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Claims  |
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What is claimed is:
1. A retinal projection system for projecting an image onto a selected area
of a retina of an eye, said retinal projection system comprising:
a light source for providing a projection beam of at least partially
coherent light;
modulation optics for modulating said projection beam with a desired image
to form a modulated projection beam; and
wide angle imaging optics for focusing said modulated projection beam,
whereby said modulated projection beam passes through the pupil of the eye
and diverges to project an image on the retina, said image having a
field-of-view that includes an area anterior to the posterior 25.degree.
of the visual field of the retina.
2. A retinal projection system in accordance with claim 1, wherein said
field-of-view of said image includes an area 25.degree.-40.degree.
anterior to the posterior-most point of the retina.
3. A retinal projection system in accordance with claim 1, wherein said
modulation optics include an aperture for blocking a selected portion of
said projection beam, whereby said image is projected onto a selected
segment of the retina.
4. A retinal projection system in accordance with claim 1, wherein said
modulation optics include an interferometer for modulating said projection
beam with an interference pattern to create an ERG pattern, and for
selectively alternating said interference pattern to create an ERG pattern
shift.
5. An ERG pattern projection (PERG) system for projecting an ERG pattern
onto a selected area of a retina of an eye, said PERG system comprising:
a light source for providing a projection beam of at least partially
coherent light;
modulation optics for modulating said projection beam to create a desired
ERG pattern and for selectively shifting said ERG pattern to create an ERG
response from said retina;
imaging optics for focusing said ERG pattern, whereby said ERG pattern
passes through the pupil of the eye and diverges to project an ERG pattern
image on a selected area of the retina, said ERG pattern image having a
field-of-view that includes an area anterior to the posterior 25.degree.
of the visual field of the retina.
6. A PERG projection system in accordance with claim 5, wherein said
field-of-view of said ERG pattern image includes an area
25.degree.-40.degree. anterior to the posterior-most point of the retina.
7. A PERG projection system in accordance with claim 5, wherein said
modulation optics include an interferometer for modulating said projection
beam with an interference pattern to form an ERG pattern, and for
selectively altering the interference pattern to cause an ERG pattern
shift.
8. A PERG projection system in accordance with claim 7, wherein said
interferometer modulates said 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 from said retina.
9. A PERG projection system in accordance with claim 7, wherein said
interferometer is a shearing interferometer comprising:
first and second prisms having a gap therebetween defined by respective
first and second opposing, non-parallel prism surfaces formed on said
first and second prisms, said first and second opposing, non-parallel
prism surfaces defining a shearing angle, said second prism being
pivotally mounted relative to said first prism for selectively changing
said shearing angle, whereby said projection beam is partially reflected
from said first opposing prism surface to form a first reflection beam,
and whereby said projection beam, after transiting said gap defined by
said first and second opposing, non-parallel prism surfaces, is partially
reflected from said second opposing prism surface to form a second
reflection beam, such that said first and second reflection beams
interferometrically combine to form an interference pattern, and such that
said interference pattern is precision angle tuned by selectively changing
said shearing angle.
10. The PERG projection system of claim 9, wherein said gap defined by said
first and second opposing, non-parallel prism surfaces is filled with a
Kerr fluid having a refractive index, and wherein said first and second
opposing, non-parallel 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 said gap
defined by said first and second opposing, non-parallel prism surfaces for
controlling the refractive index of said Kerr fluid so as selectively to
shift said interference pattern.
11. A PERG projection system in accordance with claim 7, wherein said
interferometer is a Newton's ring interferometer comprising:
a plano convex lens having an antireflective coating on one side thereof
and a reflective optical flat, whereby a first portion of said projection
beam is reflected by said plano convex lens to form a first reflection
beam, and whereby a second portion of said projection beam is reflected by
said optical flat to form a second reflection beam, such that the first
and second reflection beams interferometrically combine to form an
interference pattern.
12. A PERG projection system in accordance with claim 11, said system
further comprising a translation means for selectively translating the
optical flat to shift the interference pattern.
13. A PERG projection system in accordance with claim 5, further comprising
segmentation optics for selectively segmenting said ERG pattern image.
14. A PERG projection system in accordance with claim 13, wherein said
segmentation optics comprise:
a ring aperture for segmenting said projection beam into posterior, medial,
and anterior regions; and
a sectoring aperture for segmenting at least the medial and anterior
regions into angular sectors.
15. A PERG projection system in accordance with claim 5, wherein said
imaging optics include aspheric surfaces to provide a short focal length.
16. A PERG projection system in accordance with claim 15, wherein the
imaging optics include zoom optics for selectively adjusting a diameter of
said projection beam.
17. A retinal projection method for selectively projecting an image onto a
selected area of a retina, comprising the steps:
generating a projection beam of at least partially coherent light;
modulating said projection beam with a desired image to form a modulated
projection beam; and
focusing said modulated projection beam, such that said modulated
projection beam passes through the pupil of the eye and diverges to
project an image on the retina, said image having a field-of-view that
includes an area anterior to the posterior 25.degree. of the visual field
of said retina.
18. A retinal projection method in accordance with claim 17, wherein said
field-of-view of said image includes an area 25.degree.-40.degree.
anterior to the poster-most point of the retina.
19. A retinal projection method in accordance with claim 17, said method
further comprising the step of blocking a selected portion of said
projection beam, such that said image is projected onto a selected segment
of the retina.
20. An ERG pattern projection (PERG) method for projecting an ERG pattern
onto a selected area of the retina, comprising the steps:
generating a projection beam of at least partially coherent light;
modulating said projection beam to create a desired ERG pattern,
focusing said ERG pattern, such that said ERG pattern passes through the
pupil of the eye and diverges to project an ERG pattern image on the
retina, said ERG pattern image having a field-of-view that includes an
area anterior to the posterior 25.degree. of the visual field of the
retina; and
selectively shifting a phase of said ERG pattern to create an ERG response
from said retina.
21. A PERG projection method in accordance with claim 20, wherein said
field-of-view of said ERG pattern image includes an area
25.degree.-40.degree. anterior to the posterior-most point of the retina.
22. A PERG projection method in accordance with claim 20, wherein the step
of modulating the projection beam comprises the step of modulating said
projection beam with an interference pattern to form said ERG pattern.
23. A PERG projection method in accordance with claim 22, wherein said
projection beam is modulated with a selected spatial frequency to produce
a selected fringe line spacing, and wherein said projection beam is
modulated with a selected alternation frequency to produce an ERG response
from said retina.
24. A PERG projection method in accordance with claim 20, wherein the step
of modulating said projection beam is accomplished by an interferometer.
25. A PERG projection method in accordance with claim 20, further
comprising the step of selectively segmenting said ERG pattern.
26. A PERG projection method in accordance with claim 20, further
comprising the step of selectively adjusting a diameter of said projection
beam. |
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Claims |
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Description |
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FIELD OF THE INVENTION
Invention relates to an ophthalmic instrument and a method for projecting
an image onto a selected area of the retina. In particular, the invention
relates to a pattern electroretinogram (PERG) projector and a method for
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 tests, 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 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 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 a narrow field-of-view within the
posterior retina.
The above-referenced field-of-view limitation, i.e., a field-of-view
limited to the posterior-most regions of the retina, is particularly
disadvantageous because the posterior retina survives best to the final
stage of glaucoma, while the ganglion cells in the more anterior regions
of the retina 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 employing laser interferometers and potential acuity meters have
been used to evaluate visual acuity in some subjects. However, the
interferometer projects an interference pattern, while the potential
acuity meter projects an eye chart. Thus, these projection systems have
not been used for PERG testing, and are capable only of projecting into
limited areas of the posterior retina.
Accordingly, a specific need exists for a PERG system capable of
stimulating the posterior, medial, and anterior portions of the 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, 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 Optlthalmol 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 Petsson, 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 present invention is directed to a retinal projection system and method
for projecting an image onto a selected area of the retina with a
field-of-view that includes retinal areas anterior to the posterior
25.degree. of the visual field of the retina. In one aspect of the
invention, the retinal projection system includes a light source for
generating a projection beam which is at least partially coherent.
Modulation optics modulate the projection beam with a desired image, and
wide angle imaging optics adjacent the eye focus the modulated projection
beam, such that the projection beam passes through the pupil and diverges
to provide a predetermined field-of-view. By appropriately controlling the
wide angle focusing optics, the field-of-view can be adjusted to include
the medial and anterior sections of the retina, including 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 onto a selected area of the retina that
includes an area anterior to the posterior 25.degree. of the field-of-view
of the retina.
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 to be probed. Interferometry modulation optics modulate the
projection beam with an ERG interference pattern characterized by a
selected fringe line spacing, and selectively alternate the interference
pattern to create the ERG pattern shift that stimulates an ERG response.
Segmentation optics allow the ERG pattern to be selectively segmented for
stimulating a selected area of the retina, including an area anterior to
the poster 25.degree. of the field-of-view of the retina.
In one embodiment, the interferometry optics comprise 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 include a plano convex
lens and a reflective optical flat. The lens has an antireflective coating
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 to diverge to fill substantially the entire field-of-view of the
retina.
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: (1) 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 that can extend significantly anterior to the posterior area
of the retina. (2) Laser light can be used for projection to bypass
potential ocular media opacities, and to obviate the need for any
refractive correction. (3) 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). (4) 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. (5) 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 the invention, taken in conjunction with the accompanying
Drawings. Although the Detailed Description, and the Drawings, are
provided with respect to specific, exemplary embodiments 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 DRAWING
FIG. 1 is a functional block diagram of the retinal projection system of
the invention;
FIG. 2 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. 3 illustrates an exemplary segmentation of the ERG pattern from the
PERG projector, to allow selected segments of the entire retina to be
probed;
FIG. 4 is a functional block diagram of a PERG performing under computer
control PERG testing using a PERG projector according to the invention;
FIG. 5 illustrates an alternative embodiment using a Newton's ring
interferometer to generate an ERG interference pattern.
FIG. 6 illustrates an alternative embodiment of the interferometry optics
using a Michaelson interferometer; and
FIG. 7 illustrates an alternative embodiment of the interferometry optics
using a pressure driven interferometer.
FIG. 8 depicts a map of the normal visual field of the retina.
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 provided 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
extended into areas anterior to the posterior-most portions of the retina,
including the periphery.
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 bring 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 or any preselected portion thereof. That is, the retinal
projection technique of the present invention enables a pattern or other
image to be projected with a field-of-view that includes the anterior,
medial and/or posterior areas of the retina, including the far periphery
regions 25. In one embodiment of the present invention, the field-of-view
includes an area anterior to the posterior 25.degree. of the retina,
thereby facilitating earlier detection of glaucoma and other diseases.
2. Exemplary PERG Projector
The exemplary PERG projection system provides wide angle ERG pattern
projection onto the entire retina or any preselected portion thereof. The
ERG pattern can be selectively segmented to probe selected areas of the
retina for degeneration caused by glaucoma or other diseases.
FIG. 2 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 PERG pattern onto the retina.
The ERG response is detected by an ERG detection system 37 that includes
at least one 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. PERG processor 38 also receives the resulting ERG responses
from the ERG detection system 37.
The limits of the normal visual field upon maximum target stimulation
measure approximately 60.degree. above and nasal of center, 70.degree.
below center, and 100.degree. temporal of center, thereby creating a
normal full visual field that can be depicted as a horizontal oval
identified 200 in FIG. 8. A central visual field 202 is commonly defined
by drawing an arc 60.degree. from the axis normal to the posterior-most
point of the retina 212, thus creating a circular 120.degree. field. The
posterior region 204 of retina commonly is defined between 0.degree. and
40.degree. while the medial region 206 and the anterior region 208
commonly are defined between 40.degree.-80.degree. and 80.degree.-
120.degree., respectively. Regions lying beyond the central visual field
202 are referred to as peripheral.
FIG. 3 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 control the modulation optics
34 and the segmentation optics 35, thereby determining 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. 2, 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 a
projection beam of at least partially coherent light.
The projection beam is directed through a polarizer 53 and an analyzer 54,
which are configured as polarized sheets. The projection beam 52 is
adjusted in intensity by rotating the polarizer relative to the plane of
polarization established by the analyzer.
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
In one embodiment of the present invention, modulation optics 34 comprise a
shearing interferometer. Alternate interferometer configurations are
described in Section 2.7. Other suitable interferometers include, but are
not limited to, the Michaelson, the Mach Zender, and the Twyman-Green
interferometers (see Section 2.7).
The exemplary shearing interferometer depicted in FIG. 2 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 such that the prism
surfaces define a shearing angle Theta.
The incident projection beam 33 enters prism 61 and is partially reflected
from prism surface 61a to provide a first reflection beam 67. The portion
of incident projection beam 33 not reflected from prism surface 61a is
transmitted across gap 63, and a portion of this beam is reflected from
prism surface 62a, back across gap 63 to provide a second reflection beam
68.
The first and second reflection beams 67, 68 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.
In the embodiment of the present invention depicted in FIG. 2, 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 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 120 Hz signal from PERG
processor 38. As a result, an electric field is established in the gap 63
and applied to the Kerr fluid, thereby changing the refractive index of
the Kerr fluid. Thus, by controlling the electric field in the gap, the
effective path length through the gap for the second reflection beam 68
can be changed. This path modulation results in a selective shift in the
light/dark areas 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 provide the desired segmentation for the projected
ERG pattern. The time-varying PERG interference beam 69 is first expanded
by lenses 82 and 84 and then 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. 3.
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 into the anterior region (42 in FIG. 3), or only
into one or more sectors of that region (47 in FIG. 3).
As set forth in the Background of the Invention, it is preferable that the
ERG interference pattern passes into more than just the most posterior
sections of the retina in order to facilitate earlier diagnosis of
glaucoma. Thus, in one embodiment of the present invention, at least a
portion of the PERG interference pattern is directed to a portion of the
retina anterior to the posterior-most 25.degree. of the field-of-view,
thereby providing ERG responses in such regions. The posterior-most
25.degree. of the retina is identified as area 210 in FIG. 8.
By way of example and not by way of limitation, the present invention can
be used to diagnose retinal diseases such as glaucoma in the posterior
region of the retina, i.e., those areas lying within a 40.degree. zone as
indicated at 204 in FIG. 8. In this application of the present invention,
the projection beam preferably is focused such that at least a portion of
the beam is projected onto the retina at a position 25.degree.-40.degree.
from the posterior-most point of the retina, identified at 212 in FIG. 8,
in order to ensure that ERG responses are generated throughout a
representative section of the posterior retina. By focusing the projection
beam to include this area, earlier diagnosis of glaucoma is possible, as
discussed above. Similarly, if the present invention is used to diagnose
retinal diseases in the medial region of the retina, it is preferable that
at least a portion of the projection beam is projected onto the retina at
a position 40.degree.-80.degree. from the posterior-most point 212 of the
retina. Finally, if the present invention is used to diagnose retinal
diseases in the anterior retina, at least a portion of the projection beam
is projected onto the retina at a position 80.degree.-120.degree. from the
posterior-most point 212 of the retina. It is to be appreciated that the
present invention can be used to project a projection beam to the entire
field-of-view of the retina, including the peripheral areas, thereby
facilitating the diagnosis of disease throughout the retina.
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 project the
PERG interference beam into the eye, projecting the segmented ERG pattern
into the entire field-of-view of the retina, or to any preselected area.
Wide angle imaging optics 36 preferably include two lenses 92, 94 located
adjacent, but not in contact with, the eye. Parabolic aspheric lens 92
preferably has a back focal length of about 30 mm, whereas positive
meniscus lens 94 preferably has 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 to project onto the entire retina, filling the field-of-view
out to the far periphery or filling any preselected area of the retina.
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 t | | |
