Biomedical Engineering Reference
In-Depth Information
isoplanatic.patch,.the.aberrations.incurred.are.similar.and.a.single.correction.at.the.active.element.is.
suicient.to.correct.the.image..Conversely,.if.the.two.objects.have.a.spatial.separation.greater.than.the.
isoplanatic.patch,.the.rays.emanating.from.the.objects.will.see.diferent.aberrations.as.they.pass.back.
through.the.eye.to.the.detector..Since.the.aberrations.difer,.it.will.not.be.possible.to.correct.for.both.
objects.using.only.one.active.element..Although.research.has.been.carried.out.so.that.a.single.active.ele-
ment.adopts.a.best.it.correction,.the.increased.ield.of.view.of.the.retina.is.only.at.3°.[63]..It.is,.of.course,.
possible.to.image.small.sections.of.the.retina.and,.by.creating.a.montage,.build.up.a.whole.picture.of.the.
retina..his.is.diicult.in.in.vivo.studies.as.not.all.patients.can.hold.their.eyes.steady.for.the.extended.
period.of.time.required.to.build.the.montage..here.is,.therefore,.a.drive.toward.extending.this.ield.of.
view,.which.can.be.corrected.by.AO.
he.current.solution.to.this.problem.is.multiconjugate.adaptive.optics.(MCAO).and.this.has.its.roots.
in.astronomy.[64,.65]..he.key.idea.behind.MCAO.is.that.the.diferent.layers.of.the.aberrating.medium.
will.have.a.corresponding.active.element.at.its.conjugate.plane..In.a.study.using.a.modeled.human.eye,.
Bedggood.et.al..[40,.66].found.that.with.ive.calibration.points.positioned.at.various.parts.of.the.eye,.
it.was.possible.to.identify.the.aberrations.in.the.various.assumed.aberrating.layers..hese.planes.cor-
responded.to.the.anterior.corneal.plane,.which.has.the.greatest.variation.in.the.index.of.refraction.and.
thereby.contributes.most.to.aberrations,.the.pupil.of.the.eye,.and.other.arbitrarily.chosen.positions.in.
anticipation.of.the.ease.of.conjugating.the.active.elements.to.the.selected.planes..It.was.found.through.
modeling.that.by.placing.ive.active.elements.conjugate.to.these.positions,.it.was.possible.to.increase.the.
size.of.the.isoplanatic.patch.by.up.to.six.times..he.practical.implementation.of.ive.correctors.in.their.
work.would.have.been.diicult,.so.they.gave.a.general.rule.of.thumb.that.for.a.two-corrector.system,.the.
best.locations.would.be.the.anterior.corneal.position.and.the.posterior.surface.of.the.lens.
7.3 Biological imaging with Microscopes
Yet. another. application. exists. in. incorporating. an. AO. system. into. a. microscope. for. biological. imag-
ing..he.primary.aim.of.an.AO.microscope.is.to.correct.for.either.the.system-induced.aberrations,.the.
specimen-induced.aberrations,.or.both..Various.approaches.have.been.devised.and.this.section.covers.
some.of.these.approaches.
7.3.1 Wavefront Sensorless
his. chapter. has. touched. on. many. various. aspects. of. AO. systems. in. biological. imaging.. One. com-
mon.thread.that.runs.through.them.is.the.presence.of.a.wavefront.sensor.to.detect.and.feed.the.mea-
sured.wavefront.to.the.active.element.and.together.they.form.a.feedback.loop..Curiously.enough,.AO.
in. microscopy. tends. not. to. incorporate. a. wavefront. sensor. [67].. his. is. because. the. detection. of. the.
relected.or.backscattered.light.causes.problems.with.the.aberration.detection,.simply.because.the.aber-
rations.of.even.orders.are.added,.while.the.odd.orders.are.canceled.[67,.68]..he.net.result.is,.of.course,.
under-.or.over.correction..At.the.same.time,.most.of.the.aberrations.are.specimen-induced.and.the.cor-
rection.of.lower.order.modes.is.suicient.to.improve.the.image.quality.[69].
To.determine.the.appropriate.amount.of.correction.to.apply.via.the.active.element,.most.of.these.wave-
front. sensorless. methods. rely. on. the. maximization. of. the. detected. luorescence,. oten. through. an. opti-
mization. algorithm. [70,. 71].. Such. algorithms. include. genetic. algorithms,. hill-climbing,. random. search,.
and.adaptive.random.search..What.the.system.does.is.basically.impose.a.sequence.of.aberrations.as.initial.
“guesses”.on.the.DM.and.detect.the.subsequent.change.in.luorescence..Recent.research.into.the.optimiza-
tion.of.such.wavefront.sensorless.techniques.has.focused.on.the.decomposition.of.the.correcting.aberration.
imposed.onto.the.DM.in.terms.of.modes,.such.as.the.Zernike.polynomials..Booth.[72].has.proposed.an.opti-
mization.approach.using.model-based.sphere.packing.that,.under.certain.conditions,.requires.only.
N
+ 1.
measurements.for.
N
.modes..Alternatively,.it.is.possible.to.expand.as.Lukosz.rather.than.Zernike.modes,.and.
a.detailed.exposition.can.be.found.in.the.works.by.Debarre,.Booth,.and.Wilson.[73].and.Booth.[74].
