MR Pulse Sequences - AAPM HQamos3.aapm.org/abstracts/pdf/115-31754-387514-118259... · 2016. 8....

8/3/2016

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©2014 MFMER | slide-1

MR Pulse SequencesRobert A. Pooley, PhD

AAPM 2016, Washington, DC

August 3, 2016


Disclosure

• No conflicts of interest

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Outline

• Spin echo

• Inversion recovery

• Gradient echo

• Echo planar

• K-space trajectories


Spin Echo

• Uses 90° and 180° flip angles

• Spin density contrast: short TE, long TR

• T1 contrast: short TE, short TR

• T2 contrast: long TE, long TR

• Rephases effects from B0 inhomogeneity, chemical shift, and magnetic susceptibility

• Scan time = TR x NP x NSA TR: Repetition timeNP: # phase encodesNSA: # signal averages = NEX: # excitations

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90° 180°

Spin Echo

RF

Signal

ADC

Gslice

Gfreq

Gphase

TE TR


Filling K-space

• The signal from the echo is sampled; the values are placed in k-space.

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Image space k-space

FT


RFSignal

ADCGslice

Gfreq

Gphase

Single Slice Spin Echo

1 Slice

TE

TR

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RFSignal

ADCGslice

Gfreq

Gphase

Multi-Slice Spin Echo

Slice 1

TE

TR

Slice 2

TE

Slice 3

TE


Multi-Echo Spin Echo

• Multiple images, each with different contrast

• Multiple 180° pulses, to create multiple echos

• Each echo is used to form a separate image

• Each image will have different contrast ranging from spin density to strong T2, depending on TE for that echo

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90° 180°

RF

Signal

ADC

Gslice

Gfreq

Gphase

180°

Multi-Echo Spin Echo

TE2 TE1


Turbo (Fast) Spin Echo

• Multiple 180° pulses to create multiple echos

• All echos used to create single image

• Scan time decreased by echo train length

• T1 and T2 weighting

• Scan Time = TR x NP x NSA / ETL

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90° 180°RF

Signal

ADC

Gslice

Gfreq

Gphase

180° 180° 180°

Turbo Spin Echo

Echo Train Length (ETL) = 4


256 TRPeriods

64 TRPeriods

SE FSE

ETL = 4is 4 Times

Faster!

FT

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Inversion Recovery

• Can be used to produce strong T1 weighted images

• Can be used to suppress fat, e.g. short TI inversion recovery (STIR)

• Does not depend on B0 inhomogeneity

• Can be used to suppress fluid, e.g. fluid-attenuated inversion recovery (FLAIR)


90°

RF

Signal

ADC

Gslice

Gfreq

Gphase

180° 180°

Inversion Recovery

TI

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180°

Long

itudi

nal M

agne

tizat

ion

TR

Apply 90°pulse

TI

Fat

Other

Inversion Recovery - STIR


Gradient Echo

• Partial flip angle, no 180° refocusing pulse

• This allows very short TR

• Uses gradients to rephase echo

• Produces T1 or T2* contrast

• Scan time = TR x NP x NSA

• Options include spoiled GE, steady-state GE

• Does not rephase effects of B0 inhomogeneity, chemical shift, or magnetic susceptibility

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°

RF

Signal

ADC

Gslice

Gfreq

Gphase

No 180°

Gradient Echo


Flip Angle - 90°

x

y

z

Transverse (x,y) Plane

Longitudinal Direction

B0

Direction of mainmagnetic field

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Flip Angle - °

x

y

z

Transverse (x,y) Plane

Longitudinal Direction

B0

Direction of mainmagnetic field


Gradient Echo (Spoiled / Incoherent)

• “Spoiling” is used to remove residual transverse magnetization at end of sequence, before next RF pulse

• Spoiling is accomplished by using extra gradient pulses, RF pulses, or both to completely dephase the spins

• E.g. Fast Low Angle Shot (FLASH); Spoiled Gradient Recalled Echo (SPGR); T1 Fast Field Echo (T1-FFE)

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°

RF

Signal

ADC

Gslice

Gfreq

Gphase

Gradient Echo – Spoiled / Incoherent°


Gradient Echo (Steady-state / Coherent)

• Residual transverse magnetization at end of sequence is “rewound”

• This signal then combines coherently with new signal in the transverse plane, increasing overall signal

• E.g. Gradient Recalled Acquisition in the Steady State (GRASS); Gradient Recalled Echo (GRE); Fast Imaging with Steady-state Precession (FISP); Fast Field Echo (FFE)

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°

RF

Signal

ADC

Gslice

Gfreq

Gphase

Gradient Echo – Steady State / Coherent°

E.g. FISP


°

RF

Signal

ADC

Gslice

Gfreq

Gphase

Gradient Echo – Steady State / Coherent°

E.g. True FISP

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Echo Planar

• Method of ultrafast MR signal acquisition

• All of k-space can be acquired in a “single shot” by rapid gradient reversal and echo collection after a single set of RF pulses

• EPI is a fast readout mechanism; the excitation pulses produce the contrast, the signal is read with EPI


90° 180°

RF

Signal

ADC

Gslice

Gfreq

Gphase

Echo Planar

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Echo Planar – Filling K-space


K-space Trajectories

• Cartesian• Linear• Centric

• Non-cartesian• Radial• Propeller• Spiral

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Cartesian - Linear

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Cartesian - Centric

975312468

Image contrast at center ofk-space

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Non-Cartesian - Radial


Non-Cartesian - Propeller

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Non-Cartesian – Spiral


Recommended Reading

• Handbook of MRI Pulse Sequences. Bernstein, King, Zhou. 2004

• Breast MRI – Fundamentals and Technical Aspects. R.Edward Hendrick. 2010

• AAPM/RSNA Physics Tutorial for Residents –Fundamental Physics of MR Imaging. RAPooley. 2005

• MRI From Picture to Proton. McRobbie, Moore, Graves, Prince. 2007

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Thank YouMayo Clinic Florida

[email protected]

MR Pulse Sequences - AAPM HQamos3.aapm.org/abstracts/pdf/115-31754-387514-118259... · 2016. 8....

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Transcript of MR Pulse Sequences - AAPM HQamos3.aapm.org/abstracts/pdf/115-31754-387514-118259... · 2016. 8....