Patent Publication Number: US-8988518-B2

Title: Medical imaging system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image capturing system, and more particularly to a medical imaging system achieving a smaller volume and higher luminous efficiency. 
     2. Description of the Prior Art 
     Image capturing technology, particularly that for a biological cavity, is required to have an adequate external illumination that can pass through the opening of the cavity to provide illuminating light with sufficient luminance, for there is no light source illuminating the interior of the biological cavity and the opening of the biological cavity is often relatively small, so that the imaging system may capture reflected image forming light to render a clear and accurate image within the cavity for observation or filming. 
     A prior art illuminating system employed a ring-shaped hollow reflective mirror to reflect light rays of a light source in coordination with a lens set to direct illuminating light into an opening of a cavity. The image light passed through the hollow region of the ring-shaped hollow reflective mirror to form an image. However, normally the luminous intensity distribution of a light source is a Lambertian distribution or Gaussian distribution that has higher intensities in the central region. The ring-shaped hallow reflective mirror was unable to utilize this central portion of the light to provide illumination, thereby causing a low luminous efficiency. 
     Another prior art illuminating system used a point light source with a reflective mirror and lens set to focus illuminating light at an opening of a cavity, and then captured image forming light from the cavity with an image forming system off axially disposed with respect to the illuminating system to form an image. Although the problem of low luminous efficiency was solved, the off-axis configuration of the illuminating system and the imaging system required more optical components, and raised the manufacturing cost. Besides, the volume and length of the system were also increased, which limited the field of view of the system, and hindered the design of a large field of view and large aperture system. As a result, the illuminating view and the imaging view were different, lowering the capturing accuracy. 
     In summary, it is highly desirable to provide a system with a reduced volume and higher luminous efficiency. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a medical imaging system which configures a light source according to the object-image relationship of lens, thereby allowing illuminating light to enter an interior of a cavity sufficiently, and integrates an imaging component and illuminating component into one system, thereby achieving advantages of smaller volume and lower cost. 
     According to an embodiment, the medical imaging system for capturing an image of a cavity with an opening comprises a first lens set, a light source and an image forming module. The first lens set defines a focal point, a first optical axis position and a second optical axis position, wherein the first optical axis position and the second optical axis position are respectively located on the opposite sides of the first lens set on an optical axis and satisfy the following relationship: 
     
       
         
           
             
               
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     wherein D1 is a distance from the first optical axis position to the first lens set, D2 is a distance from the second optical axis position to the first lens set, f is a focal length of the first lens set, and the distance from the first optical axis position to the first lens set is larger than twice the focal length of the first lens set. The light source is disposed off the optical axis of the first lens set, has a distance with the first lens set larger than the focal length and smaller than the distance between the first optical axis position and the first lens set, and is coupled optically with the first lens set to provide illuminating light, wherein the illuminating light directly irradiates on the first lens set, passes through the first lens set, converges in the cavity and then diverges to illuminate the interior of the cavity. The image forming module is disposed on the optical axis for receiving an image forming light reflected by the interior of the cavity through the first lens set to form the image. 
     The objective, technologies, features and advantages of the present invention will become more apparent from the following description in conjunction with the accompanying drawings, wherein certain embodiments of the present invention are set forth by way of illustration and examples. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating the medical imaging system according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram illustrating the method for determining the position of the light source of the medical imaging system according to an embodiment of the present invention; 
         FIG. 3  is a schematic diagram illustrating the first optical polarizing sheet and the second optical polarizing sheet of the medical imaging system according to an embodiment of the present invention; 
         FIG. 4A  is a schematic diagram illustrating the light source of the medical imaging system according to an embodiment of the present invention; and 
         FIG. 4B  is a schematic diagram illustrating the light source of the medical imaging system according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1  and  FIG. 2  simultaneously,  FIG. 1  is a schematic diagram of the medical imaging system according to an embodiment;  FIG. 2  is a schematic diagram illustrating the method for determining the position of a light source of the medical imaging system according to an embodiment. The medical imaging system of the present invention is for capturing an image in a cavity  1  with an opening  11 . In the description below, the directions up, down, right and left accord with the relative positions shown in figures for the convenience of illustration, and should not be considered as limitations of the present invention. The medical imaging system according to an embodiment includes: a first lens set  20 , a light source  30  and an image forming module  40 . The first lens set  20  has a focal point F with a focal length f. A optical axis X passes through the first lens set  20  and has a first optical axis position A and a second optical axis position B defined thereon, wherein the first optical axis position A and the second optical axis position B are respectively on the opposite sides of the first lens set  20  and satisfy the following relationship: 
     
       
         
           
             
               
                 
                   
                     
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     It can be understood that eq. 1 is the image formation formula of a lens, wherein D1 is the distance from the first optical axis position A to the first lens set  20  and D2 is the distance from the second optical axis position B to the first lens set  20 . The light source  30  includes at least a point light source, which may be a light-emitting diode or a light-emitting fiber optics. The light source  30  is disposed off the optical axis X and its distance to the first lens set  20  is greater than the focal length f and smaller than the distance from the first optical axis position A to the first lens set  20 . Preferably, the distance D1 from the first optical axis position A to the first lens set  20  is greater than twice the focal length f. The cavity  1  is disposed on the left side of the first lens set, and the distance from the opening  11  of the cavity  1  to the first lens set  20  is substantially equal to the distance D2 from the second optical axis position B to the first lens set  20 . 
     Continuing the above description, the light source  30  is optically coupled with the first lens set  20  and provides illuminating light  301 , wherein the illuminating light  301  passes through the first lens set  20 , converges inside the cavity  1  and then diverges, thereby illuminating a surface  12  of the interior of the cavity  1 . The image forming module  40  is disposed on the optical axis X. According to an embodiment, the image forming module  40  includes a second lens set  41  and an image sensing element  42 , and is for accepting image forming light  302  reflected by the surface  12  of the cavity  1 . The image forming light  302  passes through the opening  11  of the cavity  1 , the first lens set  20  and the second lens set  41  in sequence and reaches the image sensing element  42  to form an image. The image sensing element  42  may be a CCD (charged coupled device), CMOS (complementary metal oxide semiconductor) sensor, film or the combination thereof. According to an embodiment, the image forming module  40  further includes an diaphragm  43  disposed between the first lens set  20  and the light source  30  for limiting light outside the optical axis X from entering the image forming module  40 . It can be understood that the first lens set  20  and/or the second lens set  41  may be variable focal length lens sets so that the focal length may be adjusted to form a clear image. 
     Referring  FIG. 1  and  FIG. 3  simultaneously,  FIG. 3  is a schematic diagram of a first optical polarizing sheet and a second optical polarizing sheet of the medical imaging system according to an embodiment. As illustrated in  FIG. 1 , according to an embodiment, the first medical imaging system further includes a first optical polarizing sheet  71  and a second optical polarizing sheet  72 , wherein the first optical polarizing sheet  71  is disposed between the light source  30  and the first lens set  20 , and is optically coupled with the illuminating light  301  to polarize the illuminating light  301 ; the second optical polarizing sheet  72  is disposed between the image forming module  40  and the first lens set  20  and is optically coupled with the image forming light  302  to polarize the image forming light  302 . The polarization directions of the first optical polarizing sheet  71  and the second optical polarizing sheet  72  are different. For example, the first optical polarizing sheet  71  is a horizontal polarizing sheet and the second optical polarizing sheet  72  is vertical polarizing sheet. The light source  30  emits light passes through the first optical polarizing sheet  71  and then reflected and directed by the first lens set  20  towards the image forming module  40 . Because the polarization directions of the light emitted and the second optical polarizing sheet  72  are orthogonal to each other, the light emitted is not able to pass through the second optical polarizing sheet  72  and only the image forming light  302  with the same polarization direction as the second optical polarizing sheet  72  is allowed to pass, thereby enhancing image quality. As illustrated in  FIG. 3 , according to an embodiment, the first optical polarizing sheet  71  may be annularly disposed around the second optical polarizing sheet  72 , but the present invention is not limited to be implemented as such. It can be understood that the first optical polarizing sheet  71  may be integrated with the light source  30  as a single module for emitting the illuminating light  301  with a specific polarized direction. 
     According to an embodiment, the medical imaging system further includes a light filtering sheet optically coupled with the light source for emitting the illuminating light  301  with a specific range of wavelengths. Alternatively, in another embodiment, the light filtering sheet may be optically coupled with the image forming module  40 , so that the image forming module  40  only receives image forming light  302  with a specific range of wavelengths. Preferably, as illustrated in  FIG. 1 , the medical imaging system according to an embodiment further includes two light filtering sheets  80 ,  81  disposed respectively for coupling optically with the illuminating light  301  and the image forming light  302 . For example, in the embodiment illustrated in  FIG. 1 , the light filtering sheet  80  is disposed between the light source  30  and the first optical polarizing sheet  71 ; the light filtering sheet  81  is disposed between the second lens set  41  of the image forming module  40  and the image sensing element  42 , but the present invention is not limited to be implemented as such. The light filtering sheets  80 ,  81  are for filtering out a wavelength range of light. Based on the characteristics of the light source  30  and the surface  12  of the cavity  1 , only the illuminating light  301  and image forming light  302  with a specific wavelength is allowed to pass through, thereby enhancing the application range and the image quality. 
       FIG. 2  and the description below illustrate the method for determining the position of the light source in the medical imaging system of the present invention. It is noted that for convenience of illustration, the image forming module  40  is omitted in  FIG. 2 . Suppose there is an image I with an image height r on the second optical axis position B. Then according to the image formation formula of a lens (eq. 1), there is a virtual object O in the direction of the first optical axis position A. The apex of the object O is defined to be an off-axis position C, and the projected position of the off-axis position C on the optical axis X is the first optical axis position A. The distance between the off-axis position C and the first optical axis position A is the height of the object O and the value of which is equal to the image height r multiplied by the magnification factor of the first lens set  20 . 
     Suppose the object O emits light of a first light cone  501  from the off-axis position C. The first light cone  501  includes upper edge light  5011  of the first light cone  501  and lower edge light  5012  of the first light cone  501 , wherein the upper edge light  5011  of the first light cone  501  intersects with the first lens set  20  at the first optical border  201 ; the lower edge light  5012  intersects with the first lens set  20  at the second optical border  202 . The off-axis position C and the first optical border  201  are on the opposite sides of the optical axis X, and the first optical border  201  and the second optical border  202  are on the opposites of the optical axis X. It has to be clarified that the first optical border  201  and the second optical border  202  refer to the borders within which the first lens set  20  may generate optical effects, e.g. the light emitted by the light source  30  may couple optically with the object O on the first optical axis position A and form the image I on the second optical axis position B. 
     Continuing the above description, the object O emits the light of a second light cone  502  on the first optical axis position A. The second light cone  502  includes upper edge light  5021  of the second light cone  502  and lower edge light  5022  of the second light cone  502 , respectively intersect with the first lens set  20  at the first optical border  201  and the second optical border  202 . The light source  30  is disposed at the intersecting point E of the connecting line of the off-axis position C and the first optical border  201  of the first lens set  20 , and the connecting line of the first optical axis position A and the second optical border  202  of the first lens set  20 . Then, the illuminating light  301  emitted by the light source  30  disposed at the intersecting point E would cover the range between the first optical border  201  and the second optical border  202  of the first lens set  20 , i.e., the illuminating light  301  travels along the upper edge light  5011  of the first light cone  501  and the lower edge light  5022  of the second light cone  502  to couple optically with the first lens set  20 , and converges between the focal point F of the first lens set  20  and the surface  12  of the interior of the cavity  1 , rather than converges at the second optical axis position B or the focal point F of the first lens set  20 . Therefore, the height of the image I is smaller than half of the inner radius of the opening  11  whereby the illuminating light  301  may sufficiently enter the interior of the cavity  1 , significantly increasing the luminous efficiency. 
     Besides, as shown in  FIG. 2 , the deflection angle of the illuminating light  301  along the path of the upper edge light  5011  of the first light cone  501  after coupling optically with the first lens set  20  is smaller than the deflection angle of the illuminating light  301  along the path of the lower edge light  5022  of the second light cone  502  after coupling optically with the first lens set  20 . Hence, the illuminating light  301  within the space between the second optical axis position B and position where the illuminating light  301  converges in the cavity  1  has a smaller height variation, thereby allowing the illuminating light  301  to have a larger tolerance moving forward or backward. That is, as long as the opening  11  of the cavity  1  is disposed within this range, the illuminating light  301  may sufficiently enter the interior of the cavity  1 . It can be understood that the medical imaging system of the present invention may adjust the position where the light source  30  is disposed (the intersecting point E) along the direction perpendicular to the optical axis X according to the size of the opening  11  of the cavity  1 . When the intersecting point E is located further from the optical axis X, a wide field and large aperture image forming module  40  may be used. Since the medical imaging system of the present invention integrates the image formation and illuminating elements within a system, it has the advantages of smaller volume and lower cost. In addition, the cavity  1  may be an eye, and the opening  11  may be a pupil. The cavity  1  may also be organs such as an ear, nose, throat, skin, abdomen and stomach. The medical imaging system of the present invention may be widely applied in the fields of endoscopy (optical tube type or capsule type) and biological microscopy or any class of digital medical imaging system. 
     Moreover, the illuminating light  301  of the light source  30  is directly optically coupled with the first lens set  20 , so the distribution of luminance would not be non-uniform when the light source  30  is disposed off the optical axis X, thereby increasing the luminous efficiency. Also, because the light source  30  is disposed off the optical axis X, there will be no phantom image problem generated during image formation. According to an embodiment, the opening  11  of the cavity  1  is symmetrical with respect to the optical axis X, and the inner radius of the opening  11  is twice the image height r, and the illuminating light  301  pass through the range of half the cross section of the opening  11 . Preferably, the illuminating light  30  is an annular light source such as one shown in  FIG. 4A , a schematic diagram illustrating the light source of the medical imaging system according to an embodiment. The light source  30  may be a plurality of light-emitting diodes  31  arranged annularly around and symmetrically with respect to the optical axis X, thereby the illuminating light  301  may sufficiently cover the opening  11 , and providing uniform illumination. The illuminating light  301  converges inside the cavity  1  into an annular light spot and then diverges to illuminate the interior of the cavity  1 . It can be understood that the aforementioned annular light source may also be formed by a plurality of light-emitting fiber optics arranged annularly. In another embodiment, the light source  30  further includes an annular light guide element (not illustrated), forming an annular light source. According to another embodiment, the medical imaging system further includes a reflecting device  60 . As illustrated in  FIG. 4B , the light source  30  is an annular light source, and the reflecting device  60  may be a hallow reflector cover coupled optically with the light source  30  for reflecting the light emitted by the light source  30  to the first lens set  20  to increase the luminous efficiency. 
     In summary, in the medical imaging system of the present invention, the off-axis position of the light source is determined according to the intersecting point of the edges of the first light cone and the second light cone that form an image on the second optical axis position after an object on the first optical axis position is optically coupled with the first lens set. The light source is off axially disposed and the illuminating light emitted is directly optically coupled with the first lens set, and therefore does not results in phantom images or non-uniform luminance distribution. The illuminating light passes through the first lens set and converges inside the cavity, providing sufficient illumination at the opening of the cavity, and greatly enhancing the luminous efficiency. In addition, disposing the light source on an off-axis position allows a greater tolerance for moving the light source back and forth, and integrating the imaging and illuminating components within one system may provide the advantages of reduced volume and lower cost. 
     While the invention is susceptible to various modifications and alternative forms, a specific example thereof has been shown in the drawings and is herein described in detail. It should be understood, however, that the invention is not to be limited to the particular form disclosed, but to the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims.