Abstract:
Disclosed is a probe for obtaining geometrical data related to a cavity, the probe comprising a probe body comprising a first coupling means; at least one magnetic field generating device comprising a support member and a means for generating a magnetic field; wherein said means for generating a magnetic field being connected to the support member so as to fix a position of said means for generating a magnetic field relative to said support member and wherein said support member comprises a second coupling means configured to engage the first coupling means so as to connect the at least one magnetic field generating device to said probe body. 
     Thereby is achieved to improve positional accuracy of geometrical data related to an internal surface of a cavity.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention is related to a method of fixedly position a magnetic generating means to a probe or similar body, and a system for obtaining geometrical data related to a cavity, such as for example the ear and ear canal of the human body. 
       BACKGROUND OF THE INVENTION  
       [0002]    In order to make a shell which fits a cavity such as for example the ear and ear canal of the human body, an apparatus enabling generation of a data mapping of the internal surface of the ear and ear canal may be utilized, so that 3-dimensional data or a digital model of the internal surface of the ear and ear canal can be obtained. Such a 3-dimension model can be used to produce the shell, which may have the exact shape of the canal and the shell may form the basis for an e.g. In-The-Ear (ITE) or Completely-In-The-Canal (CIC) hearing aid. Also ear moulds or shells for other purposes such as a hearing protection or for headsets may be produced from the data model. The shell can be produced on the basis of the data model in different ways, such as by recent developed rapid prototyping methods or by well known machining, e.g. in a Computer Numerically Controlled (CNC) machining centre. 
         [0003]    It remains a problem to improve the position accuracy of devices for data mapping of internal surfaces such as the ear and/or ear canal of a person. A lack of positional accuracy in geometrical 3-D data obtained from the apparatus generating a data mapping may, for example, give rise to discomfort to the person using the shell e.g. a CIC hearing aid. This problem is also at hand in connection with other tracking systems, where the location of a body can only be determined with high precision if magnetic field generating elements are positioned precisely thereto and in strictly orthogonal relationship. 
       SUMMARY  
       [0004]    Disclosed is a probe and a coil fixed thereto system for establishing the spatial location of the probe wherein the probe and coil comprises a probe body with a first coupling means; at least one magnetic field generating device comprising a bobbin and a coil for generating a magnetic field; wherein the coil for generating a magnetic field is connected to the bobbin so as to fix a position the coil for generating a magnetic field relative to the bobbin and wherein the bobbin comprises a second coupling means configured to engage the first coupling means so as to connect the bobbin and coil to the probe body. 
         [0005]    Consequently, it is an advantage that the bobbin (e.g. a base plate  402 ) comprises a second coupling means adapted to engage the first coupling means of the probe (e.g. a probe comprising a distal light-emitting end) because it enables accurate positioning of the bobbin with respect to the probe. Additionally, it is an advantage that the coil for generating a magnetic field is connected to the bobbin so as to fix a position of the coil relative to the bobbin because it enables accurate placement of the coil with respect to the probe. Thereby an accurate position of the magnetic field generated by the means coil is obtained. This accurate positioning is with respect to, for example, the probe and/or a distal light-emitting end of the probe. 
         [0006]    For example, the distal light-emitting end of the probe obtains the geometrical data. Accurate placement of the coil with respect to the distal light-emitting end and a constant distance between the coils enables accurate determination of the position of the geometrical data, because the position of the geometrical data is determined with respect to the location of the coil, and with respect to distal light-emitting end distance and with respect to the coil&#39;s distance to, for example, an external sensor detecting at least one magnetic field generated by said coil. Thus, an improved determination of the position of geometrical data with respect to e.g. an external sensor may be obtained by placing the coil with a fixed position on a base plate and the base plate with a fixed position on the probe. 
         [0007]    Further, when the probe comprise two or more spaced apart coils for generating the magnetic field in one direction, these coils should be aligned to ensure that the magnetic fields are also aligned both with respect to angle and spatial placement. This is especially important when two orthogonal fields is desired, as mis-alignment will inevitably cause cross coupling between the two orthogonal fields. If the coils are Idelly aligned having fields in precisely right angles with respect to each other, the two fields will be independent, but if one coil is mis-aligned, this coil will be influenced by the orthogonally placed other magnetic field and vice-versa. 
         [0008]    The probe is not as such essential to the invention, as the means for locating the coil or coils at a fixed position may be used at other devices when there is a need to establish the location thereof. 
         [0009]    In an embodiment, one of the first and second coupling means comprises a protrusion and the other one of the first and second coupling means comprises a hole adapted to receive said protrusion. 
         [0010]    In a further embodiment, the size of the hole adapted to receive the protrusion is identical to or larger than the size of the protrusion. 
         [0011]    An advantage of this embodiment is that the first and second coupling means may fit together to a high degree, e.g. by frictional engaging each other, thereby increasing the accuracy by which the at least one magnetic field generating device is placed on the probe. 
         [0012]    In a further embodiment, the size of a hole in the first coupling means are greater than the size of a corresponding protrusion of the second coupling means and/or the size of a protrusion in the first coupling means are smaller than the size of a corresponding hole in the second coupling means. 
         [0013]    An advantage of this embodiment is that production errors in the first and second coupling means may be encompassed. 
         [0014]    In a further embodiment, the first coupling means comprises a first plurality of protrusions and/or a second plurality of holes and wherein the second coupling means comprises the first plurality of corresponding holes and/or the second plurality of corresponding protrusions. 
         [0015]    An advantage of this embodiment is that at least two protrusions and/or two holes are in the probe and the at least one magnetic field generating device which enables accurate placement of a means for generating a magnetic field where the magnetic field generated is not rotational symmetric around the at least two holes and/or at least two protrusions of the probe. Thereby, a non-rotational symmetric magnetic field generated by the means for generating a magnetic field may be placed as demanded e.g. by probe design. 
         [0016]    In a further embodiment, the probe comprises at least bobbins with each their coil adapted to generate respective magnetic fields in substantially the same direction. 
         [0017]    An advantage of this embodiment is that two means for generating a magnetic field e.g. two coils may be oriented such that their respective magnetic fields are oriented in the same direction and thus add up to a resulting magnetic field which is greater than the respective magnetic fields. 
         [0018]    The present invention relates to different aspects including the method described above and in the following, and corresponding methods and system, each yielding one or more of the benefits and advantages described in connection with the first mentioned aspect, and each having one or more embodiments corresponding to the embodiments described in connection with the first mentioned aspect and/or disclosed in the appended claims. 
         [0019]    In particular, disclosed herein is a method of fixedly position a magnetic generating device to a probe body wherein the probe comprises a first coupling means and at least one magnetic field generating device wherein the magnetic field generating device comprises a bobbin with a second coupling means and a coil for generating the magnetic field; wherein the method comprises, connecting the coil for generating a magnetic field to the bobbin so as to fix a position of said coil for generating a magnetic field relative to said bobbin and connecting the at least one magnetic field generating device to the probe body by engaging the second coupling means of the bobbin to the first coupling means of the probe. 
         [0020]    Further in particular, disclosed herein is a system for obtaining geometrical data related to a cavity, the system comprising at least one probe body with magnetic generating means fixedly positioned thereon according to the present invention and the system further comprising at least a plurality of magnetic sensor for detecting the at least one probe. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0021]    The above and/or additional objects, features and advantages of the present invention, will be further elucidated by the following illustrative and non-limiting detailed description of embodiments of the present invention, with reference to the appended drawings, wherein: 
           [0022]      FIG. 1  shows a schematic view of an example of an apparatus for obtaining geometrical data relating to the internal surface of a cavity. 
           [0023]      FIG. 2  shows a schematic view of another example of an apparatus for obtaining geometrical data relating to the internal surface of a cavity. 
           [0024]      FIG. 3  shows a sectional view of the distal end of a probe of an apparatus for obtaining geometrical data relating to the internal surface of a cavity. 
           [0025]      FIG. 3   a  shows a cross sectional view of a distal light-emitting probe end comprising a number of coils. 
           [0026]      FIG. 3   b  shows an image of a probe comprising four coils placed in the distal light-emitting end of the probe. 
           [0027]      FIG. 4   a  shows a distal light-emitting end portion of a probe comprising at least one protrusion. 
           [0028]      FIG. 4   b  shows a distal light-emitting end portion of a probe comprising at least one cut-out. 
           [0029]      FIG. 4   c  shows a coil housing comprising a coil, a base plate and a cover. 
           [0030]      FIG. 5   a  shows an example of how two coils in two coil housings of a probe may be connected in series. 
           [0031]      FIG. 5   b  shows an example of the magnetic fields generated by two coils connected in series and mounted on a probe. 
           [0032]      FIG. 6  shows an example of a series resonance circuit. 
           [0033]      FIG. 7  shows an example of a probe containing a printed circuit board contained in a shielding box. 
           [0034]      FIG. 8  shows a side view of a human ear and illustrating the use of the apparatus described herein. 
       
    
    
     DETAILED DESCRIPTION  
       [0035]    In the following description, reference is made to the accompanying figures, which show by way of illustration how the invention may be practiced. 
         [0036]      FIG. 1   a  shows a schematic view of an example of an apparatus for obtaining geometrical data relating to the internal surface of a cavity. The apparatus, generally designated  100 , comprises a distal end portion  103  including an optical system  116  for emitting and receiving light, a light source  101  and light guides  102  for directing the light from the light source  101  to a rear surface  104  of the optical system  116 . The optical system  116  may comprise one or more lenses and/or one or more reflecting surfaces  105  for directing the light from the light guides  102  as one or more beams  106  to the internal surface  107   a, b  of a cavity. 
         [0037]    The light beams  106  are emitted at an acute angle  115  relative to the optical axis  113  of the optical system  116  which also defines the direction of insertion of the distal end portion  103  into the cavity. The light beams  106  are emitted from respective exit positions  117  radially displaced from the optical axis. Furthermore, the exit position  117  and the position where the light beam intersects with the cavity wall  107   a, b,  are positioned on opposite sides of the optical axis  113 . The light beams  106  thus cross the optical axis. Furthermore, the light beams  106  intersect at a point  118  in front of the probe; in the example of  FIG. 1  the beams intersect with each other on the optical axis. 
         [0038]    In the example of  FIG. 1 , two light guides  102  are shown. However, it will be appreciated that generally a different number of light guides may be used, e.g. 3, 4, 5, 6, or even a larger number of light guides. In some embodiments a uniform illumination over a full circumference is achieved by providing a relatively large number, e.g. between 60 or 80, fibres. The fibres are divided into bundles, e.g. 6 bundles, and each bundle is illuminated with a respective light source. This gives the opportunity to make an automatic control of the emitted power at different sections of the probe. This is advantageous as it allows staying in the dynamic range of the light detector, when measuring surfaces at different distances in different sections at the same time. 
         [0039]    For example, the light guides may be arranged such that the emitted light beams  106  intersect with each other as shown in  FIG. 1 , so as to define a double cone where the point of intersection  118  defines the apex of the cone. In the example of  FIG. 1 , the beams  106  intersect each other on the optical axis. 
         [0040]    The emitted light beams  106  are reflected from the internal surface  107   a, b  of the cavity and at least a portion of the reflected light will be reflected back in the direction of the distal end portion  103  as indicated by reflected beams  108   a, b  in  FIG. 1 . The optical system  116  of the distal end portion  103  thus also functions as a light receiving system and receives the reflected light  108   a, b  from the cavity walls  107   a, b.  The optical system  116  directs the received light  108   a, b  towards a light sensitive element  110 , e.g. an array of light sensitive elements such as a CCD or another position sensitive light detector. 
         [0041]    Generally, if the CCD is a colour sensitive CCD element, colour information may be used when analyzing the light reflected from the surface of an ear canal. If white light is used, it is possible to determine the relative content of red, green and blue light in the received signal, and thereby foreign objects such as earwax may be identified. This is because earwax will reflect the light in other wavelength ranges than the naked skin of the ear canal. 
         [0042]    Furthermore, the detected signal from the CCD may be displayed on a display, via an eyepiece, or the like, as a received image, thus allowing the probe described herein to be used in a fashion similar to an endoscope. Such an image may be valuable for the person conducting an ear scan, e.g. for a visual inspection of the measured cavity and/or so as to provide a visual control as to how close the probe is to the end wall, e.g. the tympanic membrane. 
         [0043]    The light source  101  may be any suitable light source, e.g. a one or more light emitting diodes (LED) or diode lasers. LEDs provide a low noise level as they avoid noise from speckle, while diode lasers provide a high output power. 
         [0044]    In  FIG. 1 , the cavity surface  107   a, b  is shown as an example in a first distance at  107   a  from the tip of the probe and in a second distance at  107   b  closer to the tip of the probe. Light reflected from the surface at  107   a  will enter the optical system  116  as light  108   a  at an incident angle  114   a  relative to the optical axis, while light reflected from the surface at  107   b  will enter the optical system  116  as light  108   b  at an incident angle  114   b  relative to the optical axis. Consequently, the optical system  116  directs the incoming light  108   a  and  108   b  to respective positions  109   a  and  109   b  on the light sensitive element  110 , thereby allowing the determination of the distance to the points  107   a  and  107   b  in the direction of the emitted light  106  from the position of the detected light on the light sensitive element  110 .  FIG. 1   b  illustrates positions of constant distance on the light sensitive area of the detector  110  for two different distances  109   a  and  109   b  respectively. 
         [0045]    The apparatus  100  further comprises a signal analysis circuit  111  which generates an output signal  112 , e.g. an analogue signal or a digital data signal. In some embodiments, the output signal is indicative of an intensity distribution of the detected light across the light sensitive area  110  of the detector. Alternatively or additionally, the signal analysis circuit may perform additional signal processing steps, e.g. including the actual distance calculation based on an incident angle determined from the detected locations  109   a  and  109   b.  The distance may be calculated by means of a conventional triangulation resulting in a distance from the probe to the locations where the beams  106  intersect with the cavity wall  107   a, b,  i.e. a distance in a direction having a component in a radial direction from the optical axis  113 . Alternatively, the distance calculation and/or further signal processing may be performed by a separate signal/data processing unit, e.g. on a computer such as a PC to which the apparatus  100  may be connected. 
         [0046]      FIG. 2  shows a schematic view of another example of an apparatus, generally designated  200 , for obtaining geometrical data relating to the internal surface of a cavity. The apparatus  200  is similar to the apparatus described in connection with  100 , and will therefore not be described in detail again here. However, while the light sensitive element  110  of the apparatus shown in  FIG. 1  was arranged adjacent to the distal end portion  103 , such that the reflected light is captured at the distal end of the probe, the light sensitive element  110  of the apparatus  200  is arranged remote from the distal end  103 . To this end, the optical system  116  at the distal end portion  103  directs the reflected light into the distal end of a light guide  220 , which in the following will be referred to as an imaging guide, the imaging guide  220  thus directs the received light, e.g. via a further lens or lens system  221  to the light sensitive element  110 . 
         [0047]    While the apparatus of  FIG. 100  provides a simpler construction, the apparatus  200  does not require a light sensitive element, e.g. a CCD which is small enough to be mounted at the tip of the probe, which is going to enter the cavity. For example, the light guides  102  and the imaging guide  220  may be arranged in a flexible tube connecting the distal end portion  103  with a proximal unit including the light source  110 , the detector  110 , and, optionally a signal processing unit  111 . 
         [0048]      FIG. 3  shows a sectional view of the distal end of a probe of an apparatus for obtaining geometrical data relating to the internal surface of a cavity. The probe, generally designated  300 , has a distal light-emitting end portion  103  and a rod portion  336 , which connects the distal portion to a proximal part (not explicitly shown). The rod portion  336  comprises a flexible pipe  337 , a set of light guides  102  and an image guide  220 . The image guide  220  is placed centrally in the pipe  337 , and the light guides  102  are arranged between the pipe  337  and the image guide  220 . Near the tip of the probe the image guide  220  and the light guides  102  are connected to a bushing  330  surrounded by an outer tube  339 . 
         [0049]    The probe further comprises an optical system including an annular lens  331  arranged at the bushing  330  to capture the light emitted from the light guides  102  and to direct the light towards a cavity wall, e.g. as a collimated or focussed light beam  106 , at an acute angle from the longitudinal axis of the probe which also defines the optical axis  113  and the direction of insertion. 
         [0050]    Light reflected from the cavity wall will enter the tube  339  through a central receiving lens  332  of the optical system. From the lens  332  the light is directed via an aperture  333  and a further imaging lens  334  towards a surface of the image guide  220 . The aperture  333  increases the depth of field and prevents stray light from reaching the sensor. The light received on the surface of the image guide  220  is transmitted through the image guide  220 , and will appear at the other end thereof. Here the image is captured by a CCD array (not shown). The signal from the CCD is transferred to a signal processing unit for further processing in order to calculate the distance from the probe to the canal wall. This is done by a triangulation method well known as such in the art. 
         [0051]    In general, even though the light  106  directed towards the cavity wall may be unfocussed or uncollimated, the use of focused or collimated light provides better contrast and thus results in a more precise detection of the distance between the probe and the cavity wall. 
         [0052]    The probe  300  further comprises one or more coils  335  used to generate a magnetic field, which is picked up by sensors arranged outside the cavity so as to determine the position of the probe relative to the external sensors. Thus, when the sensors are arranged in a fixed spatial relationship to the cavity, a signal/data processing unit can compute spatial coordinates of positions on the inner surface of the cavity from the optical distance measurements relative to the probe and from the position measurements of the position of the probe relative to the external sensors. 
         [0053]    The probe  300  may comprise one or more coils  335  such as for example one coil or two coils or three coils or four coils or any number of coils  335  greater than or equal to one. The one or more coils may, for example, be placed such that the magnetic field generated by each of the one or more coils is directed in one or more directions lying in a plane perpendicular to a longitudinal axis  301  of the probe  300 . 
         [0054]      FIG. 3   a  shows a cross section of a probe  300  comprising four coils  335   a - 335   d,  where said cross section is made perpendicular to the longitudinal axis  301  of the probe  300  and through the four coils  335   a - 335   d.  The coils  335   a - 335   d  are placed such that the magnetic field generated by each of the four coils is directed in a plane perpendicular to the longitudinal axis  301  of the probe  300  such as for example indicated by the magnetic field vectors B 1 -B 4  generated by the coils  335   a - 335   d,  respectively. Additionally, the coils  335   a - 335   d  may be placed such that an angle of 90 degrees (or substantially 90 degrees e.g. 88-92 degrees) may be between the magnetic field vectors B 1  and B 2  of coils  335   b  and  335   c,  respectively, 90 degrees (or substantially 90 degrees e.g. 90±0.25 degrees) may be between the magnetic field vectors B 2  and B 3  of coils  335   c  and  335   d,  respectively, 90 degrees (or substantially 90 degrees e.g. 90±0.25 degrees) may be between the magnetic field vectors B 3  and B 4  of coils  335   d  and  335   a,  respectively, and 90 degrees (or substantially 90 degrees e.g. 90±0.25 degrees) may be between the magnetic field vectors B 4  and B 1  of coils  335   a  and  335   b,  respectively. 
         [0055]    If the probe  300  comprises more than one coil for generating a magnetic field, such as for example four coils generating magnetic fields vectors B 1 -B 4 , respectively, then the four coils may be identical to each other e.g. generating identical magnetic fields when a current is passed through the coils and/or comprising the same number of turns and/or comprising the same wire diameter size etc. Alternatively, the four coils may be different from each other e.g. generating different magnetic fields when a current is passed through the coils and/or comprising a different number of turns and/or comprising the different wire diameter size etc. Alternatively, the probe may comprise a number of identical coils, e.g. two identical coils, and a number of different coils, e.g. two coils different from each other and different from the two identical coils. 
         [0056]    As shown in  FIG. 3 , the one or more coils  335  may be positioned in the distal light-emitting end portion  103  of the probe  300 . 
         [0057]      FIG. 3   b  shows an image of a probe  300  comprising four coils  335   a - 335   d  (of which three are visible in the image). The four coils may, for example, generate four magnetic fields. 
         [0058]      FIG. 4   a  shows a distal light-emitting end portion  103  of a probe  300  comprising at least one protrusion  410  and at least one coil bobbin  401 . For example, the probe  300  may comprise two protrusions  410  one coil bobbin  401  for each coil  335  to be mounted on the probe  300 . For example, if four coils  335  are to be mounted on the probe  300 , then the probe  300  may comprise four times two protrutions  410 . 
         [0059]    The at least one protrusion  410  may be constructed as an object extending outwards from the probe  300  e.g. extending vertically outwards in a direction perpendicular to the longitudinal axis  301  of the probe  300 . The at least one protrusion  410  may have any geometrical form such as for example cylindrical as indicated in  FIG. 4   a.  Alternatively, the at least one protrusion  410  may have the geometrical form of a box or a cone or any other geometrical form suitable for having a bobbin  401  with coil  335  or a coil housing with a substantially corresponding cut-out (or hole) mounted on it. 
         [0060]    The bobbin  401  may comprise a base plate  402  (a support member) 
         [0061]    The base plate  402  may, for example, comprise a cylindrical promontory, around which a coil  335  is wound as e.g. shown in  FIG. 4   c.  Alternatively, the coil  335  may be pre-wound and have an internal diameter enabling the coil  335  to be mounted around the promontory of the base plate  402 . Additionally, a cover  490  may be cast around the coil  335  for example in order to protect the coil from damage and/or to enable easier handling of the bobbing  401 . Alternatively, the cover  490  may be pre-cast and subsequently mounted around the coil 
         [0062]    The bobbin  401  including a coil  335  may further include a cut-out  405  or a hole  405  of similar geometrical form as the protrusion  410 . The cut-out  405  may, for example, be in the base plate  402 . For example, if the probe  300  comprises a protrusion  410  which is cylindrical, then the bobbin  401  may comprise a cylindrical cut-out  405  or a cylindrical hole  405  enabling the bobbin  401  to be placed onto the probe such that the cylindrical cut-out  405  (or cylindrical hole  405 ) of the bobbin  401  is penetrated and filled by the protrusion  410 . 
         [0063]    The cut-out  405  (or the hole  405 ) in the bobbin  401  may be extending all the way through the bobbin  401  e.g. through the base plate  402  and the cover  490 . Alternatively, the cut-out  405  (or the hole  405 ) may be of a type not extending all the way through the bobbin  401  e.g. only in the base plate. 
         [0064]    If, for example, the probe  300  comprises two protrusions  410  which are cylindrical, then a bobbin  401  may comprise two cylindrical holes  405  enabling the bobbin  401  to be placed onto the two protrusions  410 . If, for example, the probe  300  comprises two protrusions, e.g. a cylindrical and a conical protrusion, the a bobbin  401  may comprise a corresponding cylindrical and a conical cut-out enabling the bobbin to be placed onto the cylindrical and the conical protrusions  410 . 
         [0065]    In an embodiment, the dimensions of the volume of the one or more holes  405  in a bobbing  401  may correspond to or substantially correspond to the dimensions of the volume of the at least one protrusion  410  onto which the bobbing  401  is to be mounted or is mounted. 
         [0066]    For example, if a bobbing  401  is to be mounted onto two protrusions  410  on a probe  300 , each of the protrusions  410  having a cylindrical shape with a volume of dimension Height×Diameter and the two protrusions  410  being separated by a distance Separation, e.g. measured from center to center of the two protrusions  410 , then the bobbing  401  to be mounted onto the two protrusions  410  may comprise two cylindrical holes  405  each with a dimension of Height×Diameter and the two holes  405  being separated by the distance Separation such that the bobbing  401  may be fitted onto the protrusions  410 . 
         [0067]    In an additional or alternative embodiment, the holes  405  of the bobbing  401  may have a volume slightly larger than the volume of the protrusions  410  in order to ensure that the bobbing  401  may be placed onto the protrusions  410  e.g. in order to encompass production uncertainties of the protrusions  410 . For example, if the dimensions of two cylindrical protrusions  410  are Height×Diameter, then the dimensions of the holes  405  of the bobbing  401  to be mounted onto the protrusions  410  may be (Height+ε)×(Diameter+ε). Alternatively, one of the dimensions of the bobbing  401  and volume of the hole  405  may be slightly larger than the corresponding protrusion  410  dimension e.g. if the dimensions of two cylindrical protrusions  410  are Height×Diameter, then the dimensions of the holes of the bobbing  401  to be mounted onto the protrusions  410  may be Height×(Diameter+ε). In the above, ε may be a real number greater than zero representing a distance. For example, ε may be chosen in the range of 0.01 mm-0.05 mm. 
         [0068]    By mounting the one or more bobbins  401  on the at least one protrusion  410  by means of said holes  405  or cut-outs  405  in said one or more bobbins  401 , e.g. by mounting four bobbins  401  each comprising two holes  405  on four times two protrusions  410 , enables the probe  300  to ensure good alignment of the coils  335  with respect to the probe  300 . 
         [0069]    The coil winding has to be done so the wires are arranged with equal number of turns in each layer. Also the placement of the wires has to be done nicely and tight to the coil bobbin. The reason for this, is to achieve the highest possible symmetrical magnetic system when placed on the probe head, this will lead to a minimum of cross coupling. A safe way to achieve this is to wind the coil directly onto the bobbin. 
         [0070]      FIG. 4   b  shows an additional or alternative embodiment in which a distal light-emitting end portion  103  of a probe  300  comprises at least one cut-out  452 . For example, the probe  300  may comprise two cut-outs  452  for each coil  335  to be mounted on the probe  300 . For example, if four coils  335  are to be mounted on the probe  300 , then the probe  300  may comprise four sets of two cut-outs  452  e.g. placed in a plane perpendicular to the longitudinal axis  301  of the probe  300  and with 90 degrees or substantially 90 degrees (e.g. 88-92 degrees) between each of the sets. Alternatively, the probe may comprise any number of cut-outs  452  greater than or equal to one for each coil  335  to be mounted on the probe  300 . 
         [0071]    The at least one cut-out  452  may be constructed as a hole extending inwards in the probe  300  e.g. in a direction perpendicular to the longitudinal axis  301  of the probe  300 . The at least one cut-out  452  may have any geometrical form such as for example a cylindrical hole as indicated in  FIG. 4   b.  Alternatively, the at least one cut-out  452  may have the geometrical form of a box or a cone or any other geometrical form cut-out from the probe  300 . 
         [0072]    Each of the one or more coils  335  may be included in a respective bobbing  401 . The bobbing  401  (a magnetic field generating device) may, for example, comprise a base plate  402  (a support member) and a coil  335  (means for generating a magnetic field). 
         [0073]    The base plate  402  may, for example, comprise a promontory e.g. a cylindrical promontory, around which a coil  335  may be wound as e.g. shown in  FIG. 4   c.  Alternatively, the coil  335  may be pre-wound and have an internal diameter enabling the coil  335  to be mounted around the promontory of the base plate  402 . Additionally, a cover  490  may be cast around the coil  335  for example in order to protect the coil from damage and/or to enable easier handling of the bobbing  401 . Alternatively, the cover  490  may be pre-cast and subsequently mounted around the coil 
         [0074]    Further, the bobbing  401  may further include one or more protrusions  451  of similar geometrical form corresponding to cut-outs  452 . For example, if the probe  300  comprises a cut-out  452  which is cylindrical, then a bobbing  401  may comprise a cylindrical protrusion  451  enabling the protrusions  451  of the bobbing  401  to be placed into the cut-out  452  such that the cylindrical protrusion  451  of the bobbing  401  penetrates and fills the cylindrical cut-out  452  of the probe. 
         [0075]    Alternatively, if the probe comprises a conical cut-out  452 , then the bobbing  401  may comprise a conical protrusion  451  enabling the protrusions  451  of the bobbing  401  to be placed into the cut-out  452  such that the conical protrusion  451  of the bobbing  401  penetrates the conical cut-out  452 . 
         [0076]    The cut-out  451  in the probe  300  may be extending all the way through the probe  300 . Alternatively, the cut-out  451  in the probe  300  may be of a type not extending all the way through the probe  300 . 
         [0077]    If, for example, the probe  300  comprises two cut-outs  451  which are cylindrical, then a bobbing  401  may comprise two cylindrical protrusions  451  enabling the protrusions  451  of the bobbing  401  to be placed into the two cut-outs  452 . If, for example, the probe  300  comprises two cut-outs  452 , e.g. a cylindrical and a conical cut-out, the bobbing  401  may comprise a corresponding cylindrical and a conical protrusion enabling the protrusions  451  of the bobbing  401  to be placed into the cylindrical and the conical cut-outs  452 . 
         [0078]    In an embodiment, the dimensions of the volume of the one or more protrusions  451  in a bobbing  401  may correspond to or substantially correspond to the dimensions of the volume of the at least one cut-out  452  into which the protrusions  451  of the bobbing  401  is to be mounted or is mounted. 
         [0079]    For example, if two protrusions  451  of a bobbing  401  are to be mounted into two cut-outs  452  in a probe  300 , each of the cut-outs  452  having a cylindrical shape with a volume of dimension Height×Diameter and the two cut-outs  452  being separated by a distance Separation, e.g. measured from center to center of the two cut-outs  452 , then the two protrusions  451  of the bobbing  401  to be mounted into the two cut-outs  452  may comprise two cylindrical protrusions  451  each with a dimension of Height×Diameter and the two cylindrical protrusions  451  being separated by the distance Separation, e.g. measured from center to center of the protrusions  451 , such that the two protrusions of the bobbing  401  may be fitted into the cut-outs of the probe  300 . 
         [0080]    In an additional or alternative embodiment, the protrusions of the bobbing  401  may have a volume slightly smaller than the volume of the cut-outs  452  in order to ensure that the protrusions  451  of the bobbing  401  may be placed into the cut-outs  452  of the probe  300  e.g. in order to encompass production uncertainties of the cut-outs  452 . For example, if the dimensions of two cylindrical cut-outs  452  are Height×Diameter, then the dimensions of the two protrusions  451  of the bobbing  401  to be mounted into the cut-outs  452  may be (Height−ε)×(Diameter−ε). Alternatively, one of the dimensions of the bobbing  401  protrusion volume may be slightly smaller than the corresponding cut-out  452  dimension e.g. if the dimensions of two cylindrical cut-outs  452  are Height×Diameter, then the dimensions of the two protrusions of the bobbing  401  to be mounted into the cut-outs  452  may be Height×(Diameter−ε). In the above, ε may be a real number greater than zero representing for example a distance. For example, ε may be chosen in the range of 0.01 mm-0.05 mm. 
         [0081]    By mounting the one or more bobbins  401 , each bobbing comprising a coil  335 , in the at least one cut-out  452  of the probe  300 , e.g. by mounting four bobbins  401 , each bobbing  401  comprising two protrusions  451 , in four times two cut-outs  452 , enables the probe  300  to ensure good alignment of the coils  335  with respect to the probe  300 . 
         [0082]      FIG. 5   a  shows an example of how two coils  335  in two bobbins  401  of a probe  300  may be connected in series e.g. via an electrical connection such as a wire. For example, if the probe of  FIG. 4  comprises four coils  335  in respective bobbins  401 , then the four coils  335  may be connected two and two in series e.g. via two electrical connections, one electrical connection for each coil pair. 
         [0083]    In  FIG. 5   a,  a first coil on a first bobbin  501  and a second coil on a second bobbin  502  are shown electrically connected in series. Using Amperes Law (and/or the right hand grip rule), the magnetic field of the first coil may be determined to be directed out of the figure plane and the magnetic field of the second coil may be determined to be directed into the figure plane when a (steady) current is flowing through the respective coils of the two bobbins  501  and  502 . 
         [0084]    If the second bobbin  502  is placed as coil  335   b  of  FIG. 3   a  and the first bobbin  501  is placed as coil  335   d  of  FIG. 3   a,  then the first and the second bobbins  501  and  502  may be placed as for example shown in  FIG. 5   b,  and then the magnetic field B 3  generated by the first bobbin  501  and the magnetic field B 1  generated by the second coil on the second bobbin  502  may add up to Btotal=B 1 +B 3  thereby increasing the magnetic field generated by the two coils on the bobbins  501  and  502  of probe  300 . 
         [0085]    Correspondingly, if a second set of two bobbins connected electrically in series are placed with a third bobbin, comprising a third coil generating a third magnetic field B 2 , as coil  335   c  of  FIG. 3   a  and a fourth bobbin, comprising a fourth coil generating a fourth magnetic field B 4  as coil  335   a,  then the magnetic field B 2  generated by a third coil on the second bobbin and the magnetic field B 4  generated by a fourth coil on the fourth bobbin may add up to B 2   tal =B 2 +B 4  thereby increasing the magnetic field generated by the two coils. The described addition of the magnetic vectors an orientation of the magnetic fields as indicated in  FIG. 5   a.  In order to achieve the maximum size of the B 2   tal  magnetic field the B 2  and B 4  should be aligned to be both parallel and on the same axis. The protrusions and cut-outs in bobbin and the probe ensure this alignment. 
         [0086]    Thereby, the probe  300  may provide a first Btotal and a second B 2 tal magnetic fields, both magnetic fields Btotal and B 2 tal directed in a direction perpendicular to a longitudinal axis  301  of the probe  300  and Btotal being perpendicular (or substantially perpendicular e.g. 90±0.25 degrees) to B 2   tal.    
         [0087]    The electrical wires connecting two coils in series may be twisted. Additionally, the electrical wires connecting the two coils in series with the probe may be twisted. As an alternative to twisted wires, coaxial cables may be used for one or more of the above named wirings. 
         [0088]    In an embodiment, the coils in each of the four coil housings  401  of e.g.  FIG. 4   a  may be identical. The coils may, for example, have the following values: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Parameter 
                 Value 
                 Unit 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Turns: 
                 40 
                   
               
               
                   
                 Wire diameter: 
                 50 
                 μm 
               
               
                   
                 Inductance: 
                 5.5 
                 μH 
               
               
                   
                 Copper 
                 3.7 
                 Ohm 
               
               
                   
                 resistance: 
               
               
                   
                 Q-factor: 
                 0.9 
               
               
                   
                   
               
             
          
         
       
     
         [0089]    In an embodiment, one or more coils  335  (e.g. in respective coil housings  401 ) may be driven as a series resonance circuit  600  as shown in  FIG. 6 . For example, if two coils  335  are driven in series, two coils  335  may be included in the resonance circuit  600 . The resonance circuit  600  may further comprise a resonance capacitor  601 . Further, the resonance circuit may comprise the two coils e.g.  603  and  604 . 
         [0090]    The capacitor  601  may, for example, be included in a printed circuit board (PCB)  605 . The distance between the PCB  605  and the coils  603  and  604  may, for example, be in the order of 120 mm thereby providing an effective inductance of 11.6 μH and 10.30 Ohm copper resistance of the resonance circuit  600 . 
         [0091]    In  FIG. 7 , the resonance circuit is included in a handheld probe  700 . The handheld probe includes a distal light-emitting end portion  103 , said distal light-emitting end portion comprising the one or more coils  335  (e.g. four coils) generating one or more magnetic fields (e.g. four magnetic fields B 1 -B 4 ). Additionally, the distal light-emitting end portion  103  of the handheld probe  700  may comprise an optical system  116 . 
         [0092]    Further, the handheld probe  700  may comprise a rod portion  336  which connects the distal light-emitting end portion  103  to a proximal part. The distal light-emitting end portion  103  is rigidly fixed to the rod portion  336  and the rod portion  336  is rigidly fixed to the proximal part  710  e.g. by use of glue or screws or any other means of rigidly fixing. 
         [0093]    A handheld probe may, for example, have a volume of 6 dm̂3. For example, the distal light-emitting end portion  103  and the rod portion  336  may have dimensions of approximately 3 mm in diameter and 100 mm in length. The proximal part  401  may have dimensions of approximately 100 mm in length, 55 mm in depth and 100 mm in height. 
         [0094]    The proximal part  710  may be rigidly fixed in connection with a number of light sources  720 , e.g. a number of light sources  720  may be contained in the proximal part  710 . For example, if the proximal part  710  contains four laser diodes  720 , then the four laser diodes  720  may be contained in the proximal part  710  e.g. by glue to one or more inside walls of the proximal part  710 . Alternatively or additionally, the four laser diodes may be fixed to the inside of the proximal part by one or more screws for each of the four laser diodes  710  contained in the proximal part  710 . 
         [0095]    The handheld probe  700  may further comprise a number of light guides  102  such as one or more optical fibers. In an embodiment, the handheld probing device  700  comprises a light guide  102  for each of the light sources  720  rigidly fixed in connection with the proximal part  710 . For example, if the handheld probe  700  comprises four laser diodes  720 , then the handheld probe  700  may further comprise four optical fibers  102 . One or more of the optical fibers may, for example, be single mode optical fibers. Alternatively or additionally, one or more of the optical fibers may be multimode optical fibers. Alternatively, the proximal part  710  may comprise one or more single mode fibers and one or more multimode fibers. 
         [0096]    A first end of each of the light guides  102  may be optically coupled to a rear surface  104  of the optical system  116  in the distal light-emitting end portion  103 . For example, the first end of each of the light guides  102  may be glued to the optical system  116  via an optical adhesive. For example, if the handheld probing device comprises four multimode optical fibers, the first end of each of the four multimode optical fibers may be glued to the rear surface  104  of the optical system  116 . Alternatively or additionally, the optical system  116  may comprise one or more recesses such that the first end of each light guide  102  may be frictionally coupled to the optical system  116 . 
         [0097]    A second end of each of the light guides  102  may be optically coupled to a laser diode  720 . For example, the second end of each of the light guides  102  may be glued to a respective one of the one or more light sources  720 . For example, if the handheld probing device comprises four laser diodes  720  and four multimode optical fibers  102 , then the second end of each of the four multimode optical fibers may be glued to a respective one of each of the four laser diodes  720 . Alternatively or additionally, the second end of each of the four multimode optical fibers may be frictionally coupled to respective ones of the four laser diodes  720  e.g. via a frictional coupling. 
         [0098]    Additionally, the probe  700  may comprise one or more PCBs, each PCB including a resonance capacitor  601 . For example, if the probe comprises four coils  335   a - 335   d,  then the probe  700  may comprise two PCBs, each PCB being in connection with two coils. The one or more PCBs may be included in a shielding box  701  e.g. a box made of aluminium. If the probe comprises four coils  335 , the shielding box  701  may, for example, comprise the two PCBs. The shielding box  701  may prevent one or more magnetic fields generated by the one or more PCBs in the shielding box  701  from perturbing the magnetic fields generated by the one or more coils  335  and vice versa. A perturbing magnetic field from the electrical circuit in the shielding box  701  could disturb the accuracy of the determination of the position of the probe  700  relative to the external sensors. 
         [0099]    If the probe  700  is connected to a signal/data processing unit  702 , then the electrical connection between the probe  700  and the signal/data processing unit  702  may be made via a twisted pair cable for each coil pair in the probe  700 . Alternatively, the electrical connection between the probe  700  and the signal/data processing unit  702  may be made via a coax cable of a commercially available make, such as the RG-158 coax cable with one RG-158 coax cable for each coil pair (e.g. sine wave modulating a first coil and a cosine wave modulating a second coil in each of the coil pairs is used for measuring the cross coupling) in the probe  700 . Thereby, cross coupling between the two coil pairs in a four coil probe may be minimized which is important to ensure a noise free signal from the probe. 
         [0100]    The table below shows an example of the cross coupling levels with either the use of twisted pair cable or RG-158 coax cable for a probe system as shown in  700 . 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Cable type 
                 Cross Coupling [dB] 
               
               
                   
                   
               
             
             
               
                   
                 Twisted pair 
                 −53 
               
               
                   
                 RG-158 Coax cable 
                 −69 
               
               
                   
                   
               
             
          
         
       
     
         [0101]    A cross coupling level below −60 dB may be required in order to calibrate the probe shown in  700 . With the above measures and the accurate mounting of the coils in the probe this may be achieved. When calibrating the sine modulated transmitter coil, a small signal may be coupled into the cosine modulated transmitter coil due to cross coupling. The small signal coupled into the cosine modulated transmitter coil may disturb the calibration of the sine modulated transmitter coil. Therefore, it may be required to minimize the cross coupling e.g. by using RG-158 coax cables. 
         [0102]    In the above embodiments, the light guides and the image guide have been shown as separate light guides. It will be appreciated that alternatively a single light guide may be used for both directing light to the tip of the probe and for transmitting the reflected light back to the CCD element. Such a combined guide may require an additional beam splitter, and may further have the disadvantage of a reduced signal to noise ratio. 
         [0103]    Hence, in the above, a probe of an apparatus has been disclosed which is suitable for obtaining geometrical data of the inner surface of a cavity, such as an ear canal. 
         [0104]      FIG. 8  is a side view showing the human ear and illustrating the use of the apparatus described herein. In use the probe  300  or the distal light-emitting end  103  and part of the rod portion  336  of handheld probe  700  is gently inserted into the ear  850 , and magnetic sensors  851  are placed in close relation to the patient&#39;s head. Placing the probe in the ear is done while objects in front of the probe are monitored as described above. A real picture may be obtained, and/or the distance to the tympanic membrane is measured as described herein. The picture captured this way is displayed on a monitor, such that the operator may know when the probe is approaching the tympanic membrane. Once the region near the tympanic membrane is reached, the measurements may commence. This may be done while retracting the probe as corresponding values of the distances to the canal wall and the position of the probe are recorded. The recording is continued until the probe reaches the outer regions of the outer ear. 
         [0105]    In the example of  FIG. 8 , at each sensor position A, B and C two ore more sensors  851  are located, which are designed to register the magnetic field in each their direction. Through this arrangement the exact location and orientation of the tip of the probe can be determined at any time. In the case shown in  FIG. 8 , the probe  300  or the distal light-emitting end  103  and part of the rod portion  336  of the handheld probe  700  is located inside the canal of a human ear  850 , shown schematically in the figure. The three sensor locations are arranged in a fixed construction, which in use is held immobilized relative to the patient&#39;s head. In the embodiment shown in  FIG. 8 , the fixed construction comprises a tripod, whereby each of the sensor positions are placed at the outer end of each of the branches  852  of the tripod. The tripod is only schematically indicated in  FIG. 8  and will be designed such that it does not cover the ear opening of the user when the sensors A, B and C are mounted to the head. In use, a coil at the tip of the probe is driven at a fixed frequency and by using a lock-in procedure, thereby allowing any noise coming from other magnetic fields in the surroundings to be cancelled out from the sensor signals. 
         [0106]    In the embodiments of  FIG. 3   a  and  FIG. 5   a,  dimensions in the figures are given in millimetres. 
         [0107]    Although some embodiments have been described and shown in detail, the invention is not restricted to them, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilised and structural and functional modifications may be made without departing from the scope of the present invention. 
         [0108]    In device claims enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage. 
         [0109]    It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.