Abstract:
The invention provides a hybrid OCT scanning device which improves OCT scanning of targets, including adaption for a ophthalmic targets (i.e. cornea, retina and fovea areas). The invention provides a movable beam modifying element which is adapted to be interposed in the probe radiation directed to the target, and the interposition of the movable element is synchronous with the periodicity of the OCT scan beam. The beam modifying element may include any of angular deflection regions, optical delay regions, or combinations of deflection and delay regions. Such regions of deflection and/or delay translate into a governable scan range, both lateral and depth, of a target region.

Description:
CROSS REFERENCES TO RELATED PATENTS OR APPLICATIONS 
       [0001]    This patent application, docket number CI120121US, claims priority from U.S. provisional patent application No. 61/461,975 (docket number FP110125PR) titled “Hybrid Scanning System” filed on 26 Jan. 2011 and also claims priority from US provisional patent application No. 61,572,433 (docket number CI110701PR) titled “Hybrid Angular Scanning System” filed on 14 Jul. 2011 and is related to U.S. Pat. No. 7,526,329 titled Multiple reference non-invasive analysis system and U.S. Pat. No. 7,751,862 titled Frequency resolved imaging system, the contents of both of which are incorporated herein as if fully set forth herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention described and illustrated in this application relates to non-invasive imaging and analysis techniques such as Optical Coherence Tomography (OCT). In particular it relates scanning mechanisms suitable for both time domain and Fourier domain OCT systems. 
         [0003]    This invention also relates to the use of such OCT systems for non-invasive imaging and analysis of targets including, but not limited to: components of an eye; tissue; seeds; other biological or non-biological entities; concentrations of specific components or analytes in a target, such as the concentration of glucose in blood or tissue fluids. This invention also relates to analysis or monitoring for manufacturing defects in components for improved quality control. 
       BACKGROUND OF THE INVENTION 
       [0004]    Non-invasive imaging and analysis of targets is a valuable technique for acquiring information about systems or targets without undesirable side effects, such as damaging the target or system being analyzed. In the case of analyzing living entities, such as human tissue, undesirable side effects of invasive analysis include the risk of infection along with pain and discomfort associated with the invasive process. In the case of quality control, it enables non-destructive imaging and analysis on a routine basis. 
         [0005]    Optical coherence tomography (OCT) is a technology for non-invasive imaging and analysis. OCT typically uses a broadband optical source, such as a super-luminescent diode (SLD), to probe and analyze or image a target. It does so by applying probe radiation from the optical source to the target and interferometrically combining back-scattered probe radiation from the target with reference radiation also derived from the optical source. 
         [0006]    The typical OCT optical output beam has a broad bandwidth and short coherence length. The OCT technique involves splitting the output beam into probe and reference beams, typically by means of a beam-splitter, such as a pellicle, a beam-splitter cube or a fiber coupler. The probe beam is applied to the system to be analyzed (the target). Light or radiation is scattered by the target, some of which is back scattered to form a back-scattered probe beam, herein referred to as signal radiation. 
         [0007]    The reference beam is typically reflected back to the beam-splitter by a mirror. Light scattered back from the target is combined with the reference beam, also referred to as reference radiation, by the beam-splitter to form co-propagating reference radiation and signal radiation. Because of the short coherence length only light that is scattered from a depth within the target whose optical path length is substantially equal to the path length to the reference mirror can generate a meaningful interferometric signal. 
         [0008]    Thus the interferometric signal provides a measurement of scattering properties at a particular depth within the target. In a conventional time domain OCT system, a measurement of the scattering values at various depths can be determined by varying the magnitude of the reference path length, typically by moving the reference mirror. In this manner the scattering value as a function of depth can be determined, i.e. the target can be scanned. 
         [0009]    There are various techniques for varying the magnitude of the reference path length. Electro-mechanical voice coil actuators can have considerable scanning range, however, there are problems with maintaining the stability or pointing accuracy of the mirror. Fiber based systems use fiber stretchers, however, they have speed limitations and have size and polarization issues. Rotating diffraction gratings can run at higher speeds, however, are alignment sensitive and have size issues. 
         [0010]    Piezo devices can achieve high speed scanning and can have high pointing accuracy, however to achieve a large scanning range requires expensive controls systems and have limited speed. A scanning method that effectively amplifies the scan range of a piezo device is described in U.S. Pat. Nos. 7,526,329 and 7,751,862 incorporated herein. 
         [0011]    The technique described in these incorporated references uses multiple reference signals with increasing scan range and correspondingly increasing frequency interference signals. This scanning method can achieve large scan range at high speed with good pointing stability. 
         [0012]    In swept source Fourier domain OCT systems depth scanning is accomplished by repeatedly sweeping the wavelength of the optical source. The wavelength range over which the optical source is swept determines the depth resolution. The period of the sweep repetition rate determines the period of the depth scans. 
         [0013]    In addition to depth scanning, lateral scanning is required for many imaging and analysis applications. There are many conventional techniques for lateral scanning, such as the use of stepper or linear motors. Some applications require angular scanning, which is typically accomplished by electro-mechanical oscillating mirrors, typically referred to as salvo-scanners. 
         [0014]    An example of an imaging application that requires angular scanning is the ophthalmic application of imaging the retina of the eye. The lens of the eye focuses collimated light on the retina. As a consequence lateral scanning by stepper or linear motors is not and effective retinal scanning technique. An angular scanning technique is required. 
         [0015]    Conventional salvo-scanners are physically bulky and are inefficient because of their oscillating nature. They are therefore not suitable for compact or portable devices. There is therefore an unmet need for a method, apparatus and system for angular scanning over an extended depth range in a compact and power efficient manner. 
       SUMMARY OF THE INVENTION 
       [0016]    The invention taught herein meets at least all of the aforementioned unmet needs. The invention provides a hybrid OCT scanning device which improves OCT scanning of targets, including adaption for a ophthalmic targets (i.e. cornea, retina and fovea areas). The invention provides a movable beam modifying element which is adapted to be interposed in the probe radiation directed to the target, and the interposition of the movable element is synchronous with the periodicity of the OCT scan beam. The beam modifying element may include any of angular deflection regions, optical delay regions, or combinations of deflection and delay regions. Such regions of deflection and/or delay translate into a governable scan range, both lateral and depth, of a target region. One embodiment of the invention provides hybrid scanning method and device which provides a means to insert angular deflection and optical delays into an OCT scanning system. The invention provides a hybrid OCT scanning device which improves OCT scanning of targets, including adaption for a ophthalmic targets (i.e. cornea, retina and fovea areas). The invention provides a movable beam modifying element which is adapted to be interposed in the probe radiation directed to the target, and the interposition of the movable element is synchronous with the periodicity of the OCT scan beam. The beam modifying element may include any of angular deflection regions, optical delay regions, or combinations of deflection and delay regions. Such regions of deflection and/or delay translate into a governable scan range, both lateral and depth, of a target region. 
         [0017]    Such angular deflections translate into a governable scan range of some target region. Such optical delays extend the depth range scanned. In a preferred embodiment, the means of introducing angular displacement and optical delay is a rotating disc in the optical path of an OCT beam, where the disc contains multiple displacement elements effecting predetermined degrees of angular displacement and multiple optical delays effecting predetermined degrees of extended depth scan range or combinations of angular displacement elements and optical delays. 
         [0018]    The movement of the disc is synchronized so that the set of scans obtained provide information that permits measurement or imaging of a predetermined scan area. In another embodiment of the invention, an axial displacement of the disc is included, which results in further governable aspects of the target site to be scanned. Examples of the inventive application include application in ophthalmology generally, and more specifically as a means of obtaining scan of cornea, retina and fovea areas. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a schematic type illustration of a preferred embodiment of an OCT system that includes an angular scanning device according to the invention. 
           [0020]      FIG. 2  is a more detailed illustration of a configuration of the angular scanning device. 
           [0021]      FIG. 3  is a more detailed illustration of an additional scanning aspect of the angular scanning device. 
           [0022]      FIG. 4  is an illustration of aspects of an ophthalmic application. 
           [0023]      FIG. 5  is an illustration of aspects of a configuration of the stepped optical delay elements and their relationship with scan segments. 
           [0024]      FIG. 6  is a an alternate example of movable beam modifying element (disc and linear) and an example of the effect of two beam modifying elements with the same angular deflection but different optical delays. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The preferred embodiment includes a time domain OCT system, an example of which is included in the  FIG. 1  illustration where a broadband optical source  101  generates broadband output radiation  103 , referred to as source radiation, that is directed at a beam-splitter  105 . The beam-splitter  105  transmits a component of the source radiation as initial probe radiation  107  through a rotating disc  109  towards a target  111 . 
         [0026]    The rotating disc  109  can allow the initial probe radiation  107  to go directly to the target  111  or can introduce one or more beam modifying optical elements that can modify the optical radiation and allow at least a portion of the modified optical radiation to go to the target  111 . While the preferred embodiment the beam modifying elements are housed in a rotating disc, in other embodiments the beam modifying elements could be housed in holders with many different configurations. For example, beam modifying elements could be housed in linear holder that moves laterally. Importantly the beam modifying elements are movable in a synchronous manner with respect to the repetitive scans of the OCT system. 
         [0027]    The rotating disc, the beam modifying optical elements and their effect on the radiation directed at the target are all further described with respect to  FIG. 2 . Returning to  FIG. 1 , a portion of the initial probe radiation that is back-scattered from the target is directed by the beam-splitter towards a detector  113  and is referred to as back-scattered probe radiation. 
         [0028]    The beam-splitter  105  also directs a component of the source radiation referred to as reference radiation  115  through an optical element  117  to a reference mirror  119  mounted on a translational device  121 , such as a piezo device or an electro-mechanical translational device. 
         [0029]    In the case of a conventional OCT configuration the optical element  117  is typically an attenuator used to optimize signal to noise (SNR) aspects. In the case of a multiple reference OCT configuration, further described in U.S. Pat. No. 7,526,329 titled Multiple reference non-invasive analysis system and U.S. Pat. No. 7,751,862 titled Frequency resolved imaging system (incorporated herein by reference), the optical element  117  is a partial reflective mirror which may also include an attenuator and in the case of a polarization OCT configuration may include a polarization rotating characteristic. 
         [0030]    In the preferred embodiment the translational device  121  is a piezo device whose length is modulated by a periodic electronic drive signal (which determines the OCT scan periodicity). 
         [0031]    A portion of the reference radiation  115 , modulated by the translational device  121 , propagates back through the beam-splitter  105  to the detector  113  where, in combination with the as back-scattered probe radiation it can generate one or more interference signals. 
         [0032]    The interference signals are detected by at least one opto-electronic detector  113  whose electronic output is processed by a processing module  123  in conjunction with a control module  125  that provides electronic timing signals and drive signals to the piezo device  121 , to a motor  127  rotating the disc  109  and optionally to the optical source  101 . 
         [0033]    The rotation of the disc  109  is synchronized with the repetitive length modulation of the piezo device. The disc is further illustrated in  FIG. 2  where a disc  201  has 16 holes (of which  203 , numbered  1 , is an example) in which beam modifying optical elements can be located. The rotation of the disc  201  may be achieved by means of a shaft  205  at the center of the disc. 
         [0034]    In the preferred embodiment the synchronization of the rotation of the disc with the repetitive length modulation of the piezo device causes the time the optical probe radiation encounters the solid regions between the holes in the disc (of which  207  is an example between holes number  1  and  2 ) to coincide with the turn around regions of the piezo device. 
         [0035]    Rotation of the disc may be accomplished by either a linear or a stepper motor. In the case of a linear motor the beam modifying optical elements would typically be symmetric about the path that the optical beam traversed through the optical elements as the disc continuously rotated. 
         [0036]    In the case of a stepper motor, the motor would move the beam modifying optical elements such that the optical beam is substantially centered on one of the beam modifying optical elements and then holds the disc stationary for a substantial percentage of the period of the repetitive length modulation of the piezo device. 
         [0037]    Such synchronization ensures that the back-scattered optical signal is available for generating one or more interference signals during the most useful regions of the piezo scan (i.e. the regions between the turn around points). 
         [0038]    In one preferred embodiment a movable beam modifying optical element would consist of an optical angular deflection element such as a holographic optical element whose design would pre-determine the magnitude of the optical angular deflection. Optical elements that do not cause angular deflection could be located in some holes (or they could be left empty or blank) and multiple different optical angular deflection elements could be located in the other holes. 
         [0039]      FIG. 2B  illustrates the case of the optical beam  215  propagating through a blank hole or an optical element that does not cause angular deflection in hole location numbered  1  of the disc  201  (of  FIG. 2  A). (note using a non-deflecting optical element, rather than an empty hole, enables selecting or compensating for optical path length delays in order to depth align scans). The optical beam  215  propagates un-deflected to the target  209 , which in this example is an eye. In this example the beam  215  is laterally aligned with the center of the cornea  211  and propagates in a straight line to be focused on the center of the foveal pit region  217  of the eye. (This lateral alignment can be accomplished with conventional positioning techniques.) 
         [0040]    The disc  213  and shaft  205  (of  FIGS. 2B and 2C ) rotate to position an optical angular deflection element located in hole numbered  2  of the disc  201  (of  FIG. 2  A) in the beam path. In this case the optical beam  221  is deflected by the optical element in the disc  213  resulting in an angularly deflected beam  223 . Further rotation of the disc  213  causes multiple different optical angular deflection elements to be sequentially inserted into the path of the beam  221  resulting in the angularly deflected beam  223  to be deflected by multiple different angles. 
         [0041]    In the target  209  example illustrated in  FIG. 2C  the angularly deflected beam  223  is further deflected by the lens of the eye (the target). This further deflected beam  225  is focused on the retina at a location  219  that is laterally displaced from the center of the fovea region  217 . Similarly multiple different optical angular deflection elements cause the optical beam to be focused on different laterally displaced locations on the retina. 
         [0042]    The figures Sheet  3  are in many respects similar to those of Sheet  2 . In particular  FIG. 3A  illustrates the same rotation device  301  as  201  of  FIG. 2A  and  FIG. 3B  illustrates the same configuration as  FIG. 2C .  FIG. 3C  illustrates an axial displacement of the rotating disc  313  of  FIGS. 3B and 3C  and thereby axial displacement of the pre-selected holographic optical element located in the rotating disc in the path of the optical beam. 
         [0043]    While the magnitude of angular deflection achieved by introducing the pre-selected holographic optical element in the rotating disc  313  remains the same as that of  FIG. 3B , the deflected beam  327  intersects a more outer region of the lens of the eye than does the deflected beam  323 . The resulting further deflected beam  329  has an angular deflection of greater magnitude than does the further deflected beam  325 . 
         [0044]    The resulting lateral displacement along the retina to location  331  has a greater magnitude of displacement from the center location  217  of  FIG. 2B  than the displacement of location  333  from the center location  217 . 
         [0045]    The magnitude of the axial displacement  319  of the rotating disc  313  of both  FIGS. 3B and 3C  (and hence the magnitude of the axial displacement of the holographic optical element) can be systematically varied at a rate that is slow with respect to the period of the piezo modulation, while maintaining the same pre-selected angular deflecting holographic optical element, to generate a set of depth scans. 
         [0046]    Sheet  4  illustrates scan location patterns suitable for ophthalmic applications.  FIG. 4A  is similar to  FIGS. 2C and 3B . A cross-section  403  of the retina centered on the foveal pit is depicted in  FIG. 4B . The center of the foveal pit is shown as  401  in both  FIGS. 4A and 4B . 
         [0047]      FIG. 4C  depicts a suitable pattern for mapping a region of the retina of the eye and locating the center of the foveal pit by repeated sets of scans accompanied by iterative lateral alignment of the OCT system with respect to the target (the eye). It shows six locations substantially symmetrically positioned on a circle indicated by the dashed circle  411  and a seventh location ideally in the center of the foveal pit. 
         [0048]    The seventh and centered location would correspond to location  401  of  FIGS. 4A and 4B . The laterally displaced location  423  of  FIG. 4A  would correspond to  413  of  FIG. 4C  (note drawings are not to scale for clarity). 
         [0049]    A second scan pattern is depicted in  FIG. 4D  that includes the same depth scan locations as  FIG. 4C  with an additional six depth scan locations symmetrically positioned on the dashed circle  405 . The pattern depicted in  FIG. 4D  could be achieved by loading the rotating disc  201  of  FIG. 2  with non or zero deflecting optical elements in disc holes  1 ,  5 ,  9 ,  13  and twelve different angular deflecting optical elements in disc holes  2 ,  3 ,  4 ,  6 ,  7 ,  8 ,  10 ,  11 ,  12 ,  14 ,  15 ,  16 . 
         [0050]    In an alternative embodiment a disc with eight holes could be used with non or zero deflecting optical elements in disc holes  1  and  5  and six different angular deflecting optical elements in disc holes  2 ,  3 ,  4 ,  6 ,  7 ,  8 . An axial displacement of the rotating disc  413  to scan at the additional outer six locations depicted on the dashed circle  405 . 
         [0051]    There are many variations of the described embodiments possible. For example, the number of optical elements in the disc could be other than sixteen or eight and their geometry could be other than circular. Other arrangements are possible such as two counter rotating discs. Instead of a rotating disc other geometries are possible, such as a rectangular synchronously sliding device more generally referred to as a carrier. Such a configuration is depicted in  FIG. 6B  where  605  is the first of a set of 16 beam modifying elements. One or more discs with more than one sets of beam modifying optical elements centered on circles of different radii could be used, as is depicted in  FIG. 6A  where a disc  601  ha a second set of beam modifying optical elements, one of which is  603 , centered on a second smaller radius. 
         [0052]    Holographic diffractive elements or other optical elements loaded in the rotating disc could also address focusing, wave front distortion, attenuation aspects. The beam modifying optical elements can be comprised of individual elements or various combinations of different types of elements that include, but are not limited to: an angular deflection element; an optical delay element; an attenuation element; a focusing element; a wave front distortion element. 
         [0053]    While the preferred embodiment describes a time domain OCT system, other OCT configurations are included. Fourier domain swept source or spectral domain OCT systems could also avail of the invention. In these configurations rotation of the disc or carrier would be substantially synchronized with at least one of: periodicity of a piezo motion of an OCT reference mirror; periodicity of path length change of an OCT reference arm; periodicity of a Fourier domain OCT swept source. 
         [0054]    Some applications, including some ophthalmic applications, require illumination of the target by light which may be of a similar wavelength to that used by the OCT system (typically to assist in lateral alignment of the OCT system with respect to the center of the cornea of the eye). Embodiments with such requirements, with or without a rotating disc, could eliminate undesirable optical cross-talk or interference between the OCT light and the target illuminating light by pulsing the illuminating light synchronously with the periodicity of the piezo motion (or other length modifying device) of an OCT reference mirror and phase aligned with un-useful turn around time of the piezo device. 
         [0055]    Alternatively the pulsed illuminating light could be pulsed synchronously with a multiple of the period of the piezo motion or in the case of a Fourier domain OCT system, the period of the swept source. In such an approach, OCT information acquired during periods when the illuminating light is pulsed on would be discarded or at least processed differently. 
         [0056]    In alternative embodiments the patterns depicted in  FIGS. 4C and 4D  could be depth aligned with a surface other than the retina, such as a surface of the cornea or a surface of the intra-ocular lens. They could also be aligned to locate the center region of the optic nerve. 
         [0057]    The axial displacement approach could be used to repeatedly scan radially across the retina to generate a set of scans to image one or more (radially rotated) cross-sections of the retina, typically centered on the foveal pit. 
         [0058]    The axial displacement approach could be used to scan from the center of the foveal pit to the optic nerve to determine the distance between the foveal pit and the optic nerve. 
         [0059]    Multiple locations, either randomly distributed or according to a pre-determined pattern, on the retina could be depth scanned to monitor for malignant characteristics of the retina. Pre-determined pattern could be used to generate one or more images of retina regions. 
         [0060]    In another preferred embodiment a beam modifying optical element would consists of an optical delay element such as a glass element whose thickness and refractive index would pre-determine the magnitude of the optical delay. This embodiment is illustrated in  FIG. 5 . Locating a similar optical delay element in all of the odd numbered holes ( 1 ,  3 ,  5 ,  7 ,  9 ,  11 ,  13 ,  15  in the example of disc  501 ) and leaving the even numbered holes empty would enable the scan depicted in  FIG. 5B . 
         [0061]    In this embodiment the system would perform a scan at a deeper region  511  of the target  509  during the time that the (empty) even numbered holes of the rotating disc  513  are aligned with the probe radiation. Rotation of the disc to align the odd numbered holes containing similar optical delay elements with the probe radiation introduces the pre-determined delay in the optical path and as a consequence as second region  515  of the target  509  to be scanned where the second region  515  is offset in depth from the first region  511  by an amount determined by the optical delay. 
         [0062]    As depicted in  FIG. 5B  the system could be aligned to have first region  511  be the main region of interest within the target and the optical delay chosen to locate the second region  515  to scan the front surface of the target, thus providing a method of registering the location of the first scan with respect to the front surface. 
         [0063]    In this embodiment one of the two scans is used to register the location of the other scan by having one scan scanning a region of the front surface, and the other scan at a predetermined (by virtue of the predetermined delay introduced into the optical path), known depth. 
         [0064]    Another embodiment is illustrated in  FIG. 5C  which again has a target  517  and the rotating disc  519  and where a set of nine overlapping scan segments are depicted in the oval  521 . Every second of the scan segments within the oval  521  is offset for illustration clarity only. 
         [0065]    In a preferred embodiment the nine scan segments would be accomplished by having pre-determined optical delays have more than one value. In this embodiment again the odd numbered holes of the rotating disc  513  contain similar optical delay elements (to provide the same optical delay) and are depth aligned to scan the front surface of the target  517 . 
         [0066]    The eight odd numbered holes contain different optical delay elements with eight different values that are systematically smaller in magnitude (with the smallest optionally being empty). As the disc  519  rotates delays are introduced that have eight different values with a ninth value introduced between each of the eight other values. The resulting scan consists of nine scan segments at different, known depths, as depicted in the oval  521 . 
         [0067]    In this manner the rotating disc  519  containing the optical delay elements is a device that is synchronously interposed in the optical path, which effectively permits controlled and selectable delay magnitudes. 
         [0068]    The different optical delay elements may also include attenuation (such as ND filters) with different magnitudes that are associated with different magnitudes of delay. A preferred embodiment would include attenuation that increases in magnitude with increasing optical delay magnitude to allow reducing of the magnitude of the less deep signals (whose magnitude are typically larger) in order to optimize dynamic range. 
         [0069]    In the preferred embodiments described above, in addition to (or in place of) different attenuation being associated with different optical delays different focusing properties could be associated with different optical delays to compensate for different target associated focusing properties. 
         [0070]    In the preferred embodiments described above, the odd numbered optical delay elements would correspond to forward motion of the piezo device while the even numbered optical delay elements would correspond to reverse motion of the piezo device (or visa versa). Other arrangements are possible, for example the rotating disc could be synchronized such that the opaque disc regions (one of which is  507 ) coincides with the same turn around region of the piezo scan (at one extreme rather than alternating between the two extremes). 
         [0071]    Many other embodiments of the invention are possible. For example, angular deflection elements could be combined with optical delay elements as a single beam modifying optical element. An example is depicted in  FIG. 6  C which in many respects is similar to  FIG. 3  C however the portion of the retina  607  which is shown in greater detail in the circle  609  shows a scan  611  of the surface region of the retina and a scan  613  of a deeper region of the retina. In this example one beam-modifying element would accomplish an angular displacement and an optical delay, while a second beam modifying element would accomplish the same angular displacement but a larger optical delay. 
         [0072]    Other examples will be apparent to persons skilled in the art. The scope of this invention should be determined with reference to the specification, the drawings and the appended claims, along with the full scope of equivalents as applied thereto.