As X-ray CT apparatuses are known a single slice type X-ray CT apparatus for obtaining one tomogram by one X-ray exposure and a multi-slice type X-ray CT apparatus for simultaneously obtaining plural tomograms by one X-ray exposure.
According to the single slice type X-ray apparatus, an X-ray detector having many X-ray detection elements arranged on a line, that is, in a one-dimensional direction (channel direction) is used, a fan beam, that is, a sectorial X-ray beam is emitted from an X-ray tube to an examinee and X-ray which has passed through the examinee is measured to obtain projection data of the examinee.
On the other hand, according to the multi-slice type X-ray CT apparatus, a cone beam, that is, a conical or pyramidal X-ray beam is emitted from an X-ray tube to an examinee, and X-ray which has passed through the examinee is measured by an X-ray detector having many X-ray detection elements arranged in a two-dimensional direction (channel direction and line direction), thereby obtaining projection data.
In both the X-ray CT apparatuses, the X-ray tube and the X-ray detector which are opposed to each other are rotated around the examinee to collect projection data in many directions, and these collected projection data are subjected re-constructing filtering processing for correction of blurring, and then the data are inversely projected to reconstruct a tomogram(s) of the examinee.
The projection data is collected at a discrete projection angle (hereinafter referred to as “view”) of an X-ray beam emitted from the X-ray tube, and the thus-collected projection data is referred to as “the projection data at the corresponding view”). The number of views per rotation of the X-ray tube and the X-ray detector rotating around the examinee, which is required to reconstruct one CT image, normally ranges from several hundreds to about one thousand. Furthermore, the projection data of one view comprises data corresponding to the number of channels×the number of lines of the X-ray detector (in the case of the single slice type X-ray CT apparatus, the number of lines=1 as described above).
Recently, a method of analyzing the composition of an examinee on the basis of images picked up by irradiating the same cross-sectional plane with X-ray beams having plural different energies has been used in the X-ray CT apparatuses as described above, and this method is called as a multi-energy imaging method.
Particularly, when imaging is performed by using two different kinds of energies, it is called as a dual energy imaging method.
A method of measuring tomograms of an average atomic number and an average density of an examinee by applying the dual energy imaging method is disclosed in Non-patent Document 1.
Furthermore, a method of performing the dual energy imaging while the voltage between the anode and the cathode of the X-ray tube (hereinafter referred to as “tube voltage”) is varied every X-ray projection angle to change the energy spectrum of the X-ray (hereinafter referred to as “tube voltage modulation method”) is disclosed in Patent Document 1.
Non-patent Document 1: R. E. Alvarez and A. Macovski, “Energy-selective Reconstructions in X-ray Computed Tomography, ”Phys. Med. Biol. Vol. 21, No. 5, pp. 733-744, (1976)
Patent Document 1: JP-A-10-73544
In order to pick up an image of a fast-moving site of an examinee such as a heart, coronary artery or the like, it has been proposed to increase the rotating speed (that is, the scan speed of an imaging system comprising a pair of an X-ray tube and an X-ray detector), in an X-ray CT apparatus for medical application, and X-ray CT apparatuses of about 0.33 to 0.4 [second/revolution] have been practically used.
In such X-ray CT apparatus whose scan speed is increased, the number of views per rotation of the imaging system in normal X-ray CT imaging is equal to about 1,000, and when the scan speed is equal to 0.33 second, imaging is once carried out per rotation of 0.36° (=360°/1,000).
On the other hand, when dual energy imaging is applied to the conventional X-ray CT apparatus as described above by using the technique of the patent document 1, the following problems occur.
(1) Current flowing between the anode and the cathode of the X-ray tube (hereinafter referred to as “tube current”) is fixed irrespective of the magnitude of the tube voltage.
This is because it is difficult to switch the tube current at high speed due to thermal inertia of the filament temperature of the X-ray tube and thus the tube current is controlled to be fixed at a high energy (high tube voltage) and at a low energy (low tube voltage), so that the tube current is not switched.
Normally, the X-ray amount absorbed in an examinee increases when imaging is carried out at a low tube voltage, and thus the tube current is required to be larger than that at the high tube voltage.
Furthermore, when the tube current runs short at the low tube voltage, the quantum noise in the pickup image increases, and the quality of the pickup image is lowered.
Still furthermore, it is possible to obtain sufficient projection data at the high tube voltage, and thus it is desired that the tube current is set to be less than that at the low tube voltage from the viewpoint of reduction of exposure.
For the foregoing reason, it is originally desired to vary the tube current (reduce the tube current/increase the tube current) in connection with the variation of the tube voltage.
(2) In the technique described in the patent document 1, as described in paragraph number [0058] of the patent document 1, the number of views which are obtained in each of the high energy case and the low energy case during one rotation of a scanner is equal to 600 and this is a small number. Accordingly, when each view data obtained in each of the high energy case and the low energy case is subjected to image reconstruction, there is concern about occurrence of a radial artifact.
This is because a delay occurs in a process of supplying the tube voltage from a power source portion to the X-ray tube due to parasitic impedance (parasitic inductance and parasitic electrostatic capacitance) possessed by wires and thus increase of the switching speed of the tube voltage is restricted, thereby limiting the number of views.