wagshul y học

Pulse Sequences
Mark Wagshul, PhD
Director, MR Research Center
Department of Radiology
Stony Brook University


MRI Pulse Sequences


Spin echo and gradient echo
sequences
basic methods of MRI contrast

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3D techniques
Preparation techniques
secondary contrast methods

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Fast imaging
Gating and k-space segmentation


Important basic concepts


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Images are not acquired in real space,
but in inverse-space  k-space
Image contrast is obtained by manipulating
magnetic or biological properties of spins (e.g.
relaxation, precession frequency, magnetic
susceptibility, diffusion)

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Image contrast is (almost) always obtained at
the expense of signal size
“Static” fields (e.g. B0,G) are along z, RF fields
are
to z


Basic Spin
Echo
Sequence
TE/2

RF
pulses
slice
select

Position of BOTH
spin-echo and
gradient echo

phase
encode

readout
TE


The 3 step encoding process
z =
z =  / Gz
BW/ Gz

 y) =  Gyy ef
encoded
PRIOR
to
x)readout
=  Gxxt
encoded
DURING
readout


Double-echo spin echo - PD/T2

Basic
Gradient
Echo
Sequence
RF Pulse
Slice Select

Phase Encode

e-iGx x eiGxxt
Readout


Major diferences between
spin-echo and gradient echo


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SE – 90 degree pulse, uses all Mz per pulse
GE – variable pulse angle, partial use of Mz,
allows for faster TR’s
SE – refocuses T2*, allows longer TE’s
GE – T2* weighting, generally requires short TE’s
SE – usually used for T2 weighted images
GE – usually used for T1 weighted images, or for

speed
SE - T1 weighting adjusted with TR
GE - T1 weighting adjusted with TR AND flip angle


3D imaging
Reduced gradient
for “slab” selection


secondary phase encode:
 (z) =  Gzzton

b
a
l
S
e
m
u
l
Vo


2D vs. 3D imaging
Slice selection replaced with large slab selection
Slice selection  secondary phase encoding
Repetition times:
2D – slices excited serially, TR = Nsl * TR’, imaging time = NPE * TR
3D – all slices excited simultaneously, TR = TR ’, imaging time = NPE *

Nsl * TR

SNR:
2D – signal contributions from the slice, but noise from the entire
volume
3D – signal and noise contributions from the entire volume
Slice fidelity:
2D – imperfect slice profiles, slice crosstalk
3D – perfect slice profiles, no slice crosstalk
3D sequences essentially trade-off speed for SNR
secondary phase encode serves as a signal averaging process


Magnetization Preparation
Preparation schemes applied prior to
initial RF excitation, generally every
TR
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Fat saturation
Presaturation
Spin Tagging
Inversion recovery
Magnetization transfer

Prep. phase

general prep. scheme




Fat saturation – selective saturation
of fat signal based on chemical shift

Presaturation, tagging – selective
saturation of all signal based on
position
 IR
– selective nulling of signal
based on T1
 Mag. transfer – saturation of water
signal based on exchange with
nearby macromolecules



Water-Fat Chemical Shift
Water

Fat

Also possible to utilize in-phase/out-ofphase techniques to separate water/fat

Presaturation

Magnetization in this
slab
is completely
destroyed prior
to imaging RF pulse
•Applications:
motion suppression
spin labeling


Inversion recovery

S(t) = S0 (1 – 2 e-TI/T1)


IR example –
FLAIR (FLuid Attenuated IR)

TI – 2.2 s
TE – 140
ms
TR – 10 s


Mag. transfer

Preparation
pulse

Intra-sequence contrast


Components added within sequence
to obtain additional contrast

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Diffusion
Flow
Motion suppression


Phase/Difusion contrast
Spin Phase =  G z
RF Pulse
Slice Select

Phase Encode

Read Encode

Spin Phase =  G z+ z)


Bo
v




Fast Imaging


SE methods
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GE methods
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Fast spin echo (FSE)
Echo planar imaging (EPI)
Steady state imaging (PSIF, CE-FAST)
Spoiled GE (SPGR)
Echo planar imaging (EPI), Spiral
Steady state imaging (True-FISP, FIESTA,
Balanced FFE)

Parallel imaging (SENSE, SMASH)

Fast Spin Echo
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Collect multiple k-space lines per 90 degree pulse
New parameters – interecho spacing and echo
train length (ETL), will determine overall slice TR
and attainable TE’s
Typical ETL – 8, but can collect the entire image in
a single shot (e.g. 128 PE’s)
TE determined by echo position of center of kspace
For large ETL  increasing T2 signal loss in later
echoes lead to image blurring (loss of edge of kspace)


4 ETL Fast
Spin
Echo
Inter
echo
spaci
ng

Phase
unwrap

k1

k2

k3

k4