unit 120 see fig8 as a result ct image data referred to 34 post data 34 is acquired then the pre data ct image data is subtracted from the post data ct image data to obtain such a three dimensional contrast medium distribution as a subtraction image map 34 dif 34 as shown in fig1 since this subtract image map 34 dif 34 corresponds to variation amount of the ct value this variation amount is converted into an amount of blood 34 cbv 34 in accordance with the following calculation method where symbol 34 bpost 34 denotes a ct value of the blood acquired before injection of the x ray contrast medium and symbol 34 bpre 34 shows a ct value of the blood after injection of the x ray contrast medium b 1 when the blood is collected at the time instants 34 t 2 34 and 34 t 3 34 see fig1 the ct values of this blood are measured by operating the second helical dynamic scanner and a difference between these ct values is calculated as 34 u03b4b 34 b 2 at the time instant 34 t 3 34 the blood is collected and then processed by the chemical analysis so as to obtain density of the x ray contrast medium contained therein this density is converted into a variation amount of a ct value b 3 no blood collection is carried out alternatively a measurement is carried out for u03b4ct value of a large blood vessel contained in the subtraction image map 34 dif 34 as described above according to the second preferred embodiment the cbv of the brain of the patient 120 can be measured in a similar helical dynamic scanning operation of the first x ray ct imaging system shown.