KR20140134183A - Compensation circuit and Method for controlling bias voltage supplied photodiode - Google Patents

Abstract

A compensation circuit to control bias voltage supplied from a plurality of photodiodes contained in a pixel comprises: a controller to monitor a dark count generated from photodiodes, and determining photodiodes requiring the change of bias voltage among the photodiodes based on the dark count; and a bias voltage supplier to supply the bias voltage to the determined photodiodes in accordance to the control of the control part.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compensation circuit for controlling a bias voltage supplied to a photodiode,

A compensation circuit for controlling a bias voltage supplied to the photodiode, and a method of controlling the same.

Medical imaging devices that acquire images of the inside of the human body to diagnose patients provide information necessary for diagnosis of diseases. The medical imaging methods currently being used or developed in hospitals are largely divided into methods of obtaining anatomical images and physiological images. First, MRI (Magnetic Resonance Imaging) or CT (Computed Tomography) are examples of imaging techniques that provide detailed anatomical images of the human body at high resolution. These two-dimensional images of a human body, or a plurality of two-dimensional images using three-dimensional images with high resolution to produce the exact position and shape of the organs in the body. Second, examples of physiological imaging techniques include positron emission tomography (PET), which captures the metabolic processes in the body and contributes to the diagnosis of metabolic abnormalities.

Positron emission tomography (CT) produces a special radioactive tracer that emits a positron in the form of a component that participates in human metabolism. The tracer is injected into the human body by intravenous injection or inhalation, and the positron emitted by this tracer is combined with electrons , Which are emitted in the opposite direction to each other, are detected by using an external device to observe the position of the tracer and to observe the distribution pattern and the change of the distribution pattern with time. At this time, a scintillator for converting the gamma rays into light and a photodiode for detecting the converted light are used. The photodiode is biased and operates in a breakdown region or Geiger region. Therefore, a method for controlling the photodiode to operate in a desired region by biasing is being studied.

And a compensation circuit for controlling a bias voltage supplied to the photodiode and a method of controlling the same.

A compensation circuit for controlling a bias voltage supplied to a plurality of photodiodes included in the pixel monitors a dark count generated from the photodiodes, A controller for determining a photodiode requiring a change in a bias voltage among the photodiodes based on the count; And a bias voltage supplier for supplying a bias voltage to the determined photodiode under the control of the control unit.

The control unit sets a reference value for the rate of the dark count and determines a photodiode for which a bias voltage needs to be changed based on a result of comparing the ratio of the dark count and the reference value .

Wherein the control unit sets the maximum value and the minimum value as the reference value in advance and controls the bias voltage supplied to the determined photodiode to be lower when the ratio of the dark count is higher than the maximum value, The bias voltage to be supplied to the determined photodiode is increased.

Wherein the plurality of microcells included in the pixel each include a voltage adder and a photodiode and further comprise a reference voltage supplier for supplying a reference voltage to the voltage adder .

And the voltage adder changes a bias voltage supplied to the photodiode by raising or lowering the reference voltage supplied from the reference voltage supply device.

And the voltage adder changes the bias voltage under the control of the controller.

The reference voltage supply device includes a reference diode connected in series and a current source.

The control unit monitors the dark counts generated from the photodiodes in real time and changes the bias voltage supplied to the photodiodes based on the monitoring in real time.

The photodiode is a silicon photomultiplier (SiPM).

The dark count generated from each of the plurality of photodiodes included in the pixel can be monitored to control the bias voltage supplied to each of the photodiodes.

In addition, a bias voltage to be supplied to the photodiodes can be set using a reference diode located outside the microcell.

In addition, a bias voltage supplied to each photodiode can be changed using a voltage adder located in each microcell.

1 is a view showing an embodiment of a medical image generating apparatus.
2 is a view for explaining the structure of the detector.
3 is a view for explaining a compensation circuit according to an embodiment of the present invention.
4 is a view for explaining a compensation circuit according to another embodiment of the present invention.
5 is a diagram for explaining a compensation circuit according to another embodiment of the present invention.
6 is a flowchart illustrating a method of controlling a bias voltage according to another embodiment of the present invention.
7 is a graph for explaining a method of controlling a bias voltage applied to the photodiode.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a medical image generating apparatus, which shows an overall system for generating an image of a cross section of a target object. Referring to FIG. 1, a medical image generating apparatus includes a radiographing apparatus 100, a computer 200, a display apparatus 300, a user input apparatus 400, and a storage apparatus 500.

The medical image generating apparatus of FIG. 1 can generate an image of a cross section of a target body, as well as a system response of a detector 110 used for generating a medical image. The system response may represent a blur model of the detector 110. The correction model of the detector 110 is used to generate a high-resolution image in generating an image using the signal obtained from the detector 110, or to use a low-resolution image as a high-resolution image. , There may be a blur model for correcting the spread of the image.

The medical image generation apparatus acquires a signal emitted from the positron emission material located in each of the position coordinates in the detection space of the detector 110 to generate a PSF for each of the position coordinates, (Or a blur model) for the detector 110 by generating a PSF model for the entire detection space of the detector 110. [

For example, in the case where an image of a cross section of a target object is generated using the medical image generating apparatus of Fig. 1, the image capturing apparatus 100 detects a signal emitted from a tracer injected into the target object. The tracer is used to refer to a substance that emits a positron. For example, the image capturing apparatus 100 detects two gamma rays emitted by a positron emitted from a positron emitting material injected into a target in combination with surrounding electrons. The image capturing apparatus 100 transmits the LOR data for the detected gamma rays to the computer 200.

The computer 200 generates a medical image for a target object by using the data acquired from the image capturing apparatus 100. For example, in the case of generating a medical image of a target object by using the medical image generating apparatus of Fig. 1, the computer 200 generates a medical image representing a section of the target object by using the data acquired from the image capturing apparatus 100 do. For example, when the blur model of the detector 110 is generated by using the medical image generating apparatus of Fig. 1, the computer 200 calculates the blur model of the detector 110 using the data acquired from the image photographing apparatus 100, Create a model.

The display device 300 displays the medical image or the blur model generated from the computer 200 on the display panel.

The user can input information necessary for the operation of the computer 200 using the user input device 400. [ For example, the user can use the user input device 400 to instruct the computer 200 to start or end the operation.

The detector 110 acquires a signal that occurs in the detection space. The detection space of the detector 110 represents the inside of the cylinder shape, as shown in Fig. The signal may be a signal emitted from a point source located within the detection space, or a signal emitted from an object injected with a tracer. Also, in the case of a ladar system, the signal may be a laser originating from a radar.

In a positron emission tomography (PET) device, a signal can be two gamma rays, in which a positron emitted from a positron emitter injected into the body of a subject combines with surrounding electrons to emit.

In the case of a detector of a positron emission tomography apparatus, a plurality of detector arrays may be arranged on the surface of the detector in connection with each other. Further, each detector array may include a plurality of detection elements, and an area divided by the detection elements is referred to as a pixel. Since each detector array is arranged at a predetermined angle apart, and the inner surface of each detector array is not a curved surface, the detector cross-section may be in the form of a polygon that is not actually a complete circle.

2 is a diagram for explaining the structure of the detector 110. As shown in FIG. Referring to FIG. 2, detector 110 includes a plurality of detector arrays 210. Looking at one of the detector arrays 210 of the plurality of detector arrays 210, the detector array 210 is divided into a plurality of pixels 211-215. The pixel may represent a unit for detecting radiation, and may be used as a unit that distinguishes the detector array 210.

One pixel 215 may include a curser 220 and a plurality of micro-cells 230. In one embodiment, The curser 220 is an element that converts incident radiation into light. Accordingly, the radiation generated in the detector 110 is converted into light through the cyrillator 220, and the converted light is output to the microcell 230. A plurality of microcells 230 are located in one pixel 215. The microcells 230 each include at least one photodiode.

Photodiodes are devices that convert light into electrical signals. For example, the photodiode may be a silicon photomultiplier (SiPM). There is also an Avalanche photodiode (APD) that operates in the avalanche region as a silicon photomultiplier, or a Geiger photodiode (GAPD) that operates in the Geiger region. In the avalanche or Geiger region, photodiodes generate carriers (electrons or holes, electron or holes) by light or heat. Silicon photomultiplier is strong against the influence of magnetic field because electrons move only in a short silicon channel, and it has the advantage of requiring a voltage as low as 50v or less. These photodiodes are biased to operate in the desired region. The bias voltage may be different for each photodiode. Thus, there is a need to control the bias voltage applied to operate the photodiode in the desired region.

The breakdown voltage of the photodiode is affected by factors such as the magnetic field, temperature, and pressure. Therefore, the breakdown voltage of the photodiode is not constant. A method of controlling the voltage applied to the photodiode will be described in more detail in FIGS. 6 and 7. FIG.

The process of detecting whether or not radiation is detected in the pixel 215 will be described. When the radiation is incident on the pixel 215, the ciliary light 220 converts the incident radiation into light and outputs the light to the micro cells 230. Among the micro cells 230, the micro cells 230 to which light is incident output electric signals.

There are analog and digital methods for counting radiation. First, the analog system accumulates the electrical signals output from the microcells included in one pixel 215 and counts the radiation incident on the pixel 215 when the accumulated electrical signal is equal to or greater than a reference value Method. On the other hand, in the digital type, when the number of the micro cells 230 outputting the electrical signal among the micro cells 230 included in one pixel 215 is more than a specific number, it is counted that the radiation is incident on the pixel 215 Method.

The digital method will be described in more detail. In the case of the digital method, it is determined whether or not the radiation is incident based on the output of the electric signal from the photodiode included in each microcell 230. Therefore, it is necessary to control the bias voltage applied to the photodiodes so that the photodiodes output an electric signal when the light is incident. The photodiode outputs an unwanted electrical signal in the avalanche region or Geiger region, which is referred to as the dark count. If the dark count rate at which the dark count occurs is high, it is difficult to distinguish whether the electrical signal output from the photodiode is caused by dark count or light. In addition, if the rate at which dark counts occur is too low, an electric signal may not be output from the photodiode even if light is incident. Therefore, the ratio of the dark count output from the photodiode is monitored, and the ratio of the dark count is controlled by adjusting the bias voltage applied to the photodiode. The compensation circuit and method for adjusting the bias voltage applied to the photodiode will be described in detail below.

3 is a view for explaining a compensation circuit according to an embodiment of the present invention. Referring to FIG. 3, the compensation circuit includes a control unit 310 and a bias voltage supply unit 320. The compensation circuit controls the ratio of the dark counts output from the plurality of microcells 230 contained in the pixel 215. In other words, the compensation circuit controls the bias voltage applied to the photodiode 330 to control the ratio of dark counts output from the photodiode 330 included in the microcell 230. [ At this time, the control unit 310 controls each of the photodiodes 330 individually.

Although only two microcells 230 are illustrated in FIG. 3 in a pixel 215, a single pixel 215 includes a plurality of microcells 230, for example, 3600 microcells 230 .

The control unit 310 is connected to the photodiode 330 and the bias voltage supply unit 320 included in each microcell 230. The control unit 310 is connected to each photodiode 330 and receives a ratio of dark counts output from the photodiode 330. In other words, the control unit 310 may monitor the voltage or current output from the photodiode 330 to measure the dark count ratio. The control unit 310 is connected to each bias voltage supply unit 320 and outputs a control signal to the bias voltage supply unit 320.

The controller 310 monitors the ratio of the dark counts output from the photodiodes 330. The control unit 310 monitors the ratio of the dark count in real time. The dark count output from the photodiode 330 varies depending on the voltage applied to the photodiode 330 or the temperature of the photodiode 330. Accordingly, the control unit 310 monitors the ratio of the dark count output from the photodiode 330 in real time, and detects a change in the dark count ratio.

The control unit 310 controls the bias voltage supply unit 320 based on the result of monitoring the dark count ratio. The control unit 310 determines the microcell 230 to which the bias voltage is to be changed based on the result of monitoring the dark count ratio. For example, when the ratio of the dark counts output from the microcell 230 is higher or lower than the set reference value, the controller 310 may determine to change the bias voltage of the microcell 230. The reference value may be set to two or more values in advance. The control unit 310 outputs a control signal to the bias voltage supply unit 320 located in the determined microcell 230 to change the bias voltage output from the bias voltage supply unit 320 to the photodiode 330.

The bias voltage supply unit 320 supplies a bias voltage to the photodiode 330. The bias voltage supplier 320 supplies a reference bias voltage to the photodiode 330 and receives a control signal from the controller 310. The bias voltage supplier 320 supplies the reference bias voltage to the photodiode 330 And supplies the changed bias voltage. The bias voltage supply unit 320 may be implemented with various types of voltage supply devices.

The bias voltage supply unit 320 is controlled by the control unit 310. The bias voltage supplier 320 supplies the set reference bias voltage at an initial stage of the operation, but operates according to the control of the controller 310 after the control signal is input by the controller 310.

A bias voltage supply 320 is included in each microcell 230. The microcell 230 may include at least one bias voltage supply unit 320 and a photodiode 330. Therefore, the bias voltage applied to the photodiode 330 included in each microcell 230 may be different from each other. Since the characteristics of the photodiodes 330 are different, the bias voltages applied to the respective photodiodes 330 are individually controlled to ensure the performance of the photodiodes 330.

4 is a view for explaining a compensation circuit according to another embodiment of the present invention. Fig. 4 is a diagram showing another example of the compensation circuit shown in Fig. 3. Fig. Therefore, the contents described above with respect to the compensation circuit are applied to the compensation circuit according to the embodiment of FIG. 4, even if omitted below.

Referring to FIG. 4, the compensation circuit further includes a reference voltage supplier 410. The reference voltage supply unit 410 supplies a reference bias voltage to the voltage adder 420. The reference voltage supplier 410 supplies the reference bias voltage to the voltage adder 420 located in each of the microcells 230. The reference voltage supply unit 410 supplies a voltage of a predetermined magnitude to the voltage adder 420. The supplied voltage may be initially applied to the photodiode 330 and the voltage adder 420 may supply the photodiode with a raised or lowered reference bias voltage. The reference voltage supply 410 may be implemented outside the pixel.

The voltage adder 420 is an example of the bias voltage supply unit 320. [ The voltage adder 420 is included in each of the micro cells 230 and receives the reference bias voltage from the reference voltage supply unit 410 and is controlled by the control unit 310.

5 is a diagram for explaining a compensation circuit according to another embodiment of the present invention. 5 is a diagram showing another example of the compensation circuit shown in FIG. Therefore, the contents described above with respect to the compensation circuit are applied to the compensation circuit according to the embodiment of FIG. 5, even if omitted below.

The reference voltage supply unit 410 includes a current source 411 and a reference diode. The current source 411 supplies a constant current to the reference diode 412. Therefore, the voltages at both ends of the reference diode 412 are always applied with a constant voltage. The reference voltage supplier 410 may supply a voltage applied to both ends of the reference diode 412 to the voltage adder 420 and supply a reference bias voltage having a predetermined magnitude to the voltage adder 420 .

Referring to FIG. 5, the microcell 230 includes a voltage adder 420, a resistor 510, and a photodiode 330. The bias voltage output from the voltage adder 420 is supplied to the photodiode 330 via a resistor. A node between the photodiode 330 and the resistor 510 is connected to the control unit 310. Accordingly, the controller 310 can monitor the voltage across the photodiode 330 or the current output from the photodiode 330.

6 is a flowchart illustrating a method of controlling a bias voltage according to another embodiment of the present invention. FIG. 6 is a flow chart for explaining that the compensation circuit of FIG. 3 controls the bias voltage of the photodiode. Therefore, the contents described above with respect to the compensation circuit are applied to the control method according to the embodiment of Fig. 6, even if omitted below.

The compensation circuit controls a bias voltage supplied to a plurality of photodiodes included in the pixel.

In operation 610, the control unit 310 of the compensation circuit monitors a dark count generated from the photodiodes. The control unit 310 monitors the dark count in real time. For example, the control unit 310 monitors the dark counts at regular intervals during the operation of the compensation circuit. That is, the control unit 310 counts how many dark counts generated from the respective photodiodes occur for a predetermined time.

In operation 620, the control unit 310 determines a photodiode that requires a change in the bias voltage in the photodiodes based on the dark count. The controller 310 monitors the ratio of the dark counts generated from all the photodiodes and determines the photodiodes that are higher or lower than the reference values for which the dark count ratio is set. For example, monitor whether there is a photodiode that has a dark ratio lower than the minimum reference value or a dark ratio higher than the maximum reference value.

In step 630, the compensation circuit changes the bias voltage supplied to the determined photodiode. The control unit 310 determines whether to increase or decrease the bias voltage of the determined photodiode. The control unit 310 also determines how much the bias voltage is to be changed, and outputs a control signal to the bias voltage supply unit 320. The bias voltage supplier 320 receiving the control signal applies a voltage of a predetermined magnitude to the reference bias voltage and supplies the voltage to the photodiode. For example, if the ratio of the dark count to the minimum reference value is low, the control unit 310 outputs a control signal to the bias voltage supply unit 320 instructing the bias voltage to be lowered by a predetermined voltage. If the ratio of the dark count to the maximum reference value is high, the control unit 310 outputs a control signal to the bias voltage supply unit 320 indicating that the bias voltage should be increased by a predetermined voltage.

The control unit 310 repeats steps 610 to 630. [ The controller 310 repeats steps 610 to 630 at predetermined intervals. The control unit 310 repeats the process of changing the bias voltage supplied to the photodiode, and controls the bias voltage so that the dark count ratio is maintained within the range of the set reference values. For example, the control unit 310 changes the bias voltage until the ratio of the dark counts generated in the photodiodes is larger than the minimum reference value and smaller than the maximum reference value.

7 is a graph for explaining a method of controlling a bias voltage applied to the photodiode. Referring to FIG. 7, FIG. 7 shows a dark count generated according to a bias voltage Vbias applied to the photodiode 330.

The first to third dark counts 710 to 730 represent voltages or currents output from the photodiode 330 when the bias voltages applied to the photodiode 330 are V1 to V3, respectively. Referring to FIG. 7, the first dark count 710 indicates that three dark counts have occurred for a predetermined time, the second dark count 720 indicates that five dark counts have occurred for a predetermined time, (730) indicates that eight dark counts have occurred during a predetermined time. In other words, the ratio of the second dark count 720 is higher than the ratio of the first dark count 710, and the ratio of the third dark count 730 is higher than the ratio of the second dark count 720.

For example, the ratio of the first dark count 710 may be a set minimum reference value, and the ratio of the third dark count 730 may be a set maximum reference value. Accordingly, the control unit 310 lowers the bias voltage of the photodiode that outputs a dark count lower than the ratio of the first dark count 710, and outputs a dark count of a ratio higher than the ratio of the third dark count 730 to the output The bias voltage of the photodiode is increased.

For example, a region where a dark count occurs as a ratio between the first dark count 710 and the third dark count 730 may be the Geiger region 740, and the ratio of the first dark count 710 and the dark count The ratio of three dark counts 730 may be varied to maintain the desired characteristics.

7 shows that the first to third dark counts 710 to 730 occur when the bias voltage is V1 to V3, but this is due to the characteristics of the specific photodiode, and the other photodiodes are different from each other And even if the same diode is used, the characteristics can be changed by factors such as a change in temperature or pressure, and the like. Accordingly, the controller 310 monitors the ratio of the dark count generated from the photodiode 330 and repeats the process of changing the bias voltage in real time to maintain the desired dark count ratio.

Meanwhile, the above-described method can be implemented in a general-purpose digital computer that can be created as a program that can be executed by a computer and operates the program using a computer-readable recording medium. In addition, the structure of the data used in the above-described method can be recorded on a computer-readable recording medium through various means. The computer-readable recording medium includes a magnetic storage medium (e.g., ROM, RAM, USB, floppy disk, hard disk, etc.), optical reading medium (e.g., CD-ROM, DVD, etc.).

215: Pixel
230: Microcell
310:
320: bias voltage supply unit
330: photodiode

Claims (14)

A compensation circuit for controlling a bias voltage supplied to a plurality of photodiodes included in a pixel, the compensation circuit comprising:
A controller for monitoring a dark count generated from the photodiodes and determining a photodiode requiring a change in a bias voltage in the photodiodes based on the dark count; And
And a bias voltage supplier for supplying a bias voltage to the determined photodiode under the control of the control unit. The method according to claim 1,
The control unit sets a reference value for the rate of the dark count and determines a photodiode for which a bias voltage needs to be changed based on a result of comparing the ratio of the dark count and the reference value A compensation circuit characterized by: 3. The method of claim 2,
Wherein the control unit sets the maximum value and the minimum value as the reference value and controls the bias voltage to be supplied to the determined photodiode to be lower when the ratio of the dark count is higher than the maximum value,
And controls the bias voltage supplied to the determined photodiode to be higher when the ratio of the dark count is lower than the lowest value. The method according to claim 1,
Wherein the plurality of microcells included in the pixel each include a voltage adder and the photodiode,
And a reference voltage supplier for supplying a reference voltage to the voltage adder. 5. The method of claim 4,
Wherein the voltage adder changes the bias voltage supplied to the photodiode by raising or lowering the reference voltage supplied from the reference voltage supply device. 5. The method of claim 4,
And the voltage adder changes the bias voltage under the control of the controller. The method according to claim 1,
Wherein the reference voltage supply comprises a reference diode and a current source connected in series. The method according to claim 1,
Wherein the controller monitors a dark count generated from each of the photodiodes in real time and changes a bias voltage supplied to the photodiode based on the monitoring in real time. The method according to claim 1,
Wherein the photodiode is a Silicon Photomultiplier (SiPM). A method for controlling a bias voltage supplied to a plurality of photodiodes included in a pixel,
Monitoring a dark count occurring from the photodiodes;
Determining a photodiode requiring a change in bias voltage among the photodiodes based on the dark count; And
And changing a bias voltage to be supplied to the determined photodiode. 11. The method of claim 10,
Wherein a reference value for the rate of the dark count is set in advance and a photodiode which requires a change in the bias voltage is determined based on a result of comparing the ratio of the dark count and the reference value Control method. 12. The method of claim 11,
Wherein the control circuit controls the bias voltage to be supplied to the determined photodiode to be lower when the ratio of the dark count is higher than the maximum value,
And when the ratio of the dark count is lower than the minimum value, control is performed to increase the bias voltage supplied to the determined photodiode. 11. The method of claim 10,
Monitors a dark count generated from each of the photodiodes in real time and changes the bias voltage supplied to the photodiode based on the monitoring in real time. A method of controlling a bias voltage supplied to a plurality of photodiodes included in a pixel, the method comprising the steps of:
Monitoring a dark count occurring from the photodiodes;
Determining a photodiode requiring a change in bias voltage among the photodiodes based on the dark count; And
And changing a bias voltage supplied to the determined photodiode.

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