X ray computed tomography apparatus and scan start timing determination method

An X-ray computed tomography apparatus, comprising:

• an X-ray tube configured to generate X-rays;
• an X-ray detector configured to detect X-rays generated from the X-ray tube and having passed through an object;
• projection data generation circuitry configured to generate a plurality of projection data values respectively corresponding to a plurality of channels in the X-ray detector based on an output from the X-ray detector;
• setting circuitry configured to set a region of interest on a slice image generated by a first scan for the object;
• and
• scan start timing determination circuitry configured to determine, based on a plurality of projection data values of interest corresponding to the region of interest of the plurality of projection data values generated by a second scan at a dose lower than a dose in the first scan, a timing of terminating the second scan and starting a third scan at a dose higher than a dose in the second scan.