X-ray CT apparatus and optimal cardiac phase determining method

An X-ray CT apparatus, comprising:

• an X-ray irradiation unit that irradiates X-rays from periphery of an object;
• an X-ray detection unit that detects information of X-rays transmitted through the object;
• an electrocardiographic information acquisition unit that acquires electrocardiographic information of the object;
• an image creation unit that creates an image of the object from the X-ray information and the electrocardiographic information;
• a display unit that displays the image;
• a tomographic data creation unit that creates a plurality of pieces of tomographic data with different cardiac phases from the X-ray information and the electrocardiographic information;
• a region data generation unit that generates region data by extracting a predetermined region for each piece of the tomographic data;
• a variation distribution calculation unit that calculates a variation distribution within the tomographic data by calculating a variation between cardiac phases regarding the region data;
• and
• a degree-of-harmony calculation unit that calculates a degree of harmony, which is an index indicating whether local locations of a target portion of the object move similarly, on the basis of the variation distribution.