Biomedical Engineering Reference
In-Depth Information
In. an. implementation. of. a. smart. microscope,. Albert. et. al.. [71]. used. a. genetic. algorithm. to. deter-
mine. the. optimal. shape. of. the. DM. only. through. maximizing. the. detected. two-photon. luorescence..
he.setup.utilized.an.
f
/1.parabolic.mirror.to.focus.an.expanded.beam.from.a.pulsed.titanium.sapphire.
laser..Since the.intensity.of.the.two-photon.signal.scales.quadratically.with.the.intensity.at.the.focus,.
the. closer. the. DM. was. to. correcting. for. the. unknown. aberrations,. the. greater. the. two-photon. signal.
detected.. In. this. manner,. the. system. was. able. to. learn. beforehand. the. needed. corrections. for. every.
position.of.the.scanned.beam..With.certain.assumptions.and.modiications.to.the.algorithm,.the.smart.
microscope.was.able.to.learn.the.necessary.corrections.in.one.minute,.and.thereby.extend.the.scanning.
range. of. the. parabolic. mirror. ninefold.. he. optimization. of. luorescence. signal. is. well. suited. to. laser.
scanning.microscopy.because.unlike.other.modes.that.form.the.image.directly.on.the.detector,.only.the.
signal.strength.is.detected.with.the.image.formed.electronically.
A. recent. study. [75]. compared. four. methods. of. optimizing. the. shape. of. the. deformation. through.
maximizing.the.signal.intensity..In.this.study,.the.four.methods.compared.were.genetic.algorithm.(GA),.
hill-climbing. algorithm. (HC),. random. search. (RS),. and. adaptive. random. search. (ARS).. hese. algo-
rithms.were.tested.for.the.repeatability.and.reliability.of.the.various.algorithms,.the.time.taken.to.reach.
a.solution.corresponding.to.a.set.level.of.signal.recovery,.and.the.inal.axial.resolution.of.the.system..
For. a. predetermined. aberration,. it. was. found. that. each. of. the. diferent. algorithms. resulted. in. a. dif-
ferent.shape.of.the.DM,.thereby.indicating.that.there.was.no.one.unique.solution..However,.repeated.
runs.of.the.algorithm.with.a.ixed.amount.of.aberration.to.correct.indicated.that.of.the.four,.only.the.
ARS.appeared.to.not.reach.the.predetermined.level.of.improvement.required..Of.the.three.algorithms.
capable.of.reaching.an.optimal.solution.repeatedly,.HC.was.the.quickest.at.a.constant.48.seconds,.com-
pared.to.a.maximum.of.12.min.30.s.for.GA,.with.the.others.falling.somewhere.in.between..Among.the.
methods,.the.algorithm.that.gave.the.best.axial.resolution.was.RS..he.drawback.of.using.RS.was.that.
it.took.between.2.and.6.minutes.to.reach.a.solution..It.is.obvious.that.the.use.of.optimization.routines.
limits.the.applicability,.especially.if.imaging.speed.is.a.concern.
Since.the.imaging.of.biological.samples.oten.requires.a.3D.stack.to.be.built.up,.the.signal.strength.
also.varies.throughout.the.sample,.making.the.use.of.signal.intensity.as.the.optimizing.metric.ill-suited..
Oliver,. Debarre,. and. Beaurepaire. [76]. recently. proposed. the. use. of. image. sharpness. as. an. alternative.
metric.. Using. image. sharpness. to. build. an. inluence. matrix,. they. found. that. they. were. able. to. apply.
the previously. obtained. inluence. matrix. to. samples. diferent. from. the. initial. sample.. his. implies.
that the.technique.was.relatively.insensitive.to.intersample.variations.and.could.mean.a.faster.AO.sys-
tem. since. the. inluence. matrix. could. be. predetermined. beforehand. and. still. be. used. for. subsequent.
samples.
One.current.example.of.AO.using.a.RS.algorithm.that.may.be.of.particular.signiicance.in.biological.
imaging. is. that. of. signal-enhanced. coherent. Stokes. Raman. spectroscopy. (CARS). microscopy.. In. this.
work,.Wright.et.al..[77].used.adaptive.optics.with.CARS.microscopy.and.demonstrated.that.they.could.
correct.for.both.the.system.and.the.specimen.aberrations..With.a.test.sample,.they.reported.that.they.
were.able.to.obtain.signals.up.to.700.μm.deep,.with.a.trebling.of.the.signal..When.the.same.system.was.
applied.to.muscles,.they.reported.a.more.modest.imaging.depth.of.260.μm,.but.a.sixfold.increase.in.signal.
7.3.2 Wavefront Sensors and Guide Stars
Although.many.implementations.of.AO.in.microscopes.adopt.a.sensorless.approach,.there.are.systems.
that.incorporate.a.wavefront.sensor..One.of.the.reasons.why.wavefront.sensors.like.the.SHWS.have.not.
been.incorporated.is.because.these.rely.on.a.guide.star.to.determine.the.aberrations.picked.up.by.the.
rays.as.they.travel.through.the.intervening.medium..his.places.restrictions.on.using.the.backscattered.
excitation.beam.as.the.input.beam.for.the.SHWFS.for.the.reason.that.odd.aberrations.are.canceled.on.
specular. relection. and. that. even. aberrations. are. summed. [68].. Another. alternative. is. to. use. a. luo-
rescent.source,.which,.because.it.is.sensitive.only.to.the.incoming.excitation.beam.(irst.pass),.can.be.
used.as.a.measure.of.the.aberrations.[67]..Such.a.system.has.been.described.in.a.wide-ield.luorescence.
