Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type
Reexamination Certificate
2002-09-30
2004-08-03
Manuel, George (Department: 3737)
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Objective type
C356S497000
Reexamination Certificate
active
06769769
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a dual channel optical mapping apparatus and to methods which can be used to supply images from essentially transparent objects or tissue using different depth resolutions, or, sequentially, images with adjustable depth resolution at the same or at different wavelengths, as required to observe fluorescence or Raman radiation emitted by the object. The two channels of the dual channel apparatus could be either a confocal channel and an optical coherence tomography channel, two optical coherence tomography channels, or two confocal channels.
BACKGROUND OF THE INVENTION
In the description which follows, reference is made primarily to the eye as the object. This has to be understood as merely a way to help the description and not as a restriction of the application of the present invention. As such, where the term “eye” is used, a more general transparent and scattering object or organ may be sought instead.
High depth resolution imaging of the eye fundus can be achieved by optical coherence tomography (OCT) as shown in the paper “Optical coherence tomography” by D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee. T. Flotte, K. Gregory, C. A. Puliafito and J. G. Fujimoto, Science 254, (1991), pp. 1178 and in the paper “Optical coherence tomography” by A. F. Fercher, in J. Biomed. Opt., 1(2), (1996), pp. 157-173. OCT has the potential of achieving much better depth resolution, as the limit in this case is not set by the eye, but by the coherence length of the source. For example, optical sources, such as superluminiscent diodes and mode-locked lasers are now available with coherence lengths below 20 &mgr;m.
An OCT apparatus is now commercially available (e.g. from Humphery), which produces longitudinal images only, i.e. images in the planes (x,z) or (y,z), where the z axis is perpendicular to the patient's face and x and y axes are in the plane of the patient's face. Examples of such apparatus for longitudinal imaging are described in U.S. Pat. Nos. 5,493,109, 5,537,162, 5,491,524, 5,469,261, 5,321,501 and 5,459,570.
OCT has also been reported as being capable of providing en-face (or transversal) images, as reported in “Coherence Imaging by Use of a Newton Rings Sampling Function⇄ by A. Gh. Podoleanu, G. M. Dobre, D. J. Webb, D. A. Jackson, published in Opt. Lett., Vol. 21, No. 21, (1996), pp. 1789-1791, “Simultaneous En-face Imaging of Two Layers in Human Retina” Opt. Letters, by A. Gh. Podoleanu, G. M. Dobre, D. J. Webb, D. A. Jackson, published in Opt. Lett., 1997, vol.22, No. 13, pp. pp. 1039-1041 and “En-face Coherence Imaging Using Galvanometer Scanner Modulation” by A. Gh. Podolenu, G. M. Dobre, D. A. Jackson, Opt. Lett. 23, pp. 147-149, 1998. When applied to the eye, however, the en-face OCT images look fragmented, as demonstrated in “Transversal and Longitudinal Images from the Retina of the living Eye Using Low Coherence Reflectometry”, by A. Gh. Podoleanu, Mauritius Seeger, George M. Dobre, David J. Webb, David A. Jackson and F. Fitzke, published in the Journal of Biomedical Optics, 3(1), pp. 12-20, 1998. These papers also demonstrate that, owing to the low coherence length, the OCT transversal images show only fragments of the retina and are difficult to interpret.
To improve the usefulness of en-face OCT images, a dual presentation of images was proposed as described in U.S. Pat. No. 5,975,697. Two en-face images are produced and displayed simultaneously; one of which is an OCT image and the other is a confocal image (similar to the image produced by a scanning laser ophthalmoscope (SLO)). The dual presentation allows the fragments sampled by the OCT of the fundus to be uniquely placed in correspondence with fundus images displayed by the confocal channel. The confocal channel, however, has a much larger depth resolution and the images look continuous, offering good guidance of the part of the eye investigated.
The dual display is generally essential not only for guidance, but for subsequent alignment and processing of the stack of en-face images prior to reconstruction of the 3D volume investigated. In addition, it enables the same location in the eye to be accessed easier on subsequent examinations. However, a practical problem of the dual channel imaging instruments is that focusing has to be adjusted simultaneously for both channels. No provisions were presented in U.S. Pat. No. 5,975,697 with respect to this feature.
Another problem with the prior art technique is that the confocal channel taps some of the possibly already weak return signal from the tissue, which results in a lower signal to noise ratio in the OCT channel. For instance, when 10% of the signal is tapped, the loss in the OCT channel can be more than 19%. Therefore, it would be desirable, especially when the target returns a weak signal, to eliminate confocal tapping and return all of the signal to the OCT channel. On the other hand, there are situations when the presentation of an OCT image is not needed and depth analysis using the confocal channel only may be required. Unfortunately, however, the beam-splitter ratio in U.S. Pat. No. 5,975,697 is fixed and therefore, such versatility cannot be achieved.
Another problem is that when this technique is used for OCT of skin or teeth, longer wavelengths are recommended for providing better penetration depth. However, the gain of photocathodes and avalanche photodetectors at longer optical wavelengths is much poorer than that for visible light or, for example, the 800 nm band which is preferred for the retina. Therefore, at longer wavelengths, poorer performance of the confocal channel of the dual instrument as presented in U.S. Pat. No. 5,975,697 is expected. Another problem with the dual channel imaging instrument as described in U.S. Pat. No. 5,975,697 is that the wavelengths of the two channels are the same. The system as such cannot be used to generate a confocal image at a different wavelength from that used in the OCT channel. This prevents the utilisation of the system in fluorescence and autofluorescence imaging, or for Raman studies.
In U.S. Pat. No. 5,459,570, the beam-splitter shared by the confocal and the OCT channel is used in transmission. It is known that the dispersion of the optical material used in beam-splitters, if left uncompensated, leads to deterioration of the depth resolution in the OCT channel. An on-axis fixation lamp is also required for the investigation of the fovea. This requires other beam-splitters to be introduced in the system, which adds further dispersion to the OCT channel.
A still further problem with en-face scanning using galvanometer scanners is that the fly-back of the galvanometer scanner is finite and consequently, more than 20% of the period time of the ramp signal driving the galvanometer scanners, at kHz rates, may be wasted.
In terms of transverse resolution, this feature depends on how well the focus is matched to the coherence position (wherein tracking of the focusing and zero optical path difference are referred to as dynamic focus). Dynamic focus was described in PCT patent publication No. WO 92/19930, but only in principle. Possible optical configurations to simultaneously scan the depth and the position of the focus in the depth are described in U.S. Pat. No. 4,589,773 and in U.S. Pat. No. 6,057,920. These solutions however, require mechanical synchronism of elements or adjustment of ratios of focal lenses. The prior art method works only when the index of refraction of the tissue is known. If the tissue consists of layers of different index of refraction, different adjustments are required. The methods described are devised especially for longitudinal OCT, where B-scan images are generated by fast scanning along the depth coordinate with a slower scanning along a transverse coordinate. As such, the method needs to be fast, and operational at the depth scanning rate of, for example, a rate on the order of 100-1000 Hz. Once different solutions have been devised as described in U.S. Pat. No.
Cucu Radu G.
Dobre George M.
Jackson David A.
Podoleanu Adrian Gh.
Rogers John A.
(Marks & Clerk)
Manuel George
OTI Ophthalmic Technologies Inc.
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