Biomedical Engineering Reference
In-Depth Information
7.2.2 optical coherence tomography
Optical. coherence. tomography. (OCT). was. developed. by. Huang. et. al.. [47],. using. an. in. vitro. retinal.
sample..Since.then,.OCT.has.seen.widespread.use.as.an.in.vivo.ophthalmic.imaging.device.for.tracking.
macular-related.diseases.[48-53]..he.major.advantage.of.OCT.is.that.the.axial.and.transverse.resolu-
tions.are.decoupled.and.diferent.strategies.can.be.adopted.to.maximize.both..his.is.not.true.for.imag-
ing.systems.that.rely.on.conventional.focusing.to.achieve.high.spatial.resolutions,.where.the.transverse.
resolution,.axial.resolution,.and.depth.of.focus.vary.with.the.numerical.aperture.as.well.as.wavelength..
here.are.two.modes.in.which.OCT.works..One.is.the.time-domain.OCT.and.the.second.is.the.Fourier.
domain.OCT..Currently,.AO.has.been.incorporated.into.both.types.of.OCT,.with.the.irst.instance.of.
AO.time-domain.OCT.in.2003.[54].
Since. the. transverse. the. axial. resolution. is. inversely. proportional. to. the. wavelength. spread. of. the.
source,.whereas.the.transverse.resolution.is.determined.independently,.it.possible.to.introduce.a.broad-
band.source.to.improve.the.axial.resolution.while.maintaining.the.transverse.resolution.and.the.depth.
of.focus..his.led.to.the.development.of.ultrahigh-resolution.OCT.(UHR-OCT),.where.the.irst.version.
used.a.pulsed.titanium-sapphire.laser.with.a.spectral.bandwidth.of.350.nm.[55].
In.2004,.Hermann.et.al..[56].demonstrated.an.adaptive.optics.system.based.on.a.commercial.UHR-
OCT,.wherein.the.transverse.resolution.of.the.system.reached.5-10.µm.and.a.3.µm.axial.resolution.with.
a.130.nm.spectral.bandwidth..In.this.implementation,.they.utilized.an.AO.setup.consisting.of.an.SHWS.
and.a.MEMs.membrane.DM,.all.itting.within.a.300.mm.×.300.mm.footprint..Unlike.the.commercial.
UHR-OCT. used,. they. were. able. to. use. an. input. beam. diameter. of. 3.8. mm. (from. the. original. 1. mm).
while.still.being.able.to.obtain.an.improvement.in.the.transverse.resolution.
While.AO.is.capable.of.correcting.monochromatic.aberrations.at.wider.pupil.diameters,.it.still.runs.up.
short.against.chromatic.aberrations,.which.are.aberrations.that.occur.as.a.function.of.wavelength..his.
becomes.important.as.ultrawide.broadband.light.sources.have.been.developed.and.their.application.to.
UHR-OCT.becomes.obvious.[57]..To.this.end,.Fernandez.et.al..[58].have.investigated.the.correction.of.
polychromatic.light.(as.used.in.UHR-OCT).using.an.LC-SLM.as.the.active.element..hey.found.that.they.
were.able.to.correct.for.very.high-frequency.aberrations.due.to.the.density.of.the.pixels.on.the.LC-SLM,.
but.the.drawback.was.that.the.speed.of.correction.was.still.below.that.of.conventional.MEMs.DM.
he. Fourier. domain. OCT. is. faster. than. the. time-domain. OCT. in. terms. of. acquisition.. It. is. also.
more.sensitive.and.was.irst.demonstrated.for.retinal.imaging.by.Wojtkowski.et.al..[59]..Because.of.this.
improvement.in.speed.and.sensitivity,.it.becomes.possible.to.image.live.cells.in.vivo..Such.a.system.has.
been.implemented.by.Zhang.et.al..[60]..In.their.work,.they.combined.an.AO.loodlit.OCT.(for.locating.
their.imaged.areas.with.higher.precision).with.an.AO.Fourier.domain.OCT..In.this.system,.they.were.
able.to.image.single.cells.with.three-dimensional.(3D).resolutions.of.3.×.3.×.5.7.µm.and.speeds.of.up.to.
100.A-scans/ms.in.a.single.shot..hey.were.thus.able.to.demonstrate,.for.the.irst.time,.simultaneously.
resolved.images.in.the.laterally.and.axially.directions.of.the.interface.between.the.outer.and.inner.seg-
ments.of.individual.cones..Some.limitations.they.encountered.included.the.presence.of.speckle,.which.
has.an.average.size.close.to.that.of.the.theoretical.point.spread.function.(PSF).of.the.imaging.system..
As.a.result,.objects.of.interest.that.are.close.to.the.PSF.of.the.system.sufer.more.from.speckle..However,.
speckle. contrast. is. inversely. correlated. to. the. difusion. speed. of. the. scatterers.. he. temporal. averag-
ing.of.images.is,.therefore,.a.possible.way.to.overcome.this.limitation.at.the.expense.of.the.acquisition.
speed.[61],.and.image-processing.techniques.have.been.described.in.the.removal.of.the.motion.artifacts.
incurred.by.the.moving.eye.[62].
7.2.3 Multiconjugate Adaptive optics for Wide Field of View
As. mentioned. previously,. the. ability. to. image. a. large. portion. of. the. retina. simultaneously. faces. sev-
eral.challenges.even.with.AO..Good.correction.and.imaging.are.oten.limited.because.the.isoplanatic.
patch.is.limited.in.the.human.eye..his.means.that.if.the.two.objects.are.spaced.so.as.to.be.within.the.
