Abstract:
A volume holographic imaging system, apparatus, and/or method enables the projection of a two-dimensional (2D) slice of a four-dimensional (4D) probing object A 4D probing source object is illuminated to emit or scatter an optical field A holographic element having one or more recorded holograms receives and diffracts the optical field into a diffracted plane beam having spectral information A 4-ftelecentπc relay system includes a pupil filter on the relayed conjugate plane of the volume hologram and images the pupil of the volume hologram onto the front focal plane of the collector lens A collector lens focuses the diffracted plane beam to a 2D slice of the 4D probing source object The focused 2D slice is projected onto a 2D imaging plane The holographic element may have multiple multiplexed holograms that are arranged to diffract light from the corresponding slice of the 4D probing source object.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 61/250,306, entitled “Phase Contrast Multi-Focal Microscope” filed Oct. 9, 2009, U.S. Provisional Application Ser. No. 61/264,432, entitled “Wavelength-Coded Multi-Focal Microscope” filed November 25, 2009 and U.S. Provisional Application Ser. No. 61/381,369, entitled “System, Method and Apparatus for Contrast Enhanced Multiplexing of Images” filed Sep. 9, 2010, each application in its entirety is incorporated herein by reference. This application is related to International Application PCT, Attorney Docket Number 118648-00220, entitled “System, Method and Apparatus for Contrast Enhanced Multiplexing of Images” filed Oct. 8, 2010, and International Application PCT, Attorney Docket Number 118648-00420, entitled “System, Method and Apparatus for Wavelength-Coded Multi-Focal Microscopy” filed Oct. 8, 2010, each application in its entirety is incorporated herein by reference. 
     
    
     STATEMENT OF GOVERNMENT INTEREST 
       [0002]    The United States government has rights in this application as a result of financial support provided by governmental agencies in the development of aspects of the disclosure. Parts of this work were supported by a grant from the National Institutes of Health, Grant No.: R21CA118167 and the National Science Council Contract No.: NSC-97-2917-1-564-115. 
     
    
     BACKGROUND 
       [0003]    This disclosure relates generally to imaging systems, methods and apparatus, and more particularly to volume holographic imaging systems, methods and apparatus that obtain enhanced images from multiple depths within an object. 
         [0004]    Microscopic imaging systems are beneficial for biomedical and clinical applications. Volume holographic microscopy (VHM) has been developed as a microscopic instrument for biological samples. Volume imaging systems have many useful applications such as spectral and three spatial dimensional biological imaging (known as four-dimensional (4D) imaging), endoscope imaging systems, spectrometers, and the like. 
         [0005]    VHM incorporates multiplexed holographic gratings within a volume hologram to visualize structures at different focal planes in an object. Each focal plane within the object is projected to a different lateral location on the camera. Thus, the entire object volume is imaged slice-wise onto the camera without the need for electrical or mechanical scanning. However, many objects of interest are composed of weak phase features with poor contrast and are barely observable with VHM. 
       SUMMARY 
       [0006]    Embodiments taught herein relate generally to imaging systems, methods and apparatus, and more particularly to volume holographic imaging systems, methods and apparatus that obtain enhanced images from multiple depths within an object. 
         [0007]    An exemplary contrast enhanced multiplexing image system taught herein obtains contrast enhanced information from multiple depths within an object without scanning. A phase filter is introduced into the Fourier plane of a 4-f telecentric relay system to enhance weak phase information from a volume holographic imaging system. The exemplary system can be expanded to include additional multiplexed holographic gratings within a single volume hologram and, hence, simultaneously image more object slices onto non-overlapping locations on an imaging plane without scanning. 
         [0008]    An exemplary microscope as taught herein includes focusing lenses, a holographic element, relay lenses, a phase filter and an imaging plane. The lenses, holographic element and phase filter together project an image onto the imaging plane. The phase filter is advantageously located at the conjugate plane of the holographic element&#39;s pupil. The holographic element is a volume hologram with one or more multiplexed hologram gratings therein. The multiplexed holographic gratings are located at the Fourier plane of the microscope and are Bragg matched to a different focal plane within an object and simultaneously projected to a different lateral location on the imaging plane. In the exemplary embodiments, the holographic element is recorded in phenanthrenquinone doped poly methyl methacrylate. 
         [0009]    An exemplary volume imaging system for imaging a source object as taught herein includes a holographic element, collector optics and a phase filter. The holographic element is capable of recording one or more holograms of the source object and is configured to receive and diffract an optical field emitted or scattered from the source object onto one or more diffracted plane beams. The collector optics are configured to focus each of the one or more diffracted plane beams to a two-dimensional slice of the source object, and simultaneously project the focused two-dimensional slice along an optical path onto an imaging plane. The phase filter is disposed along the optical path to eliminate the DC component in the spatial frequency domain of the focused two-dimensional slice of the source object. 
         [0010]    An exemplary method for imaging an object in four-dimensions and real time in which an emitted or scattered optical field of an object is received by a holographic element which diffracts the received optical field into one or more diffracted plane beams. The diffracted plane beams are focused into a two-dimensional slice of the object and filtered. The filtered two-dimensional slice is projected onto an imaging plane. When two or more slices of the object are projected, the slices are simultaneously projected onto non-overlapping regions on the imaging plane. The filtering step is performed using a phase filter. The diffraction is based on one or more Bragg degeneracy properties. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The foregoing and other objects, aspects, features, and advantages of exemplary embodiments will become more apparent and may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which: 
           [0012]      FIG. 1  depicts an illustrative diagrammatic view of a recording arrangement for multiplexing holographic gratings within a volume hologram as taught herein. 
           [0013]      FIG. 2  depicts an illustrative diagrammatic view of a volume holographic microscope as taught herein. 
           [0014]      FIG. 3  depicts an illustrative diagrammatic view of the 4-f telecentric relay system of the volume holographic microscope of  FIG. 2 . 
           [0015]      FIG. 4  is a flow diagram depicting an illustrative method for practicing an embodiment of a volume holographic imaging system as taught herein. 
           [0016]      FIG. 5  illustrates an image of a mouse colon sample obtained by a conventional volume holographic microscopy. 
           [0017]      FIG. 6  illustrates an image of a zoomed section of the mouse colon image obtained by conventional volume holographic microscopy in  FIG. 5 . 
           [0018]      FIG. 7  illustrates an image of another zoomed section of the mouse colon image obtained by conventional volume holographic microscopy in  FIG. 5 . 
           [0019]      FIG. 8  illustrates an image of the mouse colon sample used in  FIG. 5  obtained by an exemplary volume holographic microscope as taught herein. 
           [0020]      FIG. 9  illustrates an image of a zoomed section of the mouse colon image obtained by the exemplary volume holographic microscope in  FIG. 8  corresponding to the section of the mouse colon shown in  FIG. 6 . 
           [0021]      FIG. 10  illustrates an image of another zoomed in image of the mouse colon image obtained the exemplary volume holographic microscope in  FIG. 8  corresponding to the section of the mouse colon shown in  FIG. 7 . 
           [0022]      FIG. 11  is an illustrative graphical representation of the improvement in the on-axis modulation transfer function measurement resulting from the use of an exemplary volume holographic imaging microscope, as taught herein, taken along the x-direction. 
           [0023]      FIG. 12  is an illustrative graphical representation of the improvement in the on-axis modulation transfer function measurement resulting from the use of an exemplary volume holographic imaging microscope, as taught herein, taken along the y-direction. 
           [0024]      FIG. 13  is an illustrative graphical representation of the improvement in the off-axis modulation transfer function measurement resulting from the use of an exemplary volume holographic imaging microscope, as taught herein, taken along the x-direction. 
           [0025]      FIG. 14  is an illustrative graphical representation of the off-axis modulation transfer function measurement resulting from the use of an exemplary volume holographic imaging microscope, as taught herein, taken along the y-direction. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0026]    In accordance with various embodiments taught herein are single sideband edge enhancement volume holographic imaging systems that employ a phase filter to obtain phase contrast enhanced images from multiple depths within an object. An exemplary volume holographic imaging system can obtain contrast enhanced information from multiple depths within biological samples without scanning. An exemplary volume holographic imaging system enhances weak phase information of the displayed images which are from different depths within biological samples by introducing a phase filter at the plane conjugate to the volume holographic pupil during imaging. This enhances weak phase features from multiple depths. An exemplary volume holographic imaging system images the entire object volume in real time without electrical or mechanical scanning, and provides enhanced edge and phase information at all slices simultaneously. The volume holographic imaging system may be a microscope, spectroscope, endoscope, and the like and may be known as single sideband edge enhancement volume holographic microscope. 
         [0027]    A mouse colon placed in the exemplary imaging system as taught herein results in two-depth resolved images separated by approximately 50 μm simultaneously displayed on an imaging plane. With the exemplary volume hologram imaging method for weak phase enhancement, the exemplary system improves phase contrast of the object by up to 89.0 times over conventional VHM methods. 
         [0028]      FIG. 1  illustrates an exemplary recording arrangement  100  for multiplexing holographic gratings, or recording multiple holographic gratings, within a volume hologram  124  using a source of electromagnetic radiation such as a collimated laser beam. A holographic grating may be created in a transmissive volume hologram by recording the interference pattern of two mutually coherent light beams. In an exemplary embodiment, a collimated laser beam, not shown, is split into a reference arm  115  and a signal arm  117 . A point source  120  along the reference arm  115  is formed by lens  116 . The point source  120  provides the source of electromagnetic radiation along the reference arm  115  which interferes with the signal arm  117  to record a grating in the multiplexed volume hologram  124 . More than one grating is formed in multiplexed volume hologram  124  by varying the position of the point source  120  in the reference arm, for example, by moving lens  116  while lens  118  stays fixed, between different exposures of electromagnetic radiation from the collimated laser beam. The nominal inter-beam angle θ is the angle between signal arm  117  and reference arm  115  at the volume hologram  124  surface and is changed by Δθ between exposures. 
         [0029]    In some embodiments, the nominal inter-beam angle in air is 68°, Δθ is 1°, and Δz is 50 μm. In the same embodiment, the recording medium of volume hologram  124  is phenanthrenquinone doped poly methyl methacrylate (PQ-doped PMMA) and the collimated laser beam is an argon-ion (Ar + ) laser operating at a wavelength of approximately 488 nm. 
         [0030]    Other materials may be used as a recording medium. By way of example, Aprilis ULSH-500, LiNbO 3  including Zn-doped LiNbO 3  and DuPont photopolymers may be used as recording material. (See Atsushi Sato et al, Applied Optics vol. 42, pp. 778-784, (2003), Yasuo Tomita et al, Optics Express vol. 14, pp. 5773-5778 (2006), and Raymond K. Kostuk et al, Applied Optics vol. 38, pp. 1357-1363 (1999)). Those skilled in the art will appreciate that each material has a range of sensitivity for recording and that another source of electromagnetic radiation with appropriate wavelength in the corresponding range of sensitivity may be used for recording. With proper fabrication, the multiplexed holographic gratings within a volume hologram can operate at wavelengths longer than the recording wavelength of signal arm  117  and reference arm  115 . (See Y. Luo, P. J. Gelsinger, J. K. Barton, G. Barbastathis, and R. K. Kostuk, Opt. Lett. Vol. 33, 566-568 (2008) which is incorporated by reference herein in its entirety). In the same embodiment, the diffraction efficiencies of the two multiplexed gratings are approximately 40% and approximately 35%, the thickness of the PQ-doped PMMA recording material is approximately 1.5 mm, and the numerical apertures of lens  116  and lens  118  are 0.65 and 0.55, respectively. 
         [0031]      FIG. 2  illustrates an exemplary imaging system  200  which may be a SSEE-VHM system as taught herein. The system  200  includes an optional source of electromagnetic radiation  201 , an objective lens  222 , the multiplexed volume hologram  124 , relay lenses  232  and  234 , a phase filter  236 , a collector lens  226 , and an imaging plane  240 . Source  201  emits an electromagnetic field along signal arm  203  to object  210 . An objective lens  222  acts to collimate the optical field emitted or scattered from the object  210 . The collimated field passes through the multiplexed volume hologram  124  towards relay lenses  232  and  234 . The emitted holographic representation from the multiplexed volume hologram  124  is relayed by lenses  232  and  234  towards the knife filter  236 . The filtered representation from the knife filter  236  is collected by the collector lens  226  which projects images to the imaging plane  240 . In an exemplary embodiment, the multiplexed volume hologram  124  has two multiplexed gratings. Each grating is Bragg matched to a different two-dimensional (2D) slices of the object  210  taken along the y-axis at first focal plane  212  and second focal plane  214 . Thus, in the same embodiment, images of focal planes  212  and  214  are simultaneously projected by the system  200  to non-overlapping lateral locations,  242  and  244 , respectively, on the image plane  240 . The gratings are diffractive elements consisting of a periodic phase or absorption perturbation throughout the entire volume of the holographic element. When a beam of incident light satisfies the Bragg phase matching condition it is diffracted by the periodic perturbation. Those skilled in the art would appreciate that Bragg matched refers to satisfying the Bragg matching condition which occurs when the diffraction efficiency of a transmissive volume hologram is maximized. 
         [0032]    In an exemplary embodiment, the multiplexed volume hologram  124  is located at the Fourier plane of the objective lens  222 . Similarly, the imaging plane  240  is located at the Fourier plane of the collector lens  226 . In the same embodiment, the distance f o  is the distance between the second focal plane  214  and the objective lens  222 . Those skilled in the art would appreciate that the grating within multiplexed volume hologram  124  that is Bragged matched to the second focal plane  214  is located a distance of f o  from the objective lens  222 . Relatively positioned between the multiplexed volume hologram  124  and the collector lens  226  is a relay system composed of relay lenses  232  and  234 . Phase filter  236  is located such that it images the pupil of the multiplexed volume hologram onto the front focal plane of the collector lens  226 . The distance f c  is the distance between the phase filter  236  and the collector lens  226 , which is the same distance between the collector lens  226  and the imaging plane  240 . 
         [0033]    In exemplary embodiments, the source of electromagnetic radiation may be a plurality of coherent light sources, a broadband light source such as a dispersed white-light source with chromatic foci, a plurality of light emitting diodes or the like. The imaging plane  240  may be part of a charge couple device or camera which may be connected to or part of a computer, projector, or other such device. In some embodiments, the phase filter may be a knife edge filter, Zernike filter, or the like. 
         [0034]      FIG. 3  depicts an exemplary placement relationship of the relay system located between lenses  232  and  234  of the imaging system of  FIG. 2 . The relay system located between lenses  232  and  234  is a 4-f telecentric system. The distance f R  is the distance between the multiplexed volume hologram  124  and the relay lens  232 . The distance between the relay lenses  132  and  134  is two times the length of distance f R . The distance f R  is also the distance between the relay lens  234  and the phase filter  236 . Phase filter  236  is therefore located on the conjugate plane of the multiplexed volume hologram  124  relayed through the 4-f telecentric relay system, i.e. on the 4-f telecentric relay system&#39;s Fourier plane. The phase filter  236  eliminates all components to the one side of the DC component in the spatial frequency domain to achieve the single sideband edge enhancement method, as taught herein. The one-dimensional transmittance of the phase filter  236  at the Fourier space is given in Equation 1 as: 
         [0000]        t   filter ( f   y )=1+sgn( f   y )   (1)
 
         [0000]    where sgn is the signum function and sgn(f y )=1 at f y &gt;0; sgn(f y )=0 at f y =0; sgn(f y )=−1 at f y &lt;0. For a weak phase object, exp[jφ(y)]≈1+jφ(y) where φ(y) is the phase in the y-direction. When a weak phase object is placed in the exemplary imaging system, the resultant image, centered at the appropriate transverse location on the image plane, can be written in Equation 2 as: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    where I i  is the irradiance distribution of the image and FT is the Fourier transform. The Hilbert transform reduces the DC component and significantly enhances the detection sensitivity of phase jumps or edges. This enhancement is observed in parallel at all the multiplexed focal planes (slice-wise images from multiple depths within object  210 ) of the imaging system  200 . 
         [0035]      FIG. 4  depicts an exemplary method of imaging an object defined in four-dimensional space and real time using an exemplary volume holographic imaging system as taught herein. In step  400 , multiplexed volume hologram  124  receives an optical field that has been scattered or emitted from the object  210  of interest. In some embodiments, the scattered or emitted optical field may be processed by one or more optical elements, such as the objective lens  222 , to focus the received optical field onto the volume hologram  124 . In step  410 , a grating within the multiplexed volume hologram  124  diffracts the received optical field into one or more plane beams. The plane beam is a holographic representation of a 2-D slice of the object  210  taken at a plane within the object  210  that is Bragg matched to the grating in the volume hologram  124 . In step  420 , the Fourier transform of the plane beam is formed by the relay lenses  232  and  234  at an intermediate plane located at the phase filter  236 . In step  430 , the Fourier transform of the plane beam is filtered by the phase filter  236  to a filtered 
         [0036]    Fourier transform of the plane beam. In step  440 , the Fourier transform of the plane beam diffracted from the phase filter  236  is projected onto an imaging plane  240 . In some embodiments, the volume hologram  124  has two or more gratings recorded therein. In the same embodiment, the number of 2-D images that are simultaneously projected onto the imaging plane  240  in a non-overlapping manner corresponding to the number of gratings. Advantageously, the multiple images are simultaneously projected to non-overlapping portions of the imaging plane. 
         [0037]      FIG. 5  depicts a depth-resolved image of a mouse colon obtained using conventional VHM.  FIG. 6  depicts an arbitrarily zoomed-in section of the mouse colon of  FIG. 5  with interesting features visible in it.  FIG. 6  is the correspondingly noted portion of  FIG. 5 .  FIG. 7  depicts another arbitrarily zoomed-in section of the mouse colon of  FIG. 5  with interesting features visible in it.  FIG. 7  is also the correspondingly noted of  FIG. 5 . 
         [0038]      FIG. 8  depicts a depth-resolved image of mouse colon obtained using the exemplary imaging system as taught herein.  FIG. 9  depicts a zoomed-in section of the mouse colon of  FIG. 8  corresponding to the zoomed-in section of the mouse colon of  FIG. 6 .  FIG. 10  depicts a zoomed-in section of the mouse colon of  FIG. 8  corresponding to the zoomed-in section of the mouse colon of  FIG. 7 . 
         [0039]    The images in  FIGS. 5-8  were obtained by the same PQ-doped PMMA volume hologram with two multiplexed gratings, each grating imaging in parallel a different slice through the object: one slice just below the tissue surface and one approximately 50 μm in to the tissue. The mouse colon specimen was illuminated using a red LED with central wavelength of approximately 630 nm and spectral bandwidth of approximately 25 nm using the exemplary imaging system of  FIG. 2 . An Olympus objective lens (ULWDMSPlan50X), a Mitutuyo collector lens (MPlanAPO20X), and an Andor iXon CCD array (Andor X-2647) were used to produce the images of  FIGS. 5-8 . The field of view of this embodiment was approximately 1 mm by 0.8 mm. 
         [0040]      FIGS. 5 and 8  further include the contrast ratio of different tissue features calculated along an arbitrarily selected vertical line on the same corresponding locations between the conventional VHM and exemplary imaging system&#39;s images (right-hand side inset of  FIG. 5  and  FIG. 8 ). At four arbitrarily selected features,  501 ,  503 ,  505  and  507  in the two images, the contrast ratio in  FIG. 5  with conventional VHM was 4.6%, 0.1%, 2.6%, and 0.5%, respectively, while at corresponding locations,  801 ,  803 ,  805  and  807 , in  FIG. 8  using the exemplary imaging system as taught herein the contrast ratio was 15%, 8.9%, 8.5%, and 8.6%, respectively. The improvement in contrast ratio over the conventional VHM system varied from 
         [0000]    
       
         
           
             
               
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         [0041]    Advantageously the exemplary imaging systems taught herein increase the identification of structures, such as the turbid media depicted in  FIGS. 5-8 . The resulting images are contrast-enhanced, two-dimensional and observable in real time. Furthermore, exemplary imaging systems as taught herein can be applied to both fluorescence and non-fluorescence imaging and collects both spectral and spatial information of an object without mechanically scanning in the X-Y-Z direction for a given field of view. 
         [0042]      FIGS. 11-14  are graphical representations of the Modulation Transfer Function (MTF) against line pairs per millimeter which shows the improvement in contrast ratio between a conventional VHM system and an exemplary imaging system as taught herein. Those skilled in the art will understand that MTF, also known as spatial frequency response, is used to measure the contrast and resolution of an imaging system. The higher the MTF values the greater the contrast and resolution of an imaging system. MTF is the modulus, or absolute value, of the Optical Transfer Function (OTF) of an imaging system. Those skilled in the art will understand that the OTF describes the spectral variation of a electromagnetic signal as a function of spectral angular frequency. 
         [0043]      FIG. 11  depicts exemplary on-axis MTF measurements along the x-direction or sagittal direction for both a conventional VHM and an exemplary imaging system, as taught herein. The results for a conventional VHM system are depicted by trace  1101 . The results for an exemplary imaging system are depicted by trace  1103 . Trace  1103  quantifies that the exemplary imaging system has a higher MTF value for almost all number of line pairs per millimeter. 
         [0044]      FIG. 12  depicts exemplary on-axis MTF measurements along the y-direction or tangential direction for both a conventional VHM and an exemplary imaging system, as taught herein. The results for a conventional VHM system are depicted by trace  1201 . The results for an exemplary imaging system are depicted by trace  1203 . Trace  1203  quantifies that the exemplary imaging system has a higher MTF value for almost all number of line pairs per millimeter. 
         [0045]      FIG. 13  depicts an off-axis MTF measurements along the x-direction or sagittal direction for both a conventional VHM and an exemplary imaging system, as taught herein. The results for a conventional VHM system are depicted by trace  1301 . The results for an exemplary imaging system are depicted by trace  1303 . Trace  1303  quantifies that the exemplary imaging system has a higher MTF value for almost all number of line pairs per millimeter. 
         [0046]      FIG. 14  depicts an off-axis MTF measurements along the y-direction or tangential direction for both a conventional VHM and an exemplary imaging system, as taught herein. The results for a conventional VHM system are depicted by trace  1401 . The results for an exemplary imaging system are depicted by trace  1403 . Trace  1403  quantifies that the exemplary imaging system has a higher MTF value for almost all number of line pairs per millimeter.  FIGS. 11-14  quantify that the exemplary imaging system displays significantly enhanced information in the higher frequencies when compared to conventional VHM systems. 
         [0047]    Although the teachings herein have been described with reference to exemplary embodiments and implementations thereof, the disclosed methods, systems and apparatus are not limited to such exemplary embodiments/implementations. Rather, as will be readily apparent to persons skilled in the art from the description taught herein, the disclosed methods, systems and apparatus are susceptible to modifications, alterations and enhancements without departing from the spirit or scope hereof. Accordingly, all such modifications, alterations and enhancements within the scope hereof are encompassed herein.