Patent Publication Number: US-9412257-B2

Title: Electronic cassette management system, method of operating electronic cassette management system, and electronic cassette management device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2014-063998, filed Mar. 26, 2014. Each of the above application(s) is hereby expressly incorporated by reference, in its entirety, into the present application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electronic cassette management system, a method of operating the electronic cassette management system, and an electronic cassette management device. 
     2. Description Related to the Prior Art 
     An electronic cassette is known as an X-ray image detector that detects an X-ray image while receiving radioactive rays, e.g. X-rays. The electronic cassette has a structure wherein a sensor panel for converting incident X-rays to an electric signal is contained in a transportable housing. Some electronic cassettes are equipped with a function for wireless communication, wherein a battery and a wireless communicator are integrated within the housing to allow wireless transmission of the X-ray image and other data. 
     Japanese Laid-open Patent Application No. 2011-120813 discloses a round-visit vehicle for X-ray imaging in a patient bedroom during the ward-round, wherein an X-ray source for projecting X-rays toward an imaging subject (a patient) and a console for operating an electronic cassette, which is equipped with function for wireless communication, are boarded on a mobile wagon. The round-visit vehicle is provided with a container box for containing the electronic cassette. 
     The above-mentioned round-visit vehicle is equipped with a function for preventing misplacement of the electronic cassette in the patient bedroom. The function preventive against the misplacement is constituted of a detector for detecting whether or not the electronic cassette is contained in the container box while the imaging is not being conducted, and an alerting section, such as a display panel or a speaker, for warning that the electronic cassette is not contained in the container box when the detector detects that the electronic cassette is not contained in the container box. 
     The detector monitors the state of wireless communication between the electronic cassette and the console, and detects that the electronic cassette is not contained in the container box when the electronic cassette is apart such a distance from the round-visit vehicle that the wireless communication between the electronic cassette and the console gets impossible. The alerting section stops alerting when the electronic cassette is contained in the container box. 
     Because electronic cassettes are relatively expensive, even a medical facility that has multiple radiography rooms and multiple round-visit vehicles is not always equipped with a sufficient number of electronic cassettes as compared to the number of radiography rooms or round-visit vehicles. Therefore, in these medical facilities, electronic cassettes are shared among a plurality of service zones, including radiography rooms and round-visit vehicles. For example, an electronic cassette located in a radiography room may be taken out for use in another radiography room, or an electronic cassette boarded on a round-visit vehicle may also be brought to use in a radiography room. 
     Due to the mobility and the relative expensiveness of the electronic cassette, it is necessary to use abundance of caution against the theft of the electronic cassette. In the invention disclosed in the above Japanese Laid-open Patent Application No. 2011-120813, when the electronic cassette is apart a certain distance from the console and hence the wireless communication between the electronic cassette and the round-visit vehicle becomes impossible, an alert is given to warn that the electronic cassette is not contained in the container box electronic cassette. Therefore, the alert would be given not only when the electronic cassette is left in a patient bedroom, but also when an unauthorized person takes out the electronic cassette with the intention to steal. Thus, it is possible to prevent the theft of the electronic cassette. 
     However, if the feature disclosed in the above patent document is applied to the case where an electronic cassette is shared among a plurality of service zones and moved across these service zones, the alert would be given also when an authorized user brings an electronic cassette out from a service zone into another service zone, and the alert would not stop in that case. This would be definitely bothering. It would be possible to provide the electronic cassette with an button to stop the alert or set the alert to stop automatically when a time is over. However, these solutions would disable the preventive effect against the theft because the alert would be interrupted even while the electronic cassette is being improperly taken out. 
     Thus, in the case where an electronic cassette is moved across a plurality of service zones to share the electronic cassette among these zones, it is required to give a warning when the electronic cassette is taken out of any of the service zones, but if the electronic cassette is taken out of the service zone for the purpose of using the same in another service zone or for other proper purposes or applications, it is necessary to stop the warning at the right time. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the present invention has an object to provide an electronic cassette management system, a method of operating the electronic cassette management system, and an electronic cassette management device, which make it possible to give an alert when an electronic cassette is taken out of any of a plurality of service zones in a situation where the electronic cassette is shared among and moved across the service zones, but also make it possible to stop the alert appropriately at the right time if the electronic cassette is taken out within a range of proper operations. 
     In order to achieve the above object of the present invention, an electronic cassette management system in accordance with the present invention comprises first and second detectors, a data taking section, and an alert controller. The first and second detectors detect the come and go of an electronic cassette with respect to at least first and second predetermined service zones. The data taking section takes detection results from the first and second detectors. The alert controller starts an alert when the alert controller determines, on the basis of detection results taken by the data taking section, that the electronic cassette has gone out either of the first and second service zones. The alert controller stops the alert when the alert controller determines, on the basis of detection results taken by the data taking section after the start of alerting, that the electronic cassette has come in either of the first and second service zones. 
     It is preferable that the electronic cassette management system further comprises a recording controller for recording a current position of the electronic cassette, which is determined by the detection results, in a recording section. The recording controller may record tracking records in the recording section in addition to the current position, tracking records indicating former positions of the electronic cassette, which have been determined by former detection results taken by the data taking section. 
     It is preferable that the electronic cassette management system further comprises a display controller for displaying at least the current position on a display among the current position and the tracking records. 
     It is preferable that the electronic cassette management system further comprises a home position setting section for setting up a home position of the electronic cassette. The recording controller may delete at least part of the tracking records when the electronic cassette comes back to the home position that is set up by the home position setting section. The home position setting section may also refer to the tracking records to set up a place as the home position where the electronic cassette is frequently located. 
     It is preferable that the data taking section, the alert controller and the recording controller are provided in a console that is provided for operating the electronic cassette. 
     In a situation where one of the service zones is supervised by a plurality of consoles, it is preferable that the electronic cassette management system further comprises an enable/disable setting section for enabling the alert controller and the recording controller in at least one of the plurality of consoles. 
     In a situation where the plurality of consoles include a portable console that a user of the electronic cassette is carried about, the enable/disable setting section may preferably enable the alert controller and the recording controller of the portable console. 
     It is preferable that the electronic cassette management system further comprises a range setting section for setting up the range of the service zone according to operational conditions of the electronic cassette. It is preferable that the operational conditions include a non-selected condition in which the electronic cassette is not selected to use for radiography and a selected condition selected to use for radiography, and the range setting section sets the range of the service zone narrower in the selected condition than in the non-selected condition. 
     It is possible to register a plurality of electronic cassettes as ones to be detected by the detectors. 
     The detectors are preferably wireless receivers that a receive radio frequency signal from a wireless communicator provided in the electronic cassette. 
     A method of operating the electronic cassette management system in accordance with the present invention comprises the steps of detecting the come and go of an electronic cassette by first and second detectors with respect to at least first and second predetermined service zones, taking detection results from the first and second detectors, starting an alert when it is determined on the basis of the detection results that the electronic cassette has gone out either of the first and second service zones, and stopping the alert when it is determined that the electronic cassette has come in either of the first and second service zones on the basis of detection results taken from the first and second detectors after the start of alerting. 
     Furthermore, an electronic cassette management device in accordance with the present invention comprises a first detector for detecting the come and go of an electronic cassette with respect to a predetermined first service zone, a data taking section for taking first detection results from the first detector and second detection results from a second detector for detecting the come and go of the electronic cassette with respect to a second service zone that is assigned to at least another electronic cassette management device, and an alert controller that starts an alert when the alert controller determines, on the basis of the first detection results taken by the data taking section, that the electronic cassette has gone out the first service zone. The alert controller stops the alert when the alert controller determines, on the basis of the first detection results or the second detection results taken by the data taking section, that the electronic cassette has come in the first service zone or the second service zone. 
     According to the present invention, detection results indicating the come and go of an electronic cassette with respect to at least first and second predetermined service zones are obtained, so that an alert is started when it is determined that the electronic cassette has gone out either of the first and second service zones, and the alert is stopped when it is determined that the electronic cassette has come in either of the first and second service zones after the start of alerting. Therefore, in a situation where an electronic cassette is shared among and moved across a plurality of service zones, the present invention can provide an electronic cassette management system, a method of operating the electronic cassette management system and an electronic cassette management device, which make it possible to give an alert when the electronic cassette is taken out of any of the service zones and also stop the alert at the right time if the electronic cassette is taken out for proper operations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects of the present invention will be more apparent from the following detailed description of the preferred embodiments when read in connection with the accompanied drawings, wherein like reference numerals designate like or corresponding parts throughout the several views, and wherein: 
         FIG. 1  is a diagram illustrating the interior of a medical facility having first and second radiography rooms; 
         FIG. 2  is a diagram illustrating a cassette registration table; 
         FIG. 3  is a diagram illustrating a cassette selection window; 
         FIG. 4  is a perspective view of an electronic cassette; 
         FIG. 5  is a block diagram illustrating an internal structure of the electronic cassette; 
         FIG. 6  is an explanatory diagram illustrating the operation sequences of the electronic cassette, a sensor panel and a determining section; 
         FIG. 7  is a block diagram illustrating a computer constituting a console; 
         FIG. 8  is a block diagram illustrating functions of a CPU of the console; 
         FIG. 9  is an explanatory diagram illustrating the timing of starting and stopping the alert by an alert controller; 
         FIGS. 10 and 11  show a flowchart illustrating the operation sequence of the console and a wireless tag reader; 
         FIG. 12  is a diagram illustrating a situation wherein first to fourth service zones are allocated to first to third radiography rooms and a round-visit vehicle, respectively; 
         FIG. 13  is a block diagram illustrating functions of a CPU of a console, which is provided with a recording controller and a display controller in accordance with second and third embodiments; 
         FIG. 14  is a diagram illustrating a cassette registration table including an item on current positions of electronic cassettes; 
         FIG. 15  is a diagram illustrating a window showing the current positions of the electronic cassettes; 
         FIG. 16  is a diagram illustrating a cassette registration table including items on current positions and tracking records of the electronic cassettes; 
         FIG. 17  is a diagram illustrating a window showing the current positions and tracking records of the electronic cassettes; 
         FIG. 18  is a diagram illustrating another example of a window showing the current positions and tracking records of the electronic cassettes; 
         FIG. 19  is a block diagram illustrating functions of a CPU of a console, which is provided with a home position setting section in accordance with a fourth embodiment; 
         FIG. 20  is a diagram illustrating a cassette registration table including an item on home positions of the electronic cassettes; 
         FIG. 21  is a diagram illustrating a tally table; 
         FIG. 22  is an explanatory diagram illustrating a situation wherein one service zone is supervised by a console, whereas another service zone is supervised by multiple consoles; 
         FIG. 23  is a block diagram illustrating functions of a CPU of a console, which is provided with an enable/disable setting section in accordance with a sixth embodiment; 
         FIG. 24  is an explanatory diagram illustrating an example wherein a portable console is set up as a master console; 
         FIG. 25  is a block diagram illustrating functions of a CPU of a console, which is provided with a range setting section in accordance with a seventh embodiment; 
         FIG. 26  is a diagram illustrating a range setup table; 
         FIG. 27  is an explanatory diagram illustrating the variations in the range of the service zone depending on the operational conditions of the electronic cassette; 
         FIG. 28  is a diagram illustrating another example of a range setup table; and 
         FIG. 29  is a diagram illustrating an electronic cassette management system in accordance with another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     In  FIG. 1 , a first radiography room  2 A and a second radiography room  2 B, which are for X-ray imaging, are located in adjacent to each other in a medical facility. The first radiography room  2 A is provided with a first doorway  6 A through which a patient P as an imaging subject comes in from a waiting room  5  or goes out to the waiting room  5 , and a staff entrance  8 A for a first user RA or a second user RB to go to and come from the second radiography room  2 B through a passageway  7 . The first user RA or the second user RB may be a doctor, a radiologist or a staff who is stationed in the first radiography room  2 A or the second radiography room  2 B, respectively. 
     The first radiography room  2 A is installed with a first X-ray source  10 A for projecting X-rays toward the patient P, a first radiographic stand  11 A for use in imaging the patient P in an upright position, a first radiographic table  12 A for use in imaging the patient P in a recumbent position, a first console  13 A operated by the first user RA, and a first rack  14 A. Note that the structure and equipment of the second radiography room  2 B are equal to those of the first radiography room  2 A, including the first doorway  6 A and the first X-ray source  10 A, and thus the description of the structure and equipment of the second radiography room  2 B will be omitted. In the following explanation, structural elements and devices as for the second radiography room  2 B are designated by the same terms and reference numerals as those for the first radiography room  2 A and, as needed, differentiated therefrom by putting “second” and “B” at the beginning of each term and at the end of each numeral, respectively, whereas “first” and “A” are put at the beginning of each term and at the end of each numeral, respectively, as for the structural elements and devices of the first radiography room  2 A. In case where the differentiation therebetween is unnecessary, “first” and “A” or “second” and “B” will be omitted from the designations and, for example, the wording “X-ray source  10 ” will be used. The same applies to the first user RA and the second user RB. 
     The medical facility is equipped with four electronic cassettes  15 A,  15 B,  15 C and  15 D. The electronic cassettes  15 A to  15 D each detect an X-ray image from X-rays received thereon, which have been projected from the X-ray source  10  and penetrated the patient P. In the medical facility, the electronic cassettes  15 A to  15 D are shared between the radiography rooms  2 A and  2 B. In this example, among the electronic cassettes  15 A to  15 D, the electronic cassettes  15 A and  15 B are of the same type, and the electronic cassette  15 A is mainly used in the first radiography room  2 A, whereas the electronic cassette  15 B is mainly used in the second radiography room  2 B. The electronic cassettes  15 C and  15 D are of a different type. 
     The same type means that the cassettes are equal in dimensions and features, and the different type means that the cassettes are different in dimensions or features or both. For example, the electronic cassettes  15 A and  15 B have the same dimensions of 14×17 inches and the same features, whereas the electronic cassette  15 C has dimensions of 17×17 inches and the electronic cassette  15 D has dimensions of 10×12 inches. 
     The electronic cassettes  15 A and  15 B are set in the radiographic stand  11 A of the first radiography room  2 A and in a radiographic stand  11 B of the second radiography room  2 B, respectively. In the first radiography room  2 A, the X-ray imaging of the patient P is being carried out. The electronic cassette  15 C is stored in the first rack  14 A of the first radiography room  2 A, and the electronic cassette  15 D is stored in a second rack  143  of the second radiography room  2 B. In spite of the differences in size and features, the electronic cassettes  15 A to  15 D are equal in the fundamental structure, and therefore are collectively called “the electronic cassettes  15 ” in case where the differentiation therebetween is unnecessary. 
     In the radiographic stand  11  or the radiographic table  12 , the electronic cassette  15  is held in a posture where a front face  45  thereof (refer to  FIG. 4 ) faces the X-ray source  10 . The patient P is positioned by the user R such that, the body site to be imaged is placed in between the X-ray source  10  and the electronic cassette  15 . The X-ray source  10  can be set in a desired orientation at a desired location by means of an X-ray source transporting device (not shown), so that the X-ray source  10  may be shared by the radiographic stand  11  and the radiographic table  12 . The electronic cassette  15  is also usable independently of the radiographic stand  11  and the radiographic table  12  in such a manner that the electronic cassette  15  is put on a bed on which the patient P lies or the electronic cassette  15  is held by the patient P. 
     A service zone  20  is predetermined in each radiography room  2 . The service zone  20  is a spatial range within which the use of the electronic cassette  15  is permitted. The waiting room  5  and the passageway  7  are excluded from the service zone  20 . 
     The console  13  is connected to a tag reader  21  (corresponding to a wireless receiver unit of the present invention). The wireless tag reader  21  is mounted to a wall, a ceiling or the like of each radiography room  2 . The wireless tag reader  21  is driven by a different power supply from that for the console  13 , and periodically sends a radio search signal. Responding to the search signal, the wireless tag reader  21  receives a response signal from a wireless tag  50  (corresponding to a wireless transmitter; refer to  FIG. 4 ) of each of those electronic cassettes  15  which are located within the service zone  20  across which the wireless tag reader  21  is searching, to thereby detect passage of the electronic cassettes  15  into and out of said service zone  20 . The wireless tag reader  21  thus serves as a detector. The wireless tag  50  is attached to each electronic cassette  15 . 
     The console  13  and the wireless tag reader  21  constitute an electronic cassette management apparatus of the present invention. The first console  13 A and the first wireless tag reader (first detector)  21 A supervise the come and go of the electronic cassettes  15  into and out of the first service zone  20 A that is predetermined within the first radiography room  2 A. The second console  13 B and the second wireless tag reader (second detector)  21 B supervise the come and go of the electronic cassettes  15  into and out of the second service zone  203  that is predetermined within the second radiography room  2 B. The first console  13 A, the second console  13 B, the first wireless tag reader  21 A and the second wireless tag reader  21 B constitute an electronic cassette management system of the present invention. 
     So long as the wireless tag reader  21  receives the response signal from the wireless tag  50  of one of the electronic cassettes  15 , the wireless tag reader  21  outputs a detection result indicating that the one electronic cassette  15  is present in the supervised service zone  20 . Meanwhile, when no response signal is received from one of the electronic cassettes  15 , the wireless tag reader  21  outputs a detection result indicating that the one electronic cassette  15  is absent (or brought out) from the supervised service zone  20 . Each detection result is attended by a reader ID of the wireless tag reader  21  that outputs the detection result. In addition to the reader ID, the detection result indicating that one electronic cassette  15  is present in the service zone  20  is attended by a cassette ID of the one electronic cassette  15  from which the wireless tag  50  sends the response signal. 
     The service zone  20  is determined depending upon the signal reach or transmission range of the search signal and the response signal. In this example, a frequency band around 900 MHz (e.g., 868 MHz), so-called UHF (ultra-high frequency) band, is used for the search signal and the response signal, wherein the signal reach thereof is about 2 to 5 meters. In order to discriminate between the respective electronic cassettes  15 A to  15 D, a different channel is allocated to each of the respective electronic cassettes  15 A to  15 D for the search signal and the response signal. 
     The first service zone  20 A and the second service zone  20 B are determined to cover the entire space in the first radiography room  2 A and the entire space in the second radiography room  2 B, respectively, and do not overlap each other. Therefore, the search signals and the response signals will not be confused between the radiography rooms  2 A and  2 B. As a method of determining the first service zone  20 A and the second service zone  20 B so as to cover the respective entire spaces of the radiography rooms  2 A and  2 B but not to overlap each other, it is possible to employ a method of electromagnetically shielding the individual radiography rooms  2 A and  2 B. Note that the wireless tag reader  21  may be attached to the door of the doorway  6  or the staff entrance  8 , or a base for disposing the console  13  thereon, etc. The wireless tag reader  21  may also be integrated in the console  13 . 
     The X-ray source  10  has an X-ray tube for radiating X-rays and an irradiation field limiter (that may also be called collimator) for limiting the irradiation field of X-rays from the X-ray tube. The X-ray source  10  is connected to a source controller  22 , and an activation switch  23  is connected to the source controller  22 . The source controller  22  controls tube voltage applied to the X-ray tube, tube current and X-ray irradiation time. The source controller  22  previously stores a number of kinds of image acquisition settings, including tube voltage, tube current and irradiation time, adjusted to individual body sites to be imaged, such as chest and abdomen, so that the user R can select and input an appropriate one from among the stored settings. 
     The activation switch  23 , which is operated by the user R, may be a two-step push button switch. When the activation switch  23  is pushed to the first step (to the half), the source controller  22  causes the X-ray source  10  to start a preparatory operation for X-ray irradiation. When the activation switch  23  is pushed further to the second step (to the full), the source controller  22  causes the X-ray source  10  to start X-ray irradiation. The source controller  22  has a timer that starts time-counting with the start of X-ray irradiation, to stop the X-ray source  10  from X-ray irradiation when the counted time reaches an irradiation time that is determined according to the image acquisition settings. 
     The console  13  includes a display device  25 , an input device  26 , a speaker  27 , a storage device  28 , etc., and is installed in each radiography room  2 . The display device  25  displays a variety of operation menu screens according to the operation through the input device  26 , such as a keyboard and a mouse. The operation menu screens are provided with GUI (graphical user interface) through which the console  13  receives inputs of operational instructions from the input device  26 . 
     The display device  25  also displays X-ray images detected through the electronic cassette  15  besides the operation menu screens, such as a screen for inputting imaging conditions. The speaker  27  sounds an alarm when any of the electronic cassettes  15  is moved out of the service zone  20 . The storage device  28  may for example be a hard disc drive, and stores the X-ray images and various information necessary for X-ray imaging. 
     The console  13  receives inputs of examination orders and displays the received orders. Each examination order includes patient information, such as gender and age of the patient P, information on the body site to be imaged, such as head, chest, abdomen, hand or finger, the position in imaging, such as upright or recumbent, and the imaging direction, such as frontal, back or lateral. The examination orders are input by means of external systems (not shown), such as a hospital information system (HIS) and a radiological information system (RIS), which manage patient information and information on radiological examinations. 
     The storage device  28  stores a number of kinds of image acquisition settings corresponding to respective types of patient information, body sites to be imaged, imaging positions and imaging directions, for example, in the form of a data table. As described above, the image acquisition settings include a tube voltage applied to the X-ray tube, a tube current and an X-ray irradiation time, which are determined taking consideration of the patient information, the body site to be imaged, the imaging position and the imaging direction. The user R checks the content of each examination order on the display device  25  to determine the image acquisition settings according to the content. The determined image acquisition settings are transferred from the console  13  to one electronic cassette  15  used for imaging. The user R sets up the source controller  22  with the same image acquisition settings as in the console  13 . In place of a tube current and an irradiation time, the product of the tube current and the radiation time (mAs value) may be input as one of the image acquisition settings. 
     Referring to  FIG. 2 , the storage device  28  stores a cassette registration table  30  for identifying the individual electronic cassettes  15 A to  15 D, in which the respective cassette IDs and the respective cassette names are registered in combinations. For example, at most five electronic cassettes  15  can be registered in the cassette registration table  30 . The cassette IDs are entered in the cassette registration table  30  by reading the wireless tags  50 , which are respectively attached to the respective electronic cassettes  15 A to  15 D, through the wireless tag reader  21  at the registration of the respective electronic cassettes  15 A to  15 D. The cassette names may be given to the respective electronic cassettes  15 A to  15 D by the user R. In this example, the electronic cassette  15 A, the electronic cassette  15 B, the electronic cassette  15 C and the electronic cassette  15 D are named “Cassette A”, “Cassette B”, “Cassette C” and “Cassette D”, respectively. 
     When one electronic cassette  15  is selected to use for imaging by operating the input device  26 , the console  13  determines which of the respective electronic cassettes  15 A to  15 D is selected with reference to the cassette ID registered in the cassette registration table  30 . Furthermore, the respective electronic cassettes  15 A to  15 D registered in the cassette registration table  30  are recognized by the wireless tag reader  21  as subjects to monitor. 
     As shown in  FIG. 3 , the display device  25  displays a cassette selection window  31  for selecting one electronic cassette  15  to use for imaging from among the electronic cassettes  15 A to  15 D. The cassette selection window  31  lists the names of the respective electronic cassettes  15 A to  15 D registered in the cassette registration table  30  and radio buttons  32  are provided beside the cassette names. The radio buttons  32  constitute a GUI which allows to choose only one button and cancels the choice of another button upon one button being chosen, thereby allowing to select only one of the electronic cassettes  15 A to  15 D. In the illustrated example, the electronic cassette  15 A, named Cassette A, is selected. By acting on an OK button  33  after selecting any of the electronic cassettes  15  by the corresponding radio button  32 , the selection of the one electronic cassette  15  to use for imaging is complete. 
     When one electronic cassette  15  has been selected to use for imaging on the cassette selection window  31 , the console  13  sends a selection signal, which indicates the selection for imaging (refer to  FIG. 6 ), to the selected one of the electronic cassettes  15 A to  15 D. 
     In  FIG. 4 , the electronic cassette  15  has a sensor panel  40 , a control circuit board  41 , a battery  42 , a communicator section  43 , and a flat box-shaped portable housing  44  housing these components. A rectangular opening is formed through a front face  45  of the housing  44 , and X-rays are incident through this opening. An X-ray permeable plate  46  as a top panel is fitted in the opening. 
     The battery  42  supplies power to the components of the electronic cassette  15  via a power supply circuit (not shown). The battery  42  is unloadable from the housing  44  and rechargeable by a specific charger (not shown). The communicator section  43  is connected to the console  13  wirelessly or in a wired manner, to exchange various kinds of data with the console  13 , including the image acquisition settings for X-ray imaging and X-ray images. The communicator section  43  also receives the selection signal from the console  13  when one electronic cassette  15  is selected to use for imaging. 
     The sensor panel  40  consists of a scintillator  47  and a photo detector circuit board  48 . The scintillator  47  and the photo detector circuit board  48  are laminated in this order from the X-ray incident side. The scintillator  47  includes a phosphor, such as thallium-activated cesium iodide (CsI:T1) or terbium-activated gadolinium oxy sulfide (Gd 2 O 2 S:Tb), converts X-rays entering through the X-ray permeable plate  46  to visible rays and emits the visible rays to electric charges through the pixels  55 . It may be possible to use another sensor panel in which a scintillator  47  is disposed on the opposite side of a photo detector circuit board  48  from the X-ray incident side. It may also be possible to use a sensor panel of a direct conversion type in which X-rays are directly converted to electric charges through a photoconductive layer, such as amorphous selenium. 
     The photo detector circuit board  48  detects the visible rays emitted from the scintillator  47  and converts the visible rays to an electric signal. The control circuit board  41  controls driving the photo detector circuit board  48  and produces an X-ray image on the basis of the electric signal output from the photo detector circuit board  48 . 
     A power switch  49  is disposed on one side of the housing  44 . The power switch  49  is operated by the user R when turning the power of the electronic cassette  15  on and off. 
     The wireless tag  50  is attached to one side of the housing  44 . The wireless tag  50  stores information including the cassette ID. The wireless tag  50  is a passive tag that is activated upon receipt of the search signal from the wireless tag reader  21 , and sends back a response signal that carries information including the cassette ID in response to the search signal. The search signal is periodically sent out. Thus, so long as one electronic cassette  15  stays in the service zone  20 , the wireless tag  50  of the one electronic cassette  15  repeatedly receives the search signal and sends back the response signal. The wireless tag  50  may also be integrated in the housing  44 . 
     Referring to  FIG. 5 , the photo detector circuit board  48  is provided with pixels  55  arranged in a two-dimensional matrix of N lines and M columns on a glass substrate (not shown), N scanning lines  56  and M signal lines  57 . “N” and “M” represent plural integers, e.g., M, N=around 2000. The scanning lines  56  extend in X-direction along the lines of the pixels  55  and are spaced at predetermined intervals from each other in Y-direction along the columns of the pixels  55 . The signal lines  57  extend in the Y-direction and are spaced at predetermined intervals in the X-direction. The scanning lines  56  and the signal lines  57  orthogonally intersect with each other, and the pixels  55  are provided corresponding to the cross points between the scanning lines  56  and the signal lines  57 . The arrangement of the pixels  55  is not limited to the square matrix array as shown in the illustrated example, but may be a honeycomb array. 
     The pixels  55  each have a photoelectric converter section  58  and a thin film transistor (TFT)  59  as a switching element, as well known in the art. The photoelectric converter section  58  generates electric charges (pairs of electrons and positive holes) according to incident visible rays and accumulates the generated electric charges. The photoelectric converter section  58  is constituted of a semiconductor layer and upper- and lower electrodes disposed on the top and bottom of the semiconductor layer. For example, the semiconductor layer is of a PIN (p-intrinsic-n) type wherein an n-type layer is formed on the side of the upper electrode and a P-type layer is formed on the side of the upper electrode. The TFT  59  is connected at its gate electrode to the scanning line  56 , at its source electrode to the signal line  57  and at its drain electrode to the lower electrode of the photoelectric converter section  58 . Note that a CMOS (complementary metal oxide semiconductor) type sensor panel may also be used instead of an TFT type. 
     The upper electrodes of the photoelectric converter sections  58  are connected to bias lines (not shown). The bias lines are provided in same number as the number of lines of the pixels  55  (N lines). The bias lines are connected together to a bus line, which is connected to a bias power source. Through the bus line and the respective bias lines, a positive bias voltage is applied to the upper electrodes of the photoelectric converter sections  58 . The applied positive bias voltage induces an electric field inside the semiconductor layer of each photoelectric converter section  58 . The photoelectric converter section  58  is used under a reverse bias condition. Among the pairs of electrons and holes, which are generated through the photoelectric conversion in the semiconductor layer, the electrons move to the upper electrode and are absorbed in the bias line, whereas the holes move to the lower electrode and are collected as signal charges. 
     The control circuit board  41  is provided with a gate driver  60 , a signal processor  61 , a memory  62 , a determining section  63  and a controller  64  for controlling these components. The gate driver  60  is connected to one terminals of the respective scanning lines  56  and outputs gate pulses G(K) (K=1 to N) for driving the TFTs  59 . The controller  64  drives the TFTs  59  through the gate driver  60 , to thereby cause the sensor panel  40  to perform a pre-irradiation reading operation for reading out signal charges from the pixels  55  before the start of an X-ray irradiation, an accumulating operation for accumulating signal charges in each pixel  55  according to the amount of X-rays incident on the pixel  55 , and an image reading operation for reading out the accumulated signal charges from the pixels  55  after the end of the X-ray irradiation. 
     In the accumulating operation, the gate driver  60  does not give the gate pulses G(K) to any of the scanning lines  56 . Thus, the TFTs  59  are off during the accumulating operation. While the TFT  59  is off, signal charges are accumulated in the pixel  55  according to the incident amount of X-rays. In the pre-irradiation reading operation and the image reading operation, the gate driver  60  sequentially outputs the gate pulses G(K) one to one scanning line  56  at regular intervals from the first to the last of the N scanning lines  56 , thereby turning on those TFTs  59  connected to the same scanning line  56  at a time and thus turn on the TFTs  59  line-sequentially. 
     The signal processor  61  is connected to one terminals of the respective signal lines  57 . The signal processor  61  coverts the signal charges read out from the pixels  55  in the pre-irradiation reading operation or the image reading operation to digital voltage values (signal voltages), and outputs these values to the memory  62 . The memory  62  stores the digital signal voltages output from the signal processor  61 . Hereinafter, the digital signal voltages stored in the memory  62  as a result of the pre-irradiation reading operation will be referred to as a dose signal (refer to  FIG. 6 ) and the digital signal voltages stored in the memory  62  as a result of the image reading operation will be referred to as an image signal (refer to  FIG. 6 ). 
     The determining section  63  is driven under the control of the controller  64 . The determining section  63  determines whether the X-ray source  10  starts an X-ray irradiation or not on the basis of the dose signal. When the determining section  63  determines that an X-ray irradiation has started, the determining section  63  outputs an irradiation start determination signal to the controller  64  (refer to  FIG. 6 ). 
     The controller  64  has an integrated timer  65 . The timer  65  is set up with the irradiation time that is contained in the image acquisition settings determined on the console  13 . The timer  65  starts counting upon receipt of the irradiation start determination signal from the determining section  63 . The controller  64  determines that the X-ray irradiation stops when the time counted by the timer  65  reaches the irradiation time set in the timer  65 . 
     In  FIG. 6 , the electronic cassette  15  has five operation modes: power-off mode, standby mode, sleep mode, ready mode and image acquisition mode. 
     Upon the power switch  49  being turned on, the electronic cassette  15  transmits a power-on signal to the console  13  via the communicator section  43 , indicating that the power source is turned on, and the electronic cassette  15  gets in the standby mode. If no operation is applied to the electronic cassette  15  for a certain time in the standby mode, the electronic cassette  15  moves to the sleep mode. When the electronic cassette  15  in the sleep mode receives the selection signal from the console  13 , indicating that the electronic cassette  15  is selected to use for X-ray imaging, the electronic cassette  15  moves back to the standby mode. When the electronic cassette  15  in the standby mode receives data of image acquisition settings from the console  13 , the electronic cassette  15  moves to the ready mode. When the determining section  63  determines the start of an X-ray irradiation and outputs the irradiation start determination signal to the controller  64  in the ready mode, the electronic cassette  15  moves to the image acquisition mode. 
     In the standby mode and the sleep mode, the controller  64  stops supplying power to the gate driver  60  and the signal processor  61 , and drives the communicator section  43  only. Accordingly, the sensor panel  40  does not work in the standby mode and the sleep mode. In the ready mode, the controller  64  controls the sensor panel  40  to execute the pre-irradiation reading operation and output the dose signal to the memory  62 . In the image acquisition mode, the controller  64  controls the sensor panel  40  to execute the accumulating operation. When the controller  64  receives the irradiation start determination signal from the determining section  63 , the controller  64  immediately stops the pre-irradiation reading operation and starts the accumulating operation even while the pre-irradiation reading operation is being conducted on an intermediate line of the pixels  55 . Thus, the starting time of the accumulating operation is synchronized with the starting time of the X-ray irradiation. 
     When the time counted by the timer  65  reaches the irradiation time, which is set as one image acquisition setting, the controller  64  determines that the X-ray radiation is terminated. Then the controller  64  controls the sensor panel  40  to execute the image reading operation to output the image signal to the memory  62 . The image signal output to the memory  62  is transmitted as an X-ray image to the console  13  via the communicator section  43 . 
     At the end of the image reading operation, the electronic cassette  15  returns to the standby mode. When the power switch  49  is turned off, the electronic cassette  15  transmits a power-off signal (refer to  FIG. 27 ) to the console  13  via the communicator section  43 , indicating that the electronic cassette  15  gets in the power-off mode. 
     The power-off mode, the standby mode from the power-on to the receipt of the selection signal and the sleep mode are comprehended in a non-selected condition of the electronic cassette  15 , wherein the electronic cassette  15  is not selected to use for imaging. The standby mode after the receipt of the selection signal, the ready mode and the image acquisition mode are comprehended in a selected condition of the electronic cassette  15 , wherein the electronic cassette  15  is selected to use for imaging. The standby mode after the image acquisition mode is comprehended in the selected condition of the electronic cassette  15  until another electronic cassette  15  is selected on the cassette selection window  31  or the electronic cassette  15  moves to the sleep mode. 
     At each output of the dose signal concerning one line of the pixels  55  to the memory  62 , the determining section  63  compares a representative value of the dose signal concerning one line with a predetermined threshold value. The greatest value among the dose signal values for one line is preferable as the representative value to be compared with the threshold value, but may also be the mean value or the sum of the dose signal values for one line. The dose signal begins to increase with the start of X-ray irradiation, and the representative value of the dose signal will reach a level above the threshold value in a certain time. At the time when the representative value of the dose signal exceeds the threshold value, the determining section  63  determines that the X-ray irradiation has started. 
     In  FIG. 7 , the console  13  is constituted of a computer which is equipped with the display device  25 , the input device  26 , the speaker  27  and the storage device  28 , as described above, and also with a memory  70 , a CPU (central processing unit)  71  and a communicator section  72 . These components are interconnected through a data bus  73 . 
     The storage device  28  stores control programs, such as operating systems, and a variety of application programs, including an electronic cassette management program  74 , in addition to the cassette registration table  30 . The electronic cassette management program is for making the computer that constitutes the console  13  function as the electronic cassette surveillance apparatus in cooperation with the wireless tag reader  21 . 
     The memory  70  is a work memory for the CPU  71  to execute processing. The CPU  71  loads the programs from the storage device  28  onto the memory  70  and executes processing according to the loaded programs, thereby overall controlling the components of the computer. 
     The communicator section  72  is connected to the communicator section  43  of each electronic cassette  15  wirelessly or in a wired manner. Furthermore, within each service zone  20 , the communicator section  72  is connected to the wireless tag reader  21 , so that the communicator section  72  receives the detection results from the wireless tag reader  21  on the come and go of the respective electronic cassettes  15  into and out of the service zone  20 . Moreover, the communicator section  72  functions as a network interface for transmission of various kinds of information between the consoles  13  and other external devices over a network  75  such as a LAN (local area network) laid in the medical facility. The communicator section  72  receives the power-on signal, the power-off signal and X-ray images from the electronic cassette  15 , and sends the selection signal and the image acquisition settings to any one of the electronic cassettes  15 . The communicator section  72  also receives the detection results, which have been obtained by the wireless tag reader  21  for another service zone  20 , from the console  13  of another radiography room  2 , and sends the detection results obtained by the wireless tag reader  21 , which is connected to the communicator section  72 , to the console  13  of another radiography room  2 . 
     Referring to  FIG. 8 , when the electronic cassette management program is activated, the CPU  71  serves as a data taking section  80  and an alert controller  81  in cooperation with the memory  70 . 
     A first data taking section  80 A of the first console  13 A takes the detection results on the come and go of the electronic cassettes  15  into or out of the first service zone  20 A (the first detection results), which are obtained by the first wireless tag reader  21 A. The first data taking section  80 A also takes the detection results on the come and go of the electronic cassettes  15  into or out of the second service zone  20 B (the second detection results), which are obtained by the second wireless tag reader  21 B and sent from the second console  13 B via the network  75  and received on the first communicator section  72 A. The first data taking section  80 A transfers the first and second detection results to the first alert controller  81 A. 
     The first alert controller  81 A controls driving the first speaker  27 A on the basis of the first and second detection results from the first data taking section  80 A. More specifically, when the first alert controller  81 A determines on the basis of one first detection result that one of the electronic cassettes  15  is brought out of the first service zone  20 A, the first alert controller  81 A drives the first speaker  27 A to start warning, as shown in  FIG. 9A . After the first speaker  27 A starts alerting, when the first alert controller  81 A determines on the basis of another first detection result that the one electronic cassette  15  is brought back into the first service zone  20 A, the first alert controller  81 A stops driving the first speaker  27 A to stop the alert, as shown in  FIG. 9B . Furthermore, when the first alert controller  81 A determines on the basis of one second detection result that the one electronic cassette  15  is brought into the second service zone  20 B, the first alert controller  81 A stops driving the first speaker  27 A to stop the alert, as shown in FIG.  9 C. After the first alert controller  81 A determines that one electronic cassette  15  is brought out of the first service zone  20 A, the first alert controller  81 A drives the first speaker  27 A to keep the alert so long as the first alert controller  81 A determines on the basis of the following first and second detection results that the one electronic cassette  15  does not exist either in the first service zone  20 A or in the second service zone  20 B. The above description referring to  FIGS. 8 and 9  mainly relates to the first console  13 A, but the same applies to the second console  13 B. For example, a second  81 B of the second console  13 B drives the second speaker  27 B to start warning when the second alert controller  81 B determines on the basis of one second detection result that one of the electronic cassettes  15  is brought out of the second service zone  20 B. 
     Now the operation of the above-described configuration will be described with reference to  FIG. 9  and the flow chart in  FIGS. 10 and 11 , showing an operation sequence for the first console  13 A and the first wireless tag reader  21 A while the electronic cassette management program is being executed. First, as shown in step S 10 , the first console  13 A and the first wireless tag reader  21 A are actuated. As shown in step S 11 , the first wireless tag reader  21 A starts detecting passage of the electronic cassettes  15  into and out of the first service zone  20 A by periodically sending out the search signals and receiving the response signals from the wireless tags  50  of the electronic cassettes  15 . The first data taking section  80 A starts taking the first detection result, which the first wireless tag reader  21 A obtains on the basis of the response signal received thereon, through the first communicator section  72 A (step S 12 ). 
     In the same way as the second wireless tag reader  21 B, the second wireless tag reader  2 B also starts detecting passage of the electronic cassettes  15  into and out of the second service zone  20 B. The second detection result obtained by the second wireless tag reader  21 B is transferred from the second communicator section  72 B of the second console  13 B through the network  75  and the first communicator section  72 A to the first data taking section  80 A. Thus the first data taking section  80 A starts taking the second detection result (step S 13 ). In the manner as above, the first console  13 A and the first wireless tag reader  21 A start monitoring passage of the electronic cassettes  15  into and out of the first service zone  20 A (step S 14 ). 
     After the monitoring starts in step S 14 , when the first alert controller  81 A determines that one of the electronic cassettes  15  is present in the first service zone  20 A (“Yes” in step S 15 ), the program proceeds to step S 16 . If it is determined that one of the electronic cassettes  15  is not in the first service zone  20 A (“No” in step S 15 ), the program proceeds to step S 30 , as shown in  FIG. 11 . Note that the step S 15  and the following steps are executed with respect to each of the registered electronic cassettes  15 . 
     In step S 16 , when the first alert controller  81 A determines on the basis of the first detection result that one electronic cassette  15  is brought out of the first service zone  20 A (“Yes” in step S 16 ), the first alert controller  81 A drives the first speaker  27 A to start warning (step S 17 ) (refer to  FIG. 9A ). 
     Accordingly, if, for example, the second user RB of the second radiography room  2 B brings one of the electronic cassettes  15  out of the first radiography room  2 A through the first staff entrance  8 A to the passageway  7  with intent to use the one electronic cassette  15  in the second radiography room  2 B, or if an unauthorized person fraudulently takes any of the electronic cassettes  15  out of the first radiography room  2 A, the first speaker  27 A starts sounding an alert. Thus, if any of the electronic cassettes  15  is taken out of the first radiography room  2 A without being noticed by the first user RA in the first radiography room  2 A, the first user RA is quickly warned of the fact that the electronic cassette  15  is taken out of the first service zone  20 A. 
     After the start of an alert, the first speaker  27 A keeps the alert so long as the first alert controller  81 A does not determine that the one electronic cassette  15  enters the first service zone  20 A or the second service zone  20 B (“No” in step S 18 ) (refer to  FIG. 9D ). Thus, in the case where someone other than the users R fraudulently takes the electronic cassette  15  out of the first radiography room  2 A, e.g., with intent to steal, the alert continues. Thereby, the first user RA can notice the smuggle of the electronic cassette  15  and take measures against it, e.g., alerting it to security personnel of the medical facility. 
     When the electronic cassette  15  is returned to the first service zone  20 A, as shown in  FIG. 9B , or is brought into the second service zone  20 B, as shown in  FIG. 9C , the first alert controller  81 A determines on the basis of the first or the second detection result that the electronic cassette  15  enters the first service zone  20 A or the second service zone  20 B (“Yes” in the step S 18 ), and stops driving the first speaker  27 A to cease the alert (step S 19 ). Thus, the alert will stop insofar as the electronic cassette  15  is within a proper operational range, including the transportation of the electronic cassette  15  between the radiography rooms  2 A and  2 B. Accordingly, the user R would not be bothered by the continuous alert within the proper operational range of the electronic cassette  15 . 
     After the alert stops in step S 19 , the first console  13 A and the first wireless tag reader  21 A continues monitoring passage of the electronic cassettes  15  into and out of the first service zone  20 A (“Yes” in step S 20 ), and if the first alert controller  81 A determines on the basis of the first detection result that the one electronic cassette  15  that has been taken out of the first service zone  20 A is present in the first service zone  20 A (“Yes” in step S 21 ), the program returns to the step S 16 . If the first alert controller  81 A determines on the basis of the second detection result that the electronic cassette  15  is in the second service zone  20 B not in the first service zone  20 A (“No” in the step S 21 ), the program proceeds to step S 31  (refer to  FIG. 11 ). 
     As shown in  FIG. 11 , when the program proceeds from the step S 15  to the step S 30 , the first alert controller  81 A determines on the basis of the second detection results whether the one electronic cassette  15  is in the second service zone  20 B or not. If it is determined that the one electronic cassette  15  is not in the second service zone  20 B (“No” in the step S 30 ), the program returns to the step S 17  in  FIG. 10  and the first speaker  27 A is driven to start warning. If, on the contrary, the first alert controller  81 A determines that the one electronic cassette  15  is in the second service zone  20 B (“Yes” in the step S 30 ) and thereafter determines on the basis of the second detection result that the one electronic cassette  15  is taken out of the second service zone  20 B (“Yes” in the step S 31 ), the first alert controller  81 A does not drive the first speaker  27 A to start warning. In this case, the second alert controller  81 B drives the second speaker  27 B to start warning from the second speaker  27 B. 
     Thereafter when the first alert controller  81 A determines on the basis of the first or the second detection result that the one electronic cassette  15  enters the first service zone  20 A or the second service zone  20 B (“Yes” in the step S 32 ), the first alert controller  81 A determines on the basis of the first detection result whether the one electronic cassette  15  is in the first service zone  20 A or not (step S 33 ). Simultaneously, the second alert controller  81 B also determines that the one electronic cassette  15  enters the first service zone  20 A or the second service zone  20 B, in the same way as the first alert controller  81 A, and the alert from the second speaker  27 B is stopped. 
     If the first alert controller  81 A determines on the basis of the first detection result that the one electronic cassette  15  is in the first service zone  20 A (“Yes” in the step S 33 ), the program returns to the step S 16  in  FIG. 10 . If the first alert controller  81 A determines on the basis of the second detection result that the one electronic cassette  15  is in the second service zone  20 B not in the first service zone  20 A (“No” in the step S 33 ), the program returns to the step S 31 . 
     It is to be noted that, in addition to or in place of sounding an alert by the speaker  27 , it may be possible to blink the display device  25  or light a warning lamp that is provided separately from the console  13 . In the first embodiment, the first speaker  27 A is driven to give an alert only when any of the electronic cassettes  15  is moved out of the first service zone  20 A, but it may be possible to drive all speakers  27  to sound an alarm when any of the electronic cassettes  15  is moved out of any service zone  20 , including the second service zone  20 B. It may also be possible to provide the electronic cassette  15  with a speaker or an indicator to give an alert from the speaker or indicator of the electronic cassette  15 . 
     The console  13  may not necessarily be located in the service zone  20 . For example, in a case where the console  13  is installed in an operation room separately from the radiography room  2 , the service zone  20  need not cover the operation room. The serves zones  20  may partly overlap, so far as each service zone  20  is supervised by at least one console  13 . 
     In the case where the service zones  20  partly overlap, when one electronic cassette  15  enters an overlap area of two service zones  20 , the second data taking sections  80  of the consoles  13  that supervise the two service zones  20  take detection results indicating that the one electronic cassette  15  is in the two service zones  20 , so that the alert controllers  81  of these consoles  13  determine that the one electronic cassette  15  is in the two service zones  20 . Thereafter when the one electronic cassette  15  is moved from the overlap area to one of the two service zones  20 , the second data taking sections  80  once determine that the one electronic cassette  15  has gone out the other service zone  20  but immediately determine that the one electronic cassette  15  is moved in one service zone  20 . Therefore, no alert is given in that case. If the electronic cassette  15  is moved from the overlap area out of these service zones  20 , the speakers  27  of the consoles  13  that supervise the two service zones  20  are driven to start warning. 
     The consoles  13  illustrated in the above first embodiment are installed in the radiography rooms  2 , but a console may be of a portable type that the user R can handle for the X-ray imaging while carrying about the same. Examples of portable consoles are mobile phones, smart phones, tablet terminals, PDA (personal digital assistant), laptop computers, etc. Unlike the installed console  13 , the service zone  20  supervised by the portable console is not fixed but defined in a region where the portable console is moved in. 
     In the above first embodiment, two service zones  20 A and  20 B are defined in the medical facility, but this is for the sake of simplifying the explanation, and it may be possible to define more than two service zones. 
     For example, as shown in  FIG. 12 , a first service zone  20 A, a second service zone  20 E, a third service zone  20 C and a fourth service zone  20 D may be defined with respect to a first radiography room  2 A, a second radiography room  2 B, a third radiography room  2 C and a round-visit vehicle  85 , respectively. In the first to third radiography rooms  2 A to  2 C, a first console  13 A, a second console  13 B and a third console  13 C as well as first to third electronic cassettes  15 A,  15 B and  15 C are disposed, respectively. The round-visit vehicle  85  has a structure wherein an X-ray source  10 , a fourth console  13 D and a container box  86  are integrally board on a mobile wagon  87 . An electronic cassette  15 D is contained in the container box  86 . 
     The consoles  13 A to  13 D are interconnected through a network  75  (refer to  FIG. 7 ). In addition, a first wireless tag reader  21 A, a second wireless tag reader  21 B, a third wireless tag reader  21 C and a fourth wireless tag reader  21 D are disposed in the radiography rooms  2 A to  2 C and the round-visit vehicle  85 , respectively. Note that the doorways  6 , the passageway  7 , the X-ray sources  10  and the radiography stand or table  11  or  12  are omitted from the drawings in the following embodiments. 
     The fourth service zone  20 D allocated to the round-visit vehicle  85 , for example, has a range of approximately 2-5 m diameter around the round-visit vehicle  85 , which is the transmission range of an UHF band signal. The electronic cassettes  15 A to  15 D may be equal to those of the first embodiment, wherein the electronic cassettes  15 A and  15 B are of the same type, whereas the electronic cassettes  15 C and  15 D are of a different type. 
     In the same as in the first embodiment, when it is determined that one of the electronic cassette  15  has gone out any of the service zones  20 A to  20 D, at least a speaker  27  raises an alert in the one service zone  20 . Thereafter when it is determined that the one electronic cassette  15  is moved in any of the service zones  20 A to  20 D, the alert is interrupted. 
     By defining a service zone  20  with respect to a mobile subject such as the round-visit vehicle  85 , the alert will be given not only when the electronic cassette  15  is taken out of the round-visit vehicle  85  but also when the round-visit vehicle  85  is moved away from a patient bedroom while leaving the electronic cassette  15  behind in the patient bedroom, thereby preventing the electronic cassette  15  from being left behind. 
     The following second to fifth embodiments will be described on the assumption that four electronic cassettes  15 A to  15 D are shared among and moved across the service zones  20 A to  20 D, as is shown in  FIG. 12 . 
     Second Embodiment 
     Since one wireless tag reader  21  is provided in each service zone, and each detection result includes an ID of the individual wireless tag reader  21  that outputs the detection result, the detection results not only indicate the come and go of the electronic cassettes  15  into and out the service zones  20 , but also indicate respective service zones in which the electronic cassettes  15  currently exist, i.e., current positions of the electronic cassettes  15 . For example, a detection result indicating that one electronic cassette  15  enters the fourth service zone  20 D also indicates that the one electronic cassette  15  is in the fourth service zone  20 D and hence the current position of the one electronic cassette  15  is on or around the round-visit vehicle  85 . Accordingly, in the present embodiment shown in  FIG. 13 , a CPU  71  of each console  13  is also served to function as a recording controller  90  for recording current positions of the electronic cassettes  15  on a storage device  28  and as a display controller  91  for displaying the respective current positions on a display device  25 . 
     In this case, a data taking section  80  transfers the detection result, which is obtained through a communicator section  72 , to an alert controller  81  and the recording controller  90 . Furthermore, as shown in  FIG. 14 , a cassette registration table  92  including an item on the current position in addition to items included in the cassette registration table  30  in  FIG. 2  is stored in the storage device  28 . The recording controller  90  records the current position of each electronic cassette  15 , which is determined by the detection result from the data taking section  80 , in the cassette registration table  92 . The recording controller  90  revises the current position data in the cassette registration table  92  each time the current position changes. In the present example, the first radiography room  2 A (the first service zone  20 A), the second radiography room  2 B (the second service zone  20 B), the third radiography room  2 C (the third service zone  20 C) and the round-visit vehicle  85  (the fourth service zone  20 D) are recorded as the current positions of the electronic cassettes  15 A,  15 B,  15 C and  15 D, respectively. 
     The display controller  91  reads out the recorded current positions from the cassette registration table  92  in response to an operation command entered by the user R through an input device  26 , and controls the display device  25  to display a current position window  93  shown in  FIG. 15  on the basis of the read current positions. The current position and tracking record window  93  shows the names of the electronic cassettes  15 A to  15 D and the respective current positions. From the current position and tracking record window  93 , the user R can confirm the current positions of the electronic cassettes  15 . 
     Thus, enabling the user R to be informed of the current positions of the electronic cassettes  15  eliminates the labor of the user R to walk around the service zones in search of the electronic cassette  15  that the user R wants to use in the case where the electronic cassettes  15  are shared among and moved across a plurality of service zones. Since the users R can instantly see the current positions of the electronic cassettes  15 , the users R may freely carry about the electronic cassette  15  without the need for permission or reporting thereafter. Accordingly, it becomes possible to make full use of the mobility of the electronic cassettes  15 . 
     In  FIG. 13 , “Detection Result” represents the detection results on the come and go of the electronic cassettes  15  into and out of one service zone  20  that is supervised by one console  13 , whereas “Other Detection Result” represents those detection results on the come and go of the electronic cassettes  15  into and out of other service zones  20  supervised by other consoles  13 , and those transmitted from other consoles  13  (the same applies to  FIGS. 19 and 23 ). For example, provided that the one console  13  is the first console  13 A, the “Detection Result” represents the first detection results from the first tag reader  21 A, whereas the “Other Detection Result” represents the second to fourth detection results transmitted from the second to fourth consoles  13 B to  13 D. Provided that the one console  13  is the second console  13 B, the “Detection Result” represents the second detection results from the second tag reader  21 B, whereas the “Other Detection Result” represents the first, third and fourth detection results transmitted from the first, third and fourth consoles  13 A,  13 C and  13 D. 
     Third Embodiment 
     The second embodiment is configured to record and display the current positions of the electronic cassettes  15 . The third embodiment is configured to record and display not only the current positions but also former positions of the electronic cassettes  15  as tracking records. By successively recording the current position and former positions of each cassette, the tracking record will be acquired. For example, a detection result indicating that the first electronic cassette  15 A is moved from the first service zone  20 A to the service zone  20 C indicates that the current position of the first electronic cassette  15  changes from the first service zone  20 A to the service zone  20 C. Note that other functional sections of a CPU  71  of each console  13  are equivalent to those shown in  FIG. 13  with respect to the second embodiment, and thus the description of the functional sections will be omitted. 
     As shown in  FIG. 16 , according to the third embodiment, a storage device  28  stores a cassette registration table  95  that contains an item on tracking records of the electronic cassettes  15  (data relating to the electronic cassettes  15 C and  15 D is omitted in the drawing) in addition to the items of the cassette registration table  92  in  FIG. 14 . The item on tracking records of each electronic cassette  15  includes last four service zones, which have been recorded as the current positions, and the date and time of movement out of each of said service zones (the dates and times when the alert controller  81  determined that the electronic cassette  15  was moved out of these service zones). When a new current position of one electronic cassette  15  is determined by the detection results from the data taking section  80 , the recording controller  90  rewrites the current position data of the cassette registration table  95  with the new current position, and revises the tracking records with the service zone that has been recorded as the current position and the date and time when it is determined that the one electronic cassette  15  was moved out of said service zone  20 . 
     Taking the first electronic cassette  15  for example, if the current position thereof is in the first radiography room  2 A, as shown in  FIG. 16 , and changes, for instance, to the round-visit vehicle  85 , then the round-visit vehicle  85  is newly recorded as the current position of the first electronic cassette  15  in the cassette registration table  95 . Furthermore, the tracking records are revised by deleting the oldest record, i.e. the round-visit vehicle and the move-out date and time therefrom (at 11:06 on Mar. 12, 2014) in this example, rewriting the second oldest record, i.e. the first radiography room and the move-out date and time therefrom (at 14:20 on Mar. 12, 2014), as the oldest record, and sequentially rewriting the following records of former positions and move-out dates and times in this way, and finally recording the first radiography room that has been recorded as the current position and the move-out date and time therefrom as the newest tracking record. 
     The display controller  91  reads out the current positions and the tracking records of the electronic cassettes  15  from the cassette registration table  95  in response to an operation command entered by the user R through an input device  26 , and displays on the basis of the read current positions and the tracking records a current position and tracking record window  96  on a display device  25 . As shown for example in  FIG. 17 , the current position and tracking record window  96  displays the cassette name (e.g., “Cassette A” corresponding to the first electronic cassette  15 ), the current position and the tracking records of each individual electronic cassette  15 . Beside the current position are displayed the present date and time. As the tracking records, last four former positions of the electronic cassettes  15  and the respective move-out dates and times therefrom are displayed. On this current position and tracking record window  96 , the user R can confirm the current position and the tracking records of each electronic cassette  15 . 
     Thus, making it possible to inform the user R of the tracking records of each electronic cassette  15  in addition to the current position thereof enables the user R to see at one view the track of the electronic cassette  15  having been traveled around for operations. If it is necessary to switch over the operation modes of the electronic cassette  15  manually according to the applications, e.g. depending on whether the electronic cassette  15  is carried on the round-visit vehicle  85  or used in the radiography room  2 , the tracking records help the user R instantly determine whether to switch over the operation modes or not, thereby smoothing the procedure of operation. 
     The number of former positions of each electronic cassette  15 , which are to be recorded as the tracking records, is not limited to four. Furthermore, it may be possible to record the former positions of each electronic cassette  15  over a certain period, e.g. over the past month. By thus recording and storing a certain number of former positions, it becomes possible to know the trend of application of each electronic cassette  15 , i.e., in which service zones and how long the electronic cassette  15  has been used. This can help planning the installation of the electronic cassette  15 . 
     If it is unnecessary to switch over the operation modes of the electronic cassette  15  manually depending upon whether the electronic cassette  15  is used in the radiography room  2  or on the round-visit vehicle  85 , the user R has to know only the current position of at least one electronic cassette  15  that the user R intends to use. Therefore, it is also possible to allow the user R to choose whether to display the current position only, like in the current position and tracking record window  93 , or to display the tracking records in addition to the current position, like in the current position and tracking record window  96 . 
     In a modification, each console may display the tracking records discriminating between those indicating the move-out of the electronic cassette  15  from one of the service zones  20  which is supervised by said console  13  and those indicating the move-out from other service zones  20 . Taking the first console  13 A for example, as supervising the first service zone  20 A, the first console  13 A displays the tracking records indicating the move-out of the electronic cassette  15  from the first service zone  20 A distinguishably from those indicating the move-out from the second to fourth service zones  20 B to  20 D. More specifically, a first display controller  91 A built in a first CPU  71 A of the first console  13 A causes a first display device  25 A to display a current position and tracking record window  100  in a manner as shown in  FIG. 18 . 
     Referring to  FIG. 18 , on the current position and tracking record window  100 , the tracking records are discriminated between two kinds: “move” and “move-out”, and the tracking record of “move-out” is asterisked. The tracking record of “move” indicates that the electronic cassette  15 A (Cassette A) was moved from any of the second to fourth service zones  20 B to  20 D (the second and third radiography rooms  2 B and  2 C and the round-visit vehicle  85 ), whereas the tracking record of “move-out” indicates that the first electronic cassette  15  was moved out the first service zone  20 A (the first radiography room  2 A). In addition, the asterisk indicates that the first speaker  27 A of the first console  13 A gave an alert as the electronic cassette  15 A was moved out of the first service zone  20 A. 
     Thus, the console  13  can remarkably display those tracking records which indicate that the electronic cassette  15  was moved out of the service zone  20  that is supervised by said console  13 . Any appropriate method for displaying the tracking records in a distinguishable manner is applicable to this embodiment, including differentiation by font color, font type, font style and blinking. 
       FIGS. 17 and 18  show examples displaying the current position and tracking records of the first electronic cassette  15  (Cassette A), but each console  13  can display the current position and tracking records of any of the electronic cassettes  15  which have been registered in the cassette registration table  95 . 
     The way of displaying the current position or the tracking records is not limited to the current position and tracking record window  93  in  FIG. 15  or the current position and tracking record window  96  or  100  in  FIG. 17 or 18 . For example, it is possible to display the current position and tracking records of a designated electronic cassette  15  by popping up a window when the name of the designated electronic cassette  15  is selected on the cassette selection window  31  shown in  FIG. 3 . This is preferable as the user R can see the current position and tracking records of each electronic cassette  15  concurrently when selecting any of the electronic cassettes  15  to use for radiography. 
     By applying a portable console to the above second or third embodiment, the user of the portable console can check the current position of any electronic cassette  15  or the tracking records thereof on the portable console without the need for accessing an installed console. Therefore, the user can move directly to the current position of a desired electronic cassette  15  to capture it, which facilitates the work procedure. 
     Fourth Embodiment 
     In some cases, as set forth above in the description of the electronic cassettes  15 A and  15 B, a main service zone of each individual electronic cassette  15 , in which said electronic cassette is mainly used, may be determined within a medical facility. Therefore, according to the fourth embodiment, as shown in  FIG. 19 , a CPU  71  of a console  13  is provided with a home position designating section  105  for designating a home position to an electronic cassette  15 , wherein the home position may be a service zone in an imaging room or around a round-visit vehicle, in which the electronic cassette  15  is determined to be mainly used. Also in the present embodiment, a recording controller  90  records the current position and tracking records of each electronic cassette in the same way as in the above third embodiment. 
     In this embodiment, as shown in  FIG. 20 , a storage device  28  stores a cassette registration table  106  containing an item on the home position in addition to the items of the cassette registration table  95  in  FIG. 16 . The designation of the home position may be done by the user R through an input device  26 , for example, by selecting one of the first to third radiography rooms  2 A to  2 C or the round-visit vehicle  85  as the home position of one of electronic cassettes  15 A to  15 D registered in the cassette registration table  106 . The home position setting section  105  records the designated home positions in the cassette registration table  106 . In the illustrated example, the first to third radiography rooms  2 A to  2 C and the round-visit vehicle  85  are designated as the home positions of the electronic cassettes  15 A to  15 D, respectively. 
     The recording controller  90  deletes the tracking records of one electronic cassette  15  except the last former position thereof when the one electronic cassette  15  is moved back to the designated home position from another position.  FIG. 20  shows an example in which the first electronic cassette  15 A has moved back to the home position, the first radiography room  2 A, and hence the recording controller  90  has deleted the tracking records of the first electronic cassette  15 A except but the last former position (the round-visit vehicle  85 ). Since the second electronic cassette  15 B is not moved back to the home position, the second radiography room  2 B, the tracking records thereof remain undeleted. 
     If one electronic cassette  15  is in the home position thereof and the user R who intends to use the one electronic cassette  15  is also in that home position, the tracking records of the one electronic cassette  15  is not so important information for this user R. Accordingly, there is no problem in partly deleting the tracking records. Since some parts of the tracking records are automatically deleted, as being not so important for the user R, the capacity load on the storage device  28  is reduced. Furthermore, the deleted tracking records are not displayed on a current position and tracking record window  96  or  100  in this embodiment, thereby simplifying the display of tracking records and thus improving the visibility thereof. 
     Instead of deleting tracking records of one electronic cassette except the last former position when the current position of the one electronic cassette returns to the designated home position, it is possible to delete all the tracking records or delete the tracking records excluding the last two former positions. Alternatively, it may be possible for one console  13  to selectively delete a particular kind of tracking records, e.g. those tracking records indicating that the electronic cassette  15  was moved out other service zones  20  that are supervised by other consoles  13 . Furthermore, it is possible to allocate multiple home positions to one electronic cassette. For example, as for such an electronic cassette that is evenly shared among all service zones, and no main service zone is assigned thereto, it is possible to designate all service zones as home positions. 
     Fifth Embodiment 
     In the above fourth embodiment, the home position is set up by the user R through the input device  26 . In an alternative, such a location in which one electronic cassette  15  most frequently exists is determined with reference to tracking records of the one electronic cassette  15 , to set up the location as the home position of said electronic cassette  15 . Specifically, in the fifth embodiment, a home position designating section  105  analyzes tracking records written in a cassette registration table  106 , and counts the respective numbers of times at which each of the electronic cassettes  15 A to  15 D has been located in the service zones  20 A to  20 D (the radiography rooms  2 A to  2 C and the round-visit vehicle  85 ), to create a tally table  107  shown in  FIG. 21  (data being omitted with respect to the electronic cassettes  15 C and  15 D). Then, with reference to the tally table  107 , the home position setting section  105  sets up one service zone where one electronic cassette has been most frequently located as the home position of the one electronic cassette. 
     In the illustrated example, the first electronic cassette  15  has been located most frequently on or around the round-visit vehicle  85 , as indicated by hatching, and hence the home position setting section  105  sets up the round-visit vehicle  85  as the home position of the first electronic cassette  15 . Since the second electronic cassette  15 B has been located most frequently in the second radiography room  2 B, the home position setting section  105  sets up the second radiography room  2 B as the home position of the second electronic cassette  15 B. As for other electronic cassettes, the respective home positions are set up in the same way. Thus, it is possible to determine the home positions of the electronic cassettes in accordance with the actual operational positions. 
     Like the above fourth embodiment, it is possible to allocate multiple home positions to one electronic cassette according to the present embodiment. For example, the home position may be two locations in which one electronic cassette has been located most and second most frequently. In the present embodiment, as the home position of each electronic cassette is determined on the basis of the frequency of presences of said electronic cassette in each service zone, it is preferable to store the tracking records over a relatively long period, without deleting the records even when the electronic cassette  15  is returned to the home position, in order to secure the adequacy or validity of the determined home position. To avoid frequent changes of the home positions, it is preferable to configure the home position setting section  105  to update the home positions periodically at regular intervals, e.g. every month. 
     It is also possible to combine the above fourth embodiment and the present embodiment such that the home position is determined by the user R and on the basis of the frequency of presences of each electronic cassette in each location in a combined manner. In this case, the home positions are initially determined by the user R and, after a certain number of tracking records are acquired, the determination of the home positions are switched to be based on the frequency. 
     In the above embodiments, one service zone  20  is supervised by one console  13 , like a first service zone  20 A supervised by a first console  13 A in  FIG. 22 , but one service zone  20  may be supervised by multiple consoles  13 , like a second service zone  20 B supervised by a couple of consoles  13 B- 1  and  13 B- 2 . 
     In this case, the consoles  13 B- 1  and  13 B- 2  are interconnected. Data taking sections  80 B- 1  and  80 B- 2  of the consoles  13 B- 1  and  13 B- 2  take second detection results from a second wireless tag reader  21 B and first detection results from a first wireless tag reader  21 A as well. Alert controllers  81 B- 1  and  81 B- 2  of the consoles  13 B- 1  and  13 B- 2  operate in the same way as in the above first embodiment. Accordingly, when one electronic cassette  15  is moved out the second service zone  20 B, speakers  27 B- 1  and  27 B- 2  of the consoles  13 B- 1  and  13 B- 2  sound an alarm. Recording controllers  90 B- 1  and  90 B- 2  of the consoles  13 B- 1  and  13 B- 2  individually record the current position of each cassette and store the tracking records. 
     Sixth Embodiment 
     In one service zone  20  which is supervised by multiple consoles  13 , like the second  20 B in  FIG. 22 , it may be bothering that all the speakers  27  of the multiple consoles  13  sound an alarm when one electronic cassette  15  has gone out said service zone  20 . It would be sufficient to sound an alarm from one of the speakers  27  in one service zone  20 . Furthermore, it would be sufficient to record the current positions and tracking records in one of the multiple consoles  13  for one service zone  20 . 
     Therefore, according to the present embodiment, as shown in  FIG. 23 , in one service zone  20  which is supervised by multiple consoles  13 , CPUs  71  of the multiple consoles  13  include a function of an enable/disable setting section  110  for designating one of the multiple consoles  13  as a master console of which the alert controller  81  and the recording controller  90  are enabled. 
     In this embodiment, the user R previously sets up one of the multiple consoles  13  as the master console through an input device  26  of the one console  13 . Thereafter when the communication between these consoles  13  is established, the master console transmits a slave setting signal (refer to  FIG. 24 ) to other consoles (hereinafter referred to as slave consoles), instructing the slave consoles to disable the alert controller  81  and the recording controller  90 . 
     The data taking section  80  of each slave console receives the slave setting signal through a communicator section  72 , and transfers the signal to the enable/disable setting section  110 . The enable/disable setting section  110  disables the alert controller  81  and the recording controller  90  in response to the slave setting signal. Accordingly, the slave consoles do not give the alert from the speaker  27  or record the current position or make the tracking records by the recording controller  90 . Meanwhile, the enable/disable setting section  110  of the master console enables the alert controller  81  and the recording controller  90 . Thus, one master console takes charge of giving the alert from the speaker  27 , recording the current position and making the tracking records, thereby resulting the problem of bothering alerts from multiple speakers  27 . 
     If the multiple consoles  13  include a portable console, it is also possible to automatically assign the portable console as the master console regardless of the setting by the user R, so as to enable the alert controller  81  and the recording controller  90  of the portable console. 
     Specifically, as shown in  FIG. 24 , where one service zone  20  is supervised by a stationary console  13 X and a portable console  13 Y, the portable console  13 Y automatically sends the stationary console  13 X the slave setting signal when the communication between the stationary console  13 X and portable console  13 Y is established. Then, the stationary console  13 X sends back a responding signal (not shown) to the portable console  13 Y, acknowledging receipt of the slave setting signal. 
     Then, an enable/disable setting section  110 X of the stationary console  13 X disables an alert controller  81 X and a recording controller  90 X, to make the stationary console  13 X function as a slave console. Meanwhile, an enable/disable setting section  110 Y of the portable console  13 Y enables an alert controller  81 Y and a recording controller  90 Y, to make the portable console  13 Y function as a master console. 
     The portable console  13 Y will be handled by the user R during the X-ray imaging, and is generally located closer to the user R than the stationary console  13 X. Therefore, the user R can more easily be aware of the alert from the portable console  13 X or refer to the current position and tracking records on the portable console  13 X. Thus, it would be more convenient if the portable console  13 Y is automatically designated as the master console. 
     Seventh Embodiment 
     Although the range of each service zone  20  is fixed in the above embodiments, the present invention, as shown in  FIG. 25 , provide a CPU  71  of a console  13  with a range setting section  115  for setting the range of the range of an allocated service zone  20  according to the operational condition of the electronic cassette  15 . The range setting section  115  modifies the range of the service zone  20  by changing the coverage (frequency band or wave intensity or both) of a search signal from a wireless tag reader  21 . In the example in which the frequency band is changed to modify the range of the service zone  20 , the wireless tags  50  of the electronic cassettes  15  should be configured to be compatible with multiple frequency bands. 
     The range setting section  115  determines the current operational condition of the electronic cassette  15  on the basis of various signals and information relating to the operational condition of the electronic cassette  15 . The signals relating the operational condition of the electronic cassette  15  include the power-on and power-off signals and the selection signal. The information relating to the operational condition of the electronic cassette  15  include whether any X-ray image is received from the electronic cassette  15  or not. 
     In a period from when the communicator section  72  receives the power-off signal from the electronic cassette  15  to when the communicator section  72  thereafter receives the power-on signal from the electronic cassette  15  and sends back the selection signal to the electronic cassette  15 , the range setting section  115  determines that the electronic cassette  15  is currently in the non-selected condition. After the communicator section  72  has sent back the selection signal to the electronic cassette  15 , the range setting section  115  determines that the electronic cassette  15  is currently in the selected condition till when the communicator section  72  sends the selection signal to another electronic cassette or when the range setting section  115  determines that the electronic cassette  15  gets into the sleep mode. Whether the electronic cassette  15  gets into the sleep mode or not is determined by counting the time from when the communicator section  72  has sent the selection signal to the electronic cassette  15  and the time from when the communicator section  72  has received the X-ray image from the electronic cassette  15 . The range setting section  115  determines that the electronic cassette  15  is in the sleep mode when no action relating to the electronic cassette  15  is newly made before the counted time reaches a predetermined length. 
     The range setting section  115  defines the range of the service zone  20  in accordance with a range setup table  116  shown in  FIG. 26 . The range setup table  116  show a relation between the operational condition of the electronic cassette  15  and the range of the service zone  20 , and is stored in a storage device  28 . The range of the service zone  20  is defined “5 m” in the case of the non-selected condition, and “2 m” in the case of the selected condition. 
     Because the electronic cassette  15  in the power-off mode will not be used in a short while, the user R does not usually pay attention to the electronic cassette  15  in the power-off condition. Therefore, especially for the sake of alerting the user R that the electronic cassette  15  is taken out by an unauthorized person, the range of the service zone  20  is set wider in the non-selected condition of the electronic cassette  15  than in the selected condition. Also for the electronic cassette  15  in the standby mode immediately after the power-on and in the sleep mode, as being not used soon, the range of the service zone  20  is set wider than in the selected condition. 
     Meanwhile, in the standby mode after the selection signal is received on the electronic cassette  15 , while the user R is handling the electronic cassette  15 , positioning the patient P and setting the imaging conditions, the electronic cassette  15  is under the supervision of the user R. Also in the ready mode of the electronic cassette  15 , while the user R completes positioning the patient P and setting the imaging conditions and is ready to start imaging, as well as during the imaging, the electronic cassette  15  is under the supervision of the user R. Accordingly, it is not usually expected that the electronic cassette  15  is unrighteously taken out while the electronic cassette  15  is in the standby mode or the ready mode or taking the image. Therefore, the range of the service zone  20  is set narrower than in the non-selected condition. 
     Indeed the electronic cassette  15  might not usually be taken out of the service zone  20  while the electronic cassette  15  is in the standby mode or the ready mode or is taking an image, as the user R usually keeps an eye on the electronic cassette  15  under these circumstances, but there is a little possibility that the electronic cassette  15  will be stolen while the user R is busy setting for the image acquisition. To avoid this, the range of the service zone  20  is set to be 2 m for the selected condition of the electronic cassette  15 . 
     By defining a narrower range of the service zone  20  for the electronic cassette  15  in the selected condition, while the electronic cassette  15  is selected to use for imaging, in comparison with the range of the service zone  20  for the non-selected condition while the electronic cassette  15  is not selected to use for imaging, the surveillance by the tag reader  21  ensures the security of the electronic cassette  15  while the user R does not pay attention to the electronic cassette  15 , but the surveillance by the tag reader  21  is limited to the minimum range while the electronic cassette  15  is under the supervision of the user R. The narrower range of the service zone  20  for the selected condition makes it possible to alert the user R to the theft of the electronic cassette  15  quickly before the electronic cassette  15  is taken out of eyeshot of the user R. 
     The range of the service zone  20  may be defined more finely, as shown for example in a range setup table  117  in  FIG. 28 , wherein the range of the service zone  20  is narrowed to 3 m for the sleep mode as compared to the range of 5 m for the power-off condition among those for the non-selected condition, and the range of the service zone  20  is narrowed to 1 m for the ready mode as compared to the range of 2 m for the standby mode immediately after the receipt of the selection signal among those for the selected condition. 
     The feature of “narrowing the range of the service zone  20 ” includes setting the range of the service zone  20  to be zero meter, that is, not-executing the detection of passage of the electronic cassette  15  into and out of said service zone  20 . In this case, the console  13  is configured to control the power of the tag reader  21  on and off, so that the range of the service zone  20  is set to be 0 m by turning the power of the tag reader  21  off. Alternatively, the data taking section  80  may discard the detection results from the tag reader  21 , to thereby set the range of the service zone  20  to be virtually 0 m. It is possible to define the range of the service zone  20  to be 0 m while the electronic cassette  15  is in the selected condition because the electronic cassette  15  in the selected condition is under the supervision of the user R and would not usually taken out. 
     As the method for defining the range of the service zone  20 , it is possible to provide different kinds of wireless tag readers  21  each having a different transmission range of the search signal such that the range setting section  115  switches over these wireless tag readers  21  to change the range of the service zone  20 . 
     Narrowing the range of the service zone  20  in the manner as described above would cause no problem while there is only one electronic cassette  15  in the service zone  20 . However, if another electronic cassette  15  that is not used for imaging exists in the same service zone as the electronic cassette  15  to be used for imaging, the other electronic cassette can be excluded from the coverage of the wireless tag reader  21  when the range of the service zone  20  is narrowed according to the condition of the electronic cassette  15  to be used for imaging. This is a problem because no alert would be given if the other electronic cassette  15  should be taken out. To avoid this problem, it is preferable to allow the range setting section  115  to narrow the range of the service zone  20  only while there is a single electronic cassette  15  in the service zone  20 , and interrupt narrowing the range of the service zone  20  if there is another electronic cassette  15  in the same service zone besides the electronic cassette  15  selected for imaging. 
     In a case where there are multiple electronic cassettes  15  in one service zone  20  and one of these electronic cassettes  15  is in the selected condition, it is not a problem if one electronic cassette  15  that is not selected is taken out for a proper purpose. Therefore, under these circumstances, it may be preferable to define a wider range of the service zone  20  than usual in order to monitor unauthorized takeout of the electronic cassettes  15  while permitting authorized transportation of the electronic cassettes  15  within a proper operational range. 
     Although examples where applicable electronic cassettes  15  are previously registered in the console  13  have been described in the above embodiments, the previous registration of the electronic cassettes  15  in the console  13  is not essential to the present invention. However, it is preferable to register the electronic cassettes  15  previously in the console  13  because it allows determining which of these cassettes should be supervised or not. 
     In the above embodiments, a wireless tag reader  21  which receives the radio frequency signal s from wireless tags  50  on electronic cassettes  15  is referred to as a wireless receiver which constitutes the detector section for detecting passage of the electronic cassettes  15  into and out of each service zone  20 . The detector section may also be a wireless LAN access point. In this case, the wireless LAN access point communicates signals with a wireless LAN interface that constitutes a wireless transmitter provided in each electronic cassette  15 , to thereby detect passage of the electronic cassette  15  into and out of each service zone  20 . 
     As another example of the detector section, a surveillance camera may be used besides the wireless receiver. In the example where the surveillance camera is used as the detector section, the come and go of the electronic cassettes  15  is detected by analyzing images captured by the surveillance camera. In this case, the imaging field of the surveillance camera constitutes the service zone  20 . Alternatively, in place of the wireless receiver or the surveillance camera, a sensor for detecting passage of the electronic cassettes  15  may be disposed at the doorway  6  or the staff entrance  8 . 
     Furthermore, a detector section (a wireless receiver), a data taking section and an alert controller may be provided in the electronic cassette  15 . In this case, for example, a sender for sending radio frequency signal s is disposed at the doorway  6  or on the door of the staff entrance  8 , so that the radio frequency signal is received on the detector section of the electronic cassette  15 . When it is determined that the radio frequency signal is not received on the detector section, the alert controller determines that the electronic cassette  15  has gone out the service zone  20 , and controls a speaker of the electronic cassette  15  or other device to give an alert. Thereafter when the detector section begins to receive the radio frequency signal again, the alert controller determines that the electronic cassette  15  returns to the service zone  20  and stops the alert. The detection results obtained by the detector section may be transmitted from the electronic cassette  15  to a console  13  that supervises the service zone  20  in which the electronic cassette  15  has been present and then from said console  13  to other consoles  13  that supervise other service zones  20 , at the moment when the electronic cassette  15  is moved out and hence the detector section cannot receive the radio frequency signal, or at the moment when the detector section begins to receive the radio frequency signal again as the electronic cassette  15  is moved back to the service zone  20 . The detection result should be transmitted in accordance with a different communication protocol from that applied to the radio frequency signal for the detector section. 
     In the above embodiments, the consoles  13  that supervise respective service zones  20  individually take on the functions of the electronic cassette management device. In an alternative, as shown for example in  FIG. 29 , a computer other than the consoles  13  may serve as an electronic cassette management device. 
     In  FIG. 29 , an electronic cassette management system  120  is constituted of an electronic cassette management device  121 , and a first wireless tag reader  21 A, a second wireless tag reader  21 B, . . . for supervising passage of electronic cassettes  15  into and out of a first service zone  20 A, a second service zone  20 B, . . . , respectively. The wireless tag readers  21 A,  21 B, . . . are connected to the electronic cassette management device  121 . Furthermore, the service zones  20 A,  20 B, . . . are supervised by a first console  13 A, a second console  13 B, . . . , respectively, and the consoles  13 A,  13 B, . . . and the electronic cassette management device  121  are interconnected through a network  75 . 
     A CPU of the computer that constitutes the electronic cassette management device  121  functions as a data taking section for taking the first, the second, . . . detection results from the respective wireless tag readers  21 A,  21 B, . . . , and as an alert controller that outputs an alert start command to at least one of the consoles  13  when the alert controller determines that one of the electronic cassettes  15  is moved out of any of the service zones  20 A,  20 B, . . . , to thereby cause at least a speaker  27  to start warning. When the alert controller determines that the one electronic cassette  15  is moved into any of the service zones  20 A,  20 B, . . . after the alert starts, the alert controller outputs an alert stop command to the console  13  to stop the alert by the speaker  27 . 
     In this configuration, a controller for recording the current position and tracking records is established in the CPU of the computer that constitutes the electronic cassette management device  121 , the current position and tracking records are stored in a storage device of the electronic cassette management device  12 . The electronic cassette management device  121  transmits the current position and tracking records from the storage device to the console  13  in response to a request from the console  13  for the current position and tracking records. The CPU of the computer that constitutes the console  13  serves as a display controller, and the display controller controls the display device  25  to display the current position window  93  or the current position and tracking record window  96  on the basis of the current position and tracking records from the electronic cassette management device  121 . Thus, the electronic cassette management device  121  of the present invention may be functionally divided into a plurality of computers, or integrated into a computer. 
     In this example, the home position setting section, the enable/disable setting section and the range setting section are established in the CPU of the computer that constitutes the electronic cassette management device  121 . 
     Establishing the functions of the electronic cassette management device integrally in one computer, not in the individual consoles  13  for supervising the individual service zones  20 , will save the time for installing the electronic cassette management program  74  in the individual consoles  13 . 
     The present invention is applicable to a case where there is only one electronic cassette  15 . Furthermore, the wireless tag  50  may also be an active tag that has an integrated power supply and generates a radio frequency signal by itself. Moreover, in place of the wireless tag reader  21 , a wireless tag reader/writer having a function of writing information may be used. The wireless tag reader/writer makes it possible to write the current position and tracking records in the wireless tag  50 . 
     It is to be noted that the present invention may be embodied as a program, as set forth above with reference to the above embodiments, and includes a storage medium storing the program in the scope. Furthermore, the present invention is not to be limited to the above embodiments, but various modifications are possible without departing from the gist of the present invention. For example, the above embodiments may be implemented individually or in combination.