JP2005168961A - Radiation imaging equipment - Google Patents

Abstract

Provided is a radiographic imaging apparatus capable of correcting a phenomenon in which the sensitivity of a detector decreases with the cycle of X-ray pulse exposure and data reading after a detector reset (refresh) operation. .
In a radiographic imaging apparatus that captures an X-ray image using a detector whose sensitivity varies upon receiving X-ray irradiation, a sensitivity deterioration function of the detector is maintained and the detector is reset (refreshed). In the correction of the data of the multiple X-ray exposure and detector readout cycles since then, the deterioration function determines the cumulative X-ray detection amount and / or readout cycle number and sensitivity since the detector is reset (refreshed). It is a sensitivity degradation function that represents the relationship of degradation, and has means for calculating the cumulative X-ray detection amount and cycle number of multiple X-ray exposure and detector readout cycles, and based on the cumulative detection amount and cycle number Correct the read data of each cycle.
[Selection] Figure 3

Description

  The present invention relates to a radiographic image capturing apparatus, and more particularly to an X-ray digital tomographic apparatus using a flat panel or a moving image capturing apparatus. More particularly, the present invention relates to a method or apparatus for correcting sensitivity depending on the accumulated read charge amount from sensor reset (refresh).

  When the X-ray detector is used continuously, dynamic sensitivity correction may be necessary. In Patent Document 1, in a solid state detector for CT, a second detector that does not change with time is used to measure in advance a solid-state detector temporal sensitivity deterioration function and a temporal sensitivity return function from the immediately preceding scan. A technique for correcting data received by a detector is disclosed.

  Patent Document 2 uses an output of a second X-ray monitor that detects a dose variation at the time of radiography by storing a nonlinear characteristic with respect to the X-ray intensity and using it as a sensitivity correction function. Thus, a technique for correcting the dose fluctuation and correcting the sensitivity is disclosed.

JP-A-8-243101 Japanese Patent Laid-Open No. 10-272125

  However, the data correction means depends on the structure of the X-ray detector, and it is not possible to correct the sensitivity of a detector such as a detector having a MIS structure with a correction function as conventionally proposed. It was. Specifically, the MIS structure sensor requires a reset operation, and the sensitivity differs depending on the factor from the reset operation, and correction related to moving image shooting in which a plurality of light inputs and a plurality of readout operations are performed from one reset. It was necessary to develop a method.

  The present invention has been made in view of the above problems, and its intended process is that the sensitivity of the detector increases with the cycle of X-ray pulse exposure and data reading after the reset (refresh) operation of the detector. An object of the present invention is to provide a radiographic imaging apparatus capable of correcting a phenomenon that decreases.

  In order to achieve the above object, the present invention, in a radiographic imaging apparatus that captures an X-ray image using a detector that varies in sensitivity upon receiving X-ray irradiation, maintains a sensitivity deterioration function of the detector, In correcting data for a plurality of X-ray exposure and detector read cycles since the reset (refresh) operation of the detector, the deterioration function is accumulated X since the reset (refresh) operation of the detector. A sensitivity deterioration function representing a relationship between sensitivity detection and a line detection amount and / or number of readout cycles, and means for calculating a cumulative X-ray detection amount and cycle number of the plurality of X-ray exposure and detector readout cycles. And reading data of each cycle is corrected based on the accumulated detection amount and the number of cycles.

  According to the present invention, it has become possible to correct a phenomenon in which the sensitivity of the detector is lowered with the cycle of X-ray pulse exposure and data reading after the reset (refresh) operation of the detector.

  FIG. 1 shows an equivalent circuit of one MIS structure photodetector. In the following example, a two-dimensional amorphous silicon sensor will be described. However, the detection element is not particularly limited, and can be similarly applied to a sensor having different sensitivity characteristics depending on a factor from the reset operation.

  The configuration of one element is composed of a light detection unit 21 and a switching TFT 22 that controls charge accumulation and reading, and is generally formed of amorphous silicon (α-Si) disposed on a glass substrate. In this example, 21C in the light detection unit 21 may be simply a photodiode having a parasitic capacitance, and is regarded as a photodetector including an additional capacitor 21C in parallel so as to improve the dynamic range of the photodiode 21D and the detector. May be.

  The anode A of the diode 21D is connected to a bias line Lb that is a common electrode, and the cathode K is connected to a controllable switching TFT 22 for reading out the electric charge accumulated in the capacitor 21C. In this example, the switching TFT 22 is a thin film transistor connected between the cathode K of the diode 21D and the charge readout amplifier 26.

  The signal charge is generated by operating the switching TFT 22 and the reset switching element 25 to reset the capacitor 21C, and then radiating X-rays according to the amount of radiation by the photodiode 21D (converting the X-rays into visible light). The scintillator to be converted is not shown) and is stored in the capacitor 21C. Thereafter, the signal charge is transferred again to the capacitive element 23 by operating the switching TFT 22 and the reset switching element 25 again. Then, the amount accumulated by the photodiode 21D is read out by the preamplifier 26 as a potential signal, and the incident radiation dose is detected by performing A / D conversion.

  FIG. 2 is an equivalent circuit diagram showing a two-dimensionally arranged photoelectric conversion device. A photoelectric conversion operation in a case where the photoelectric conversion element shown in FIG. 1 is specifically expanded in two dimensions will be described.

  The pixels of the photodetector are composed of about 2000 × 2000 to 4000 × 4000 pixels, and the sensor area is about 200 mm × 200 mm to 500 mm × 500 mm. In FIG. 2, the photodetector is composed of 4096 × 4096 pixels, and the sensor area is 430 mm × 430 mm. Therefore, the size of one pixel is about 105 μm × 105 μm. Each pixel is arranged two-dimensionally by wiring 4096 pixels in one block in the horizontal direction and arranging 4096 lines in order vertically.

  In the above example, an example in which a 4096 × 4096 pixel photodetector is configured by one substrate is shown, but a 4096 × 4096 pixel photodetector is detected by four optical detectors having 2048 × 2048 pixels. It can also be configured with a vessel. When one photo detector is configured by four 2048 × 2048 detectors, there is an advantage that the yield is improved by manufacturing it separately.

  As described above, one pixel includes the photoelectric conversion element 21 and the switching TFT 22. 21 (1, 1) to 21 (4096, 4096) correspond to the photoelectric conversion element 21 described above, and K represents the cathode side and A represents the anode side of the photodetecting diode. Reference numerals 22 (1, 1) to 22 (4096, 4096) correspond to the switching TFT 22.

  The K electrodes of the photoelectric conversion elements 21 (m, n) in each column of the two-dimensional photodetector are connected to common column signal lines (Lc1 to Lc4096) for the column by the source and drain conductive paths of the corresponding switching TFT 22 (m, n). )It is connected to the. For example, the photoelectric conversion elements 21 (1, 1) to 21 (1,4096) in the column 1 are connected to the first column signal wiring Lc1. The A electrodes of the photoelectric conversion elements 21 in each row are commonly connected to a bias power source 31 that operates the above-described mode through a bias wiring Lb. The gate electrode of the TFT 22 in each row is connected to the row selection wiring (Lr1 to Lr4096). For example, the TFTs 22 (1, 1) to 22 (4096, 1) in the row 1 are connected to the row selection wiring Lr1. The row selection wiring Lr is connected to the imaging control unit 33 through the line selector unit 32. The line selector unit 32 includes, for example, an address decoder 34 and 4096 switch elements 35. With this configuration, any line Lrn can be read. The line selector unit 32 can be configured simply by a shift register used in a liquid crystal display or the like if it is most simply configured.

  The column signal wiring Lc is connected to a signal readout unit 36 that is controlled by an imaging control unit 33 (not shown). 25 is a switch for resetting the column signal wiring Lr to the reference potential of the reset reference power supply 24, 26 is a preamplifier for amplifying the signal potential, 38 is a sample and hold circuit, 39 is an analog multiplexer, and 40 is an A / D. Each converter is represented. The signal of each column signal line Lrn is amplified by the preamplifier 26 and held by the sample hold circuit 38. The output is sequentially output to the A / D converter 40 by the analog multiplexer 39, converted into a digital value, and transferred to the image processing unit 10 (not shown).

  The photoelectric conversion device of the present embodiment divides 4096 × 4096 pixels into 4096 lines Lcn and simultaneously transfers the output of 4096 pixels per column, and 4096 preamplifiers 26 through the column signal wiring Lc, The signals are sequentially output to the A / D converter 40 by the analog multiplexer 39 through the 4096 sample hold units 38. In FIG. 2, the A / D converter 40 is represented as if constituted by one, but in reality, A / D conversion is simultaneously performed in 4 to 32 systems. This is because it is required to shorten the reading time of the image signal without unnecessarily increasing the analog signal band and the A / D conversion rate.

  FIG. 3 is a block diagram conceptually illustrating the present invention.

  The charge information from the sensor is amplified by the readout circuit and then AD converted. The digitized data is sent to the offset circuit, and at the same time, the accumulation value is calculated for each channel (element) by the accumulation circuit. The sensor is reset by a reset circuit prior to reception of X-rays. Here, the refresh and reset described in FIG. 1 are used synonymously.

  FIG. 4 shows an offset characteristic from reset of the MIS type sensor used in the present invention. The offset value is largely exponentially attenuated immediately after reset (refresh). For this reason, since it is 100-200 ms immediately after the refresh, it is used after the offset value becomes small. The drive history of the drive circuit records the time until the radiation image capturing apparatus that can be recorded by the drive recording circuit is issued for each readout operation. This recording is performed separately in the calibration mode for creating the correction coefficient and in the actual photographing mode.

  5 and 6 show schematic characteristics of data recorded in the calibration mode.

  FIG. 5 shows how the gain fluctuates with respect to the accumulated charge read from reset. In the calibration mode, a fixed amount of X-ray exposure is continuously performed after resetting, and the output value during that period is recorded for each channel. This drift in gain cannot be clearly separated as to whether it depends on (1) drift in the charge generation process of the detector or (2) drift in the charge extraction process from the capacitor. This will be described as a characteristic depending on.

  FIG. 6 similarly shows data depending on the number of readings for each element.

  In the data collection process of FIG. 5, attention is paid to the fact that gain fluctuation occurs with respect to the same cumulative readout when the incident X-ray dose (light quantity) is reduced and increased with respect to one readout.

  FIG. 6 shows that the gain decreases as the number of readouts increases even with the same accumulated readout charge. It should be noted that in FIG. 6, there is no gain drift until the number of readings is a certain number. That is, it is indicated that it is not necessary to perform gain correction according to the number of readings until a certain number of readings. Such a limit reading number is stored in the control circuit, and the gain correction based on the reading number is not performed at the reading number that does not require correction.

  The recording by the drive recording circuit at the time of calibration is the recording of time data for creating the correction coefficients shown in FIGS. The characteristics shown in FIGS. 4 to 6 are calculated from the actual read data based on the time axis data. However, storing the characteristics with a fine time resolution or a fine accumulated charge resolution means that the correction coefficient section Requires a huge amount of memory.

  Therefore, the coefficients can be stored with coarse resolution, and the coefficients corresponding to the stored points can be obtained by interpolation calculation. The creation of a recording coefficient at the time of calibration data and the calculation of a corresponding correction coefficient at the time of shooting mode can be performed by a DSP (Digital Signal Processor) or CPU without preparing a special circuit.

  The driving at the time of photographing depends on the photographing engineer who operates the apparatus through the console. That is, the imaging dose and the moving image capturing speed differ depending on the imaging region (heart, brain, stomach). It also depends on whether the patient is a child or an adult.

  In accordance with an instruction from the console, the control circuit controls the X-ray exposure timing and intensity via the X-ray control circuit, controls the sensor drive via the drive circuit, and outputs the corrected data to the image output circuit. This is done by controlling

The offset correction circuit is realized by difference calculation. Assume that the data Dorg read in the shooting mode (the drive time is recorded in the drive recording circuit) is the integration from the time t1 to the time t2 from the reset, and the offset characteristic of the element (corresponding to FIG. 4) ) Is offset (t1) and offset (t2) respectively, the offset correction value Doff is

Doff = Dorg- (offset (t1) + offset (t2)) / 2
It can be expressed as

Depending on the storage format of the correction factor,

Doff = Dorg-offset ((t1 + t2) / 2)
It is also good. Since the offset characteristic is stored for each element, it is performed for each element.

The gain correction circuit is realized for each element by multiplication. The output of the accumulation circuit when the offset-corrected Doff is read is Dcul. Note that the latest read Dorg is also added to the cumulative value Dcul. Since the gain correction is basically correction between elements, the correction coefficient for adjusting the gain of all elements constituting the sensor is Gij.
Dgain = Gij ・ C (Dcul) ・ Doff
Here, C (Dcul) is a cumulative charge component gain correction coefficient when the output of the cumulative circuit is Dcul. Note that the data is offset corrected before calculating the gain drift when creating C (Dcul) coefficients during calibration.
Furthermore, when the gain correction of the read count component is performed,
Dgain = Gij ・ C (Dcul) ・ T (n) ・ Doff
It becomes.

  Here, T (n) is a read count component gain correction coefficient when the read count is n.

  As a reminder, the above expression shows the operation in linear space, not the operation in LOG space after LOG conversion. To convert the X-ray data to the attenuation coefficient space, it is necessary to perform LOG conversion on the data obtained by the above equation.

  In the above description, the cumulative read data value and the cumulative read charge amount are used synonymously. In addition to the above cases, the offset value may vary depending on the incident dose. In this case, at the time of calibration, readout with X-ray exposure and offset readout are repeatedly performed, and the variation in offset value is converted into a coefficient using the accumulated charge readout amount after reset as a parameter and stored for correction in the imaging mode. It is possible to use.

  The present invention is particularly applicable to an X-ray digital tomography apparatus or a moving image imaging apparatus using a flat panel.

It is a figure which shows a sensor equivalent circuit. It is a circuit diagram of a flat sensor panel. It is a block diagram of the Example of this invention. It is a figure which shows the example of an offset characteristic. It is a figure which shows the example of the gain characteristic drift depending on the accumulation read charge amount. It is a figure which shows the example of the gain characteristic drift depending on the frequency | count of reading.

Explanation of symbols

10 Image processing unit 21 Photodetection unit 22 Switching TFT
DESCRIPTION OF SYMBOLS 23 Capacitance element 25 Reset switching element 26 Preamplifier 31 Bias power supply 32 Line selector 33 Imaging control part 39 Analog multiplexer 40 A / D converter

Claims (4)

In a radiographic imaging apparatus that captures an X-ray image using a detector that varies in sensitivity upon receiving X-ray irradiation,
The sensitivity deterioration function of the detector is retained with respect to the read data of the detector in a plurality of X-ray exposure and detector read cycles since the detector's sensitivity deterioration function is retained and the detector is reset (refreshed). A radiographic imaging apparatus characterized by correcting data by applying a function.   The deterioration function is a sensitivity deterioration function that represents the relationship between the accumulated X-ray detection amount and the sensitivity deterioration since the detector was reset (refreshed), and the deterioration function includes a plurality of X-ray exposure and detector readout cycles. The radiographic imaging apparatus according to claim 1, further comprising means for calculating a cumulative X-ray detection amount, and correcting read data of each cycle based on the cumulative detection amount.   The deterioration function is a sensitivity deterioration function that represents the relationship between the number of read cycles since the reset (refresh) operation of the detector and the sensitivity deterioration, and the number of cycles of the multiple X-ray exposure and detector read cycles. The radiographic image capturing apparatus according to claim 1, further comprising: a calculating unit that corrects read data of each cycle based on the number of cycles.   The deterioration function is a sensitivity deterioration function that represents the relationship between the cumulative X-ray detection amount and the number of read cycles since the reset (refresh) operation of the detector and the number of read cycles, and sensitivity deterioration. 2. The apparatus according to claim 1, further comprising means for calculating a cumulative X-ray detection amount and a cycle number of the device readout cycle, and correcting read data of each cycle based on the cumulative detection amount and the cycle number. Radiation imaging device.

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