Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B3/00—Apparatus for testing the eyes; Instruments for examining the eyes
  • A61B3/10—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions
  • A61B3/1025—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions for confocal scanning
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61F—FILTERS IMPLANTABLE INTO BLOOD VESSELS; PROSTHESES; DEVICES PROVIDING PATENCY TO, OR PREVENTING COLLAPSING OF, TUBULAR STRUCTURES OF THE BODY, e.g. STENTS; ORTHOPAEDIC, NURSING OR CONTRACEPTIVE DEVICES; FOMENTATION; TREATMENT OR PROTECTION OF EYES OR EARS; BANDAGES, DRESSINGS OR ABSORBENT PADS; FIRST-AID KITS
  • A61F9/00—Methods or devices for treatment of the eyes; Devices for putting-in contact lenses; Devices to correct squinting; Apparatus to guide the blind; Protective devices for the eyes, carried on the body or in the hand
  • A61F9/007—Methods or devices for eye surgery
  • A61F9/008—Methods or devices for eye surgery using laser
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61F—FILTERS IMPLANTABLE INTO BLOOD VESSELS; PROSTHESES; DEVICES PROVIDING PATENCY TO, OR PREVENTING COLLAPSING OF, TUBULAR STRUCTURES OF THE BODY, e.g. STENTS; ORTHOPAEDIC, NURSING OR CONTRACEPTIVE DEVICES; FOMENTATION; TREATMENT OR PROTECTION OF EYES OR EARS; BANDAGES, DRESSINGS OR ABSORBENT PADS; FIRST-AID KITS
  • A61F9/00—Methods or devices for treatment of the eyes; Devices for putting-in contact lenses; Devices to correct squinting; Apparatus to guide the blind; Protective devices for the eyes, carried on the body or in the hand
  • A61F9/007—Methods or devices for eye surgery
  • A61F9/008—Methods or devices for eye surgery using laser
  • A61F9/00821—Methods or devices for eye surgery using laser for coagulation
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B17/00—Surgical instruments, devices or methods, e.g. tourniquets
  • A61B2017/00681—Aspects not otherwise provided for
  • A61B2017/00694—Aspects not otherwise provided for with means correcting for movement of or for synchronisation with the body
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61F—FILTERS IMPLANTABLE INTO BLOOD VESSELS; PROSTHESES; DEVICES PROVIDING PATENCY TO, OR PREVENTING COLLAPSING OF, TUBULAR STRUCTURES OF THE BODY, e.g. STENTS; ORTHOPAEDIC, NURSING OR CONTRACEPTIVE DEVICES; FOMENTATION; TREATMENT OR PROTECTION OF EYES OR EARS; BANDAGES, DRESSINGS OR ABSORBENT PADS; FIRST-AID KITS
  • A61F9/00—Methods or devices for treatment of the eyes; Devices for putting-in contact lenses; Devices to correct squinting; Apparatus to guide the blind; Protective devices for the eyes, carried on the body or in the hand
  • A61F9/007—Methods or devices for eye surgery
  • A61F9/008—Methods or devices for eye surgery using laser
  • A61F2009/00844—Feedback systems
  • A61F2009/00846—Eyetracking
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61F—FILTERS IMPLANTABLE INTO BLOOD VESSELS; PROSTHESES; DEVICES PROVIDING PATENCY TO, OR PREVENTING COLLAPSING OF, TUBULAR STRUCTURES OF THE BODY, e.g. STENTS; ORTHOPAEDIC, NURSING OR CONTRACEPTIVE DEVICES; FOMENTATION; TREATMENT OR PROTECTION OF EYES OR EARS; BANDAGES, DRESSINGS OR ABSORBENT PADS; FIRST-AID KITS
  • A61F9/00—Methods or devices for treatment of the eyes; Devices for putting-in contact lenses; Devices to correct squinting; Apparatus to guide the blind; Protective devices for the eyes, carried on the body or in the hand
  • A61F9/007—Methods or devices for eye surgery
  • A61F9/008—Methods or devices for eye surgery using laser
  • A61F2009/00861—Methods or devices for eye surgery using laser adapted for treatment at a particular location
  • A61F2009/00863—Retina
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61F—FILTERS IMPLANTABLE INTO BLOOD VESSELS; PROSTHESES; DEVICES PROVIDING PATENCY TO, OR PREVENTING COLLAPSING OF, TUBULAR STRUCTURES OF THE BODY, e.g. STENTS; ORTHOPAEDIC, NURSING OR CONTRACEPTIVE DEVICES; FOMENTATION; TREATMENT OR PROTECTION OF EYES OR EARS; BANDAGES, DRESSINGS OR ABSORBENT PADS; FIRST-AID KITS
  • A61F9/00—Methods or devices for treatment of the eyes; Devices for putting-in contact lenses; Devices to correct squinting; Apparatus to guide the blind; Protective devices for the eyes, carried on the body or in the hand
  • A61F9/007—Methods or devices for eye surgery
  • A61F9/008—Methods or devices for eye surgery using laser
  • A61F2009/00878—Planning
  • A61F2009/0088—Planning based on wavefront

Definitions

• the invention relates generally to instruments for examining and treating the eye and specifically to a scanning laser ophthalmoscope equipped with external laser sources for the purpose of retinal microphotocoagulation under the condition of minimal wavefront aberrations of the eye optics.
• the ophthalmoscope is well known as an important device for examining the eye, and in particular the retina
• ophthalmoscopes of various constructions have been built
• the latest version of the ophthalmoscope, a scanning laser ophthalmoscope is particularly appealing because of 2 its unique capability of combining infra-red and angiographic imaging of the retina with psychophysical procedures such as the study of visual fixation characteristics, visual acuity measurements and micrope ⁇ metry
• a precise correlation between retinal anatomy and retinal functioning can be established with the scanning laser ophthalmoscope
• This retinal function mapping is now known to be very helpful to the surgeon when applying therapeutic laser Until now however, these therapeutic laser applications have been delivered to the retina with an instrument other than the scanning laser ophthalmoscope
• the use of different instruments renders the comparison of images the interpretation of psychophysical testing and precision of treatment more difficult
• Another solution that may be considered for effective thermal coagulation whether pulsed or continuous m nature is to combine the scanning laser ophthalmoscope with a traditional external non-scanning therapeutic laser source
• ophthalmoscopes exemplified by the biomicroscope are optically combined with a non-scanning therapeutic laser source for the purpose of retinal photocoagulation
• a contact glass is placed on the cornea to be able to view the retina with the instrument and a mirror is used for reflecting the therapeutic laser beam onto the desired retinal location through a small part of the pupillary area
• the retina is illuminated and observed through different parts of the pupillary area to avoid reflexes l e Gullstrand's principle of ophthalmoscopy
• This optical arrangement makes the art of precise focusing of a therapeutic laser beam m the retina more difficult This is certainly the case in the presence of wavefront 3 aberrations, small pupil diameter or large diameter entrance beam of the treating laser.
• Small therapeutic applications are often desired because they save retinal tissue, also they can be tailored to the shape of the lesion and they can take a variability in absorption more easily into account.
• photocoagulating ophthalmoscopes have been limited when consistent small or localized laser applications in the retina are desired because the anatomical changes caused by the therapeutic laser are often very difficult to visualize during treatment in the presence of photocoagulating light. This is even more the case if minimal intensity, i.e. threshold applications are desired.
• the critical endpoint of the laser application is often exceeded.
• the surgeon upon recognizing the minimal anatomical changes on the retina, is also handicapped by a substantial human reaction time delay before he can interrupt the therapeutic laser. During this delay the laser continues to deliver energy to the retina and changes in the subject's fixation may occur. Since the reaction time of the surgeon may exceed 200 ms , a 100 s laser application can be delivered to the wrong place on the retina in the case of misalignment.
• the object of this invention is to combine in one instrument the capabilities of advanced imaging and precise psychophysics with microphotocoagulation of retinal tissue under conditions of minimal wavefront aberrations and documentation of small therapeutic laser applications for later reference. This is basically accomplished by selecting an entrance location of the external therapeutic laser beam that is subject to minimal wavefront aberrations, through observation of the retina with the scanning laser ophthalmoscope using the same entrance location for the scanning lasers. As documented in the prior art, Gullstrand's principle is used differently in scanning laser ophthalmo ⁇ copy, hence the necessity to use a similar optical pathway for both the external therapeutic or diagnostic and scanning laser beams. Seven features of the invention are :
• a special coupling system between a confocal scanning laser ophthalmoscope and external laser sources comprises an optimally coated beamsplitter and optomechanical linkage device.
• the linkage device allows the spatial matching of the pivot point for the fast scanning diagnostic laser beams of the scanning laser ophthalmoscope with the pivot point of the non-scanning external therapeutic or diagnostic laser beams.
• Optimizing the Maxwellian viewing of a retinal location will 4 then also result in a minimal wavefront aberration for the external laser beams because the same pivot point is used.
• the amount of prefocusing necessary to image with the scanning laser ophthalmoscope on a specific retinal layer is a reference, if needed, for focusing the therapeutic laser beam with its proper telescopic optics.
• a non-confocal or co-pupillary scanning laser ophthalmoscope cannot be used to image the external laser spot on the retina.
• the confocal instrument can do this, however not in a straightforward manner. It is important to realize that the image of the external laser spot on the monitor, is actually a convolved image of the external laser spot with the confocal aperture. Often, the confocal aperture of the scanning laser ophthalmoscope is larger and hence the backscatter image cannot be used directly to determine size or adjust focusing.
• a second synchronized detector images only the backscattered light from the external laser beams, without a background of moving retinal details.
• This image can be localized using simple image processing techniques such as look-up table manipulation.
• the implementation of a two detector pathway therefore allows registration of external laser beams referenced on the retinal image, and the use of a safety shutter in case of excessive misalignment. It should be noted that this specific part of the invention could equally well be applied to traditional photocoagulating systems if they are equipped with two video cameras, as long as the detector images are made spatially congruent .
• An aiming or diagnostic beam of different wavelength than the actual therapeutic laser source is typically polarized. Its light is partially transmitted after backscattering from the retina, through the beamsplitter. Only the aiming or diagnostic beam wavelength is reaching one of the photodetectors . Polarization removes 5 the strong corneal reflections that may appear as a second confusing spot or veiling on the retinal image
• Pulsing of the aiming or diagnostic beam allows higher but still safe peak power to be used Pulsing of the aiming beam requires anti-aliasing relative to its confocal detection on the retina
• the same opto-mechanical linkage device can also determine the wavefront aberrations of the complete eye optics by systematically varying the entrance location of the external laser beam in the plane of the anatomical pupil The differences in retinal location of the diagnostic laser beam for each entrance location are measured imaged, or neutralized with small angulat ons of the external diagnostic laser beam It is well known m the prior art how to reconstruct the wavefront aberrations over the pupil from individual slope measurements (Zernike) Precise knowledge of the higher orders of the wavefront aberrations is then useful to correct the shape of a wide entrance Gaussian external laser source thereby permitting even more localized thermal applications
• a major advantage of the invention is the ability to accurately deliver and document small, minimal intensity therapeutic laser applications to selected layers in the retina subject to minimal wavefront aberrations hence the term microphotocoagulation
• microphotocoagulation has the ability to remove temporarily or permanently a percentage of the metabolically very active photoreceptors or retinal pigment epithelium cells, while minimizing damage to other anatomical structures, especially the choriocapillary layer, Bruch s membrane, ganglion cell layer and neural tissue in between applications
• Virtual oxygen windows , reducing relative hypoxia, can for example oe established through reduction of the demanding metabolic load of the central retina This approach is useful in the retardation of onset or prevention of drusen related and neovascular age-related maculopathy
• Possible mechanisms are an accelerated removal of material that thickens Bruch' s membrane and the reduced production of angiogenetic factors
• Fig 1 is a diagrammatic representation, illustrating the different components of the confocal scanning laser ophthalmoscope optimized for microphotocoagulation
• AOM acousto-optic
• EAM direct electrical amplitude modulation
• External therapeutic or diagnostic non-scanning lasers with modulation options coupled to the scanning laser ophthalmoscope with the help of a beamsplitter and opto-mechanical linkage device, safety shutter, collimator-telescope, and interface electronics
• the computer with one or more linked overlay framegrabber graphic cards, capable of digital image processing, and monitor
• Fig 2 details the opto-mechanical linkage device coupling external laser sources and confocal scanning laser ophthalmoscope Angulation of the laser beam is possible mechanically or with the help of motors under CPU control
• the pivot points of external lasers and scanning lasers can be adjusted relative to each other Overview image and different components of one embodiment are given Reduced embodiments only permit parallel positioning of a diagnostic laser beam
• Fig 3 details the ray tracing of the scanning laser ophthalmoscope and external therapeutic or diagnostic laserbeams
• a common pivot point s used to avoid lens changes, thereby resulting in minimal wavefront aberrations and predictable focusing of the laser beam in the retina Wavefront aberrations cause a difference in slope of refraction
• spurious reflections are demonstrated at the anterior corneal interface They are minimized by polarization techniques and optical stops
• Fig 4 shows a block diagram of an overlay frame grabber card capable of advanced image processing
• the one or more overlay frame grabber graphic cards have an input interface frame memory, display interface and CPU interface
• the host bus accommodates a I/O for interaction with several components of the therapeutic laser assembly and opto-mechanical linkage device
• Essential electronic pathways include (1) Synchronized video-m pathway from SLO detectors (2) Video-out pathway to monitor and laser modulators (3) System timing generator with genlockmg of the other components including A/D converters and D/A converters of the different boards 7
• Fig. 5 details the contents of part of frame memory of an OFG card with video coming from a first detector.
• a search window within the video retinal image contains a gray scale pattern that is used to determine the coordinates of fiducial landmarks of the retina with a normalized gray-scale correlation algorithm.
• Fig. 6 details the contents of part of frame memory of an OFG card with video coming from a second synchronized detector.
• the coordinates of the location of the external beams can be determined with simple digital image processing techniques and indicated on the retinal image with the help of overlay graphics.
• Fig. 7 is a schematic that illustrates the importance of a minimal fiber optic aperture and low N.A. to match the f/# of the receiving optics, i.e. the eye. Entrance pupil is limited to 2 mm. Back focal length of the eye is assumed to be 22.28 mm.
• Fig. 8 illustrates a stationary raytracing of the scanning lasers through the eye.
• Gaussian beam optics The 1.0 mm beam focuses to a small spot of about 25 ⁇ in the retina. Not to scale.
• the different retinal layers, virtual retinal conjugate aperture of about 100 ⁇ and backscattered rays from a non-scanning external laser source are indicated.
• the 75 ⁇ backscatter predominantly originates from the internal limiting membrane and pigment epithelium.
• Fig. 9 is a schematic that illustrates a double wheel with confocal apertures for the external laser beam and scanning laser beams respectively.
• the thin confocal apertures for use with the external laser beam are smaller than those for use with the scanning laser beams and they are constructed of a thin material that passes the scanning laser beam wavelength used for imaging the retina. Separation of confocal apertures can neutralize the small longitudinal chromatic aberration of the system.
• Fig. 10 illustrates a video image used for modulating a Gaussian therapeutic laser source with an AOM or EAM.
• Example pulse characteristics are 100 % pass during 50 pixels or 5 ⁇ s, every 1 ms or approximately 15 video lines. Aliasing, especially of the aiming or diagnostic beam, can be eliminated with an additional stimulus graphic in the appropriate location.
• Scanning optics including polygon and galvanometer, additional lenses, mirrors 8 Similar Maxwellian view of scanning and therapeutic beams, common pivot point Collimator-telescope for scanning laser beams of ophthalmoscope Lens changes scattering elements in the optical media of the eye Backscattered light returning from the retina Beamsplitter or aperture for separation of incident and backscattered laser light Combining beamsplitter (polarizing) Avalanche photodiode detectors Video and sync generating electronics of scanning laser ophthalmoscope Computer Overlay frame grabber graphic cards Video display monitor SLO diode infra-red 792 nm laser for imaging purposes SLO He-Ne 633 nm laser for microperimet ⁇ c purposes Pair of adjustable linear polarizers, attenuators Acousto-optic modulator Overlay on retinal image indicating characteristics of external laser spot Barrier, interference or polarizing filters, optionally with pinhole in 200 or 202 Beamsplitter for separating scanning (imaging) and external (aiming)
• Adjustable mirror hinges interconnected with ball-bearing adjustable cylinders Support arc for positioning distal element 100 Pair of support arcs for positioning element 102 Supporting framework for elements 100, 102 and 104 Adjustable leadscrew support for positioning element 106 Framework with adjustable lead screw to position element 108 Motors screws or mechanics to position the elements 100 104, 106, 108 Fixed attachment to the scanning laser ophthalmoscope of element 58 116 Ball-bearing cylinders with rotational and translational capabilities 118 One half of clamp around torical support arc 120 Torical support arc, completely closed
• FIG. 1 A representative embodiment of the confocal scanning laser ophthalmoscope for retinal microphotocoagulation is illustrated in Fig 1
• the principles of scanning laser ophthalmoscopy are described in detail in the prior art (Pomerantzeff, Saban, Webb Plesch) Features of the confocal scanning laser ophthalmoscope that are relevant to the invention are further discussed
• a prefocused Gaussian beam of laser light e g consisting of a combined polarized He-Ne 633 nm and diode 792 nm wavelength, and having an approximate diameter of 1 0 mm at the entrance of the eye, is further focused by the eye optics of approximately 60 units of dioptric power to a spot m the retina
• This spot typically between 10 and 30 ⁇ in diameter at a retinal plane is scanned over the posterior pole of the eye 12 in a sawtooth manner with the help of scanning optics, currently comprising a polygon and galvanometer driven mirror 14
• Fast horizontal 15 KHz and slower vertical 60 Hz deflections of this flying laser spot are synchronized to a standard video R ⁇ -170 signal They create the rectangular laser beam raster on the retina
• a rectangular area of about 0 5 en on the retina is illuminated in the 40 degree field of view of the instrument
• a Maxwellian view illumination is used m the scanning laser ophthalmoscope
• the pivot point 16 of the scanning laser beam
• the light that is backscattered and reflected from the retina 22 now distributed in the anatomical pupil is descanned over the same optics and separated from the illuminating beam at a mirror-pmhole 24
• This mirror-pmhole 24 is optimally positioned with regard to light collection, e g close to a pupillary conjugate plane and the pinhole also blocks most of the perpendicularly and specularly reflected light coming from the anterior surface of the cornea along the same path that was used for the illumination
• Other options are 1 1 available and have been used before
• a polarizing beamsplitter can be applied in combination with a polarizing filter Th s would allow the use of axially backscattered light from the retina with some advantages, but puts other optical constraints, especially with regard to polarization and intensity on the illuminating sources
• the returning light is focused to pass through a small aperture 200, 202
• This aperture for example having a diameter of 1 mm, is conjugate with a virtual aperture 172 of
• a high intensity diode infra-red 792 nm laser 38 under electrical modulation control and vertically polarized, is nearly invisible to the observer It produces the retinal image on the display monitor 36
• An aligned and low intensity He-Ne 6 3 2 8 nm laser 40, horizontally polarized is modulated with a pair of linear polarizers 42 and acousto-optic modulator 44
• the 633 nm laser 40 is used to draw visible graphics in the laser raster These visible graphics are created by amplitude modulation of the laser 40
• the acousto-optic modulator 44 is usually driven by the same computer overlay frame grabber graphics card 32
• the graphics which are seen by the observer are usually not visible m the retinal image, unless when they are very bright
• the exact position and characteristics of the graphics can however be indicated in real-time on the retinal image with the help of computer generated overlays 46, because the image video that
• External therapeutic laser sources 52, 54 are well known in the prior art, e.g. argon 488 nm, 514 nm or currently diode laser 52, having the possibility of emitting different wavelengths, for example 532 nm, 810 nm and 1064 nm.
• a variable part of the optical transmission usually occurs in fiber optics.
• the advantage of fiber optics is flexibility and a more even intensity profile at the exit aperture of a multimode fiber. Multimode propagation of higher intensity laser beams and fiber optics transmission are more difficult to focus to a small spot size when compared with a fundamental mode Gaussian beam due to the M 2 -factor of propagation or limitations in N.A. of the optical fiber.
• a pure Gaussian profile of the external laser would be used for the purpose of measuring wavefront aberrations of the eye or very small therapeutic applications, as is briefly explained in the following section.
• the external therapeutic lasers can be pulsed, for example with the help of an acousto-optic modulator 62 or Pockel cell. Either first diffracted or undiffracted zero order beam can be used. If different wavelenghts pass through the AOM, achromatizing prisms can be used. It is also possible to use an attenuated and zero order as aiming beam, after optical recombination with the modulated first order beam. It is difficult to modulate a multimode laser beam of 50 mrad divergence. A large Bragg angle and therefore high frequency carrier signal e.g.
• a Gaussian profile 300 mW output at 532 nm (Crystalaser, Irvine CA) however is easy to modulate with the AOM.
• the light can be then fed into a special optical fiber for mode scrambling.
• a mechanical light chopper 62 can also be inserted in the optical path of the therapeutic laser beam.
• An example is a circular disc of 150 mm 13 diameter pierced with 0.5 mm holes at the outer edge.
• a motor with rotation speed of up to 5000 RPM can be used.
• Several pulse characteristics can be created with adjacent trajectories on the wheel. The wheel can then be moved to select one.
• Direct electrical modulation using video for the aiming beam diode laser and diode pumped doubled frequency laser 52 or Q-switchmg of a continuously pumped Nd-YAG laser are other options The modulation characteristics are further discussed This pulsing is done to selectively target absorbing layers, for example the pigment epithelium 168 Therapeutic lasers can be used in association with absorbing dyes
• a photosensitizing drug is first injected and laser of appropriate wavelength is then applied w th the aim of closing off small bloodvessels
• conventional 514 or 532 nm photocoagulation can penetrate more completely in deeper layers because the blood flow has been halted before with the photodynamic therapy
• Hemoglobin molecules that are fixed are far more effective in absorbing energy, together with the melanin pigment
• an aiming beam and therapeutic beam of e.g 664 nm or 695 nm can be the same Typically, the therapeutic beam is left on for 120 seconds or longer, and larger areas on the retina are illuminated
• a large Gaussian beam diameter 1064 nm source 52 could also selectively coagulate layers in the retina, e.g. photoreceptors, with proper focusing
• the main absorbant element will be water as 50 percent of the incident 1064 nm light will reach the retina. Water is ubiquitous, hence the danger of damaging the innermost retina or deeper layers Therapeutic wavelengths e.g.
• a low power diagnostic beam 54 of different wavelength typically a diode laser m the 630-700 nm range, is provided for aiming purposes.
• the high power therapeutic laser beam 52 could also serve as an aiming » beam at much lower intensities, but would be required to continuously emit power
• Other elements in tne optical construction of the therapeutic laser include a safety shutter, aperture stops at the end of the fiber optic, various filters and the modulating devices 62 allowing specific pulsating patterns of energy, typically in the ⁇ s domain.
• the foregoing components are controlled by electronic circuitry 64 known in the prior art.
• an I/O link 66 often a combination of TTL circuits exists between the control electronic circuitry 64 of the external lasers and the computer 34 This electronic connection 66 can signal to the computer 34 14 when the external lasers 52 and 54 are used and also allows activating the modulating means 62 under control of the computer 34
• an external 670 nm diode s used as aiming laser 54 can be electrically or AOM modulated using computer 32 and anti- aliased with each update of the retinal image.
• the aiming beam has to be on only when the confocal aperture of the SLO is m its neighborhood. For this purpose, the aiming beam location is already approximately known by the computer because it knows the coordinates of the joystick-manipulator and the previous location of the aiming beam Higher peak powers can then be safely used if necessary
• External diagnostic and non-scanning laser sources 52, 54 include He-Ne red, He-Ne green, and optimized, collimated mono-modal fiber diodes They are used for the reconstruction of wavefront aberrations across the anatomical pupil of the eye Clean and small diameter beam profiles are important. They produce a relatively small spot on the retina with a large depth of focus External diagnostic lasers 52, 54 differ from the therapeutic sources in the maximum power output they can produce Surface- emitting quantum-well laser diodes are of increasing interest, and offer the advantages of high packing densities on a wafer scale.
• the external therapeutic laser beams 52, 54 are non-scanning, however their orientations are allowed to change using a special transmission system comprising an appropriately coated beamsplitter 56 and opto-mechanical linkage device 58.
• Fig. 2 illustrates the essential components of the opto-mechanical linkage device 58 between the external therapeutic laser sources 52, 54 and the confocal scanning laser ophthalmoscope for the purpose of microphotocoagulation Also, the methods are described by which (1) the position and characteristics of the external laser beams 52, 54 are referenced on the retinal image, (2) a precise focusing is obtained for the purpose of microphotocoagulation, and (3) wavefront aberrations influencing the shape of the therapeutic laser beams are minimized Also (4) the optical constraints are discussed with regard to optical throughput, as are (5) the characteristics of the different possible Darner, bandpass interference and polarizing beamsplitters. (6) 15
• a part of the opto-mechanical linkage device 58 that transmits the therapeutic laserbeams is often realized with a combination of extendable mirror hinges 100 Enough degrees of freedom are available with the mirror hinges 100 as to permit an unimpeded movement of the laserbeams m a rotational and translational fashion
• the hinges themselves are connected with ball bearing elements 114 for smooth rotation and extension
• the opto-mechanical linkage device 58 consist further of a framework or base 106 that permits a support arc 102 to slide across using another pair of support arcs 104
• the support arcs are part of a toroid 118 and can contain a groof or thread
• the last mirror hinge 100 can contain additional optical elements and is attached to the support arc 102
• the proximal or first mirror hinge 100 is fixed to the SLO
• the terminal part of the transmission optics can slide along the support arc 102 w th the help of an attachment 116 and reflects the therapeutic laser light 52 54 coming from the other components in the opto-mechanical linkage device 58 towards the posterior pole of the eye 12
• the two sliding movements allow the external laser beams 52 54 to move perpendicular to a curved surface e g a part of a sphere such that the external laser beams 52 54 will have a pivot point 16 that is very similar in location to the pivot point of the Maxwellian view of the scanning laser ophthalmoscope
• the prefocused external laser beam 52 usually has a 0 5 mm to 2 mm diameter at the entrance position m the eye depending on the desired spot size on the retina the diameter of the fiber optic and its numerical aperture
• the external beams 52, 54 are prefocused by a collimator-telescope 60 using as a reference the amount of prefocusing of the scanning laser beams 38, 40 for the same retinal location and using the same pivot point 16
• the optical pathway is a collimator-telescope 60 and optional field lenses for precisely selecting the spot size and focusing of the external laser beams 52 54 in a specific retinal plane
• the amount of focusing that is needed for the 16 external laser beams 52 and 54, is related to the amount of prefocusmg of the scanning lasers 38 and 40 for the same retinal area since the same optical pathway s used through the ocular media
• This focusing can further take into account the dispersion of light that is caused by differences in wavelength Longitudional chromatic aberration is more important than lateral chromatic aberration because of the fact that the angular displacements in the eye measured from normal incidence are usually small for our purposes
• the longitudinal differences have been tabulated before and amount e g to approximately 0 75 D for 532 nm and 792 nm, approximately 0 5 D for the difference between 670 nm and 792 nm
• biomet ⁇ c data derived from the keratometer and biomet ⁇ c ultrasound to further refine the raytracmg in
• Fig 3a illustrates the different possibilities that can exist an eye having some lens changes or scattering elements 20 in the dioptric media that cause wavefront aberrations Such changes however can occur at the cornea or in the vitreous as well
• This pivot point 16 can be chosen by the observer so that a particular retinal location is seen in good focus with minimal aberrations on the monitor 36
• the therapeutic external laser beams 52, 54 will also undergo minimal aberrations and will be
• Fig 7 Focusing optics are illustrated Fig 7 and Fig 8 In Fig 7 the aperture of a multimodal cladded fiber optic with diameter of 75 ⁇ is in the object plane 18
• the backfocal length of the eye being 22.28 mm results in an entrance pupil of about 2 mm, which is a reasonable limit. Larger spot sizes will then require a smaller entrance diameter. From the discussion above it is clear that a fiber with minimal N.A. i.e. divergence, smallest diameter and minimal power loss is advantageous. The mode mixing within the fiber creates a reasonable flat profile at the exit aperture, thus avoiding the peaked central power of a Gaussian beam.
• Beamsplitters 50, 56 are specifically coated to reflect for the wavelength and polarization of the external lasers 52, 54, but are highly transparent for the wavelengths of the imaging scanning laser 38 of the scanning laser ophthalmoscope.
• Beamsplitter 03BTF011 from Melles-Griot reflects 70 % of the 532 nm light, transmits 95 % of the properly polarized 792 nm light, reflects 70 % of the s-component of 670 nm, transmits 40 % mixed polarization of 670 nm.
• An alternative beamsplitter, cold mirror Melles-Griot 03 MCS007 transmits 90% of 792 nm, reflects over 97 % of 532 nm, and reflects 20 % or transmits 80% of 670 nm light.
• the task of the beamsplitter 56 is to direct the external laser beams 52, 54 and various scanning laser beams 38, 40 of the scanning laser ophthalmoscope towards the posterior pole of the eye 12. Some external diagnostic laser light 52, 54 is however permitted to pass the beamsplitter 56 in order to reach a photodetector 28 after returning from the retina.
• the task of beamsplitter 50 is to direct the returning light from scanning lasers 38, 40 and external lasers 52, 54 to different detectors 28. E.g.
• Melles-Griot long wave pass filter 03 BDL001 separates the 532 and 670 nm from the 792 nm.
• the remaining transmitted therapeutic light 52, 54 after the beamsplitter 50 is absorbed by a filter 48 in front of one of the detectors, to avoid confusion with the descanned laser light 38 or 40 that is returning from the retina.
• both detectors 28 and circuitry 30 generate video that is genlocked to a common master timing signal derived from the spinning polygon. This property will be exploited in referencing the external laser beam location onto the retina.
• one detector 28 is provided with a barrier filter 48 to eliminate all wavelengths of light returning from the retina except the external laser wavelength of 670 nm. Image processing is applied to determine the position of the light from the external laser beams in the video-signal. Because of the 19 synchronization of the video generated from the detectors 28 within the overlay frame grabber graphic card(s) memory 32, a precise indication of the position of the external laser beams 52, 54 is possible using overlays 46 on the retinal image produced by the second detector 28.
• the overlay can be pseudo-colored and semi- transparent to imitate the look and feel of an aiming beam on the retina using the regular optical slitlamp. It is important to realize that this separation of laser beams at the detectors 28 is desired for multiple reasons. First, it is easier to see the progression of retinal coagulation in the absence of the treating light. Second, the high power of the external laser beams makes their precise localization on the retina impossible because of oversaturation of the video. Third, tracking algorithms as explained below cannot work efficiently in the presence of a retina that can move independently in a different direction when compared with the position on the retina of the external laser beam that can move in another direction.
• Fig. 5 and Fig. 6 summarize what is seen by the two synchronized detectors 28.
• Fig. 5 only the scanning lasers 38, 40, and most often only the scanning IR laser 38 responsible for the retinal image, is contributing.
• the graphics however can be seen as overlays 46 as previously explained.
• Diverse retinal features can be used as fiducial landmarks, e.g. the branching pattern of vessels or pigmentary changes.
• Fig. 6 only the external lasers 52, 54 will contribute to the image.
• Conventional laser delivery systems can also be equipped with e.g. two monochrome CCD cameras that are attached to the slitlamp with the help of beamsplitters.
• One color CCD camera that registers the attenuated green light in one channel with the help of filters and the red fundus light on another channel can also be used.
• the spatial congruence is equivalent to the temporal alignment found in the SLO system.
• Overlay graphics can then be employed in a similar fashion and registration can also be performed.
• the external laser beams 52, 54 will generate a convolved image with the confocal aperture on the monitor 36 This can be understood by examining Fig 8 In this Fig 8, the scanning laser beam 176 is flying across the retina while transvers g the dioptric media of the eye 152, 156, 154, 158 Further visualized are the virtual confocal aperture 172 situated at the waist of the scanning laser beam 176, and the different parts of the retina, 164, 170, 162, 168, 166 An external laser beam 52, 54 produces the stationary retinal spot 174 This retinal spot backscatters light, e g from the retinal pigment epithelium 168 and internal limiting membrane If the confocal aperture is larger than the retinal spot then a blurred image of the opening of the confocal aperture 172 will be generated on the monitor 36 through convolution of the external laser spot 174 with the confocal aperture It is possible to enhance the resolution of the therapeutic spot by using a confocal aperture that is significantly smaller
• One solution uses
• each significant entrance position of the external diagnostic beam should be documented together with the entrance position of the scanning laser rays that correspond to a fiducial landmark on the retina
• the test should be short enough to keep the latter entrance position constant by preventing lateral eye movements
• a bite-bar or data correction may be necessary if the test duration is longer.
• Indispensable for processing the video that is generated by the scanning laser ophthalmoscope laser sources 38, 40 and external laser source 52, 54, and for the production of graphics that w ll be projected onto the retina is an overlay frame grabber graphics card 32, schematized m F g. 4
• a good example is the Imaging Technology OFG card in a 90 Mhz Pentium PC
• This overlay frame grabber graphics card 32 can accept four different video input sources, and digitizes the incoming video signals to eight bits of accuracy, at a rate of 60 fields per second (RS-170) .
• On 22 board frame memory can store two 512 by 480 pixel video frames or one larger 640 by 512 pixel video frame.
• Two or more overlay frame grabber cards 32 which are I/O mapped, can reside in one computer 34.
• This versatility is advantageous when combining the signals from a multidetector scanning laser ophthalmoscope, e.g. in simultaneous recording of the aiming laser beam location on the retina witn one detector 28 and the retinal image itself with another detector 28
• the analog-to-digital converter of the frame grabber card 32 has programmable negative and positive reference values for calibrating the white and black video signal levels .
• a look-up table (LUT) controls the input video and can be used for preprocessing contrast and intensity Th s feature is particularly useful in facilitating normalized gray scale correlation, a digital image processing technique further explained It can also be used to separate the aiming beam pixels from noise pixels
• Non-destructive graphic overlays 46 drawn over the incoming video signal, generate the graphics visible in the laser raster of the scanning laser ophthalmoscope and indicate the position on the retina of the aiming laser beam
• the overlay 46 can be semi-transparent to imitate the look and feel of a real aiming beam when using the slitlamp coagulator
• the green output video channel sends the retinal image to the monitor 36, overlaid with graphics indicating the aiming therapeutic laser beam location
• the blue output channel of the original video signal is transformed into pure graphics.
• the acousto- optic modulator 44 controls the acousto- optic modulator 44 and defines what is visible to the observer in the scanning laser ophthalmoscope. This is typically a reference fixation target or test stimulus The remaining red output channel can be used for different purposes.
• One option is the control of the acousto-optic modulator to create a pulsating external laser beam 54
• An important feature of the overlay frame grabber card is the capability to synchronize frame grabber memory D/A, A/D converters, to the external video source using a phase-locked loop This s important since the timing signals provided by the high speed rotating polygon are slightly irregular
• Digital image processing techniques are used for the tracking of a fiducial landmarK in the retinal image as in Fig 5.
• the overlay frame grabber card 32, or an external faster card connected to the computer 34 can perform this task using for example a technique called two-dimensional normalized gray-scale correlation.
• Such software is provided by Imaging Technology, Inc, Bedford, Ma
• a characteristic search pattern such as the branching of retinal 23 vessels or the actual area of treatment is located with the video images provided by a detector 28 of the scanning laser ophthalmoscope.
• a higher resolution imaging board or digital oscilloscope is another obvious solution to help to increase the resolving power beyond 0 1 mill radians , a value desirable for wavefront reconstruction techniques
• Locating the aiming laser beam on the retina, as in Fig 6, and modulating the aiming beam in an anti-aliased fashion, as in Fig 10, is realized using a combination of foregoing techniques
• the aiming or diagnostic laser beam location is already approximately known by the computer if it takes into account the mechanical displacement of the joystick-manipulator 78
• the reference image m frame memory contains no retinal details as m Fig. 5, for facilitating the tracking of a fiducial landmark on the retina, the fundus image in frame memory is devoid of the therapeutic laser light, as in Fig 6
• the same method of locating an external laser spot on the retina is used to calculate the wavefront aberrations of the complete eye optics, by systematically varying the entrance location of an external aiming laser source across the extent of the anatomical pupil, in a parallel fashion, relative to scanning laser ophthalmoscope
• the differences in retinal location and shape characteristics of the external laser beam spot on the retina, and optionally a reference location m the scanning laser raster on the retina are image processed Alternatively, the differences in position can be neutralized with an adjustment of the postion of the fixation spot in the scanning laser raster.
• Well-known Zernike polynomial analysis can then reconstruct the wavefront aberrations from this data of slope determinations of the incident rays at about 30 to 60 positions.

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