MRI: A historical perspective

¡ Magne&cResonanceImaging§  Past§  Present§  Glimpseofthefuture

¡  JohnBap&steFourier§  OfficerinNapoleon'sarmy

§  Mathema&csforthermodynamics

§  InventedtheFourierTransformwhichliesatthecoreofMRItoday

[1]TalGevaJCMR2006

¡  NikolaTesla§  Serbian§  InventorofAC§  Teslacoil§  Sta&onaryterrestrialwaves

§  Unitofmagne&cfieldstrengthnamedaQerhim

[1]TalGevaJCMR2006

¡  Independentlydiscoveredthenuclearmagne&cresonanceeffect

¡  AwardedNobelPrizein1952fortheirdiscoveries[1]TalGevaJCMR2006

Vivekananda

Memorial

Kargil1999

§  1971:RaymondDamadian–relaxa&on&mesfortumorsinratmodelsofcancer

§  1973:AbeZenuemonandothers,filepatentforthefirsttargetedNMRforevalua&onofinforma&oninsidefromoutside

§  1974:Magne&cResonanceImagingisbornbytheuseofgradients,thankstoPaulLauterburandPeterMansfield

§  1975:RichardErnst(AnilKumaret.al)describedtheuseofFTforMRI

[1]TalGevaJCMR2006

[2]hfp://www.cis.rit.edu/htbooks/mri/

§  1980:Average&metomakeanMRimageis5minutes

§  1985:Average&metomakeanMRimageis5seconds

§  1983:T2weightedimagingbeferforhighligh&ngpathology

§  1984-85:CardiacMRI,bloodflow,CE-MRI,Steadystatefreeprecession

Researchlabstoclinicalprac&ce

[1]TalGevaJCMR2006[3]hfp://www.fonar.com/&melineofmri.htm

¡  1988:EchoPlanarImagerforpediatrics,LarryCrooksatUCSF

¡  1987:Mul&pleRFcoilsusedforMRI

¡  1987–90:ParallelimagingisbornthankstoCarlson

• Gooddataqualitybuttakesalong&me!• Hence,maynotbesuitableforcertainimagingprotocols.• Limitsspa&alandtemporalresolu&ons• Higherspa&alresolu&onaidsinmorphologicalanalysisoftumors–breastDCE-MRI• Temporalresolu&onisimportantforaccuratepharmacokine&canalysis.• Severalapproacheslikekeyhole,parallelimagingandotherfastsequenceshavebeenused.2D FFT

2D IFFT

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1 0 DataprovidedbyBaek

[4]Larkmanet.alPMBS2007

¡  2008:CompressedsensingisappliedtoMRI

¡  2012:KeynotespeechesatISMRMoncompressedsensing

¡  CSappliedtoMRSI,DCE-MRI,cardiacimaging,brainimaging,almosteveryknownMRmethod

X2D IFFT

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X2D IFFT

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Completedatareconstruc0on

Wavelet

Transform

DataprovidedbyBaek

[1]DavidL.Donoho,IEEETransac0onsonInforma0ontheory,Vol.52,no.4,April2006[2]Candes,E.J.etal.,IEEETransac0onsonInforma0ontheory,Vol.52,no.2,Feb.2006

• Mostobjectsinnatureareapproximatelysparseinatransformeddomain.• U&lizeaboveconcepttoobtainveryfewmeasurementsandyetreconstructwithhighfidelity [5,6]

Only33%ofcompletedataX

¡  CoilsinIndia

¡  Combina&onofparallelimagingandcompressedsensing

¡  ParalleltransmitMRI

[7] Andrew Thomas et. al.,2007 International Journal of Biomedical Imaging

MRVOLUMECOILSfor

0.5Tesla

BIRDCAGE1:2ra&o,8rung

VOLUMEcoil1:2ra&o

BIRDCAGE1:1ra&o,4rung

BIRDCAGE-1:2ra&o,4rung

§  Tohaveminimumaliasing due to sampling below theNyquist rate, thefollowingproper&esofideasamplinghavetobemet.§  Thenear zero regionaround themain lobeof thePSF shouldbeas largeas

possibleandoutsidethatregion,PSFshouldresemblewhitenoise.

§  The samples should be placed randomly but with a restricted maximum

distancebetweensamples.

§  These two condi&ons are met by Poisson disc sampling but it hasimprac&calgradientrequirements.

§  Constrained random pafern is a normal lasce pafern with samplesshiQed along one dimension randomly by -1, 0 and +1 i.e. constrainedrandomiza&onaddedalongonedirec&on.

§  Ithasmoderategradientrequirements.

§  Performance of compressed sensing (CS) algorithms depends onthesparsity levelof thesignal, thetypeofsamplingpafernusedandthereconstruc&onmethodapplied.

§  The higher the incoherence of the sampling pafern used forundersampling, less aliasing is no&ced resul&ng in befer CSreconstruc&on.

§  ThetheoryofCSrequiresacquisi&onofrandomizedsetofmeasurements(randomsampling),leadingtoincoherantaliasingar&facts.

§  ButrandomsamplingrequiresbiggerchangesinamplitudesandpolarityofMRsystemgradients,whichisnotfeasibleinMRsystem.

Three candidate sampling patterns and their corresponding PSFs: top to bottom: random, Poisson disc and constrained random

Usman et. al., Sampta 2009

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¡  Joint k-space trajectory allows for the specification of a region of interest(ROI), which improves excitation accuracy at high speedup factors

¡  It allows for magnetic field inhomogenetiy compensation during excitation k-space

¡  Optimized accelerated selective excitation is useful for reducing specific absorption rate(SAR) and shortening multidimensional RF pluses

¡  Normal transmission §  In a normal transmission, the data/signals are sent one at a time

over the transmission channel

¡  Parallel transmission §  Parallel transmission means simultaneous transmission

of N signals. These signals are sent simultaneously over N different channels

Radartutorial.eu

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Reconstruc&onofimage

Pulse sequence to generate k-space

1.  Target organ 2.  K-space trajectory focus on

heart and use spiral

2D-IFT

Pa&ent

Spiral k-space trajectory

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SAR §  Specific absorption rate (SAR) is a measure of the rate at

which energy is absorbed by the body when exposed to a radio frequency (RF) electromagnetic field

§  It is defined as the power absorbed per mass of tissue and has units of watts per kilogram(W/kg)

¡  SAR can be calculated from the electric field within the tissue as:

where ¡  σ is the sample electrical conductivity ¡  E is the RMS electric field ¡  ρ is the sample density

( ) ( )( )

drrrEr

SARsample∫=

2

2ρσ

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¡  Errors and SAR will be increasing, we need to decrease them ¡  So we have a formula as

C= є + λ (SAR) …2 Here C= cost, є= excitation error, λ = Lagrange multiplier ¡  As λ increases SAR reduces to some extent then remains constant and

error also remain constant L-curve

L-curve Subset virtual observation points for RF

¡  Facultyandmanagement§  Prof.A.N.N.Murthy,Principal,DSCE§  Dr.PremchandraSagar,CEO,DSCE

¡  Students§  PadmaC.R.(coils)§  SnehaPotdar(Paralleltransmitbackground)

¡ MIRCfacultyandstudents,collaboratorsandindustrialpartners

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¡  The image of the target organ in which we are interested is taken using 2D-FT

¡  The scanned image obtained by parallel transmit from MRI is called as k-space, obtained k-space undergoes sampling at ROI

¡  Then 2D-IFT is done to achieve a desired excitation pattern

Echo-Planar Imaging (EPI) k-space raster. Brazilian journal of physics