Abstract:
This invention relates to mutual positioning of an x-ray beam and an intra-oral x-ray sensor in connection with an imaging process by utilizing at least one measurement signal, which is achieved by arranging means for producing a magnetic field resp. for measuring the magnetic field in connection with a radiation source resp. the sensor.

Description:
TECHNICAL FIELD 
     The present invention relates to positioning of an X-ray beam and an intra-oral X-ray sensor with respect to each other in connection with a dental imaging event. 
     BACKGROUND OF THE INVENTION 
     Dental intra-oral X-ray images are taken by using an X-ray examination apparatus which typically include a multi-jointed arm construction and an X-ray source placed inside a housing. Typically, an elongated collimator limiting the X-ray beam has been attached or arranged to be attached to the housing. The imaging process includes placing the X-ray device in the proximity of the object area to be imaged and aiming the X-ray beam so that it will meet the sensor in a correct orientation and at a desired angle. Typically, the X-ray beam is arranged perpendicularly with respect to a film, or some other sensor placed inside the patient&#39;s mouth. 
     Dental professionals do generally recognize the problems which relate to aiming and orienting the X-ray beam concentrically as well as so that, for eliminating geometric distortions and unsharpness of the image, the X-ray beam is not inclined or turned with respect to the means for receiving image data. Thus, different aiming arrangements have been developed to facilitate correct positioning of the X-ray source with respect to the sensor. One approach according to prior art is to attach the X-ray source and the means for receiving image data such as a film, a phosphorous imaging plate, a CCD sensor or other digital sensor physically to each other for the duration of irradiation. 
     The junction assemblies designed for physical connection of imaging means typically include an aiming arm, which may be attached both to a sensor holder/bite block and to the housing of the X-ray source. The latter connection is typically made by means of an aiming ring attached to a collimator tube of the X-ray device housing. Since there are several imaging modes in intra-oral imaging, such as the left- and right-side anterior, posterior, endodontic and bitewing imaging, assemblies comprising components of special shapes are required for supporting these special imaging modes for enabling different sensor positions and aimings of the sensor with respect to the X-ray beam. Some prior-art systems and assemblies utilising this approach are presented in patent specifications U.S. Pat. Nos. 6,343,875 B1, 5,632,779 A, 4,507,798 A and 4,554,676 A. 
     However, many of the dental professionals find these systems, in which the sensor placed inside the mouth should be physically connected to the X-ray device, difficult to use in practice. A reason for this is, first, that if all connections of the assembly are made prior to positioning the sensor in the mouth, it has proven difficult to direct the entire relatively heavy construction, including an X-ray tube and its arm construction, to its proper and precisely correct position. Second, if the sensor is first placed in the correct position in the mouth, assembling the construction has proven difficult—that is, e.g. connecting the aiming arm to the X-ray device so that the connecting process would not cause movement of the sensor or discomfort to the patient. 
     Because of these practical problems related to the abovementioned operations, the technical advantages of these systems are frequently ignored and aiming is done by simply visually estimating the correct place and orientation of the X-ray device, possibly by using as a help the position and orientation of the aiming arm protruding from patient&#39;s mouth. One has also tried to utilise the thin aiming arm to facilitate aiming by connecting it manually e.g. to the outer surface of the collimator of the X-ray tube, with limited success, however. This is not the least because of the fact that it has proven quite difficult to keep the sensor in a correct position by keeping the thin aiming arm between one&#39;s fingers and, at the same time, direct the arm construction of the X-ray source, especially into contact with the thin aiming arm. The probability for a repeated success in achieving the same distance between the X-ray source and the image forming plane, not to mention the proper and precise orientation of the X-ray beam, is clearly not extremely high by these methods. 
     A further problem of prior-art aiming assemblies is that, because of the great number of different components required for supporting different intra oral imaging modes, a lot of experience or learning by trial-and-error is required in order to be able to assemble the jigsaw puzzle according to each imaging mode. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to offer a novel intra-oral X-ray imaging arrangement and a method for positioning an X-ray source and a sensor with respect to each other in connection with intra-oral X-ray imaging, by means of which it may be possible to even totally avoid the need to use an aiming arm attachable to the sensor, or a corresponding device for bringing the sensor and the X-ray tube to a desired mutual position. This and other objects and advantages of the invention with respect to prior art will become apparent in the description to follow and are achievable by solutions presented in the accompanying patent claims. 
     The preferable embodiments of the present invention are presented by means of example in the accompanying drawings, which may be considered to be adequate for describing the invention in full. The purpose of the exemplary embodiments in question is not to show all the possible different forms and modifications by which the invention may be implemented, but the characteristics of the invention are defined in the patent claims to follow. 
    
    
     
       SHORT DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 ,  2   a  and  2   b  show a typical intra-oral X-ray device. 
         FIG. 3  shows a sensor arrangement usable in the imaging arrangement according to the invention. 
         FIG. 4  shows a typical appropriate position of the sensor with respect to the position of the radiation source with respect to the X-ray beam in connection with intra-oral X-ray imaging. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 ,  2   a  and  2   b  show a typical intra-oral X-ray device ( 1 ) which includes a control panel ( 2 ), a jointed arm construction ( 3 ) and an X-ray source ( 4 ).  FIG. 2   b  shows additionally an elongated collimator ( 4 ′) which may be attached to the housing of the X-ray source ( 4 ) for limiting of the X-ray beam more precisely and thus minimising the radiation dose received by the patient. The multi-element arm-joint constructions ( 3 ) of intra-oral X-ray devices create a lot of degrees of freedom for positioning the X-ray source ( 4 ) in a desired manner. 
       FIG. 3  describes one sensor arrangement usable in the imaging arrangement according to the invention. Therein, MRI (magnetic resonance imaging) detectors ( 31 ), which measure the strength of a magnetic field, are arranged to the intra-oral sensor ( 30 ), to the corners of an area forming a rectangle which essentially does not resemble a line. When, correspondingly, a permanent magnet, or e.g. according to one preferable embodiment of the invention, an inductive transmitter is arranged to the X-ray source ( 4 ) or to its essential proximity, signals corresponding the strengths of the magnetic field measured by the MRI detectors ( 31 ) are received from them, which signals depend on the position and orientation of the sensor ( 30 ) in the magnetic field, such as the distance and the angle of inclination of the detectors ( 31 ) with respect to the inductive transmitter or the permanent magnet. Instead of MRI detectors ( 31 ), it is also possible to use small coils ( 41 ) and connect to them a means for measuring the signal received from each coil ( 41 ), the strength of which signal thus being dependent on the strength of the magnetic field in the position the coil ( 41 ) is located at a given time. 
     Typically, the distance of the sensor ( 30 ) and the radiation source ( 4 ) with respect to each other is desired to be able to be set the same, repeatedly, and the sensor ( 30 ) essentially perpendicularly with respect to the X-ray beam produced by the radiation source ( 4 ), and into the middle of it. Typically one tries to collimate (limit) the X-ray beam to correspond to the form and size of the sensor ( 30 ) used. The positioning or the reproductionability of positioning of the sensor ( 30 ) and the radiation source ( 4 ) with respect to each other may be facilitated already by a single signal received from one MRI detector ( 31 ) or coil ( 42 ) arranged in connection with the sensor ( 30 ), especially if the mutual distance between the sensor ( 30 ) and the radiation source ( 4 ) may be standardised by some other means. For more complete information, signals received from at least three, preferably four different coils ( 41 ) or detectors ( 31 ) are required. In the case of four coils ( 41 ) or detectors ( 31 ), they may be preferably arranged e.g. to the corners of an area forming a rectangle which essentially does not resemble a line, and three e.g. to the corners of an area forming essentially an equilateral triangle. At least one of the detectors ( 31 ) or coils ( 41 ) may also be arranged to a different plane or angle with respect to one of their mounting surfaces, with respect to at least one other detector ( 31 ) or coil ( 41 ), e.g. so that they are arranged perpendicularly with respect to one focus point. 
     Signals including measured values representing strengths of the magnetic fields may be sent e.g. to a display device pertaining to the user interface of the control system of the imaging arrangement or some other display device, from which one is able to monitor changing of the values in question when changing the mutual position of the sensor ( 30 ) and the X-ray source ( 4 ), and deduce from it when the positioning is as desired. The arrangement may also include a computer program, e.g. for converting the measured values in question to visually presenting the position of the sensor ( 30 ) and the X-ray source ( 4 ) with respect to each other on the screen, or some other computer program by means of which one is able to show signal data in a form which facilitates the positioning. 
     As partly already referred to above, one preferable embodiment of the invention comprises a solution which utilises an inductive transmitter arranged to the radiation source ( 4 ) for other purposes, i.e., for transmitting energy. Such an inductive transmitter, or a coil ( 42 ) pertaining therein, may be attached or integrated e.g. to an adapter ( 40 ) of e.g. ring-like or rectangular shape, to be attached to the collimator ( 4 ′) of the X-ray source ( 4 ).  FIG. 4  shows such an adapter ( 40 ) and a coil ( 42 ) of an inductive transmitter integrated therein, as positioned with respect to the sensor ( 30 ) according to  FIG. 3  in such a manner one typically tries to position it in connection with intra-oral imaging. The adapter ( 40 ) may be arranged to be attached to the end of e.g. an X-ray tube ( 4 ), or a collimator tube ( 4 ′) used therein, or as integrated therewith. 
     Partly because of this embodiment of the invention, also the coil ( 32 ) of the inductive receiver is shown in the sensor ( 30 ) according to  FIGS. 3 and 4 , which coil may thus be used not only in accordance with one preferable embodiment of the invention to form an energy transmission link e.g. with an inductive transmitter arranged to the X-ray source ( 4 ) or its essential proximity, but also in a corresponding manner than described above in connection with corner coils ( 41 ), as a signal source to provide data on position of the sensor ( 30 ) in the magnetic field produced by the inductive transmitter. The coil ( 32 ) of the inductive receiver may be arranged as e.g. according to  FIGS. 3 and 4  in the form of a rectangle so that it essentially imitates the form of the sensor ( 30 ) perimeter and is located in essential proximity to at least part of the sensor ( 30 ) edges. Such a coil ( 32 ) is preferable to arrange in connection with the sensor housing so that the winding will not limit the active detector surface available in the sensor ( 30 ). 
     One preferable embodiment of the invention thus comprises an arrangement in which an inductive transmitter is placed in the X-ray source ( 4 ), and in which small receiver coils ( 41 ) or MRI detectors ( 31 ) are arranged e.g. essentially to the corners of a sensor ( 4 ) essentially of rectangular shape, or to at least one of them. When the sensor ( 30 ) is being positioned with respect to the radiation source ( 4 )—or in connection with intra-oral imaging, more frequently perhaps vice versa, when the X-ray source ( 4 ) is being positioned with respect to the sensor ( 30 )—the signals received from the detectors ( 31 ) or the coils ( 41 ) change according to how their position changes in the magnetic field produced by the inductive transmitter attached to the X-ray source ( 4 ). Also the coil ( 32 ) of the inductive receiver may be arranged to be used for this purpose as well. These signals may be arranged to be sent e.g. via a wireless link (not shown in  FIGS. 3 and 4 ) such as an RF link, as signals indicating the mutual position of the sensor ( 30 ) and the X-ray source ( 4 ), which signals may be utilised for achieving a desired mutual position of the sensor ( 30 ) and the radiation source ( 4 ). 
     The invention is described above mainly so that the magnetic field is produced in connection with the radiation source ( 4 ), and its strength is measured by a means arranged in connection with the sensor ( 30 ). The arrangement may also be realised in such a way that measuring of the magnetic field is arranged to be made in connection with the radiation source ( 4 ), i.e., so that the MRI detectors ( 31 ) or coils ( 41 ,  32 ) are arranged in connection with it, whereby the magnetic field may be produced e.g. in a permanent magnet arranged in connection with the sensor ( 30 ). 
     It is self evident for a man skilled in the art that the present invention may be implemented also in accordance with other embodiments than the ones presented above within the scope of protection defined by the accompanying patent claims.