Biomedical Engineering Reference
In-Depth Information
by.a.very.small.splitting.energy..Thus,.phase.refers.to.the.position.of.a.magnetic.moment.
on.its.circular.precessional.path.and.is.expressed.as.an.angle..However,.the.sums.of.the.
magnetic. vectors. are. in. longitudinal. magnetization. (z. axis).. Radiofrequency. (RF). pulse.
synchronizes. the. phases. and. rotates. all. the. longitudinal. magnetization. into. the. trans-
verse.plane..To.make.it.easy,.imagine.that.the.external.magnetic.ield.aligns.the.protons.
in.the.z.axis.(longitudinal.magnetization).and.RF.puts.them.in.the.xy.plane.(transverse.
magnetization).
After.excitation,.the.xy.plane.of.the.magnetic.moments.gradually.realigns.with.the.z.axis.
of.the.external.magnetic.ield.B0..This.process.is.known.as.longitudinal.relaxation.recov-
ery.and.occurs.exponentially.with.a.time.constant.T1..T1.values.depend.on.the.strength.of.
the.external.magnetic.ield.and.the.internal.motion.of.the.molecules.(Brownian.motion)..
Biological.tissues.have.T1.values.of.half.a.second.to.several.seconds.at.1.5.T.
Immediately.after.the.RF.pulse,.the.spins.are.in.the.same.phase..This.state.is.called.phase.
coherence..This.phase.coherence.is.gradually.lost.as.some.spins.are.in.a.high.level.while.
others.fall.to.a.low.level..So.the.transverse.magnetization,.which.comes.from.the.vector.
sums,.becomes.smaller.and.smaller.and.inally.disappears.because.spins.lose.coherence.
(dephasing)..Dephasing,.because.of.energy.transfer.between.spins,.occurs.with.the.time.
constant.T2.and.is.more.or.less.independent.of.the.strength.of.the.external.magnetic.ield,.
B0..T1.is.thus.associated.with.the.enthalpy.of.the.spin.system.(the.number.of.nuclei.with.
parallel.vs..antiparallel.spin),.while.T2.is.associated.with.its.entropy.(the.number.of.nuclei.
in.phase)..However,.intrinsic.inhomogeneties.exist.due.to.the.magnetic.ield.generator.and.
the. person. who. is. being. imaged. and. contribute. to. dephasing. even. faster. than. T2.. This.
second.type.of.decay.occurs.with.the.time.constant.T2*..The.T2*.effect.occurs.at.tissue.bor-
ders,.particularly.at.the.air-tissue.interface,.or.is.induced.by.the.local.magnetic.ield.(e.g.,.
iron.particles)..Depending.on.the.tissue.and.the.external.magnetic.ield,.T1,.T2,.and.T2*.
values.are.different,.but.T1.and.T2.relaxation.are.completely.independent.of.each.other,.
although. they. occur. more. or. less. simultaneously.. MR. signal. decay. due. to. T2. relaxation.
occurs.within.the.irst.100.to.300.ms,.which.is.long.before.there.has.been.T1.relaxation.at.
0.5.to.5.s.
MRI.is.a.well-established.clinical.tool.for.noninvasive.imaging.of.anatomical.changes.
of. the. whole. body.. It. offers. several. advantages. over. other. imaging. techniques,. includ-
ing. greater. speed,. more. direct. anatomical. correlation,. and. lower. cost.
186
. Although. not.
able.to.match.the.resolution.of.photon.base.imaging,.a.number.of.strategies.have.been.
employed.to.improve.its.resolution.and.sensitivity.by.using.contrast.agents.or.increas-
ing.the.magnitude.of.induced.nuclear.magnetism..Due.to.limited.resolution,.the.pres-
ence.of.a.cluster.of.cells.ranging.from.1.×.10
5
.to.5.×.10
5
.is.required.to.be.imaged.by.MRI.
techniques.
187,188
In. addition. to. proton. density,. which. determines. the. maximum. signal. obtained. from.
a. given. tissue,. two. other. intrinsic. features. of. a. biological. tissue. contribute. to. its. signal.
intensity.or.brightness.on.MR.images:.T1.and.T2.times,.which.alters.with.contrast.agents..
Proton. density. and. T1. and. T2. times. may. vary. widely. from. ones,. tissue. to. the. next. one.
therefore,.depending.on.which.of.these.parameters.is.focused,.the.results.of.MRI.are.dif-
ferent.in.a.tissue..As.explained.later,.the.T1.time.of.a.tissue.is.the.time.that.the.excited.spin.
takes.to.recover.and.is.available.for.the.next.excitation,.and.T2.is.the.time.referred.to.how.
quickly.an.MR.signal.fades.after.excitation..Images.with.contrasts.that.are.mainly.deter-
mined.by.T1.or.T2.are.called.T1-weighted.images.(T1w).and.T2-weighted.images.(T2w),.
respectively.
Manipulating.the.chemical.environment.around.the.protons.can.alter.the.signals..MR.
contrast.agents.have.been.developed.as.a.way.to.manipulate.the.inside.an.organism,.and.
