Biomedical Engineering Reference
In-Depth Information
PMT
PMT
PMT
PZT
PZT
PZT
OBJ
OBJ
OBJ
(a)
(b)
(c)
FIGuRE 14.2
he.precise.position.of.a.single.scatterer.determines.whether.its.scattered.light.reaches.the.detector..
(a).A.scatterer.exactly.in.the.focus..(b).A.scatterer.axially.displaced.by.λ/4.from.the.focus..(c).A.scatterer.far.from.
focus.
leaving.the.interferometer.is.almost.identical.in.both.directions.and.hardly.changes.when.the.reference.
arm.length.is.modulated.by.λ/2.
In.addition,.for.a.scatterer.outside.the.depth.of.ield.of.the.objective.(Figure.14.2c),.the.backscattered.
wave. is. noticeably. curved,. causing. constructive. and. destructive. interference. to. depend. on. the. radial.
position.in.the.light.beam..his.decreases.the.modulation.of.the.signal.on.the.detector.to.decrease.for.
scatterers.outside.the.depth.of.ield,.which.is.oten.referred.to.as.the.confocal.efect.of.OCT.
Since.the.signal.from.a.scatterer.near.the.optimal.path.length.is.modulated.when.the.reference-arm.
length.is.modulated,.lock-in.detection.can.be.used.to.separate.signal.from.the.scatterers.within.a.coher-
ence.length.of.the.focus.from.signal,.from.scatterers.further.away,.and.from.excess.reference.light.or.any.
stray.light.the.detector.might.receive.
When.we.later.discuss.the.coherence.volume.(CV),.we.refer.to.the.volume.where.scatterers.can.con-
tribute.to.the.lock-in.signal,.keeping.in.mind.that.there.is.no.sharp.edge.to.this.volume,.but.that.detec-
tion.eiciency.drops.of.gradually.for.typical.spectral.shapes..he.CV.is.limited.laterally.by.the.focusing.
of.the.incident.light.and.axially.by.the.coherence.length.
Two.important.efects.need.to.be.taken.into.account.when.thinking.about.image.formation.in.OCM..
First,.the.density.of.scatterers.inside.biological.samples.is.generally.high.enough.such.that.many.of.them.
are.found.inside.the.CV..Since.their.backscattered.signals.superimpose.coherently,.they.can.either.add.
up.to.form.a.strong.signal.or.interfere.destructively.and.thus.efectively.cancel.backscattering.in.certain.
directions,.depending.on.the.relative.phases.of.their.individual.contributions.to.the.backscattered.elec-
tromagnetic.ield..In.OCT.images,.this.efect.is.visible.as.speckle.whenever.the.distribution.of.scatterers.
is.random,.causing.zones.of.constructive.and.destructive.interference.to.alternate.rapidly.
Second,.when.high-numerical-aperture.(NA).objective.lenses.are.used,.the.diference.between.paths.
with.diferent.angles.to.the.optical.axis.needs.to.be.taken.into.account.even.for.small.distances.from.
the.focus..For.a.scatterer.at.the.focus,.the.optical.path.length.to.the.back.focal.plane.(BFP).of.the.objec-
tive.lens.is.by.deinition.independent.of.the.ray.angle,.but.as.scatterers.are.further.and.further.above.or.
below.the.focus,.the.path.length.will.increasingly.depend.on.the.direction..In.other.words,.the.spheri-
cal.wave.emitted.by.a.scatterer.in.focus.is.transformed.into.a.plane.wave.by.the.objective,.whereas.all.
spherical.waves.with.axially.displaced.origins.are.transformed.into.more.or.less.curved.spherical.waves.
(Figure 14.2c)..On.the.detector,.these.waves.are.not.all.in.focus,.so,.due.to.lack.of.spatial.overlap,.the.
interference. term. diminishes,. decreasing. the. signal. modulation. and. thereby. detectability. for. scatter-
ers. far. from. focus.. his. efect. of.
z
-selectivity. is. completely. independent. of. the. coherence. length. and.
