Medical Photonics Lecture 1.2 Optical Engineering

Transcription

1 Medical Photonics Lecture 1.2 Optical Engineering Lecture 10: Instruments III Michael Kempe Winter term

2 2 Contents No Subject Ref Detailed Content 1 Introduction Gross Materials, dispersion, ray picture, geometrical approach, paraxial approximation 2 Geometrical optics Gross Ray tracing, matrix approach, aberrations, imaging, Lagrange invariant 3 Diffraction Gross Basic phenomena, wave optics, interference, diffraction calculation, point spread function, transfer function 4 Components Kempe Lenses, micro-optics, mirrors, prisms, gratings, fibers 5 Optical systems Gross Field, aperture, pupil, magnification, infinity cases, lens makers formula, etendue, vignetting 6 Aberrations Gross Introduction, primary aberrations, miscellaneous 7 Image quality Gross Spot, ray aberration curves, PSF and MTF, criteria 8 Instruments I Kempe Human eye, loupe, eyepieces, photographic lenses, zoom lenses, telescopes 9 Instruments II Kempe Microscopic systems, micro objectives, illumination, scanning microscopes, contrasts 10 Instruments III Kempe Medical optical systems, endoscopes, ophthalmic devices, surgical microscopes 11 Optic design Gross Aberration correction, system layouts, optimization, realization aspects 12 Photometry Gross Notations, fundamental laws, Lambert source, radiative transfer, photometry of optical systems, color theory 13 Illumination systems Gross Light sources, basic systems, quality criteria, nonsequential raytrace 14 Metrology Gross Measurement of basic parameters, quality measurements

3 Key Limitation of Optical Imaging in Medicine II II 0 = eeeeee μμ ss + μμ aa dd μμ ss : reduced scattering coefficient (typ cccc 1 ) μμ ss : scattering coefficient (typ cccc 1 ) μμ aa : absorption coefficient Penetration / Resolution: Ballistic light (μμ ss ) few mm / several µm Diffuse light (μμ ss ) depth d

4 Optical Imaging in Medicine Ophthalmology Diagnostic Imaging Dermatology Others Neuro/Spine Optical Medical Imaging Open Surgery Visualization Gynaecology /Urology ENT Ophthalmology Dental Gastroenterology Cardiology Endoscopy Urology Pulmonology Others

5 Endoscopes: Relay Systems Endoscopes use various light guiding principles to relay the image over distance Ref.: M. Rill Rigid endoscopes slab lens relay Combination of several relay subsystems GRIN lenses may be used Large field-angle objective lens objective 1. relay 2. relay 3. relay

6 Rigid Endoscopes W rms [λ] nm 587 nm 656 nm diffraction limit y' [mm] Example: Systems by Storz diameter 3.7 mm

7 Flexible Endoscopes Use of fiber bundle array as relay Each fiber transmits one image point Diameter: typ mm for 4k to 18k fibers (data points = pixels) Pixel size: typ µm Example: System by Storz Helen D. Ford and Ralph P. Tatam, "Characterization of optical fiber imaging bundles for swept-source optical coherence tomography," Appl. Opt. 50, (2011)

8 Historical Development of Surgical Microscopes 1. Head worn loupe (1876) 4. OPMI (Littmann 1953) 2. Corneal loupe (von Zehender/Westien 1887) 5. Contraves Stativ (Yasargil 1972) 3. Corneal loupe (Schanz/Czapski 1899)

9 Surgical Microscope Modern surgical microscopes are stereo systems combining ocular and digital imaging Surgical Microscope Computer Data Transfer Power Supply Ref.: M. Kaschke et al. Ophthalmology Ref.: ZEISS

10 Zoom Systems Motivation for zooming: Enlargement of image details Foveated imaging Adaptation of field of view

11 Basic Principle Two thin lenses in a certain distance t: Focal length Refractive power f F = f = F 1 f + 1 f f 2 2 t 1 + F2 t F1 F2 Many types of zoom system layouts a) Finite-finite (F-F) b) Infinite-finite (I-F) c) Infinite-infinite (I-I)

12 Change of Focal Length Distance t increased First lens fixed changed distance t moved lens changed focal length f

13 Change of Focal Length Distance t increased Image plane fixed two lenses moved t f image plane

14 Mechanical Compensated Zoom Systems Simple explanation of variator and compensator Movement of variator arbitrary Compensator movement depends on variator, nonlinear Perfect invariance of compensator nonlinear variator linear relay lens objective lens image plane image plane possible P P P

15 Modular F-F-Setup Finite-finite configuration wth three parts : 1. Focusing lens 2. Zoom group with movable components 3. Realy lens object focusing lens movable zoom lenses relay lens image

16 Symmetrical Three Component I-I Setup Telescope angle magnification : f 1 f 2 f 1 w ' Γ = = w h h first last asymmetric 1 Γ > 1 t max Major positions symmetric Magnification First Second distance distance Γ = Γ max > 1 t max 0 Γ = 1 t m t m Γ = 1 asymmetric 2 t m t m Γ = 1/ Γ max < 1 0 t min Γ < 1 t min Symmetrical layout

17 Example: Three Groups I-I Optical Compensated Mechanical compensation Variable distances: d 3 and d 4 Zoom factor 6 Γ = 2.44 Γ = 1.92 Γ max / Γ min = 6 Γ = 1.41 Γ = 0.92 Γ = 0.41

18 18 Optical Ophthalmic Diagnosis Imaging Measuring Anterior Segment Posterior Segment Refractive Power Wavefront Visual Field Cornea Topography Eye lengths Retinal layers Slit lamp Slit lamp Objective Refraction: Autorefractor Aberrometer Perimeter Topographer (Placido) Biometer (OCT) OCT OCT Ophthalmoscope Subjective Refraction: Phoropter Keratometer SL Polarimeter Fundus Camera OCT

19 19 Slit Lamp Stereo microscope CMO type Greenough type Ref.: ZEISS Köhler Illumination ( slit lamp ) a) from below (Zeiss type) b) from above (Haag Streit type)

20 20 Slit Lamp Projection of a slit onto the cornea with small NA Scattering in the eye Scanning in the anterior of the eye to detect inhomogeneities With the use of (neg.) contact lens or (pos.) auxilliary lens imaging of the fundus is possible Diffuse illumination Slit illumination Parfocal Swivel Ref.: M. Kaschke et al. Ophthalmology

21 21 Direct Ophthalmoscope Inspection of an illumination path reflected on the retina without microscope Selection of different apertures by a rotatable wheel Compensation lens forces a coincidence with the observation Ref.: M. Kaschke et al. Ophthalmology

22 22 Indirect Ophthalmoscope Pupil mismatch between patient and observer reduces field of view in direct ophthalmoscope Indirect ophthalmoscope: additional ophthalmoscopy lens close to the eye creates an enlarged image of the patients pupil Ref.: M. Kaschke et al. Ophthalmology

23 23 Fundus Camera Observation and photographic inspection of the retina Inspection of the fundus structural analysis to detect morphological deceases Separation of illumination and observation beam path to avoid disturbing reflections Typically ring-shaped illumination Ref.: M. Kaschke et al. Ophthalmology

24 24 Fundus Camera Modalities Ref.: M. Kaschke et al. Ophthalmology

25 25 Confocal Laser Scanning Ophthalmoscope Confocal imaging of a fundus spot by scanning (CSLO) Pinhole mirror separates illumination and detection Confocal pinhole suppresses straylight Ref.: M. Kaschke et al. Ophthalmology

26 26 Optical Coherence Tomography (OCT) Using of a low-coherence source enables 3D imaging Time-domain OCT Ref.: M. Kaschke et al. Ophthalmology

27 27 Optical Coherence Tomography (OCT) Spectral-domain OCT Better sensitivity by simultaneous detection of spectral components Depth information obtained by Fourier transform Ref.: ZEISS Ref.: M. Kaschke et al. Ophthalmology

28 28 Optical Coherence Tomography (OCT) Depth information enables measurement of layer thickness for diagnosis For Glaucoma diagnostics: Either measurement of topology of the blind spot or the thickness of the RNFL Topology of the nerve head Thickness of RNFL is measured by a circular OCT scan OCT-Scan Measurement against normative database: OCT-Scan RNFL = Retina Nerve Fiber Layer The yellow band represents healthy persons Ref.: Zeiss

29 29 Refractometer Autorefraction measurement of the eye power Test pattern projected onto the retina (mire = target pattern) Fundus reflected light is observed (Ophthalmoscope) z-differences corresponds to focal power errors Ref.: M. Kaschke et al. Ophthalmology

30 30 Aberrometer Measurement of the human eye wavefront with a Hartmann-Shack wavefront sensor Illumination spot on the fundus reflected Ref.: M. Kaschke et al. Ophthalmology

31 31 Keratometer Measuring the refractive power of the cornea Main contribution: curvature, only R measured Principle: Determination of image size y 1 sss = 1 ss + 1 fff yyy yy = sss ss rr cc = 2ff = 2 ss yy yy To correct for motion a double image is used as reference y y Ref.: M. Kaschke et al. Ophthalmology

32 32 Keratometer Helmholtz-type keratometer Littmann keratometer Achieved accuracy: r c = mm Ref.: M. Kaschke et al. Ophthalmology

33 33 Cornea Topography by Placido Disk Projection of a ring mask onto the cornea (Placido mask) Imaging the rings onto a camera Evaluation of the imaged ring widths Reconstruction of the topology of the cornea real deformed image projected pattern image Ref.: M. Kaschke et al. Ophthalmology reconstructed topology

34 34 Corneal Topographer Realization of the Placido-projection and imaging of the reflected light Ring-by-ring reconstruction of the cornea surface Ref.: M. Kaschke et al. Ophthalmology

35 35 Biometer (OCT) Special OCT technique to determine: Measurement of full eye along optical axis possible with timedomain OCT (double beam technique) or swept source OCT Low lateral resolution ensures large depth of focus Ref.: ZEISS