Patent Number: 054992834
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 6, one embodiment of an X-ray CT apparatus according to the present invention will be described in detail. In this embodiment, an X-ray CT apparatus 1 has a scanner unit 2 for scanning with respect to a body P to be examined, including a bed plate 3 for carrying the body P to be examined along a direction of the body axis of the body to be examined P, an X-ray tube 4 for irradiating X-rays on the body to be examined P on the bed plate 3, and a detector 5 for detecting the X-rays irradiated by the X-ray tube 4 and penetrated through the body to be examined P, where the bed plate 3 is linearly movable along the direction of the body axis of the body to be examined P, while the X-ray tube 4 and the detector 5 are integrally rotatable around the body to be examined P at a predetermined constant angular speed. In this scanner unit 2, the helical scan imaging is carried out by moving the body to be examined P located on the bed plate 3 along the direction of the body axis of the body to be examined P while rotating the X-ray tube 4 and the detector 5 around the body to be examined P, such that the X-ray tube 4 moves along a helical trajectory relative to the body to be examined P. In addition, this X-ray CT apparatus 1 further comprises a bed plate driving unit 6 for driving the bed plate 3 in the linear motion along the direction of the body axis of the body to be examined P; a bed plate controller 7 for controlling the driving operation by the bed plate driving unit 6 in order to control the linear motion of the bed plate 3 appropriately; a data collection unit 8 for collecting data from the X-rays detected by the detector 5; an image reconstruction unit 9 for reconstructing the tomographic images according to the data collected by the data collection unit 8; a display unit 10 for displaying the tomographic images reconstructed by the image reconstruction unit 9; a data collection controller 11 for controlling the data collection operation of the data collection unit 8 appropriately; and the input unit 12 from which an operator enters a desired scanning region, according to which the bed plate controller 7 and the data collection controller 11 control the bed plate driving unit 6 and the data collection unit 8. In carrying out the helical scan imaging in this X-ray CT apparatus, the operator enters the desired imaging region in which the tomographic images are to be obtained for the slice planes located therein, through the input unit 12, and places the body to be examined P on the bed plate 3 such that a scan start side end of the imaging region is located at a predetermined scanning position. Then, in accordance with the desired imaging region entered at the input unit 12, the bed plate controller 7 and the data collection controller 11 controls the bed plate driving unit 6 and the data collection unit 8 as follows. As shown In FIG. 7, when the imaging region J is set up with respect to the body to be examined P with a scan start side end K located at the scanning position, the bed plate controller 7 automatically determines extra data regions L and R. Each of regions L and R includes a half main data region covering a half rotation (180.degree.) part of a main data region for a first slice plane in the imaging region J and an associated supplementary data region covering additional half rotation (180.degree.) adjacent to the half main data region, as well as an initial acceleration region M for accounting an initial acceleration of the bed plate 3 and a final deceleration region S for accounting a final deceleration of the bed plate 3. The bed plate controller 7 controls the bed plate driving unit 6 to move the bed plate 3 in a direction opposite to a scanning direction for a predetermined distance such that a scan start side end N of the initial acceleration region M is moved to the scanning position. Here, the extra data regions L and R, the initial acceleration region M, and the final deceleration region S can be determined in advance according to the predetermined constant rotational speed of the X-ray tube 4 and the detector 5 and a linear motion characteristic of the bed plate 3. Then, as the helical scan imaging starts, the X-ray tube 4 and the detector 5 are integrally rotated at a predetermined constant angular speed around the body to be examined P at the scanning position while the bed plate controller 7 controls the bed plate driving unit 6 to move the bed plate 3 in the scanning direction such that the scanning is carried out for the entire scanning region formed by the imaging region J, extra data regions L and R, initial acceleration region M, and final deceleration region S. This is done until a scan finish side end T of the final deceleration region S stops at the scanning position. As a result, the bed plate 3 initially accelerates for a distance covered by the initial acceleration region M, moves at a predetermined constant linear speed through a distance covered by the extra data regions L and R and the imaging region J, and finally decelerates for a distance covered by the final deceleration region S, such that the X-ray tube 4 moves along a helical trajectory relative to the body to be examined P. Meanwhile, when the imaging region J is set up with respect to the body to be examined P with a scan start side end K located at the scanning position, the data collection controller 11 also similarly determines extra data regions L and R as well as the initial acceleration and the final deceleration region S around the imaging region J automatically. The data collection controller 11 controls the data collection unit 8 such that the data from the X-rays detected by the detector 5 are collected only in the extra data regions L and R and the imaging region J. In other words, the data are collected by the data collection unit 8 between a scan start side end U of the extra data region L and a scan finish side end W of the extra data region R. Then, the data collected by the data collection unit 8 from the extra data regions L and R and the imaging region J are fed to the image reconstruction unit 9 in which image data necessary to reconstruct the tomographic images at desired slice planes are derived by using the interpolation on the collected data, and the tomographic images at the desired slice planes are reconstructed by using the derived image data. Here, the interpolation can be achieved similarly to a conventional manner described above in the background of the invention section. The reconstructed tomographic images are then displayed on the display unit 10. Thus, according to this embodiment, the initial set up operation for the helical scan imaging can be achieved by simply specifying the desired imaging region J at the input unit 12, so that the scanning region can be set up accurately, while reducing the burden of the operator. It is to be noted that in a case where the imaging region J is too wide to be covered by a single scan such that more than one scans are required, the bed plate controller 7 may control the bed plate driving unit 6 as follows. In such a case, as shown in FIG. 8, when the first scan finishes at a position X, the bed plate controller 7 determines a readjustment region Y containing another extra data region adjacent to the position X, correction data region, and another initial acceleration region, and controls the bed plate driving unit 6 to move the bed plate 3 in a direction opposite to a scanning direction for such a distance that a scan start side end Z of the readjustment region Y is moved to the scanning position. Hence, the next scan can be started at the scan start side end Z of the readjustment region Y. Here, the correction data region is provided in the readjustment region Y in order to obtain additional data necessary in removing the inconsistency in the collected data due to the displacement of the slice plane caused by the physical motion of the body to be examined P during the readjustment between the successive scans. Thus, in this case, the data collected at the additional extra region and the correction data region overlap with the data collected in the previous scan, so as to enable the effective data correction for the subsequent scan. Furthermore, the readjustment may be achieved such that the data collected at a part of the main data region in the subsequent scan also overlap with the data collected in the previous scan, in order to account for the physical motion of the target portion of the body to be examined P due to breathing or some other cause during the readjustment between the successive scans. It is also to be noted that, in the above embodiment, any one of the initial acceleration region M, final deceleration region S, and a supplementary data region in the extra data regions L and R may be omitted if its omission is preferred. Moreover, although the above embodiment has been described for a third generation type X-ray CT apparatus, the present invention is equally applicable to a fourth generation type X-ray CT apparatus. Similarly, although the above embodiment has been described for a case of a full scan using a full 360.degree. rotation of the X-ray tube and a detector around the body to be examined, the present invention is also equally applicable to an X-ray CT apparatus for carrying out a half scan in which the data for one rotation are obtained by using scans of 90.degree. plus a fan angle on both sides of the desired image center position. Besides these, many modifications and variations of the above embodiment may be made without departing from the novel and advantageous features of the present invention. Accordingly, all such modifications and variations are intended to be included within the scope of the appended claims.