Single Echo Multi Echo

GRE SE GRE SESTE STE

Turbo* FLASH GRASS

SE TurboSTEAM

EPI FSE FIESTA

The PSD Family Tree

Sequences Across Vendors

Sequence Name GE Siemens PhilipsSpin-echo MEMP,VEMP Spin-Echo Spin-EchoFast spin-echo FSE TSE TSESingle-shot technique SSFSE HASTE Single Shot TSECoherent gradient-echo GRASS, GRE, FGR, FMPGR FISP, ROAST FFEIncoherent gradient-echo (RF spoiled) SPGR, FSPGR T1 FFEIncoherent gradient-echo (gradient spoiled)MPGR FLASH Contrast-enhanced gradient-echo sequenceSSFP, DE FGR PSIF T2 FFEBalanced coherent gradient-echo FIESTA, SSFP True FISP Balanced-FFEUltrafast gradient-echo FAST, GRASS, SPGR (IR/DE prep), IR FGRTurbo FLASH, 3D MP RAGETFEGradient and spin-echo GRACE GSE GRACEInversion recovery MPIR,TIR IR,TIR IR, IR-TSE, IR-TFEShort T1 inversion recovery STIR STIR STIRPhase-contrast sequence Phase Contrast Phase Contrast Phase ContrastParallel imaging technique ASSET IPAT SENSE

Options Across VendorsOption Name GE Siemens PhilipsSignal averaging NEX AC NSAPartial averaging Fractional NEX Half Fourier Half ScanPartial echo Fractional Echo Echo PartialRectangular field-of-view RFOV HFI RFOVOff-center shifting slices Off Center FOV Shift, Offset Off Center ShiftSpacing between slices Spacing Distance Factor in % Slice GapPresaturation Spatial SAT SAT RESTFat saturation FAT SAT, CHEM SAT FAT SAT SPIR, SPAIR, WaterSELMoving saturation pulse Walking SAT Travel SAT Travel RESTGradient moment rephasing FC GMR FCRespiratory compensation Respiratory Compensation, Respiratory TriggeringRespiratory Gated PEAR, Respiratory TriggerECG synchronization Cardiac Gated, Triggering ECG Triggered ECG TriggeredDelay after R wave Trigger Delay Trigger Delay Trigger DelayAutomatic bolus detection Smart Prep Care Bolus Bolus TrackNumber of echoes ETL ETL, Turbo Factor TSE TFTime between echoes Echo Spacing Echo Spacing Echo SpacingOversampling in frequency direction Always On Over-sampling Always OnOversampling in phase direction No Phase Wrap Over-sampling Fold Over SuppressionBandwidth Received Bandwidth Bandwidth Water/Fat ShiftVariable bandwidth VB Optimized Bandwidth Optimized Water/Fat ShiftSegmented k-space data acquisition Views per Segment Lines, Segments Views, SegmentsMultislice imaging Multi Slice Multi Slice Multiple Slice3D Imaging 3D 3D Volume 3DOrientation scan Localizer Localizer, Scout Plan Scan, Survey

SI

slice selection gradients

FID

RF "sinc" pulse - flip angle

4 msec

sampling time

dephase

Gradient-recalled-echo (GRE)

slice

SI

FID

RF "sinc" pulse

4 msec

sampling echodephase

Spin-echo (SE)

RF "sinc" pulse

sliceslice

Single Echo

GRE SE STE

Turbo* FLASH GRASS

SE TurboSTEAM

SubsecondEasy to runFlexiblePopularPoor SNR

Popular...ConventionalKeyholed...

SecondsMore difficultVery flexiblePopular

Multi Echo

GRE SE STE

EPISpiral

FSE FIESTA

Subsecond 10HzDifficult/expensiveVery flexibleGood SNRIntense interest

Seconds to minutesEasy, SARVery flexibleGood SNRVery popular

Fast but sensitiveTo field inhomogeneities

K-space covers Frequency and Phase

Frequency

Phase

Sequences: GRE SE

FSE Turbo GRE

EPI Spiral

SamplingVB

1/2NEX1/2 Echo Key Hole

k-Space and MRI Physics

Phase

Frequency

hi

lo

+

-

hi

FT

lo

"sample spin echo"

slice

phase

frequency

90 rf 180 rf

Frequency

Phase

The MR Sequence Determines How k-spaceis Sampled...

90 180

view 1

192

The "spin-echo"

128

64

256

view 1

view 256

Frequency

Slice selection

High-Speed MRI Families

GRE SE

Turbo

single echo

multiecho

EPI

multiecho

FSE

~ 1 secez to do

~ 32 msecstrong gradientsneeded...

10 sec - 6 min.

SE tissue contrasts

High-Speed GRE Family

GRE

SPGR

spoiled

Refocused GRE

GRASS MPGR

Appears T1-wtUseful for GM-WM3D GRE uses SPGR for anatomy - MP-RAGE

Appears T2-wt.Useful for MRA, flow, CSF, MS

GRE vs. EPISlice

Frequency

Phase

Partial flip

Partial flipecho

echo echo echoecho

GRE vs. EPIOne view every TR...

All views in one TR!

Note where the "center

echo" - this is the key to

TEf - the effectve TE!

Alternate K-Space samplings

MR projections MR “spirals”

Spiral Scanning

Segmented samplingDone with less than 1 gauss/cmNon-linear ADC sampling

Incredible applications (fast, flow sensitvie)

slice

x-gradient

y-gradient

LAD

Spiral-scan

Meyer et al.

Stanford

Transverse Magnetization builds up for

short TR values in Gradient-echo MRI!!

REFOCUS OR SPOIL

REWIND OR REFOCUS - GRASS, MPGR

SPOIL - SPGR

SE

FOV 16, 4 NEX, 128X256, 5 mm

MPGR 50/10

SI

slice selection gradients

FID

RF "sinc" pulse

4 msec

sampling time

dephase

TR 10-100

gradient-echo still dephasing for 4XT2*gradient-echo still dephasing for 4XT2*

Transverse magnetization builds up from Transverse magnetization builds up from view to view.view to view.

SI

slice selection gradients

FID

RF "sinc" pulse

4 msec

sampling time

dephase

"rewinds" phase

from view to view

REWIND OR REFOCUS

SI

slice selection gradients

FID

RF "sinc" pulse - VARY PHASE OF EACH PULSE

4 msec

sampling time

dephase

"rewinds" phase

from view to view

SPOILED GRASS

KILLER

Contrast in GRE

TR 200TE 15MPGR

5o

45o

75o

For long TR:

Increase inflip addsT1-wting.....

Contrast in GRE

TR 200, Flip 30, MPGR

For long TE

in GRE:

Increase in

TE adds

T2-wting.....

and MS artifacts

6 ms

30 ms

Basic Rules of GRE Tissue Contrast

GRASS, MPGR - Gives T1, T2, T2* mixed contrasts

SPGR - Gives (nearly always) T1-weighted contrast

Basic Rules of GRE Tissue Contrast

SPGR - Gives (nearly always) T1-weighted contrast

For best T1-weighting:

TR short 20- 100

TE short as possible < 10

Flips from 30-45

In General...

Small flip angles enhances proton density (PD).

Increasing flip leads to more T1-weighting.

Increasing TE leads to more T2, T2*-weighting.

Increasing TR, decreasing flip leads to more PD.

Mechanisms in Action

Steady-state transverse phase loss - T2, T2*Longitudinal recovery - T1FRE, magnetic susceptibility - FRE, MS

GRE - Mutants

"Fast" GRE - Done with short TR = "Turbo"

"Center out" - Do the center views first to minimize T1 saturation...

"DE" - Driven equilibrium - does a 90 - 180 - GRE to create a "T2- like" tissue contrast...

"IR" - Inversion recovery to give T1 weighting...

F/W phase - adjusts TE to make fat/water in phase useful to help minimize lipids

SE vs. FSE

Slice

Frequency

Phase

90 180 echo

90 180 180 180 180 180

sample frequency

incrementphase

change

FSE k-space sampling is simply view by view

FSE Advantages

Short scan times!!

TR 2000, 512X512, 2 NEX = 34 minutes

Scan time = TR x #views x NEX

TR 2000, 512X512, 2 NEX , ET 16 = 2 minutes!!

The "effective TE" in FSE is determined bywhere the "center views" (LEAST AMOUNTOF PHASE ENCODING!) are collected...

Effective TE in FSE

FSE

90 180 echo

Phaseencoding

Where is the center view? What is the eff. TE??

Effect of TETR 5000

TE 85

TE 119 TE 136

TE 102

FSE allows longTR's... helps TEeffect by reducingT1 contributions!!

FSE Issues

Total TR

8 echoes give 6 slices...

16 echoes give 3 slices...

SI

Time (msec)

17

34

5168

85102

136153 msec

Long ET trains increase T2-weighting...

For longer ET trains, thelater echoes contributegreatly...

For longer ET trains, thelater echoes contributegreatly...

This is ET 16!!

Effect of TE

TR 5000

TE 85

TE 119 TE 136

TE 102

FSE allows longTR's... helps TEeffect by reducingT1 contributions!!

Effect of ET

TR 4000 TE 102

ET 4

ET 16

ET 8

Increasing ET increasesT2 effects...

By increasing the contributionfrom the later echoes...

FSE has unique applicatons in the body..

TR 2000TE 85ET 16

128X2561 NEX

BREATHHELD!

FSE hasbright fat..

Virtues of FATSAT...

TR 6000TE 119

Chem Sat (FatSat)1. Wide range of clinical advantages: Anatomy free of lipids (or water!) Better SNR (dynamic range is increased, as is amplifier gain) Definition of hyperintensity as fat/fluid. Reduction of fat-enhanced respiratory artifacts!! NO chemical shift misregistration VB now possible2. Clinical roles: Better depiction of joint fluids Improved Gd enhancement

How does FATSAT work?

The chemical shift is due to the different protonenvironments of water and lipids...

frequency

Water Lipids

220 Hz+ +

This chemcial shift is responsible for the "Chemical shift artifact"

How does FATSAT work?

Apply a long (16 msec) rf pulse exactly at thelipid resonance but miss the water resonance...

frequency

Water

Lipids

+ +

This chemcial shift is responsible for the "Chemical shift artifact"

Bandwidth of rf pulse = 100Hz

How does FATSAT work?

Apply a long (16 msec) rf pulse exactly at thelipid resonance but miss the water resonance...

The "sinc" pulse profile has a square bandwidth..

Time

Frequency

RF pulse length related to RF bandwidth

centerfrequency

bandwidth

How does FATSAT work?

Now that lipids are excited... Crush phase coherence with a strong GRADIENT!

SAT RF "Crush" 90 RF180 RF

gradient

Excites only lipids 220 Hz from water

Spoils phase coherence

Begin next viewwith 90 - 180 - echo...

FATSAT SPGR, FOV 16, 4 NEX, 128X256, 5 mm

SAT off SAT on

Steady-state Free Precession - 1Improved gradient capabilities -> ultrashort TRsSSFP for cardiac imaging, flow imaging T1/T2 quantification, whole body imaging

SSFP is unique - both spin-echo and gradient-echo

In SSFP A gradient-echo is acquired like FLASH or FISP But, gradients are symmetrically balanced in slice, phase-encoding, & read directions No RF spoiling is implemented.

Steady-state Free Precession - 2Coherences are maintained in successive TRs.Transverse magnetization from one TR contributes to the next TRs.

The RF echoes generated by the train of α pulses in the steady state are then added to the gradient-echo (assuming a uniform main field, this is the catch).

Signal is mostly from long-T2* and long-T2 tissues.

No spoiling, so no saturation effects as in spoiled techniques, and high flip angles can be used in SSFP.

Balanced gradients also mean motion insensitivity.

Steady-state Free Precession - 3As long as TR is shorter than T2 without RF spoiling, we have a coherent steady state.That is a combination of the longitudinal and transverse components. It takes time to reach SS.

In steady state, the SSFP signal is T2/T1 weighted.

SSFP signal is a complicated function of parameters such as TR, T1, T2, flip angle, and off-resonance angle β (β = γ×ΔB0 ×TR).

Advantages: unique contrast, high SNR compared to spoiled GRE & high imaging efficiency, but sensitivity to off-resonance is a major limitation.

Figure 5-32 SSFP pulse sequence diagram. The gradients are balanced along all three axes so that steady-state effects related to long-T2* species are emphasized. The gradient-echo and the RF echo are superimposed at TE, and the gradient structure is motion insensitive .

Downloaded from: Clinical Magnetic Resonance Imaging, 3rd edition (on 3 April 2007 05:38 PM)© 2007 Elsevier

Off-resonance Artifacts in SSFPOff-resonance Artifacts in SSFP

Off-resonance artifacts are usually bands in SSFP.

Main sources of off-resonance artifact B0 inhomogeneity.

Static B0 field varies within an object.

Artifacts can be minimized by careful shimming, high BW, shortest TR possible.

Approach to Steady StateApproach to Steady State

3 x T1 to reach steady state. Long T1 tissues may show artifact.

FIESTA Example

T2 FSE FIESTA T1 FSE

Reduced Scan time Data Acquisition Strategies

Fractional EchoPartial ViewsSMASH

Partial FOVSENSE

Increased SNR Data Acquisition Strategies

SMASH - Simultaneous Acquisition of Spatial HarmonicsSodickson & Manning, MRM 38(4):591-603 (Oct. 1997)

“Linear combinations ofsimultaneously acquired signals from multiple surface coils with different spatial sensitivities to generate multiple data sets with distinct offsets in k-space.”

Huh?

Add signals from multiple coilsusing information about the coil’sspatial location

SMASH - Simultaneous Acquisition of Spatial HarmonicsSodickson & Manning, MRM 38(4):591-603 (Oct. 1997)

S(kx,ky)=∫ ∫ C(x,y)M(x,y) e-I(kx·x+ky ·y) dxdy

Usually C(x,y) = 1

For SMASH,Construct coils withsinusoidal sensitivities,

Like a gradient shift.

5 lines per readout with4 spatial harmonics per readout

SMASH achieves a reduction in scantime, R, given by the number of simultaneously acquired spatial frequency harmonics.

SMASH Procedure:1. Determine sensitivity profile for each coil.2. Determine the number of spatial harmonics that can be generated using the coil array.3. Acquire data from coil array - these are aliased component coil images.4. Determine weights for linear combinations of component coil signals.5. Form composite k-space signals corresponding to the spatial harmonics.6. Interleave the composite signals then Fourier transform.

Weakness: Measurement & manipulation of sensitivity profiles.

SENSE: Sensitivity encodingPruessmann, Weiger,Scheidegger,Boesigner. MRM 42(5):952-969 (Nov. 1999)

“Knowledge of coil sensitivity implies information about the detected MR signal which may be used in imagegeneration.”

Coil 2Coil 1

Coil 4Coil 3

SMASH requires the combination of coil sensitivity.

SENSE is the generalizationof SMASH for any geometry.

Aliasing: MRI data is collected in the frequency domain, MRI data is collected in the frequency domain, so objects outside of the FOV fold back into the image.so objects outside of the FOV fold back into the image.

AnalogSignal

Over-sampled

Under-sampled

f1

f2

f1

Aliasing or Wrap-around Aliasing or Wrap-around in standard coil seen when

FOV is smaller than the object being imaged.FOV xFOV x

FOV yFOV y

TrueTruepositionspositions

AliasedAliasedpositionspositions

SENSE: Sensitivity encodingPruessmann, Weiger,Scheidegger,Boesigner. MRM 42(5):952-969 (Nov. 1999)

Coil 2Coil 1

Coil 4Coil 3

Reconstruction of an image from N receiver coils:

Undersampled k-space from each receiver (aliasing).Undo signal superposition caused by fold-over (aliasing).Undo signal superposition byusing weighting caused by varied coil sensitivities.

FOV=24cm, 384x256, 5mm slice, TR/TE=4400/97.4Ef msEC=1/1, BW= 31.2 kHz, 2 NEX, VBW/TRF/Z512

WVU Twin-speed ACR Uniformity Slice: ASSET compatible FRFSE-XL/90

After ASSET cal. - ASSET turned on

without SCIC with SCIC

8 Channel Brain Coil

FOV=24cm, 384x256, 5mm slice, TR/TE=4400/97.4Ef msEC=1/1, BW= 31.2 kHz, 2 NEX, VBW/TRF/Z512

WVU Twin-speed ACR Uniformity Slice: ASSET compatible FRFSE-XL/90

After ASSET cal. - ASSET turned off

without SCIC with SCIC

8 Channel Brain Coil

Problem is that while the SNR as measured by Center Signal / stdev of background is high, uniformity is poor. In a ROI of 500 mm2 stdev = 50, or 25 w/SCIC.

WVU Twin-speed ACR Uniformity Slice: ASSET compatible FRFSE-XL/90

with SCIC

8 Channel Brain Coil Quad Head Coil

SNR w/SCIC = 195, wout/SCIC = 128 SNR= 95

WVU Twin-speed ACR Uniformity Slice: ASSET compatible FRFSE-XL/90

Coil Signal St.Dev.Head 622 118ch 677 508chSCIC 623 25

Measurements made in same location - as shown in image.