Patent Number: 
Section: description

Several embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It should be noted that the present invention is not limited to these embodiments. FIG. 1 is a block diagram of an X-ray CT apparatus, which is an embodiment of the present invention. The configuration of the apparatus represents an embodiment of the apparatus in accordance with the present invention. The operation of the apparatus represents an embodiment of the method in accordance with the present invention. As shown in FIG. 1, the apparatus comprises a scan gantry 2, an imaging table 4 and an operating console 6. The scan gantry 2 is an embodiment of the signal acquiring apparatus of the present invention. The scan gantry 2 has an X-ray tube 20. The X-ray tube 20 is an embodiment of the X-ray tube of the present invention. X-rays (not shown) emitted from the X-ray tube 20 are formed into, for example, a fan-shaped X-ray beam, i.e., a fan beam, by a collimator 22, and projected onto a detector array 24. The collimator 22 is an embodiment of the collimator of the present invention. The detector array 24 has a plurality of X-ray detector elements arranged in line as an array in the extent direction of the fan-shaped X-ray beam. The detector array 24 is an embodiment of the detector element array of the present invention. The configuration of the detector array 24 will be described in detail later. The X-ray tube 20, collimator 22 and detector array 24 together constitute an X-ray emitting/detecting apparatus, which will be described in detail later. The detector array 24 is connected with a data collecting section 26 for collecting data detected by the individual X-ray detector elements in the detector array 24. The emission of the X-rays from the X-ray tube 20 is controlled by an X-ray controller 28. The connection relationship between the X-ray tube 20 and the X-ray controller 28 is omitted in the drawing. The collimator 22 is controlled by a collimator controller 30. The connection relationship between the collimator 22 and the collimator controller 30 is omitted in the drawing. The above-described components from the X-ray tube 20 through the collimator controller 30 are mounted on a rotating section 34 of the scan gantry 2. The rotation of the rotating section 34 is controlled by a rotation controller 36. The connection relationship between the rotating section 34 and the rotation controller 36 is omitted in the drawing. The imaging table 4 is intended to carry an object to be imaged (not shown) into and out of an X-ray irradiation space in the scan gantry 2. The relationship between the object and the X-ray irradiation space will be described in detail later. The operating console 6 has a data processing apparatus 60, which is comprised of, for example, a computer. The data processing apparatus 60 is connected with a control interface 62, which is in turn connected with the scan gantry 2 and the imaging table 4. The data processing apparatus 60 controls the scan gantry 2 and the imaging table 4 via the control interface 62. The data collecting section 26, X-ray controller 28, collimator controller 30 and rotation controller 36 in the scan gantry 2 are controlled via the control interface 62. The individual connections between these sections and the control interface 62 are omitted in the drawing. The data processing apparatus 60 is also connected with a data collection buffer 64, which is in turn connected with the data collecting section 26 in the scan gantry 2. Data collected at the data collecting section 26 is input to the data processing apparatus 60 via the data collection buffer 64. The data processing apparatus 60 performs image reconstruction using signals of the transmitted X-rays for a plurality of views collected via the data collection buffer 64. The image reconstruction is performed using a filtered back projection technique, for example. The data processing apparatus 60 is an embodiment of the tomographic image producing apparatus of the present invention. The data processing apparatus 60 is also connected with a storage device 66 for storing several kinds of data, reconstructed images, programs for implementing the functions of the present apparatus, and so forth. The data processing apparatus 60 is moreover connected with a display device 68 that displays the reconstructed image and other information output from the data processing apparatus 60, and an operating device 70 that is operated by a user supplying several instructions and information to the data processing apparatus 60. The user interactively operates the present apparatus using the display device 68 and the operating device 70. FIG. 2 schematically shows a configuration of the detector array 24. As shown, the detector array 24 is a multi-channel X-ray detector having a plurality of X-ray detector elements 24(ik) arranged in an array. The plurality of the X-ray detector elements 24(ik) together form an X-ray impingement surface, curved as a cylindrical concavity. Reference symbol xe2x80x98ixe2x80x99 designates a channel index and xe2x80x98ixe2x80x99=1-1,000, for example. Reference symbol xe2x80x98kxe2x80x99 designates a row index and xe2x80x98kxe2x80x99=1, 2, for example. The X-ray detector elements 24(ik) that have the same row index xe2x80x98kxe2x80x99 together constitute a detector element row. The detector array 24 is not limited to having two rows, but may have more than two rows divided into two groups. Although the description will be made on an example the detector array 24 having two rows hereinbelow, the same holds for a detector array having more rows. A certain number of channels at the ends of the detector array 24 are reference channels 25 in each row. The reference channels 25 are situated outside a range of the object that is projected in imaging. Each X-ray detector element 24(ik) is formed of a combination of a scintillator and a photodiode, for example. It should be noted that the X-ray detector element 24(ik) is not limited thereto but may be a semiconductor X-ray detector element using cadmium telluride (CdTe) or the like, or an ionization chamber X-ray detector element using xenon (Xe) gas, for example. FIG. 3 illustrates a relationship among the X-ray tube 20, collimator 22 and detector array 24 in the X-ray emitting/detecting apparatus. FIG. 3(a) is a view from the front of the scan gantry 2 and (b) is a view from the side of the scan gantry 2. As shown, the X-rays emitted from the X-ray tube 20 are formed into a fan-shaped X-ray beam 400 by the collimator 22, and projected onto the detector array 24. In FIG. 3(a), the extent of the fan-shaped X-ray beam 400 is illustrated. The extent direction of the X-ray beam 400 is identical to the direction of the linear arrangement of the channels in the detector array 24. In FIG. 3(b), the thickness of the X-ray beam 400 is illustrated. The thickness direction of the X-ray beam 400 is identical to the direction of the side-by-side arrangement (k-direction) of the rows in the detector array 24. An object 8 placed on the imaging table 4 is carried into the X-ray irradiation space with the object""s body axis intersecting the fan surface of such an X-ray beam 400, as exemplarily shown in FIG. 4. The scan gantry 2 has a cylindrical structure containing therein the X-ray emitting/detecting apparatus. The X-ray irradiation space is formed in the internal space of the cylindrical structure of the scan gantry 2. An image of the object 8 sliced by the X-ray beam 400 is projected on the detector array 24. The X-rays after passing through the object 8 are detected by the detector array 24. The slice thickness xe2x80x98thxe2x80x99 of the X-ray beam 400 penetrating the object 8 is regulated by the openness of an aperture of the collimator 22. The X-ray emitting/detecting apparatus consisting of the X-ray tube 20, collimator 22 and detector array 24 rotates (or scans) around the body axis of the object 8 while maintaining their mutual relationships. Projection data for a plurality of (for example, ca. 1,000) views are collected per scan rotation. The collection of the projection data is performed by a system of the detector array 24, data collecting section 26 and data collection buffer 64. Based on projection data of two slices collected in the data collection buffer 64, tomographic image production, or image reconstruction, for the two slices is performed by the data processing apparatus 60. The image reconstruction is carried out such as by processing the projection data for, for example, 1,000 views obtained by one scan rotation by the filtered back projection technique. FIGS. 5 and 6 are schematic diagrams illustrating the X-ray beam 400 projected onto the detector array 24 in more detail. As shown in FIG. 5, the slice thickness xe2x80x98thxe2x80x99 of a projection image on the X-ray detectors 242 and 244 is reduced by shifting collimator blocks 220 and 222 in the collimator 22 in a direction such that the aperture is narrowed. Similarly, as shown in FIG. 6, the slice thickness xe2x80x98thxe2x80x99 is increased by moving the collimator blocks 220 and 222 in a direction such that the aperture is widened. Such regulation of the slice thickness is achieved by the collimator controller 30 under the direction of the data processing apparatus 60. Moreover, the impingement position on the detector array 24 in the k-direction is adjusted by simultaneously moving both the collimator blocks 220 and 224 defining the slice thickness xe2x80x98thxe2x80x99 in the k-direction while maintaining their relative positional relationship. The variation in the impingement position associated with the X-ray focus shift can thus be corrected and automatically controlled so that the X-ray beam 400 is always projected onto a constant position. The adjustment of the impingement position in the k-direction may be achieved by shifting the detector array 24 relative to the collimator 22 in the k-direction, as shown by broken arrow, instead of moving the collimator blocks 220 and 222. This allows the mechanism for the slice thickness adjustment and the mechanism for the impingement position control in the thickness direction to be separately provided, thereby allowing diversified control. On the other hand, if all such mechanisms are implemented by the collimator 22, the system for the control can be integrated and desired simplification of configuration can be fulfilled. It will be easily recognized that these two types of means may be combined to perform the impingement position adjustment. Such a function for automatically controlling the impingement position will be sometimes referred to as an auto collimator hereinbelow. FIG. 7 shows a block diagram of the present apparatus with respect to the auto collimator. An error of the impingement position of the X-ray beam 400 in the k-direction is detected by an error detecting section 101 as shown. The error detecting section 101 detects the impingement position error based on outputs from the reference channels 25 of the two rows in the detector array 24. The error detection is performed by using X-ray detected signals A and B of the X-ray beam 400 from the reference channels 25 of the two rows to calculate the error xe2x80x98exe2x80x99 from the following equation:                     e        =                                            A              -              B                                      A              +              B                                .                                    (        1        )             An error measurement can thus be obtained independent of the magnitude of X-ray detected signals. The error detecting section 101 is implemented by a function of the data processing apparatus 60. The error detecting section 101 is an embodiment of the error detecting means of the present invention. It is also an embodiment of the error detecting apparatus of the present invention. The error detecting signal is input to a control section 103. The control section 103 then feedback-controls the collimator 22 so that the error xe2x80x98exe2x80x99 becomes zero. The control output from the control section 103 is proportional to the error xe2x80x98exe2x80x99, as exemplarily shown in FIG. 8. The slope of this input-output characteristic curve represents a proportional gain G for the control. The proportional gain will be sometimes referred to simply as the gain hereinbelow. When the error xe2x80x98exe2x80x99 becomes zero, the following equation holds: A=B.xe2x80x83xe2x80x83(2) That is, the X-ray beam 400 impinges equally upon the reference channels of the two rows. At this time, the X-ray beam 400 is projected equally divided between the two detector element rows in the detector array 24. The control section 103 is implemented by functions of the data processing apparatus 60 and the collimator controller 30. The control section 103 is an embodiment of the control means of the present invention. It is also an embodiment of the control apparatus of the present invention. The X-ray detected signals of the two detector element rows in the detector array 24 are collected at a signal acquiring section 107, and a tomographic image is produced at a tomographic image producing section 111 based on the collected signals. Thus, two tomographic images having equal slice thicknesses can be obtained. The signal acquiring section 107 is implemented by the data collecting section 26, rotation controller 36 and data collection buffer 64. The signal acquiring section 107 is an embodiment of the signal acquiring apparatus of the present invention. The tomographic image producing section 111 is implemented by a function of the data processing apparatus 60. The tomographic image producing section 111 is an embodiment of the tomographic image producing apparatus of the present invention. The gain of the control section 103 may be varied according to the error. Specifically, as exemplarily shown in FIG. 9, the gain is set to zero when |e|xe2x89xa6xcex11,xe2x80x83xe2x80x83(3) the gain is set to G1 (xe2x89xa00) when xcex11 less than |e|xe2x89xa6xcex12,xe2x80x83xe2x80x83(4) and the gain is set to G2 ( greater than G1) when |e| greater than xcex12.xe2x80x83xe2x80x83(5) In the above equations, xcex11 is an allowed value of the error. It is also a first gain switch point. xcex12 is a second gain switch point. Accordingly, the control is not performed when the error xe2x80x98exe2x80x99 is equal to or less than the allowed value xcex11, that is, a xe2x80x98neutral zonexe2x80x99 can be provided. The control can thereby be stabilized. When the error xe2x80x98exe2x80x99 is more than the allowed value xcex11 and is equal to or less than xcex12, the feedback control is performed with the gain G1 to draw the error xe2x80x98exe2x80x99 back to the allowed value. When the error xe2x80x98exe2x80x99 exceeds xcex12, the feedback is performed with the gain G2 larger than G1 to draw the error xe2x80x98exe2x80x99 back more rapidly than with the control with G1. By thus varying the gain according to the error, collimator control possessing both stability and rapidity can be achieved. It should be noted that the switching of the gain is not limited to three steps as shown in FIG. 9, but may have two steps or more than three steps. The error xe2x80x98exe2x80x99 contains high frequency components. The high frequency components are primarily caused by very small fluctuations of the focus position incident to the rotation of the anode in the X-ray tube. Since the rotation of the anode occurs at high speed, for example, at about 8,000-12,000 rpm, the fluctuations of the focus contain the high frequency components. Since such fluctuations are extraneous to the displacement of the X-ray focus incident to the temperature change, control effected to follow such fluctuations is meaningless, or rather may degrade the stability of the control. Therefore, the high frequency components are removed prior to inputting the error xe2x80x98exe2x80x99 to the control section 103 to further increase the stability of the control. FIG. 10 shows a block diagram of the present apparatus provided with such high frequency component removal. Similar portions in FIG. 10 to those shown in FIG. 7 are designated by similar reference numerals and explanation thereof will be omitted. As shown, a high frequency removing section 105 is situated between the error detecting section 101 and the control section 103. The high frequency removing section 105 removes the high frequency components in the error xe2x80x98exe2x80x99 input from the error detecting section 101 and inputs an error signal not containing the high frequency components to the control section 103. The high frequency removing section 105 is implemented by a function of the data processing apparatus 60. The high frequency removing section 105 is an embodiment of the high frequency component removing means of the present invention. It is also an embodiment of the high frequency component removing apparatus of the present invention. The high frequency component removal in the high frequency removing section 105 is achieved by, for example, determining the average of data obtained in time series. A moving average value of, for example, 16 time-series data values is employed as the average. The number of data values for the moving averaging is not limited to 16 but may be any other appropriate one. The data of the error xe2x80x98exe2x80x99 is obtained successively at the same timing as the view data. Therefore, the error xe2x80x98exe2x80x99 is moving-averaged for, for example, every 16 views. The moving averaging may be weighted by an appropriate weight instead of the simple moving averaging. Moreover, instead of the averaging, the removal of the high frequency components may be achieved by low-pass filtering of the data values of the time-series data. By thus removing the high frequency components contained in the error xe2x80x98exe2x80x99 by the high frequency removing section 105, the control of the impingement position can be stabilized. By stabilizing the impingement position, the slice thicknesses of two tomographic images become equal and stable, thus allowing images to be obtained with good quality. Although the present invention has been described with reference to the preferred embodiments, several changes and substitutions may be made on these embodiments by those ordinarily skilled in the art without departing from the scope of the present invention. Therefore, the scope of the present invention is intended to encompass not only the aforementioned embodiments but all embodiments pertaining to the appended claims. Many widely different embodiments of the invention may be configured without departing from the spirit and the scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.