Biomedical Engineering Reference
In-Depth Information
In.most.biomedical.applications.of.adaptive.optics,.one.of.the.key.questions.is.how.to.measure.the.
specimen-induced. aberrations. that. need. to. be. corrected.. Generally,. the. samples. do. not. contain. suf-
iciently. strong. and. well-isolated. point-like. light. sources. as. they. are. used. in. astronomy.. However,. in.
the.case.of.retinal.imaging,.point-like.light.sources.can.be.created.by.the.difusive.backscattering.of.the.
retina.(Liang.et.al..1994)..Typically,.cornea.and.eye.lens.distort.the.wavefront.of.the.light.in.such.a.way.
that.high-resolution.imaging.of.the.retina.is.not.possible..Using.adaptive.optics.based.on.the.measure-
ment.of.the.wavefront.of.the.difusively.backscattered.light.spot.projected.onto.the.retina,.aberrations.
can.be.compensated.and.the.cones.of.the.retina.can.be.resolved.
However,.many.other.biological.samples.scatter.throughout.the.depth.proile;.in.other.words,.they.
lack.a.predominant.layered.structure.that.could.be.exploited.to.create.a.“guide.star”.in.the.desired.depth..
Aberrations.are.not.necessarily.dominated.by.certain.layers.but.can.originate.anywhere.in.the.sample,.
and.the.backscattered.light.comes.from. many.diferent.depths.. Since. even.the.methods.used.in.solar.
astronomy.can.deal.only.with.laterally.extended.objects,.but.not.with.axially.extended.ones,.the.light.
emanating.from.the.sample.cannot.be.easily.analyzed.to.directly.measure.aberrations.corresponding.to.
imaging.in.any.given.depth.inside.the.sample.
Because.of.these.diiculties.to.perform.direct.wavefront.sensing.in.biomedical.microscopy,.consid-
erable.work.has.been.done.to.implement.diferent.image-based.wavefront.optimization.schemes.that.
avoid.direct.wavefront.sensing..hey.rely.on.the.ability.to.modify.the.unknown.wavefront.in.a.known.
manner.with.the.active.element.and.to.analyze.the.microscope.images.taken.with.diferent.settings.
of. the. active. element. to. reconstruct,. more. or. less. eiciently,. the. wavefront.. his. category. includes.
genetic.learning.algorithms.(Sherman.et.al..2002),.hill-climbing.algorithms.(Marsh.et.al..2003),.adap-
tive.random-search.algorithms.(Wright.et.al..2005),.and.modal.wavefront.sensing.(Neil.et.al..2000;.
Booth.et.al..2002).
he.downside.of.all.of.these.image-based.methods.is.that.they.require.a.number.(which.can.be.rather.
larger).of.microscope.images.to.be.taken.to.optimize.the.optical.resolution..his.is.oten.an.issue,.since.
extensive. imaging. typically. leads. to. deterioration. of. the. contrast. agent. used. and. even. of. the. sample.
itself.. In. particular,. luorescence. microscopy. frequently. uses. chemical. or. genetically. encoded. luoro-
phores.as.contrast.agents,.which.are.subject.to.bleaching,.so.that.any.gain.in.signal.and/or.resolution.by.
the.use.of.adaptive.optics.is.no.longer.usable.as.the.luorophores.have.been.bleached.during.the.optimi-
zation.routine..Even.if.bleaching.is.not.limiting,.some.image-based.algorithms.need.so.many.images.for.
the.estimation.of.the.wavefront.that.the.imaging.frame.rate.is.severely.decreased.
Given.the.disadvantages.of.image-based.optimization.schemes,.it.seems.worthwhile.to.think.about.
direct.wavefront.sensing.and.how.it.could.be.made.to.work..As.mentioned.earlier,.in.many.samples,.
a. suicient. amount. of. light. is. scattered. back. from. all. depths.. If. the. incoming. light. is. focused. at. a.
certain.depth.and.the.backscattered.light.from.this.particular.depth.is.selected.interferometrically.in.
a.low-coherence.interferometer,.this.efectively.creates.a.point.source.inside.the.sample..his.idea.is.
at.the.heart.of.coherence-gated.wavefront.sensing.(CGWS).(Feierabend.2004;.Feierabend.et.al..2004;.
Rueckel. and. Denk. 2005,. 2006,. 2007;. Rueckel. 2006;. Rueckel. et. al.. 2006),. which. is. the. topic. of. this.
chapter.
14.2 Background/Prior Work: coherence Gating
he. idea. to. probe. the. structure. of. biological. tissue. using. coherence. gating. of. the. backscattered. light.
is. not. new,. even. though. the. previous. application. had. nothing. to. do. with. adaptive. optics.. Coherence.
gating. was. introduced. (Huang. et. al.. 1991). as. a. new. biological. imaging. modality. under. the. name. of.
optical.coherence.tomography.(OCT)..In.short,.a.light.source.with.low.temporal.coherence.is.sent.into.
a.sample.and.the.backscattered.light.is.analyzed.interferometrically..his.essentially.provides.time-of-
light.information.about.the.backscattered.light,.which.can.be.translated.into.depth.information.using.
a. known. or. assumed. refractive. index. depth. proile. of. the. sample.. In. addition. to. the. axial. resolution,.
which.depends.on.coherence.gating,.a.certain.lateral.resolution.is.usually.achieved.by.focusing.of.the.
