Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-333370, filed Nov. 24, 1999, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an X-ray computed tomography apparatus having an interlock function of monitoring the operating state of an X-ray tube device and stopping all operations pertaining to scan, including the stop of X-ray emission, when something is wrong about the X-ray tube device. 
     Helical scan is a scan scheme of continuously and quickly acquiring projection data of a subject in a wide range by continuously rotating an X-ray tube and continuously moving a top plate. 
     Almost all X-ray computed tomography apparatuses have a function of monitoring an X-ray tube to assure the safety of a subject. The state of the X-ray tube is monitored from various viewpoints using various items such as a tube voltage, tube current, filament current, tube temperature, and cool pump operating status. If any one of the plurality of items exhibits an error value, all operations pertaining to projection data acquisition operation (scanning) are forcibly stopped. More specifically, tube voltage impression, filament current supply, cool pump driving, rotation of a ring on which the X-ray tube and detector are mounted, movement of the top plate of a bed are stopped. This function is called an interlock function. 
     When scan is stopped by this interlock function, an operator tries to find a cause of the stop of scan and performs a job to remove the cause, as needed. In some case, scan must be restarted from the beginning. In radiographic examination, for example, a change in CT value over time is one of the most important pieces of information. When scan is interrupted by the interlock function, the radiographic effect mostly disappears before the restart of radiographic examination. A contrast medium must be injected again to restart scan from the beginning. In helical scan described above, as the position of the subject often changes while scan is stopped, scan must be restarted from the beginning. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to reduce the frequency of the restart of scan in an X-ray computed tomography apparatus having an interlock function. 
     An X-ray computed tomography apparatus comprises an X-ray tube configured to emit an X-ray, an X-ray detector configured to detect an X-ray passing through a subject in order to acquire projection data, a processor configured to reconstruct tomographic image data on the basis of the projection data, and an interlock unit configured to monitor generation of an arc in the X-ray tube. When an arc in the X-ray tube is detected, the interlock unit stop X-ray emission from the X-ray tube. When a predetermined period of time elapse from the stop of X-ray emission, the interlock unit restarts X-ray emission from the X-ray tube. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a view showing the arrangement of the main part of an X-ray computed tomography apparatus according to the first embodiment of the present invention; 
     FIG. 2 is a timing chart showing operation of the first embodiment; 
     FIGS. 3A,  3 B, and  3 C are a view, graph, and block diagram, respectively, for explaining detection of the period of tube voltage impression stop in an interpolation unit in FIG. 1; 
     FIG. 4 is a view for explaining the interpolation method of the interpolation unit in FIG. 1; 
     FIGS. 5A and 5B are views for explaining switching between the interpolation methods in the interpolation unit in FIG. 1; 
     FIGS. 6A and 6B are views showing operation of returning the top plate of a bed under the control of a main controller in FIG. 1; 
     FIG. 7 is a timing chart showing operation of the second embodiment; and 
     FIG. 8 is a timing chart showing operation of the third embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred embodiments of X-ray computed tomography apparatuses according to the present invention will be described in detail with reference to the accompanying drawing. Computed tomography apparatuses are classified into various types of apparatuses such as a rotate/rotate-type apparatus for integrally rotating an X-ray tube and X-ray detector around a subject and a stationary/rotate-type apparatus for rotating only an X-ray tube around a subject while a large number of detection elements are arranged in a ring. The present invention is applicable to any type and will be explained using the most popular rotate/rate-type apparatus as an example. 
     To reconstruct one tomographic image, a projection data set around the subject, i.e., through about 360° is generally required. Even in a half-scan method, a projection data set of 180°+α (α is the view angle) is required. The present invention is applicable to either scheme. The former method of reconstructing one tomographic image from the projection data set of about 360° is employed hereinafter. 
     First Embodiment 
     FIG. 1 shows the arrangement of the main part of a computed tomography apparatus according to the first embodiment. This computed tomography apparatus is comprised of a gantry unit  100  and computer unit  200 . The gantry  100  is made up of an X-ray tube  101  and a plurality of constituent components necessary for acquiring projection data. The X-ray tube  101  is mounted on a rotary ring  102 . The X-ray tube  101  has a plurality of types of sensors necessary for monitoring various operating states, such as a voltmeter  115  for sensing a tube voltage, an ammeter  116  for sensing a tube current, and a temperature sensor  117  for sensing the internal temperature of the X-ray tube  101 . 
     A multichannel type X-ray detector  103 , data acquisition system  104 , and the like are attached to the rotary ring  102  in addition to the X-ray tube  101 . The X-ray detector  103  opposes the X-ray tube  101  through a photographic area. In projection data acquisition (scanning), a subject lying on a bed is located in the photographic area. 
     Projection data acquired by the data acquisition system  104  is temporarily stored in a storage unit  201  in the computer unit  200 . In addition to the storage unit  201 , the computer unit  200  is comprised of a host controller  206 , an interpolation unit  202  for generating projection data (interpolated data), instead of projection data exhibiting an error value upon generation of an arc, on the basis of actually acquired projection data (real data), a reconstruction unit  203  for reconstructing tomographic image data on the basis of the real data of 360° or a combination of 360° real data and interpolated data, a display  205  for displaying the resultant tomographic image data, and an input device  204  having the function of allowing the operator to select an operation mode of an interlock unit  113  (to be described later). 
     As is known well, discharge is normal operation, while an arc generation is an error. 
     Referring back to FIG. 1, in addition to the X-ray tube and the like, the gantry unit  100  is comprised of a high-voltage generator  105  for impressing a tube voltage (high voltage) to the X-ray tube  101 , a rotor controller  107  for rotating and driving the rotary anode of the X-ray tube  101 , a cool pump unit  108  for cooling the X-ray tube  101 , a ring controller  109  for rotating and driving the rotary ring  102 , a bed controller  110  for moving the top plate of the bed, and a DAS controller  111  for controlling the operation of the data acquisition system  104 . 
     The gantry unit  100  further comprises a filament current controller  106  for supplying a filament current to the X-ray tube  101 . This filament current controller  106  has the feedback function of adjusting the filament current on the basis of the tube current sensed by the ammeter  116  in order to relatively stabilize the tube current to a predetermined value. 
     A scan controller  112  controls the high-voltage generator  105 , filament current controller  106 , rotor controller  107 , cool pump unit  108 , ring controller  109 , bed controller  110 , and DAS controller  111  to scan the subject. 
     The scan controller  112  is connected to the interlock unit  113 . The interlock unit  112  receives a sensing signal from the voltmeter  115 , a sensing signal from the ammeter  116 , a sensing signal from the temperature sensor  117 , and sensing signals from sensors arranged in the cool pump unit  108  to sense various operating states, e.g., the temperature and pressure of cooling oil. 
     The interlock unit  113  monitors the operating state of the X-ray tube  101  on the basis of these sensing signals from various viewpoints. When an error occurs in the X-ray tube  101 , the interlock unit  113  outputs an interlock signal for emergency stop of scan to the scan controller  112 . In addition to this original interlock function, the interlock unit  113  has the X-ray suspend function, as a characteristic feature, of outputting a suspend signal in place of the interlock signal to the scan controller  112  when the X-ray tube  101  is set in a specific state. 
     Upon receiving the interlock signal from the interlock unit  113 , the scan controller  112  urgently stops all motions pertaining to scan operation for acquiring the projection data. More specifically, tube voltage impression from the high-voltage generator  105  to the X-ray tube  101  is stopped to stop emitting X-rays. Filament current supply from the filament current controller  106  to the X-ray tube  101  is stopped. The rotary anode and rotary ring  102  stop rotating, the top plate of the bed stops moving, and the DAS  104  stops data acquisition operation. 
     Upon receiving the suspend signal from the interlock unit  113 , the scan controller  112  only stops impressing the tube voltage from the high-voltage generator  105  to the X-ray tube  101 . The scan controller  112 , however, outputs control signals to the high-voltage generator  105 , filament current controller  106 , ring controller  109 , bed controller  110 , cool pump unit  108 , and DAS controller  111  so as not to stop, i.e., so as to continue all other operations pertaining to scan, i.e., filament current supply from the filament current controller  106  to the X-ray tube  101 , rotation of the rotary anode and rotary ring  102 , movement of the top plate of the bed, operation of the cool pump, and data acquisition operation of the DAS  104 . 
     When a predetermined period of time, e.g., 100 ms, has elapsed from generation of the suspend signal, the interlock unit  113  stops generating the suspend signal. When the supply of the suspend signal is stopped, the scan controller  112  restarts impressing the tube voltage from the high-voltage generator  105  to the X-ray tube  101  and controls the high-voltage generator  105  to restart X-ray emission. 
     Of a plurality of operations pertaining to scan, operations except X-ray emission continue. When X-ray emission is restarted, normal scan can be immediately restarted. 
     The situation in which a suspend signal is generated, i.e., the specific state of the X-ray tube  101  is a state in which only an arc is generated. That is, any state other than that in which an arc is generated and overheat or the like occurs is normal. This arc is generated due to dust or fine particles in the X-ray tube  101  or a decrease in vacuum degree. 
     The above-mentioned arc generation is caused by dust, fine particles, or the decrease in vacuum degree, and is not an apparatus failure. No special repair is required, and the arc phenomenon often naturally disappears. To recover the normal state from the arc phenomenon earlier than natural disappearance, X-ray emission is stopped for a predetermined period of time (100 ms). When the predetermined period of time has elapsed upon generating the arc, X-ray emission is restarted. At the time of this restart, the arc phenomenon is often eliminated. If the arc is still being generated, all the operations pertaining to scan including X-ray emission are stopped. If the arc has disappeared, scan is continued. 
     The data acquired for the period in which X-rays are kept stopped are errors. The error data acquired in this period are replaced with interpolated data generated from the projection data (real data) actually acquired before and after the period. 
     FIG. 2 shows changes in tube current and voltage as a function of time. When an arc is generated, the tube current abruptly increases while the tube voltage abruptly decreases. To determine arc generation with high precision, arc generation is determined when the following three conditions are satisfied: 
     (1) the tube current is larger than a first threshold value TH 1 ; 
     (2) the tube voltage is lower than a second threshold value TH 2 ; and 
     (3) the descending gradient of the tube voltage is larger (steeper) than a threshold value. 
     When the operation error of the X-ray tube  101  is determined to be caused by only an arc on the basis of the above determination criteria, the interlock unit  113  generates a suspend signal. The high-voltage generator  105  stops impressing the tube voltage to the X-ray tube  101 . However, filament current supply from the filament current controller  106  to the X-ray tube  101 , rotation of the rotary anode and rotary ring  102 , movement of the top plate of the bed, operation of the cool pump, and data acquisition operation of the DAS  104  continue without any stop. 
     When the predetermined period of time (100 ms) has elapsed from generation of the suspend signal, the interlock unit  113  stops generating the suspend signal. This makes it possible to restart X-ray emission and restore a scan enable state. 
     The actual time from the stop of impressing the tube voltage to the restart of impressing the tube voltage is not limited to 100 ms. The stop time should be set in consideration of an arc disappearance possibility and interpolation accuracy. That is, the longer the stop time, the higher the arc disappearance possibility and the lower the interpolation accuracy. In consideration of both the arc disappearance possibility and interpolation accuracy, the stop time is preferably 100 ms. 
     The interlock signal and suspend signal are also supplied to the host controller  206 . In accordance with the interlock signal, the host controller  206  supplies to the display  205  a signal for displaying a message representing the stop of scan. A message displayed on the display  205  may represent an errored unit (e.g., an X-ray tube) or information (e.g., an arc) representing the cause of the error. Alternatively, the message representing the errored unit or information representing the cause of the error may be stored and so displayed as to allow a serviceman to confirm the message at the time of inspection or repair. A function of transferring to a business office or maker the message representing the errored unit or the information representing the cause of the error may be provided. This makes it possible for a serviceman to check and repair only the errored unit at the time of inspection and repair, thereby shortening the work time. 
     In accordance with the suspend signal, the host controller  206  supplies to the display  205  a signal for displaying a message representing the stop of X-ray emission. 
     The interlock unit  113  has the function of stopping scan as follows. When an arc is generated by the X-ray tube  101  at a relatively high frequency, for example, when an arc is generated three times in 10 sec, the interlock unit  113  determines a high possibility of an arc being generated due to an apparatus failure. When the third arc is detected, the interlock unit  113  generates an interlock signal in place of a suspend signal, thereby stopping scan of the subject. 
     As described above, according to this embodiment, when the X-ray tube generates an arc, only X-ray emission is stopped, and other operations pertaining to scan continue. When the predetermined period of time has elapsed, X-ray emission is automatically restarted. By this time, the arc phenomenon often disappears. Since the operations except X-ray emission have continued, the restart of X-ray generation allows the immediate restart of scan and the restart of acquiring projection data. 
     Although projection data acquisition operation continues during the X-ray stop period, data output from the detector  103  during this period are errors. For example, a slice position for reconstructing a tomographic image can be arbitrarily designated in helical scan. However, some of 360° projection data corresponding to the slice position are missing. The interpolation unit  201  must interpolate the missing projection data on the basis of the projection data actually acquired at a position near this slice position or a combination of the actually acquired projection data and their opposing data. 
     As a typical example of the interpolation method, missing data are calculated on the basis of the projection data (real data) acquired in the previous rotation and the projection data (real data) acquired in the next rotation while the angle (view) of the X-ray tube  101  remains the same. When the X-ray stop period is set relatively long, one of the projection data (real data) acquired in the previous rotation and the projection data (real data) acquired in the next rotation may be missing. 
     The interpolation unit  202  has the function of, when data to be used for interpolation is unfortunately missing, calculating the missing data using projection data (normal value) acquired for a period except the stop period of the tube voltage at a position closest to the missing data position. By this function, when scan is restarted, including the tube voltage impression stop and restart, projection data acquired between the stop and restart of tube voltage impression exhibit error values. When interpolation and reconstruction are performed using these error values, an artifact occurs in the reconstructed tomographic image. However, when the projection data exhibiting the error values are replaced with the projection data exhibiting normal values, and interpolation is performed, no artifact occurs. 
     To prevent this artifact, the interpolation unit  202  must detect the stop period of the tube voltage. This detection method is not limited to any specific method. FIGS. 3A,  3 B, and  3 C show three variations of the detection method. In the method of FIG. 3A, one- or several-channel reference detector (X-ray detector)  50  is arranged between the X-ray tube  101  and the subject. Data (reference data) from the reference detector  50  is acquired by the data acquisition system  104  together with the projection data. The reference data exhibits almost zero level when no X-rays are emitted and a large value when X-rays are emitted. The interpolation unit  202  compares the value of the reference data with a threshold value. The interpolation unit  202  detects a period in which the value of the reference data is smaller than the threshold value, as the period from the tube voltage impression stop to the tube voltage impression restart. Alternatively, the projection data accompanying reference data smaller than the threshold value is detected as projection data exhibiting an error value acquired for the X-ray stop period. 
     Based on the same idea as described above, as shown in FIG. 3B, since no X-rays are emitted during the tube voltage stop period, the projection data value of the X-ray detector  101  exhibits almost zero. The data acquisition system  104  compares the acquired projection data with the threshold value to allow detection of the tube voltage stop period. 
     As shown in FIG. 3C, a time code (attached to the projection data) corresponding to the tube voltage impression stop and a time code corresponding to the tube voltage impression restart are received from the interlock unit  113 . The period from the tube voltage impression stop to the tube voltage impression restart may be detected, or the projection data accompanying the reference data having a value smaller than the threshold value may be detected as projection data exhibiting an error value acquired during the tube voltage stop period. 
     In so-called 180° interpolation, i.e., interpolation for interpolating projection data of a designated slice on the basis of projection data actually acquired in the 180° range (hatched portion) centered on the designated slice and its opposing data (e.g., data on the same route in the opposite X-ray direction, and data obtained by interpolating data on a close route in the opposite X-ray direction), as shown in FIG. 4, projection data P 0  at a given tube angle is interpolated from adjacent projection data P 1  and P 2 . If the projection data P 2  is projection data having an error value and acquired from the tube voltage impression stop to the tube voltage impression restart, the projection data P 2  is replaced with projection data (opposing data in this case) P 3  acquired at a position closest to the acquisition position of the projection data P 2 . 
     Interpolation may be performed using the following method as well. As shown in FIG. 5A, in place of using the opposing data, so-called 360° interpolation is used. The 360° interpolation performs interpolation using only the projection data actually acquired in the 360° range (hatched portion) centered on the designated slice. When the period from the tube voltage impression stop to the tube voltage impression restart partially overlaps the above range, the interpolation method is switched to the 180° interpolation method shown in FIG.  5 B. The 180° interpolation method has a narrow range (range in the body axis direction) of data to be used for interpolation, i.e., about ½ the range of the 360° interpolation method. The possibility that the period from the tube voltage impression stop to the tube voltage impression restart partially overlaps the range is low. If the period from the tube voltage impression stop to the tube voltage impression restart partially overlaps the range of the 180° interpolation method, the projection data representing the error value and acquired for the period from the tube voltage impression stop to the tube voltage impression restart is replaced with the projection data acquired at a position closest to the acquisition position of the projection data having the error value, as shown in FIG.  4 . 
     Note that the present invention is not limited to switching between the 360° and 180° interpolation methods, but can employ any interpolation method if an appropriate interpolation method can be selected from a variety of interpolation methods. 
     In the above description, the missing projection data during the X-ray stop period is interpolated. In helical scan, as shown in FIG. 6A, it is preferable that the top plate be returned from a position corresponding to the arc generation time by a total distance of the distance required for data interpolation and the approach distance of the top plate, and scan be restarted from the return position. In this case, missing of projection data does not occur, and no data interpolation is required. 
     As shown in FIG. 6B, even if an arc is generated, scan may be continued to the end except the X-ray suspend period. Upon completion of the scan, the top plate may be returned from a position corresponding to the arc generation time by the total distance of the distance required for data interpolation and the approach distance of the top plate, and scan in the range corresponding to the X-ray suspend period may be restarted from the return position. 
     The ON/OFF mode of the method in FIG. 6A can be selected by an operator&#39;s instruction. Similarly, the ON/OFF mode of the method in FIG. 6B can be selected by an operator&#39;s instruction. 
     The ON/OFF mode of the X-ray suspend function described above can be selected by an operator&#39;s instruction. 
     As described above, according to this embodiment, it is highly probable that an arc generation as an abnormal phenomenon which abruptly increases the tube current and abruptly decreases the tube voltage as the main cause of vacuum degree degradation in the X-ray tube be naturally recovered. In such case, when impression of the tube current is restarted, often the abnormal phenomenon is naturally recovered and scan can be continued. Even if the interlock function is effected in helical scan or contrast examination, scan may not be restarted from the beginning. 
     Second Embodiment 
     The arrangement of an X-ray computed tomography apparatus according to the second embodiment is the same as that in FIG. 1, except a control mechanism of an interlock unit  113  and scan controller  112  upon generating an arc. The difference will be described below. 
     FIG. 7 shows a tube voltage curve, tube current curve, and filament current curve as a function of time under the control mechanism of the second embodiment. Most causes of arc generation are dust, fine particles, and degradation of the vacuum degree, but are not an apparatus failure. The arc phenomenon may often be naturally eliminated with a lapse of time. 
     This embodiment waits for natural recovery. 
     In the conventional monitoring function, when an arc is generated, the interlock function is effected to stop all operations pertaining to scan including X-ray emission. To the contrary, in this embodiment, even if an arc is detected, all operations pertaining to scan including X-ray emission continue. The operations include conventional feedback control for monitoring the tube current and adjusting the filament current in accordance with the monitoring result in order to stabilize the tube current. 
     The interlock unit  113  monitors the tube current, and when an arc is generated, generates a suspend signal. When the arc disappears, the interlock unit  113  stops outputting the suspend signal. Arc generation can be detected by an abrupt increase in tube current. Arc disappearance can be detected when variations in tube current converge to less than a predetermined value. 
     The interlock unit  113  keeps generating the suspend signal for a period from arc generation to arc disappearance. Unlike in the first embodiment, the scan controller  112  continues all operations pertaining to scan including X-ray emission even if the controller  112  receives a suspend signal from the interlock unit  113 . The scan controller  112  transfers the suspend signal from the interlock unit  113  to a host computer  206 . 
     The host computer  206  detects as an arc generating period a given period in which it receives the suspend signal. The host computer  206  instructs an interpolation unit  202  to interpolate the projection data of the given period using the projection data (real data) actually acquired during a period before or after the given period. The host computer  206  also instructs a reconstruction unit  203  to reconstruct a tomographic image using the resultant interpolated data and the real data. 
     In this embodiment, even if an arc is generated, scan continues. The data acquired for a period elapsed until the arc naturally disappears are not used. By this operation, even if an arc is generated, the frequency of restarting scan can be reduced. 
     Third Embodiment 
     The arrangement of an X-ray computed tomography apparatus according to the third embodiment is the same as that in FIG. 1, except the control mechanism of an interlock unit  113  and scan controller  112  upon generating an arc. The difference will be described below. 
     FIG. 8 shows a tube voltage curve, tube current curve, and filament current curve as a function of time under the control mechanism of the second embodiment. The second embodiment utilizes natural disappearance of the arc phenomenon. The third embodiment employs an implementation for positively eliminating the arc phenomenon. 
     As described above, according to the conventional monitoring function, when an arc is generated, the interlock function is effected to stop all operations pertaining to scan including X-ray emission. To the contrary, according to the third embodiment, when arc generation is detected, of all the operations pertaining to scan including X-ray emission, only feedback control for monitoring the tube current and adjusting the filament current in accordance with the tube current value in order to stabilize the tube current is stopped. 
     The interlock unit  113  monitors the tube current and when an arc is generated, generates a suspend signal. When the arc disappears, the interlock unit  113  stops outputting the suspend signal. Unlike in the second embodiment, upon receiving the suspend signal from the interlock unit  113 , the scan controller  112  continues all operations pertaining to scan including X-ray emission except the feedback control for stabilizing the tube current. That is, upon receiving the suspend signal, the scan controller  112  outputs a control signal for stopping feedback control to a filament current controller  106  and outputs no control signals to other constituent components pertaining to scan. 
     Although all the scan operations including the feedback function continue in the arc generation period as in the normal period in the second embodiment, only the feedback function is stopped and other scanning operations continue in the third embodiment. 
     The feedback function is the function of decreasing the filament current when the tube current becomes larger than a predetermined value and increasing it when the tube current becomes smaller than the predetermined value, thereby stabilizing the tube current. 
     By this function, when the tube current abruptly increases due to an arc, the filament current is abruptly decreased to decrease the tube current. The tube current abruptly decreases. When the tube current abruptly decreases, the filament current abruptly increases to abruptly increase the tube current. The tube current abruptly increases. 
     As described above, the feedback function is effective in normal operation, but may cause instability of the tube current in an abnormality such as arc generation. That is, the feedback function may prolong the period from the time when the tube current becomes unstable due to arc generation to the time when the arc disappears and the tube current returns to a relatively stable state. 
     According to this embodiment, the feedback function is stopped in synchronism with arc generation. The feedback function is restarted after the tube current recovers a relatively stable state. The period from the time when the tube current becomes unstable due to arc generation to the time when the arc disappears and the tube current returns to a relatively stable state can be shortened. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.

Technology Category: 1