1.  Field of the Invention
The present invention relates to a computer tomography apparatus (to be abbreviated as a CT hereinafter) and, more particularly, to a CT which can continuously execute a scan operation. 
2.  Description of the Related Art
In general, in a CT, three processing operations, i.e., scan processing, image re-construction processing, and image display processing are time-serially executed.  Projection data in multiple directions, which are acquired upon rotation of an X-ray tube or integral rotation of an X-ray tube and a detector array, are converted into digital data, and the digital data are subjected to pre-processing such as calibration.  Thereafter, the digital data are temporarily stored as raw data in a large-capacity storage device such as a magnetic disk. 
Upon execution of re-construction processing, the raw data are read out from the magnetic disk, and are supplied to a re-construction unit via a memory.  Tomographic image data re-constructed by the re-construction unit are stored in the magnetic disk, and are transferred to and displayed on a CRT monitor as a video signal via a display memory. 
With the advent of a slip ring, a continuous scan has become possible.  With the continuous scan, a plurality of projection data in multiple directions associated with one or a plurality of slices can be time-serially acquired.  These projection data in multiple directions are read out to the re-construction unit at an arbitrary timing via the magnetic disk, as described above, and are subjected to re-construction.  The time required for the re-construction processing is longer than the scan time, and the magnetic disk requires long storage and access times.  Therefore, it is impossible to display tomographic images in real time like cinema images while executing a continuous scan. 
In recent years, high-speed re-construction processing has been examined, and is about to reach a practical application range.  With this processing, tomographic images can be continuously displayed like cinema images as in an X-ray television system while executing a continuous scan.  However, when this real-time X-ray CT is utilized in an actual clinical application, the following problems are posed.  First, since scans are executed for an unnecessarily long period of time not only for a portion required for diagnosis but also a portion which is not required for diagnosis, the exposure amount undesirably increases.  Second, since tomographic images are continuously re-constructed in a continuous scan, when a change in specific heartbeat phase over time is to be observed, an observer must select and observe the tomographic images of the specific heartbeat phase by himself or herself, and cannot concentrate on image diagnosis.  Third, in the case of angiography diagnosis, since the flow-in timing of a contrast medium into a region of interest is unknown, the scan must be started sufficient before the actual flow-in.  Fourth, the remaining time until an X-ray tube reaches the limit heat capacity and undergoes a forcible system-down state cannot be determined.  Fifth, the scans cannot be immediately started after a scanogram used for, e.g., alignment of scans is imaged.