Download In vivo intraocular ranging by wavelength tuning interferometry

In vivo intraocular ranging by wavelength tuning interferometry
Christoph K. Hitzenberger, Manfred Kuihavy, Franz Lexer, Angela Baumgartner, and AdoifF. Fercher
Institute ofMedical Physics, University of Vienna
Wahringer StraBe 13, A-1090 Vienna, Austria
ABSTRACT
Recently, wavelength tuning interferometiy was suggested as an alternative technique for distance measurements.
Compared to partial coherence interferometry, it has the advantages of needing no high precision mechanically moving
components and the capability of measuring several distances simultaneously in veiy short time. We report on first
measurements of intraocular distances in human eyes in vivo using a distributed Bragg reflector laser diode with a tuning
range of 2 nm. We were able to measure the anterior chamber depth, the lens thickness, the vitreous depth, the axial eye
length, and to estimate the thickness ofthe retina The resolution is approximately 150 m optical distance.
Keywords: wavelength tuning interferometry, eye, intraocular distances, optical coherence tomography
1. INTRODUCTION
There is an increasing demand in ophthalmology for the precise measurement of intraocular distances. One of the most
frequenfly performed surgical interventions, the replacement of a cataractous lens by an artificial intraocular lens, depends
on the precise knowledge of the anterior chamber depth and the axial eye length These measurements are currently
performed by ultrasound or classical optical techniques.
Over the last ten years, a new optical technique, partial coherence interferometry (PCI), has been developed for measuring
intraocular distances with unprecedented precision and resolutlon1'2'3'4. This method has been extended to optical coherence
tomography (OCT), a new imaging modality capable of obtaining two-dimensional cross-sectional images of the human
retina and of other tissues5'6'7'8. An overview of this technique has been published recently9. The main advantages of these
techniques are high precision, high resolution, and the lack of need for mechanical contact between instrument and eye.
PCI and OCT work in the time domain. A broadband light source is used in conjunction with a Michelson interferometer.
The sample to be measured is placed in one interferometer arm, the other arm contains the reference mirror. Backscattering
sites within the object are located by moving the reference mirror in order to match the light transit times in reference and
sample arm.
A related technique working in the frequency domain is wavelength tuning interferometry (WTI)o' . In this case, a fixed
reference path length is used. The light source is a narrowband laser whose frequency is tunable. If its wavenumber is
changed with constant rate, the intensity of the superimposed interfering sample and reference beams oscillates with a
frequency proportional to the path difference. This intensity oscillation allows the determination of the optical path
difference. The advantage of this technique is that no moving parts are needed and several distances can be measured
simultaneously.
A first application of this technique to ophthalmology was the measurement of the axial eye length, although with poor
resolution12. With an external cavity tunable laser diode, anterior segment length, vitreous depth, and axial eye length were
measured simultaneously in vivo10"1. Because the high speed tuning range of this laser is very narrow, the resolution
obtainable for in vivo measurements was limited to about 2 mm. Another type of tunable laser is the distributed Bragg
reflector (DBR) laser diode. This type of laser can be tuned over approximately 2 nm within milliseconds. So far, DBR laser
diodes were only used for in vitro measurements13. We now present the first results obtained with this laser in human eyes
in vivo. We demonstrate the simultaneous measurement of anterior chamber depth, lens thickness, vitreous depth, axial eye
length, and retinal thickness.
SPIE Vol. 3251 • 0277-786X/98/$1O.OO
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/24/2013 Terms of Use: http://spiedl.org/terms
47
2. METHODS
2.1 Wavelength tuning interferometry
We explain the measurement of a path difference 2L in an interferometer with reference to figure 1. If the wavelength ? of
the tunable laser diode is kept at a fixed value, the intensity at the photodetector can be calculated by:
1=11 '2 +2'i.cos(2,r.W').
(1)
I and '2 are the light intensities reflected at mirror 1 and 2, respectively, AD is the phase difference of the two beams. This
phase difference is given by:
=2 =2. L
(2)
2,r
If
where k is the wavenumber corresponding to ?. the wavenumber is changed, the phase difference changes accordingly.
This causes the intensity at the photodetector to oscillate with a frequency f:
fdAdLW dkL dk
dt
dk dtff dt
—
(3)
Hence, the frequency is directly proportional to the tuning rate of the wavenumber dk/dt and to the path difference L. If
dk/dt is constant, L can be obtained by a Fourier transfonn of the time dependent intensity signal recorded by the
photodetector during tuning.
Mirror I
Mirror 2
Time dependent
signal
Figure 1. Schematic diagram of the principle of wavelength tuning interferometry.
48
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/24/2013 Terms of Use: http://spiedl.org/terms
2.2 Experimental setup
Figure 2 shows a schematic drawing of the experimental setup used for measuring intraocular distances. A DBR laser diode
(Yokogawa, YL85XTW) is used as the light source. It has a center wavelength of 852 nm and is tunable over a range of
2 urn. The beam illuminates the eye where it is reflected at several intraocular interfaces. The reflected beam components
are superimposed on photodetector 1. In this setup, the anterior cornea! surface acts as the reference surface. The optical
distances L, from the cornea to several other interfaces are measured. During tuning with constant rate, the distances L1
cause corresponding frequencies 1 according to equation (3). They are obtained from the time dependent intensity signal by
a fast Fourier transform (FFT) and converted into the distances L1 by multiplication with a calibration factor.
Auxiliary interferometer
Photodetector 2
I'
II
ii
Figure 2. Schematic diagram of wavelength tuning interferometer used for intraocular ranging.
The DBR laser diode used is a three section diode controlled by three drive current&3. The output power is determined by
the current through the active section. The wavelength is controlled by the phase control current and by the DBR current. In
order to tune the wavelength over a continuous range of 2 urn, the DBR current and the phase control current have to be
changed synchronously following a predetermined pattern. This is carried out by controlling the currents with a
programmable arbitrary function generator. During the tuning, the photodetector signal is recorded by a storage scope. One
complete scan over a 2 urn range takes 16 ms in our preliminary experiments. Faster tuning is possible in principle.
If the phase control current and the DBR current are tuned with constant rate, the wavenumber tuning rate will not be
constant. The lafter, however, is the condition for a constant frequency of intensity oscillations corresponding to a certain
path difference according to equation (3). To linearize dkidt, two methods have been suggested: the currents can be tuned in
a properly chosen nonlinear way'3 or an external auxiliaiy interferometer can be used to measure the nonlinearities of dk/dt
and correct for them numerically'0.
We chose the latter method and implemented an auxiliary Michelson interferometer into our instrument (cf. fig. 2). Its
signal is recorded by a second photodetector. It is used for numerically correcting the nonlinearities in dk/dt and as a
calibration signal for the distance measurements. Furthermore, it is used to provide a reference phase which is needed to
overcome problems with mode hopping: within the tuning range of 2 urn, three mode hops occur. The reference phase is
used to determine the contiguous points of the phase of the object signal on either side of a mode hop. For this purpose, the
whole instrument has to be calibrated with a test object and the operating conditions have to be kept stable after the
calibration.
49
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/24/2013 Terms of Use: http://spiedl.org/terms
3. RESULTS
Measurements were carried out in healthy, volunteer subjects after informed consent was obtained. The laser safety
regulations'4 were strictly obeyed. The V/TI scans were performed along the optical axis of the eye. (Scans along the vision
axis, which is inclined to the optical axis at - 5°, would not allow the detection of the lens surfaces'5). After recording the
time dependent signals they were numerically corrected for nonlinearities in dk/dt and to obtain contiguous phase across the
mode hops. The frequency components of the corrected signal were extracted by FFT. They were converted into distances
by use of a calibration factor obtained from the known path difference of the auxiliary interferometer and from the
corresponding frequency.
0,15
U)
C
I::
0,00
0
10
20
30
40
Optical distance to cornea (mm)
Figure 3. WTI scan obtained in a human eye in vivo.
Figure 3 shows the result of this process. The magnitude of the transformed signal is plotted as a function of the optical
distance from the anterior corneal surface (the reference surface). The signal peaks indicate the positions of the anterior and
posterior lens surface and of the retina. From these peaks, the optical depth of the anterior chamber (including the cornea),
the thickness of the lens, and the length of the vitreous can be determined. The position of the retinal peak indicates the
(optical) axial length of the eye. To obtain the corresponding geometric distances, the optical distances have to be divided
by the respective group indices"6. The retinal peak actually consists of two peaks, indicating probably the thickness of the
retina. The table shows preliminary results of optical intraocular distances obtained in three different eyes. For the
determination of the axial eye length, the position of the stronger second retinal peak was used. The resolution of the
technique depends on the tuning range. In our case, the distance between adjacent frequencies of the FFT corresponds to a
resolution of-- 0. 18 mm optical distance or 0.13 mm geometrical distance.
Table
Intraocular optical distances measured by wavelength tuning interferometiy. Precision (standard deviation) '-0.18 mm.
Subject
Ant. chamber depth lens thickness
(including cornea)
A.B.
4.48mm
F.L.
5.78 mm
4.70 mm
C.H.
5.70mm
4.80 mm
5.55 mm
vitreous depth axial eye length
21.64mm
24.82 mm
24.53 mm
50
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/24/2013 Terms of Use: http://spiedl.org/terms
31.82 mm
35.40 mm
34.78 mm
4. CONCLUSION
We have demonstrated the measurement of several intraocular distances in human eyes in vivo by WTI with a DBR laser
diode. Compared to earlier measurements using the piezo fine tuning of an external cavity tunable laser diode, the high
speed tuning range and therefore the resolution of in vivo measurements were increased by about an order of magnitude.
However, the resolution of - 0. 15 mm (optical distance) is still an order of magnitude worse than that obtained currently
with PCI.
The advantage of WTI, however, is that several distances can be measured simultaneously in a veiy short time.
Furthermore, no moving components are necessary. To obtain a resolution comparable to that of PCI, the tuning range of
DBR laser diodes has to be increased to a value corresponding to the spectral width of the superluminescent diodes used
with PCI. Alternatively, the speed of lull range tuning of external cavity laser diodes has to be increased by about two
orders of magnitude in order to obtain signal frequencies well separated from those caused by mechanical object motions or
vibrations. This might be the case in the near future since the tuning speed of these diodes has already been improved by a
factor of— 50 within the last two years.
5. ACKNOWLEDGMENTS
The authors would like to thank Mr. H. Sattmann and Mr. L. Schachinger for technical assistance. Financial assistance from
the Austrian Fonds zur Forderung der Wissenschafihichen Forschung (FWF grant P09781-MED) is acknowledged.
6. REFERENCES
1. A. F. Fercher and E. Roth, "Ophthalmic laser interferometry", Proc. SPIE 658, 48-51, 1986.
2. C. K. Hitzenberger, "Optical measurement ofthe axial eye length by laser Doppler interferometry", Invest. Op/it/ia/mo!.
Vis. Sci. 32, 616-624, 1991.
3 . C. K. Hitzenberger, A. Baumgartner, W. Diexler, and A. F. Fercher, "Interferometric measurement of corneal thickness
with micrometer precision", Am. J. Op/it/ia/mo!. 118, 468-476, 1994.
4. W. Drexler, A. Baumgartner, 0. Fmdl, C. K. Hitzenberger, H. Sattmann, and A. F. Fercher: "Submicrometer precision
biometry ofthe anterior segment ofthe human eye", Invest. Op/it/ia/mo!. Vis. Sci. 38, 1304-1313,1997.
5. D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Floue, K Gregoiy, C.A.
Puliafito, and J. G. Fujimoto, "Optical coherence tomography", Science 254, 1178-1 181, 1991.
6. A. F. Fercher, C. K. Hitzenberger, W. Drexier, G. Kamp, and H. Sattmann, "In vivo optical coherence tomography", Am.
J. Op/it/ia/mo!. 116, 113-114, 1993.
7. E. A. Swanson, J. A. Izatt, M. B.. Hee, D. Huan& C. P. Liii, J. S. Schuman, C. A. Puliafito, and J. G. Fujimoto,"III vivo
retinal imaging by optical coherence tomography", Opt. Left. 18, 1864-1866, 1993.
8. M. R Hee, J. A. Izatt, E. A. Swanson, D. Huang J. S. Schuman, C. P. Liii, C. A. Puliafito, and J. G. Fujimoto, "Optical
coherence tomography of the human retina", Arch. Ophtha!mo!. 113, 325-332, 1995.
9. A. F. Fercher, "Optical coherence tomography", J. Biomed. Opt. 1, 157-173, 1996.
10. F. Lexer, C. K. Hitzenberger, A. F. Fercher, and M. Kulhavy, "Wavelength-tuning interferometry of intraocular
distances",App!. Opt. 36, 6548-6553, 1997.
1 1. C. K. Hitzenberger, W. Drexier, A. Baumgartner, F. Lexer, H. Sattmann, M. Esslinger, M. Kuihavy, and A. F. Fercher,
"Optical measurement of intraocular distances: a comparison of methods", Las. Light Ophtha!mol. ,in press.
12. A. Sekine, I. Minegishi, and H. Koizumi, "Axial eye-length measurement by wavelength-shift interferometry", J. Opt.
Soc.Am.A 10, 1651-1655, 1993.
13. U. Haberland, P. Jansen, V. Blazek, and H. J. Schmitt, "Optical coherence tomography of scattering media using
frequency modulated continuous wave techniques with tunable near-infrared laser", Proc. SPIE 2981, 20-28, 1997.
14. American National Standards Institute, Safe Use ofLasers, ANSI Z 136. 1, American National Standards Institute, New
York, 1986.
15. A. Baumgartner, C. K. Hitzenberger, W. Drexler, H. Sattinann, and A. F. Fercher, "Measurement ofthe anterior
structures ofthe human eye by partial coherence interferometry", Proc. SPJE 2330, 146-151, 1995.
16. W. Drexler, C. K. Hitzenberger, A. Baumgartner, 0. Findi, H. Sattmann, and A. F. Fercher, "Investigation of dispersion
effects in ocular media by multiple wavelength partial coherence interferometry", Exp. Eye Res., in press.
51
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/24/2013 Terms of Use: http://spiedl.org/terms