Patent Publication Number: US-10334163-B2

Title: Large field of view multi-camera endoscopic apparatus with omni-directional illumination

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a United States national stage application of International patent application PCT/IB2015/051379 filed on Feb. 24, 2015 that designated the United States, and claims foreign priority to International patent application PCT/IB2014/059299 filed on Feb. 27, 2014, and also claims foreign priority to International patent application PCT/IB2014/059273 filed on Feb. 26, 2014, the contents of all three documents being herewith incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The invention generally concerns a multi-camera imaging apparatus. In particular, the present invention relates to a multi-camera, large field of view apparatus to be used in especially endoscopic applications. The present invention also relates to the real-time image processing system that is responsible of generating panoramic or stitched image from the multiple camera inputs. 
     BACKGROUND 
     Today conventional endoscope systems are capable of forward viewing with around 170°×60° field of view (FOV). There are disclosure like U.S. Pat. No. 8,419,630 B2, US 20110196200A1 to extend the field of view using pure optical methods. The present invention is using a multi-camera approach which is mimicking insect eyes. By this way, the presented invention aims a wide angle of view, uniform resolution, and high resolution imager for especially endoscopic applications. 
     SUMMARY OF INVENTION 
     In a first aspect the invention provides a multi-camera hemispherical very wide field of view imaging apparatus with omnidirectional illumination capability. The apparatus comprises a cylindrical body, a hemispherical mechanical frame arranged on one end of the cylindrical body, a plurality of imaging channels, each imaging channel comprising at least an image sensor and related optics with a fixed focus appropriate for endoscopic imaging, the plurality of imaging channels being distributed over the hemispherical mechanical frame, a light source arranged centre-down at a back part of the plurality of imaging channels and inside or at the end of the cylindrical body. Each imaging channel comprises a plurality of lightning channels around their centre, each of the plurality of lightning channels comprising at least one microfiber light guide having a determined angle of curvature arranged to transmit the light from the light source. The imaging apparatus further comprises a control and processing circuit comprising a camera control unit, an illumination control unit, an illumination unit, a sample and capture unit, an image processing unit and an output interface to a PC. The camera control unit is configured to power each of the plurality of imaging channels and make automatic gain compensation for each imaging channel, the illumination control unit is configured for automatic intensity dimming, the sample and capture unit is an interface circuit for correct sampling, extraction and capturing frames of individual imaging channels, the image processing unit is configured for constructing a spherical panoramic image by applying a determined algorithm, and the output interface is arranged to output the spherical panoramic image to a system configured to visualize it. 
     In a second aspect, the invention provides a method for distributing imaging channels over a hemispherical mechanical frame of a very wide field of view imaging apparatus The method comprises defining a first distance departing from a centre of the hemispherical mechanical frame, smaller than or equal to a second distance at which an object to be viewed is separated from the centre of the hemispherical mechanical frame, generating a virtual hemisphere and making an equal area tessellation on this virtual hemisphere with an arbitrary number of imaging directions, whereby a centre of the virtual hemisphere corresponds to the centre of the hemispherical mechanical frame, and a radius of the virtual hemisphere is equal to the first distance, where the equal area tessellation comprises dividing a surface of the virtual hemisphere into the arbitrary number of equal sized surface areas. The equal area tessellation comprises starting with the arbitrary number of imaging directions located respectively at a centre of a corresponding one of the equal sized surface areas, and iteratively checking if a coverage of the first distance is achieved by adding or subtracting one imaging direction to the tessellation at each iteration. The checking comprises ensuring that a single imaging channel angle of view is greater than or equal to a maximum radial distance on the virtual hemisphere, inside any of the equal sized tessellation areas, the angle of view of the imaging channels being assumed as constraints dictated by the optics of the imaging channels chosen to be used. The method for distributing imaging channels further comprises, when the coverage of the first distance is achieved, stopping the iterative checking and fixing all the imaging directions used in the last iterative checking as a set of locations, whereby each location corresponds to a centre of a tessellation cell on the virtual hemisphere, and back projecting the locations from the virtual hemisphere to the hemispherical mechanical frame thereby obtaining the distribution of the imaging channels on the hemispherical mechanical frame. 
     In a third aspect the invention provides a method for constructing a final panoramic image, comprising, generating an output as a panoramic Bayer Pattern image and converting this image to a full-colour image, whereby each pixel of the resulting panoramic Bayer pattern image represents an intensity value of a colour channel of either red or green or blue wavelength, whereby the sequence of the colour channels on the Bayer pattern is any known Bayer pattern, the intensity value of each pixel being a combination of sampled intensity values of real image sensors of imaging channels placed on a hemispherical frame, each combination being based on a selection of candidate intensity values of the real image sensors of the imaging channels on the hemispherical mechanical frame, and a Bayer Pattern representation of the images is used during the selection of the intensity value candidates for each combination to generate each panoramic pixel intensity value. 
     In a preferred embodiment any Bayer pattern sequence is accepted as image sensors&#39; Bayer pattern images. 
     In a further preferred embodiment the selection of intensity values for each combination is determined by a trigonometric relation of a final panorama pixel viewing direction and an imaging channel sampling direction. 
     In a further preferred embodiment a panorama pixel viewing direction is defined as a vector starting from the centre of the hemispherical mechanical frame and directed through a point in the space at infinite proximity. 
     In a further preferred embodiment each panorama pixel direction is unique and not coincident. 
     In a further preferred embodiment each imaging channel&#39;s viewing direction is determined by using its placement and orientation on the hemispherical mechanical frame. 
     In a further preferred embodiment precise relative direction vectors of all the imaging channels with respect to the hemispherical mechanical frame centre are determined afterwards by using a calibration process with a calibration checker board. 
     In a further preferred embodiment the Bayer Pattern Panorama is demosaiced after it is generated by an image processing unit and is converted to a full colour image by using a means of interpolation method by the image processing unit. 
     In a further preferred embodiment the interpolation method takes into account the object boundaries in the image. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention will be better understood in view of the description of examples of preferred embodiments and in reference to the figures, wherein 
         FIG. 1  illustrates an example assembly of a multi-camera dome with imaging channels and illumination channels according to the invention; 
         FIG. 2  contains an unrolled view of the illustration in  FIG. 1  for a part of the imaging apparatus, where 6 cameras and the related illumination channels are shown; 
         FIG. 3  illustrates another example realization of the hemispherical frame where the illumination channels are distributed on the available surface after the placement of the cameras, according to the invention; 
         FIG. 4  shows a possible implementation for a different kind of endoscopic applications: (a) capsule type with two devices on the tips of an endoscopic capsule, the body of the capsule becomes the cylindrical part, and (b) surgical type with a long support as the cylindrical part; 
         FIG. 5  schematically illustrates a control and image processing circuit for endoscopic imaging apparatus; 
         FIG. 6  contains a Final Bayer Pattern Panoramic Image representation; and 
         FIG. 7  contains a block diagram for a multi-camera true-colour panorama generation. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     The invention generally concerns a multi-camera hemispherical very wide field of view imaging apparatus with omnidirectional illumination capability. Beyond the earlier endoscopic imaging systems which either belong to optical large angle lens—single image sensor combinations or planar multi image sensor types, the inventive imaging apparatus offers a hemi-spherically placed multi camera imaging apparatus. Important features of the inventive imaging apparatus that distinguish it from the previous counterparts at least comprise: 360°×90° degree FOV imaging capability with relatively high resolution and uniform resolution imaging capability, imaging direction selection capability, and fiber-optic omni-directional illumination distribution ability. 
     The general structure of example embodiments of the inventive imaging apparatus are illustrated in  FIG. 1  and  FIG. 2 . In  FIG. 1 , a possible way of placing imaging channels  3  on a hemispherical frame  2  with the illumination channels  1  is illustrated. Square shaped components in holes on the hemispherical frame  2  represent imaging channels  3 . The imaging channels  3 , may be preferably cameras comprising a lens—not illustrated in the figures—, and an image sensor—also not illustrated in the figures—. Relatively small cylindrical components illustrate the illumination channels  1 . Imaging channels  1 , may be preferably micro-fiber light guides. In  FIG. 2 , an unrolled portion of the hemispherical frame  2  from  FIG. 1  is illustrated to visually explain how the cameras  3  and the illumination channels  1  are arranged. An illumination source—not illustrated in the figures—may be remotely connected to the illumination channels  1 , therefore the illumination channels  1 , will be starting from the illumination source and ending at the surface of the hemispherical frame  2 . In  FIG. 1 , the illumination channels  1  are attached at the zero proximity of the imaging channels  3  for simplicity and better illumination. In a further preferred embodiment illustrated in  FIG. 3 , there is another way of distributing the illumination channels  1  independent from the imaging channels  3 . In that case there will be additional holes for illumination channels on the hemispherical frame  2  distributed uniformly and in a sense to fill all the surface of the hemispherical frame  2  that is available after placement of the imaging channels  3 . The case illustrated in  FIG. 3  may be implemented in case of manufacturing restrictions. 
     The proposed device aims to provide a very wide field of view, preferably 360°×90°, as a multi-camera endoscopic imaging system. The main part of the whole system is described below. 
     1. Multichannel Imagers
     i. Each imaging channel  3  is composed of an image sensor and related optics with a fixed focus appropriate for endoscopic imaging.   ii. The distribution of the imaging channels  3  is as described above and the steps can be listed as below:
       a) Determine the distance for full coverage desired as a first distance.   b) Generate the virtual hemisphere at the determined first distance at step 1.ii.a) and make an equal area tessellation on this virtual hemisphere.   c) For the tessellation; each tessellation has a number of cells, which corresponds to a surface area portion on the total surface of the virtual hemisphere. The number of the cells are equal to number of imaging channels  3 . Then, start with a number of imaging channels  3 , and iteratively check if the coverage of the surface area of the virtual hemisphere is achieved by adding (or maybe subtracting)  1  imaging channel  3  each iteration. The coverage can be ensured as follows: the centre of the viewing direction of each imaging channel  3  is coincident with centre of one unique tessellation cell. Each cell is mapped to the one unique imaging channel  3 . If all the points of the cell is within a circle that has radial diameter equal to the angle of view of imaging channel  3 , then the area portion occupied by the related cell is said to be totally covered. If all the cells are totally covered, it is said to be full coverage of the virtual hemisphere achieved.   d) When the full coverage of the virtual hemisphere is achieved, stop the iterations and back project the centre of tessellation cells on the actual hemispherical frame  2 . These are the imaging channel  3  positions.   e) Make a validation in terms physical aspects. If there is and inconsistency, turn back to the step 1.i.a) with the information from step 1.i.e). This means to tell how much we are offset from the physical needs and then fix the tessellation centres for the cameras that are physically bounded then make the placements optimization for the rest of the tessellation cell centres. Then continue with new validation until the placement is achieved in terms of both coverage distance and physical constraints.   
       iii. In international publication WO2012056437A1, a placement for the cameras on a spherical surface is described in a layer by layer basis. This kind of arrangement becomes inefficient for a device that is miniaturized for applications like endoscopy. The surface area and the volume of the hemispherical mechanical frame is a bounding criteria for the number and the locations of the cameras. The present invention discloses a method especially for miniaturized applications like endoscopy by this way.   

     2. Multichannel Fibre-Light Guides
     i. Source of light is at the centre-down cylindrical part  4  of the apparatus (can be 1 or more LEDs or other means of light) and will be at a remote position.   ii. Imaging channels  1  will have an appropriate angle of curvature to distribute the light sufficiently.   iii. Each imaging channel  3  (image sensor with optics) will have four or more illumination channels  1  around.   iv. Each illumination channel  1  may have more than one micro-fiber light guide or the diameter of the fiber light guide may be varied accordingly to have sufficient light. The surface area and inner volume of the apparatus is a bounding criteria for the amount and the size of the micro-fiber light guides.   v. As stated above in the description of the invention section, second paragraph and illustrated in  FIG. 3 , another implementation method involves distributing sufficient number of illumination channels  1  on the hemispherical mechanical frame  2  independent from the imaging channels  3 .   vi. In international publication WO2012056437A1 a general system is described for different imaging applications, however there are no means of illumination or illumination channels since the mentioned system is not proposed for endoscopic application specifically. Here we consider endoscopic applications where the illumination channels are crucial and have a high importance.   

     3. Control and Processing Circuit 
     The control and processing circuit  5  is illustrated in  FIG. 5  and has following tasks:
     i. Camera Control  6 : Power on off the imaging channels  3  (with different patterns for still image capturing or same time for synchronised video capturing) Make automatic gain compensation for each imaging channel  3 .   ii. Illumination Control  7  and Illumination Circuit  8 : Combined with the feed-back from imaging channel  3  images, make automatic intensity dimming for illumination channels  1 .   iii. Sample and capture circuit for multi-cameras  9 : Interface circuit for correct sampling extraction and capturing as Bayer pattern frames of individual imaging channel  3 .   iv. Image processing for composite image  10 : Applying the below described algorithm in description of invention section  4 , create composite spherical panoramic image.   v. PC Interface  11 : Interfacing via a standard bus (such as Ethernet, PCIe, HDMI . . . etc) sends the needed information to a PC environment (not represented in the figure).   vi. Endoscopic Imaging Apparatus: cameras and imaging channels  12 : Is representing the apparatus composed of the hemispherical frame  2 , imaging channels  3 , and the illumination channels  1 .   

     The arrows  13 , in the  FIG. 5  is for indicating the data flow among the control and processing unit  5 , and the Endoscopic imaging apparatus  12  as well. 
     4. True Colour Panorama Image Processing Circuit 
     This algorithmic technique is based on the omega vector generation technique introduced previously in international publication WO2012056437A1. The block diagram of the control and processing circuit that will be realizing the algorithmic technique is illustrated in  FIG. 5 . 
     The main difference from previous approaches and benefits: for each imaging direction of the final panoramic image there is another degree of freedom that is the colour component of the direction. In this scheme, we have just one colour component (R/G/B) for each direction. By this way there will be a Bayer pattern for the final panorama. This Bayer pattern will be used to interpolate the final RGB panorama in one Bayer-to-RGB step. By this way the Bayer-to-RGB conversion at camera level for each camera is avoided. For circuit implementation, this eliminates the hardware cost of the individual Bayer-to-RGB conversion circuit for each camera output. This brings a significant resource saving especially when number of cameras grows. Also the colour and resolution quality of the final panorama will be improved since the sampled unprocessed true intensity inputs of the sensors are used at the final panorama construction step. That is why this technique is called “multi-camera true-colour panorama demosaicing” in this document. For endoscopic image processing applications such as bleeding detection and cancer tissue detection, real colour values have importance as features of the observed objects inside the body and colour processing capability is an important feature as well. By using high precision Bayer data instead of pre-processed colour data from the image sensor it is also aimed to achieve high precision colour processing for future applications desired in endoscopic imaging. 
     The steps of the implemented method are as follows: 
     An output is generated as a panoramic Bayer Pattern image and this image is converted to a full-colour image. 
     Each pixel of the resulting panoramic Bayer pattern image as illustrated in  FIG. 6  represents an intensity value of a colour channel of either red or green or blue wavelength. 
     The sequence of the colour channels on the Bayer pattern may be any known Bayer pattern as described in reference [2]. 
     The intensity value of each pixel is a combination of sampled intensity values of real image sensors of the imaging channels  3  placed on the hemispherical frame  2 , which are illustrated in  FIG. 3 . 
     Each combination is based on a selection of the candidate intensity values of the real image sensors of the imaging channels  3  on the hemispherical mechanical frame  2 . 
     A Bayer Pattern representation of the images is used during the selection of the intensity value candidates for each combination to generate each panoramic pixel intensity value. Any Bayer pattern sequence as described in [2] may be accepted as image sensors&#39; Bayer pattern images. 
     The selection of intensity values for each combination is determined by the trigonometric relation of the final panorama pixel viewing direction and the real image sensor sampling direction. 
     The panorama pixel viewing direction is defined as a vector starting from the centre of the hemispherical mechanical frame  2  and directed trough a point in the space at infinite proximity. 
     Each panorama pixel direction should be unique and should not be coincident. 
     Each imaging channel&#39;s  3  viewing direction is determined by using its placement and orientation on the hemispherical mechanical frame  2 . 
     If needed precise relative direction vectors of all the imaging channels  3  with respect to the hemispherical mechanical frame  2  centre may be determined afterwards by using a calibration process as defined in [ref1]. 
     The Bayer Pattern Panorama is demosaiced afterwards and converted to a full colour image by using any means of interpolation method, preferably one that takes into account the object boundaries in the image. 
     REFERENCES 
     
         
         [ref1] “http://www.vision.caltech.edu/bougetj/calib_doc/” 
       
    
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