Ophthalmic surgical microscope having an OCT-system

[0001] This application claims priority of German patent application no. 10 2007 019 680.8, filed Apr. 24, 2007, the entire which are content of which is incorporated herein by reference.

[0002] The invention relates to an ophthalmic surgical microscope having a microscope main objective and a viewing beam path which passes through the microscope main objective. The ophthalmic surgical microscope further includes an OCT-system for recording images of an object region. The OCT-system includes an OCT-scanning beam which is guided via a scan mirror unit to the object region.

[0003] A surgical microscope of the kind referred to above is known from U.S. Pat. No. 6,004,314. This surgical microscope includes an OCT-system which generates a scanning beam of short coherent laser radiation. The OCT-system includes an analyzer unit for evaluating interference signals. The OCT-system further includes a device having two scan mirrors for scanning the OCT-scanning beam. These scan mirrors can be displaced about two axes of movement. The OCT-scanning beam in the surgical microscope is coupled via a divider mirror into the illuminating beam path of the surgical microscope. The OCT-scanning beam is deflected with the illuminating beam through the microscope main objective to the object region.

[0004] Via optical coherence tomography, an OCT-system makes possible the non-invasive display and measurement of structures within a tissue. As an optical image producing method, the optical coherence tomography especially generates section and volume images of biological tissue with a micrometer resolution. A corresponding OCT-system includes a source for time-dependent incoherent and spatially coherent light having a specific coherence length which is guided to a specimen beam path and a reference beam path. The specimen beam path is directed to the tissue to be examined. Laser radiation, which is backscattered into the specimen beam path because of scatter centers in the tissue, superposes the OCT-system with laser radiation from the reference beam path. An interference signal results because of the superposition. The positions of the scattering centers for the laser radiation in the examined tissue are determined from this interference signal.

[0005] For OCT-systems, the building principles of the “time-domain OCT” and of the “Fourier-domain OCT” are known.

[0006] The configuration of a “time-domain OCT” is described, for example, in U.S. Pat. No. 5,321,501 with reference to FIG. 1a at column 5, line 40, to column 11, line 10. In a system of this kind, the optical path length of the reference beam path is continuously varied via a rapidly moving reference mirror. The light from specimen beam path and reference beam path is superposed on a photo detector. When the optical path lengths of the specimen and reference beam paths are coincident, an interference signal is provided on the photo detector.

[0007] A “Fourier-domain OCT” is, for example, described in international patent publication WO 2006/100544 A1. To measure the optical path length of a specimen beam path, light from the specimen beam path is superposed onto light from a reference beam path. In contrast to the time-domain OCT, the light from the specimen beam path and reference beam path is not supplied directly to a detector for a measurement of the optical path length of the specimen beam path but is first spectrally dispersed by means of a spectrometer. The spectral intensity of the superposed signal generated in this manner from specimen beam path and reference beam path is then detected by a detector. By evaluating the detector signal, the optical path length of the specimen beam path can be determined.

[0008] It is an object of the invention to provide a compactly configured ophthalmic surgical microscope which permits the recordation of OCT-section images of the human eye. Here, the OCT-scanning beam can be guided to an OCT-scanning plane without intensity losses.

[0009] The ophthalmic surgical microscope of the invention is for defining a viewing beam path and includes: a microscope main objective mounted so as to permit the viewing beam path to pass therethrough permitting examination of a region of an object; an OCT-system for recording images of the object region; the OCT-system providing an OCT-scanning beam and including a scanning mirror device for scanning the OCT-scanning beam; and, an optical unit mounted between the scanning mirror device and the microscope main objective for bundling the OCT-scanning beam exiting from the scanning mirror device and guiding the bundled OCT-scanning beam into a beam path passing through the microscope main objective to the object region.

[0010] In another embodiment of the invention, the ophthalmic surgical microscope is for defining a viewing beam path and includes: a microscope main objective mounted so as to permit the viewing beam path to pass therethrough permitting examination of a region of an object; an OCT-system for recording images of the object region; the OCT-system providing an OCT-scanning beam and including a scanning mirror device for scanning the OCT-scanning beam; an ophthalmoscopic magnifier lens being movably mounted; the ophthalmoscopic magnifier lens being movable between a first position whereat the viewing beam path and the OCT-scanning beam path pass through the ophthalmoscopic magnifier lens and a second position whereat the viewing beam path and the OCT-scanning beam path do not pass through the ophthalmoscopic magnifier lens.

[0011] The ophthalmoscopic magnifier lens permits bundling the OCT-scanning beam on the ocular fundus of the patient in order to scan the ocular fundus and to simultaneously make the fundus visible in the ocular via the optical viewing beam paths of the surgical microscope.

[0012] According to another feature of the invention, the optical element between the scan mirror unit and the microscope main objective is configured as a movable lens unit. In this way, different section planes of a human eye can be scanned with OCT-radiation.

[0013] According to another feature of the invention, this optical element is accommodated in a lens exchange unit. In this way, a rapid-back and forth switching between different scanning planes for OCT-radiation is possible in the object region.

[0014] According to another feature of the invention, the optical element is configured as a zoom system having variable focal length. In this way, a continuous variation of section planes examined in a patient eye is made possible with OCT-radiation.

[0015] According to another feature of the invention, means for adjusting the focal length of the main objective are provided. In this way, different planes of an object region can be examined with the ophthalmic surgical microscope without it being necessary to shift the surgical microscope.

[0016] According to another feature of the invention, the scan mirror unit for scanning the OCT-scanning beam has a first scanning mirror. Preferably, and additionally, a second scan mirror is provided. The first scan mirror can be moved about a first rotational axis and the second scan mirror can be moved about a second rotational axis. The first rotational axis and the second rotational axis are laterally offset at right angles to each other. In this way, an object can be scanned with a raster pattern running perpendicular to each other.

[0017] According to another feature of the invention, the OCT-system includes a light conductor which has a light exit section for the OCT-scanning beam. Means are assigned to the light conductor for moving the same. In this way, the OCT-system can be adapted for the use of OCT-radiation of different wavelengths.

[0018] According to another feature of the invention, an ophthalmoscopic magnifier lens is provided in the ophthalmic surgical microscope which can be pivoted into and out of the viewing beam path and the OCT-scanning beam path. Preferably, this ophthalmoscopic magnifier lens is combined with a reduction lens. The ophthalmoscopic magnifier lens and the reduction lens are arranged in an ophthalmic ancillary module which can be pivoted into and out of the OCT-scanning beam path and the viewing beam path. In this way, the anterior section of the patient eye as well as the ocular fundus can be examined with OCT-radiation with the ophthalmic surgical microscope.

[0019] According to another feature of the invention, the OCT-system in the ophthalmic surgical microscope is designed for making available a first OCT-scanning light beam at a first wavelength and for making available a second OCT-scanning light beam having a second wavelength different from the first wavelength. Preferably, a corresponding first OCT-system and a corresponding second OCT-system are provided which make available the OCT-scanning light beams of different wavelengths. In this way, body tissue having different absorption characteristics for OCT-radiation can be examined with good resolution.

[0028] The surgical microscope 100 in FIG. 1 has a microscope main objective 101 defining an optical axis 102 as well as a focal plane 103. Stereoscopic viewing beam paths 105 of a binocular tube 106 pass through the microscope main objective 101.

[0029] The surgical microscope 100 has an illumination unit in the form of an illumination module 120 for illuminating the object region in the form of a patient eye 108. This illumination module 120 includes a first light conductor 121 which makes illuminating light 122 available from a light source (not shown). A displaceable field diaphragm 124 is illuminated by the illuminating light 122 exiting from the light conductor 121. A path-folding mirror 123 is mounted on the side of the microscope main objective 101 facing away from the object. The illuminating light exiting from the light conductor 121 is directed via path-folding mirror 123 through the microscope main objective 101 and into the object region 108.

[0030] An ophthalmoscopic ancillary module 130 has a reducer lens 131 and an ophthalmoscopic magnifier lens 132 which can be pivoted into and out of the stereoscopic viewing beam path 105 of the surgical microscope 100 in correspondence to double arrows (133, 134). The ophthalmoscopic ancillary module is assigned to the ophthalmic surgical microscope 100.

[0031] A first OCT-system 140 is provided in the ophthalmic surgical microscope 100. The ophthalmic surgical microscope further contains a second OCT-system 150 which is shown in FIG. 2. These OCT-systems permit the recordation of OCT-images.

[0032] The OCT-system 140 of FIG. 1 includes a unit 141 for generating and analyzing an OCT-scanning beam 142. The unit 141 is integrated into the surgical microscope 100. The unit 141 can, however, also be mounted outside of the surgical microscope 100 in a corresponding stand console. The unit 141 is connected to a light conductor 143 which makes the OCT-scanning beam 142 available.

[0033] The OCT-scanning beam 142, which exits from the light conductor 143, is guided with a divergent beam path onto a first scan mirror 144 and a second scan mirror 145 of an OCT-scan unit 146. From there, the OCT-scanning beam passes through an optic element in the form of a converging lens 147 to thereafter pass through the microscope main objective 101. The OCT-scanning beam 142 is bundled in an OCT-scanning plane 160 in the anterior section of the patient eye 108.

[0034] The OCT-light, which is backscattered from the object region in the form of a patient eye 108 into the OCT-scanning beam path, arrives back in the unit 141 via the microscope main objective 101, the converging lens 147 and the OCT-scanning unit 146. In unit 141, the OCT-scanning light, which is backscattered from the object region, interferes with the OCT-radiation from a reference beam path. The interference signal is detected by a detector and is evaluated by a computer unit which, from the signal, determines an optical path length difference between scatter centers for OCT-light in the object region and the path length of light in the reference branch.

[0035] A displacing mechanism 148 is assigned to the converging lens 147 for adjusting the OCT-scanning plane 160. The converging lens 147 can be moved by the displacing mechanism 148 in correspondence to the double arrow 149.

[0036] The first OCT-system 140 operates at a wavelength of λ=1310 nm. The second OCT-system in the ophthalmic surgical microscope 100 is configured to correspond to the first OCT-system 140 but has a work wavelength of λ=800 nm. It is understood that the OCT-systems can also be designed for other operating wavelengths. Operating wavelengths can be realized in the range 600 nm<λ<1500 nm and are advantageous depending upon application.

[0037] FIG. 2 shows the ophthalmic surgical microscope 100 of FIG. 1 with the second OCT-system 150. The second OCT-system 150 is configured and arranged in correspondence to the first OCT-system 140.

[0038] Identical component groups of the surgical microscope are provided in FIG. 2 with the same reference numerals.

[0039] The OCT-system 150 has a unit 151 for generating and analyzing an OCT-scanning beam 152 of the OCT-system 150. The OCT-system 150 generates OCT-scanning radiation 152 at the exit end of the light conductor 153. In the same manner as the OCT-scanning beam 142 of OCT-system 140 of FIG. 1, this OCT-scanning beam 152 is guided via an OCT-scanning unit 156 having a first scan mirror 154 and a second scan mirror 155 via an optical element through the microscope main objective 101. This optical element is configured as a converging lens 157.

[0040] FIG. 2 shows the ophthalmic surgical microscope 100 in an operating mode wherein the reducer lens 131 and the ophthalmoscopic magnifier lens 132 of the ophthalmic ancillary module 130 are pivoted into the viewing beam path 205 of the surgical microscope 100. This makes possible an examination of the ocular fundus 190 of the patient eye 108 with the OCT-scanning light and with light which arrives from the ocular fundus 190 back in the viewing beam paths of the surgical microscope.

[0041] The converging lens 157 bundles the OCT-scanning beam and directs this beam to the microscope main objective 101. The OCT-scanning beam reaches the patient eye 108 via the microscope main objective 101, the reducer lens 131 and the opthalmoscopic magnifier lens 132 and is bundled in the OCT-scanning plane 160 (FIG. 6) at the ocular fundus 190 of patient eye 108.

[0042] The OCT-light, which is backscattered into the OCT-scanning beam path from the object region in the form of a patient eye 108, is guided back into the unit 151 for generating and analyzing an OCT-scanning beam. The backscattered OCT-light is guided into the unit 151 via the microscope main objective 101, the converging lens 157 and the OCT-scanning unit 156. In the unit 151, the OCT-scanning light, which is backscattered from the object region, in turn interferes with OCT-radiation from a reference beam path. As in the OCT-system 140, the interference signal is detected by a detector in the OCT-system 150 and is evaluated by a computer unit which, from the signal, determines an optical path length difference between scattering centers for OCT-light in the object region and the path length of light in the reference branch.

[0043] A displacing mechanism 158 is assigned to the converging lens 157 which can be moved by the mechanism 158 in correspondence to the double arrow 159. In this way, a focal plane can be adjusted also for the OCT-scanning beam from the OCT-system 150.

[0044] The optical path length of the OCT-scanning beam path in the operating mode of the ophthalmic surgical microscope 100 shown in FIG. 2 is longer than in the operating mode of FIG. 1. This requires an adaptation of the optical path length in the reference beam path of the OCT-system 150. For this purpose, a coupling 180 of ophthalmoscopic magnifier lens 132 and OCT-system 150 is provided which effects, with a pivoting of the ophthalmoscopic magnifier lens 132 into the viewing beam path and the OCT-beam path, the optical path length of the reference beam path in the OCT-system to increase by a specific value. This value is preferably held to be adjustable. A fixed value advantageously orients itself to the average length of the patient eye.

[0045] FIG. 3 is a section view taken along line III-III of FIG. 1. FIG. 3 shows the course of the stereoscopic viewing beam paths (105, 205) of the surgical microscope 100 of FIG. 1. Two stereoscopic component beams (105, 205) pass through the microscope main objective 101. The optical axis 102 of the microscope main objective lies at the center 310 thereof. The OCT-scanning beam 142 of the OCT-system 140 passes through the microscope main objective 101 in the region 301. The OCT-scanning beam 152 of the OCT-system 150 of FIG. 2 passes through the microscope main objective 101 in region 302 and the illuminating light 122 passes through the objective 101 in the region 303.

[0046] FIG. 4 shows the first OCT-system 140 and the second OCT-system 150 in the surgical microscope 100 of FIGS. 1 and 2. The wavelength range of the OCT-scanning beams of the two OCT-systems (140, 150) is, however, different: the first OCT-system is based on an OCT-scanning beam having the wavelength λ₁=1310 nm. The second OCT-system 150 operates with an OCT-scanning beam having the wavelength λ₂=800 nm. The same reference numerals used in FIGS. 1 and 2 to identify the component groups of the OCT-systems 140 and 150 are used in FIG. 4.

[0047] The first scan mirror (144, 154) and the second scan mirror (145, 155) of the OCT-systems (140, 150) are mounted so as to be rotationally moved by position drives (401, 402, 403, 404) about two mutually perpendicularly extending axes (405, 406, 407, 408). This permits the OCT-scanning beams (142, 152) to scan over a plane independently of each other.

[0048] The OCT-scanning beam 142 of the first OCT-system 140 is guided to the microscope main objective 101 via the converging lens 147. The OCT-scanning beam 152 of the second OCT-system 150 passes through the microscope main objective 101 via the converging lens 157.

[0049] FIG. 5 shows a front portion of the light conductor 143 of FIG. 1 having front face 502. The light conductor 143 operates as a monomode fiber for light of the wavelength λ₁=1310 nm. The diameter (d₁) of the fiber core of the light conductor 143 satisfies the relationship:

[0050] $\frac{d_1}{2}<2.4\frac{{\lambda }_1}{2\pi N{A}_1},$
wherein: NA₁ is the numerical aperture of the front face of the light conductor. Preferably, the diameter (d₁) of the fiber core of the light conductor 143 lies in the range of 5 μm<d₁<10 μm. In this parameter range, the light conductor 143 conducts the light with a Gaussian-shaped wave mode. The OCT-scanning light beam 142 exits from the light conductor 143 with an approximately Gaussian-shaped beam profile which is characterized by a waist parameter W₁ and an aperture parameter θ₁ wherein:

[0051] ${\theta }_1=\frac{{\lambda }_1}{\pi W_1}$

[0052] An aperture angle of θ₁≈0.0827 rad results thereby as an index for the beam divergence for a fiber core diameter of d₁=10 μm and a wavelength λ₁=1310 nm.

[0053] The front face 502 of the light conductor 143 is imaged into an OCT-scanning plane via the following: the scan mirrors 144 and 145 in the surgical microscope 100 of FIG. 1; the converging lens 147; and, the microscope main objective.

[0054] FIG. 6 shows the course of the intensity distribution of the OCT-scanning light beam 142 perpendicular to the OCT-scanning plane 160. In the OCT scanning plane 160, the intensity distribution of the OCT-scanning radiation has a smallest constriction. The diameter of the OCT-scanning beam path increases outside of the OCT-scanning plane. The OCT-scanning light beam 142 exits from the light conductor 143 of FIG. 5 with an approximately Gaussian-shaped beam profile. For this reason, the converging lens 147 and the microscope main objective 101 of the surgical microscope 100 of FIG. 1 effect a so-called Gaussian bundle 600 of the OCT-scanning light beam 142 for the OCT-scanning beam in the region of the OCT-scanning plane 160. This Gaussian bundle 600 is characterized by the confocal parameter (z₁) as an index for the longitudinal expansion of the constriction of the Gaussian bundle and by the waist parameter W_1,A as an index for the diameter of the smallest constriction 602 of the OCT-scanning light beam 142 in the OCT-scanning light plane, that is, for the diameter of the constriction thereof. The following applies:

[0055] $z_1=2\frac{W_{1,A}^2\pi }{{\lambda }_1},$
wherein: λ₁ is the wavelength of the OCT-scanning light beam. The following relationship applies between the waist parameter W_1,A of the Gaussian bundle 600 and the waist parameter W₁ of the scanning light beam 142 (FIG. 5) which exits from the light conductor 143:
W_1,A=β₁W₁,
wherein: β₁ is the magnification parameter or demagnification parameter of the above-mentioned geometric image of the exit end of light conductor 143 of FIG. 1 in the OCT-scanning plane. The parameter β₁ is coupled to the focal length f₁₄₇ of the converging lens 147 of FIG. 1 and the focal length f₂ of the main objective via the following relationship:

[0056] $\frac{f_2}{f_{147}}={\beta }_1$

[0057] The size of structures, which can be resolved with the OCT-scanning light beam 142, is determined by the diameter of the beam 142 in the OCT-scanning plane 160, that is, by the waist parameter W₁. If, for example, an application requires a lateral resolution of the OCT-system in the surgical microscope of approximately 40 μm, then, according to the Nyquist theorem, the cross section of the OCT-scanning light beam 142 must amount to approximately 20 μm in the OCT-scanning plane. For a given wavelength λ₁ for the OCT-scanning light beam 142 of FIG. 1, the magnification of the optical image in the OCT-beam path and the diameter of the fiber core in the light conductor 143 must be suitably selected for a desired resolution of the OCT-system 140.

[0058] The confocal parameter (z₁) as an index for the longitudinal expansion of the waist of the Gaussian bundle determines the axial depth of field from which backscattered light can be detected in the OCT-scanning beam 142 of FIG. 1. The smaller the confocal parameter (z₁), the greater is the loss of the OCT-system with respect to lateral resolution when removing an object from the OCT-scanning plane 160 with this object having been scanned with the OCT-scanning beam. The reason for this is that the location of the scatter centers can be localized only within the “funnel” defined by the waist parameter W₁ and the confocal parameter (z₁).

[0059] As the axial resolution of an OCT-system is delimited on the one hand by the specific coherence length of the light of the light source utilized in the OCT-system and, on the other hand, the lateral resolution of the OCT-system decreases when the depth index thereof exceeds the expansion given by the confocal parameter (z₁), the adjustment of the confocal parameter (z₁) to the specific coherence length of the light source utilized in the OCT-system is favorable.

[0060] For a specific wavelength λ₁ of the OCT-scanning light beam 142, the possible lateral resolution of the OCT-system of FIG. 1 results because the wavelength λ₁ and confocal parameter (z₁) determine the waist parameter W_1,A. The optical units in the OCT-scanning beam path of FIG. 1 and the dimensioning of the fiber core of the light conductor 143 are then to be selected so that the particular waist parameter results.

[0061] The converging lens 147 in the surgical microscope 100 is preferably so adjusted that the focal plane 170 of the microscope main objective 101 for the visible spectral range and the OCT-scanning plane 160 of the OCT-system 140 are coincident. Then, the waist 502 (FIG. 5) of the OCT-scanning beam lies in the focal plane 170 of the surgical microscope.

[0062] For the OCT-scanning beam 152 of the OCT-system 150 of FIG. 2, the light conductor 153 functions as a monomode fiber for light of the wavelength λ₂=800 nm. The diameter (d₂) of the fiber core of the light conductor 153 therefore satisfies the relationship:

[0063] $\frac{d_2}{2}<2.4\frac{{\lambda }_2}{2\pi N{A}_2}$
wherein NA₂ is the numerical aperture of the front face of the light conductor 153. The OCT-scanning light beam 152 exits from the light conductor 153 with an approximately Gaussian-shaped beam profile which is characterized by a waist W₂ and an aperture parameter θ₂ wherein:

[0064] ${\theta }_2=\frac{{\lambda }_2}{\pi W_2}$

[0065] For the waist parameter of the OCT-scanning beam 152 in the OCT-scanning plane 160, the following applies:
W_2,A=β₂W₂,
wherein β₂ is the demagnification parameter of the geometric image of the exit end of the light conductor 153 of FIG. 2 in the OCT-scanning plane 160.

[0066] The parameter β₂ is defined by the focal length of the converging lens 157 of FIG. 2, the focal length of the microscope main objective 101 as well as by the reducer lens 131, the ophthalmoscopic magnifier lens 132 and the cornea and lens of the patient eye 108.

[0067] The converging lens 157 is preferably so adjusted that, when the optical viewing beam images the ocular fundus 190 of the patient eye 108 through the microscope main objective 101, the OCT-scanning plane 160 for the OCT-system 150 in the surgical microscope 100 is coincident with the ocular fundus 190.

[0068] Alternatively to the described design of the OCT-systems in the surgical microscope, an offset of the OCT-scanning plane and focal plane can also be provided in the surgical microscope. Preferably, this offset is not greater than the confocal parameter (z₁) of the OCT-scanning light beam in the region of the OCT-scanning plane.

[0069] In that the OCT-scanning plane is disposed further from the microscope main objective 101 of FIG. 1 by the confocal parameter (z₁), the depth measurement index for the OCT-system can be maximized in the object region.

[0070] In FIG. 7, a modified embodiment for an OCT-system is shown which can be utilized in the ophthalmic surgical microscope 100 of FIGS. 1 and 2.

[0071] Corresponding to the OCT-system 140 of FIG. 1, the OCT-system 700 includes a unit 701 for generating and analyzing an OCT-scanning beam 702. The unit 701 is so designed that OCT-scanning beams having different wavelengths can be generated and evaluated. A control apparatus 703 is provided for adjusting the wavelength of the OCT-system 700.

[0072] The OCT-scanning beam 702 exits from a light conductor 704 which is connected to the unit 701. The OCT-scanning beam 702 reaches two scan mirrors (705, 706) which are movable in such a manner by drives (not shown) that an object region 708 can be scanned in an OCT-scanning plane 707.

[0073] For the adjustment of different OCT-scanning planes, on the one hand, a magnification exchanger 709 is provided as a lens exchange unit in the OCT-system 700 and this exchanger 709 holds converging lenses of different refractive power (710, 711). These converging lenses (710, 711) can be pivoted into and out of the OCT-scanning beam path 702. In addition, a drive unit 712 is assigned to the light conductor 704 which permits the exit end 713 of the light conductor to be moved in correspondence to the double arrow 714 in order to vary the position of the OCT-scanning plane 707 in the object region.

[0074] The OCT-scanning beam 702 reaches the OCT-scanning plane via the converging lens 711 which bundles the scanning rays and conducts the same to the microscope main objective of the corresponding surgical microscope.

[0075] FIG. 8 shows a further embodiment of an OCT-system for an ophthalmic surgical microscope. In the same manner as the OCT-system 700, the OCT-system here has a unit 801 for generating and analyzing an OCT-scanning beam 802 which exits from a light conductor 803. In the OCT-system 800, the OCT-scanning beam 802, which exits from the light conductor exit end 804, is guided by a corresponding scanning mirror system 805 through a lens unit 806 with adjustable refractive power which acts as a zoom system. Accordingly, it is in turn possible to vary the position of the OCT-scanning plane 807 in an object region 808.

[0076] A further modified embodiment of the surgical microscope 100 shown in FIG. 1 contains a focusable microscope main objective having an adjustable focal length. This measure too makes it possible to shift an OCT-scanning plane and to change the geometric imaging of the light conductor exit end in the OCT-scanning plane.

[0077] It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.