Abstract:
A method utilizing an imaging probe for registering images associated with a medical image processing application comprises the steps of: (a) providing a local tracking system that generates a field in a local area within which the imaging probe is used; (b) capturing first and second images representing substantially the same object; (c) sensing field emissions from the local tracking system to establish positional coordinates of the imaging probe during image capture; (d) using the positional coordinates to register the first and second images; and (e) utilizing the registered images to determine a characteristic of the object.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    Reference is made to commonly assigned copending applications Ser. No. 09/970,243, entitled “Method for Registering Images in a Radiography Application” and filed 03 Oct. 2001 in the names of J. T. Boland, J. P.  
         [0002]    Spoonhower and J. R. Squilla, and Ser. No. 09/894,627, entitled “Method and System for Creating Models from Imagery” and filed on 28 Jun. 2001 in the names of J. T. Boland, J. P. Spoonhower and J. R. Squilla, which are both assigned to the assignee of this application. 
     
    
     
       FIELD OF THE INVENTION  
         [0003]    The invention relates generally to the field of medical imagery, and in particular to the field of image registration as related to dental imagery.  
         BACKGROUND OF THE INVENTION  
         [0004]    Image registration in general is concerned with determining a precise geometric match between two or more images of the same object or area which are from different times or taken from different positions relative to the image content. In the present invention, the primary emphasis is on dental images (e.g., obtained from intra-oral digital imagery or dental radiography) taken on different dates or times. Comparison of such imagery, after registration, allows detailed analysis of any changes that may have occurred due to, e.g., new or larger cavities, bone loss, loosened fillings, etc.  
           [0005]    The registration process often relies on tie points, which are points (image positions) of the same object in different images. Tie points must be accurately placed, and must be unambiguously identified. The tie points are then used to generate a polynomial function that is used to warp one image to another. Tie point selection can be an arduous process, requiring users to repeatedly cycle between overviews of the imagery and close-up views as they attempt to identify and then precisely indicate the common locations. The process of “zooming-in” and “zooming-out” can be time consuming, as well as disconcerting, frequently resulting in the user losing context, i.e., not being sure of which part of the image is being viewed.  
           [0006]    In the aforementioned commonly assigned copending application Ser. No. 09/970,243, entitled “Method for Registering Images in a Radiography Application”, an image registration method is described for x-ray imagery, in which specific views of the x-rays are provided to optimize tie point selection accuracy and efficiency. The first view maintains context during initial point selection. The second view provides a detailed view of each point pair, to allow for fine adjustment, while automatically presenting each point pair in sequence to the user. After the tie points are refined and the images are registered, a third view is provided which allows direct comparison of the registered images.  
           [0007]    Image registration thus is an important element in isolating historical changes in film or digital imagery. Change detection, in this context, is an image based concept, and refers to the process of comparing imagery over an area of interest taken at two different times. Images are compared either manually or automatically to determine those places where some change in the scene content has occurred. Imagery-based change detection can be performed on a variety of image types, including panchromatic, color, IR and multi-spectral image types. In some applications, the size, location and type of change can be determined.  
           [0008]    Image registration is also an important element in three-dimensional modeling of intra-oral objects, e.g., in effecting imagery of a prepared cavity in a tooth followed by automatic generation of a model to control automatic fabrication of a dental inlay for the cavity. For instance, U.S. Pat. No. 4,837,732 (Brandestini et al) describes a method for a dentist to record the shape in situ of teeth prepared for repair. The method involves the acquisition of data defining the three-dimensional shape of prepared teeth and their immediate vicinity. First, a video display shows a live image from a scan head, and the scan head is manually oriented relative to the prepared teeth while observing the image of the teeth on the video display. Thereafter the data produced by the scan head in a selected orientation generates corresponding depth and contrast images, and a depth image is processed based on the contrast image. This method also includes the step of superimposing graphic markers on the image displayed on the video display to facilitate an on-line alignment of the teeth displayed in the live image with reference data from previous data acquisitions.  
           [0009]    The drawback to this method from the prior art is that it incorporates a registration scheme that can later interfere with the quality of the results, and also requires that the dentist be able to hold the scan head almost perfectly still at a specific point in the procedure. More specifically, the artifacts typically due to the 3D registration scheme (such as fringe, speckle and/or venetian blind effect) are cited in the patent as “intolerable and must be eliminated” since phase angle differences are used for measurement of the depth. Furthermore, the patent cites a need for a “quasi-instantaneous 3D acquisition following a trigger release”, the essential condition being that the orientation of the scan head must not change between the search and acquisition modes.  
           [0010]    In the aforementioned commonly assigned copending application Ser. No. 09/894,627, entitled “Method and System for Creating Models from Imagery”, a method is described for creating dental models from imagery in which errors due to the lack of certainty of knowledge about the image positions is addressed by using a method of analytical adjustment to control points. Note that image position is used here to denote both the position and the orientation of the sensor during image formation. According to this method, creating a dental model from a series of images of an intra-oral object includes the steps of (a) capturing a series of images of an intra-oral object from a plurality of capture positions, where the object includes common surface features and a control target arranged with respect to the object to provide control features; (b) measuring the common features from the series of images of the object and the control features from the control target imaged with the images of the object; (c) analytically generating a 3-dimensional model of the object by photogrammetrically aligning the measurements of the control features, thereby reducing image errors due to the variability of the capture positions; and (d) adjusting the photogrammetrically aligned 3-dimensional model of the object by aligning the common features of the model to like features on the image of the object, thereby producing an aligned dental model from the series of images.  
           [0011]    Employing a mensuration method that utilizes photogrammetric projection, the principal advantage of the method described in application Ser. No. 09/894,627 is that the use of photogrammetric projection methods and adjustment to control eliminates the need for a conventional registration scheme, such as that used in Brandestini et al, which projects stripes of light onto the target and can result in unacceptable artifacts.  
           [0012]    Notwithstanding these advantages, a robust registration procedure would remain a useful alternative. What is needed is a system that provides accurate knowledge of the sensor position and orientation during image formation. This knowledge can be used to either eliminate the need for manual provision of tie points, or for the adjustment to control described above, to provide more reliable initial position estimates to that process.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, a method utilizing an imaging probe for registering images associated with a medical image processing application comprises the steps of: (a) providing a local tracking system that generates a field in a local area within which the imaging probe is used; (b) capturing first and second images representing substantially the same object; (c) sensing field emissions from the local tracking system to establish positional coordinates of the imaging probe during image capture; (d) using the positional coordinates to register the first and second images; and (e) utilizing the registered images to determine a characteristic of the object.  
           [0014]    In yet another aspect of the invention, an image processing system utilizing an imaging probe for registering images associated with a medical image processing application comprises: a local tracking system that generates a field in a local area; an imaging probe utilized within the local area for capturing first and second images representing substantially the same object; one or more sensors associated with the imaging probe for sensing field emissions from the local tracking system to establish positional coordinates of the imaging probe during image capture; and one or more processing stages using the positional coordinates to register the first and second images, said one or more processing stages utilizing the registered images to determine a characteristic of the object.  
           [0015]    With accurate knowledge of the sensor position and orientation during image formation, the advantage of the invention is that manual provision of tie points is eliminated. In addition the adjustment to control described in connection with the prior art can be used to refine the position estimates, thereby providing more reliable initial position estimates for the subtractive process.  
           [0016]    These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is an illustration of a dental office outfitted according to the invention to capture and record local positioning information along with imagery of an intra-oral object.  
         [0018]    [0018]FIG. 2 is a perspective diagram of a computer system that is useful in practicing the present invention.  
         [0019]    [0019]FIG. 3 shows an intra-oral imaging probe and display system that is useful in practicing the present invention.  
         [0020]    [0020]FIG. 4 shows a block diagram of the electronics in the integral base associated with the imaging probe shown in FIG. 3.  
         [0021]    [0021]FIG. 5 is an illustration of imagery performed according to the invention on the lower jaw and teeth of a typical patient by use of a digital intra-oral imaging probe.  
         [0022]    [0022]FIG. 6 is a an illustration of imagery performed according to the invention on the lower jaw and teeth of a typical patient by use of an x-ray source.  
         [0023]    [0023]FIG. 7 shows a block diagram of the various stages of a registration method according to the invention.  
         [0024]    [0024]FIG. 8 is an illustration of a standard analytical photogrammetric technique which uses a sensor geometry model to project from an image space to an object space for one image, and then to project from an object space to an image space for another image.  
         [0025]    [0025]FIG. 9 is a block diagram of a subtractive process that is performed on the images, once the projective process illustrated in FIG. 8 is completed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    Because image registration systems employing tie points are well known, the present description will be directed in particular to attributes forming part of, or cooperating more directly with, a method and system in accordance with the present invention. Method and system attributes not specifically shown or described herein may be selected from those known in the art. In the following description, a preferred embodiment of the present invention would ordinarily be implemented as a software program, although those skilled in the art will readily recognize that the equivalent of such software may also be constructed in hardware. Given the system as described according to the invention in the following materials, software not specifically shown, suggested or described herein that is useful for implementation of the invention is conventional and within the ordinary skill in such arts. If the invention is implemented as a computer program, the program may be stored in conventional computer readable storage medium, which may comprise, for example; magnetic storage media such as a magnetic disk (such as a floppy disk or a hard drive) or magnetic tape; optical storage media such as an optical disc, optical tape, or machine readable bar code; solid state electronic storage devices such as random access memory (RAM), or read only memory (ROM); or any other physical device or medium employed to store a computer program. The computer program could also be made available to the operator&#39;s computer via a network; the use of the program could be provided as a service for which the operator pays a fee.  
         [0027]    Before describing the present invention in detail, it is helpful to understand that the present invention is preferably utilized on any well-known computer system, such a personal computer. Consequently, the computer system will not be discussed in detail herein. It is also instructive to note that the images are either directly input into the computer system (for example, from a digital intra-oral imaging probe or a digital radiographic source) or digitized before input into the computer system (for example, by scanning an original, such as a silver halide x-ray film or other form of radiographic image).  
         [0028]    Referring first to FIG. 1, the basic concept of the invention is illustrated in relation to a dental office  1  incorporating a predetermined, constrained patient location  2 , e.g., a conventional dental chair with head restraints, where a patient is positioned for a dental procedure. In connection with such a procedure, and according to one aspect of the invention, dental imagery is captured by an intra-oral imaging probe  3  connected to a hand held display unit  4 , as is disclosed in co-pending, commonly assigned U.S. patent application Ser. No. 09/796,239, entitled “Intra-Oral Camera with Integral Display” and filed 28 Feb. 2001 in the names of J. P. Spoonhower, J. R. Squilla and J. T. Boland. The display unit  4  includes a transceiver for communicating image data to a computer system  10 . Alternatively, the display  4  (or the imaging probe  3 ) could be physically tethered to the computer  10 , as shown by the dotted connection  5 , in order to transfer the captured and/or processed image data from the display  4  (or imaging probe  3 ) to the computer system  10 .  
         [0029]    In accordance with the invention, a local real time tracking system  6  is fixedly located as a base unit in the room for emitting a field  7  generally in the direction of the imaging probe  3  and the patient location  2 . One or more miniature field sensors  8  are incorporated into the handheld imaging probe  3  in order to sense the field emitted by the base unit of the local tracking system  6 . This will give the location of the probe  3  for each image relative to the base unit. In addition, an additional sensor  8  may be placed in the mouth of the patient to give the position of the probe  3  relative to the mouth and the base unit of the local tracking system  6 . From this sensed information, which is transferred to the computer  10  for processing, it is possible to record the location and orientation of the imaging probe while images of the oral cavity are captured by the imaging probe  3  and recorded. Thus, the system provides a means for recording evidence of imaging probe position with a high degree of accuracy and precision. Imaging probe position can be recorded with the captured images as location and orientation metadata, and thereby used to implement an image registration process to facilitate subtraction and other processing of the captured images. For example, images of the same portion of the mouth taken at different times (perhaps in sequential visits to a dentist) could be more easily registered and subtracted to improve the process of rendering difference images critical to identifying changes in the images, such as bone loss.  
         [0030]    The local tracking system  6  may be a receiver for a conventional global positioning system (GPS) that is commercially available for local applications. Such systems are well-known, and may be incorporated in a hybrid installation with pseudolites to improve reception within a building (see, for example, U.S. Pat. No. 5,686,924, entitled “Local-Area Position Navigation System with Fixed Pseudolite Reference Transmitters” and which issued 11 Nov. 1997). Pseudolites can be the basis for an entirely independent navigation system, see for example the article by E. A. LeMaster and S. M. Rock entitled “A Local-Area GPS Pseudolite-Based Mars Navigation System”,  IEEE  10 th   International Conference on Advanced Robotics , Budapest, Hungary, August 2001, pp. 1-6. It is also known to use localized real time tracking systems, such as described in International Patent Application WO 01/89405 A1, entitled “Fully-Automatic, Robot-Assisted Camera Guidance Using Position Sensors For Laparoscopic Interventions”, published 29 Nov. 2001. One commercially available real-time tracking system is the miniBIRD™ tracking system offered by Ascension Technology Corporation, Burlington, Vt. The field generated by the local tracking system  6  may be field radiation of whatever form is suitable under the circumstances; for example, because of the enclosed space of the dental office  1 , the emitted radiation may be a magnetic field emission, such as provided by the miniBIRD™ tracking system, and the field sensor would therefore be a magnetic field sensor. Alternatively, the emitted field may be a radio-frequency electromagnetic emission, such as provided by a GPS or a pseudolite transmitter, and the field sensor would therefore be an RF field sensor.  
         [0031]    Referring to FIG. 2, there is illustrated a typical configuration of the computer system  10  for implementing aspects of the present invention. Although the computer system  10  is shown for the purpose of illustrating a preferred embodiment, the present invention is not limited to the computer system  10  shown, but may be used on any electronic processing system. The computer system  10  includes a microprocessor-based unit  12  for receiving and processing software programs and for performing other processing functions. A display  14  is electrically connected to the microprocessor-based unit  12  for displaying user-related information associated with the software, e.g., by means of a graphical user interface (GUI)  15 . A keyboard  16  is also connected to the microprocessor based unit  12  for permitting a user to input information to the software. As an alternative to using the keyboard  16  for input, a mouse  18  may be used for moving a selector (cursor)  20  on the display  14  and for selecting an item on which the selector  20  overlays, as is well known in the art.  
         [0032]    A compact disk-read only memory (CD-ROM)  22  is connected to the microprocessor based unit  12  for receiving software programs and for providing a means of inputting the software programs and other information to the microprocessor based unit  12  via a compact disk  24 , which typically includes a software program. In addition, a floppy disk  26  may also include a software program, and is inserted into the microprocessor-based unit  12  for inputting the software program. Still further, the microprocessor-based unit  12  may be programmed, as is well known in the art, for storing the software program internally. The microprocessor-based unit  12  may also have a network connection  27 , such as a telephone line, to an external network such as a local area network or the Internet. Accordingly, the software program may be received over the network, perhaps after authorizing a payment to a network site. A printer  28  is connected to the microprocessor-based unit  12  for printing a hardcopy of the output of the computer system  10 .  
         [0033]    Images may also be displayed as part of the graphical user interface  15  on the display  14  via a personal computer card (PC card)  30 , such as, as it was formerly known, a PCMCIA card (based on the specifications of the Personal Computer Memory Card International Association) which contains digitized images electronically embodied in the card  30 . The PC card  30  is ultimately inserted into the microprocessor based unit  12  for permitting visual display of the image on the display  14 . Images may also be input via the compact disk  24 , the floppy disk  26 , or the network connection  27 . Any images stored in the PC card  30 , the floppy disk  26  or the compact disk  24 , or input through the network connection  27 , may have been obtained from a variety of sources, such as a digital intra-oral imaging probe  3  or an x-ray image scanner (not shown).  
         [0034]    The invention is useful in a subtractive radiography process where change detection is used to identify areas of differences among images of the same region that were collected at different times. Registration of the images is a prerequisite for the change detection process. In accordance with the invention, an image capture position and orientation system for the purpose of facilitating both image registration and extraction of 3D data is described. When used in visible medical imaging (recording and analyzing photographic images), this system facilitates both the imaging and the re-construction of the 3-D topology of an object, such as a tooth, when visible light is sensed in conjunction with the capture position data. Additionally, by using the invention described herein, a capture position and orientation system, in conjunction with the invention described in the aforementioned commonly assigned copending application Ser. No. 09/970,243, would allow recording temporal differences in both optical and x-ray images. The image capture position and orientation measurement system would facilitate the registration of images and enable a more automatic version of the software assisted process described in the aforementioned commonly assigned copending application Ser. No. 09/970,243 to be applied. For example, this capability could be used to map surface wear in a tooth if a patient had a grinding bite, or to monitor bone loss through periodontal disease in a patient.  
         [0035]    The invention incorporates and modifies any conventional image capture system, such as a conventional intra-oral imaging probe or an x-ray or ultrasound source. A preferred implementation is a imaging probe of the type disclosed in the aforementioned U.S. patent application Ser. No. 09/796,239, entitled “Intra-Oral Camera with Integral Display”. Referring to FIG. 3, an intra-oral dental imaging probe system of the type disclosed in the above application includes a portable dental imaging probe  40  and a power source, illumination source and a display unit integrally located in a portable enclosure (hereinafter referred to as the integral base  42 ) tethered to the imaging probe  40 . The imaging probe  40  and the integral base  42  thus constitute an intra-oral imaging probe with integral display. The dental imaging probe  42  includes a handpiece  44  and a cable  46  connecting the dental imaging probe  40  to the integral base  42 . As shown for illustrative purposes in FIG. 3, the integral base  42  can be easily cradled in a hand, and includes a display monitor  48  that can be easily hand positioned relative to the dentist&#39;s and/or patient&#39;s line of sight. A set of user controls  50  are provided on the integral base  42  that can be easily hand-navigated for controlling the illumination and the images displayed on the monitor, as well as communicating with peripheral devices. The handpiece  44  supports a removable lens unit  52  that includes a lens  54  and light emitting apertures  56 . The handpiece  44  is generally elongated and cylindrical with a central axis. The lens  54  is positioned to receive light impinging on the handpiece in a direction substantially perpendicular and normal to the central axis of the handpiece. In accordance with the invention, one or more field sensors  58  are located on the handpiece  44  to sense the field emissions from the local tracking system  6 . Despite this preferred implementation, it should be clear that the invention can be implemented on many other types of imaging probes, including imaging probes of various shapes and capabilities, and without any portable or attached display capability.  
         [0036]    Referring to FIG. 4, the integral base  42  in the preferred implementation includes a central processing unit (CPU)  60 , a CPU memory  62 , a power supply  64 , a wireless transceiver  66 , and flash memory (RAM)  68 . The user controls  50  interface with a video control unit  70  and an illuminator control unit  72 . The illuminator control unit  72  connects with an illumination source  74 , which provides illumination to the handpiece  44  through a fiber optic  46   a  that is part of the cable  46 . The illumination source may take a variety of forms known to those of skill in this art, such as a halogen arc lamp lighting system or a tungsten/halogen lamp. The power supply  64  is connected by a power cable (not shown) to a power source, such as a wall socket. The image signal communication between the handpiece  44  and the CPU  60  is maintained through an electrical connection  46   b , which is also in the cable  46 . While not shown in detail, the handpiece  44  also supports a connection of the fiber optic  46   a  with the light emitting apertures  56  and a connection of the electrical conductor  46   b  to an image sensor  76 , such as a conventional charge coupled device (CCD), and the field sensor(s)  58 . The image sensor  76  is arranged in a conventional optical path, with mirrors and other optical components as might be necessary, such that the lens  54  can form an image of an intra-oral object on the image sensor  76 . The field signals from the field sensor(s)  58  are transferred via the electrical connection  46 b to a field receiver  84  in the integral base  42 .  
         [0037]    It should be noted that portability is facilitated by incorporating into the dental imaging probe system both a high quality image display  48  along with means to transfer image data to a physically separate and distinct data storage associated with an image printing capability. The means to accommodate a transfer of image data may include (a) wireless RF or microwave transceiver technology, (b) wireless infra-red transmission technology, and/or (c) removable memory technology embodied in physically small elements  80 , such as flash RAM cards or small hard drives, that are easily removed from an interface  82  in the imaging probe part of the system and subsequently plugged into either the image data storage or printer parts of the system.  
         [0038]    Accordingly, the dental imaging probe system can, through the transceiver  66  in its integral base  42 , initiate communication via wireless links  78  with a variety of peripheral units. Each of these units would have its own data storage for receiving the transmitted images. Without intending to be exhaustive as to type of peripheral unit that may be accessed, such peripheral units include a larger monitor or television receiver, a printer, and a computer system, such as any conventional desktop PC, where the images may be processed and stored. With this arrangement, a dental practitioner may view an image on the integral base  42  and immediately initiate its transfer to any one of the peripheral units by means of the user controls  50 . The incorporation of the transceiver  66  and the display monitor  48  into the dental imaging probe system further enables the practitioner to view the results of an image recording, and conveniently display the captured image(s) either for the practitioner&#39;s or patient&#39;s benefit. For this purpose, the transceiver  66  would receive images from a storage peripheral, such as a computer system, and display the stored images on a display monitor. Importantly, such viewing occurs without the requirement of producing a physical print of the image.  
         [0039]    In operation, the handpiece  44  is maneuvered into the region of the patient location  2 , where it is exposed to the field emissions  7  from the local tracking system  6 . The field sensor(s)  58  on the handpiece  44  senses the presence of the field  7  and registers a signal that is transmitted over the electrical conductor  46   b  to the field receiver  84  in the integral base  42 . The field receiver  84  detects and converts the field emissions received by the field sensor into field measurements that are sent to a suitable peripheral unit, such as a computer, for processing (alternatively, the processing may occur within the integral base  42  or within the tracking system  6 ). Basically, the local tracking system  6  tracks the location of the one or more field sensors  58  in the designated field  7 . The tracking occurs in real time, with six degrees-of-freedom—three position coordinates and three orientation angles. (This tracking technique is based on well known technology exemplified by systems such as the aforementioned commercially available miniBIRD™ tracking system offered by Ascension Technology Corporation). Using the known position of the sensor(s)  58 , the coordinates of the imaging probe  3  within the designated field space can be determined, and this coordinate data is included and stored with the captured images as metadata. Using the metadata, the position of the images captured by the imaging probe  3  can be inferred from these coordinates.  
         [0040]    Although there are applications of the present invention throughout medical imaging, consider for example, an application in dentistry. FIG. 5 shows the lower jaw  100  and teeth  102  of a typical patient, and an imaging probe  3  having means for both image recording and position/orientation detection. The imaging probe  3  is shown taking an image of tooth  104  from a first position # 1 . That image is saved and then a second image is taken by moving the imaging probe  3  to a second position # 2 . Then, conventional software may be used to register the two (or more) images using the position data captured at the time of exposure (and stored as metadata with the images) so that a proper overlap of the two images is obtained. Given the proper registration of the two images, a calibration process enables accurate derivation of the distances involved in measuring the intra-oral object (e.g., the tooth  104 ). FIG. 5 also illustrates the use of two field sensors, a first field sensor  58   a  and a second field sensor  58   b . The position information obtained by the tracking system  6  for the two sensors enables the two different aspects of the projected images to be obtained. That is, the image at the first position # 1  has a projection aspect determined by its axis of view  110   a  and its angular relationship to the object and the second position # 2  has a projection aspect determined by its axis of view  110   b  and its angular relationship to the object. Knowing the position coordinates of the two sensors  58   a  and  58   b  enables the angular relationship  112  to be determined between the two images and from that aspect information the two pictures can be adjusted, e.g., by conventional morphing techniques, such that they precisely overlap.  
         [0041]    Referring to FIG. 6, in the case of an X-ray investigation, an x-ray source  114  for recording x-ray images is shown in two different positions # 1  and # 2 . The x-ray source  114  has attached field sensors  58   a  and  58   b  to record the field information, from which the position/orientation of the source  114  is determined. This system can be used with a photosensitive receiver  116 , in this case either a frame of x-ray film or a digital radiographic sensor, to capture the image information. As was described in connection with FIG. 5, software would again normalize the differences in position of the x-ray emissions from the two positions of the x-ray source  114 , thus enabling accurate registration of the images. Since the film or the sensor would be held in fixed orientation to the tooth, only position information of the source relative to the tooth would be required to calibrate the system.  
         [0042]    In another embodiment of the invention, another field sensor  58   c  (see FIG. 5) may be placed on a patient&#39;s tooth (or jaw) for instantaneous monitoring of the actual position coordinates of the tooth (or jaw) position. This would further enable the calibration process by allowing an accurate derivation of the distance between the probe and the tooth (or jaw), through calculation of the difference in x,y,z coordinates in reference to the magnetic field. This approach has the further advantage of simplifying the image registration process by tracking, and thereby allowing the elimination of, any movement between the images of the tooth relative to the probe. The registration process is simplified due to identification and removal of rotation/translation of the tooth relative to the probe. In yet another extension of the concepts embodied in the present invention, a fine mechanical tip may be formed on the imaging probe  3  or x-ray source  114 , which contacts the object to be recorded at a specific point. Then, by using the aforementioned methodology of the invention, the x,y,z coordinates of that specific point may be determined and recorded in reference to the magnetic field.  
         [0043]    An advantageous use of the invention is in conjunction with the methodology described in the aforementioned commonly assigned copending application Ser. No. 09/970,243, where custom software described in that application would allow recording temporal differences in both optical and x-ray images. The process described in Ser. No. 09/970,243 includes the use of manually selected tie points (seed points) followed by autocorrelation to find additional tie points, leading to the warping of one image to another which necessarily includes an image resampling step. The image capture position and orientation measurement system described according to the present invention would simplify and improve the subtractive radiography process described in Ser. No. 09/970,243, thus facilitating the registration of images and applying a more automatic version of the software assisted process described in that application.  
         [0044]    [0044]FIG. 7 shows an automated method for placing reference points in a radiography application, which is based on a version of the method shown in Ser. No. 09/970,243 that has been modified according to the current invention. The method is shown in its several stages in FIG. 7 for the two images  140  and  142 , which represent before and after dental images or radiographs of an oral object, such as a tooth (“before” and “after” is meant to represent a time sequence that would reveal, e.g., changes in the tooth or bone structure caused by a cavity or disease). Using the coordinates developed by the tracking system  6 , the images are processed in a tracking stage  150  to locate potential tie points. The images may be presented to the user via the graphical user interface  15 , whereupon the user may signal an acceptance decision  154  through manipulation of the mouse  18  or the keyboard  16  (if, for any reason, the results are unacceptable, the process is returned to a manual refinement stage, not shown, until the result is acceptable). The result is a set of refined, automatically-determined tie points that are suitable for the registration process.  
         [0045]    Once accepted, the refined tie points can be used in conjunction with optional automatically correlated points in the correlation stage  156 . These optional points may then be reviewed by the user. In the auto registration stage  158 , a polynomial function is generated to relate the tie points. In its simplest form, the polynomial (alignment equation) is of the form 
           X=a   1   +a   2   X′+a   3   Y ′ 
         [0046]    with only three constants (and a similar equation for Y). Hence, locating three reference (tie) points that are common to two sequential images allows one to be rotated and stretched (warped) to align with the other. (See pages 201-208 on Alignment in  The Image Processing Handbook, Second Edition , by John C. Russ, CRC Press, 1995). Typically, more tie points are involved in the registration process. For instance, in commonly-assigned U.S. Pat. No. 6,163,620 (entitled “Automatic Process for Detecting Changes Between Two Images”), which is incorporated herein by reference, between five and one hundred tie points are used. The polynomial function is then used in the auto registration stage  158  to warp the right image  142  to the left image  140  (or vice versa). Once registration is completed, the results are aligned side by side for review in the registration review stage  160 . Known alignment techniques may be employed to render the left and right images for this view with the same zoom level and image centering. (cf., The Image Processing Handbook). If the user deems the registration adequate, acceptance is signaled by the acceptance decision  162  through manipulation of the mouse  18  or the keyboard  16 .  
         [0047]    The use of the field sensor(s)  58  provides the precise position and attitude of the sensor for each image. This knowledge allows a suitable analytical geometry model for the specific sensor(s) to be used to predict the corresponding position of each pixel from one image to another. This is a standard analytical photogrammetric technique (see FIG. 8 and  9 ) which uses the sensor geometry model (providing data  168  of the sensor position and orientation during image formation) to project (stage  176 ) from the image space  170  (for image  1 ) to object space  172 , and then to project (stage  178 ) from object space  172  to image space  174  (for image  2 ). This projective process precludes the need for manually selected tie points (seed points), minimizes, and could in some instances eliminate, the need for autocorrelation, and eliminates the need for an image warping step; along with the associated resampling. Once the projective process determines the corresponding pixel in the second image, the subtractive process (stage  180 ) is performed, directly yielding a difference image  182  (see FIG. 9).  
         [0048]    Another application of the positioning system of the invention is found in the aforementioned commonly assigned copending application Ser. No. 09/894,627, entitled “Method and System for Creating Models from Imagery”, in which a method was described for creating dental models from imagery in which errors due to the lack of certainty of knowledge about the image positions were addressed using a method of analytical adjustment to control points. Note that image position is used here to denote both the position and the orientation of the sensor during image formation. That method corrects the errors by analytically projecting a known 3-D model into an existing image or multiple images, assuming initial estimates for the image positions, then determining the misalignment of the control points between the model and the image, and refining the estimates of image position through photogrammetric adjustment. The projection is an analytical process, meaning that it is accomplished mathematically, and the determination of misalignment may be accomplished interactively or automatically with appropriate image processing algorithms.  
         [0049]    The current invention employs a position determination system, which provides knowledge of the sensor position and orientation during image formation. This knowledge can be used to either eliminate the need for the adjustment to control described above, or to provide more reliable initial estimates to that process. In the former case, the entire process is made more efficient by eliminating the need for interactive or automatic control point measurement followed by photogrammetric adjustment. In the latter case, the adjustment process is enhanced through the use of improved initial estimates, which allows the process to be initialized in a more accurate state, i.e., closer to the true condition.  
         [0050]    Adjustment should be understood as a process that includes adjustment to control—as well as adjustment to conjugate measurements—which will work very well when there are excellent starting estimates (as would be obtained from the position sensors). In such a case, the positional estimates can be used to automatically look for tie points via software by (a) using the software to locate a definable point in a first image (b) projecting the position of that point into a second image (c) searching for the exact conjugate point in the second image, thereby producing a set of tie points, and (d) repeating these steps until the required number of tie points are obtained. Then, adjustment can be made to the tie points.  
         [0051]    The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention.  
       Parts List  
       [0052]    [0052] 1  dental office  
         [0053]    [0053] 2  patient location  
         [0054]    [0054] 3  intra oral imaging probe  
         [0055]    [0055] 4  hand held display unit  
         [0056]    [0056] 5  tethered connection  
         [0057]    [0057] 6  local tracking system  
         [0058]    [0058] 7  field  
         [0059]    [0059] 8  miniature field sensor  
         [0060]    [0060] 9  computer system  
         [0061]    [0061] 10  microprocessor based unit  
         [0062]    [0062] 12  display  
         [0063]    [0063] 14  graphical user interface  
         [0064]    [0064] 16  keyboard  
         [0065]    [0065] 18  mouse  
         [0066]    [0066] 20  cursor  
         [0067]    [0067] 22  CD-ROM memory  
         [0068]    [0068] 24  compact disk  
         [0069]    [0069] 26  floppy disk  
         [0070]    [0070] 27  network connection  
         [0071]    [0071] 28  printer  
         [0072]    [0072] 30  PC card  
         [0073]    [0073] 40  dental imaging probe  
         [0074]    [0074] 42  integral base  
         [0075]    [0075] 44  handpiece  
         [0076]    [0076] 46  cable  
         [0077]    [0077] 46   a  fiber optic  
         [0078]    [0078] 46   b  electrical connection  
         [0079]    [0079] 48  display monitor  
         [0080]    [0080] 50  user controls  
         [0081]    [0081] 52  lens unit  
         [0082]    [0082] 54  lens  
         [0083]    [0083] 56  light emitting apertures  
         [0084]    [0084] 58  field sensor(s)  
         [0085]    [0085] 58   a  first field sensor  
         [0086]    [0086] 58   b  second field sensor  
         [0087]    [0087] 58   c  third field sensor in patient&#39;s mouth  
         [0088]    [0088] 60  CPU  
         [0089]    [0089] 62  CPU memory  
         [0090]    [0090] 64  power supply  
         [0091]    [0091] 66  wireless transceiver  
         [0092]    [0092] 68  RAM  
         [0093]    [0093] 70  video control unit  
         [0094]    [0094] 72  illuminator control unit  
         [0095]    [0095] 74  illumination source  
         [0096]    [0096] 76  image sensor  
         [0097]    [0097] 78  wireless link  
         [0098]    [0098] 80  removable memory element  
         [0099]    [0099] 82  memory interface  
         [0100]    [0100] 84  field receiver  
         [0101]    [0101] 100  lowerjaw  
         [0102]    [0102] 102  teeth  
         [0103]    [0103] 104  tooth  
         [0104]    [0104] 110   a  first view axis  
         [0105]    [0105] 110   b  second view axis  
         [0106]    [0106] 112  aspect angle  
         [0107]    [0107] 114  x-ray source  
         [0108]    [0108] 116  photosensitive receiver  
         [0109]    [0109] 140  first image  
         [0110]    [0110] 142  second image  
         [0111]    [0111] 150  tracking stage  
         [0112]    [0112] 154  acceptance decision  
         [0113]    [0113] 156  correlation stage  
         [0114]    [0114] 158  registration stage  
         [0115]    [0115] 160  registration review stage  
         [0116]    [0116] 162  acceptance decision  
         [0117]    [0117] 168  position and orientation data  
         [0118]    [0118] 170  image space  
         [0119]    [0119] 172  object space  
         [0120]    [0120] 174  image space  
         [0121]    [0121] 176  projection stage  
         [0122]    [0122] 178  projection stage  
         [0123]    [0123] 180  subtraction stage  
         [0124]    [0124] 182  difference image