Imaging and visualization systems, instruments, and methods using optical coherence tomography

CROSS REFERENCE TO RELATED APPLICATION

0001This application claims the benefit of U.S. provisional patent application No. 61/434,242, filed Jan. 19, 2011, the disclosure of which is incorporated herein by reference in its entirety.

GOVERNMENT RIGHTS NOTICE

0002This invention was made with government support under grant numbers EY019411 and RR024128, awarded by the National Institutes of Health. The government may have certain rights in the invention.

TECHNICAL FIELD

0003The presently disclosed subject matter relates to surgical instruments and imaging equipment, and more specifically, to surgical imaging and visualization systems, instruments, and methods using optical coherence tomography.

BACKGROUND

0004Optical coherence tomography (OCT) has emerged as a promising imaging modality for micrometer-scale noninvasive imaging in biological and biomedical applications. Its relatively low cost and real-time, in vivo capabilities have fueled the investigation of this technique for applications in retinal and anterior segment imaging in ophthalmology (e.g., to detect retinal pathologies), early cancer detection and staging in the skin, gastrointestinal, and genitourinary tracts, as well as for ultra-high resolution imaging of entire animals in embryology and developmental biology.

0005Conventional OCT systems are essentially range-gated low-coherence interferometers that have been configured for characterization of the scattering properties of biological and other samples. By measuring backscattered light as a function of depth, OCT fills a valuable niche in imaging of tissue ultrastructure, and provides subsurface imaging with high spatial resolution (˜1-10 μm) in three dimensions and high sensitivity (>110 dB) in vivo with no contact needed between the probe and the tissue. OCT is based on the one-dimensional technique of optical coherence domain reflectometry (OCDR), also called optical low-coherence reflectometry (OLCR). See Youngquist, R. C., S. Carr, and D. E. N. Davies, <i>Optical Coherence Domain Reflectometry: A New Optical Evaluation Technique</i>. Opt. Lett., 1987. 12: p. 158; Takada, K., et al., <i>New measurement system for fault location in optical waveguide devices based on an interferometric technique</i>. Applied Optics, 1987. 26(9): p. 1603-1606; and Danielson, B. L. and C. D. Whittenberg, <i>Guided</i>-<i>wave Reflectometry with Micrometer Resolution</i>. Applied Optics, 1987. 26(14): p. 2836-2842. In some instances of time-domain OCT, depth in the sample is gated by low coherence interferometry. The sample is placed in the sample arm of a Michelson interferometer, and a scanning optical delay line is located in the reference arm.

0006The time-domain approach used in conventional OCT has been used in supporting biological and medical applications. An alternate approach involves acquiring as a function of optical wavenumber the interferometric signal generated by mixing sample light with reference light at a fixed group delay. Two methods have been developed which employ this Fourier domain (FD) approach. The first is generally referred to as spectrometer-based or spectral-domain OCT (SDOCT). SDOCT uses a broadband light source and achieves spectral discrimination with a dispersive spectrometer in the detector arm. The second is generally referred to as swept-source OCT (SSOCT) or alternatively as optical frequency-domain imaging (OFDI). SSOCT time-encodes wavenumber by rapidly tuning a narrowband source through a broad optical bandwidth. Both of these techniques can provide improvements in signal-to-noise ratio (SNR) of up to 15-20 dB when compared to time-domain OCT, because SDOCT and SSOCT capture the complex reflectivity profile (the magnitude of which is generally referred to as the “A-scan” data or depth-resolved sample reflectivity profile) in parallel. This is in contrast to time-domain OCT, where destructive interference is employed to isolate the interferometric signal from only one depth at a time as the reference delay is scanned.

0007Surgical visualization has changed drastically since its inception, incorporating larger, more advanced optics toward increasing illumination and field-of-view (FOV). However, the limiting factor in vitreoretinal surgery remains the ability to distinguish between tissues with subtle contrast, and to judge the location of an object relative to other retinal substructures. S. R. Virata, J. A. Kylstra, and H. T. Singh, Retina 19, 287-290 (1999); E. Garcia-Valenzuela, A. Abdelsalam, D. Eliott, M. Pons, R. Iezzi, J. E. Puklin, M. L. McDermott, and G. W. Abrams, Am J Ophthalmol 136, 1062-1066 (2003). Furthermore, increased illumination to supplement poor visualization is also limited by the risks of photochemical or photothermal toxicity at the retina. S. Charles, Retina 28, 1-4 (2008); J. R. Sparrow, J. Zhou, S. Ben-Shabat, H. Vollmer, Y. Itagaki, and K. Nakanishi, Invest Ophthalmol Vis Sci 43, 1222-1227 (2002). Finally, inherent issues in visualizing thin translucent tissues, in contrast to underlying semitransparent ones, require the use of stains such as indocyanine green, which is toxic to the retinal pigment epithelium. F. Ando, K. Sasano, N. Ohba, H. Hirose, and O. Yasui, Am J Ophthalmol 137, 609-614 (2004); A. K. Kwok, T. Y. Lai, K. S. Yuen, B. S. Tam, and V. W. Wong, Clinical &amp; experimental ophthalmology 31, 470-475 (2003); J. Lochhead, E. Jones, D. Chui, S. Lake, N. Karia, C. K. Patel, and P. Rosen, Eye (London, England) 18, 804-808 (2004).

0008SDOCT has demonstrated strong clinical success in retinal imaging, enabling high-resolution, motion-artifact-free cross-sectional imaging and rapid accumulation of volumetric macular datasets. N. A. Nassif, B. Cense, B. H. Park, M. C. Pierce, S. H. Yun, B. E. Bouma, G. J. Tearney, T. C. Chen, and J. F. de Boer, Optics Express 12, 10 (2004); M. Wojtkowski, V. J. Srinivasan, T. H. Ko, J. G. Fujimoto, A. Kowalczyk, and J. S. Duker, Optics Express 12, 2404-2422 (2004). Current generation SDOCT systems achieve greater than 5 μm axial resolutions in tissue, and have been used to obtain high resolution datasets from patients with neovascular AMD, high risk drusen, and geographic atrophy. M. Stopa, B. A. Bower, E. Davies, J. A. Izatt, and C. A. Toth, Retina 28, 298-308 (2008). Other implementations of OCT including SSOCT may offer similar performance advantages.

0009Intraoperative guidance of surgical procedures using optical coherence tomography (OCT) holds promise to aid surgeons visualize microscopic tissue structures in preparation for and during surgery. This potential includes visualization of the critical interface where surgical tools (e.g., scalpels, forceps, needles, scrapers) intersect and interact with tissue surfaces and sub-surface structures. In many cases, critical aspects of this dynamic interaction exceed the spatial or temporal resolution of conventional imaging devices used during surgery (e.g., surgical microscopes, endoscopes, ultrasound, CT, and MRI). A particularly compelling case for OCT guidance is in ophthalmic surgery, since OCT is already a widely accepted diagnostic in ophthalmology, and real-time visualization of delicate and translucent tissues during intrasurgical maneuvers could be of great benefit to surgeons and patients. In ophthalmic surgery of both anterior and posterior segments of the eye, the typical imaging modality used to visualize microscopic tissue structures is stereo zoom microscopy. Surgical microscopes provide real-time natural color imaging to the surgeon, however the quality of the imagery is often severely limited by the available illumination and the quality of the patient eye's own optics, particularly for retinal surgery. Additionally, conventional surgical microscopy only provides en-face imagery of the surgical field, which bears little to no depth information, forcing the surgeon to infer when instruments are in contact with tissue surfaces, how deep instruments are penetrating, how thick tissue structures are, etc. As a cross-sectional imaging modality, OCT is particularly well suited to providing critical depth-resolved information in ophthalmic surgery. The advent of Fourier domain OCT (FDOCT) approaches including both SDOCT and SSOCT is especially promising for providing real time feedback because of their enhanced SNR compared to prior time-domain. OCT methods, thus enabling much faster imaging than previously available.

0010FDOCT systems have been developed for use during surgery, including breast cancer biopsy and surgery and ophthalmic surgery of the anterior segment and retina. High speed FDOCT systems are available, including research SSOCT systems now operating in excess of 5,000,000 A-scans/sec, corresponding to tens of volumes per second. However, these systems are very complex and expensive, and require illumination light levels which may not be safe for human ocular exposure. Even with advances in higher speed OCT scanning, real-time complete volumetric imaging in the intrasurgical setting, where OCT imaging must be safely compatible with other bright light sources, may not be achievable. Thus, there is a need for systems, equipment, and techniques for using current or near-future generation OCT systems to provide useful, real time feedback to the surgeon.

SUMMARY

0011This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

0012Disclosed herein are microscope-integrated OCT (MIOCT) systems that integrate OCT imaging into the optical path of a surgical microscope for direct and OCT imaging. As a result, fast feedback is provided to the surgeon with less intrasurgical disruption than previous intrasurgical OCT protocols that require the surgeon to use a hand-held OCT probe during intermittent pauses during surgery. According to an aspect, an MIOCT system utilizes a standard current-generation SDOCT engine which images at a rate of 15,000-40,000 A-scans/sec, ˜20 B-scans/sec (with 1000 A-scans/B-scan), and acquires a complete 3D volumetric image in ˜5 sec. The systems and methods disclosed herein provide useful, near real-time intrasurgical imaging by using current or near-future generation OCT hardware in combination with a feedback control system to localize OCT image data acquisition to the region of the tip of the surgical tool, or the site of its interaction with the tissue. This occurs by tracking some predetermined feature of the surgical tool (such as its tip, some markings made upon it or indented into it, or a reflective spot or light source located on it) and using that position information to direct the OCT system to concentrate image acquisition in a region relative to that position which is of anticipated importance during surgery. A variety of OCT scan patterns, protocols, and displays are disclosed, which may be of particular value for guiding surgery, such as small numbers of B-scans (which can still be acquired in real time as perceived by the surgeon) acquired with specific orientation relative to the tool tip, small volumetric datasets (which can still be acquired in real time as perceived by the surgeon) localized to the tool tip location, or other novel combinations of A-scans. In addition to providing real-time imaging capability, the image data thus acquired can also be used in combination with the saved instrument track data and image processing techniques to build up and maintain an evolving three-dimensional (3D) rendition of the entire operative field of view. The control system may also perform adaptive sampling of the field of view, for example directing the OCT scanner to prioritize filling in missing information when tool movement reveals a previously unsampled region of retina, which had until then been shadowed by the tool. Thus, in this disclosure, an intelligent feedback control system is disclosed that can be readily modified to the surgical style of the surgeon.

0013According to an aspect, a method for OCT imaging includes receiving multiple OCT B-scans of a field of view area that includes an instrument. The method also includes applying spatial compounding to the B-scans to generate an OCT image of the area of the field of view area.

0014According to another aspect, a method for OCT image capture includes determining a location of a feature of interest within an operative field. The method also includes determining a relative positioning between the feature of interest and an OCT scan location. Further, the method includes controlling capture of an OCT image at a set position relative to the feature of interest based on the relative positioning.

0015According to another aspect, a surgical microscope system includes a heads-up display (HUD). The system also includes an ocular eyepiece unit having the HUD integrated therein for display via the ocular eyepiece unit. Further, the system includes a user interface controller configured to determine surgical information associated with a surgical site image projected for view through the ocular eyepiece unit. The user interface controller is also configured to control the HUD to display the surgical information.

0016According to yet another aspect, a surgical instrument for use in optical coherence tomography (OCT)-imaged surgical procedures is disclosed. The surgical instrument comprises a body having a predefined shape for improving capture of OCT images of nearby tissue during a surgical procedure, or being made from a combination of materials which optimize its appearance in OCT or video images or reduce the amount by which the instrument shadows the underlying tissue in OCT imaging.

BRIEF DESCRIPTION OF THE DRAWINGS

0017The foregoing summary, as well as the following detailed description of various embodiments, is better understood when read in conjunction with the appended drawings. For the purposes of illustration, there is shown in the drawings exemplary embodiments; however, the presently disclosed subject matter is not limited to the specific methods and instrumentalities disclosed. In the drawings:

0018<figref idref="DRAWINGS">FIG. 1</figref> is a schematic diagram of the microscope-attached components of an example microscope-integrated OCT (MIOCT) system according to embodiments of the presently disclosed subject matter;

0019<figref idref="DRAWINGS">FIG. 2</figref> is a schematic diagram of an OCT unit and a microscope having a vitreoretinal viewing unit;

0020<figref idref="DRAWINGS">FIG. 3</figref> is a schematic diagram of an example microscope-integrated OCT (MIOCT) system in accordance with embodiments of the present disclosure;

0021<figref idref="DRAWINGS">FIG. 4</figref> is a flowchart of an example method for OCT imaging in accordance with an embodiment of the present disclosure;

0022<figref idref="DRAWINGS">FIGS. 5A-5F</figref> illustrate various B-scan patterns with respect to a surgical instrument in accordance with embodiments of the present disclosure;

0023<figref idref="DRAWINGS">FIG. 6</figref> is an image of a volumetric dataset acquired of an eye by scanning the general location of instrument-tissue interaction and associated B-scans;

0024<figref idref="DRAWINGS">FIG. 7</figref> shows a time lapse and lateral position montage of sequentially acquired volume images of intrasurgical manipulations using OCT and spatial compounding in accordance with embodiments of the present disclosure;

0025<figref idref="DRAWINGS">FIG. 8</figref> is a flowchart of an example method for tracking a feature of interest using OCT in accordance with embodiments of the present disclosure;

0026<figref idref="DRAWINGS">FIG. 9</figref> illustrates a schematic diagram of an example model for image feature tracking in accordance with an embodiment of the present disclosure;

0027<figref idref="DRAWINGS">FIG. 10</figref> illustrates images in which two sparsely sampled sequences are fused together creating a dense representation of the underlying pathological structures;

0028<figref idref="DRAWINGS">FIG. 11</figref> depicts various images of example data obtained from simulated surgery on an excised porcine eye with the anterior segment removed; and

0029<figref idref="DRAWINGS">FIG. 12</figref> illustrates orthogonal B-scans acquired using the scan pattern shown in <figref idref="DRAWINGS">FIG. 5C</figref> during simulated porcine surgery and orthogonal B-scans.

DETAILED DESCRIPTION

0030The presently disclosed subject matter is described with specificity to meet statutory requirements. However, the description itself is not intended to limit the scope of this patent. Rather, the inventors have contemplated that the claimed subject matter might also be embodied in other ways, to include different steps or elements similar to the ones described in this document, in conjunction with other present or future technologies.

0031Like numbers may refer to like elements throughout. In the figures, the thickness of certain lines, layers, components, elements or features may be exaggerated for clarity.

0032The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the presently disclosed subject matter. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

0033Further, it is noted that although the term “step” may be used herein to connote different aspects of methods employed, the term should not be interpreted as implying any particular order among or between various steps herein disclosed unless and except when the order of individual steps is explicitly described.

0034Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present subject matter belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Well-known functions or constructions may not be described in detail for brevity and/or clarity.

0035The presently disclosed subject matter is described below with reference to block diagrams and/or flowchart illustrations of methods, apparatus (systems) and/or computer program products. It is understood that each block of the block diagrams and/or flowchart illustrations, and combinations of blocks in the block diagrams and/or flowchart illustrations, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, and/or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer and/or other programmable data processing apparatus, create means for implementing the functions/acts specified in the block diagrams and/or flowchart block or blocks.

0036In some embodiments, OCT imaging of a surgical specimen or operating field in conjunction with standard surgical microscopy may be used to give the surgeon an additional, two- or three-dimensional view of structures which may be difficult or impossible for the surgeon to visualize with a standard microscope alone. These structures may be difficult to visualize because they are beyond the resolution limit of the microscope optics or of the surgeon's eye, or are poorly lit, translucent, opaque, or buried in a translucent or opaque structure. OCT 2D images may be acquired in a cross-sectional view which complements the en-face view which the surgeon sees through the surgical microscope. OCT 3D volume images convey more information regarding the structures and their spatial orientation and relationships than is available in the surgeon's standard view through the microscope.

0037<figref idref="DRAWINGS">FIG. 1</figref> illustrates a schematic diagram of the microscope-attached components <b>100</b> of an example microscope-integrated OCT (MIOCT) system according to embodiments of the presently disclosed subject matter. Referring to <figref idref="DRAWINGS">FIG. 1</figref>, the system is implemented as an attachment to an ophthalmic surgical microscope <b>102</b> with a binocular indirect ophthalmo-microscope (BIOM) attachment for imaging the retina. The microscope-attached components <b>100</b> include an OCT scanning unit <b>104</b> which is part of the sample arm of an OCT system. The OCT scanning unit <b>104</b> includes a collimating lens <b>128</b>, two-axis galvanometer scanners (G_x, G_y) <b>108</b>, <b>110</b>, a beam-forming assembly comprising a 12.5× beam expander <b>114</b>, <b>116</b> (f₁, f₂), and a beamsplitter <b>118</b>. The OCT scanning unit <b>104</b> may direct an OCT beam through a microscope objective <b>120</b> shared with the surgical microscope, and a BIOM retinal viewing attachment including a reduction lens <b>122</b> and a non-contact wide-field lens <b>124</b>, in order to scan the OCT beam across a 12 mm field of view on a retina of a patient's eye <b>126</b> or another surgical site. The backscattered OCT light may be returned back through the same optical path and re-focused by the collimating lens <b>128</b> into an optical fiber which conveys the light back to the OCT interferometer. The BIOM unit is just one option for converting a standard operating room microscope for retinal viewing, the other primary means used in retinal surgery is imaging through a contact lens designed to compensate for the optical power of the cornea. Both the BIOM unit and contact lens are for conversion of a standard surgical microscope for retinal imaging. Without such conversion, a standard surgical microscope may be used for imaging of the anterior segment of the eye or for any other exposed tissue surface in the body. All of the same methods for OCT scanning through the BIOM for retinal imaging may equivalently be accomplished through a contact lens for retinal imaging, or with no other optics between the shared objective and the tissue surface for non-retinal imaging. Although not shown in <figref idref="DRAWINGS">FIG. 1</figref>, it is noted that the MIOCT system may also include an OCT engine and computer as described herein, such as the computer and OCT engine shown and described with respect to <figref idref="DRAWINGS">FIG. 3</figref>.

0038<figref idref="DRAWINGS">FIG. 2</figref> illustrates an OCT scanning unit <b>200</b> and a microscope <b>202</b> having a vitreoretinal viewing unit. The OCT scanning unit <b>200</b> includes a collimating lens <b>204</b>, scanners <b>206</b>, a beam forming optical assembly having optics or lenses <b>208</b>, <b>210</b>, and a beamsplitter <b>212</b>. The microscope <b>202</b> includes a shared main objective <b>214</b>, an illumination unit <b>216</b>, microscope optics <b>218</b> having dual channels <b>220</b>, <b>222</b> and respective eyepieces <b>224</b>. The vitreoretinal viewing unit includes a non-contact widefield lens <b>226</b> and a reduction lens <b>228</b>. In this configuration, OCT may be incorporated into an intrasurgical microscope for obtaining OCT images of the retina and macula with increased resolution and brightness supported by the physical dimensions of the microscope, while maintaining compatibility for matching the field of view of the OCT scanning unit <b>200</b> and microscope views and avoiding OCT image vignetting. The OCT sample arm beam is introduced into the optical path of the microscope in the infinity space above the shared main objective <b>214</b> of the microscope, by use of a dichroic beamsplitter <b>212</b> which reflects OCT wavelengths (typically in the near infrared) while transmitting visible wavelengths carrying the white light microscope image to the optical imaging channels above. Such a dichroic mirror is typically referred to as a “hot mirror.” The OCT sample arm beam is assumed to be delivered to the vicinity of the microscope via a single mode optical fiber and impinging upon a collimating lens <b>204</b> with focal length f₇, as depicted in <figref idref="DRAWINGS">FIG. 2</figref>, or it may impinge directly from a bulk-optic OCT system lacking optical fibers. The narrow OCT sample arm beam is incident on a pair of orthogonal optical scanners <b>206</b>, depicted as a bolded cross in <figref idref="DRAWINGS">FIG. 2</figref>. Many different scanning technologies can and have been used in OCT systems, although the most typical are limited-angle galvanometer scanners. Most scanning technologies however share the attributes that their cost and performance is typically inversely proportional to the size of their optical aperture; this is why they are typically employed in OCT systems where the sample arm beam is narrow. In <figref idref="DRAWINGS">FIG. 2</figref>, an orthogonally oriented pair of scanners is indicated, capable of scanning the narrow OCT sample arm beam over a limited range of angles in two dimensions under computer control. It will be understood to one skilled in the art of optical scanning systems that such a pair may be constructed from a single mirror having the capability of pivoting in two dimensions, or a pair of single-axis galvanometer scanners located in close proximity (which is the most common arrangement), or a pair of single-axis galvanometer scanners with the optical aperture of one being imaged into the optical aperture of the other using, for example, a standard 4f optical imaging system comprised of a pair of lenses. It is also to be understood that any of these technologies may be used and equivalently represented by the cross symbol depicted in <figref idref="DRAWINGS">FIG. 2</figref>.

0039As illustrated, the beam-forming optical assembly is positioned between the 2D scanners <b>206</b> and the OCT beamsplitter <b>212</b> above the shared main objective <b>214</b> of the microscope <b>202</b>. The design purpose of the optical assembly is to match the size, divergence, and scanning geometry of the OCT sample arm beam to the maximum capabilities supported by the microscope <b>202</b>. In <figref idref="DRAWINGS">FIG. 2</figref>, this is accomplished using a simple, adjustable Keplerian telescope as a beam expander which is placed at the proper position with respect to the microscope, although other possible optical designs could be used to accomplish the same objective (such as, most simply, a Galilean teslescope). For the beam former depicted in <figref idref="DRAWINGS">FIG. 2</figref>, the optical design parameters are as follows. The ratio of the focal lengths of the two lenses <b>210</b> (f₅) and <b>208</b> (f₆) may be chosen so that the narrow OCT beam with 1/e²beam diameter a₆is expanded to approximately fill the back aperture of the shared main microscope objective a₃. This is accomplished when:

0000<maths id="MATH-US-00001" num="00001"><math overflow="scroll"><mtable><mtr><mtd><mrow><mfrac><msub><mi>f</mi><mn>5</mn></msub><msub><mi>f</mi><mn>6</mn></msub></mfrac><mo>=</mo><mrow><mfrac><msub><mi>a</mi><mn>3</mn></msub><msub><mi>a</mi><mn>6</mn></msub></mfrac><mo>.</mo></mrow></mrow></mtd><mtd><mrow><mi>Eq</mi><mo>.</mo><mstyle><mspace width="0.8em" height="0.8ex" /></mstyle><mo></mo><mrow><mo>(</mo><mn>10</mn><mo>)</mo></mrow></mrow></mtd></mtr></mtable></math></maths><img file="US2012184846A1_D0001.tif" />

0040In order that the divergence of the OCT sample beam generally match the divergence of the microscope image light above the shared objective which may not be exactly parallel, and therefore provide the capability to closely match the position within the patient's retina of the OCT focus to that of the microscope, some variability should be provided in the position of either or both of lenses <b>210</b> (f₅) and <b>208</b> (f₆), as depicted in <figref idref="DRAWINGS">FIG. 2</figref>. This can be accomplished by mounting one or both lenses on a mechanical stage which can be adjusted manually or electronically by the surgeon or an assistant while viewing both images. For this purpose, an adjustment range of either lens of approximately 5% of its focal length will suffice.

0041In order to reduce or prevent vignetting of the OCT beam during scanning, while simultaneously making use of the entire optical aperture of the shared objective at the same time, optimal design of the beam-forming optical assembly includes provision that the OCT beam pivot through the main objective rather than scan across it. In general, this can be accomplished by designing the optical assembly in such a way as to optically image the optical scanner plane into the main objective back aperture. For the Keplerian telescope depicted in <figref idref="DRAWINGS">FIG. 18</figref>, this is accomplished by placement of the lenses of the telescope relative to the scanner and objective location, such that the following relationship among the distances between the lenses is satisfied:

0000<br /><i>d</i>₆<i>=f</i>₆<i>; d</i>_s<i>=f</i>_s<i>+f</i>₆<i>; d</i>₄<i>=f</i>_s  Eq. (1)

0042Furthermore, for OCT imaging of a patient's retina with increased or maximum resolution and image brightness, the OCT sample arm beam may be designed at the position of the patient's cornea to have a beam diameter which is the maximum over which a human's eye is approximately diffraction limited (typically 2-3 mm without use of adaptive optics, and up to 6-7 mm if adaptive optics are used to compensate for the eye's aberrations), and that the scanning OCT beam pivot through the patient's iris plane rather than scan across it, so that the entire available optical aperture is used for all scan widths without vignetting. The first condition on beam size at the cornea is satisfied by realizing that the lenses <b>226</b> (f₂), <b>228</b> (f₃), and <b>214</b> (f₄) operate essentially as a Keplerian beam reducer. As a simplifying assumption, the reducing lens <b>228</b> (f₃) typically has much less optical power than the main objective <b>214</b> (f₄), and is located directly adjacent to it. Then, these two lenses can be considered as operating together as a single lens, with optical power given by the sum of the optical powers of the lenses individually, and located at the position of the main objective <b>214</b>. According to this approximation, the main objective <b>214</b> is replaced by a modified main objective with focal length f₄according to:

0000<maths id="MATH-US-00002" num="00002"><math overflow="scroll"><mtable><mtr><mtd><mrow><mfrac><mn>1</mn><msubsup><mi>f</mi><mn>4</mn><mi>′</mi></msubsup></mfrac><mo>=</mo><mrow><mfrac><mn>1</mn><msub><mi>f</mi><mn>3</mn></msub></mfrac><mo>+</mo><mfrac><mn>1</mn><msub><mi>f</mi><mn>4</mn></msub></mfrac></mrow></mrow></mtd><mtd><mrow><mi>Eq</mi><mo>.</mo><mstyle><mspace width="0.8em" height="0.8ex" /></mstyle><mo></mo><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow></mrow></mtd></mtr></mtable></math></maths><img file="US2012184846A1_D0002.tif" />

0043Now the design condition on the choice of lenses f₂and f₄′ to ensure the correct beam size on the patient's cornea is given by:

0000<maths id="MATH-US-00003" num="00003"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><mfrac><msubsup><mi>f</mi><mn>4</mn><mi>′</mi></msubsup><msub><mi>f</mi><mn>2</mn></msub></mfrac><mo>=</mo><mfrac><msub><mi>a</mi><mn>3</mn></msub><msub><mi>a</mi><mn>1</mn></msub></mfrac></mrow><mo>,</mo></mrow></mtd><mtd><mrow><mi>Eq</mi><mo>.</mo><mstyle><mspace width="0.8em" height="0.8ex" /></mstyle><mo></mo><mrow><mo>(</mo><mn>3</mn><mo>)</mo></mrow></mrow></mtd></mtr></mtable></math></maths><img file="US2012184846A1_D0003.tif" />

0044where a1 is the desired collimated beam size on the cornea. Finally, in order to have the OCT beam pivot through the patients' iris plane rather than scan across it, the position of lens f₂should be set so that it forms a real image of the aperture of the shared main objective at the location of the patient's iris plane. Thus, the distances d₁and d₂(which equals f_4′+f₂) should be set according to:

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0045As a practical design procedure, f₂and f₃should be chosen according to Eq. (2) and (3) given the constraint the microscope imposes on f₄, then d₁may be chosen according to Eq. (4).

0046In the foregoing, it is to be understood that the distances d₄, d₅, etc. correspond to the center-to-center distances between the lenses referred to. Also, it is to be understood that the relationships given in all of these design equations are simplified according to the assumption that all of the lenses act as ideal lenses which is appropriate for first-order design. For production design, professional lens design software may be used to optimize higher order performance, however following the principles described here.

0047In the configuration illustrated in <figref idref="DRAWINGS">FIG. 2</figref>, all of the OCT optical elements are generally positioned either within the microscope body itself, or else sufficiently high up on the microscope body that the OCT assembly will not interfere with the surgeon's view of or access to the patient. However, this configuration may lengthen the overall height of the surgical microscope by displacing the optical imaging channels upward in order to make room for the beamsplitter, which may be inconvenient for the surgeon. Secondly, the configuration illustrated in <figref idref="DRAWINGS">FIG. 2</figref> utilizes the main objective for OCT in addition to white light imaging.

0048<figref idref="DRAWINGS">FIG. 3</figref> illustrates a schematic diagram of an example microscope-integrated OCT (MIOCT) system <b>300</b> in accordance with embodiments of the present disclosure. Referring to <figref idref="DRAWINGS">FIG. 3</figref>, the system <b>300</b> includes the microscope <b>202</b>, the OCT scanning unit <b>200</b>, a video fundus camera <b>302</b>, and one or more computers <b>304</b>. In this example, the microscope <b>202</b> is configured with a port for the video fundus camera <b>302</b>, which may capture video of a surgical site <b>306</b>, which is a retina in this example. The view of the surgical site <b>306</b> captured by the camera <b>302</b> may be the same as that viewed by an operator of the microscope <b>202</b> through oculars <b>308</b>. In an example, the camera <b>302</b> may be a 1.3 megapixel CMOS color camera configured to digitize the top-down view of the surgical site <b>306</b>. Further, in this example, an intensity peak-finding algorithm may be implemented by the computer <b>304</b> for capturing images of the surgical site <b>306</b> and a surgical instrument positioned within a field of view. The frame rate of the camera <b>306</b> may operate at 15 frames per second (fps) that is comparable to a B-scan rate of the OCT unit <b>200</b>. The surgical instrument may be a diamond-dusted silicone-tipped instrument (e.g., a Tano scraper) having a non-reflective tip, for example, and marked with a predefined color marking for tracking in accordance with embodiments of the present disclosure. An OCT engine <b>307</b> may interface the OCT scanning unit <b>200</b> with the computer <b>304</b> and may include, but is not limited to, a light source, an interferometer, a reference arm, and a detector. In the case of an OCT engine based on SDOCT technology, the light source may be a broadband light source such as a superluminescent diode, and the detector may be a spectrometer coupled to a linear detector array. In the case of an OCT engine based on SSOCT technology, the light source may be a frequency swept laser and the detector may be a single or multiple receivers arranged in a dual-balanced configuration.

0049The computer <b>304</b> may be any suitable computing device and include a processor <b>308</b> and memory <b>310</b> for implementing surgical imaging and visualization of a surgical site and instruments using OCT in accordance with embodiments of the present disclosure. In accordance with an embodiment, <figref idref="DRAWINGS">FIG. 4</figref> illustrates a flowchart of an example method for OCT imaging. The method of <figref idref="DRAWINGS">FIG. 4</figref> is described in the following example as being implemented by the system of <figref idref="DRAWINGS">FIG. 3</figref>; however, it should be understood that the method may be implemented by any suitable system. In this example, the method includes receiving OCT B-scans of a field of view area that includes an instrument (step <b>400</b>). For example, the B-scans captured by the OCT unit <b>200</b> may be communicated to the computer <b>304</b>. The computer <b>304</b> may include an input/output (I/O) module <b>312</b> configured to receive the B-scans for storage in memory <b>310</b> and processing by the processor <b>308</b>. A-scans or any other suitable images may be captured by the OCT unit <b>200</b> and communicated to the computer <b>304</b>. Example surgical instruments include, but are not limited to, scalpels, scrapers, and forceps.

0050The computer <b>304</b> may operate as a controller for controlling a timing and scan pattern of scans captured by the OCT unit <b>200</b>. The computer <b>304</b> may control the OCT unit <b>200</b> to capture scan patterns based on a surgical tool position and orientation. For example, the scan pattern of the B-scans may be substantially aligned with an axis of the surgical instrument. In an example, the B-scans may have a pattern with a long-axis aligned substantially with an axis of the instrument and a short-axis substantially collinear with the surgical instrument. <figref idref="DRAWINGS">FIGS. 5A-5F</figref> illustrate various B-scan patterns, indicated by broken lines <b>500</b>, with respect to a surgical instrument <b>502</b> in accordance with embodiments of the present disclosure. Referring to <figref idref="DRAWINGS">FIGS. 5A and 5B</figref>, B-scans can be acquired in real time in orientations either perpendicular as shown in <figref idref="DRAWINGS">FIG. 5A</figref> or parallel as shown in <figref idref="DRAWINGS">FIG. 5B</figref> with respect to an axis of the surgical instrument <b>502</b>. In another example, <figref idref="DRAWINGS">FIG. 5C</figref> shows a rapidly captured set of orthogonal B-scans. In another example, the orientation of the B-scans may be arbitrary based on a preference of a surgeon or a type and/or shape of the surgical instrument. In yet another example, two B-scans may be oriented directly orthogonal and intersecting at the middle, and with one B-scan being directly aligned with an axis of the surgical instrument and the other B-scan being perpendicular. These B-scans can provide desired information about the instrument angle along or relative to tissue, as well as have a second view (e.g., perpendicular view) such that the surgeon has a different perspective. <figref idref="DRAWINGS">FIGS. 5E and 5F</figref> show scan patterns in which small volumes may be acquired with a fast-axis either parallel (<figref idref="DRAWINGS">FIG. 5E</figref>) or perpendicular (<figref idref="DRAWINGS">FIG. 5F</figref>) to the instrument axis. Also, scanning the OCT beam along the tool initially, and subsequently scanning directly beside the instrument may be useful because the scanning beam may not be obstructed by the partially or fully opaque instrument.

0051Referring again to <figref idref="DRAWINGS">FIG. 4</figref>, the method includes applying spatial compounding to the B-scans to generate an OCT image of the area of the field of view area (step <b>402</b>). For example, spatial compounding may include combining the B-scans to generate an OCT image. The B-scans may be combined to generate the OCT image by averaging the B-scans. In another example, the B-scans may be combined to generate the OCT image by performing a weighted average of the B-scans. The processor <b>308</b> and memory <b>310</b> of the computer <b>304</b> shown in <figref idref="DRAWINGS">FIG. 3</figref> may implement the step of spatial compounding. The generated OCT image may subsequently be displayed by control of a display of a user interface <b>314</b>. The OCT unit <b>200</b> may be controlled to capture OCT B-scans at a lateral spacing of a predetermined distance.

0052<figref idref="DRAWINGS">FIG. 6</figref> is an image (a) of a volumetric dataset acquired of an eye by scanning the general location of instrument-tissue interaction. The image capture occurred over an area of 8×0.4 mm. The area was sampled with 1024×256 pixels (spectral×lateral) at 20 kHz line-rate, and with 10 B-scans across the short scanning axis as shown in image (a) of <figref idref="DRAWINGS">FIG. 6</figref>. The pattern of the scan is indicated by the broken arrows shown in image (a) of <figref idref="DRAWINGS">FIG. 6</figref>. The long-axis of the scan pattern was positioned approximately along the axis of a surgical instrument, and the short-axis of the scan pattern was approximately collinear with the surgical instrument. Images (b)-(e) shown in <figref idref="DRAWINGS">FIG. 6</figref> represent B-scans, separated by 0.12 mm across the short-axis of the scan patter, demonstrating the instrument cross-section moving into and out of view. The scans of images (b)-(e) show regions of tissue deformation and discrete portions of the surgical instrument, which is a subretinal needle in this example, that overlap with the OCT field of view. In spatial compounding, all of the 10 B-scans in the short scanning axis are averaged to allow for visualization of the complete instrument and underlying tissue without the sectioning and shadowing artifacts which are apparent in the individual B-scans. In this example, the long scanning axis is large enough such that any instrument motion remains at a temporal spacing of 25.6 ms per frame, or at a frame rate of 39 Hz. Here, cross-sections of intrasurgical instrument-tissue interactions acquired at video rates over the entire region of interest are shown. As an additional benefit, spatial compounding may serve to improve signal-to-noise (SNR) of the images by averaging a series of B-scans.

0053<figref idref="DRAWINGS">FIG. 7</figref> shows a time lapse and lateral position montage of sequentially acquired volume images of intrasurgical manipulations using OCT and spatial compounding in accordance with embodiments of the present disclosure. Along the lateral position direction, for each time point, evenly spaced raw B-scans across the short scanning axis are shown in the top four rows of frames in the vertical axis to illustrate inadequate visualization of the entire instrument and its interaction with the tissue by sampling at a single lateral position. Depending on the orientation of the instrument relative to the scanning axis, only portions of the instrument overlap with the field of view of the OCT in each B-scan, and the instrument moves relative to the OCT field of view over time (see the top four rows of frame in the horizontal axis). All 10 B-scans across the short scanning axis may be averaged (see the bottom row of frames) to demonstrate the advantages of spatial compounding for visualizing the entire region of interest. In this example, the surgical site is a porcine eye which was lifted using a subretinal needle and imaged using MIOCT. The surgical procedure was performed by viewing through a surgical microscope with concurrent acquisition of MIOCT volumes. A 8×0.4 mm volumetric dataset was acquired with 10 B-scans, sampled with 1024×256 pixels (spectral×lateral) at 20 kHz line-rate. The long-axis of the scan pattern was positioned approximately along the axis of the instrument and the short-axis of the scan pattern was approximately collinear with the instrument. 100 consecutive volumes were then acquired to capture tissue-instrument interactions. Here, individual raw B-scans are shown in the frames of the top 4 rows for different time points at evenly spaced lateral positions across the short scanning axis. These individual frames show incomplete visualization of the surgical tool as the instrument cross-section moves into and out of the interrogation path of the OCT beam. By averaging these raw frames across all 0.4 mm of the short scanning dimension using spatial compounding, complete instrument cross-sections can be visualized for all time points in the dataset shown in the bottom row of frames. Over the course of the time lapse, these averaged frames show retinal deformation due to compression of the vitreous by the subretinal needle and to lifting of the retinal membrane by the subretinal needle. Given the sampling parameters used in this example acquisition, the temporal spacing between each average frame is 25.6 ms, resulting in an effective spatial compounding frame rate of 39 Hz, demonstrating video rate interrogation of intrasurgical manipulations. These sampling parameters can be optimized to specific region of interest sizes and spatial and temporal sampling densities.

0054In accordance with embodiments of the present disclosure, methods for tracking OCT image capture to the location of a feature of interest in an operative field are provided. As an example, such tracking methods may be used to track a location of a feature of interest within one or more captured images. The images may be captured, for example, by the camera <b>302</b> shown in <figref idref="DRAWINGS">FIG. 3</figref>. Alternatively, for example, the images may be OCT summed voxel projection (SVP) images, OCT B-scans, or a scanning laser ophthalmoscopy (SLO) image. Further, the captured images may be stored in the memory <b>310</b> of the computer <b>304</b>. The feature of interest may be identified and tracked for controlling the subsequent capture of OCT images such that these images include the feature of interest. The feature of interest may be, for example, a surgical image, tissues, or portions thereof. Alternatively, the location of the feature of interest within the surgical field which is desired to be tracked with OCT may be obtained through means independent of any optical imaging modality. Such means may include localization of features or instruments by alternative imaging modalities such as ultrasound, computed tomography, or magnetic resonance imaging (including microscopic versions of each modality), or by non-imaging means such as instrument tracking via radiofrequency triangulation.

0055<figref idref="DRAWINGS">FIG. 8</figref> illustrates a flowchart of an example method for tracking a feature of interest using OCT in accordance with embodiments of the present disclosure. The method of <figref idref="DRAWINGS">FIG. 8</figref> is described in the following example as being implemented by the system of <figref idref="DRAWINGS">FIG. 3</figref>; however, it should be understood that the method may be implemented by any suitable system. In this example, the method includes determining a location of a feature of interest within an operative field (step <b>800</b>). For example, images may be captured that include a feature of interest. Example images include, but are not limited to, OCT images, such as B-scans, SVP images, or SLO images, captured by the OCT unit <b>200</b> shown in <figref idref="DRAWINGS">FIG. 3</figref>. In another example, the captured images may be videos captured by the camera <b>302</b>. The images may be communicated to the computer <b>304</b>.

0056A feature of interest may be, for example, a surgical instrument or a tissue feature within a surgical site view. In an example, a surgical instrument may include a straight edge, a color, a recognizable shape of an area of a surgical instrument, a homogenous texture, a bright area, a light source attached to a surgical instrument, and/or another identifiable feature that may be recognized by suitable analysis performed by the computer <b>304</b>. The feature of interest may also be a marking of a region of interest on a surgical instrument. The computer <b>304</b> may store information about characteristics of surgical instruments for use in recognizing the feature of interest.

0057The method of <figref idref="DRAWINGS">FIG. 8</figref> includes determining a relative positioning between the feature of interest and an OCT scan location (step <b>802</b>). For example, the processor <b>308</b> and memory <b>310</b> of the computer <b>304</b> may process captured images to recognize or identify the feature of interest. In addition, the processor <b>308</b> and memory <b>310</b> may be configured to determine a position and orientation of the feature of interest with respect to the OCT scan location. For example, an image capture including the feature of interest may be calibrated or coordinated with an OCT scan. It may be determined whether the position of the feature of interest is co-located with coordinates of a most recent OCT scan.

0058Methods for determining an OCT scan location include those based on determining the absolute position of a pre-set OCT scan location with respect to the field of view of the surgical microscope and attached imaging devices attached to it, as well as those based on on tracing the history of previous scan locations since an absolute determination was last made. Methods for determining the absolute OCT scan location include performing careful calibration between the OCT scanner unit and the surgical microscope field of view prior to surgery, utilizing an infrared camera mounted to the surgical microscope which is capable of directly viewing the OCT beam, and localizing the OCT scan location based on visualizing a common feature of interest in both the OCT image and the video image simultaneously.

0059The method of <figref idref="DRAWINGS">FIG. 8</figref> includes controlling capture of an OCT image at a set position relative to the feature of interest based on the relative positioning (step <b>804</b>). For example, the computer <b>304</b> may control the OCT unit <b>200</b> to acquire one or more OCT scans based on the relative positioning. The computer <b>304</b> may determine the difference between the position of the feature of interest and a current OCT scan location for adjusting the OCT scan location to acquire scans at the position of the feature of interest.

0060The OCT scans acquired at a position of a feature of interest may be B-scans having a predetermined scan pattern. For example, the predetermined scan pattern may be a single B-scan such as the scan pattern shown in <figref idref="DRAWINGS">FIGS. 5A and 5B</figref>. In the example of <figref idref="DRAWINGS">FIGS. 5A and 5B</figref>, the scan patterns have a particular position and orientation with respect to the surgical instrument <b>500</b>. In examples of <figref idref="DRAWINGS">FIGS. 5A-5F</figref>, the computer <b>304</b> determines both a position and orientation of the instrument such that the scan patterns can have a particular position and orientation with respect to the surgical instrument. In other examples, the scan pattern may be a pair of B-scans (e.g., the scan pattern of <figref idref="DRAWINGS">FIG. 5C</figref>), or a multiple B-scans oriented in a radial or raster scanning pattern (e.g., the scan patterns of <figref idref="DRAWINGS">FIGS. 5D-5F</figref>).

0061In an embodiment, capture of an OCT image may be controlled based on a surgical procedure or an instrument being used during a surgical procedure. For example, the computer <b>304</b> shown in <figref idref="DRAWINGS">FIG. 3</figref> may determine a type of surgical procedure or surgical instrument. An operator of the computer <b>304</b> may, for example, enter input into the user interface <b>314</b> (e.g., keyboard or mouse) for indicating a type of surgical procedure being performed. In another example, an operator may enter input into the user interface <b>314</b> for indicating a type of surgical instrument being used during a surgical procedure. Based on the identified surgical procedure and/or surgical instrument, the OCT unit <b>200</b> may be controlled to capture one or more OCT images having a particular pattern within a predetermined area. The OCT images may be captured within an area with respect to a position of the surgical instrument and based on the surgical procedure. For example, it may be desired to have a particular view of a surgical site and an instrument within the surgical site in the case of a particular surgical procedure and/or surgical instrument being used.

0062Referring again to <figref idref="DRAWINGS">FIG. 8</figref>, the steps <b>800</b>, <b>802</b>, and <b>804</b> may be repeated for tracking OCT image acquisition to the feature of interest. For example, subsequent to B-scans being captured of a feature of interest, a position of the feature of interest may be determined again such that OCT image acquisition can be controlled to track the feature of interest. In this way, for example, an OCT unit can be controlled to track a surgical instrument or a tissue feature for facilitating view of these features by a surgeon.

0063In accordance with an embodiment for tracking a feature of interest using OCT, <figref idref="DRAWINGS">FIG. 9</figref> illustrates a schematic diagram of an example model for image feature tracking. The model represents a closed-loop control system that may be implemented by the computer <b>304</b> shown in <figref idref="DRAWINGS">FIG. 3</figref> or any other suitable controller. Referring to <figref idref="DRAWINGS">FIG. 9</figref>, the input to the control system is a reference signal r(t) corresponding to a position of a feature of interest within the surgical field, such as a surgical instrument. The output y(t) corresponds to an OCT scan location. The error signal e(t) represents the difference between the position of the feature of interest and an OCT scan location, which in a closed-loop tracking system is desired to be minimized. The difference error signal e(t) is fed into the controller Gc(s), which applies well known control algorithms to construct the signal x(t) which is optimized to drive the OCT scanner unit represented as the plant Gp(s) to obtain OCT images at a position which minimizes the difference between the OCT scan position and the position of the feature of interest. The variable t represents time, and the variable s represents complex frequency as is common in linear systems and control theory. The purpose of the control system is to drive the OCT scanner unit, such as the OCT unit <b>200</b> shown in <figref idref="DRAWINGS">FIG. 3</figref>, to track OCT image acquisition to the feature of interest.

0064The reference signal r(t) may represent a specified feature of a surgical instrument, which may be detected or otherwise identified by the computer <b>304</b> or any suitable instrument localizing sub-system. The computer <b>304</b> may identify a position and orientation of the instrument with sufficient resolution to direct the system to perform OCT imaging in its vicinity. In an embodiment, the computer <b>304</b> may extract the instrument position information by computerized analysis of the color video camera signal which may be configured to provide a duplicate of the surgeon's view through the surgical microscope. In an example, the video camera image may be aligned and calibrated with respect to the OCT scanner coordinate system, and control provided for such alignment to be maintained throughout surgery. Live image processing can be performed on the video image signal to recognize a feature of interest, such as a surgeon's instrument. The instrument may be configured with features that are distinct from the rest of the surgical view, such as straight edges, non-tissue colors, homogeneous texture in its body, or brightly reflective points. The computer <b>304</b> may implement a computer vision algorithm that recognizes these features and estimates the position and orientation of a tool in view. For example, such an algorithm may limit the search region by observing a limited square view in the most recently acquired image frame of the camera. In this example, a limited square is centered on the initially-estimated tool location, beginning either in the center of the field or by a user who is simultaneously operating the tracking software computer. Within the limited square, the location of the peak intensity is determined, and the limiting square is re-centered at this location for the next image frame. Because the surgical instruments may have brightly reflecting tips, the brightest part of the view will be at the tool tip in this case.

0065In another embodiment, an infrared (IR) sensitive video camera may be used for capturing images. As a result, visualization of the OCT illumination may be provided, rather than the visible illumination. This configuration may allow the system to verify that the beam is actually coincident with the location of interest, such as a surgical instrument tip. In this case, a computer vision algorithm may be configured to recognize the known and expected shape of the beam focused on the sample. An alternative embodiment uses an additional imaging modality which provides a higher quality en-face view of the surgical field than a color video camera, such as a scanning laser ophthalmoscope (SLO) which can be built into the OCT system for simultaneous OCT/SLO imaging. An alternative embodiment may use an aspect of the OCT imaging system itself to localize the instrument position. For example, surgical tools constructed of metal or plastic may be identified in OCT B-scans by their specific reflectance or shadowing profile. Since OCT B-scans are highly localized in the direction perpendicular to their scan direction, they would not provide any localization information in that dimension. However, the instrument may be equipped with a predetermined marking (such as a divot) which may be recognizable in an OCT B-scan such that a computer vision algorithm can distinguish between the subject tissue and features of a surgical instrument in the cross sectional view of a B-scan. To find the instrument in the view within a view such as a surgical site view, the OCT system may scan in a “search mode” until the instrument is recognized. The OCT system may determine whether a feature of interest is not contained within captured image, and capture images of a different area of a view to search for the instrument within the view in response to determining that the image is not contained within the captured image. Once the instrument is recognized, the system may enter a “locked-on” mode, such as the example method shown in <figref idref="DRAWINGS">FIG. 8</figref> which always keeps scanning near the instrument. To remain locked on, the OCT system may regularly scan in locations just outside the plane of a central B-scan to check if the instrument has been displaced. Fast scanning and tight search range may be used for continuing to lock onto the instrument. Once the instrument has escaped the view, the OCT system may return to the search mode. These processes may be implemented by, for example, the computer <b>304</b> controlling the OCT unit <b>200</b>.

0066Various surgical instruments may be tracked in accordance with embodiments of the present disclosure. These instruments may be used in OCT-imaged surgical procedures as disclosed herein. Example surgical instruments that may be tracked include, but are not limited to, scalpels, forceps, scrapers, scissors, picks, vitrectomy tips, and other instruments and tools in common use in microsurgery including vitreoretinal surgery. These instruments may include a body having a predefined shape for improving capture of OCT images of nearby tissue during a surgical procedure. Examples body shapes include, but are not limited to, flat or sharp-edged shapes. Such shapes may have varying reflectance and shadowing patterns. The body of the instrument may be constructed from a variety of materials, including metals, plastics, polyamide, silicone, and composites. Other example materials include clear or tinted plastic, polymers, glass, ceramics, or other materials to allow control of the transmission and reflectance of an OCT beam. Such instruments may be compatible with intrasurgical OCT imaging as described herein and optimized for such factors as transparency to OCT light, or having localized or distributed reflecting material embedded within them for enhanced visibility with OCT. Additionally, instruments may be specifically designed or modified for compatibility with the tool localizing sub-system as disclosed herein. For example, instruments may be designed for detection by a color video camera, IR camera, or SLO image and may be specially marked in a certain pattern of one or more markings or color which uniquely identifies the instrument position and orientation in the en-face camera view. In an example, the body of an instrument may have multiple markings on its surface that have a predefined spacing for use in determining a distance in a capture OCT image including the markings. Such instruments may also be modified or designed to have a small light source or emitter, such as a light emitting diode or optical fiber tip embedded within them, so that the emitted light can be detected and localized by the en-face camera image analysis software. In an example, an instrument body may define an interior channel such that a optical fiber may be embedded within it and connected to a light source at one end. The opposing end of the fiber optic may be positioned to terminate at a pre-defined location within the surgical instrument for view from an exterior of the body such that when the light source is activated, light is emitted and viewable for tracking the instrument. Further, all or a portion of the body or the surface may be modified to selectively increase or decrease OCT transmission and reflectance, such as through abrading or diamond dusting the surface or alternatively embedding reflectors within the body to increase reflectance on OCT and increase visualization of the instrument. Modification on the surfaces can be performed to decrease reflectivity and further improve visibility of underlying structures. In an example, an instrument tip may have portions of reflective and non-reflective material to optimize the view of surrounding structures while at the same time, maintaining optimal view of the instrument for OCT control or for view by a surgeon.

0067The error signal e(t) may be a difference between the reference signal r(t) and output signal y(t). A controller sub-system G_c(s) may employ predetermined information about the characteristics of the instrument localizing sub-system and plant G_P(s), along with suitable feedback control algorithms, to process the error signal e(t) to produce an input signal x(t) which directs the OCT scanner unit <b>200</b>, represented by the plant G_p(s), to perform OCT imaging at the location y(t) which the controller Gc(s) causes to track the feature of interest. The OCT scan unit <b>200</b> comprises the plant which is controlled by the computer <b>304</b> implementing the model of <figref idref="DRAWINGS">FIG. 9</figref> to capture a desired OCT scan pattern, such as any of the scan patterns illustrated in <figref idref="DRAWINGS">FIGS. 5A-5F</figref>.

0068The OCT scan unit <b>200</b> may be represented by the plant G_p(s) shown in <figref idref="DRAWINGS">FIG. 9</figref>. In an example, control unit <b>200</b> may be any suitable OCT scanning apparatus. OCT scanning apparatus may include orthogonal galvanometer-driven optical mirrors. The OCT scanning apparatus may implement its own spatial coordinate system, governed by its attachment to the surgical microscope, which may be aligned and calibrated to the tool localizing sub-system. The OCT scanning apparatus may have a known transfer function which converts the plant input signal x(t) to the output signal y(t) corresponding to the desired imaging location, taking into account such factors as any gain, offset, or temporal delay inherent to the scanning hardware and electronics which control it.

0069In the example of <figref idref="DRAWINGS">FIG. 3</figref>, the OCT scanning unit <b>200</b> and the camera <b>302</b> are synchronized to provide complementary information. The camera <b>302</b> may be a wide field of view, video-rate surgical video camera, a scanning laser ophthalmoscope, or any other high frame rate ophthalmic camera. Analysis of images captured by the camera <b>302</b> may provide rough estimates of the surgical tool position (azimuthal and lateral) on the patient eye. Other sensor input is the limited field of view, low-frame rate SDOCT B-scan data captured by the OCT scanning unit <b>200</b>. The B-scan data can provide topographic information about a surgical site, such as sub-retinal structures. To simplify the computational complexity of image registration in this example, the OCT scanning unit <b>200</b> and the camera <b>302</b> are physically connected, resulting in a synchronized monotonous motion in the corresponding images. The connectivity condition may be removed by using faster computer processors. Any suitable multi-sensor image alignment algorithm, including mutual information-based registration techniques or vessel alignment based registration method, such as the method disclosed in “Enhanced Video Indirect Ophthalmoscopy (VIO) via Robust Mosaicing,” by R. Estrada, C. Tomasi, M. T. Cabrera, D. K. Wallace, S. F. Freedman, S. Farsiu. (BioMedical Optics Express, Vol. 2, Iss. 10, pp. 2871-2887, October 2011), the disclosure of which is incorporated herein by reference, may be implemented to parametrically register the SDOCT generated SVP image to the en face video sequence. To improve registration estimation accuracy and speed, calibration of the OCT unit <b>200</b> and the camera <b>302</b> may be performed accurately before the commencement of surgery. Then, during the surgery a computationally-efficient projective operation relates the position of the surgical tool on the en face video camera, to approximate position on the SDOCT camera view.

0070Various mechanisms can be utilized for guiding an SDOCT scanner to scan a feature of interest. In an example, a smooth sub-millimeter (local) displacements of a surgical instrument on the SDOCT B-scans may be tracked. The trajectory of the surgical instrument may be predicted and its motion trajectory locked onto in a Kalman-filtering framework. An en face video camera with large field of view may be used to track and compensate for large displacements, such as when a surgeon moves the instrument a large distance very quickly.

0071To fuse information from multiple scans, several repeated fast sparse 3-D scans may be rapidly captured. Subsequently, captured scan that have been affected by motion artifacts may be removed. Next, the artifact-free scans may be fused, a dynamically updated high-resolution 3-D scan generated.

0072At each time point, of the very large number of sparse scans that the MIOCT system captures, sparsely sampled volumetric scans may be captured with a significantly lower number of B-scans than the target resolution. The image fusion time-window, i.e. the optimal number of volumes to be fused (N), may be selected based on a maximum preset threshold which can be decreased in case of scene change. Since the number of frames in each sequence (K) may be relatively small, each scan is captured very fast. For this reason, it may be assumed that some of these sequences will be less affected by the abrupt patient motion. Such sequences may be detected, reordered, and interlaced for creating a densely sampled, artifact-free representation of a surgical view, such as the retina. <figref idref="DRAWINGS">FIG. 10</figref> illustrates images in which two sparsely sampled sequences are fused together creating a dense representation of the underlying pathological structures. Referring to <figref idref="DRAWINGS">FIG. 10</figref>, fusing (or interlacing) multiple sparsely sampled scan sequences can create an azimuthally higher resolution volume of B-scans. This process may be repeated for each time point in the output video. A 3-D extension of a 2-D Kalman filtering may be used to create a smoothly changing (unless there is a rapid change in the eye location) dynamic high-resolution volume of a feature of interest, such as an eye.

0073In an example, a computer, such as the computer <b>304</b> shown in <figref idref="DRAWINGS">FIG. 3</figref>, may implement a digital filter. In an example, a most recent tip location, represented as X(z), may act as a computed result of an image processing algorithm. This input may be tested against the last result of the actual scanning position Y(z)*z⁻¹, and the error signal may be scaled to define a new position. Depending on the gain, k, which determines the speed of response, the scanner may move toward the desired position over multiple frames in a smooth fashion. This gradual motion may improve stability of the tracking and reduce high-frequency fluttering of the galvanometer scanners. Computed new tracking offset voltages may be generated by a linked digital to analog converter card, and summed with an SDOCT system. This alleviated signal/image processing burden from the SDOCT computer, which operated separately from any tracking input.

0074<figref idref="DRAWINGS">FIG. 11</figref> depicts various images of example data obtained from simulated surgery on an excised porcine eye with the anterior segment removed. Particularly, <figref idref="DRAWINGS">FIG. 11</figref> shows fundus and OCT images during a simulated surgical maneuver. Image (a) shows a grayscale image of the fundus view is shown with the outline of the surgical scraper instrument outlined. A small white box with a black cross is superimposed on the tool tip in the live image. This snapshot was taken after the scraper was dragged from left to right approximately 6 mm along the surface of the retina, therefore, the box highlights the final position of the tip. Another line in image (a), indicates the calculated tool tracking position throughout the motion. Continuous B-scan images (1024×1000 pixels) were recorded by an SDOCT system with 17 B-scans per second and 50 μs integration time per A-scan. The illuminating beam power was limited to 700 μW. Images (b)-(d) show three B-scans acquired at the lateral positions indicated by points b, c, and d in image (a). As visible in image (b), the porcine retina had several detachments and folds, however the surgical tool tip was clearly visible as a bright object intersecting the retina from above right. In image (c), the tool tip is visible in the center, over the retina and causing partial shadowing of the retina. The tip appears bright due to diamond dust particles embedded in the silicone tip which strongly scatter OCT light. Also, a small, but distinct frame-breaking artifact in the OCT scan in image (c) is highlighted by a star. This discontinuity is due to a short but rapid shift in the scanner position mid-scan. In image (d), the B-scan is sectioning the transparent silicon portion of the instrument tip, which appears semicircular, and is pushing down, and causing retinal deformation. The silicone is mostly transparent to the OCT beam, and therefore retinal features below the tip are visible. The three B-scans (image s(b)-(d)) shown are separated in time by approximately 5 seconds each. The algorithm tracked the tip consistently despite the two nearby glare artifacts due to the room lighting, both visible in image (a).

0075<figref idref="DRAWINGS">FIG. 12</figref> illustrates two orthogonal B-scans, images (a) and (b), acquired using the scan pattern shown in <figref idref="DRAWINGS">FIG. 5C</figref> during simulated porcine surgery. In image (c) of <figref idref="DRAWINGS">FIG. 12</figref>, the orthogonal B-scans are rendered in their correct orientation in 3D above an image of the retinal surface.

0076In accordance with embodiments of the present disclosure, a surgical microscope system may be configured with a heads-up display (HUD) for providing surgical information to a surgeon. A HUD may integrate visualization of OCT images and/or other surgical information into the optical viewports of a surgical microscope, such as the MIOCT system <b>100</b> shown in <figref idref="DRAWINGS">FIG. 1</figref>. In this way, simultaneous display of real-time cross-section OCT imaging may be provided with direct viewing of surgical sites and instruments. Further, various surgical information may be presented to an operator via the user interface <b>314</b> shown in <figref idref="DRAWINGS">FIG. 3</figref>, such as audibly via a speaker.

0077Information displayed by the HUD may be viewed through an ocular eyepiece unit, such as the oculars <b>308</b> shown in <figref idref="DRAWINGS">FIG. 3</figref>. In an embodiment, a HUD may be implemented by attaching two assistant viewing extensions to the surgical microscope. Each assistant viewing extension may include a video camera port in addition to the assistant ocular eyepieces. On one assistant arm, the video camera can be used for archival purposes and also for providing the video fundus image view required for the SDOCT scan-directing algorithms described below. However, in the second assistant arm, beamsplitter leading to the camera port may be reversed, and the camera may be replaced with a similarly sized high-resolution organic light-emitting diode (OLED) or other type array connected as an auxiliary monitor for the SDOCT computer. This allows an observer on the second assistant arm to view whatever is displayed on the OLED display overlaid with optical view the surgical microscope is transmitting. The SDOCT display may be displayed in a “dark” area outside the primary microscope view (i.e., the dark area outside the center of view of a conventional microscope eyepiece, normally rendered black by the presence of an aperture stop).

0078Referring to the example of <figref idref="DRAWINGS">FIG. 3</figref>, the computer <b>304</b> may operate as a user interface controller for controlling a HUD <b>316</b>. For example, the computer <b>304</b> may determine surgical information associated with a surgical site image projected for view through an ocular eyepiece unit. Further, the computer <b>304</b> may control the HUD <b>316</b> to display the surgical information. In an example, surgical information may include an indication of a distance between features (e.g., a surgical instrument and tissue feature such as a retinal layer) within a surgical site image. For example, the computer <b>304</b> may be configured to identify a surgical instrument and tissue feature in an image projected to a surgeon thought the ocular eyepiece unit. Subsequent to the identification, the computer <b>304</b> may analyze the positioning to determine a distance between the instrument and tissue feature. The computer <b>304</b> may control the HUD <b>316</b> to display a number or any other indicia of the distance. In another example, the surgeon may select among different data for display via the HUD <b>316</b>. Another example of feedback includes vibratory feedback measurement guide displays.

0079It is noted that many of the examples provided herein relate to ophthalmic surgery; however, the systems, methods, and instruments disclosed herein may also be applied to other types of surgeries or any other suitable procedure. Example surgeries include, but are not limited to, neurosurgery, breast surgery, dermatologic procedures, otolaryngologic procedures, such as tympanic membrane, or any other surgeries requiring precise maneuvers to small subsurface structures (e.g., Schlemm's canal) visible on OCT or other imaging equipment.

0080The various techniques described herein may be implemented with hardware or software or, where appropriate, with a combination of both. Thus, the methods and apparatus of the disclosed embodiments, or certain aspects or portions thereof, may take the form of program code (i.e., instructions) embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or any other machine-readable storage medium, wherein, when the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the presently disclosed subject matter. In the case of program code execution on programmable computers, the computer will generally include a processor, a storage medium readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device and at least one output device. One or more programs may be implemented in a high level procedural or object oriented programming language to communicate with a computer system. However, the program(s) can be implemented in assembly or machine language, if desired. In any case, the language may be a compiled or interpreted language, and combined with hardware implementations.

0081The described methods and apparatus may also be embodied in the form of program code that is transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via any other form of transmission, wherein, when the program code is received and loaded into and executed by a machine, such as an EPROM, a gate array, a programmable logic device (PLD), a client computer, a video recorder or the like, the machine becomes an apparatus for practicing the presently disclosed subject matter. When implemented on a general-purpose processor, the program code combines with the processor to provide a unique apparatus that operates to perform the processing of the presently disclosed subject matter.

0082Features from one embodiment or aspect may be combined with features from any other embodiment or aspect in any appropriate combination. For example, any individual or collective features of method aspects or embodiments may be applied to apparatus, system, product, or component aspects of embodiments and vice versa.

0083While the embodiments have been described in connection with the various embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function without deviating therefrom. Therefore, the disclosed embodiments should not be limited to any single embodiment, but rather should be construed in breadth and scope in accordance with the appended claims.