UltrashortandZeroEchoTimeMRI

PederE.Z.Larson,Ph.D.SurbeckLaboratoryofAdvancedImaging,DepartmentofRadiologyand

BiomedicalImaging,UniversityofCalifornia,SanFrancisco,CA,UnitedStatespeder.larson@ucsf.edu

2

Semi-solid Tissues

Tendons

Meniscus

Sig

nal

t Data

Acquisition

Examples: Cortical Bone Tendons Lung Tissue Myelin

Cortical bone

3

Ultra-short/Zero Echo Time Imaging

Sig

nal

t Data

Acquisition

Potential Clinical Applications: Bone mapping Tendinopathy Cancerous Lung Nodules Fibrotic Lung Disease Demyelination/dysmyelination

Tendons

Meniscus

Cortical bone

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Dipole Field

Magnetic dipole produces local (~nm) magnetic field variations, ΔB0

µ

ΔB0(r)

z

x-y

~nm

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Dipolar Coupling

Frequency ω0

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Dipolar Coupling

∆B0,2

∆ω1

Dipoles couple, which induces resonance frequency shifts, Δω

Frequency ω0

∆ω2

Δω = γΔB0

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Dipolar Coupling

Frequency ω0

∆ω1

∆ω2

∆ω3

Dipoles couple, which induces resonance frequency shifts

Δω = γΔB0

Dipoles couple, which induces resonance frequency shifts, Δω

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Precession

•  “on-resonance” – ω = ω0 , acquires no phase •  “off-resonance” – ω = ω0 + ∆ω , dephases from resonant spins

ΔB0 > 0

Spins precess around B0 at their resonance frequency, ω Rotating Frame: Demodulate precession at Larmor frequency ω0 = γB0

ΔB0 < 0 time

ω = γ(B0+ΔB0) B0

ΔB0 – variations in magnetic field

∆ω1

∆ω2

∆ω3

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Dephasing

Net magnetization is reduced over time by varying precession rates

ω = γ(B0+ΔB0)

time B0

∆ω1

∆ω2

∆ω3

time

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T2 Relaxation

t

Tran

sver

se

Mag

netiz

atio

n

exp(-t/T2)

T2 relaxation is caused by local field shifts from dipolar coupling → Dephasing and reductions of the transverse magnetization (MXY)

B0 ∆ω1

∆ω2

∆ω3

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T2 for Mobile Spins

Long T2 relaxation time

Frequency

Num

ber

of

Spi

ns

t

MXY

ω0

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T2 for Spins in Solids

Frequency

Num

ber

of

Spi

ns

t

MXY

ω0

Short T2 relaxation time

2DUltrashortTEPulseSequence

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Half-pulse excitation

PR (radial) acquisition

Conventional UTE

Full-pulse excitation

Cartesian acquisition

ConvenMonalExcitaMon

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RF t

GZ t

RF

kZ

kZ = 0

Always ends at center of excitation k-space

2DUTEExcitaMon

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RF t

GZ t

RF

kZ

kZ = 0

Half-pulses

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Half-pulse Slice Profile Positive Gradient Negative Gradient

UTEAcquisiMons

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Start at k-space origin for minimum TE

echo echo

kx

ky

t

GX, GY

t

kx

ky

GX, GY

Ultra-short t

DAQ

t

DAQ

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3D UTE Excitation •  Excites everything •  Short •  Minimum TE = 0

•  Excites a slab •  Minimum TE

and duration increased (~100 µs)

•  Limits FOV and artifacts (gradient non-linearity)

RF t

RF t

t GZ

Images courtesy of Paul Gurney

Hard Pulse

Minimum-phase Slab Pulse

3DUTEAcquisiMon

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kx

kz

ky

kz

kx ky

3D Radial Trajectory

3D Cones Trajectory Gurney et al. MRM 2006.

T2SliceProfiles

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Slice profile blurs as T2 decreases and as RF duration increases

T2DecayduringAcquisiMon

•  LoseResoluMonwithT2decay•  Tread«T2requiresmassive

gradientsandlowersSNR(sqrt(Tread))

•  UseTread≈T2foropMmalSNR

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t

Gread Fourier Transform

kx

exp(-t / T2)

t

Tread « T2

Tread = T2 Tread = 2T2

Sig

nal

Nor

mal

ized

PSF

Pixels

3D UTE image, TE=229 µsa b

TE=2.2 ms

c T1-weighted FSE T2-weighted FSE

d

VaryingTEAnkleImages

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Sig

nal

t

Achilles’ tendon (T2 ≈ 1 ms)

TE=226µs

Sig

nal

t

TE=2.2ms

ProblemwithUTEImaging

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Poor short-T2 component contrast Signal from all tissues

Myelin? tendons (T2 ≈ 1 ms)

cortical bone (T2 ≈ 500 µs)

meniscus (T2 ≈ 4 ms)

Long-T2Suppression

24 Rahmer J, et al. Magn Reson Mater Phy.

Fat T2 ≈ 80 ms

Muscle T2 ≈ 50 ms

RFPulseT2Contrast

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Short-T2

RF Pulse

Long-T2

Frequency

Num

ber

of

Spi

ns

z

x-y

RFPulseT2Contrast

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RF Pulse

Frequency

Num

ber

of

Spi

ns

z

x-y

Short-T2 Long-T2

Long-T2SuppressionPulses

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Dephasing gradient: destroys MXY

Suppression pulse: contrast preparation

z

x-y

Half-pulse excitation

Signal = MXY

Contrast in MZ

ImprovedSuppressionPulses•  DesignedwithShinnar-Le

Roux(SLR)Transform(PaulyJMetal.IEEE-TMI10:53-65(1991).)

•  Preciselymatchdesiredspectralprofile

•  Time-bandwidth(TBW)-controlssharpnessofprofile

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RF t

TBW = 2.0

TBW = 3.0

Off-resonance Frequency

MZ

Rectangular (TBW ≈ 1.0)

SuppressionResults

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Rectangular pulse failures

No Suppression Rectangular Suppression Pulse

SLR-designed Suppression Pulse

TE = 80 us, TR = 500 ms, flip = 60 degrees, 5 mm slice thickness, 1 mm in-plane resolution, transmit/receive head coil, 1.5T GE Scanner 4:15 min acquisition

Consistent white matter contrast (likely Myelin)

Falx Cerebri

Dura Matter

AddingFatSuppression

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z

x-y

ω0

(water)

Num

ber

of

Spi

ns

Frequency ω0 - Δωfat

(fat)

Short-T2 Fat

Long-T2

Water Long-T2

Dual-BandSuppressionPulses

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RF

Using SLR pulse design

TBW = 3.4 +  Fat suppression

band

-  Additional short-T2 signal loss

t

Off-resonance Frequency

MZ

0 -Δωfat

real imag

3DAnkleImages(1.5T,2005)

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No Suppression Dual-band Long-T2 Suppression Pulse

Achilles’ tendon

Fat

3DAnkleImages(1.5T,2005)

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tendons

plastic boot

Achilles’ tendon

Peroneal tendons

No Suppression Dual-band Long-T2 Suppression Pulse

3DAnkleImages(7T,2014)

34Han et al. Inv Radiology, 2014.

UTELungImaging

35Jiang et al. Proc ISMRM 2016.

1.25mm resolution, Resolved over 5 respiratory phases

UTELungImaging

36Jiang et al. Proc ISMRM 2016.

ZeroEchoTime(ZTE)MRI

•  ZeroechoMme:crosscenterofk-spaceduringRFpulse

•  Butdon’tacquirecenterofk-space–  AlgebraicreconstrucMon–  FillinwithsinglepointacquisiMons–  Fillinwithlow-resoluMonprojecMons

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RF

DAQ

GX

GY

GZ

RF

DAQ¨7

GX

UTE

GZ

GY

ZTE

kx

ky

kx

ky

¨7� �7(

UTEor

ZTE(1 TR)

Fat Saturationx Nsat x Nproj

/Nsat

Inversion Pulse

TI TDUTEor

ZTE(1 TR)

Fat Saturationx Nsat x Ninv

x Nproj/Nsat/Ninv

a

b

c

d

RF

G

100º

RF

DGLDEDWLF�����

RF

G

100º

Miss center of k-space

ZeroEchoTime(ZTE)MRI

•  Silentscanduetoslowrampingofgradients

•  Limitedflipangleandvolumetriccoveragedueto“sliceselecMon”duringRFpulse

•  “Slice”changeseveryTR!BlurringarMfacts38

y

x

ZTEBoneImaging

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Wiesinger,etal.MRM(2015).Delso,etal.JNucMed(2015).Leynes,etal.ProcISMRM2016.

June 3, 2015

midres=2.4mm highres=1.35mm (39sec) (2min:53sec)

09/01/2015

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ZTE -log(ZTE) Pseudo-CT

Fat sat, 0.65 mm isotropic resolution, 32 projections/fat sat, 1.3ms readout duration, TR = 2.3ms (ZTE), 2.2ms (UTE) 4:45 5:15

ZTE Tendon Imaging

§ Excellent tendon visualization, including fascicular structure of Achilles’ tendon

§ 0.7mm isotropic resolution

§ 4:45 scan time

Achilles tendon (T2* ≈ 0.85 ms)

tendons

40 Larson et al. MAGMA 2015.

ZTE Knee Imaging

§ Excellent tendon, ligament, cartilage, and meniscus visualization

§ 0.6 mm isotropic resolution

§ 4:45 scan time

Larson et al. MAGMA 2015. 41

OtherUTEImages

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3DUTEViolinImages

Barral et al. Proc 49th ENC 2008.

3D PR acquisition with FOV tailored to the violin shape Larson et al. IEEE Trans Med Imag 2008; 27:47-57.

TE = 64 us, TR = 5.2 ms, 1.3-1.8 mm in-plane resolution, 3” surface coil, transmit/receive head coil, 1.5T GE Scanner 1 min 43 s and 19 min acquisition

k-space PSF