half and read each half-column independently. This cutting of the column requires doubling the number of readout circuits, which increases the cost of the detector. Moreover, even if the column noise is reduced, the noise due to the number of readout circuits is conversely increased.

[0013] It is also possible to control noise by modifying the duration of the image capture phase depending on the application. For example, in a mode where static images are captured, it is possible to lengthen the duration of the image capture phase compared to a mode where a video sequence is recorded. It is even possible to define intermediate modes between static mode and video mode.

[0014] On the other hand, for the reading phase, it is customary to implement only one timing diagram associated with the detector reading circuit.

[0015] 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. When the readout phase of all the 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 implies that the noise level is set to a maximum.

[0016] Subsequently, the solution was to create additional specific modes to reduce noise. However, this multiplies the modes and adds operating points, without providing continuous noise optimization.

[0017] One object of the invention is to automatically optimize electronic noise to satisfy all current and future system builders' demands.

[0018] 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, from a reading frequency of at least one image of the received electromagnetic radiation, and an integration capacitance of a detector reading circuit among a plurality of integration capacitance values, to determine the maximum reading time of a group of at least one line of the matrix, and to use the entirety of this maximum reading time to read one line of the matrix.

[0019] Increasing the maximum reading time to read a line of the matrix allows for automatic optimization of electronic noise.

[0020] In one embodiment, the detector is configured to use the entire maximum time available to read a row of the matrix, by means of a time loop adjustable in the number of iterations, inserted into a single adjustable timing diagram.

[0021] According to one embodiment, the adjustable time loop comprises at least one iteration.

[0022] In one embodiment, the detector is configured to include a dynamic noise-adaptive radiation detection activation / deactivation device.

[0023] According to one embodiment, the radiation is X-ray, gamma, visible radiation, or infrared radiation.

[0024] 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.

[0025] 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:

[0026] [Fig. la] and [Fig.lb] schematically illustrate the operation of a semiconductor detector for X-ray detection, according to the state of the art;

[0027] [Fig.2a] and [Fig.2b] schematically illustrate the operation of a semiconductor detector for X-ray detection, according to one aspect of the invention;

[0028] [Fig. 3a] and [Fig. 3b] schematically illustrate the operation of a semiconductor detector for X-ray detection, according to one aspect of the invention; and

[0029] [Fig.4], schematically illustrates the operation of a semiconductor detector for X-ray detection, according to one aspect of the invention; In all the figures, elements with identical references are similar.

[0030] Figs.1a and 1b schematically illustrate the operation of a semiconductor detector for X-ray detection, according to the state of the art.

[0031] An X-ray imaging system comprising a detector and an X-ray emitting 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, a duration of the X-ray window (xrw) is performed, during which X-rays are emitted. This is followed by a pixel matrix readout phase lasting a duration of the pixel matrix readout (LP), or photodiode matrix readout. In the case of [Fig. 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.

[0032] In the case of [Fig.lb], 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 acquisition.

[0033] The invention relates to a semiconductor detector for X-ray detection with dynamic noise adaptation, equipped with a sensor comprising an array of X-ray 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, an emission characteristic of an X-ray source from pulsed or continuous, and a capacitance of a detector readout circuit from a plurality of capacitance values, to determine the maximum readout time LPML of a row of the array, and to use the entirety of this maximum readout time to read a row of the array.

[0034] Figures [Fig.2a] and [Fig.2b] schematically illustrate the operation of a semiconductor detector for X-ray detection, according to one aspect of the invention. Whether at high image acquisition frequencies, as illustrated in [Fig. 2a], or at low image acquisition frequencies, as illustrated in [Fig. 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 duration of maximum read LPM of an image and use it in its entirety, and consequently configured to determine the maximum read time LPML of a line of the matrix and use it in its entirety.

[0035] Figures 3a 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 of Figures 2a and 2b.

[0036] In these two cases shown, an initialization or reset SIR sequence is represented, having a fixed duration independent of the number of lines. Each line is then read using the entire maximum possible LPML reading time for that line, as determined by the detector.

[0037] Each possible maximum LPML reading 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).

[0038] Fig.4 represents a fusion of Fig.2b and Fig.3b.

[0039] An example of application to explain the invention may be the following.

[0040] 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 reading area ('windowing' in English), number of lines, and and column 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).

[0041] The detector calculates the maximum time or duration LPM it has to read the pixel matrix: 1 / f - xrw. In a register, the detector knows that it has a fixed drive duration of dfp ms and a minimum line read duration of dll ms. It therefore knows that the fastest it can read the pixel matrix is ​​LPMmax = dfp + dll x ni (in ms).

[0042] He therefore knows that he has a time to distribute of tr = LPM - LPMmax (in ms) over ni lines, which makes tir = tr / ni (in ms) to add.

[0043] The detector is configured to use the entire maximum time used 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.

[0044] To vary the line time, there is, for example, a loop, allowing the variable time SD2 to be implemented. 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.

[0045] Thus, with a single timing diagram, it is possible to manage several time lines.

[0046] 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 can be offered to a user: one where reading is performed without grouping, and another where contiguous pixels are grouped. In the binning mode, for pixels belonging to two contiguous rows, binning consists of simultaneously closing the reading switches of the pixels in question. The reading time then relates to the grouping of the two contiguous rows. It is, of course, possible to bin pixels belonging to more than two contiguous rows.

Claims

Demands

1. A dynamically noise-adaptive matrix detector for the detection of electromagnetic radiation, 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, from a reading frequency (f) of at least one image of the received electromagnetic radiation, and an integration capacitance of a detector reading circuit among a plurality of integration capacitance values, to determine the maximum read time (LPML) of a group of at least one line of the matrix, and to use the entirety of this maximum read time (LPML) to read the group of at least one line of the matrix.

2. Detector according to claim 1, 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.

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

4. Detector according to any one of the preceding claims, configured to include a radiation detection on / off device with dynamic noise adaptation.

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

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

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