ultrashort and zero echo time mriinst.eecs.berkeley.edu/~ee225e/sp16/notes/ute_zte.pdfultrashort and...

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

4

Dipole Field

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

µ

ΔB0(r)

z

x-y

~nm

5

Dipolar Coupling

Frequency ω0

6

Dipolar Coupling

∆B0,2

∆ω1

Dipoles couple, which induces resonance frequency shifts, Δω

Frequency ω0

∆ω2

Δω = γΔB0

7

Dipolar Coupling

Frequency ω0

∆ω1

∆ω2

∆ω3

Dipoles couple, which induces resonance frequency shifts

Δω = γΔB0

Dipoles couple, which induces resonance frequency shifts, Δω

8

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

9

Dephasing

Net magnetization is reduced over time by varying precession rates

ω = γ(B0+ΔB0)

time B0

∆ω1

∆ω2

∆ω3

time

10

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

12

T2 for Spins in Solids

Frequency

Num

ber

of

Spi

ns

t

MXY

ω0

Short T2 relaxation time

2DUltrashortTEPulseSequence

13

Half-pulse excitation

PR (radial) acquisition

Conventional UTE

Full-pulse excitation

Cartesian acquisition

ConvenMonalExcitaMon

14

RF t

GZ t

RF

kZ

kZ = 0

Always ends at center of excitation k-space

2DUTEExcitaMon

15

RF t

GZ t

RF

kZ

kZ = 0

Half-pulses

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Half-pulse Slice Profile Positive Gradient Negative Gradient

UTEAcquisiMons

17

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

18

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

19

kx

kz

ky

kz

kx ky

3D Radial Trajectory

3D Cones Trajectory Gurney et al. MRM 2006.

T2SliceProfiles

20

Slice profile blurs as T2 decreases and as RF duration increases

T2DecayduringAcquisiMon

•  LoseResoluMonwithT2decay•  Tread«T2requiresmassive

gradientsandlowersSNR(sqrt(Tread))

•  UseTread≈T2foropMmalSNR

21

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

22

Sig

nal

t

Achilles’ tendon (T2 ≈ 1 ms)

TE=226µs

Sig

nal

t

TE=2.2ms

ProblemwithUTEImaging

23

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

25

Short-T2

RF Pulse

Long-T2

Frequency

Num

ber

of

Spi

ns

z

x-y

RFPulseT2Contrast

26

RF Pulse

Frequency

Num

ber

of

Spi

ns

z

x-y

Short-T2 Long-T2

Long-T2SuppressionPulses

27

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

29

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

30

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)

33

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

37

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

39

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

42

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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