1. An X-ray CT apparatus comprising: processing circuitry configured to collect pieces of image data in a plurality of time phases that contain at least a part of a coronary artery of a heart; acquire image indexes regarding a plurality of pieces of the image data; extract a set of image data from combinations of pieces of image data having a larger time interval than a predetermined time interval, of the pieces of image data, based on the image indexes in the respective pieces of image data; and perform fluid analysis regarding the coronary artery based on the extracted set of image data to obtain a fluid parameter regarding the coronary artery.