1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT) apparatus that collects projection data by exposing X-rays to a subject and generates an image based on the projection data, and relates to a method of controlling the X-ray CT apparatus
2. Description of the Related Art
The X-ray CT apparatus includes an X-ray tube and an X-ray detector, which are arranged opposite to one another by sandwiching a subject. The X-ray CT apparatus rotates the X-ray tube and the X-ray detector about the subject while exposing X-rays from the X-ray tube and detecting, in the X-ray detector, X-rays transmitted through the subject. The data detected by the X-ray detector is collected as projection data in a data acquisition system (DAS), and the collected projection data is used to generate an image of the subject. For example, the projection data is collected by rotating the X-ray tube and the X-ray detector by 360° or (180°+fan angle) and the image is generated based on the projection data over 360° or (180°+fan angle).
A set of detection data detected at an angle relative to the subject by the X-ray detector having a plurality of detection elements is referred to as a “view”. A single rotation of the X-ray tube and the X-ray detector about the subject to collect the projection data of a plurality of views necessary to reconstruct an image is referred to as a “scan”. For example, when the projection data for 1 view is collected for each 1°, the projection data for 360 views is obtained for 1 scan and the image is reconstructed using the projection data of these 360 views.
The collection of the projection data is continuously performed by the data acquisition system (DAS) while X-rays may be intermittently exposed from the X-ray tube. When the image is reconstructed using the projection data collected by such imaging, it is necessary to reconstruct the image using the projection data collected over a period during which X-rays have actually been exposed. For example, in cardiac testing, imaging may be performed by switching the X-ray exposure on and off according to the subject's heartbeat (prospective gating scan) (e.g., Japanese Unexamined Patent Application Publication No. 2009-28065). In this case, it is necessary to reconstruct the image using the projection data collected by the data acquisition system (DAS) over a period during which X-rays have actually been exposed.
In the prior art, an X-ray detector (Ref detector) that is to be a reference is provided to the X-ray CT apparatus and a detection value (count value) detected by the Ref detector is added to the projection data for each view. Then, based on the count value detected by the Ref detector and added to each projection data for 1 view, it is determined whether X-rays have been exposed and it is thereby determined whether the projection data has been collected during a period in which X-rays have been exposed. In addition, when the exposure amount of the subject during the actual imaging is obtained, the dose is calculated by counting the views for which X-rays have been exposed.
In addition, in a scan mode in which X-rays are exposed while modulating the X-ray output (mA), the level of X-ray output (mA) can be judged by a detection value detected by the Ref detector but the actual X-ray output value (mA value) cannot be judged. Therefore, the dose is calculated using the X-ray output value that has been set by the X-ray CT apparatus during imaging planning.
In the prior art, by setting a threshold relative to the detection value (count value) of the Ref detector, it is determined whether the projection data for each view has been collected during a period in which X-rays have been exposed. However, the count value of the Ref detector is varied according to each Ref detector. In addition, the count value of the Ref detector is varied according to the conditions for X-ray exposures. Therefore, it is difficult to set a threshold suitable for determining whether X-rays have been exposed or not. For example, it is necessary to take into consideration the individual differences of the Ref detectors and the variations in the individual differences of the X-ray tube. In addition, the count value of the Ref detector is varied according to the tube voltage. Therefore, it is difficult to set a suitable threshold. In addition, by taking into consideration the offset value of the Ref detector and the data acquisition system (DAS), it is necessary to decide the threshold for determining the ON and OFF states of the X-ray exposure. Moreover, it is possible to make an error in determining the ON and OFF states of the X-ray exposure if the X-ray tube is unstable and may, for example, instantaneously generate a discharge or undergo current fluctuations.
If there is an error in determining the ON and OFF states of the X-ray exposure, the number of views necessary for reconstruction may not be obtained or the image may be reconstructed using the projection data of views that have been collected when the X-rays were in the OFF state.
Furthermore, using the detection value (count value) of the Ref detector makes it possible to determine only the ON and OFF states of the X-ray exposure. In a scan mode in which the X-ray output (mA) is modulated, the actual X-ray output value cannot be judged. Therefore, it is difficult to accurately calculate the actual exposure dose.