JP6305080B2 - Information processing apparatus, radiation imaging system, information processing method, and program - Google Patents

Description

  The present invention relates to an information processing apparatus, a radiation imaging system, an information processing method, and a program, and more particularly to a technique for performing sensitivity correction processing on radiation image data.

  In recent years, FPDs (Flat Panel Detectors) in which solid-state image sensors composed of single crystal silicon, amorphous silicon, or the like are two-dimensionally arranged have been widely put into practical use. Such a solid-state imaging device is also used when taking a radiation image in a medical device. In a typical FPD, radiation such as X-rays transmitted through a subject is captured by an image sensor directly or through light emission of a phosphor that reacts to the radiation, and converted into electric charge. Then, by driving a TFT (thin film transistor) connected to the image sensor, electrical signals based on the electric charges accumulated in the image sensor are sequentially read out, and radiation image data reflecting subject information based on the electrical signals Is generated.

  It is known that the sensitivity of an image sensor temporarily decreases due to residual charges generated in the image sensor when a high dose of radiation is irradiated. In order to cope with this problem, Patent Document 1 discloses that an increase in the offset component caused by X-ray irradiation is obtained, and a sensitivity correction coefficient is calculated according to information indicating a relationship between the offset increase and the sensitivity characteristic. ing. By using this sensitivity correction coefficient, image correction is performed so as to cancel out a decrease in sensitivity of the image sensor due to radiation irradiation. Furthermore, Patent Document 2 discloses that a sensitivity at a predetermined time after X-ray irradiation is obtained using a mathematical model representing sensitivity deterioration and recovery characteristics caused by X-ray irradiation. In accordance with the sensitivity thus obtained, image correction is performed so as to compensate for sensitivity deterioration of the image sensor due to radiation irradiation.

JP2008-237920A JP2003-156567

  On the other hand, the sensitivity of the image sensor decreases with time during imaging standby. This is because the charge due to dark current accumulates in the capacitance existing in the image sensor, resulting in a potential difference, resulting in a decrease in the net bias voltage applied to the photoelectric conversion unit in the image sensor. This is thought to be due to a decrease in conversion efficiency. Here, the imaging standby state refers to a state in which a reverse bias is applied to the image sensor after the FPD is turned on, and the image sensor can accumulate signals when radiation or light is incident.

  However, the methods described in Patent Documents 1 and 2 compensate for a decrease in sensitivity of the image sensor caused by radiation irradiation, and do not compensate for a decrease in sensitivity over time during standby for imaging.

  An object of the present invention is to compensate for a decrease in sensitivity over time during radiography in radiography.

In order to achieve the object of the present invention, for example, an information processing apparatus of the present invention comprises the following arrangement. That is,
Time acquisition means for acquiring a standby time from when the radiation imaging apparatus starts imaging standby until imaging of a radiation image;
Determining means for determining a correction amount of the radiation image according to the waiting time;
It is characterized by providing.

  In radiography, it is possible to compensate for a decrease in sensitivity over time during imaging standby.

FIG. 3 is a diagram illustrating a functional configuration of the image acquisition device according to the first embodiment. 1 is a diagram illustrating a functional configuration of a radiation imaging system according to Embodiment 1. FIG. FIG. 3 is a diagram illustrating a hardware configuration of the image acquisition apparatus according to the first embodiment. 5 is a flowchart of processing performed by the image acquisition apparatus according to the first embodiment. The figure explaining the time characteristic data regarding the fall of imaging sensitivity. The figure explaining the time characteristic data regarding the fall of imaging sensitivity. FIG. 4 is a diagram illustrating a functional configuration of an image acquisition apparatus according to a second embodiment. FIG. 5 is a diagram illustrating a functional configuration of a radiation imaging system according to a second embodiment. FIG. 4 is a diagram illustrating a hardware configuration of an image acquisition apparatus according to a second embodiment. 10 is a flowchart of processing performed by the image acquisition apparatus according to the second embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the scope of the present invention is not limited to the following embodiments.

[Embodiment 1]
First, Embodiment 1 of the present invention will be described. FIG. 1A shows a functional configuration of an image acquisition apparatus 100 according to the present embodiment. As illustrated in FIG. 1A, the image acquisition device 100 includes an imaging unit 101, a storage unit 102, a monitoring unit 103, a parameter acquisition unit 104, an estimation unit 105, and a correction unit 106.

  The imaging unit 101 captures a radiographic image and acquires radiographic image data. The storage unit 102 stores time characteristic data indicating the sensitivity change of the photographing sensitivity by the photographing unit 101 that is acquired in advance. This time characteristic data indicates a change in photographing sensitivity by the photographing unit 101 in accordance with a photographing standby time from when the photographing unit 101 starts photographing standby to when photographing is performed. In the present embodiment, the time characteristic data more specifically indicates the relationship between the imaging standby time and the amount of sensitivity reduction of imaging by the imaging unit 101. The storage unit 102 outputs this time characteristic data as necessary.

  The monitoring unit 103 measures the elapsed time since the imaging unit 101 started the imaging standby, and outputs this elapsed time. That is, the monitoring unit 103 can acquire and output a shooting standby time from when the shooting unit 101 starts shooting standby until shooting. Normally, when the standby for photographing is started, a refresh operation, that is, an operation for discharging the charge accumulated in the image pickup device included in the photographing unit 101 is performed. In other words, this elapsed time can be considered as the elapsed time since the last refresh operation, and the shooting standby time is the shooting standby time from the last refresh operation to shooting. Can think.

  The estimation unit 105 and the correction unit 106 determine the correction amount of the radiation image according to the imaging standby time counted by the monitoring unit 103. In the present embodiment, the estimation unit 105 and the correction unit 106 further acquire time characteristics, that is, acquire time characteristic data of the photographing unit 101, and determine the correction amount with reference to the time characteristic data. That is, the estimation unit 105 estimates the amount of reduction in sensitivity of photographing by the photographing unit 101 using the photographing standby time and time characteristic data, and the correction unit 106 calculates a correction amount that compensates for the estimated sensitivity reduction.

  In another embodiment, time characteristic data indicating the relationship between the imaging standby time and the correction amount can be used. In this case, the correction unit 106 can also calculate a correction amount to be applied to the radiation image using the imaging standby time and time characteristic data. In a further embodiment, the estimation unit 105 and the correction unit 106 may calculate the sensitivity reduction amount or the correction amount according to the shooting standby time according to a predetermined mathematical formula without using the time characteristic data.

  In this embodiment, referring to shooting information indicating the status of the shooting unit 101, time characteristic data used for estimation of the sensitivity reduction amount is selected from a plurality of time characteristic data. In the present embodiment, the shooting information includes at least one of sensor internal temperature information and gain setting information at the time of shooting. The parameter acquisition unit 104 acquires such shooting information from the shooting unit 101. The storage unit 102 stores a plurality of time characteristic data corresponding to, for example, the sensor internal temperature and the gain setting according to the status of the photographing unit 101. The estimation unit 105 acquires time characteristic data corresponding to the shooting information acquired from the parameter acquisition unit 104 from the storage unit 102.

  That is, in the present embodiment, the estimation unit 105 acquires time characteristic data from the storage unit 102, elapsed time from the monitoring unit 103, and information indicating the sensor internal temperature and gain setting from the parameter acquisition unit 104. And the estimation part 105 estimates the sensitivity fall amount at the time of imaging | photography by the imaging | photography part 101 using these information, and outputs an estimation result. The correction unit 106 acquires the sensitivity reduction amount estimated by the estimation unit 105 and corrects the radiation image data received from the imaging unit 101 according to the sensitivity reduction amount. However, the estimation unit 105 may acquire time characteristic data from the outside of the image acquisition device 100 or may be acquired from the imaging unit 101.

  As illustrated in FIG. 1B, the function of the present embodiment may be realized by a radiation imaging system 110 including a radiation imaging apparatus 120 and an information processing apparatus 130. The radiation imaging apparatus 120 includes the imaging unit 101, the monitoring unit 103, and the parameter acquisition unit 104 described above. Further, the information processing apparatus 130 includes the storage unit 102, the estimation unit 105, and the correction unit 106 described above. In this case, the radiation imaging apparatus 120 outputs the captured radiation image and the measured imaging standby time to the information processing apparatus 130. Then, the information processing apparatus 130 calculates a correction amount for sensitivity correction according to the acquired shooting standby time. The information processing apparatus 130 can perform correction processing of a radiographic image using this correction amount. That is, the information processing apparatus 130 and the radiation imaging system 110 include a determination unit 131 configured by the estimation unit 105 and the correction unit 106 that determines the correction amount of the radiographic image according to the standby time. Further, the information processing apparatus 130 and the radiation imaging system 110 include a time acquisition unit that acquires a standby time from when the radiation imaging apparatus 120 starts imaging standby until imaging of a radiation image and supplies the standby time to the determination unit 131. Yes.

  FIG. 2 shows a hardware configuration of the image acquisition apparatus 100 according to the present embodiment. As illustrated in FIG. 2, the image acquisition apparatus 100 is connected to a radiation generation apparatus 210 and an image display apparatus 220. The radiation generator 210 irradiates the subject with radiation. The image acquisition device 100 acquires radiation image data of a subject by detecting the radiation that has passed through the subject and converting it to an electrical signal. The image acquisition device 100 outputs the radiation image data to the image display device 220 and causes the image display device 220 to display the radiation image.

  The image acquisition apparatus 100 includes an imaging unit 1010, a storage unit 1020, a control unit 1030, and a parameter acquisition unit 1040.

  The imaging unit 1010 generates radiation image data by converting the radiation transmitted through the subject into an electrical signal. Further, the imaging unit 1010 generates dark image data according to an electrical signal obtained when the radiation generator 210 is not radiating radiation in order to correct an offset component of an image such as a dark current component.

The imaging unit 1010 includes a drive circuit 1011, a sensor 1012, and a readout circuit 1013. Under the control of the control unit 1030, the drive circuit 1011 drives each unit included in the imaging unit 101 so as to perform imaging. The sensor 1012 converts incident radiation into an electrical signal. In one embodiment, the sensor 1012 includes a solid-state imaging device arranged in a grid on a two-dimensional plane made of amorphous silicon, and a phosphor that converts a radiation signal into a visible light signal. The phosphor is not particularly limited, CsI: can be used Tl or _Gd 2 _{O 2} S or the like. The phosphor converts the radiation transmitted through the subject into visible light, and the solid-state imaging device further converts the visible light into a charge signal. The readout circuit 1013 reads out an electrical signal based on the charge signal accumulated in the solid-state imaging device.

  The parameter acquisition unit 1040 acquires status information from the imaging unit 1010 and generates imaging information including the acquired status information. Here, the shooting information includes status information such as the internal temperature of the sensor 1012 during or before shooting by the shooting unit 1010 and the gain setting of the shooting unit 1010. However, the photographing information may include other information. In the present embodiment, the imaging unit 1010 and the parameter acquisition unit 1040 are independent, but the imaging unit 1010 may include the parameter acquisition unit 1040.

  The storage unit 1020 includes a radiation image storage unit 1021, a dark image storage unit 1022, a sensitivity data storage unit 1023, and a parameter storage unit 1024. The radiation image storage unit 1021 stores the radiation image data acquired by the imaging unit 1010. The dark image storage unit 1022 stores dark image data acquired by the imaging unit 1010. The parameter storage unit 1024 stores the shooting information acquired by the parameter acquisition unit 1040. After the shooting is finished, a sensitivity reduction amount estimation circuit 1051 described later acquires the stored shooting information.

  The sensitivity data storage unit 1023 stores sensitivity correction data used for sensitivity correction and time characteristic data related to a decrease in shooting sensitivity by the shooting unit 101 that is acquired in advance before shooting by the shooting unit 101. Here, the sensitivity correction data includes white image data and manufacturing variation data. Here, the white image data is radiation image data obtained when radiation is uniformly applied to the entire effective pixel region of the sensor 1012 without setting a subject. This white image data corresponds to variation information that reflects variations in sensitivity of each of the plurality of imaging elements included in the sensor 1012. The white image data can be acquired when necessary under the control of the control unit 1030.

  The manufacturing variation data is data indicating manufacturing variations in sensitivity for each photographing unit 101. The manufacturing variation data may be data representing the ratio of the sensitivity of the sensor 1012 to the sensitivity target value set for each model of the image acquisition device 100, for example. The manufacturing variation data stored in the sensitivity data storage unit 1023 may be acquired in advance when the image acquisition apparatus 100 is shipped from the factory. This data indicates the manufacturing variation in sensitivity immediately after the start of photographing standby. In the present embodiment, the manufacturing variation data does not reflect a decrease in sensitivity over time in the imaging standby state.

  The control unit 1030 controls driving of each unit by the drive circuit 1011. In addition, the control unit 1030 monitors an elapsed time after the photographing unit 1010 starts photographing standby. After the shooting is finished, the sensitivity reduction amount estimation circuit 1051 described later acquires from the control unit 1030 an elapsed time (shooting standby time) from when shooting standby is started until shooting is performed.

  The image processing unit 1050 includes a sensitivity reduction amount estimation circuit 1051, an offset correction processing circuit 1052, a sensitivity correction processing circuit 1053, a frequency processing circuit 1054, a gradation processing circuit 1055, and a defect correction processing circuit 1056.

  The sensitivity reduction amount estimation circuit 1051 acquires time characteristic data from the sensitivity data storage unit 1023, imaging information from the parameter storage unit 1024, and imaging standby time from the control unit 1030. Then, the sensitivity reduction amount estimation circuit 1051 estimates the sensitivity reduction amount of photographing by the photographing unit 1010 at the time of photographing based on such information.

  The offset correction processing circuit 1052 receives the radiation image data and dark image data acquired by the imaging unit 1010 from the radiation image storage unit 1021 and the dark image storage unit 1022. Then, the offset correction processing circuit 1052 performs offset correction processing on the radiation image data using the dark image data. The offset correction process can be performed according to a known technique, and this process can reduce the influence of the electrical signal generated on the radiographic image even when no radiation is incident on the sensor 1012.

  The sensitivity correction processing circuit 1053 corrects the radiographic image data so as to compensate for the variation in the pixel value of the radiographic image due to the difference in the sensitivity of the pixels of the sensor 1012. Specifically, the sensitivity correction processing circuit 1053 acquires sensitivity correction data from the sensitivity data storage unit 1023 and acquires the sensitivity decrease amount estimated from the sensitivity decrease amount estimation circuit 1051. Then, the sensitivity correction processing circuit 1053 performs sensitivity correction processing on the radiation image data using the sensitivity correction data and the sensitivity decrease amount. Details of the processing by the sensitivity correction processing circuit 1053 will be described later.

  The frequency processing circuit 1054 performs image processing such as frequency enhancement processing and noise suppression processing on the radiation image data. The gradation processing circuit 1055 performs gradation processing on the radiation image data based on dose information at the time of imaging, part information on the subject, and the like. The defect correction processing circuit 1056 performs defect correction processing on the radiation image data based on previously acquired defect data indicating defective pixels.

  At least a part of the functions of the image processing unit 1050 realized by the circuits 1051 to 1056 may be realized by a CPU provided in the image processing unit 1050 executing a computer program. For example, the image processing unit 1050 can be realized using a general-purpose computer. In this case, a storage medium storing a computer program including instructions for causing the CPU to perform processing realized by the image processing unit 1050 is supplied. Then, the function of the image processing unit 1050 can be realized by loading the program into a memory such as a RAM provided in the image processing unit 1050 and the CPU operating according to the program. In another embodiment, at least a part of the functions of the image processing unit 1050 may be realized by a GPU or a dedicated processing board.

  The imaging unit 101 in FIGS. 1A and 1B corresponds to the imaging unit 1010 in FIG. The storage unit 102 in FIGS. 1A and 1B corresponds to the storage unit 1020 in FIG. Further, the monitoring unit 103 in FIGS. 1A and 1B corresponds to the control unit 1030 in FIG. Also, the parameter acquisition unit 104 in FIGS. 1A and 1B corresponds to the parameter acquisition unit 1040 in FIG. Further, the estimation unit 105 and the correction unit 106 in FIGS. 1A and 1B correspond to the sensitivity decrease amount estimation circuit 1051 and the sensitivity correction processing circuit 1053 in FIG.

  Next, processing of the image acquisition apparatus 100 according to the present embodiment will be described with reference to FIG. In step S300, the photographing unit 101 is turned on. At this time, the sensor 1012 is similarly turned on. After the power is turned on, the sensor 1012 may wait for a certain time until the characteristics of the sensor 1012 are stabilized.

  In step S301, the photographing unit 101 starts photographing standby. Specifically, the control unit 1030 waits in a state where a reverse bias voltage is applied to each imaging element in the sensor 1012 so that the signal can be accumulated when a radiation signal is incident. The drive circuit 1011 is controlled. If this photographing standby is continued, the charge due to the dark current accumulates in the capacitance existing in the sensor 1012, and thus the net bias voltage applied to each image sensor in the sensor 1012 decreases. For this reason, the sensitivity of each image sensor in the sensor 1012 decreases with time.

  In step S <b> 302, the monitoring unit 103 monitors the elapsed time from the start of shooting standby.

  In step S <b> 303, the monitoring unit 103 determines whether radiation incident on the imaging unit 101 has been detected. If radiation is detected, the process proceeds to step S304. If radiation has not been detected, the process returns to step S302 and imaging standby is continued.

  The monitoring unit 103 may detect the incidence of radiation when a radiation irradiation signal is received from the radiation generator 210. This radiation irradiation signal is transmitted from the radiation generation apparatus 210 when the radiation generation apparatus 210 starts irradiation of radiation.

  Further, the image acquisition device 100 may have a function of detecting radiation. For example, the image acquisition apparatus 100 may include a detection unit for detecting the incidence of radiation separately from the sensor 1012. Further, the image acquisition apparatus 100 may detect the incidence of radiation by a method such as reading out charges from the image sensor of the sensor 1012 and determining whether the amount of charges exceeds a threshold value.

  For example, the image acquisition apparatus 100 can detect the incidence of radiation by reading out charges from the image sensor for each row during imaging standby. Specifically, after a bias voltage is applied to each row so that charge can be accumulated, the charge is sequentially read out row by row. When the incidence of radiation is detected, the readout of charges is stopped, and the drive for causing all image sensors to accumulate charges is performed. In particular, when an MIS type image sensor is used, it is known that even if the charge is read out, the sensitivity gradually decreases unless the refresh operation is performed. Therefore, sensitivity correction is performed as in the present embodiment. It is effective.

  However, it is not essential to detect the incidence of radiation, and the sensor 1012 may continuously accumulate charges and read out the charges after receiving a signal indicating that radiation irradiation has ended.

  In step S304, the imaging unit 101 acquires radiation image data. At this time, the drive circuit 1011 reads out the charge signal based on the radiation by sequentially performing the drive for accumulating the charge by radiation and the drive for reading out the accumulated charge signal. The drive for charge accumulation includes a drive for controlling the charge accumulation time after the start of radiation incidence, a drive for stopping the readout of the charge for detecting the incidence of radiation, and the like. It is.

  In addition, after acquiring the radiographic image data, the imaging unit 101 acquires dark image data in order to remove the dark current component included in the radiographic image data by offset correction. Specifically, the imaging unit 101 accumulates charges for the same accumulation time as when radiation image data is acquired in a state in which radiation is not irradiated, and reads out the accumulated charge signal, thereby obtaining dark image data. get. The acquired radiation image data and dark image data are stored in the radiation image storage unit 1021 and the dark image storage unit 1022.

  Further, the status information of the imaging unit 101 when the radiation image data is acquired is acquired by the parameter acquisition unit 104 and stored in the parameter storage unit 1024 as imaging information. As described above, the imaging information includes the internal temperature information of the sensor 1012 and the gain setting information. Furthermore, the monitoring unit 103 acquires an elapsed time from the start of shooting standby to the start of shooting. In one embodiment, the time from the start of imaging standby to the start of driving for charge accumulation is used as the elapsed time. However, the time from the start of imaging standby until the detection of radiation incidence may be used as the elapsed time.

  In step S305, the offset correction processing circuit 1052 acquires radiation image data and dark image data from the radiation image storage unit 1021 and the dark image storage unit 1022, respectively. Then, the image data after the offset correction is generated by subtracting the dark image data from the radiation image data.

  In step S <b> 306, the sensitivity reduction amount estimation circuit 1051 selects appropriate time characteristic data from the sensitivity data storage unit 1023 according to the imaging information received from the parameter storage unit 1024. In the present embodiment, this time characteristic data indicating the sensitivity reduction is a look-up table (LUT) that represents the correspondence between the elapsed time from the start of shooting standby to the start of shooting and the amount of reduction in sensitivity. This correspondence can be expressed by a curve shown in FIG. However, the configuration of the time characteristic data is not limited to that shown in FIG. 4, and for example, a threshold value indicating whether the sensitivity decrease amount is the first value or the second value may be indicated.

  The sensitivity data storage unit 1023 stores a plurality of time characteristic data corresponding to the status of the imaging unit 101 at the time of imaging. This time characteristic data is created in advance by measuring the sensitivity of the sensor 1012 while changing shooting parameters such as the elapsed time, the internal temperature of the sensor 1012 and the gain setting.

The amount of decrease in sensitivity varies depending on the status of the photographing unit 101. For example, when the internal temperature of the sensor 1012 is T ₂ higher than T ₁ , the dark current component increases, so that charge accumulation in the electrostatic capacitance in the imaging element is accelerated, and as a result, the internal temperature is T _1. Compared with the case where the sensitivity is reduced, the decrease in sensitivity becomes faster. Relationship between the elapsed time and sensitivity decrease amount at this time, the curve shown in T = T ₂ in FIG. On the other hand, when the internal temperature of the sensor 1012 is T ₂ which is lower than T ₁ , the dark current component decreases, and therefore, the decrease in sensitivity is moderate as compared to the case where the internal temperature is T ₁ . Relationship between the elapsed time and sensitivity decrease amount at this time, the curve shown in T = T ₀ in FIG. In the present embodiment, the sensitivity reduction amount is expressed as a ratio to the sensitivity value at the start of shooting standby, and the sensitivity reduction amount at the start of shooting standby is 1, which decreases as time elapses. Further, the amount of decrease in sensitivity varies depending on the gain setting of the photographing unit 101.

  Furthermore, the sensitivity decrease amount estimation circuit 1051 refers to the selected time characteristic data, and acquires the sensitivity decrease amount corresponding to the elapsed time acquired from the monitoring unit 103. When the amount of decrease in sensitivity corresponding to the elapsed time is not included in the time characteristic data, the amount of decrease in sensitivity can be obtained by interpolation. For example, interpolation may be performed using straight line interpolation or the like, or an approximation curve may be used when approximation can be performed with high accuracy. Even when there is no time characteristic data corresponding to the photographing information, it is possible to similarly obtain the amount of decrease in sensitivity by interpolation using a plurality of time characteristic data.

In step S307, the sensitivity correction processing circuit 1053 acquires sensitivity correction data from the sensitivity data storage unit 1023, and acquires the sensitivity decrease amount estimated from the sensitivity decrease amount estimation circuit 1051. Then, the sensitivity correction processing circuit 1053 performs sensitivity correction on the image data after the offset correction using the sensitivity correction data and the sensitivity decrease amount. In the present embodiment, sensitivity correction is performed according to the following equation.
I _g (x, y) = (I _off (x, y) × M _Iw ) / (I _w (x, y) × R _t × R _d ) (1)
In equation (1), (x, y) is the x and y coordinates of the pixel, I _g (x, y) is the image data after sensitivity correction, and I _off (x, y) is the image after offset correction. Each represents data. I _w (x, y) represents white image data, M _Iw represents the average pixel value of the white image data, R _t represents the ratio of the sensor sensitivity to the sensitivity target value, and R _d represents the amount of sensitivity decrease. .

The parameter (1 / R _d ) represents a correction amount for sensitivity correction determined according to the sensitivity decrease amount. In the present embodiment, a uniform correction amount (1 / R _d ) is used for each pixel of the radiation image, and this correction amount (1 / R _d ) is multiplied by the pixel value I _off (x, y) of each pixel. By doing so, sensitivity correction is performed. However, different correction amounts can be applied to each pixel of the radiation image.

  As shown in Expression (1), in sensitivity correction, variation in sensitivity of each image sensor of the sensor 1012 is compensated using white image data. That is, in the present embodiment, both correction of sensitivity reduction according to the imaging standby time and correction for compensating for variations in sensitivity of the image sensor are performed. In the present embodiment, correction for compensating manufacturing variations in sensitivity for each photographing unit 101 is also performed. However, it is not essential to perform all corrections.

  In step S308, the frequency processing circuit 1054, the gradation processing circuit 1055, and the defect correction processing circuit 1056 perform various types of image processing on the sensitivity-corrected image data. In the sensitivity-corrected image data, since the relationship between the pixel value and the irradiation dose is more accurate, the accuracy of the image processing depending on the irradiation dose is increased.

式（２）において、Ｋ_ｑはＸ線強度からノイズ量を計算する際の変換係数である。 For example, since the amount of random quantum noise largely depends on the irradiation dose, noise suppression processing can be performed more accurately by performing sensitivity correction. When the amount of random quantum noise is σ _q and the X-ray dose is X, these relations are expressed by Expression (2).

In Equation (2), K _q is a conversion coefficient when calculating the noise amount from the X-ray intensity.

The noise amount σ of the entire image data is obtained according to Equation (3) from the random quantum noise amount σ _q depending on the X-ray dose and the electrical noise amount σ _s fixed for each system.

  According to the equations (2) and (3), the noise amount of the radiographic image according to the X-ray dose can be estimated. By using the image data whose sensitivity has been corrected, the accuracy of the value of the X-ray dose X increases, so that the estimation accuracy of the noise amount σ also increases. As a result, the effect of the noise suppression process according to the estimated noise amount σ on the radiographic image is increased. The noise suppression process can be performed, for example, by applying a low-pass filter around the target pixel. That is, the pixel value of the target pixel after the noise suppression process is determined by performing a convolution operation between the pixel value of the peripheral pixel in the filter region and the filter coefficient. At this time, in order to leave an edge, the filter coefficient (weight) when the peripheral pixel is noise can be made larger than the filter coefficient (weight) when the peripheral pixel is an edge. Whether the peripheral pixel is noise or located at the edge can be determined by comparing the difference between the pixel values of the target pixel and the peripheral pixel and the estimated noise amount σ.

式（４）において、αはノイズ抑制力を調整するパラメータである。（ｘ＋ｉ，ｙ＋ｊ）はフィルタ領域内の周辺画素を表し、ｉ，ｊはフィルタの大きさに応じた範囲の値をとる。式（４）に示すように、対象画素と周辺画素との差分をとり、この差分をノイズ量σと比較することにより、周辺画素がノイズであるかエッジに位置するのかを判定する。一実施形態においては、エッジと判定された周辺画素に関してはその画素に対するフィルタ係数を０にする。一方で、ノイズと判定された周辺画素に関しては、適当なフィルタ係数が与えられる。例えば、対象画素と周辺画素との差分とノイズ量σとの差に対応する、任意の関数に従う値を、フィルタ係数として用いることができる。この関数は連続的な関数であってもよいし、ある範囲の値に対して１つのフィルタ係数を与える離散的な関数であってもよい。 The determination can be performed, for example, using the noise amount σ as the determination threshold according to the determination formula of Expression (4).

In Expression (4), α is a parameter for adjusting the noise suppression force. (X + i, y + j) represents peripheral pixels in the filter area, and i and j take values in a range corresponding to the size of the filter. As shown in Expression (4), the difference between the target pixel and the peripheral pixel is taken, and this difference is compared with the noise amount σ to determine whether the peripheral pixel is noise or positioned at the edge. In one embodiment, for a peripheral pixel determined to be an edge, the filter coefficient for that pixel is set to zero. On the other hand, an appropriate filter coefficient is given to surrounding pixels determined as noise. For example, a value according to an arbitrary function corresponding to the difference between the target pixel and the peripheral pixel and the noise amount σ can be used as the filter coefficient. This function may be a continuous function or a discrete function that provides one filter coefficient for a range of values.

式（５）において、ｇはＶを空気カーマに変換するための係数である。また、Ｖは適切に設定した領域の中心における感度指標の傾向を示す値として規定されており、多くの場合には被写体の中心付近における指標の平均値を用いる。感度補正がなされた画像データを用いてＶを算出すると、照射線量に対してより適切な値が得られるため、式（５）に従って算出されるＥＩ値もより正確となる。また、感度補正がされていない画像データからＶを計算した場合であっても、感度補正の補正量を用いてＶを補正する、例えば感度補正の補正量をＶに乗じることにより、同様の効果を得ることができる。 Another example of image processing depending on the irradiation dose is calculation of Exposure Index (hereinafter referred to as EI value). The EI value is a value proportional to the irradiation dose to the sensor 1012 and is defined as shown in Expression (5).

In Expression (5), g is a coefficient for converting V into air kerma. V is defined as a value indicating the tendency of the sensitivity index at the center of the appropriately set region, and in many cases, the average value of the index near the center of the subject is used. If V is calculated using image data that has been subjected to sensitivity correction, a more appropriate value can be obtained with respect to the irradiation dose. Therefore, the EI value calculated according to the equation (5) becomes more accurate. Even when V is calculated from image data that has not been subjected to sensitivity correction, the same effect can be obtained by correcting V using the correction amount of sensitivity correction, for example, by multiplying V by the correction amount of sensitivity correction. Can be obtained.

  Thereafter, the image processing unit 1050 transfers the radiation image data obtained by the image processing to the image display device 220.

  As described above, according to the first embodiment, sensitivity correction can be performed in consideration of a decrease in sensitivity over time in an imaging standby state, so that more accurate radiation image data can be obtained. In addition, since the relationship between the radiation image data and the irradiation dose becomes accurate as a result, the accuracy of image processing and the like depending on the irradiation dose can be improved, and a higher-quality radiation image or the like can be output.

[Embodiment 2]
Next, Embodiment 2 of the present invention will be described. FIG. 6A shows a functional configuration of the image acquisition apparatus 600 according to the present embodiment. As illustrated in FIG. 6A, the image acquisition apparatus 600 is the same as the image acquisition apparatus 100 according to the first embodiment except that the image acquisition apparatus 600 includes a refresh determination unit 606. As illustrated in FIG. 6B, the function of the present embodiment may be realized by a radiation imaging system 610 including a radiation imaging apparatus 620 and an information processing apparatus 630 as in the first embodiment. In this case, the radiation imaging apparatus 620 may include the refresh determination unit 606, and the information processing apparatus 630 may include the refresh determination unit 606.

  FIG. 7 shows a hardware configuration of the image acquisition apparatus 600 according to the present embodiment. As illustrated in FIG. 7, the image acquisition apparatus 600 includes a refresh determination circuit 6042, and the image acquisition according to the first embodiment is performed except that the sensitivity data storage unit 1023 further stores sensitivity decrease amount threshold data. This is the same as the device 100. The refresh determination circuit 6042 in FIG. 7 corresponds to the refresh determination unit 606 in FIGS. 6A and 6B.

  The sensitivity decrease amount threshold data stored in the sensitivity data storage unit 1023 represents a limit value of the sensitivity decrease amount that is determined and stored in advance. That is, when the amount of decrease in sensitivity of the sensor 1012 becomes larger than the limit value, a refresh operation is performed to restore the sensitivity. The limit value of the sensitivity reduction amount can be arbitrarily determined. In this embodiment, when the amount of sensitivity decrease is smaller than the limit value, sufficient image quality can be obtained by sensitivity correction. However, when the amount of sensitivity decrease is larger than the limit value, sufficient image quality can be obtained even if sensitivity correction is applied. A limit value is determined so that it cannot be obtained. That is, if the sensitivity reduction amount is larger than the limit value, it means that image quality deterioration cannot be avoided.

  For example, when the sensitivity reduction amount is large, the output value (pixel value) in the radiation image data is relatively low with respect to the noise component mixed in the signal transmission path located downstream from the image sensor. The image quality is degraded. In addition, a decrease in the output value (pixel value) in the radiographic image data leads to deterioration in the image quality of the radiographic image because an arithmetic error in image processing increases. Based on these points, the sensitivity decrease amount threshold data is determined in advance by examining whether or not the image quality standard is satisfied at various sensitivities.

  FIG. 8 is a flowchart illustrating processing of the image acquisition apparatus 600 according to the second embodiment. In FIG. 8, the same processes as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted.

  In step S <b> 801, the parameter acquisition unit 1040 acquires the status of the imaging unit 101 and stores it as imaging information in the parameter storage unit 1024. In addition, the control unit 1030 monitors the elapsed time from the start of shooting standby. Further, the estimation unit 105 acquires the amount of decrease in sensitivity corresponding to the shooting information and the elapsed time, as in step S306 of the first embodiment. The process in step S801 may be performed repeatedly at all times.

  In addition, the refresh determination unit 606 determines whether to cause the imaging unit 101 to perform a refresh operation according to the imaging standby time measured by the monitoring unit 103. In the present embodiment, the refresh determination unit 606 acquires sensitivity decrease amount threshold data from the sensitivity data storage unit 1023. In addition, the refresh determination unit 606 obtains the sensitivity reduction amount estimated according to the imaging standby time from the estimation unit 105. Then, the refresh determination unit 606 compares the sensitivity reduction amount limit value with the sensitivity reduction amount. If the amount of decrease in sensitivity is equal to or less than the limit value, the process proceeds to step S103. If the sensitivity decrease amount is larger than the limit value, the process proceeds to step S802. However, in another embodiment, the refresh determination unit 606 may determine whether or not to cause the imaging unit 101 to perform a refresh operation by comparing the imaging standby time with a threshold value.

  In step S802, the control unit 1030 controls the drive circuit 1011 to cause the photographing unit 1010 to perform a refresh operation. The refresh operation refers to an operation of discharging charges derived from dark current components accumulated in the capacitance of the image sensor in the sensor 1012. For example, the discharge can be performed by changing the bias voltage applied to the sensor 1012. The details of the refresh operation can be appropriately selected according to the type of the image sensor to be used. For example, a method of setting the bias voltage applied to the image sensor to 0 V can be mentioned. The refresh operation eliminates the sensitivity decrease with time due to the dark current, and the sensitivity of the sensor 1012 is restored. After the refresh operation, the process proceeds to step S801, and photographing standby driving is started again. In this case, the control unit 1030 monitors the elapsed time after the refresh operation is performed.

  In the present embodiment, the image acquisition device 600 performs both sensitivity correction according to the shooting standby time and control of the refresh operation according to the shooting standby time. However, in another embodiment, the image acquisition apparatus 600 may control only the refresh operation according to the shooting standby time.

  As described above, according to the second embodiment, the sensitivity is automatically recovered when the amount of decrease in sensitivity increases. By combining such a configuration with a configuration for performing sensitivity correction, a sufficient image quality can be ensured.

(Other embodiments)
The present invention is also realized by executing the following processing. That is, software (program) that implements the functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media. Then, the computer (or CPU, MPU, etc.) of the system or apparatus reads and executes the program code. In this case, the program and the storage medium storing the program constitute the present invention.

Claims (16)

Time acquisition means for acquiring a standby time from when the radiation imaging apparatus starts imaging standby until imaging of a radiation image;
Determining means for determining a correction amount of the radiation image according to the waiting time;
An information processing apparatus comprising: Characteristic acquisition means for acquiring time characteristic data indicating a sensitivity change of the radiation imaging apparatus according to the standby time;
The information processing apparatus according to claim 1, wherein the determination unit determines the correction amount with reference to the acquired time characteristic data.   The information processing apparatus according to claim 2, wherein the time characteristic data indicates a relationship between the standby time and a sensitivity reduction amount of the radiation imaging apparatus. Means for obtaining imaging information indicating the status of the radiation imaging apparatus;
The information processing apparatus according to claim 2, wherein the characteristic acquisition unit acquires the time characteristic data corresponding to the acquired photographing information selected from a plurality of time characteristic data. .   The information processing apparatus according to claim 4, wherein the imaging information includes internal temperature information of the radiation imaging apparatus.   The information processing apparatus according to claim 4, wherein the imaging information includes gain setting information of the radiation imaging apparatus.   The information processing apparatus according to claim 1, further comprising a correction unit that corrects each pixel value of the radiation image according to the correction amount.   The correction means performs the correction by multiplying each pixel of the radiographic image by the correction amount uniformly determined for each pixel of the radiographic image. The information processing apparatus described in 1. Means for obtaining variation information indicating variations in sensitivity of each of a plurality of image sensors included in the radiation imaging apparatus;
The correction unit according to claim 7 or 8, wherein the correction unit performs both correction according to the correction amount and correction for compensating the sensitivity variation according to the variation information on the radiation image. Information processing device.   The apparatus further comprises processing means for estimating a noise amount of a corrected radiographic image according to the correction amount, and performing a noise suppression process on the corrected radiographic image according to the estimated noise amount. The information processing apparatus according to any one of 7 to 9.   11. The information processing apparatus according to claim 7, further comprising a calculation unit configured to calculate an EI value (Exposure Index) from a corrected radiographic image according to the correction amount.   12. The apparatus according to claim 1, further comprising a calculation unit that calculates an EI value from the captured radiographic image and corrects the calculated EI value using the correction amount. The information processing apparatus described.   The elapsed time from the start of the imaging standby is an elapsed time since the last operation for discharging the charge accumulated in the imaging device of the radiation imaging apparatus. The information processing apparatus according to any one of 1 to 12. A radiographic means for taking radiographic images;
Time acquisition means for measuring the elapsed time since the radiation imaging means started imaging standby;
A determining unit that determines a correction amount of the radiographic image according to a standby time from when the radiation imaging unit starts imaging standby until imaging is performed;
A radiation imaging system comprising: A time acquisition step of acquiring a standby time from when the radiation imaging apparatus starts imaging standby until radiographic imaging is performed;
A determining step of determining a correction amount of the radiation image according to the waiting time;
An information processing method characterized by comprising: On the computer,
A time acquisition step of acquiring a standby time from when the radiation imaging apparatus starts imaging standby until radiographic imaging is performed;
A determining step of determining a correction amount of the radiation image according to the waiting time;
A program characterized by having executed.

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