Abstract:
The invention provides a system ( 102 ) for accurately detecting a patient&#39;s ( 114 ) movement during imaging procedures. The system comprises a camera ( 126 ) for providing a stream of camera images of a part of a patient&#39;s exterior ( 206 ). The system ( 102 ) furthermore comprises a fiducial element ( 116 ), which fiducial element ( 116 ) is mountable on said part of the patient&#39;s exterior ( 206 ), and which fiducial element ( 116 ) is detectable in the stream of camera images, and an image processor ( 128 ) for detecting a displacement of the fiducial element based on consecutive camera images comprised in at least the stream of camera images and for generating an output signal ( 129 ) indicative for said displacement. Herein, the fiducial element ( 116 ) has an in-plane stiffness which is substantially larger than an in-plane stiffness of said part of the patient&#39;s exterior. In addition, the fiducial element ( 116 ) and said part of the patient&#39;s exterior are provided with substantially equal outer in-plane dimensions.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a system for detecting a patient&#39;s movement during imaging procedures. 
       BACKGROUND OF THE INVENTION 
       [0002]    US-A 2008/0287807 A1 discloses a method for breast cancer screening. For the purpose of tumor detection, digital imaging of an actuated breast is employed to determine tissue surface motion. On the basis of said tissue surface motion, an internal stiffness distribution is reconstructed, whereby regions of high stiffness suggest cancer. The method according to US-A 2008/0287807 A1 comprises a step of placing a plurality of fiducial element markers on a tissue surface, a step of actuating the tissue surface, a step of imaging the tissue surface with an array of digital cameras, a step of choosing motion invariant properties of the fiducial element markers, a step of tracking the fiducial element markers from image to image and a step of using the tracked motion in each camera and the camera calibration to measure tissue surface motion. 
         [0003]    During imaging procedures performed by e.g. Magnetic Resonance Imaging (MRI) or X ray devices, it is essential for the medical professional to be provided with accurate information regarding global movements of the patient&#39;s exterior. The method disclosed in US-A 2008/0287807 A1 is typically arranged for detecting local movements of the patient&#39;s exterior, but is not capable of detecting global movement of the patient&#39;s exterior. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object of the invention to provide a system of the kind defined in the opening paragraph capable of accurately detecting a global movement of a patient&#39;s exterior during imaging procedures. This object is achieved by the system according to the invention. The system according to the invention comprises a camera for providing a stream of camera images of a part of a patient&#39;s exterior, a fiducial element mountable on said part of the patient&#39;s exterior, wherein the fiducial element is detectable in the stream of camera images, wherein the fiducial element has an in-plane stiffness substantially larger than an in-plane stiffness of said part of the patient&#39;s exterior, and wherein the fiducial element and said part of the patient&#39;s exterior are provided with substantially equal outer in-plane dimensions, and an image processor for detecting a displacement of the fiducial element based on consecutive camera images comprised in the stream of camera images, and for generating an output signal indicative for said displacement. 
         [0005]    Through providing the fiducial element with a large in-plane stiffness compared to the in-plane-stiffness of the part of the patient&#39;s exterior, and by providing the fiducial and the part of the patient&#39;s exterior with substantially equal outer in-plane dimensions, local movements of the patient&#39;s exterior, i.e. the patients skin, relative to a patient&#39;s skeleton along a direction parallel to the fiducial element&#39;s plane are effectively prevented from, as will be explained hereinafter. As a result, presuming the patient maintains a stationary position, the patient&#39;s exterior is limited to move along a direction perpendicular to the fiducial, which is a global movement. The latter global movement is made observable via the fiducial element. By making the fiducial element detectable in a stream of camera images, the system according to the invention effectively increases the accuracy of detecting global movement of the patient&#39;s exterior. 
         [0006]    In addition, the system according to the invention enables detecting global movement of the patient&#39;s exterior in a painless and efficient way. Namely, the system according to the invention effectively circumvents rigidly attaching fiducial elements to the patient&#39;s skeleton for the purpose of accurately detecting global movement of the patient&#39;s exterior, which would be an invasive procedure requiring additional preparation of the patient, making it time consuming and costly. Further, the system according to the invention effectively circumvents the application of a significant large number of relatively small fiducial elements attached to the patient&#39;s exterior, whereby local movements are compensated for by leaving out of consideration mutual displacements of said relatively small fiducials, which is a rather difficult procedure subject to inaccuracies. That is, rather than counterbalancing local movements afterwards, the system according to the invention prevents the origination of local movements beforehand. 
         [0007]    The fiducial element is typically manufactured from an adhesive and an additional layer mounted on the adhesive. The additional layer is preferably a metal, and more preferably aluminum. 
         [0008]    In this document, a stream of camera images implies at least two camera images, but usually a plurality of camera images. It is stressed that in this document, a camera not necessarily implies an optical camera; the camera is responsive to a wavelength, which wavelength may be either in the visible spectrum, the infrared spectrum or the ultraviolet spectrum. 
         [0009]    In this document, outer in-plane dimensions are considered dimensions defining a contour of said plane. 
         [0010]    In this document, stiffness is defined as a resistance of a deformable body to deformation through an applied force or an applied torque. An in-plane stiffness means a stiffness along any imaginary axis comprised in the plane, hence in-plane stiffness does not include stiffness along any other imaginary axis having a component perpendicular to the plane. In this document, the fiducial element&#39;s in-plane stiffness is considered to be substantially larger than the in-plane stiffness associated with said part of the patient&#39;s exterior, in case the ratio of the fiducial element&#39;s in-plane stiffness to the in-plane stiffness of said part of the patient&#39;s exterior exceeds 10 to 1. Preferably, the aforementioned ratio exceeds 50 to 1, and more preferably, the latter ratio exceeds 100 to 1. Namely, by increasing the latter ratio, local movements of the patient&#39;s exterior along a direction parallel to the fiducial element&#39;s plane will be further decreased. Consequently, the accuracy of determining the patient&#39;s exterior global movements will be additionally increased. It is stressed that unlimitedly increasing said ratio is not a feasible option. Namely, it would result in a fiducial element having an in-plane stiffness such large compared to the patient&#39;s exterior, that the fiducial element would be barely mountable on the patient&#39;s exterior. Herein, mounting the fiducial element on the patient&#39;s exterior implies placing said fiducial element on the patient&#39;s exterior, and subsequently securing said fiducial element to the patient&#39;s exterior. It is furthermore noted that the in-plane stiffness associated with the part of the patient&#39;s exterior that is under examination, is mainly determined by its Young&#39;s modulus of elasticity, which modulus is dependent on the patient at hand, the body part at which the part of the patient&#39;s exterior is located, the patient&#39;s gender and parameters such as temperature and relative humidity. It is to be noted that in e.g. D. L. Bader and P. Bowker, “ Mechanical characteristics of skin and underlying tissues in vivo”, Biomaterials,  4:305-308, 1983, a range of Young&#39;s moduli is presented for a human&#39;s exterior. For instance, the Young&#39;s modulus of a male patient&#39;s exterior at the forearm, amounts 1.51*10 −3  [MPa] in case of indentation. Employing an equal way of loading, a female patient&#39;s exterior at the forearm is determined to amount 1.09*10 −3  [MPa]. 
         [0011]    In this document, the in-plane dimensions of the fiducial element are in any case considered to be substantially equal to the in-plane dimensions of said part of the patient&#39;s exterior, if the fiducial element&#39;s outer in-plane dimensions at least exceed 1 cm by 1 cm. Preferably, the fiducial element&#39;s outer in-plane dimensions exceed 5 cm by 5 cm, and more preferably, the fiducial element&#39;s outer in-plane dimensions exceed 10 cm by 10 cm, provided that the fiducial element&#39;s outer in-plane dimensions remain substantially equal to the further outer in-plane dimensions of said part of the patient&#39;s exterior. Herein, the outer in-plane dimensions of the fiducial element are considered to be substantially equal to the further outer in-plane dimensions of the part of the patient&#39;s exterior in case the fiducial element&#39;s outer in-plane dimensions amount to 75% up to and including 125% of said further outer in-plane dimensions. Through increasing the fiducial element&#39;s in-plane dimensions, local movements of the patient&#39;s exterior along a direction parallel to the fiducial element&#39;s plane will be further decreased. Consequently, the accuracy of determining the patient&#39;s exterior global movements will be additionally increased. Furthermore, increasing the fiducial element&#39;s in-plane dimensions will improve a visibility of said fiducial element in the camera images. 
         [0012]    In a preferred embodiment of the system according to the invention, the fiducial element has a buckling stiffness which is substantially larger than a buckling stiffness of the part of the patient&#39;s exterior. In this document, buckling is interpreted to be a deformation mode under a compressive mechanical stress, which deformation mode is characterized by exhibiting displacements along orientations different a direction in which the compressive mechanical stress is applied. It is to be noted that a buckling fiducial element manifests a deformation pattern perpendicular to the plane of the fiducial element, however, this deformation pattern is induced by a local deformation of the patient&#39;s exterior, and is therefore to be suppressed by the fiducial element. In this document, the fiducial element&#39;s buckling stiffness is considered to be substantially larger than the buckling stiffness associated with said part of the patient&#39;s exterior, in case the ratio of the fiducial element&#39;s buckling stiffness to the buckling stiffness of said part of the patient&#39;s exterior exceeds 10 to 1. Preferably, the aforementioned ratio exceeds 50 to 1, and more preferably, the latter ratio exceeds 100 to 1. Namely, by increasing the latter ratio, local movements of the patient&#39;s exterior will be decreased to a further extend by the fiducial element. Consequently, the accuracy of determining the patient&#39;s exterior global movements will be additionally increased. It is stressed that unlimitedly increasing the latter ratio is not a feasible option. Namely, it would result in a fiducial element having a buckling stiffness such large compared to the patient&#39;s exterior, that the fiducial element would be barely mountable on the patient&#39;s exterior. For the purpose of providing substantial buckling stiffness, the aforementioned additional layer has a thickness ranging from 50 μm to 500 μm, a.o. depending on the material employed to manufacture the additional layer. 
         [0013]    In a further preferred embodiment of the system according to the invention, the system comprises a plurality of fiducial elements, wherein the fiducial elements are installable in mutually substantially non-parallel planes. This embodiment is advantageous in that it increases the accuracy with which global movement of the patient&#39;s exterior is detectable. Namely, by installing a plurality of fiducial elements in mutually substantially non-parallel planes, information regarding multiple directions along which the global movement of the patient&#39;s exterior may evolve will be obtained. 
         [0014]    In a further preferred embodiment of the system according to the invention, the system comprises a further camera for providing a stream of further camera images of said part of the patient&#39;s exterior, wherein the camera and the further camera are being mutually rigidly supported for establishing a mutually predetermined spatial relationship, and wherein the fiducial element is detectable in the further camera image. The system furthermore comprises a data processor for rendering the stream of camera images and the stream of further camera images into a stream of composite camera images on the basis of a further spatial correlation between the stream of camera images and the stream of further camera images, which further spatial correlation is established by each fiducial element. Given a distance between optical axes associated with the camera and the further camera, a parallax is introduced. Consequently, three-dimensional information regarding the patient&#39;s exterior is obtained. As a result this embodiment advantageously increases an accuracy of detecting the global movement of the patient&#39;s exterior. 
         [0015]    In a further preferred embodiment of the system according to the invention, the system comprises an X ray device for generating an X ray image of said part of a patient&#39;s interior, wherein at least the camera is supported by the X ray device for establishing a predetermined spatial relationship between the stream of camera images and the X ray image, and wherein the fiducial element is detectable in the X ray image. The system furthermore comprises a further data processor for updating the X ray image on the basis of the output signal generated by the image processor, and on the basis of a spatial correlation between at least the stream of camera images and the X ray image, which spatial correlation is established by the fiducial element. Preferably, provided the further camera is present, the further camera is supported by the X ray device as well. The spatial correlation between the stream of camera images and the X ray image, i.e. a correlating of the stream of camera images and the X ray image in spatial respect, is a onetime spatially correlating. Namely, since a predetermined spatial relationship between the camera and the X ray device is acquired through supporting the camera by the X ray device, the spatial correlation established through the fiducial, which fiducial is detectable in both the stream of camera images and the X ray image, is applicable for an indefinite time span thereafter. This embodiment has the advantage that it guarantees the X ray image to be consistent with an actual position and orientation of the patient&#39;s exterior, while exposing the patient to a minimum amount of potentially harmful X rays. Namely, the X ray image is allowed to be a onetime X ray image indeed through spatially correlating the stream of camera images and the X ray image and by subsequently updating the X ray image in conformance with a global movement of the patient&#39;s exterior on the basis of the output signal generated by the image processor. Nonetheless, the data processor may be configured for rendering a stream of X ray images and the stream of camera images into a stream of composite images. Herein, a camera refresh rate need not necessarily equal an X ray image refresh rate. 
         [0016]    In a further preferred embodiment of the system according to the invention, the data processor is arranged for rendering the X ray image and at least the stream of camera images into a stream of composite images on the basis of said spatial correlation between the stream of camera images and the X ray image. This embodiment has the advantage that it enables a medical professional&#39;s ability to efficiently and precisely perform an image based medical intervention. Namely, this embodiment provides said medical professional with an image comprising information of both the patient&#39;s interior and exterior in a simultaneous way. Preferably, the data processor is arranged for rendering the X ray image and a stream of composite camera images into a stream of composite images. 
         [0017]    In a preferred embodiment of the system according to the invention, the X ray device comprises a movable geometry, wherein at least the camera is being supported by said movable geometry. This embodiment has the advantage that the medical professional is permitted to freely choose a patient&#39;s location or orientation with regard to the camera, since a position or a viewing angle of the camera allows for adjustment, without cancelling the predetermined spatial relationship between the stream of camera images and the X ray image. This feature is of large assistance in generating the stream of camera images. Preferably, the movable geometry is embodied by a movable C arm which is usually present in nowadays X ray devices. In that way, the system according to the invention advantageously allows for seamless integration with a medical professional&#39;s convenient way of working. Furthermore, the movable C arm has the advantage of providing an entire rotational degree of freedom for the camera with respect to the surgical area. In addition, the movable C arm has the advantage that it enables a three dimensional reconstruction for the X ray image. 
         [0018]    In a further preferred embodiment of the system according to the invention, the system comprises an instrument for performing a medical intervention, wherein the instrument is detectable in the stream of camera images and in the X ray image. This embodiment advantageously enables image guided surgery in a convenient and effective way. Namely, a geometry of the medical instrument is deducible by generating the X ray image of both the patient and the medical instrument. As a consequence, no elaborate programming of the instrument&#39;s geometry is required. Since the medical instrument is detectable in the stream of camera images, information regarding an instrument&#39;s location and orientation, can be updated in the stream of composite images on the basis of the spatial correlation between the X ray image and the stream of camera images. Hence, the medical professional is provided with information regarding the instrument&#39;s location and orientation, with respect to the patient&#39;s interior and exterior, by way of the stream of composite images. Preferably, the instrument comprises pulsed Light Emitting Diodes (LEDs) for enhancing its detectability in the stream of camera images hence in the stream of composite images. 
         [0019]    In a further preferred embodiment of the system according to the invention, the camera is arranged for providing a beam of electromagnetic radiation for excitation of a contrast agent supplied to the patient. As a result, the stream of camera images is advantageously provided with a fluorescence characteristic, which fluorescence characteristic provides information regarding a patient&#39;s circulatory system. The latter information will preferably be available real time in the in the stream of camera images, hence the medical professional is provided with real time information regarding e.g. the patient&#39;s blood circulatory and lymphatic systems for e.g. detecting tumors. The contrast agent for example comprises dyes in small molecule form, which dyes remain in a patient&#39;s blood flow for a limited amount of time, typically a few minutes. 
         [0020]    In a further preferred embodiment of the system according to the invention, the system comprises an illumination device arranged for projecting information comprised in the X ray image onto said part of the patient&#39;s exterior on the basis of the spatial correlation between the stream of camera images and the X ray image. This embodiment advantageously enables the medical professional to perform the image guided medical intervention even more safely and effectively. Namely, this embodiment effectively circumvents the need for translating the stream of composite images to said part of the patient&#39;s exterior through providing information regarding the patient&#39;s interior at the patient&#39;s exterior. Preferably, a projection of information comprised in the X ray image is compensated for a possible curvature of the patient&#39;s exterior on the basis of the stream of camera images. 
         [0021]    In a further preferred embodiment of the system according to the invention, the illumination device is supported by the X ray device for establishing a further predetermined spatial relationship between the illumination device and the X ray device. This embodiment has the advantage that a projection of information comprised in the X ray image is easily performed, that is, without further spatial correlating. Since the camera is supported by the X ray device as well, a determined spatial relationship between the camera and the illumination device is obtained. Because the camera image has been spatially correlated to the X ray image, information comprised in the X ray image is projected to the patient&#39;s exterior without further calibration of the illumination device. 
         [0022]    In a further preferred embodiment of the system according to the invention, the illumination device is arranged for radiation sterilization. For this purpose the illumination device is arranged to transmit a beam of electromagnetic radiation, which electromagnetic radiation has a wavelength at which the electromagnetic radiation is absorbable by the DNA of infectious agents such as bacteria and other pathogenic cells. For instance, UV radiation at a wavelength of about 250 [nm] is employed. This embodiment has the advantage of being capable to sterilize an environment of the system according to the invention, e.g. a surgical table, more effectively. Namely, compared to sterilization based on solvents, the chance of successful sterilization is significantly larger. An additional advantage of this embodiment is in the fact that the sterilization is quickly and easily performed, that is, without interference of other systems. Preferably the illumination device is attached to a movable C arm presumably comprised in the X ray device. In that case, the sterilization is performed by making a rotation, preferably a full rotation, employing the movable C arm geometry. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  schematically displays a first embodiment of the system according to the invention comprising an X ray device to which a camera is mounted. 
           [0024]      FIG. 2  schematically displays an X ray image, a stream of camera images and a stream of composite images as generated by the embodiment displayed in  FIG. 1 . 
           [0025]      FIG. 3  schematically displays a head of a patient, which patient is provided with a plurality of fiducial elements, wherein fiducial elements have been installed in mutually substantially non-parallel planes. 
           [0026]      FIG. 4  schematically displays a second embodiment of the system according to the invention comprising an X ray device to which a camera and a further camera are being mounted. 
           [0027]      FIG. 5  schematically displays an X ray image, a stream of camera images, a stream of further camera images and a stream of composite images as generated by the embodiment displayed in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0028]      FIG. 1  schematically displays a system  102  comprising an X ray device  104  for providing an X ray image  202  of a patient&#39;s interior  204 , as displayed in  FIG. 2 . The X ray device  104 , see  FIG. 1 , has a base frame  106  supported by wheels  108 , a movable C arm  110  and a surgical table  112  for supporting a patient  114 , which patient  114  is a human being in this particular example. In operational conditions, a fiducial element  116  is mounted on a patient&#39;s exterior  206 , as depicted in  FIG. 2 . The movable C arm  110  is rotatable with regard to an axis  118 , which axis  118  has a direction corresponding to a main orientation of the surgical table  112 , and with regard to an axis  120 , which axis  120  is perpendicular to the axis  118  and perpendicular to the surgical table  112 . An X ray source  122  and an X ray detector  124 , which is preferably a rectangular and flat detector, are mounted on the C arm  110  such that the X ray source and the X ray detector reside opposite one another with respect to the axis  118 . A camera  126  for providing a stream of camera images  208  of the patient&#39;s exterior  206 , as displayed in  FIG. 2 , is mounted on the C arm  110  aside the X ray source  122 . In that way, a predetermined spatial relationship between the X ray image  202  and the stream of camera images  208  is established. In this specific example, the camera  126  is sensitive to electromagnetic radiation having wavelengths in the visible spectrum. An image processor  128  generates an output signal indicative for a movement of the fiducial element  116  based on consecutive camera images. 
         [0029]    Referring to  FIG. 2 , the fiducial element  116  is installed on the patient&#39;s exterior  206  at a part  210  of the patient&#39;s exterior  206 , for making a global movement of the patient&#39;s exterior  206  perceptible by at least the camera  126 . The fiducial element  116  is detectable in both the X ray image  202  and the stream of camera images  208 . In this example, the X ray image  202  furthermore displays a tumor  212  present in the interior  204  of the patient  114 . The stream of camera images  208  additionally displays a body contour  214  of the patient  114 . Through spatially correlating the fiducial element  116  in the X ray image  202  with said fiducial element  116  in the stream of camera images  208 , the X ray image  202  and the stream of camera images  208  allow for spatially correlating. 
         [0030]    Referring to  FIG. 1 , a further data processor  130  renders, during operation, the X ray image  202  and the stream of camera images  208  into a stream of composite images  216  based on the spatial correlating provided by the fiducial element  116 . Based on the output signal  129  generated by the image processor  128 , the X ray image  202  and hence the stream of composite images  216  is updated regarding a global movement of the fiducial element  116 . The stream of composite images  216  display the patient&#39;s interior  204  and the patient&#39;s exterior  206  in a geometrically overlapping sense, and furthermore the fiducial element  116 , the part  210  of the patient&#39;s exterior  206 , the tumor  212  and the patient&#39;s body contour  214 . 
         [0031]    The camera  126  is configured for providing a beam of electromagnetic radiation for excitation of a contrast agent supplied to the patient  114  in order to provide the stream of camera images  204  and consequently the stream of composite images  216  with a fluorescence characteristic. A monitor  132 , as depicted in  FIG. 1 , displays the stream of composite images  216  to a medical professional (not shown). The system  102  furthermore comprises an illumination device  134  mounted on the C arm  110  aside the X ray source  122 , hence aside to the camera  126 , for establishing a spatial relationship between the X ray device  104  and the illumination device  126 . The illumination device  134  is configured for projecting information comprised in the X ray image  202 , for example the tumor  214 , on the patient&#39;s exterior  206 . The illumination device  134  is further arranged for radiation sterilization of e.g. the surgical table  112  and its environment, by way of transmitting a beam of electromagnetic radiation having a wavelength at which the electromagnetic radiation is absorbable by the DNA of infectious agents. In this particular example, a wavelength of about 250 [nm] is employed. The sterilization of the surgical table  112  is performed in between interventions performed at the system  102 , preferably by making a full rotation with the rotatable C arm  110 . 
         [0032]      FIG. 3  schematically depicts a patient&#39;s head  302 . A plurality of fiducial elements  304  and  306  is installed in mutually non-parallel planes at a patient&#39;s exterior. Namely, the fiducial element  304  is mounted on a patient&#39;s forehead, more specifically at a part  308  of the patient&#39;s exterior whereas the fiducial element  306  is mounted on a side of the patient&#39;s head, more specifically at a further part  310  of the patient&#39;s exterior. In other cases, one fiducial element or more than two fiducial elements may be installed. The fiducial element  304  and the part  308  of the patient&#39;s exterior are provided with substantially equal outer in-plane dimensions. Namely, a width w 1  of the fiducial element  304  nearly equals a width w 2  of the part  308  of the patient&#39;s exterior. Further, a height h 1  of the fiducial element  304  nearly equals a height h 2  of the part  308  of the patient&#39;s exterior. Likewise, the fiducial element  306  and the part  310  of the patient&#39;s exterior are provided with substantially equal outer in-plane dimensions. In this particular example, the fiducial elements  304  and  306  are provided with additional layers having a thickness of 100 μm. 
         [0033]      FIG. 4  schematically displays a system  402  comprising an X ray device  404  for providing an X ray image  502  of a patient&#39;s interior  504 , as displayed in  FIG. 5 . The X ray device  404 , see  FIG. 4 , has a base frame  406  supported by wheels  408 , a movable C arm  410  and a surgical table  412  for supporting a patient  414 . In this particular example, the patient  414  is a human being. In operational conditions, a fiducial element  416  is mounted on a patient&#39;s exterior  506 , as depicted in  FIG. 5 . Referring to  FIG. 4 , the C arm  410  is rotatable with regard to the axis  418 , which axis  418  has a direction corresponding to a main orientation of the surgical table  412 , and with regard to an axis  420 , which axis  420  is perpendicular to the axis  418  and perpendicular to the surgical table  412 . An X ray source  422  and an X ray detector  424 , which is preferably a rectangular and flat detector, are mounted on the C arm  410  such that the X ray source and the X ray detector reside opposite one another with respect to the axis  418 . A camera  426 , i.e. a first camera, for providing a stream of camera images  508 , i.e. a stream of first camera images, of a patient&#39;s exterior  506 , as depicted in  FIG. 5 , is mounted on the C arm  410  aside the X ray source  422 . A further camera  428 , i.e. a second camera, for providing a stream of further camera images  510 , i.e. a stream of second camera images, of the patient&#39;s exterior  506 , as displayed in  FIG. 5  is additionally mounted on the C arm  410  aside the X ray source  422 . In that way, a predetermined spatial relationship between the X ray image  502  and both the stream of first camera images  508  and the stream of second camera images  510  is established. In addition to that, a mutual predetermined spatial relationship between the stream of first camera images  508  and the stream of second camera images  510  is established. The first camera  426  is responsive to electromagnetic radiation having a first range of wavelength while the second camera  428  is responsive to electromagnetic radiation having a second range of wavelengths. In this specific example, both the first and the second ranges of wavelengths are in the visible part of the electromagnetic spectrum. The first camera  426  and the second camera  428  are mounted at different positions on the C arm  410 . Thereby a parallax is introduced between optical axes associated with the first and second camera  426  and  428 . 
         [0034]    Referring to  FIG. 5 , the fiducial element  416  is installed on the patient&#39;s exterior  506  at the part  512  of the patient&#39;s exterior  506 , for making a global movement of the patient&#39;s exterior  506  perceptible by the first camera  426  and the second camera  428 . The fiducial element  416  is detectable in the X ray image  502 , the stream of first camera images  508  and the stream of second camera images  510 . Through spatially correlating the fiducial element  416  in the X ray image  502  with said fiducial element  416  in the stream of first camera images  508  and the stream of second camera images  510 , the X ray image  502 , the stream of first camera images  508  and the stream of second camera images  510  allow for spatially correlating. In this particular example, the X ray image  502  displays the patient&#39;s interior  504 , a part  512  of the patient&#39;s exterior  506 , a contour  514  of the patient&#39;s exterior  506 , a tumor  516  or another medical deficiency present in the interior  504  of the patient  414 , and a medical instrument  518 , which medical instrument  518  is partly present in the patient&#39;s interior  504  in this particular example. For the purpose of displaying the contour  514 , an amount of X ray radiation provided by the X ray source  422  to the patient  414  must be sufficiently large, i.e. the amount of X ray radiation is to enable a detectability of a patient&#39;s soft tissue in the X ray image  502 . Both the stream of first camera images  508  and the stream of second camera images  510  display the patient&#39;s exterior  506 , the part  512  of the patient&#39;s exterior  506 , the contour  514  of the patient&#39;s exterior  506 , and a part  520  of the medical instrument  518 , which part  520  is not present in the patient&#39;s interior  504 . 
         [0035]    Referring to  FIG. 4 , a data processor  430  renders the stream of first camera images  508  and the stream of second camera images  510  into a stream of composite camera images (not shown). Based on the parallax between the optical axes of the first and second cameras  426  and  428 , the stream of composite camera images allows for displaying three dimensional characteristics. An image processor  432  generates an output signal  433  indicative for a movement of the fiducial element  416  based on consecutive images comprised in the stream of composite camera images. A further data processor  434  renders the X ray image  502  and the stream of composite camera images into a stream of composite images  522  based on the spatial correlating provided by the fiducial element  416 . A monitor  436  displays the stream of composite images  522  to a medical professional. 
         [0036]    Referring to  FIG. 5 , the stream of composite images  522  display the patient&#39;s interior  504  and the patient&#39;s exterior  506  in a geometrically overlapping sense, and furthermore the part  512  of the patient&#39;s exterior  506 , the contour  514  of the patient&#39;s exterior  506 , the tumor  516  and the medical instrument  518 . Based on the output signal  433  generated by the image processor  432 , the X ray image  502 , and hence the stream of composite images  522 , is updated regarding a global movement of the fiducial element  416 . 
         [0037]    While the invention has been illustrated and described in detail in the drawings and in the foregoing description, the illustrations and the description are to be considered illustrative or exemplary and not restrictive. It is noted that the system according to the invention and all its components can be made by applying processes and materials known per se. In the set of claims and the description the word “comprising” does not exclude other elements and the indefinite article “a” or “an” does not exclude a plurality. Any reference signs in the claims should not be construed as limiting the scope. It is further noted that all possible combinations of features defined in the set of claims are part of the invention.