Patent Description:
As a radiation imaging apparatus for use in medical diagnostic imaging and nondestructive inspection with radiation, such as X-rays, a radiation imaging apparatus including a flat panel detector (FPD) formed of a semiconductor material is currently in widespread use. For example, in medical diagnostic imaging, such a radiation imaging apparatus is used as a digital radiation imaging apparatus to perform still image capturing such as general image capturing and moving image capturing such as fluoroscopic image capturing.

Some radiation imaging apparatuses are configured to monitor the irradiation dose (cumulative dose) of radiation and stop irradiation of radiation (e.g., transmit an irradiation stop signal for stopping irradiation of radiation to the radiation generation apparatus) in a case where the cumulative dose reaches a threshold. This operation is referred to as automatic exposure control (AEC). The AEC can prevent, for example, excessive radiation irradiation.

As an example of such a radiation imaging apparatus, <CIT> discusses a radiation imaging apparatus including a dose detection unit that is provided in an imaging region of an FPD and is configured to detect the dose of radiation that reaches the imaging region. In the radiation imaging apparatus discussed in <CIT>, a timing for stopping irradiation of radiation in the radiation imaging apparatus is predicted based on the dose detected by the dose detection unit and a preset dose target value. Additionally, an irradiation stop timing notification for notifying the radiation generation apparatus of the timing for stopping irradiation of radiation is issued before the irradiation stop timing is reached.

However, in the technique discussed in <CIT>, the radiation generation apparatus cannot accurately perform control processing to stop irradiation of radiation. Specifically, in the technique discussed in <CIT>, if the dose rate of radiation that is transmitted through an object is high, the cumulative dose of radiation reaches the threshold in a short period of time, for example, about several milliseconds (ms). This may cause an issue that the cumulative dose of radiation exceeds the threshold before the irradiation stop timing notification is made to cause the radiation generation apparatus to stop irradiation of radiation. Known exposure control systems and methods are described in <CIT>.

The present invention has been made in view of the above-described issue, and is directed to providing a mechanism for enabling a radiation generation apparatus to accurately perform control processing to stop irradiation of radiation.

According to a first aspect of the present invention, there is provided a radiation imaging apparatus as specified in claims <NUM> to <NUM>. According to a second aspect of the present invention, there is provided a radiation imaging system as specified in claim <NUM>. According to a third aspect of the present invention, there is provided a control method for a radiation imaging apparatus as specified in claims <NUM> to <NUM>. According to a fourth aspect of the present invention, there is provided a program as specified in claim <NUM>.

A radiation imaging apparatus includes sensor means for detecting radiation from a radiation generation apparatus, and control means for transmitting a signal for stopping irradiation of radiation to the radiation generation apparatus using a cumulative dose of the radiation detected by the sensor means, an irradiation time for the radiation, and an irradiation stop threshold. The control means measures the irradiation time based on the cumulative dose and transmits the signal for stopping irradiation of radiation to the radiation generation apparatus based on the irradiation stop threshold, wherein the irradiation stop threshold is changed according to the irradiation time.

The control means may start measurement of the irradiation time when the cumulative dose reaches a predetermined amount.

The control means may control setting of the irradiation stop threshold based on a target value of the cumulative dose in radiation imaging, the irradiation time, and a delay time from transmission of the signal for stopping irradiation of the radiation to stopping of irradiation of the radiation in the radiation generation apparatus.

The radiation imaging apparatus may further include communication means for transmitting the signal for stopping irradiation of the radiation to the radiation generation apparatus. The control means may derive the delay time based on a communication delay time in communication between the communication means and the radiation generation apparatus.

The communication means may include a plurality of communication modes. The control means may derive the delay time based on the communication delay time in a communication mode for the radiation imaging among the plurality of communication modes.

The control means may acquire a plurality of the communication delay times before the radiation imaging is performed.

The control means may derive the delay time based on one of the plurality of communication delay times.

The control means may derive the delay time based on an average value of some or all of the plurality of communication delay times.

The control means may derive the delay time based on a minimum value of the plurality of communication delay times.

In a case where no response is received from the radiation generation apparatus for a predetermined period in response to the transmission of the signal for stopping irradiation of the radiation, the control means may retransmit the signal for stopping irradiation of the radiation to the radiation generation apparatus.

The control means may determine a timing of the transmission of the signal for stopping irradiation of the radiation based on the irradiation time and an irradiation start threshold.

A radiation imaging system may include the radiation imaging apparatus, and the radiation generation apparatus.

In a control method for a radiation imaging apparatus, the radiation imaging apparatus includes sensor means for detecting radiation from a radiation generation apparatus, and control means for transmitting a signal for stopping irradiation of radiation to the radiation generation apparatus using a cumulative dose of the radiation detected by the sensor means, an irradiation time for the radiation, and an irradiation stop threshold. The control method includes measuring the irradiation time based on the cumulative dose, and transmitting the signal for stopping irradiation of radiation to the radiation generation apparatus based on the irradiation stop threshold, wherein the irradiation stop threshold is changed according to the irradiation time.

Measurement of the irradiation time may be started when the cumulative dose reaches a predetermined amount.

The irradiation stop threshold may be set depending on the irradiation time after the measurement. The irradiation stop threshold may be set based on a target value of the cumulative dose in radiation imaging and a delay time from transmission of the signal for stopping irradiation of the radiation to the radiation generation apparatus to stopping of irradiation of the radiation in the radiation generation apparatus.

A communication delay time in communication between the radiation generation apparatus and communication means may be measured before the setting, the communication means being configured to include a plurality of communication modes and transmit the signal for stopping irradiation of the radiation to the radiation generation apparatus. In the setting, the delay time may be derived based on the communication delay time in a communication mode for the radiation imaging among the plurality of communication modes.

In the setting, a timing of the transmission of the signal for stopping irradiation of the radiation may be determined based on the irradiation time and an irradiation start threshold.

A computer program may include instructions which, when executed by a computer, cause the computer to perform the control method.

Exemplary embodiments of the present invention will be described below with reference to the drawings. Assume herein that X-rays can be suitably used as radiation according to the present invention. However, the radiation is not limited only to X-rays. Examples of the radiation may also include α-rays, β-rays, and γ-rays.

<FIG> is a block diagram illustrating a schematic configuration example of a radiation imaging system <NUM> according to a first exemplary embodiment of the present invention. In the first exemplary embodiment, the radiation imaging system <NUM> can be suitably used for medical use. As illustrated in <FIG>, the radiation imaging system <NUM> includes a radiation generation apparatus <NUM>, a radiation imaging apparatus <NUM>, and an irradiation control apparatus <NUM>.

The radiation generation apparatus <NUM> emits radiation to an object (not illustrated) based on control processing performed by the irradiation control apparatus <NUM> (specifically, an imaging control unit <NUM>). The radiation generation apparatus <NUM> includes a radiation tube serving as a radiation generation unit to generate radiation, and a collimator for defining the beam spread angle of the radiation generated by the radiation tube.

The radiation imaging apparatus <NUM> includes a flat panel detector (FPD), for example, and also includes a sensor unit <NUM> including two-dimensionally distributed image sensors. The sensor unit <NUM> is configured to detect the radiation that is emitted from the radiation generation apparatus <NUM> and enters the sensor unit <NUM>.

Specifically, the radiation imaging apparatus <NUM> detects information (dose information) about a two-dimensional distribution of doses of radiation that reaches the image sensors in the sensor unit <NUM>, and generates radiation image data. After that, the radiation imaging apparatus <NUM> transmits the generated radiation image data to an image processing unit <NUM> of the irradiation control apparatus <NUM> through a communication unit <NUM>. The irradiation control apparatus <NUM> controls operations of the radiation generation apparatus <NUM> and the radiation imaging apparatus <NUM>, and acquires and processes radiation image data generated by the radiation imaging apparatus <NUM>.

The radiation imaging apparatus <NUM> also includes a control unit <NUM>. The control unit <NUM> includes a threshold determination unit <NUM>, a threshold decision unit <NUM>, a timer unit <NUM>, and a dose calculation unit <NUM>. The threshold determination unit <NUM> includes a function for determining whether the dose detected by the sensor unit <NUM> has reached a threshold. If the threshold determination unit <NUM> determines that the radiation dose (cumulative dose) that is detected by the sensor unit <NUM> and is calculated by the dose calculation unit <NUM> has reached a predetermined irradiation stop threshold, the threshold determination unit <NUM> transmits an irradiation stop signal <NUM> to the irradiation control apparatus <NUM> through the communication unit <NUM>. When the irradiation control apparatus <NUM> receives the irradiation stop signal <NUM> from the radiation imaging apparatus <NUM> through a communication unit <NUM>, the imaging control unit <NUM> performs an irradiation stop control <NUM> to stop irradiation of radiation from the radiation generation apparatus <NUM>.

At the same time, the communication unit <NUM> returns a response indicating receipt of the irradiation stop signal <NUM> to the radiation imaging apparatus <NUM>. If the radiation imaging apparatus <NUM> cannot receive a response for a certain period of time after the irradiation stop signal <NUM> is transmitted, the radiation imaging apparatus <NUM> retransmits the irradiation stop signal <NUM> to the irradiation control apparatus <NUM>.

When the dose calculation unit <NUM> calculates the cumulative dose that has reached a predetermined amount from start of imaging, the threshold determination unit <NUM> determines that the irradiation has been started and causes the timer unit <NUM> to start measurement of an irradiation time. In this case, a threshold for the dose is referred to as an irradiation start threshold. In a case where the dose detected by the sensor unit <NUM> has reached the irradiation start threshold, the threshold determination unit <NUM> transmits information indicating that the dose has reached the irradiation start threshold to the threshold decision unit <NUM>. Upon receiving, from the threshold determination unit <NUM>, the information indicating that the dose has reached the irradiation start threshold, the threshold decision unit <NUM> sets the time point when the dose reaches the irradiation start threshold as a threshold change reference point. The predetermined amount corresponding to the irradiation start threshold is a dose value as a reference with which it is determined that the irradiation of radiation from the radiation generation apparatus <NUM> starts to be stabilized. A suitable value is set to the threshold determination unit <NUM> according to the properties of the radiation generation apparatus <NUM>.

The timer unit <NUM> is a timer for measuring the irradiation time for radiation from a predetermined timing. In the present exemplary embodiment, the irradiation time is measured from the threshold change reference point set by the threshold decision unit <NUM>. The threshold decision unit <NUM> successively changes a timing of transmission of the irradiation stop signal <NUM> based on a time variation from the threshold change reference point, the irradiation start threshold, and a communication delay time Tdc to be described below, with reference to the irradiation time measured by the timer unit <NUM> and the irradiation start threshold of the threshold determination unit <NUM>.

The communication unit <NUM> is configured to communicate with the irradiation control apparatus <NUM> by wired communication and wireless communication. Prior to imaging, the communication unit <NUM> communicates with the communication unit <NUM> of the irradiation control apparatus <NUM> and the timer unit <NUM> measures a communication response time between the radiation imaging apparatus <NUM> and the irradiation control apparatus <NUM>, to thereby calculate the communication delay time Tdc. The communication delay time Tdc is held in the threshold decision unit <NUM>. The threshold decision unit <NUM> reflects the calculated communication delay time Tdc in the setting of the irradiation stop threshold.

Assume herein that the communication delay time Tdc is held in a table format or the like in the threshold decision unit <NUM> so that a number of communication delay times Tdc corresponding to the number of combinations of communication modes between the communication unit <NUM> and the communication unit <NUM> can be held. The communication delay time Tdc to be reflected in the setting of the irradiation stop threshold by the threshold decision unit <NUM> is switched depending on the communication mode between the communication unit <NUM> and the communication unit <NUM>. Instead of holding the communication delay time Tdc in a table format, the communication delay time Tdc may be calculated based on a formula, or may be calculated by measurements.

Next, the functions of the imaging control unit <NUM>, an imaging condition setting unit <NUM>, the communication unit <NUM>, the image processing unit <NUM>, and a display unit <NUM> included in the irradiation control apparatus <NUM> will be described.

The imaging condition setting unit <NUM> sets imaging condition data including imaging condition information about, for example, an object imaging area, a tube voltage and a tube current in the radiation generation apparatus <NUM>, and a target value Dref of the dose (cumulative dose) of radiation that is transmitted through the object and reaches the radiation imaging apparatus <NUM>. The term "dose" used herein generally refers to the cumulative dose of radiation during irradiation of radiation. A dose value similar to the dose and a dose value linked to the dose can also be used, and these values are hereinafter referred to as "cumulative dose", as needed.

The communication unit <NUM> is configured to communicate with the radiation imaging apparatus <NUM> by wired communication and wireless communication.

The image processing unit <NUM> performs image processing, such as gradation processing and noise reduction processing, on the radiation image data transmitted from the radiation imaging apparatus <NUM>. The image processing unit <NUM> transmits the radiation image data subjected to the image processing to the display unit <NUM>.

The display unit <NUM> outputs a radiation image based on the radiation image data transmitted from the image processing unit <NUM> to a monitor or the like and displays the radiation image.

In the radiation imaging system <NUM> according to the present exemplary embodiment, a timing for transmitting the irradiation stop signal <NUM> from the radiation imaging apparatus <NUM> to the irradiation control apparatus <NUM> is based on a period of time for transmitting the irradiation stop signal <NUM> from the above-described radiation imaging apparatus <NUM> to the irradiation control apparatus <NUM>. Further, it may be desirable to set the timing by taking into consideration a delay time from a time when the irradiation control apparatus <NUM> performs the irradiation stop control <NUM> to stop irradiation of radiation from the radiation generation apparatus <NUM> to a time when irradiation of radiation is stopped in the radiation generation apparatus <NUM>.

The time when irradiation of radiation is stopped in the radiation generation apparatus <NUM> corresponds to the time when the tube voltage in the radiation tube of the radiation generation apparatus <NUM> has started to drop or the tube voltage has dropped to a minimum voltage level. In the case of setting the delay time based on the time when the tube voltage has dropped to the minimum voltage level, it may be desirable to set the delay time by adding a period of time obtained by multiplying a coefficient set in consideration of change in the dose and quality of radiation by a non-stationary period from a time when the tube voltage in the radiation tube has started to drop to a time when the tube voltage has dropped to the minimum voltage level.

More specifically, a delay time Td is divided into two periods, i.e., a stationary period Ta and a non-stationary period Tb. The stationary period Ta is a period from a time when a signal is transmitted to a time when the tube voltage has started to drop. The non-stationary period Tb is a period from a time when the tube voltage has started to drop to a time when the tube voltage has dropped to the minimum voltage level. In this case, since the tube voltage in the radiation tube drops in the non-stationary period Tb, the delay time Td is obtained by multiplying a coefficient k (k is less than or equal to "<NUM>") and adding the multiplication result. That is, the delay time Td to which the non-stationary period Tb is added can be determined based on the following expression.

In the present exemplary embodiment, it may be desirable to acquire the delay time Td before irradiation of radiation in object radiation imaging (before imaging) for each radiation generation apparatus. A value obtained by actual measurements when the radiation imaging apparatus <NUM> is installed can be used as the delay time Td. Alternatively, an imaging environment and the radiation generation apparatus <NUM> to be used may be preliminarily registered in a database and the delay time Td may be calculated with reference to the database. It may be desirable to provide a notification about the delay time Td in advance to the threshold decision unit <NUM> and to set the irradiation stop threshold in consideration of the delay time Td.

Next, object imaging processing will be described with reference to <FIG>.

<FIG> is a flowchart illustrating an example of a processing procedure in a series of control methods from start of imaging of an object to end of imaging in the radiation imaging system <NUM> according to the first exemplary embodiment.

In this imaging processing, object radiation imaging is performed by setting an irradiation stop threshold Dth for the dose (cumulative dose) and a time variation of the irradiation stop threshold Dth based on the preliminarily held delay time Td, the preliminarily acquired communication delay time Tdc, the irradiation start threshold Dobs, and the target value Dref of the dose (cumulative dose).

First, in step S201, the imaging condition setting unit <NUM> receives an imaging start instruction input by an operator through an input unit (not illustrated), and sets, for example, imaging condition information (irradiation condition information) input by the operator. In this case, the imaging condition setting unit <NUM> sets the tube voltage and tube current in the radiation tube, the target value Dref of the dose (cumulative dose), the irradiation start threshold Dobs, the delay time Td, and the like as the imaging condition information (irradiation condition information).

After that, the imaging condition setting unit <NUM> transmits the acquired imaging start instruction and the set imaging condition information (irradiation condition information) to the radiation imaging apparatus <NUM>. The value of the delay time Td may be stored in any one of the apparatuses constituting the radiation imaging system <NUM>, and the stored value may be referenced.

Next, in step S202, the threshold decision unit <NUM> sets the irradiation stop threshold Dth for the dose (cumulative dose) and the time variation of the irradiation stop threshold Dth based on the target value Dref of the dose (cumulative dose), the irradiation start threshold Dobs, the delay time Td, and the communication delay time Tdc set in step S201. Settings of the irradiation stop threshold Dth for the dose (cumulative dose) and the time variation of the irradiation stop threshold Dth will be described below with reference to <FIG>.

Next, in step S203, the imaging control unit <NUM> transmits an irradiation execution signal for executing irradiation of radiation, as well as the imaging condition information (irradiation condition information) received from the imaging condition setting unit <NUM> in step S201, to the radiation generation apparatus <NUM>. In response to this, the radiation generation apparatus <NUM> irradiates the object with radiation under irradiation conditions based on the imaging condition information (irradiation condition information) received from the imaging condition setting unit <NUM>.

Next, in step S204, the dose calculation unit <NUM> first calculates a value D that is representative of the dose (cumulative dose) of radiation detected by the sensor unit <NUM>. In this case, a maximum value, an average value, a median, or the like of the doses (cumulative doses) may be used as the value D that is representative of the dose (cumulative dose) of radiation. The value D that is representative of the dose (cumulative dose) of radiation is hereinafter referred to as the "radiation dose (cumulative dose) D".

The threshold determination unit <NUM> compares the radiation dose (cumulative dose) D with the irradiation stop threshold Dth set in step S202 to determine whether the radiation dose (cumulative dose) D is less than the irradiation stop threshold Dth. As a result of this determination, if the radiation dose (cumulative dose) D is less than the irradiation stop threshold Dth (YES in step S204), the processing of step S204 is repeated.

On the other hand, as a result of determination in step S204, if the radiation dose (cumulative dose) D is more than or equal to the irradiation stop threshold Dth (radiation dose (cumulative dose) D is more than or equal to the threshold) (NO in step S204), the processing proceeds to step S205.

In step S205, since the radiation dose (cumulative dose) D has reached the irradiation stop threshold Dth, the radiation imaging apparatus <NUM> transmits the irradiation stop signal <NUM> for stopping irradiation of radiation from the radiation generation apparatus <NUM> to the irradiation control apparatus <NUM>. Upon receiving the irradiation stop signal <NUM>, the irradiation control apparatus <NUM> causes the imaging control unit <NUM> to perform the irradiation stop control <NUM> on the radiation generation apparatus <NUM>.

In this case, the radiation is continuously irradiated by the amount corresponding to the communication delay time Tdc associated with the transmission of the irradiation stop signal <NUM> and the delay time Td from the time when the irradiation stop control <NUM> is performed to the time when irradiation of radiation is actually stopped in the radiation generation apparatus <NUM>. This makes it possible to set the actual radiation dose (cumulative dose) D to be approximate to the target value Dref of the dose (cumulative dose).

Next, in step S206, the radiation imaging apparatus <NUM> controls the image sensors in the sensor unit <NUM> to stop conversion into dose information, and transmits the generated radiation image data to the image processing unit <NUM>.

Next, in step S207, the image processing unit <NUM> performs image processing, such as gradation processing and noise reduction processing, on the radiation image data received from the radiation imaging apparatus <NUM>. After that, the image processing unit <NUM> transmits the radiation image data subjected to the image processing to the display unit <NUM>.

Next, in step S208, the display unit <NUM> outputs a radiation image based on the radiation image data received from the image processing unit <NUM> to the monitor or the like and displays the radiation image to thereby present the radiation image to the operator.

After completion of the processing of step S208, the processing in the flowchart for object radiation imaging illustrated in <FIG> is terminated.

Next, a method for acquiring the communication delay time Tdc before imaging will be described with reference to <FIG>.

In a state where the radiation imaging apparatus <NUM> and the irradiation control apparatus <NUM> are activated and are ready to communicate with each other by wired communication or wireless communication, the radiation imaging apparatus <NUM> causes the communication unit <NUM> to send an inquiry about acquiring the communication delay time Tdc to the communication unit <NUM> of the irradiation control apparatus <NUM>.

In this case, the radiation imaging apparatus <NUM> refers to a counter of the timer unit <NUM>. The irradiation control apparatus <NUM> sends a response immediately after receiving the inquiry about the communication delay time Tdc. Upon receiving the response from the irradiation control apparatus <NUM>, the radiation imaging apparatus <NUM> refers to the counter of the timer unit <NUM> and holds a period of time obtained by calculating a round-trip communication time (t1) ÷ <NUM> as a temporary communication delay time (T1).

The radiation imaging apparatus <NUM> repeatedly and continuously performs this operation before imaging is started, to thereby acquire a plurality of temporary communication delay times (T2, T3, T4, and •••). If the number of acquired temporary communication delay times exceeds a maximum number, the oldest information is overwritten, as needed, with new information. At an imaging start timing, a single communication delay time is calculated based on the plurality of acquired temporary communication delay times, and the calculated communication delay time is set as the communication delay time Tdc. In this case, a single communication delay time may be calculated using an average value (e.g. median, mean, mode, etc.) of some or all of the plurality of communication delay times, or may be calculated using a minimum value of some or all of the plurality of communication delay times.

The threshold decision unit <NUM> uses the communication delay time Tdc to set the irradiation stop threshold Dth for the dose (cumulative dose) and the time variation of the irradiation stop threshold Dth for the target value Dref of the dose (cumulative dose). Settings of the irradiation stop threshold Dth for the dose (cumulative dose) and the time variation of the irradiation stop threshold Dth will be described below with reference to <FIG>.

If the following expression can be established based on the relationship among the calculated communication delay time Tdc, the delay time Td in the radiation generation apparatus <NUM>, and a nominal minimum irradiation time, it can be determined prior to imaging that the irradiation cannot be stopped at a desired timing due to an extremely large communication delay time. In this situation, for example, control processing to prohibit imaging is performed. Additionally or alternatively, for example, an operation to issue a warning to a user may be performed.

Since the communication delay time Tdc is a delay time in communication from the radiation imaging apparatus <NUM> to the irradiation control apparatus <NUM>, the communication delay time Tdc includes a time for internal processing in units such as an access point, a hub, and a wireless relay. In addition, the communication delay time Tdc includes an application layer delay, a transmission delay, a propagation delay, a processing delay in a wireless management server (encryption processing etc.), and a delay time in using a plurality of hubs and the like. The communication delay time including these times is referred to as the communication delay time Tdc.

The value of the communication delay time Tdc varies depending on the difference in the communication mode between the radiation imaging apparatus <NUM> and the irradiation control apparatus <NUM>. Specifically, the value of the communication delay time Tdc varies depending on which one of wired communication and wireless communication is established. In a system in which a technique for improving the communication quality using a plurality of antennas in wireless communication is incorporated, the communication delay time Tdc varies depending on which one of the plurality of antennas is used. Depending on the difference between communication modes, different communication delay times Tdc are held in a table format in the threshold decision unit <NUM> as illustrated in <FIG>.

The threshold decision unit <NUM> determines the communication mode between the communication unit <NUM> and the communication unit <NUM> at an imaging timing, and reflects the value of the communication delay time Tdc depending on the communication mode in the setting of the irradiation stop threshold using the table of the communication delay times Tdc.

For example, if a wired communication is disconnected and switched to a wireless communication immediately before imaging and the previously acquired delay time Tdc during the wired communication is reflected in the setting of the irradiation stop threshold, automatic exposure control (AEC) cannot be accurately performed. Accordingly, if the communication delay times Tdc corresponding to the respective communication modes are held in a table format as illustrated in <FIG>, the accurate communication delay time Tdc depending on the communication mode set at the imaging timing can be reflected in the setting of the irradiation stop threshold, thereby making it possible to accurately perform AEC.

While Table 4A illustrates an example where the communication delay times Tdc corresponding to the respective communication modes are held in a table format, differences from a base communication delay time may be held in a table format as illustrated in Table 4B.

While <FIG> illustrates an example where two antennas for wireless communication are used, the number of antennas to be used is not particularly limited. Three or more antennas may be used, or only one antenna may be used.

Next, processing for setting the irradiation stop threshold Dth for the dose (cumulative dose) and the time variation of the irradiation stop threshold Dth in step S202 illustrated in <FIG> will be described with reference to <FIG>. In the case of setting the time variation of the irradiation stop threshold Dth for the dose (cumulative dose), if it is determined that the radiation dose (cumulative dose) D is less than the irradiation stop threshold Dth (YES in step S204), the processing of step S202 is also performed.

<FIG> illustrates an example of a relationship among the irradiation stop threshold Dth for the dose (cumulative dose), the time variation of the irradiation stop threshold Dth, and the radiation dose (cumulative dose) D according to the first exemplary embodiment. <FIG> illustrates the relationship between the radiation dose (cumulative dose) D represented by the vertical axis and time (elapsed time) represented by the horizontal axis.

As illustrated in <FIG>, the threshold decision unit <NUM> performs control processing to change the irradiation stop threshold Dth for the dose (cumulative dose) depending on the elapsed time from start of irradiation of radiation. Specifically, as illustrated in <FIG>, the threshold decision unit <NUM> performs control processing to increase the irradiation stop threshold Dth for the dose (cumulative dose) with elapsed time.

A starting point when the elapsed time is measured corresponds to the threshold change reference point illustrated in <FIG>. As described above, when the threshold determination unit <NUM> determines that the radiation dose (cumulative dose) D detected by the sensor unit <NUM> has reached the irradiation start threshold held in the threshold determination unit <NUM>, a notification about the time point when the radiation dose (cumulative dose) D reaches the irradiation start threshold is provided to the threshold decision unit <NUM> as the threshold change reference point.

The threshold decision unit <NUM> counts the elapsed time from the threshold change reference point, and performs control processing to change the irradiation stop threshold Dth for the dose (cumulative dose) depending on the elapsed time.

In the case of performing imaging, even when an irradiation start instruction is issued to the radiation generation apparatus <NUM> in the imaging control unit <NUM>, it may take a longer rise time depending on the tube of the radiation generation apparatus <NUM> and the activation of the tube may be unstable. If the elapsed time is counted from the time when the irradiation start instruction is issued, the actual radiation dose (cumulative dose) D does not match the irradiation dose assumed based on the dose rate, which makes it difficult to accurately change the irradiation stop threshold Dth for the dose (cumulative dose). To avoid this, a timing when the irradiation starts to be stabilized is set as the threshold change reference point, thereby achieving an improvement in the accuracy of the irradiation stop control <NUM>.

In the determination as to whether the radiation dose (cumulative dose) D detected by the sensor unit <NUM> has reached the irradiation start threshold when the threshold change reference point is set, the radiation dose (cumulative dose) D detected by the sensor unit <NUM> can be suitably used. In this case, if the signal-to-noise (S/N) ratio of the dose detected by the sensor unit <NUM> is sufficiently high, the determination as to whether the radiation dose (cumulative dose) D has reached the irradiation start threshold may be made using a sample dose instead of using the cumulative dose.

If the threshold determination unit <NUM> determines that the radiation dose (cumulative dose) D detected by the sensor unit <NUM> is more than or equal to the irradiation stop threshold Dth for the dose (cumulative dose) that varies with elapsed time illustrated in <FIG> (NO in step S204), the processing proceeds to step S205 in <FIG>. In step S205, the irradiation stop signal <NUM> is transmitted to the irradiation control apparatus <NUM> and the imaging control unit <NUM> performs the irradiation stop control <NUM> to stop irradiation of radiation from the radiation generation apparatus <NUM>.

In this case, the radiation is continuously irradiated by the amount corresponding to the communication delay time Tdc associated with the transmission of the irradiation stop signal <NUM> and the delay time Td from the time when the irradiation stop control <NUM> is performed to the time when irradiation of radiation in the radiation generation apparatus <NUM> is actually stopped.

In the example illustrated in <FIG>, at a dose rate <NUM>, which is a high dose rate of radiation entering the radiation imaging apparatus <NUM>, the radiation dose (cumulative dose) D obtained by the dose calculation unit <NUM> is more than or equal to the irradiation stop threshold Dth for the dose (cumulative dose) at an irradiation time Thigh. As a result, at the irradiation time Thigh, the radiation imaging apparatus <NUM> transmits the irradiation stop signal <NUM> to the irradiation control apparatus <NUM>. After that, irradiation of radiation in the radiation generation apparatus <NUM> is stopped.

In this case, the dose calculation unit <NUM> detects that the irradiation stop threshold Dth is exceeded, and the irradiation stop threshold Dth is determined by taking into consideration the communication delay time Tdc and the delay time Td in the radiation generation apparatus <NUM>. Accordingly, when irradiation of radiation from the radiation generation apparatus <NUM> is actually stopped, the radiation of the target value Dref reaches the radiation imaging apparatus <NUM>.

Similarly, at a dose rate <NUM>, which is a low dose rate of radiation entering the radiation imaging apparatus <NUM>, the radiation dose (cumulative dose) D obtained by the dose calculation unit <NUM> is more than or equal to the irradiation stop threshold Dth for the dose (cumulative dose) at an irradiation time Tlow.

As a result, at the irradiation time Tlow, the radiation imaging apparatus <NUM> transmits the irradiation stop signal <NUM> to the irradiation control apparatus <NUM>. After that, irradiation of radiation in the radiation generation apparatus <NUM> is stopped. The dose calculation unit <NUM> detects that the irradiation stop threshold Dth is exceeded, and the irradiation stop threshold Dth is determined by taking into consideration the communication delay time Tdc and the delay time Td in the radiation generation apparatus <NUM>. Accordingly, when irradiation of radiation from the radiation generation apparatus <NUM> is actually stopped, the radiation of the target value Dref reaches the radiation imaging apparatus <NUM>.

As illustrated in <FIG>, the dose rate <NUM> and the dose rate <NUM> of the radiation are determined based on the relationship between the radiation dose (cumulative dose) D and time. In the graph illustrated in <FIG>, for example, if the irradiation stop threshold Dth for the dose (cumulative dose) is constant, the actual radiation dose (cumulative dose) D varies depending on a change in the dose rate. This may result in a irradiation time longer than a desired timing when irradiation is to be actually stopped. Thus, the value of the actual radiation dose (cumulative dose) D is apart from the target value Dref of the dose (cumulative dose), which causes excessive irradiation.

In the present exemplary embodiment, as illustrated in <FIG>, control processing is performed to change the irradiation stop threshold Dth for the dose (cumulative dose) (specifically, increase the irradiation stop threshold Dth for the dose (cumulative dose) with elapsed time) depending on the elapsed time from start of irradiation of radiation. Accordingly, the value of the target value Dref of the dose (cumulative dose) can be obtained at each of the high dose rate <NUM> and the low dose rate <NUM>. Consequently, control processing to stop irradiation of radiation can be accurately performed regardless of the dose rate.

In the present exemplary embodiment, as illustrated in <FIG>, the time variation of the irradiation stop threshold Dth for the dose (cumulative dose) is sequentially changed with elapsed time. However, the time variation of the irradiation stop threshold Dth for the dose (cumulative dose) may be represented by a step function that varies stepwise with elapsed time, for example, in view of a memory capacity.

In this case, the length of each time segment for the step function associated with the irradiation stop threshold Dth for the dose (cumulative dose) may vary depending on the time segment.

In the present exemplary embodiment, the irradiation stop threshold Dth for the dose (cumulative dose) with respect to an elapsed time t can be set to satisfy the following Expression (<NUM>) using the elapsed time t, the delay time Td, the communication delay time Tdc, the irradiation start threshold Dobs, and the target value Dref of the dose (cumulative dose).

Specifically, as represented by Expression (<NUM>), the threshold decision unit <NUM> changes the irradiation stop threshold Dth for the dose (cumulative dose) depending on the elapsed time t. In addition, the threshold decision unit <NUM> sets the time variation of the irradiation stop threshold Dth for the dose (cumulative dose) based on the target value Dref of the dose (cumulative dose), the irradiation start threshold Dobs, the delay time Td, and the communication delay time Tdc.

In the case of using a step function as the irradiation stop threshold Dth for the dose (cumulative dose), it may be desirable to set a function such that Expression (<NUM>) and each step intersect.

According to this configuration, control processing to stop irradiation of radiation from the radiation generation apparatus <NUM> can be accurately performed. In other words, AEC can be accurately performed.

The exemplary embodiment described above illustrates an example where irradiation of radiation is stopped in a case where the cumulative dose reaches the irradiation stop threshold Dth. Alternatively, a time when the cumulative dose reaches the irradiation stop threshold Dth may be predicted based on the dose rate and irradiation of radiation may be stopped at the predicted time. The time may be predicted based on the dose rate a plurality of times until the cumulative dose reaches the irradiation stop threshold Dth, and the time when the cumulative dose reaches the irradiation stop threshold Dth may be updated in accordance with the change in the dose rate. Also, in this case, the irradiation time can be measured from the time point when the cumulative dose reaches the irradiation start threshold, thereby making it possible to accurately predict the timing for stopping irradiation of radiation.

According to the exemplary embodiment of the present invention, control processing to stop irradiation of radiation from the radiation generation apparatus can be accurately performed.

Claim 1:
A radiation imaging apparatus (<NUM>) comprising:
sensor means (<NUM>) for detecting radiation from a radiation generation apparatus (<NUM>); and
control means (<NUM>) for transmitting a signal for stopping irradiation of radiation to the radiation generation apparatus using a cumulative dose of the radiation detected by the sensor means, an irradiation time for the radiation, and an irradiation stop threshold,
wherein the control means measures the irradiation time based on the cumulative dose and transmits the signal for stopping irradiation of radiation to the radiation generation apparatus based on the irradiation stop threshold, wherein the irradiation stop threshold is changed according to the irradiation time.