Biomedical Engineering Reference
In-Depth Information
lifetime,. and. their. absorption. coeficient. is. superior. to. that. of. the. other. contrast. agents..
They.also.have.higher.density.and.atomic.number,.thus.exhibiting.greater.attenuation.
Xu.et.al.
152
.tested.the.effect.of.size.and.concentration.of.AuNPs.compared.to.Omnipaque,.
which.is.a.commercial.iodine-based.contrast.agent..The.sizes.of.AuNPs.used.in.this.study.
ranged.from.4.to.20.nm..Uptake.by.HeLa.S3.cells.was.found.to.be.dependent.on.the.size.
and. concentration. of. AuNPs. incubated. with. the. cells,. where. cellular. uptake. increased.
with.the.increase.of.concentration.and.decrease.in.size.of.the.AuNPs..Greater.x-ray.attenu-
ation.was.observed.with.AuNPs.of.all.sizes.compared.to.Omnipaque.using.the.same.con-
centrations..X-ray.attenuation.also.increased.with.smaller.AuNPs.compared.to.larger.ones,.
and.at.higher.concentrations.compared.to.lower.ones..This.may.be.due.to.the.increase.in.
surface-to-volume.ratio.as.the.AuNPs'.size.decreased.
Sun.et.al.
153
.compared.CT.images.obtained.using.eXIA.160.(another.commercial.iodinated.
contrast.agent).with.AuNPs.coated.with.heparin.to.target.liver.cells..Functionalization.was.
done. by. adding. heparin. conjugated. to. 3,4-dihydroxyphenylalanine. (DOPA). to. citrate-
capped. AuNPs,. where. each. AuNP. carried. 4. ×. 10
3
. heparin-DOPA. conjugates.. The. AuNP.
conjugates.showed.good.stability.in.PBS.containing.fetal.bovine.serum,.and.the.negative.
charge.of.heparin.prevented.their.aggregation..The.conjugates.were.also.found.to.be.non-
cytotoxic.and.biocompatible.in.human.hepatocellular.carcinoma.(HepG2).cell.lines..The.CT.
images.using.AuNPs.showed.well-deined.liver.tissue.delineated.from.background.tissues.
and.blood.vessels.in.mice,.with.3.3.times.greater.attenuation.than.eXIA.160..The.contrast.
resulting.from.the.AuNP.conjugates.was.maintained.for.up.to.24.h,.allowing.a.prolonged.
time.for.imaging..Since.the.liver.is.one.of.the.most.frequent.sites.for.metastasis,.the.use.of.
AuNPs.in.liver.CT.imaging.can.signiicantly.aid.in.early.detection.and.treatment.
Alric. et. al.
154
. used. gadolinium-coated. AuNPs. to. enhance. both. MRI. and. CT. images,.
making.it.possible.to.use.AuNPs.in.dual-modality.imaging,.while.Popovtzer.et.al.
155
.con-
jugated.AuNPs.with.UM-A9.antibodies.for.targeted.CT.imaging.of.squamous.cell.carci-
noma. of. the. head. and. neck,. where. the. attenuation. coeficient. of. targeted. cells. was. ive.
times.higher.than.that.of.untargeted.or.normal.cells..AuNPs.have.also.been.used.in.vas-
cular.CT.imaging.
148,154
3.5.3.2 Photoacoustic Imaging (PAI)
PAI,.also.known.as.optoacoustic.or.thermoacoustic.imaging,.is.a.technique.that.combines.
both.optical.absorption.and.ultrasound.waves..Tissues.are.exposed.to.short.pulses.in.the.
NIR.region,.and.the.absorbed.photons.are.converted.into.heat,.causing.thermal.expansion,.
which.leads.to.emission.of.acoustic.or.ultrasound.waves..These.waves.travel.with.minimal.
disturbance.to.the.surface.and.can.be.detected.using.wideband.detectors.or.acoustic.trans-
ducers..An.inverse.algorithm.is.then.used.to.reconstruct.the.signals.into.images,.depend-
ing.on.the.time.of.light,.amplitude,.and.time.difference.between.acoustic.wave.peaks..The.
higher.the.absorption.of.a.tissue.component,.the.stronger.the.ultrasound.signal.it.emits..
For.example,.due.to.hemoglobin's.high.optical.absorption.in.the.NIR.region.compared.to.
other.cells.and.tissue,.PAI.has.been.very.useful.in.the.imaging.of.blood.vessels..For.the.
same.reason,.it.has.been.useful.in.tumor.imaging.due.to.the.stimulation.of.angiogenesis.
by.tumors.
156
.PAI,.therefore,.gets.the.best.of.both.optical.and.ultrasound.worlds,.where.it.
depends.on.the.different.degrees.of.light.absorption.by.tissue.components.for.optical.con-
trast,.and.on.the.emitted.ultrasound.waves.for.determining.their.location.
Gold.nanospheres.have.been.deemed.unsuitable.for.use.in.PAI.due.to.their.limited.tun-
ability.in.the.NIR.region.
157
.Gold.nanoshells,.nanorods,.and.nanocages.are.preferred.due.to.
