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Claims  |
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What is claimed is:
1. A retinal-projection system for selectively projecting an image onto a
selected area of the retina, comprising:
a light source for providing a projection beam of at least partially
coherent light;
a modulation optics for modulating the projection beam with a desired
image; and
wide-angle imaging optics for focusing the modulated projection beam, such
that the projection beam passes through the pupil and diverges to provide
a projected image on the retina with a field-of-view extending beyond the
posterior region.
2. The retinal-projection system of claim 1, wherein the image is projected
over substantially the entire retina, including the anterior retina out to
the far periphery.
3. The retinal-projection system of claim 2, wherein the wide-angle imaging
optics focuses the image near the front of the eye lens.
4. The retinal-projection system of claim 2, wherein the image presents
video or printed information.
5. The retinal-projection system of claim 4, wherein the modulation optics
includes an LCD panel.
6. The retinal-projection system of claim 1, wherein the wide-angle imaging
optics images the projection beam onto substantially the entire retina,
and the modulation optics includes an aperture for blocking a selected
portion of the projection beam, such that the image is projected onto a
selected area of the retina.
7. The retinal-projection system of claim 6, wherein the image is an ERG
pattern.
8. The retinal-projection system of claim 7, wherein the modulation optics
includes an interferometer for modulating the projection beam with an
interference pattern to create the ERG pattern, and for selectively
alternating the interference pattern to create an ERG pattern shift.
9. An ERG pattern projection (PERG) system for projecting an ERG pattern
onto a selected area of the retina, comprising:
a light source for providing a projection beam of at least partially
coherent light;
modulation optics for modulating the projection beam to create a desired
ERG pattern, and for selectively shifting the pattern to create an ERG
response;
imagining optics for focusing the modulated projection beam, such that the
ERG pattern is projected onto a selected area of the retina with a
field-of-view extending beyond the posterior region.
10. The PERG projection system of claim 9, wherein the modulation optics
includes an interferometer for modulating the projection beam with an
interference pattern to form the ERG pattern, and for selectively altering
the interference pattern to cause an ERG pattern shift.
11. The PERG projection system of claim 10, wherein the interferometer
modulates the projection beam with a selected spatial frequency to produce
a selected fringe line spacing, and with a selected alternation frequency
to produce an ERG response.
12. The PERG projection system of claim 10, wherein the interferometry
optics comprises a shearing interferometer.
13. The PERG projection system of claim 12, wherein the shearing
interferometer comprises:
first and second prisms separated by a gap defined by respective opposing
first and second non-parallel surfaces that define a shearing angle;
the incident projection beam partially reflecting from the first opposing
prism surface to form a first-reflection beam, and after transiting the
prism gap, partially reflecting from the opposing prism surface to form a
second-reflection beam;
such that the first and second reflection beams interferometrically combine
to form an interference pattern;
the second prism being pivotally mounted for selectively changing the
shearing angle to provide precision angle tuning of the interference
pattern.
14. The PERG projection system of claim 3, wherein the prism gap is filled
with a Kerr fluid, and wherein:
the opposing first and second prism surfaces are coated with transparent
conductive electrode layers coupled to a controlled voltage source;
such that an adjustable time-varying electric field is produced in the gap
for controlling the refractive index in the prism gap so as to selectively
shift the interference pattern.
15. The PERG projection system of claim 10, wherein the interferometer
comprises a Newton's ring interferometer.
16. The PERG projection system of claim 15, wherein the Newton's ring
interferometer comprises:
a plano convex lens coated antireflective on one side; and
a reflective optical flat;
an incident projection beam being partially reflected by the plano convex
lens to for a first reflection beam, and then reflecting from the optical
flat to form a second reflection beam;
such that the first and second reflection beam interferometrically combine
to form an interference pattern.
17. The PERG projection system of claim 16, further including translation
means for selectively translating the optical flat to shift the
interference pattern.
18. The PERG projection system of claim 9, wherein the imaging optics
include aspheric surfaces to provide a short focal length.
19. The PERG projection system of claim 18, wherein the imaging optics
includes zoom optics for selectively adjusting the diameter of the
projection beam.
20. The PERG projection system of claim 18, wherein the imaging optics do
not contact the eye.
21. The PERG projection system of claim 9, wherein the PERG projection
system is used to evaluate retinal degeneration caused by glaucoma.
22. A retinal-projection method for selectively projecting an image onto a
selected area of the retina, comprising the steps;
generating a projection beam of at least partially coherent light;
modulating the projection beam with a desired image; and
focusing the modulated projection beam, such that the projection beam
passes through the pupil and diverges to provide a projected image on the
retina with a field-of-view extending beyond the posterior retina.
23. The retinal-projection method of claim 22, wherein the image is
projected over substantially the entire retina, including the anterior
retina out to the far periphery.
24. The retinal-projection method of claim 22, wherein the image presents
video or printed information.
25. The retinal-projection method of claim 29, wherein the modulation
optics includes an LCD panel.
26. The retinal-projection method of claim 22, further comprising the step
of blocking a selected portion of the projection beam, such that the image
is projected onto a selected area of the retina.
27. The retinal-projection method of claim 26, wherein the image is an ERG
pattern.
28. An ERG pattern projection (PERG) method of projecting an ERG pattern
onto a selected area of the retina, comprising:
generating a projection beam of at least partially coherent light;
modulating the projection beam to create a desired ERG pattern,
focusing the modulated projection beam, such that the ERG pattern is
projected onto a selected area of the retina with a field-of-view
extending beyond posterior region; and
selectively shifting the pattern to create an ERG response.
29. The PERG projection method of claim 28, wherein the step of modulating
the projection beam comprises the step of modulating the projection beam
with an interference pattern to form the ERG pattern.
30. The PERG projection method of claim 29, wherein the projection beam is
modulated with a selected spatial frequency to produce a selected fringe
line spacing, and with a selected alternation frequency to produce an ERG
response.
31. The PERG projection method of claim 29, wherein the step of modulating
the projection beam is accomplished by a shearing interferometer.
32. The PERG projection method of claim 28, wherein the step of modulating
the projection beam is accomplished by a Newton's ring interferometer.
33. The PERG projection method of claim 28, further comprising the step of
selectively adjusting the diameter of the projection beam.
34. The PERG projection method of claim 28, wherein the PERG projection
method is used to evaluate retinal degeneration caused by glaucoma.
35. An ERG pattern projection (PERG) system for projecting an ERG pattern
onto a selected area of the retina, comprising:
a light source for providing a projection beam of at least partially
coherent light;
modulation optics for modulating the projection beam to create a desired
ERG pattern, and for selectively shifting the pattern to create an ERG
response;
segmentation optics for selectively segmenting the ERG pattern into
segments that are large relative to the ERG pattern; and
imaging optics for focusing and modulated projection beam, such that the
segmented ERG pattern is projected onto a selected area of the retina.
36. The PERG projection system of claim 35, wherein the ERG pattern is
segmented into posterior, medial and anterior regions, and further into
sectors of the medial and anterior regions.
37. The PERG projection system of claim 35, wherein the segmentation optics
comprises an LCD panel.
38. The PERG projection system of claim 35, wherein the segmentation optics
comprises:
a ring aperture for segmenting the projection beam into posterior, medial
and anterior regions; and
a sectoring aperture for segmenting at least the medial and anterior
regions into angular sectors.
39. An ERG pattern projection (PERG) method for projecting an ERG pattern
onto a selected area of the retina, comprising:
generating a projection beam of at least partially coherent light;
modulating the projection beam to create a desired ERG pattern;
selectively segmenting the ERG pattern into segments that are large
relative to the ERG pattern;
focusing the modulated projection beam, such that the ERG pattern is
projected onto a selected area of the retina; and
selectively shifting the pattern to create an ERG response.
40. The PERG projection method of claim 39, wherein the ERG pattern is
segmented into posterior, medial and anterior regions, and further into
sectors of the medial and anterior regions. |
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Claims |
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Description |
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TECHNICAL FIELD OF THE INVENTION
The invention relates generally to ophthalmic instruments, and more
particularly to a retinal-projection system and method for projecting an
image onto a selected area of the retina, which can be significantly
anterior to the posterior retina. In even greater particularity, the
invention relates to such a system and method used as a pattern
electroretinogram (PERG) projector capable of projecting ERG patterns that
can be segmented to probe selected areas of the retina (posterior, medial
and/or anterior out to the far periphery) for evaluating retinal
degeneration caused by glaucoma and other diseases.
BACKGROUND OF THE INVENTION
As an improved ophthalmic technique for diagnosing and monitoring glaucoma
(and other diseases that cause retinal degeneration), electroretinogram
(ERG) measurement appears to have considerable promise. Current research
indicates that dynamic ERG response to retinal stimulation from an
alternating pattern may be a significantly better and earlier indicator of
retinal degeneration than conventional intraocular pressure, visual field
tests and cup-to-disc ratios.
The specific problem to which the invention can be applied is a pattern
electroretinogram (PERG) system capable of assessing the function of the
retinal ganglion cell layer and nerve fiber layer, and evaluating retinal
degeneration, significantly anterior to the posterior retina, such as for
glaucoma diagnosis and management. In addition to permitting the entire
retina to be evaluated, the test should have the following features: (1)
Sensitivity and specificity; (2) Reproducibility; (3) Ease of use in very
young, elderly and disabled patients; (4) Brevity of performance; (5) Easy
interpretation; (6) Minimal requirement for patient response and/or
attention; (7) Reliability in spite of ocular media opacities; and (8)
Freedom from refractive correction.
Ophthalmologists have traditionally relied on relatively few clinical tests
and findings to diagnose and follow patients with open-angle glaucoma.
Recently, the sensitivity of even the most sophisticated tests have been
questioned. One recent study has shown that glaucoma suspects may lose up
to fifty percent of their optic nerve fibers before developing visual loss
detectable by kinetic perimetry (Reference 10, listed at the end of the
Background). Moreover, in glaucoma, loss of the nerve fibers and,
subsequently, the ganglion cells themselves, have been shown by
histopathologic studies (References 1,8,9,11).
Newly developed automated threshold static perimetry has also failed to
solve this problem because, in many cases, it is difficult to distinguish
progressive visual field loss from short- and long-term fluctuations, as
well as from the effects of media opacities and miosis. Additionally,
these tests (a) require a significant amount of concentration and
participation from patients that is sometimes difficult, (b) require
administration by trained personnel, and (c) require a high degree of
understanding for accurate interpretation. Other parameters used to
evaluate glaucoma, namely cup to disc (C/D) ratios and intraocular
pressures, also have significant disadvantages. Even with the best tools
and methods, both inter- and intra-observer variability of the C/D ratio
assessment are inevitable (Reference 2). In addition, up to fifty percent
of the optic nerve fibers may be lost prior to detectable visual field
abnormalities. Elevated Intraocular Pressure (IOP) is well known to be far
more prevalent than is open-angle glaucoma as defined by visual field
loss. In addition, the IOP is extremely variable even in patients with
known glaucoma and, as a sole parameter for glaucoma management, is
grossly inadequate (Reference 12). Pattern ERG (PERG) has significant
advantages over these current approaches to diagnosing and evaluating
retinal degeneration from glaucoma. In PERG, the patient views a pattern
image (such as light and dark squares shown on a CRT), and the retina is
stimulated by alternating the pattern (such as by light/dark pattern
reversal), generating an ERG amplitude response. Many studies have
confirmed that the PERG amplitude is significantly reduced in patients
with glaucoma (References 3-7,13).
PERG is particularly advantageous because research indicates that the PERG
response signal is generated only by the proximal (inner) layers, which
are precisely the layers selectively damaged by glaucoma. Thus, PERG has
great potential for diagnosing and monitoring retinal degeneration caused
by glaucoma (References 3-7,13,14).
Moreover, at least one study has shown that abnormal PERG amplitudes are
manifested in patients with only ocular hypertension (i.e. glaucoma
suspects) (Reference 14). This group of patients have elevated IOP, but no
demonstrable visual field or C/D abnormalities. Many of these patients
later go on to develop visual field or C/D abnormalities and hence are
diagnosed to have glaucoma. The PERG abnormalities found in these patients
suggest that PERG is a more sensitive indicator of early glaucomatous
degeneration than are currently utilized tests, at least in some patients.
Conventional PERG has several important advantages: (1) Direct measurement,
i.e., minimal cognitive patient response is required for generating of
PERG data; (2) Brevity, i.e., one pattern alternation generates a
corresponding PERG response; and (3) Specificity, i.e., PERG response is
generated by the ganglion cells (the retinal component damaged in
glaucoma).
However, the current technique for performing PERG measurements based on
viewing a CRT pattern has at least two important limitations: (1) It
requires an accurate refractive correction to obtain sharp pattern
contrast; and (2) It tests only the posterior retina.
The field of view limitation is particularly disadvantageous because the
posterior retina survives best to end stage glaucoma, while the ganglion
cells in the more anterior retina, and especially in the far anterior
periphery, are frequently lost first. This phenomenon of glaucomatous
damage is demonstrated by the generalized constriction seen on standard
visual field testing of patients with glaucoma. This pattern of vision
loss progresses until only a central island of vision remains, and it is
precisely this most resistant region that is being stimulated by a
standard PERG. The entire retina can be stimulated by a flash ERG (FERG)
procedure. In a standard flash ERG (FERG), a flash of light generates a
dynamic ERG amplitude response caused by the stimulation of both the
distal (photoreceptor) and proximal (ganglion) layers of the retina in all
retinal regions (posterior, middle and anterior). This mass retinal
response is of no advantage in monitoring glaucomatous retinal
degeneration, which selectively affects only the nerve fibers and their
parent ganglion cells. Systems are available for projecting images
directly into the eye--laser interferometers and potential acuity meters
used to evaluate visual acuity in subjects for cataract surgery. The
interferometer projects an interference pattern, while the potential
acuity meter projects an eye chart. These projection systems have not been
used for PERG testing, and in any event, are only capable of projecting
into the posterior retina.
Accordingly, a specific need exists for a PERG system capable of
stimulating the retina significantly anterior to the posterior retina. A
more general need exists for an ophthalmic instrument for projecting
images (modulated light) onto a selected area of the retina.
REFERENCES
1. Apple, D. J. and Rabb, M. F. (1978) Clinico pathologic correlations of
ocular disease; a text and stereoscopic atlas. St. Louis, C. V. Mosby.
2. Balazsi, A. G., Drance, S. M., Schulzer, M., Douglas, G. R. (1984)
Neuroretinal rim area in suspected glaucoma and early chronic open-angle
glaucoma: correlation with parameters of visual function. Arch Ophthalmol
102:1011-1014.
3. Bobak, P., Bodis-Wollner, I., Harnois, C., Maffei, L., Mylin, L., Podos,
S., Thornton, J. (1983) Pattern electroretinograms and visual-evoked
potentials in glaucoma and multiple sclerosis. Am J Ophthalmol 96, 72-83.
4. Marx, M. S., Podos, S. M., Bodis-Wollner, I., Howard-Williams, J. R.,
Siegel, M. J., Teitelbaum, C. S., Maclin, E. L., Severin, C. (1986) Flash
and pattern electroretinograms in normal and laser-induced glaucomatous
primate eyes. Invest Ophthalmol Vis Sci 27:378-386, 1986.
5. Marx, M. S., Podos, S. M., Bodis-Wollner, I., Lee, P., Wang, R.,
Severin, C. (1988) Signs of early damage in glaucomatous monkey eyes: low
spatial frequency losses in the pattern ERG and VEP. Exp. Eye Res. 46,
173-184.
6. Papst, N., Bopp, M., Schnaudigel, O. E. (1984) Pattern electroretinogram
and visually evoked cortical potentials in glaucoma. Graefe's Arch Clin
Exp Ophthalmol 222:29-33.
7. Price, M. J., Drance, S. M., Price, M., Schulzer, M., Douglas G. R.,
Tansley, B. (1988) The pattern electroretinogram and visual-evoked
potential in glaucoma. Graefe's Arch Clin Exp Ophthalmol 226:542-547.
8. Quigley, H. A., Addicks, E. M., Green, W. R., Maumenee, A. E. (1981)
Optic nerve damage in human glaucoma. II. The site of injury and
susceptibility to damage. Arch Ophthalmol 99:635-649.
9. Quigley, H. A., Miller, N. R., George, T. (1980) Clinical evaluation of
nerve fiber layer atrophy as an indicator of glaucomatous optic nerve
damage. Arch Opthalmol 98:1564-1571.
10. Quigley, H. A., Sanchez, R. M., Dunkelberger, G. R., L'Hernault, N. L.,
Baginski, T. A. (1987) Chronic glaucoma selectively damages large optic
nerve fibers. Invest Ophthalmol Vis Sci 28:913-920.
11. Sommer, A., Miller, N. R., Pollack, I., Maumenee, A. E., George, T.
(1977) The nerve fiber layer in the diagnosis of glaucoma. Arch Ophthalmol
95:2149-2156.
12. Sponsel, W. E. (1989) Tonometry in Question: Can visual screening tests
play a more decisive role in glaucoma diagnosis and management? Surv
Ophthalmol 33 (suppl): 291-300.
13. Wanger, P. and Persson, H. E. (1983) Pattern-reversal
electroretinograms in unilateral glaucoma. Invest Ophthalmol Vis Sci
24:749-753.
14. Wanger, P. and Persson, H. E. (1985) Pattern-reversal
electroretinograms in ocular hypertension. Documenta Ophthalmologica 61,
27-31.
SUMMARY OF THE INVENTION
The invention is a retinal-projection system and method for projecting an
image onto a selected area of the retina with a field-of-view that can be
significantly anterior to the posterior retina. As one application, the
system can be used to project an ERG pattern for evaluating degeneration
in any area of the retina (posterior, medial and/or anterior out to the
far periphery).
In one aspect of the invention, the retinal-projection system includes a
light source, at least partially coherent, for generating a projection
beam. Modulation optics modulates the projection beam with a desired
image, and wide-angle imaging optics adjacent the eye focuses the
modulated projection beam, such that the projection beam passes through
the pupil and diverges to provide a field-of-view significantly anterior
to the posterior retina. By appropriately controlling the wide-angle
focusing optics, the field-of-view can be adjusted to include the anterior
retina out to the far periphery.
In another aspect of the invention, the retinal-projection system is used
as a pattern ERG (PERG) projector for evaluating retinal degeneration
caused by glaucoma or other disease. The PERG projector includes imaging
optics to project an ERG pattern into any selected area of the
retina--posterior, medial and/or anterior out to the far periphery.
In more specific aspects of the invention, an exemplary PERG projector
includes imaging optics that provide wide-angle projection of an
alternating, selectively segmented ERG interference pattern, enabling the
entire retina (posterior, medial and/or anterior out to the far periphery)
to be probed. Interferometry modulation optics modulates the projection
beam with an ERG interference pattern characterized by a selected fringe
line spacing, and selectively alternates the interference pattern to
create the ERG pattern shift that stimulates an ERG response. Segmentation
optics allows the ERG pattern to be selectively segmented for stimulating
a selected area of the retina.
In one embodiment, the interferometry optics comprises a shearing
interferometer. A pair of prisms are separated by a gap defined by
opposing non-parallel surfaces that define a shearing angle. One of the
prisms is pivotally mounted for selectively changing the shearing angle to
provide precision angle tuning of the interference pattern. The incident
projection beam partially reflects from one opposing prism surface, and
after transiting the prism gap, partially reflects from the opposing prism
surface. The two reflected projection beams interferometrically combine to
form an interference pattern. The prism gap is filled with a Kerr fluid
and the opposing prism surfaces are coated with transparent conductive
electrode layers, such that an adjustable time-varying electric field can
be used to change the refractive index in the prism gap, and alternate
(reverse) the interference pattern.
In another embodiment, the interferometry optics includes a plano convex
lens and a reflective optical flat. The lens is coated antireflective on a
flat side so that it partially reflects and partially transmits the
incident projection beam. The reflective optical flat reflects the
transmitted portion of the incident projection beam back through the plano
convex lens, thereby creating an interference pattern with the two
reflected projection beams. The optical flat can be selectively translated
to shift the interference pattern.
The wide-angle imaging optics can be formed by an aspheric parabolic lens,
followed by a positive meniscus lens. The lenses are configured to allow
the projection beam to be brought to a sufficiently sharp focus in the
area of the eye lens to transmit through even a constricted pupil, and
then diverge to fill substantially the entire field-of-view of the retina
(posterior and anterior out to the far periphery).
As an alternate application, the retinal-projection system can be used to
project video or printed information directly into the eye. In this
application, the modulation optics can be formed by a LCD video panel.
The technical advantages of the invention include the following. The
retinal-projection system can be used to project selected images,
including video, text and other information, onto the retina with a
field-of-view significantly anterior to the posterior retina. Laser light
can be used for projection to bypass potential ocular media opacities, and
to obviate the need for any refractive correction. In a PERG application,
an ERG pattern (such as an alternating interference pattern) can be
projected into any selected area of the retina--posterior, medial and/or
anterior out to the far periphery--providing the ability to test the
retinal elements damaged by glaucoma in the areas where they are most
likely to be affected first (i.e., the ganglion cells in the anterior
retina and especially in the periphery). The ERG pattern can be
selectively segmented to probe selected areas of the retina, allowing a
retinal map to be developed for the entire retinal field, and allowing the
different retinal regions to be compared to each other. The ERG
interference pattern can be modulated both spatially (spatial frequency or
fringe line spacing) and temporally (alternation frequency).
For a more complete understanding of the invention, and for further
features and advantages, reference is now made to the following Detailed
Description of an exemplary embodiment of the invention, taken in
conjunction with the accompanying Drawings. Although the Detailed
Description, and the Drawings, are with respect to a specific, exemplary
embodiment of the invention, various changes and modifications may be
suggested to one skilled in the art, and it is intended that the invention
encompass such changes and modifications as fall within the scope of the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a functional block diagram of the retinal-projection system of
the invention;
FIG. 2a illustrates an application of the retinal-projection system as a
PERG projector capable of projecting an ERG pattern onto the entire
field-of-view of the retina, including a shearing interferometer for
generating an ERG interference pattern;
FIG. 2b illustrates an exemplary segmentation of the ERG pattern from the
PERG projector, to allow selected segments of the entire retina to be
probed;
FIG. 3 is a functional block diagram of a PERG system for performing under
computer control PERG testing using a PERG projector according to the
invention;
FIG. 4 illustrates an alternative embodiment using a Newton's ring
interferometer to generate an ERG interference pattern.
FIG. 5 illustrates an alternative embodiment of the interferometry optics
using a Michaelson interferometer; and
FIG. 6 illustrates an alternative embodiment of the interferometry optics
using a pressure driven interferometer.
DETAILED DESCRIPTION OF THE INVENTION
The Detailed Description of exemplary embodiments of the retinal-projection
system of the invention is organized as follows:
1. Retinal-projection Technique
2. Exemplary PERG Projector
2.1. Laser Light Source
2.2. Interferometry Modulation Optics
2.3. Segmentation Optics
2.4. Wide-Angle Imaging Optics
2.5. ERG Detection System
2.6. PERG Program
2.7. Alternate Interferometry Optics
3. Alternate Applications
While the Detailed Description is in relation to an exemplary application
of the retinal-projection system as a pattern electroretinogram projector,
such as for use in evaluating and treating glaucoma, the invention has
general applicability to projecting images (modulated light) onto the
retina with a field-of-view that can be significantly anterior to the
posterior retina.
1. Retinal-projection Technique. FIG. 1 illustrates the retinal-projection
technique of the invention. A projection beam 11 is generated by a light
source 12 that is at least partially coherent. The light is modulated by
modulation optics 14 to form a modulated projection beam 15 with the
desired pattern or image.
The modulated projection beam 15 is input to wide-angle imaging optics 16,
adjacent to the eye 20. The wide-angle imaging optics brings the modulated
projection beam to a sharp focus in the area of the eye lens 22, enabling
the beam to pass through a constricted pupil. After passing through the
pupil, the beam diverges rapidly within the eye, and is projected onto the
entire retina 24.
That is, the retinal-projection technique enables a pattern or other image
to be projected with a field-of-view that is significantly anterior to the
posterior retina, including out to the far periphery 25.
2. Exemplary PERG Projector. The exemplary PERG projection system provides
wide-angle ERG pattern projection onto the entire retina--posterior,
medial and anterior out to the far periphery. Then ERG pattern can be
selectively segmented to probe selected areas of the retina for
degeneration caused by glaucoma or other diseases.
FIG. 2a illustrates an exemplary embodiment of the PERG projector 30,
including a laser light source 32 for providing a coherent projection beam
33, modulation optics 34 for generating a PERG interference pattern,
segmentation optics 35 for selectively segmenting the PERG pattern, and
wide-angle imaging optics 36 for imaging the PER pattern onto the retina.
The ERG response is detected by an ERG detection system 37 that includes
an ERG electrode 37a.
A PERG processor 38 controls modulation optics 34 and segmentation optics
35 to produce the desired interference pattern and the desired
segmentation, and receives the resulting ERG responses from the ERG
detection system 37.
FIG. 2b illustrates an exemplary segmentation of the retina. The entire
field-of-view for the retina is represented by a circle 40 divided into
seventeen representative segments. The retinal field-of-view is separated
into concentric posterior 41 and anterior 42 regions, separated by a
medial region 43. In addition, the field-of-view in the anterior and
medial regions is sectored by radial sector lines 45, defining segments
such as 47.
The PERG processor executes a PERG program to appropriately control the
modulation optics 34 and the segmentation optics 35, controlling the
retinal segments (regions and sectors) stimulated by the PERG, and the
spatial and temporal frequencies for the projected ERG pattern. As a
result, the retinal segments can be selectively probed to develop an
accurate map of the entire retinal field. Using this retinal map, the
retina can be evaluated for degeneration caused by glaucoma (or other
disease).
2.1. Laser Light Source. Referring to FIG. 2a, light source 32 includes a
laser 51 (such as helium neon or argon) that generates a projection beam
52 of coherent light. Alternatively, a point source of light could be used
to provide partially coherent light.
The projection beam is directed through a polarizer 53 and an analyzer 54,
which are polarized sheets. The projection beam 52 is adjusted in
intensity by rotating the polarizer relative to the plane of polarization
established by the analyzer to be perpendicular to the drawing sheet.
The projection beam is expanded by a lens 55, recollimated by a lens 56,
and directed toward modulation optics 34.
The intensity of the laser light can be controlled to maintain a
comfortable and safe level, but sufficient to induce an ERG response.
2.2. Interferometry Modulation Optics. Modulation optics 34 comprises a
shearing interferometer. Alternate interferometer configurations are
described in Section 2.5. Other suitable interferometers include the
Michaelson and Twyman-Green interferometers (see Section 2.6).
The exemplary shearing interferometer includes two prisms 61 and 62 with
respective opposing partially reflecting surfaces 61a and 62a, separated
by a gap 63. The prisms are mounted such that the reflecting prism
surfaces are non-parallel, and define a shearing angle Theta.
The incident projection beam 33 enters prism 61, partially reflecting from
prism surface 61a to provide a first-reflection beam 67, and partially
transmitting across gap 63. The partially transmitted projection beam
partially reflects from prism surface 62a, back across gap 63 to provide a
second-reflection beam 68.
The first- and second-reflection beams combine interferometrically into a
PERG interference beam 69, producing an ERG interference pattern of light
and dark fringe lines. The PERG pattern is controlled in spatial frequency
(i.e., the fringe line spacing) by the shearing angle Theta.
Prism 61 is stationary, while prism 62 is spring-loaded and pivotally
mounted at a pivot point 62b. Prism 62 can be selectively pivoted by a
motor-driven micrometer 64. Pivoting micrometer 64 is responsive to
control signals from PERG processor 38 to control the shearing angle Theta
between non-parallel prism surfaces 61a and 62a, providing precision
angle-tuning of the spatial frequency for the interference pattern.
To implement the pattern-shifting required to produce a PERG response, the
PERG interference pattern is alternated through a light-dark cycle with a
selected frequency of alternation. Interferometer prisms 61 and 62 are at
least partially immersed in a Kerr cell 72 filled with a Kerr fluid (such
as carbon disulfide), such that the Kerr fluid fills the gap 63 between
the prisms.
The opposing prism surfaces 61a and 62a that define the gap 63 are coated
with a transparent conductive material such as indium-tin-oxide. The
conductive coatings are coupled to a high voltage amplifier 74.
The high voltage amplifier receives a low frequency 1-20 Hz signal from
PERG processor 38. As a result, an electric field is established in the
gap 63, and applied to the Kerr fluid, which changes refractive index in
response to changes in the electric field.
By controlling the electric field in the gap, the effective path length
through the gap for the second-reflected beam 68 can be changed. This path
modulation results in a selective shift in the light/dark ares of the
interference pattern.
The PERG interference beam 69, modulated with an interference pattern
characterized by a selected angle-tuned spatial frequency, and a selected
alternation frequency, is directed to segmentation optics 35.
2.3 Segmentation Optics. Segmentation optics 35 provides the desired
segmentation for the projected ERG pattern. The time-varying PERG
interference beam 69 is first expanded by lenses 82 and 84, and directed
through a spatial light modulator 86.
The exemplary spatial light modulator 86 is conventionally formed by liquid
crystal films sandwiched between transparent electrode plates. The
modulator is configured to provide the segmentation of the retinal
field-of-view illustrated in FIG. 2b.
In response to control signals from PERG processor 38, spatial light
modulator 86 selectively passes the entire time-varying PERG interference
beam 69 (i.e., the entire ERG interference pattern), or any selected
segment. For example, the modulator could be controlled to pass the ERG
interference pattern only in the anterior region (42 in FIG. 2b), or only
in one sector of that region (47 in FIG. 2b).
After selective segmentation, the time-varying PERG interference beam is
directed to the wide-angle imaging optics 36.
2.4. Wide-Angle Imaging Optics. For the exemplary PERG system, wide-angle
imaging optics 36 injects the PERG interference beam into the eye,
projecting the segmented ERG pattern into the entire field-of-view of the
retina.
For the exemplary embodiment, wide-angle imaging optics includes two lenses
located adjacent, but not in contact with, the eye--a parabolic aspheric
lens 92 with a back focal length of about 30 mm, followed by a positive
meniscus lens 94 with a focal length of about 60 mm. Parabolic lens 92 is
made aspheric to correct for spherical aberration. Alternatively, a
contact lens could be used for at least a portion of the imaging optics.
The incident PERG interference beam is brought to a sharp focus at the eye
lens, such that the beam can pass through a constricted pupil. The beam
then diverges rapidly to project onto the entire retina, filling the
field-of-view out to the far periphery.
Since the projected PERG beam is collimated, it does not depend on the
accommodation of the subject eye. Also, if any lens opacity exists, it can
be circumvented to some extent by focal point adjustment.
A suitable fixation beam 102 may be steered into the eye along the optical
axis by a micro-miniature mirror 104. During the PERG test, the patient
fixates on the fixation spot while the PERG processor executes a PERG
program.
If it is desired to project the PERG beam only into the posterior retina,
zoom optics such as described in Section 2.6 can be included in the
optical path to control the diameter of the projected PERG pattern.
Alternatively, imaging optics that do not provide for wide-angle
projection can be used.
The resulting electrophysiological signal from the eye is detected by ERG
detection system 37, and a corresponding ERG response is provided to the
PERG processor 38.
2.5. ERG Detection System. The ERG detection system 37 converts the
electrophysiological ERG response from the eye to a corresponding digital
ERG response for input to the PERG processor.
The electrophysiological ERG response is detected by an electrode 37a, such
as a fine gold leaf placed beneath the lower eye lid. Alternatively, the
electrode may be placed on a part of a contact lens.
The electrophysiological ERG response is input to ERG detection system 37.
The ERG detection system operates conventionally in providing signal
amplification and analog-to-digital conversion, outputting a corresponding
digital ERG response signal.
2.6. PERG Program. FIG. 3 is a functional diagram illustrating a PERG
system 100 for implementing a processor-controlled PERG program. This PERG
program is a routine extension of the conventional approach to acquiring
PERG data.
A PERG projector 101 according to the invention is coordinated with a PERG
processor 102. The PERG projector receives pattern-control signals from
the PERG processor, creating a desired ERG interference pattern that is
projected onto the retina. Specifically, the PERG processor provides
pattern modulation signals that control spatial frequency (pattern fringe
line spacing) and alternation frequency (pattern phase shifting). Thus,
for a given fringe line spacing, the program determines how frequently the
interference pattern is shifted to produce an ERG response.
The resulting ERG electrophysiological response is acquired by an ERG
detection system 103. The ERG detection system is controlled by the PERG
processor, which receives the ERG response data.
A PERG program 104, executed by the PERG processor, carries out a series of
PERG pattern tests using segmented patterns to probe the retina, and
develop an ERG response map of the entire retinal field. Using the retinal
map, an ERG analysis is performed by the PERG processor, and reported in
an output device 106.
2.7 Alternate Interferometry Optics. FIGS. 4, 5 and 6 illustrate alternate
embodiments of the interferometry optics used to modulate the projection
beam with an alternating interference pattern.
FIG. 4 illustrates a PERG projector in which the interferometry optics is
based on a Newton's ring interferometer design. A laser projection beam is
expanded and collimated as in the embodiment described in Sections
2.1-2.4, and polarized normal to the plane of the FIGURE. The projection
beam is reflected from a mirror 112 to a polarizing beam splitter cube
114.
The projection beam is completely reflected by the beam splitter cube
through a quarter-wave rotating plate 116, toward a Newton's ring
interferometer 120 for modulating the beam with a Newton's ring
interference pattern. The interferometer is formed by a plano convex lens
122 and an optical flat 124. The plano convex lens is antireflective (AR)
coated on the flat side 122a.
As the projection beam propagates to the left of the beam splitter cube, it
is imparted with a circular polarization of one-quarter wave rotation by
the quarter-wave rotating plate 116. The q | | |
