Biomedical Engineering Reference
In-Depth Information
diferent.approaches..Neil.et.al..have.demonstrated.adaptive.correction.on.luorescent.beads.by.irst.mea-
suring. the. aberrations. of. the. beam. with. a. modal. wavefront. sensor. and. then. applying. the. phase. conju-
gate.of.the.measured.aberrations.with.a.liquid-crystal.spatial.light.modulator.[18]..Rueckel.et.al..applied.
adaptive. wavefront. correction. based. on. sensing. the. wavefront. of. coherence-gated. backscattered. light.
[19]..he.system.combined.a.two-photon.scanning.laser.microscope.with.an.interferometer.for.wavefront.
measurement.
Several.groups.also.tried.to.take.the.approach.of.correcting.optical.aberrations.without.measuring.
the.wavefront.error..Sherman.et.al..demonstrated.AO.correction.without.a.wavefront.sensor.by.using.
a.genetic.learning.algorithm.[20]..Wright.et.al..used.AO.in.a.CARS.microscope.with.a.random.search.
algorithm.[21]..he.setup.employs.a.deformable.membrane.mirror.and.a.random.search.optimization.
algorithm.to.improve.signal.intensity.and.image.quality.at.large.sample.depths..Marsh.et.al..showed.the.
technique.of.“hill.climbing”.to.drive.the.AO..Ji.et.al..used.image.deviation.between.a.reference.image.
and.an.aberrated.image.to.optimize.a.subregion.of.a.spatial.light.modulator.[22]..he.optimization.pro-
cess.is.repeated.to.allow.the.correction.of.optical.aberrations.across.the.entire.pupil..Débarre.et.al..used.
a.sequential.mode-based.optimization.algorithm.on.a.deformable.mirror-based.AO.system.to.optimize.
images.in.two-photon.luorescence.excitation.imaging.[23].
12.3 Method
Typical.AO.systems.use.closed-loop.feedback.control.to.correct.the.optical.aberrations.of.the.illumina-
tion. and. imaging. optics.. he. closed-loop. feedback. system. includes. a. deformable. mirror,. a. wavefront.
sensor,.and.a.computer-controlled.feedback.loop..It.is.challenging.to.apply.the.wavefront.sensor-enabled.
AO. system. to. the. TPFM.. he. main. reason. is. that. the. encoded. wavefront. aberration. is. wavelength-
dependent.. he. collected. luorescence. light. contains. undesired. wavefront. information. for. aberration.
correction.applied.to.the.illumination.light.beam..Nevertheless,.without.the.measured.wavefront.aber-
ration,.the.AO.correction.can.also.be.achieved.on.a.trial-and-error.type.of.approach.in.the.two-photon.
luorescence.imaging.setup.
In.the.sensorless.AO.system,.the.control.system.is.set.to.maximize.the.luorescence.intensity.by.arbi-
trarily.perturbing.multiple.degrees.of.freedom.of.the.deformable.mirror.(DM)..he.AO.controller.uses.
a.scalar.measure,.the.metric,.of.the.luorescence.signal.(e.g.,.intensity,.contrast,.and.resolution).as.the.
feedback.signal..he.deformable.mirror.(DM).shape.in.the.sensorless.AO.can.gradually.converge.to.an.
optimum.steady.state.by.reading.the.feedback.luorescence.metric..he.sensorless.AO.is.ineicient.in.
comparison.to.conventional.AO,.but.has.proven.to.be.efective.for.complex.AO.applications.in.which.
wavefront.sensors.are.impractical.[24,.25]..At.the.same.time,.the.sensorless.AO.system.can.signiicantly.
reduce.the.complexity.of.the.optical.design.and.construction.
Due.to.the.nonlinear.luorescence.excitation.nature.of.two-photon.microscopy,.the.sensorless.AO.can.
be.very.efective.for.optical.aberration.correction..A.small.improvement.in.the.focal.spot.radius.(
r
).can.
result.in.a.large.improvement.in.signal.intensity,.due.to.the.nonlinear.excitation.mechanism..he.illumi-
nation.intensity.is.inversely.proportional.to.
r
2
,.and.the.luorescent.intensity.is.proportional.to.the.value.of.
illumination.intensity.squared..hus,.the.luorescence.intensity.goes.up.quadratically.with.decreasing.
r
.
Various. sensorless. AO. algorithms. have. been. developed. in. recent. years. and. they. can. be. divided.
into. two. categories:. sequential. and. parallel. methods.. Hill. climbing,. genetic. algorithms,. and. random.
searches.are.a.few.examples.of.algorithms.that.achieve.the.optimal.state.by.sequentially.perturbing.the.
DM.degrees.of.freedom..On.the.other.hand,.stochastic.parallel.gradient.descent.(SPGD).is.a.more.suit-
able.approach.for.AO.in.vivo.imaging..he.SPGD,.proposed.by.Vorontsov,.perturbs.the.DM.degrees.of.
freedom.simultaneously,.yielding.faster.convergence.than.the.sequential.method.[24].
he. multi-dithering. method. also. allows. gradient. estimation. in. parallel.. Small. perturbations. are.
applied. simultaneously. to. all. channels. in. the. form. of. harmonic. signals. with. diferent. dithering. fre-
quencies. for. each. channel.. To. determine. gradient. components,. dithered. carriers. are. demodulated. by.
synchronous.detectors.and.passed.through.low-pass.ilters.[26]..he.time.required.for.metric.gradient.
