Patent Number: 056688458
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a computed-tomography (CT) apparatus for acquiring a cross-sectional image of an object by use of X-rays, ultrasonic waves or the like, and more particularly to such a CT apparatus suitable for use as a medical diagnosis apparatus. In recent years, CT apparatuses for acquiring cross-sectional images of objects have widely been used especially as medical diagnosis apparatuses for diagnosing patients. Such apparatuses include an X-ray CT apparatus which uses X-rays, a radiography isotope (RI) CT, ultrasonic CT and image intensifier (I.I) CT apparatuses in which the measurement of projection data is made from the circumferential directions of an object around the object to reconstruct an image, and so forth. In a general X-ray or other type CT apparatus, a time for measuring an object at a predetermined slice position through the scan of the entire circumference of the object (or so-called 360.degree. one-slice measurement time) differs according to a plurality of measurement modes which can be changed. Usually, this measurement time is 1 to 9 seconds. Therefore, it is general that an operator of the apparatus selectively uses the optimum measurement mode in accordance with the size of an object, a part to be subjected to diagnosis or the purpose of diagnosis. If a motion such as the motion of an object or the motion of internal organs of the object occurs during the above-mentioned measurement time, artifacts called motion artifacts are generated due to this motion. Owing to the artifacts, an accurate diagnosis from the acquired cross-sectional image becomes difficult. The well known method for solving such a technical problem is a measurement data correcting method disclosed by, for example, U.S. Pat. No. 4,580,219 issued on Apr. 1, 1986 and entitled "METHOD FOR REDUCING IMAGE ARTIFACTS DUE TO PROJECTION MEASUREMENT INCONSISTENCIES". This correction method is generally called bowel gas correction. The bowel gas correction method will now be explained briefly by use of FIG. 6. In a usual CT apparatus, a measurement start position of a scanner is fixed beforehand at a predetermined position. The scanner starts the measurement from the predetermined measurement start position and makes a 360.degree. scan over the entire circumference of an object to obtain projection data of the object. A measurement end position assumes the same position as the measurement start position. Therefore, the first image and the last image will be consistent with each other if there is no motion of the object during an interval between the measurement start and end positions. In the actual measurement, however, when the object moves during the measurement interval, discontinuities are generated between measurement data at the measurement start and end positions. Owing to the discontinuities of measurement data, inconsistencies called misregistration differences appear between the projection data measurement start and end positions. The inconsistencies cause the generation of motion artifacts after image reconstruction. For such circumstances, data correction called bowel gas correction is made in order that the continuity of data in a predetermined rotation angle range (hereinafter referred to as correction region A) near each of the measurement start and end positions is improved even if any motion of the object is involved. In general, the same result is given by CT projection data obtained by measuring an object from directions which are different by 180.degree.. In the bowel gas correction, the contribution of projection data to the reconstruction of a cross-sectional image is modified. More particularly, the contributions of projection data near the measurement (or scan) start position and projection data near the measurement end position providing the cause of artifacts are reduced while the contributions of projection data near a position opposite to the measurement start position with 180.degree. therebetween and projection data near a position opposite to the measurement end position with 180.degree. therebetween are increased. Namely, the cross-sectional image of one slice is generated by assigning a weight smaller than 1 to projection data in the predetermined rotation angle range (or the correction region A in FIG. 6) and assigning a weight greater than 1 to projection data in a rotation angle range (hereinafter referred to as correction region B) opposite to the correction region A. The correction regions A and B are positioned opposite to each other and have the same rotation angle range. The weight takes the smallest value (zero) at the measurement start position and the measurement end position and is gradually increased with the increase of a distance from the measurement start position and the measurement end position. Also, the weight takes the greatest value at the middle portion of the correction region B opposite to the measurement start position and the measurement end position and is gradually decreased with the increase of a distance from the correction region B. The case where no bowel gas correction is made corresponds to the case where the same weight (1.0) is used at any position, as shown by dotted line in FIG. 6. In a medical diagnosis CT apparatus such as X-ray CT apparatus, a measurement method is generally performed in which a contrast agent is injected into a blood vessel to emphasize constrastive differences between various tumors and normal tissues, thereby facilitating the diagnosis of a patient. In this method, the contrast agent flows away as the blood circulates. Therefore, the timing of injection of the contrasting agent and the timing of start of measurement provide important factors for accurate diagnosis. Particularly, in the tomographic imaging of a patient with an impediment in consciousness or an infant, an operator starts the measurement at a timing when the object has no motion. In such cases, the concurrency of a measurement start instruction (or operation) by the operator and the start of measurement by the diagnosis apparatus is an important task for the purpose of providing a cross-sectional image which has a high diagnostic value. Under such backgrounds, a continuously rotatable scanner having, for example, a slip ring mounted thereon has recently been used widely. In such a scanner, the measurement start position can be set freely. Therefore, it is possible to improve the concurrency of an operator's desired measurement start timing and the start of measurement by the diagnosis apparatus, thereby shortening a time for a series of measurements. SUMMARY OF THE INVENTION FIGS. 7A, 7B, 7C and 7D show the examples of cross-sectional images of a human belly subjected to bowel gas correction. As shown, linear artifacts 20 appear in the image. A plurality of short lines designated by reference numeral 21 are background noises. In the cases of FIGS. 7A and 7B, the correction region A (see FIG. 6) for bowel gas correction is set to be wide as compared with that in the cases of FIGS. 7C and 7D. As the correction region A becomes wider, the continuity between data near the measurement start position and data near the measurement end position is improved. Accordingly, the artifacts 20 in the image of FIG. 7A are reduced as compared with those in the image of FIG. 7C and therefore give a reduced influence on the diagnosis of the image of internal organs. On the contrary, in the case of FIG. 7C in which the correction region A is narrow, the artifacts 20 appear strongly in the image. However, in the case where the correction region A is set to be wide, the background noises 21 appear strongly in the image, as apparent from FIG. 7B. On the contrary, in the case where the correction region A is set to be narrow, the background noises 21 are weakened, as apparent from FIG. 7D. The reason why the background noises are increased when the correction region for bowel gas correction is wide, is that the range of data giving no contribution to image generation is increased due to the correction, thereby deteriorating the efficiency of utilization of X-rays with the relative increase of the background noises. Consequently, the bowel gas correction involves a problem that the suppression of artifacts is accompanied by the increase of background noises. In the conventional CT apparatus, a cross-sectional image obtained through measurement from an axial direction having a large attenuation of radiation is characterized in that conspicuous background noises appear along that axial direction. FIG. 8 shows projection data 24 which is obtained by irradiating an elliptic object 22 with X-rays emitted from an X-ray source suited at a position 23 on the x axis and projection data 26 which is obtained by irradiating the object 22 with X-rays emitted from an X-ray source suited at a position 25 on the y axis. As apparent from FIG. 8, the thickness of the object 22 is large in the x-axis direction and hence the attenuation of X-rays in the x-axis direction is larger than that in the y-axis direction. In this case, background noises will be conspicuous in the image data obtained through the measurement from the x-axis direction. If a direction having a large attenuation (such as the x-axis direction in the above example) coincides with a measurement start position by chance, the bowel gas correction will cause a further increase of background noises. In an image having a low contrast, the background noises make the diagnosis difficult. In addition, artifacts are an obstacle to the diagnosis. In medical diagnosis apparatuses for making the diagnosis of patients for the purpose of medical diagnosis, a variety of combinations may be employed in accordance with the shapes of objects such as patients and the measurement conditions. Therefore, it is desired to acquire a high-quality cross-sectional image by setting the optimum bowel gas correction region for all the combinations while synthetically considering merits and demerits based on the shapes of objects and the measurement conditions. In the practical state of the conventional CT apparatus, however, it is difficult to satisfy such a requirement. The present invention has been completed on the basis of the present inventors knowledge of the bowel gas correction in the prior art and the relationship between background noises and an object as mentioned above. ACT apparatus according to the present invention provides an image in which the increase of background noises necessarily associated with the bowel gas correction is reduced to the possible minimum while the motion artifact correction effect based on the bowel gas correction is maintained to the optimum and the shape of an object and the difference in attenuation depending on the axial direction of the object are taken into consideration. One object of the present invention is to provide a CT apparatus which can provide a high-quality cross-sectional image necessary for accurate diagnosis of a patient in the capacity of a medical diagnosis apparatus. ACT apparatus according to the present invention realizes the concurrency of the operation of measurement start by an operator and the actual measurement start. In the above-mentioned continuously rotatable scanner having a slip ring mounted thereon, the operation of measurement start by the operator substantially coincides with the actual measurement start. However, in a scanner based on a control method in which the operation of measurement start is performed from a certain fixed position, there may occur a situation in which a measurement instruction is issued by an operator when the scanner has already passed the measurement start position. In this case, there is generated a time delay until the scanner reaches the measurement start position again. In a measurement mode in which a long measurement (or scan) time is set for one slice, not only such a time delay but also the variations of a time from the timing of measurement instruction issuance until the actual measurement start are dominant problems in making the quantitative analysis of an image. Therefore, another object of the present invention is to provide a CT apparatus which can provide a high-quality cross-sectional image by minimizing a time delay from the designation of measurement start by an operator until the actual measurement start irrespective of the lengths of scan times determined by measurement modes which may be employed in a scanner based on a control method with the operation of measurement start performed from a fixed position. In a CT apparatus according to an embodiment of the present invention, the scan is started from a direction having a small attenuation of radiation to acquire a cross-sectional image of an object in which background noises and motion artifacts are suppressed to the possible minimum. In the CT apparatus of this embodiment, one of rotation positions of a scanner having a smaller attenuation of radiation is determined. The operation of the scanner is controlled so that the scan is started from the determined rotation position having the smaller attenuation. Individual weights are assigned to projection data in a predetermined rotation angle range including the vicinity of a scan start rotation position of the scanner and the vicinity of a scan end rotation position thereof and projection data in a rotation angle range opposite to the predetermined rotation angle range, respectively. The projection data assigned with the weights and projection data assigned with no weight are used to determine corrected data for the entire circumference of the object, thereby generating a cross-sectional image of the object on the basis of the corrected data. In a CT apparatus according to another embodiment of the present invention, the range of a correction region is changed in accordance with a scan start position. If the scan start position is not suited in a small attenuation of radiation, the correction region is narrowed to optimize the effect of suppression of background noises and artifacts. In the CT apparatus of this embodiment, one of rotation positions of a scanner having a smaller attenuation of radiation is determined. Further, a scan start rotation position of the scanner is detected. In the case where the detected scan start rotation position is not suited in the rotation position having the smaller attenuation, a predetermined rotation angle range including the vicinity of a scan start rotation position of the scanner and the vicinity of a scan end rotation position thereof is narrowed. Individual weights are assigned to projection data in the predetermined rotation angle range and projection data in a rotation angle range opposite to the predetermined rotation angle range, respectively. The projection data assigned with the weights and projection data assigned with no weight are used to determine corrected data for the entire circumference of an object, thereby generating a cross-sectional image of the object on the basis of the corrected data. In a CT apparatus according to a further embodiment of the present invention, a delay corresponding to a time from the operation of scan (or measurement) start by an operator until the actual scan start and the variations of such a time are minimized by setting a plurality of scan start positions on the locus of scan rotation of the scanner. In the CT apparatus of this embodiment, a plurality of scan start positions are detected during the rotation of the scanner. The actual scan is started from a scan start position which is first detected from the point of time when a scan start signal is issued.