Abstract:
A modified phase shifting mask is used to improve performance over traditional Zernike phase contrast imaging. The configurations can lead to an improved imaging methodology potentially with reduced artifacts and more than one order of magnitude gain in photon efficiency, in some examples. Moreover, it can be used to yield a direct representation of the sample&#39;s phase contrast information without the need for additional specialized post-acquisition image analysis. The approach can be applied to both wide-field and scanning configurations by using a phase mask including a pattern of phase elements and an illumination mask, having a pattern of holes, for example, that corresponds to a pattern of the phase mask.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 USC 119(e) of U.S. Provisional Application No. 61/869,187, filed on Aug. 23, 2013, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    X-rays, due to their high penetration power and possibility of an extended depth-of-field due to their short wavelength, are ideally suited for imaging thick and embedded soft and biological materials. In some energy ranges and/or samples including low-Z elements, phase contrast (PC) significantly dominates over absorption in transmission imaging. Thus, features that are difficult or impossible to observe in absorption contrast can be effectively studied in phase contrast mode. 
         [0003]    Various methods of phase contrast imaging exist. Zernike PC was developed by Frits Zernike (see F. Zernike, “Das Phasenkontrastverfahren bei der mikroskopischen Beobachtung.” Z. Techn. Physik. 16, 454-457 (1935)) for wide-field optical microscopy using a phase shifting annular mask in the back focal plane of the objective lens of a wide-field microscope in combination with an annular condenser illumination. This method directly reveals the phase shift introduced by the object. Schmahl et al. have used this method for wide-field x-ray microscopy (U.S. Pat. No. 5,550,887). The technique is well established in many wide-field x-ray, electron and optical microscopes. According to the principle of reciprocity in optics, this method can also be implemented on scanning microscopes. Siegel et al. describe this in U.S. Pat No. 4,953,188. 
         [0004]    Zernike phase contrast imaging is similar to absorption contrast imaging and can be achieved by modifying a microscope, which was set up for absorption contrast, by adding additional optical elements. For traditional wide-field microscopes, Zernike phase contrast is implemented by using an annular aperture in or near the plane of the condenser lens in combination with an annular phase shifting ring in or near the back focal plane of the microscope objective. The phase plate is chosen to be transparent or semi-transparent with a thickness to phase shift the radiation of +/−π/2,+/−3π/2, or generally (2n+1) π/2 where n=0,+/−1,+/−2, etc. The choice and sign of n selects either positive or negative phase contrast imaging modes. When ray tracing the path of the light in the Zernike PC microscope, the annular illumination light from the condenser is chosen to match with the phase plate in the back focal plane. According to Abbe&#39;s theory of image formation, the presence of the sample produces a scattered light field that will not pass through the phase plate in the back focal plane of the objective lens. This scattered light contains the structure information of the sample. The interference of this scattered light field with the unscattered light going through the phase ring produces on the detector the desired Zernike phase contrast image. This method has been widely used in light microscopy, electron microscopy and x-ray microscopy with great success. 
         [0005]    Scanning PC microscope systems have also been developed. Examples include differential PC using a segmented detector (see B. Hornberger et al., “Differential phase contrast with a segmented detector in a scanning x-ray microprobe,” J. Synchrotron Rad. 15, 355-362 (2008)) or more advanced schemes such as ptychography (see P. Thibault et al., “High-Resolution Scanning X-ray Diffraction Microscopy,” Science. 321, 379-382 (2008)). These methods, however, require specialized and post-acquisition image analysis in order to yield a proper sample representation and do not deliver a direct phase contrast image that can easily be interpreted. 
         [0006]    More recently, by employing methods from wide-field microscopy and the basic imaging principle of reciprocity, scanning-type Zernike PC using x-rays has been demonstrated for the first time as a new and alternative method, which directly visualizes the sample&#39;s phase contrast information with no need for image processing. See C. Holzner, M. Feser, S. Vogt, B. Hornberger, S. B. Baines and C. Jacobsen, “Zernike phase contrast scanning microscopy with X-rays,” Nat. Phys. 6, 883-887 (Nov. 2010). 
         [0007]    The major limitation of Zernike PC imaging is that its image contrast significantly decreases with increased sample feature size. Further, halo artifacts at feature edges and boundaries are inherent in Zernike PC images. The underlying cause for this is the spatial frequency dependent contrast transfer of the Zernike PC method. In particular, at low spatial frequencies (large features) the contrast transfer is very low. The artifacts can make image interpretation, and specifically quantitative analysis, difficult or even impossible. Thus, generally, Zernike phase contrast imaging is usually acceptable for observing the features&#39; morphology qualitatively, both in two (2D) and three (3D) dimensions. For quantitative and computer based image processing, these artifacts become more problematic and a solution to the non-quantitative nature of the images is desired. 
         [0008]    Moreover, with three dimensional (3D) imaging, e.g. computed tomography (CT) techniques, these artifacts lead to severe distortions and amplified artifact structures in the 3D data. This is because the CT reconstruction algorithm requires each 2D projection image to consist of the linear sum of some characteristic through the sample, e.g. the attenuation coefficient in the case of absorption contrast images. In order to effectively combine the phase-contrast imaging technique with 3D CT imaging, one must derive the linear phase shift through the sample from images that have both absorption and phase contrast signals. Another challenge is the automated separation of specimen constituents by segmentation after the tomographic reconstruction of a tilt series when these artifacts are present. 
         [0009]    More recently, M. Stampanoni described a wide-field PC system that utilizes a beam shaping condenser and a dot array as phase shifting mask, which noticeably reduces the typical Zernike artifacts and increases the photon efficiency. See M. Stampanoni et al., “Phase-contrast tomography at the nanoscale using hard x rays,” Phys. Rev. B81, 140105(R) (2010). This implementation, however, relies on the high degree of coherence (laser-like property) of the synchrotron source employed in this demonstration and could not be implemented using large spot laboratory sources. 
       SUMMARY OF THE INVENTION 
       [0010]    Scanning Zernike PC suffers from similar imaging artifacts as in the wide-field case. These artifacts are mainly due to the ring-shaped phase shifting mask, leading to the loss of low spatial frequencies in the imaging process and a subsequent haloing around sample feature edges. In the scanning-type case the phase ring represents a second disadvantage, as it is only the intensity in the phase ring&#39;s far-field projection that is meaningful to the image formation. However, this signal represents only approximately 2% of the incident probing intensity, making inefficient use of photons. 
         [0011]    The invention uses a modified phase shifting mask with increased efficiency. Using this configuration, the disadvantages of Zernike PC can be minimized and the configuration can lead to an improved imaging methodology potentially with drastically reduced artifacts and more than one order of magnitude gain in photon efficiency, in some examples. Moreover, it can be used to yield a direct representation of the sample&#39;s phase contrast information without the need for additional post-acquisition image filtering and/or analysis. The increase in photon efficiency achieved through the usage of the phase shifting mask in conjunction with the illumination mask correspondingly increases imaging throughput as compared to current systems and methods. The approach can be applied to both wide-field and scanning configurations. It also can be implemented using laboratory x-ray sources. 
         [0012]    In general, according to one aspect, the invention, which is applicable to both scanning and wide-field configurations, features a phase contrast imaging system comprising a radiation source for generating radiation, a detector for detecting the radiation after transmission through a sample, a patterned phase mask for phase shifting a portion of the radiation detected by the detector, and an illumination mask, having a pattern that corresponds to a pattern of the phase mask. 
         [0013]    In a first embodiment, the imaging system is a scanning x-ray microscope in which the illumination mask is located between the sample and the detector or implemented in the operation of the detector. The system includes an objective lens that focuses the radiation after transmission through the phase mask onto the sample. 
         [0014]    The first embodiment additionally has a number of characteristics. In one example, the detector is a spatially resolved pixelated detector and the illumination mask is implemented by summing responses of pixels to form the pattern of the illumination mask. In another example, the detector is a single element detector and the illumination mask is an opaque detector mask over the single element detector. 
         [0015]    In another characteristic, the phase mask is located between the sample and the radiation source. 
         [0016]    In a second embodiment, the imaging system is a wide-field x-ray microscope in which the illumination mask is located prior to the sample. The illumination mask comprises a membrane including transparent regions and opaque regions to form the illumination mask. The transparent regions are preferably radiation-transmitting holes. 
         [0017]    In other characteristics of the second embodiment, the phase mask is located between the sample and the detector. In addition, the system includes an objective lens that images the radiation after transmission through the sample onto the detector. 
         [0018]    The second embodiment also includes a condenser optic that illuminates the sample with the radiation from the radiation source in some examples. 
         [0019]    Additionally, the imaging system has a number of characteristics that are common to both embodiments. A laboratory x-ray source can be used to generate the radiation. In examples, the phase mask matches the pattern of the illumination mask. Alternatively, the pattern of the phase mask matches a conjugate of the pattern of the illumination mask. Preferably, the phase mask comprises phase elements distributed in the pattern that phase shift radiation of some of the radiation generated by the radiation source. The phase elements phase shift radiation scattered by the sample with respect to radiation that is not scattered by the sample. Preferably, a fill factor of the phase elements is less than 50%. 
         [0020]    Additionally, the phase elements can be spatially distributed over the phase mask in a regular array fashion for forming the pattern of the phase mask. Alternatively, the pattern is a non-regular array of the phase elements. 
         [0021]    In embodiments, the imaging system includes an image processor that creates tomographic reconstructions of the sample in response to the radiation detected by the detector. 
         [0022]    In general, according to another aspect, the invention features a phase contrast imaging method comprising generating radiation, detecting the radiation after transmission through a sample, phase shifting a portion of the radiation detected, and masking radiation from detection with a pattern that corresponds to a pattern of the phase shifting mask. 
         [0023]    The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in any claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
           [0025]      FIG. 1  is a schematic view of a scanning imaging microscope according to an embodiment of the present invention; 
           [0026]      FIG. 2  is a schematic view of a wide-field imaging microscope according to another embodiment of the present invention; 
           [0027]      FIG. 3A  is a front plan view of an illumination mask for the wide-field imaging microscope; 
           [0028]      FIG. 3B  is a front plan view of a phase shifting mask for the wide-field imaging microscope that is matched to the illumination mask of  FIG. 3A ; and 
           [0029]      FIG. 4A  is an image of a test structure generated using a conventional Zernike PC phase ring,  FIG. 4B  is an image of the test structure generated using a PC phase ring/illumination mask according to the present invention with periodic opaque elements in the illumination mask,  FIG. 4C  is an image of the test structure generated using a PC phase ring/illumination mask according to the present invention with randomly distributed opaque elements in the illumination mask with unit cell constraints, and  FIG. 4D  is an image of the test structure generated using a PC phase ring/illumination mask according to the present invention with randomly distributed opaque elements in the illumination mask. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]      FIG. 1  shows a scanning imaging microscope  100  constructed according to a first embodiment of the present invention. 
         [0031]    Radiation  108  is generated by a radiation source  106 . Typically, this radiation is intrinsically narrowband radiation or broadband radiation that is filtered by a bandpass filter to be narrowband. In the illustrated example, the radiation is generally collimated. 
         [0032]    In the preferred embodiment, the radiation  108  is x-ray radiation having an energy between 0.2 keV and 100 keV. Some specific examples of the source  106  include a sealed tube x-ray source, a rotating anode x-ray source, a micro-focus x-ray source, metal jet micro-focus x-ray source, or a synchrotron radiation source. Some of these sources include an integrated or separate collimator. 
         [0033]    In the example of an electron microscope, the source  106  generates radiation that is an electron beam, having an energy between 10 keV and 1 MeV. 
         [0034]    A phase plate or mask  110  phase shifts a portion of the radiation so that the radiation that is scattered by the sample  10  interferes with unscattered radiation to form the projection image. 
         [0035]    In the illustrated embodiment, the phase mask  110  comprises an array of dots or phase elements  112 . In the specific illustrated implementation, the array is a regular array. The material of the phase plate  110  and its thickness relative to the wavelength of the source radiation  108  has the effect of shifting the phase of the radiation  108  transmitted through the dots or phase elements  112  by typically π/2 or 3π/2 relative to the radiation that passes through the plane of the phase plate  110  but does not encounter a dot  112 . More generally, the phase shift needs to be: 
         [0000]      phase shift=((2*n+1)/2)*π+/−π/4,
 
         [0000]    where n can be any whole positive or negative number including zero. The +/- π/4 is a very conservative tolerance and if the phase shift is not exactly equal to ((2*n+1)/2)*π, some mixing of phase and absorption contrast imaging will occur. 
         [0036]    In the illustrated embodiment, the phase elements  112  are cylindrical (circle extrusions) having an axis that is parallel to the optical axis  114  of the system. In other embodiments, square phase elements  112  or other shaped extrusions are used. Alternatively, the phase shifting elements  112  can be fabricated by removing material from the substrate of the phase plate  110  to produce the relative phase shift. 
         [0037]    In the illustrated embodiment, the phase elements  112  form a regular array. Examples include arranging the phase elements in a grid or periodic fashion. In other embodiments, however, the phase elements form a non-regular array, such as when the phase elements  112  are randomly or pseudo-randomly distributed over the extent of the phase plate  110 . This is the preferred embodiment to obtain even spatial frequency contrast transfer in the scanning imaging system  100 , which correspondingly minimizes the creation of artifacts in the projection images generated by the system. 
         [0038]    Also, in the illustrated embodiment, the fill factor of the phase elements  112  compared to the portions of the phase plate  110  that include no phase elements is approximately 20%. Generally, the fill factor, or the percentage of the phase plate  110  that is covered with phase elements  112 , should be between a few percent and 50 percent. 
         [0039]    In terms of size, generally the smaller the size of the phase elements  112 , the better for reduction of artifacts in the imaging process due to uneven spatial frequency contrast transfer. Ideally the size of the phase elements  112  would be the same as or smaller than the system resolution as determined by the numerical aperture of the lens. In this ideal imaging system, no artifacts would exist and the images would be ideal phase contrast images. In practical systems, this is very difficult to achieve due to manufacturing constraints of the phase mask  110  and the requirement to keep the phase mask position stable to a small fraction of its size relative to the other optical elements. In a typical embodiment, the size of the phase elements  112  would be chosen to be in the range of 5-100 times the system resolution. 
         [0040]    In some examples, the phase elements are all approximately the same size with respect to each other. In other examples, the sizes of the phase elements  112  vary across the phase mask  110  such that some of the phase elements are two or more times larger in terms of area than other phase elements. 
         [0041]    Along the optical axis  114  is a cylindrical central stop  116 . This central stop  116  absorbs or blocks radiation along the optical axis  114 . Preferably, the area of the central stop  116  is 50 percent or less compared with the area of the objective lens  118 . 
         [0042]    A focusing objective element  118 , located at a distance p from the phase mask  110 , focuses the radiation  108  onto the sample  10 . In the illustrated embodiment, a zone plate objective  118  is used, which is a distance f from the sample  10 . In other embodiments, reflective optics such as a capillary or Wolter reflective condenser is used. In still other embodiments, focusing elements such as compound refractive lenses or KB-mirrors are used. 
         [0043]    An order-sorting aperture  120  is then provided. It has a central aperture  122  that is sized to the central stop  116 . It is chosen to be slightly smaller than the central stop  116 . This order sorting aperture  120  blocks radiation  108  that is not focused by the focusing element  118 . 
         [0044]    The sample  10  is located at the focal plane of the focusing element  118 . The sample  10  is held by a sample holder  124 . 
         [0045]    The radiation transmitted through the sample  10  is then detected by a detector  130  that is located a distance d from the zone plate objective  118 . 
         [0046]    The detector  130  includes active photosensitive regions  134  and inactive regions  132  to thereby form an illumination mask. 
         [0047]    According to the invention, in one example, the pattern of the detector&#39;s illumination mask, and specifically the active regions  134  on the detector  130 , matches the pattern of the phase elements  112  of the phase plate  110 . The size and position of the pattern of the active regions  134  are adjusted for the magnification of the system, however. Further, the pattern of the illumination mask is point mirrored (inverted) with respect to the pattern of the phase elements  112  due to the lens  118 . 
         [0048]    It should be noted that the pattern of the detector&#39;s illumination mask can match the pattern of the phase elements  112  of the phase plate  110  in terms of being its conjugate as well. 
         [0049]    In one embodiment, the detector  130  is simply a large area, single element detector. In this case, the active regions  134  correspond to radiation-transmitting hole structures of an opaque physical detector mask that is placed over a photosensitive region of the detector  130 . 
         [0050]    In another embodiment, the detector  130  is a spatially resolved, pixelated detector. In this example, summing the responses of only the pixels that fall within the active regions  134  are used in the formation of the image to thereby functionally provide or form the pattern of the illumination mask. Preferably, the spatially resolved detector  130  has a high resolution having greater than 1024×1024 pixels. Alternatively, one can use a long distance between the detector  130  and the sample  10  to further magnify the dots  112  of the phase plate  110  on the detector  130  and thus use a very coarse detector  130  with larger pixels. 
         [0051]    In some cases, a direct detection scheme is used in which a CCD or CMOS detector or other electronic detector  130  is used to detect the radiation  108 , when lower energy radiation such as soft x-rays are used, for example. However, with higher energies, intervening scintillators are employed to enable detection of the radiation  108  by conversion into the optical frequencies. In such cases, intervening fold mirrors can be added so that the electronic detector  130  is not irradiated by x-rays. 
         [0052]    In the illustrated example of a scanning system, the focal spot is scanned over the area of interest of the sample  10 . This is achieved by creating relative movement between the focal spot and the sample  10 . In one example, the focal spot is raster scanned over the sample  10 . In another example, the sample holder  124  moves the sample  10  in the radiation  108 . That is, the instrument is stationary and the sample  10  is raster scanned through the focal spot, as is most commonly the case for x-ray imaging. 
         [0053]    Preferably, the sample holder  124  further rotates the sample  10  in the radiation  108  to enable the generation of different projections through the sample, enabling tomographic reconstruction of the sample  10 . 
         [0054]    The detector  130  generates an image representation of the radiation that is scattered by the sample  10  in conjunction with radiation unscattered by the sample to form the projection image. 
         [0055]    The imaging system  100  also includes an image processor  138  that accepts the image projections from the detector  130  and creates a tomographically reconstructed volume of the sample  10  from the projection images, in one mode of operation, from the separate projection images. 
         [0056]    Operators of the imaging system  100  can choose different variations of the phase mask  110  for each scan run to induce different phase shifts for the radiation scattered by the sample  10 . Selection of positive values of n for the phase shift creates positive phase-shifted projection images of the sample  10 . Within the images, the phase-shifted light due to scattering of the radiation  108  by features of the sample  10  appears as foreground or bright spots compared to darker background features associated with unscattered light. Conversely, selection of a phase mask using negative values of n for the phase plate  110  creates negative phase-shifted projection images of the sample  10 . 
         [0057]      FIG. 2  shows a wide-field imaging microscope  100  constructed according to a second embodiment of the present invention. 
         [0058]    Radiation  108  is similarly generated by a radiation source  106 . The figure shows radiation  108  radiating out as from a point source, which is consistent with radiation generated from a laboratory source such as a sealed tube source, a rotating anode x-ray source, metal jet micro-focus source, or a micro-focus x-ray source, in examples. 
         [0059]    But here also, in other examples, the radiation  108  is generated by a synchrotron or other x-ray radiation source. In this case, a more collimated beam would be provided. 
         [0060]    In other embodiments, the radiation  108  is an electron beam. 
         [0061]    If a laboratory x-ray source is used, then typically a reflective condensing element is often preferred. In the illustrated example, a cylindrical capillary condenser  140  collects the radiation radiating from the source  106  and focuses the radiation. 
         [0062]    A converging cone of radiation  142  directed toward the sample  10  is created by including a central stop  116  aligned along the optical axis  114  and preferably centered in the exit aperture of the condenser  140 . 
         [0063]    An illumination mask  160  is located in the beam of radiation  108  preferably between the condenser  140  and the sample  10 . 
         [0064]    In the current embodiment, the illumination mask  160  has an array of transparent circular regions  164  that transmit radiation. The regions  164  are included within an opaque membrane  162 . A material of the opaque membrane  162  is selected to prevent the transmission of the radiation  108  through the membrane  162 . In one example, the membrane is metal, such as gold, and the regions  164  are holes in that gold membrane  162 . The holes enable transmission of the radiation  108  through the otherwise opaque membrane  162 . 
         [0065]    Alternatively, one can use the opposite pattern for the illumination mask  160  with opaque regions  164 , and a transparent membrane  162 . Here, the transparent membrane  162  provides mechanical support for the opaque, e.g., gold, regions  164 . 
         [0066]    The converging cone of radiation  142  passing through the sample  10  is imaged onto a spatially resolved detector  180  by an objective lens  168 , which is typically a Fresnel zone plate lens, when the radiation is x-ray radiation. In other examples, a compound refractive lens (CRL) or other image forming x-ray optic can be utilized as the objective lens  168 . The transmitted radiation includes radiation that was unscattered by the sample  10  and radiation/light that was scattered by the sample  10 . The objective lens  168  is a distance d from the condenser  140 , and the objective lens  168  is a distance f from the sample  10 . 
         [0067]    Typically, the spatially resolved detector  180  has a high resolution having greater than 1024×1024 pixels. In some cases, a direct detection scheme is used in which a CCD detector or other electronic detector is used to detect the radiation, when optical frequencies or soft x-rays are used. However, with higher energies an intervening scintillator, and possibly a fold mirror, is typically employed to enable detection of the radiation by first converting it into the optical frequencies. 
         [0068]    A phase mask  170  is a distance p from the objective lens  168  and is located between the objective lens  168  and the detector  180 . The phase mask  170  induces a phase shift between the light that is not scattered by the sample relative to the light that is scattered by the sample  10  so that they interfere with each other at the detector  180 . 
         [0069]    The phase mask or plate  170  is placed in the back focal plane of the objective lens  168 . The placement is such that the distances fulfill the lens equation 1/f=1/d+1/p. The material of the phase plate and its thickness relative to the wavelength of the source radiation  108  has the effect of shifting the phase of the radiation transmitted through the phase plate  170  by typically π/2 or 3π/2. As discussed previously, more generally, the phase shift needs to be: 
         [0000]      phase shift=((2*n+1)/2)*π+/−π/4,
 
         [0070]    where n can be any whole positive or negative number including zero. 
         [0071]    The phase mask or plate  170  comprises an array of dots or phase elements  172 . The material of the phase plate  170  and its thickness relative to the wavelength of the source radiation  108  has the effect of shifting the phase of the radiation  108  transmitted through the dots or phase elements  172  by typically π/2 or 3π/2 relative to the radiation that is passes through the plane of the phase plate  170  but does not encounter a dot or phase element  172 . 
         [0072]    In the illustrated embodiment, the phase elements  172  are cylindrical dots. In other embodiments, square phase elements  172  or other shapes are used. 
         [0073]    Also, in the illustrated embodiment, the phase elements  172  form an irregular array. In other embodiments, however, the phase elements  172  are arranged in a regular array. Preferably, the phase elements  172  are randomly distributed or pseudo-randomly distributed over the extent of the phase plate  170 . 
         [0074]    Also, in the illustrated embodiment, the fill factor of the phase elements  172  compared to the portions  174  of the phase plate  170  that have no phase elements  172  is approximately 20%. Generally, the fill factor should be between a few percent and 50 percent. 
         [0075]    According to the invention, the pattern of the phase elements  172  of the phase plate  170  matches the pattern of the transparent hole elements  164  in the opaque membrane  162  of the illumination mask  160  in terms of being the same or its conjugate. The size and position of the pattern of the phase elements  172  are adjusted, however, for the magnification of the system. In addition, the representation of the phase plate pattern is point-mirrored with respect to the optical axis  114 ; i.e. imaging through the objective lens turns the picture up-side down. As a result, the radiation  108  that is phase shifted by the phase elements  172  is only the radiation that is unscattered by features or structures within the sample  10  and thus contributes to the formation of the projection image on the image plane of the detector  180  by interference with the scattered radiation. 
         [0076]    The wide-field imaging microscope  100  similarly includes an image processor  138  for creating tomographically reconstructed volumes of the sample  10  from the projections images. 
         [0077]      FIG. 3A and 3B  illustrate the relationship between the illumination mask  160  and the phase mask  170  for the wide-field embodiment of the imaging system  100  in  FIG. 2 . Specifically, the pattern of the transparent elements  164  matches (point-mirrored) the pattern of the phase elements  172  of the phase mask  170 . 
         [0078]      FIG. 4A  though  4 D show generated PC images of a common test pattern sample. 
         [0079]      FIG. 4A  shows an image generated using a conventional Zernike PC phase ring.  FIG. 4B  through  FIG. 4D  show patterns generated using different configurations of the inventive combination phase mask/illumination mask. Because the images of  FIG. 4B-4D  were generated using a scanning configuration, the reference numbers refer to elements of the scanning configuration of  FIG. 1 . However, the images could also have been generated using the wide field configuration of  FIG. 2  with substantially similar results. 
         [0080]      FIG. 4B  shows an image generated using a periodic (regular array) arrangement of regions  134  of the illumination mask/phase elements  112  of the phase mask  112 .  FIG. 4C  shows an image generated when the regions  134 /phase elements  112  are spatially distributed in a random fashion with an additional unit cell constraint. Finally,  FIG. 4D  shows an image generated when the regions  134 /phase elements  112  are spatially distributed in a random fashion. 
         [0081]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.