1. Field of the Invention
The present invention relates to a method for controlling a radiation image pickup apparatus suitable for medical diagnosis and industrial non-destructive inspection, the radiation image pickup apparatus, and a radiation image pickup system and, in particular, to a method for controlling a radiation image pickup apparatus, the radiation image pickup apparatus, and a radiation image pickup system capable of detecting the presence of irradiation, such as start and end of irradiation.
2. Description of the Related Art
A radiation image pickup apparatus including a flat panel detector (hereinafter simply referred to as an “FPD”) performs an image capturing operation in synchronization with irradiation performed by a radiation generating apparatus. To synchronize two operations, the following techniques are employed. That is, as described in U.S. patent application Ser. No. 2007/0125952, a radiation image pickup apparatus includes a plurality of pixels. Each of the pixels includes a conversion element that converts radiation or light into electric carriers and a switch element that transfers an electric signal based on the electric carriers so as to be capable of providing a desired voltage to one of two electrodes of the conversion element. The pixel further includes a detection unit for detecting the presence of irradiation. Before the detection unit detects irradiation from the radiation generating apparatus, a conducting voltage is sequentially provided from a drive circuit to the switch elements on a row-by-row basis in order to switch the switch elements to a conducting state. In this manner, the voltage of one of the electrodes of the conversion element is reset. Thereafter, if the detection unit detects start of irradiation from the radiation generating apparatus, supply of the conducting voltage from the drive circuit is stopped. Thus, a non-conducting voltage is supplied from the drive circuit to all of the switch elements in order to switch the switch elements to a non-conducting state. Accordingly, the electric carriers generated in the conversion element is accumulated in each of the pixels. In contrast, if the detection unit detects end of irradiation, a conducting voltage is sequentially supplied from the drive circuit to the switch elements on a row-by-row basis. Thus, an electric signal in accordance with the accumulated electric carriers is transferred from the pixel.
Japanese Patent Laid-Open No. 2011-185622 describes that a noise component that is not caused by irradiation is coupled with a signal output from a detection unit for detecting irradiation due to a reset operation, and the noise component has a particular variation. Therefore, according to Japanese Patent Laid-Open No. 2011-185622, the profile of a signal used for detecting irradiation is measured in advance, and differential processing is performed on a measured signal for detecting irradiation using the profile. Thereafter, the signal subjected to the differential processing is compared with a predetermined threshold value to detect start of irradiation.
As described above, in order to accurately detect the presence/absence of irradiation, the threshold value needs to be optimally set. However, U.S. patent application Ser. No. 2007/0125952 and Japanese Patent Laid-Open No. 2011-185622 do not mention the setting of an optimal threshold value and, thus, further study is needed.
Accordingly, the present invention provides a radiation image pickup apparatus capable of setting an optimal threshold value used for instantaneously and highly accurately detecting the presence/absence of irradiation.