Patent Publication Number: US-6212256-B1

Title: X-ray tube replacement management system

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of x-ray tube radiation sources such as those used in medical diagnostic and imaging systems. More particularly, the invention relates to a technique for predicting and scheduling replacement of x-ray tubes in such systems to reduce down time and costs associated with such servicing. 
     BACKGROUND OF THE INVENTION 
     A variety of medical diagnostic and other systems are known in which x-ray tubes are employed as a source of radiation. In medical imaging systems, for example, x-ray tubes are used in both x-ray systems and computer tomography (CT) systems as a source of x-ray radiation. The radiation is emitted in response to control signals during examination or imaging sequences. The radiation traverses a subject of interest, such as a human patient, and a portion of the radiation impacts a detector or a photographic plate where the image data is collected. In conventional x-ray systems the photographic plate is then developed to produce an image which may be used by a radiologist or attending physician for diagnostic purposes. In digital x-ray systems a photo detector produces signals representative of the amount or intensity of radiation impacting discrete pixel regions of a detector surface. In CT systems a detector array, including a series of detector elements, produces similar signals through various positions as a gantry is displaced around a patient. 
     Depending upon the particular modality of the imaging system and the system configuration, the x-ray tube source may be mounted in various manners. For example, in conventional x-ray systems, anode and cathode assemblies support the x-ray tube within a casing. The anode assembly is coupled to a target within a glass or metal envelope, while the cathode assembly is coupled to a cathode plate. A metal shield or casing surrounds the glass envelope. The volume between the casing and the envelope is filled with a cooling medium, such as oil. A window is provided in the casing for emitting x-rays created by controlled discharges between the cathode plate and the target. 
     The x-ray tube is typically operated in cycles including periods in which x-rays are generated interleaved with periods in which the x-ray source is allowed to cool. A typical imaging sequence may include a number of such sequences. Moreover, the x-ray tube may have a useful life over a large number of examination sequences, and must generally be available for examination sequences upon demand in a medical care facility. 
     Given the demanding schedules to which x-ray tubes are often subjected, failure of the tubes is of particular concern. Various failure modes have been observed in x-ray tubes, and these may have a variety of sources. For example, within the glass encasement a vacuum or near vacuum is preferably maintained. However, due to leaks, degradation in the cathode or anode materials, decomposition of anode filaments, and so forth, particulates may be created or freed within the tube. These particulates may result in eventual failure of the tubes over time. Failure of the tubes can also be a function of the modes of operation and user-selected parameters, such as voltage or current. 
     Due to the stringent requirements and reliability demands placed on x-ray tubes in medical diagnostic systems, special programs may be implemented for insuring rapid replacement of the tubes upon failure. Present procedures for replacement of x-ray tubes in medical diagnostic systems are primarily reactionary. Service personnel generally monitor the performance of the tubes over time and through the various examination sequences. However, the service personnel are often made aware of tube failures only as they occur. When a tube does fail, to insure rapid replacement of failed tubes a conventional response is to expedite shipment of a replacement tube which is then installed by trained service personnel at considerable shipping and handling expense. While the x-ray tubes could be shipped in advance and stored on location or in a centralized service facility, these strategies also require inventory of relatively expensive items, again resulting in additional costs of the service program. Such inventories may also inconveniently occupy valuable storage space at the location. 
     There is a need, therefore, for an improved management and servicing approach to x-ray tube replacement. In particular, there is a need for a service system which can reduce down time in diagnostic, imaging and other systems incorporating x-ray tubes as radiation sources which can result from an anticipated failure of the x-ray tubes. The system would advantageously permit forecasting of possible tube failure and scheduling of tube replacement and shipment prior to actual failure. Such a system could also provide feedback for planning the tube manufacturing and assembly process, as well as feedback to system users for planning the replacement process. 
     SUMMARY OF THE INVENTION 
     The present invention provides a novel technique for managing x-ray tube replacement designed to respond to these needs. The technique makes use of predictive indicators of possible future tube failure. The indicators may be monitored through existing tube control or power circuitry. The parameters considered indicative of possible tube failure are then analyzed on a periodic basis, either at the scanner or at a centralized facility. The centralized facility may acquire the data through periodic sweeps of scanners subscribing to a service program. Alternatively, the scanner may monitor the data and contact the service facility to upload the data or to signal possible future failure. The operative state of the tubes may then be reported to the scanner management personnel, including the operations personnel at the scanner location and/or a field service engineer, such as through interactive messaging directly to the scanner. When the indicators suggest that tube failure is imminent, replacement is scheduled and a replacement tube is shipped. Again, the scanner management personnel can be easily informed, as can field service technicians via an interactive network connection. The technique thereby reduces the need for inventory locally near the scanner, while reducing down time resulting from unanticipated tube failure and replacement. 
     Thus, in accordance with the first aspect of the invention, a method is provided for managing of replacement of x-ray tubes. The method includes a first step of monitoring a plurality of operating parameters of a system including an x-ray tube. The monitored parameters are then analyzed in accordance with a predetermined failure prediction routine. Based upon the analysis of the monitored parameters, replacement of the x-ray tube is scheduled. Data representative of the monitored parameters may be transmitted from the system to a service facility. Such transmission may occur during periodic sweeps of the system by the service facility. The method may include a further step of commanding shipment of a replacement x-ray tube in accordance with the scheduled replacement. Messages, such as in electronic format, may be sent to a facility in which the x-ray tube is installed, as well as to field service personnel for coordinating replacement of the x-ray tube. Messages may also be provided for advising the scanner operator of possible operational considerations or changes for extending tube service until the actual replacement. 
     In accordance with another aspect of the invention, a method is provided for replacing x-ray tubes in a medical diagnostic system. The method includes steps for detecting a plurality of operating parameters of the diagnostic system, and storing values representative of the parameters. The stored values are then analyzed to determine potential tube failure and, based upon the analysis, replacement of the x-ray tube is scheduled. The stored values may be transmitted from the diagnostic system to a remote service facility where the analysis, identification of possible corrective measures, and scheduling are performed. Moreover, the diagnostic system may be linked to a local management computer, such as at a medical service provider location, and the data necessary for failure prediction transmitted to a service facility from the networked station. 
     The invention also provides a service system for managing replacement of x-ray tubes in medical diagnostic systems. The service system includes circuitry for monitoring parameters of the diagnostic systems and for storing values representative of the parameters. In a location either local to the diagnostic systems, or remote from the systems, the stored parameters are analyzed to develop a prediction of possible tube failure. Based upon the prediction, replacement of the tubes is scheduled or corrective measures are identified and communicated to the user. In a particularly preferred configuration, the system includes diagnostic systems located at separate facilities, and coupled to a central service facility via a network. The central service facility thereby schedules replacement for tubes at a number of different medical diagnostic system installations in accordance with a preset algorithm and data collected at the various system locations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatical representation of a digital x-ray imaging system incorporating an x-ray tube as a source of radiation; 
     FIG. 2 is a diagram of an exemplary x-ray tube of the type incorporated in the system of FIG. 1; 
     FIG. 3 is a detail view of a portion of the operative components of the x-ray tube of FIG. 2 illustrating events which give rise to parameters presently considered as leading indicators of possible tube failure; 
     FIG. 4 is a graphical representation of an exemplary time histogram of events presently considered indicative of future tube failure; 
     FIG. 5 is a diagrammatical representation of a service network linked to a series of scanners of the type illustrated in FIG. 1 for monitoring tube performance, predicting possible tube failure, and scheduling replacement of x-ray tubes; and, 
     FIG. 6 is a flow diagram illustrating steps in exemplary logic for monitoring and predicting failure of x-ray tubes and for scheduling their replacement. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, and referring first to FIG. 1, a diagnostic imaging system  10  is illustrated diagrammatically. System  10  includes a source of x-ray radiation  12  which employs an x-ray tube  14 . In the embodiment illustrated in FIG. 1, system  10  is a digital x-ray imaging system. However, it should be noted that the digital x-ray system is illustrated and described herein as an exemplary system only. The present technique for predicting tube failure and scheduling tube replacement may be applied to any type of imaging, diagnostic, or other system employing such x-ray tubes, such as conventional x-ray systems, CT systems, and so forth. 
     In the system shown in FIG. 1, radiation source  12  receives power and control signals from a generator or controller  16 . Generator  16  converts alternating current power to direct current power and applies controlled pulses of DC power to tube  14  to induce emissions of x-ray radiation for examination purposes. Moreover, generator  16  monitors a range of operating conditions or parameters of the tube in a manner described in greater detail below. Power and control signals from generator  16  are conveyed to tube  14  via a set of conductors  18 . 
     Under the command of generator  16 , tube  14  within the radiation source produces a stream of radiation  20 . The radiation is directed through a collimator  22  and passes through a subject  24 , such as a human patient, during examinations. A portion of the radiation impacts a detector  26 . In the case of a digital x-ray system, detector  26  converts high energy photons to lower energy photons which are detected by a series of photo diodes (not shown). The detector electronics convert the sensed signals to image data which is output as indicated at reference numeral  28 . Detector  26  conveys the image data signals to a control/data acquisition circuit  30 . Circuit  30  also provides control signals for regulating scanning of the detector. Moreover, circuit  30  may perform additional signal processing or signal filtering functions. Following such processing, circuit  30  conveys the processed image data, indicated at reference numeral  32 , to a system controller  34 . 
     System controller  34  receives the image data and performs further processing and filtration functions. In particular, controller  34  derives discrete data from the acquired signals and reconstructs useful images from the data. Controller  34  then stores the image data in a memory or storage device  36 . Device  36  may also be used to store configuration parameters, data log files, and so forth. In a presently preferred configuration, system controller  34  also provides signals to generator  16  for controlling emissions of x-ray radiation from source  12 . System controller  34  may also include circuitry for providing interactive data exchange with remote computer stations, such as a centralized service center as described more fully below. Finally, system controller  34  includes interface circuitry for exchanging configuration data, examination requests, and so forth, with an operator interface  38 . The system may also include sensors for detecting specific operating parameters, such as temperature and vibration, values of which may also be stored and analyzed as described below. Operator interface  38  preferably includes an operator work station which permits clinicians or radiologists to request and control specific examinations, review data log files, view reconstructed images, and output reconstructed images on a tangible medium, such as photographic film. 
     As will be appreciated by those skilled in the art, the foregoing system description is specific to digital x-ray imaging. Other control and interface circuitry will, of course, be included on other scanner types, such as conventional x-ray systems, CT imaging systems, and so forth. In general, however, such systems will include a generator or controller for commanding emission of x-ray radiation for examination or calibration purposes. Moreover, for implementation of the present technique, such systems will include inherent capabilities for monitoring performance of the x-ray tube during such examination or calibration sequences such that parameters considered as leading indicators of tube failure may be acquired, stored and analyzed. 
     FIG. 2 illustrates an exemplary radiation source  12 , including an x-ray tube  14 . In the embodiment shown in FIG. 2, the radiation source includes an anode assembly  40  and a cathode assembly  42 . The anode and cathode assemblies, along with x-ray tube  14  are positioned within a casing  44  which may be made of aluminum and lined with lead. Tube  14  is supported by the anode and cathode assemblies within the casing. Tube  14  includes a glass envelope  46 . Within the glass envelope, adjacent to anode assembly  40 , a rotor  48  is positioned. A stator  50  at least partially surrounds the rotor for causing rotation of an anode disc during operation, as described below. Casing  44  is filled with a cooling medium such as oil around glass envelope  46 . The cooling medium also preferably provides high voltage insulation. 
     Within envelope  46 , tube  14  includes an anode  52 , a front portion of which is formed as a target disc  54 . A target or focal surface  56  is formed on disc  54  and is struck by an electron beam during operation as described below. Tube  14  further includes a cathode  58  which is coupled to the cathode assembly  42  via a series of electrical leads  60 . The cathode includes a central shell  62  from which a mask  64  extends. The mask encloses leads  60  and conducts the leads to a cathode cup  66  mounted at the end of a support arm  68 . Cathode cup  66  serves as an electrostatic lens that focuses electrons emitted from a heated filament (not shown) supported by the cup. 
     As will be appreciated by those skilled in the art, as control signals are conveyed to cathode  58  via leads  60 , the cathode filaments within cup  66  are heated and produce an electron beam  70 . The beam strikes the focal surface  56  and generates x-ray radiation which is diverted from the x-ray tube as indicated at reference numeral  72 . The direction and orientation of beam  72  may be controlled by a magnetic field produced by a deflection coil  74 . The field produced by deflection coil  74  is also preferably controlled by the generator and controller circuitry  16  described above. Radiation beam  72  then exits the source through an aperture  76  in casing  44  provided for this purpose. 
     X-rays are produced in the x-ray tube  14  when, in a vacuum, electrons are released and accelerated by the application of high voltages and currents to the cathode assembly and are abruptly intercepted by the anode target disc. The voltage difference between the cathode and anode components may range from tens of thousands of volts to in excess of hundreds of thousands of volts. Moreover, the anode target disc may be rotated such that electron beams are constantly striking a different point on the anode perimeter. Depending upon the construction of tube  14 , the desired radiation may be emitted by substances such as radium and artificial radiotropics, as well as electrons, neutrons and other high speed particles. Within the envelope of tube  14 , a vacuum on the order of 10 −5  to about 10 −9  torr at room temperature is preferably maintained to permit unperturbed transmission of the electron beam between the anode and cathode elements. 
     As noted above, in addition to providing power and control signals for operation of tube  14 , generator  16  (see FIG. 1) monitors operating parameters of the tube. Certain of these parameters are considered as predictive of future tube failure in accordance with the present technique. Such parameters may be measured via sensors, but are preferably available from the characteristics of the control and power signals applied to the tube. FIG. 3 is a detailed representation of a portion of the tube components, and illustrates certain operational anomalies which can occur in the tube leading to detectable parameters considered to be predictive of future tube failure. 
     As shown in FIG. 3, cathode cup  66  is positioned adjacent to anode disc  54  within the interior of the x-ray tube. As power is applied to filaments within the cathode cup, an electron beam  70  is emitted which strikes the anode disc. While the beam is preferably created in a vacuum, during operation of the x-ray tube particulates  70  may be present in the tube. Such particulates may be introduced in the tube by leaks, degradation of the system components within the tube, decomposition of the tube filaments, and so forth. When electron beam  70  impacts such particulate matter, the electron beam may continue toward the anode disc as indicated by reference numeral  80 . In certain cases, however, the electron beam may be deflected from the target disc as indicated at reference numeral  82 . Both incidents create anomalies in the signals exchanged between the tube and generator  16  which can be detected by the generator. In general, such events create high current discharges. When particulate is encountered by the electron beam and the beam continues along its path to impact the anode disc, an anode overcurrent event may be recorded. Moreover, where the electron beam is diverted from the anode disc by the particulate, the high current discharge event is generally termed a “spit” in the art. In addition to detecting current anomalies of these types, generator  16  is capable of distinguishing between anode overcurrent events and spits. Such events are recorded by system controller  34  and saved within memory circuitry  36 . As will be appreciated by those skilled in the art, various other anomalies may be detected and recorded in a similar manner. 
     In addition to recording the actual number of anode overcurrent events and spits, system controller  34  preferably derives additional parameters from at least one of these. In the present embodiment, for example, the system controller records the number of spits per day of operation. Moreover, the current to the x-ray tube may be interrupted upon the occurrence of a spit, and subsequently reapplied during an examination sequence. Such events are recorded by the system controller and logged for each day of operation. However, a maximum “spit rate” may be imposed in terms of spits per unit time. If the spit rate is greater than a preset limit, a scan or examination is typically aborted. For example, in a present embodiment of the system, a spit rate of over 32 spits/second causes the current examination scan to be aborted. Such events are termed “spit rate exceeded” errors or “SREs.” The number of SREs per day is also monitored by system controller  34  and stored in memory circuitry  36 . 
     Through extensive analysis of operating parameters for a population of x-ray tubes, it has been found that certain of the parameters monitored by generator  16  and system controller  34  provide accurate predictive indicators of tube failure. From this analysis a model algorithm has been developed which permits the monitored parameters to be correlated with a potential for tube failure. While algorithms including a large number of monitor parameters may be included in such failure prediction analyses, in a present embodiment the rate of occurrence of anode overcurrent events and SREs are used to generate failure prediction values which may be compared to evaluate the potential for short term tube failure. As described more fully below, discriminant analysis is used in the present technique to identify and to properly weight such predictive parameters in the algorithm, and to relate them in a value considered predictive of tube failure. 
     By way of example, FIG. 4 is a graphical representation of a “Z-score” derived from data files of SREs for an exemplary x-ray tube. The Z-score is calculated based upon the occurrences of SREs by the following relationship:                  Z-score     =         SRE     3      d       -     SRE   L         σ   SRE         ;           (eq. 1)                         
     Where SRE 3d  is the average number of SREs per day over a previous three day period, SRE L  is the average number of SREs per day over the life of the tube, and σ SRE  is the standard deviation of the number of daily SREs over the life of the tube. 
     FIG. 4 represents a histogram or curve  84  of the Z-score over time. The Z-score may be graphed over a base line of time  86  and a magnitude on a vertical axis  88 . As indicated by the histogram, the Z-score is generally expected to remain at an extremely low or null level throughout most of the useful life of the x-ray tube. At some time during the life of the tube, however, a sharp rise will be detected in the Z-score, such as due to an increase in particular matter within the tube resulting in an increase in SREs, as indicated by the sharp rise  90  in the histogram. In many systems the rise will be followed by a peak  92  and a subsequent drop off. It is believed that such a drop off may occur due to a tendency for a particular matter to drop to the bottom of the tube. 
     As indicated above, in accordance with the present technique, discriminant analysis is used to determine weighting coefficients for the parameters considered to be predictive of failure. In the presently preferred technique, two weighted functions are obtained through the discriminant analysis as follows: 
     
       
         Idf1=C 1 +K 1 (adjrate)+K 2 (aoc)  (eq. 2); 
       
     
     and, 
     
       
         Idf2=C 2 +K 3 (adjrate)+K 4 (aoc)  (eq. 3); 
       
     
     where the value Idf1 is a first linear discriminant function value, Idf2 is a second linear discriminant function value, C 1  and C 2  are constants resulting from the discriminant analysis, K 1 , K 2 , K 3  and K 4  are coefficients resulting from the discriminant analysis, adjrate is the Z-score for the tube, and the value aoc is the count of daily anode overcurrent events. In the present embodiment, the values for the constants and coefficients applied in equations 2 and 3 are as follows: 
     C 1  −0.12588 
     C 2  −0.00937 
     K 1  0.83695 
     K 2  0.19511 
     K 3  0.1833 
     K 4  0.19962. 
     In the present embodiment, if the value of Idf2 is found to be greater than or equal to the value of Idf1 no imminent failure is predicted for the tube. On the contrary, when the value of Idf1 exceeds the value of  1 df2, the tube is considered to be near failure, and its replacement is scheduled as summarized below. 
     It should be noted that the foregoing values and correlations have been determined through extensive analysis of a variety of parameters and their fluctuations over the life of a population of x-ray tubes. In accordance with the present technique, the statistical analyses may be employed to identify the particular parameters discussed above, or additional or different parameters which may be considered indicative of impending tube failure. Similarly, the particular constant and weighting values indicated above may be altered or replaced by other values to accurately predict potential tube failure. 
     As noted above, in the present embodiment the parameters considered indicative of future tube failure are monitored at the individual diagnostic or imaging system in which the tube is installed. The analysis of these parameters may also be performed at the diagnostic system, or may be performed remotely, such as at a central service facility. FIG. 5 represents a diagrammatical representation of a number of diagnostic systems or scanners  94  coupled to such a central service facility via a remote data exchange network. In the embodiment illustrated in FIG. 5 scanners  94 , which may be similar to or different from one another, include interactive communications hardware and software for communicating over a network represented generally at reference numeral  96 . Network  96  may include an intranet, internet or other network, such as the Internet. In such cases, the scanners are preferably provided with network software, such as a graphical user interface and browser permitting operations personnel at a facility to send and receive messages with the central service facility. The network  96  permits the scanners to be coupled to a web server  98  which manages communications and data traffic between the central service facility and the scanners on the network. Alternatively, the scanners may be designed to be linked directly to the service facility by a modem-to-modem connection, as indicated by the letter M in FIG.  5 . 
     The server  98  may transmit and receive data with the scanners, and with a central service facility  102  through a firewall  100 , particularly with a Point-to-Point Protocol (PPP). Firewall  100  may include any of various known security devices for preventing access to central service facility  102  except by recognized subscribers and other users. Central service facility  102  includes one or more central computers  104  which coordinates data exchange between the network scanners and work stations  106  at the central service facility. Work stations  106  may, in turn, be staffed by service personnel. Computer  104  may also be coupled for data exchange with one or more servers  108  at the central service facility. Moreover, computer  104  or other devices at the central service facility may be coupled or configured to be coupled to other internal or external networks, such as for exchanging data with databanks  110  through an additional firewall  112 . In the presently preferred configuration, databanks  110  may be local to or remote from the central service facility, and may contain data relating to history on particular scanners, families of scanners, populations of tubes, and the like. Such data is compiled over time by transmission from computer  104 , and is subsequently accessible by computer  104  to establish or revise the particular algorithms employed for predicting future failure of the tubes. Finally, the central service facility may be coupled to a warehouse  114  or similar facility for ordering shipment of replacement tubes depending upon the outcome of the analysis summarized above. 
     It should be noted that in the presently preferred embodiment, the technique for predicting possible failure of x-ray tubes, and scheduling their replacement, may incorporate planning for production, transportation, warehousing, and similar processes. Accordingly, as illustrated in FIG. 5, the block  114  should be understood to include manufacturing and assembly operations, storage facilities, transportation infrastructure, and the like. Thus, based upon predicted failure of a particular type or types of tubes, the system may schedule manufacturing or assembly operations, cause parts or sub-components to be ordered or assembled, and the like. Similarly, tubes for which failure is predicted or possible may be transported or assigned to specific storage locations or forward staging areas at or near the locations where the tubes will be needed. In a presently preferred configuration, the system may sweep tube parameters from a variety of scanners, associate possible tube failures with a list of subscriptions stored in a database  110 , and command manufacturing, transportation, storage and other upstream replacement processes, as well as the actual tube replacement itself. 
     In operation, the central service facility  102  can access scanners  94  at will via the various network connections. Periodic sweeps of the scanners may be implemented in which the data necessary for evaluation of possible future tube failure is acquired with or without intervention from service or operations personnel at the institutions in which the scanners are installed. Moreover, similar network transfer of the data may originate at the individual scanners. Once the information has been obtained by the service facility, the computations and comparisons required for prediction of possible tube failure are made as described above. If the prediction is found to be positive, replacement of the tube is scheduled. 
     The foregoing structure also permits various alternative management procedures to be implemented. For example, the data acquisition and comparisons may be made directly at the individual scanners. In such cases, the algorithm may be stored a priori at the scanners, or may be downloaded from the service facility to the scanners. When the scanner determines that a tube failure is possible or imminent, a message can be sent from the individual scanner to the central service facility, which then schedules for tube replacement. Similarly, when multiple scanners or diagnostic systems are provided in an institution, a central management station may be linked to the scanners in an internal network. The central management station may then collect the monitored parameter data and perform the failure prediction, or may transmit the information to a service facility for analysis. 
     It is also contemplated that the central service facility may conduct the evaluations described herein and schedule tube replacement only for scanners for which a conforming service contract or agreement has been completed. Accordingly, in appropriate situations, the central service facility may only sweep data from service subscribing scanners, or may transmit updated failure analysis algorithms to such subscribing scanners. 
     It should also be noted that the present technique permits a remote field engineer station to be integrated into the tube replacement process, as shown at reference numeral  116  in FIG.  5 . As will be appreciated by those skilled in the art, field service engineers may access information on replacement of tubes through the same network used to link the scanners to the service facility. When replacement of a tube is scheduled, therefore, the field engineer may be notified of the need to attend to such replacement. 
     The foregoing procedure is summarized in FIG.  6 . As shown at step  122  in FIG. 6, subscribing scanners or facilities are periodically swept to obtain data on parameters considered indicative of possible x-ray tube failure, such as anode overcurrent events, and SREs rates or Z-scores derived from the SRE data. Alternatively, the data collection may be performed locally at the diagnostic system. All or a portion of the analysis may also be performed at the diagnostic system, which may then flag possible failure to the service facility. At step  124 , the data is compiled, either at the central service facility or at the scanners (or internal management station), to obtain the failure prediction values needed for the prediction analysis. At step  126  the predictive analysis is performed, such as through the calculations summarized above in equations 2 and 3. The predictive failure analysis concludes at step  128  wherein a comparison is made between the failure prediction values, as summarized above. Where the result of the comparison indicates that failure is not imminent, this fact may be reported to the scanner or institution in which the scanner is installed, as indicated at step  130 . The periodic sweeping and analysis summarized above then is repeated over the course of the tube life. 
     If the result of the comparison made at step  128  is affirmative, this fact is reported to the scanner or institution at step  132 . In addition, a service order is generated at step  134  and a replacement tube is ordered from a warehouse or factory as indicated at reference numeral  114  in FIG.  5 . Moreover, the service order includes an electronic message notification sent to a field service engineer, such as via a remote station  116 , to inform the field service engineer that replacement of the tube is required. Alternatively, the field service engineer may place a service order in response to receipt of a failure prediction or replacement scheduling message. 
     As noted above, the method may include coordination of other upstream operations in addition to the actual scheduling of the tube replacement. Thus, parts or subcomponents may be ordered, manufactured, or assembled based upon the predicted failure. Moreover, where local warehousing or staging areas are provided, tubes may be shipped in advance to such locations in anticipation for the predicted failure. Also, messages provided via the present technique, both to field service engineers, as well as to scanner operations personnel, may include an indication of remedial or other measures which can be implemented to avoid or forestall the predicted tube failure pending its replacement. 
     The foregoing technique thus permits effective prediction of possible tube failure by algorithms derived from actual occurrences of historic tube failures. The algorithms may be refined and altered over time as desired. Moreover, alternative algorithms may be developed for particular families or types of tubes, or for particular types of diagnostic equipment. Upon implementation, the technique facilitates planned replacement of the tubes with little or no intervention from operations personnel. At the same time, the technique allows the institutions to be kept abreast of the operational state of the x-ray tubes, and of scheduled or needed replacement as these are identified by the central service facility. Additional costs of stocking and transporting replacement of tubes after failure may thereby be reduced or eliminated, as may costs and inconvenience associated with downtime of diagnostic equipment.