Biomedical Engineering Reference
In-Depth Information
the.clinical.diagnosis.of.eye.diseases,.calls.for.a.high.level.of.imaging.quality..To.address.this.issue,.AO.
has.been.applied.to.the.imaging.of.the.eye,.with.remarkable.results..Many.of.the.above.active.elements.
have.been.incorporated.into.the.AO.imaging.of.the.eye.and.research.has.also.gone.into.the.optimization.
of.the.requirements.needed.to.optimize.the.active.element.for.retinal.imaging.[28,.29].
7.2.1 Adaptive optics ophthalmoscope
he.ophthalmoscope.is.a.device.used.to.study.the.fundus.of.the.eye..It.is.therefore.an.indispensible.tool.
for.tracking.and.diagnosing.the.state.of.health.and.diseases.of.the.eye..he.scanning.laser.ophthalmo-
scope. [30,. 31]. and. the. confocal. scanning. laser. ophthalmoscope. (cSLO). [32,. 33]. represent. a. quantum.
leap.in.the.speed.and.eiciency.over.which.the.images.of.the.fundus.could.be.imaged..he.use.of.a.laser.
scanning.confocal.system.allows.for.sectioning.of.retina.to.study.the.microstructure.at.diferent.planes.
because. the. confocal. pinhole. provides. the. optical. sectioning. required. for. depth. discrimination.. It. is.
to.be.noted.that.this.is.efectively.performing.laser.scanning.confocal.microscopy,.and.the.examining.
beam.is.efectively.focused.ideally.on.a.difraction-limited.spot.size.at.the.retina..his.requires.the.scan-
ning.to.build.up.an.image.of.the.retina,.but.it.is.also.possible.to.use.detectors.of.higher.sensitivity,.such.
as.photomultiplier.tubes,.to.increase.the.signal.detection.
he.introduction.of.wavefront.sensing.and.AO.demonstrated.that.much.better.images.of.the.fundus.
could.be.obtained,.allowing.for.a.more.precise.and.detailed.study.of.the.eye.[3,.34]..he.irst.AO.scan-
ning. laser. ophthalmoscope. (AOSLO). was. introduced. in. 2002. by. Roorda. et. al.. [35],. although. a. much.
earlier. work. by. Dreher,. Bille,. and. Weinreb. in. 1989. [1]. implemented. a. scanning. confocal. system. for.
imaging.the.retina.using.an.active.optical.system.for.axial.scanning.and.the.correction.of.aberrations.in.
the.eye.with.the.name.“laser.tomographic.scanner.”.In.their.work,.Roorda.et.al..demonstrated.the.ability.
to.optically.section.the.retina.in.vivo.while.imaging.photoreceptors,.nerve.ibers,.and.the.low.of.white.
blood.cells.within.the.capillaries.of.the.retina..Using.adaptive.optical.compensation.of.the.aberrations.
introduced.by.the.eye,.they.were.able.to.improve.the.axial.resolution.to.around.100.µm,.compared.to.
300 µm.for.cSLO..he.transverse.resolution.was.also.improved.by.a.factor.of.two.
It. is. also. known. from. population. studies. that. the. aberrations. of. the. eye. consist. of. lower-. and.
higher-order.modes.[36,.37].and.that.there.is.beneit.(in.terms.of.visual.quality).in.correcting.them.
[38].. Due. to. the. inite. number. of. actuators. on. the. active. element. as. well. as. the. limitations. to. the.
stroke.of.each.actuator,.a.single.active.element.will.sometimes.be.unable.to.compensate.for.all.the.
signiicant.orders,.especially.if.there.is.a.mix.of.low.and.high.orders..To.resolve.this.issue,.multiple.
active.elements.may.be.used.by.splitting.the.correction.for.lower.and.higher.order.modes.over.indi-
vidual.active.elements..Chen.et.al..[39].have.used.this.approach.and.used.two.DMs—one,.a.bimorph.
mirror.with.a.large.actuation.stroke.for.correcting.large-stroke.aberrations.(up.to.18.µm.stroke.over.
a.10.mm.pupil).and.the.other,.a.MEMs.continuous.sheet.DM.(up.to.1.5.µm.over.a.3.3.×.3.3.µm
2
.area)..
Both.DMs.were.placed.at.a.plane.conjugate.to.the.pupil.and.the.wavefront.sensor..he.entire.setup.
itted.onto.a.600.mm.×.900.mm.optical.breadboard,.small.enough.to.be.mounted.on.castors.and.used.
in.an.ophthalmology.clinic.
here. is. also. another. implementation,. more. akin. to. conventional. ophthalmoscopes—the. adaptive.
optical.loodlit.ophthalmoscope..In.this.setup,.a.larger.portion.of.the.retina.is.illuminated.with.a.source,.
such.as.iber-coupled,.strobing.superluminescent.and.laser.diodes..he.illuminated.patch,.however,.is.
kept.to.a.certain.size.because.of.the.need.to.satisfy.the.isoplanatic.condition.(i.e.,.where.the.aberrations.
do.vary.signiicantly.to.cause.image.degradation).[40-42]..he.size.of.the.illuminated.patch.is.still.larger.
than.that.in.the.SLO.and.the.detection.of.a.loodlit.ophthalmoscope.requires.a.CCD..Currently,.such.
setups.can.capture.images.of.the.retina.at.60Hz.and.these.studies.have.revealed.rapid.luctuations.of.
cone.relectivity.in.dark.adapted.eyes.[43].
Current.research.in.AOSLO.is.still.ongoing,.with.a.thrust.toward.compactness.and.wide-ield.imag-
ing.of.the.retina..Examples.of.such.work.can.be.found.in.Refs..[44-46].
