Biomedical Engineering Reference
In-Depth Information
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Wavefront
Zernike terms
FIGuRE 12.2
MATLAB.simulation.of.optical.aberrations.from.the.index.mismatch.
these.materials,.its.path.will.be.locally.afected.by.their.refractive.indices..he.local.structure.of.the.bone.
cavities.can.add.extra.complexity.to.the.beam.path..All.these.contribute.to.optical.aberrations.
While. the. complex. nature. of. heterogeneous. materials. in. mouse-skull. tissue. can. lead. to. complex.
aberrations,.it.might.be.expected.that.one.type.in.particular,.spherical.aberration,.will.be.prominent..
Spherical.aberrations.are.caused.by.index.mismatches.that.are.oriented.perpendicularly.to.the.optical.
axis..Booth.et.al..characterized.such.an.aberration.in.a.simpliied.geometric.approach.[11]..In..attempting.
to.show.the.efect.of.the.index.mismatch.in.a.dry.mouse.skull,.we.quantiied.the.optical.aberrations.at.
50 μm.below.the.bone.surface.in.MATLAB®.(MATLAB,.MA).according.to.Booth's.geometric.model,.
and.the.inal.results.are.shown.in.Zernike.modes.[12].to.identify.the.aberration.
he.three.terms.standing.out.on.the.Zernike.polynomials.are. defocus .(Zernike.term.no..4),. irst-order
spherical .(Zernike.term.no..12),.and. second-order spherical .(Zernike.term.no..24)..One.should.be.aware.
that.the.bone.structure.is.a.much.more.complicated.imaging.environment,.and.various.other.optical.
aberrations.are.presented.besides.spherical.aberrations.(Figure.12.2).
Despite. its. capacity. to. image. deeply. into. tissue,. two-photon. luorescence. excitation. cannot. avoid.
signal.degradation.of.the.image.with.deep.subsurface.penetration,.due.to.tissue.scattering,.absorption,.
and.optical.aberrations..Both.scattering.and.absorption.ultimately.limit.the.depth.that.can.be.imaged.
successfully.. However,. aberrations. can,. in. principle,. be. measured. and. compensated. through. an. AO.
control.system.
12.2 Background
Much.work.has.been.done.to.analyze.the.efects.of.imaging.with.a.high.NA.objective.on.a.specimen..Hell.
et. al.. illustrated. the. index. mismatch-caused. aberration. in. a. confocal. microscope. [13].. Several. meth-
ods.have.been.suggested.to.overcome.this.decrease.in.signal.and.resolution..Sheppard.et.al..illustrated.
inserting.weak.aberration.correction.lenses.and.also.altering.the.tube.length.of.the.objective.[14]..he.
latter. employed. a. static. correction. that. worked. to. restore. much. of. the. resolution. at. a. certain. depth,.
but.it.cannot.be.easily.adjusted.for.varying.depths..Hellmuth.et.al..showed.that.if.one.overcorrects.for.
the.cover-glass.thickness.or.uses.a.less-than-nominal.cover-glass.thickness,.this.would.compensate.for.
spherical.aberration.at.a.certain.depth.in.a.specimen.with.index.mismatch.[15]..Wan.et.al..changed.the.
immersion.medium.to.do.the.same.[16]..Booth.and.Wilson.used.both.of.these.methods.to.compensate.
for. spherical. aberration. in. skin. as. well. as. employing. an. iris. to. reduce. the. pupil. area. [17].. All. of. the.
.methods.discussed.use.static.means.to.correct.for.specimen-induced.spherical.aberration.at.a.speciic.
depth,.not.dynamic.means.of.compensation.as.a.beam.is.scanned.deeper.into.a.sample.
With.recent.advances.in.AO,.it.is.now.possible.to.employ.dynamic.compensation.of.aberrations,.which.
can.be.applied.during.the.scanning.process.for.a.continuous.volume.of.aberration-free.imaging..AO.has.
been. applied. to. correct. optical. aberration. in. a. multiphoton. microscope. setup. by. several. groups. with.
 
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