Basics of DWI and PWI

Basics
Basics of
of DWI
DWI and
and PWI
PWI
Mike
Mike Moseley,
Moseley, Ph.D.
Ph.D.
DWI
DWI measures
measures proton
proton motion:
motion:
5-20
5-20 µm/image.
µm/image.
Samples
Samples µenvironment.
µenvironment.
Measured
Measured as
as ADC.
ADC.
Department
Department of
of Radiology
Radiology
Stanford
Stanford University
University
http://rsl.stanford.edu/moseley/
Diffusion
Diffusion sensitive
sensitive to:
to:
Water
contentWater contentADC
ADC increases
increases with
with PD.
PD.
Water
Water compartmentscompartments-
Current
Current Concepts
Concepts
Map of
Diffusion
ADC
Intracellular
Intracellular water
water slower.
slower.
Water
Water hindranceshindrances-
Monterey
Monterey
October
October 2005
2005
Ordered
Ordered in
in WM.
WM.
Map of WM
Diffusion in an
Diffusion in an
isotropic medium (GM) ordered environment (WM)
similar molecular
greater molecular displacement
displacements in all directions
along cylinders than across
The
The Apparent
Apparent Diffusion
Diffusion is
is
quantitated
quantitated as
as the
the “ADC”
“ADC”
-- Measure
Measure of
of translation
translation
Proton Motions and DWI
5 µm
MRI
MRI and
and Acute
Acute Stroke
Stroke
44 hours
hours
T2-wt.
T2-wt.
DWI
Deviation
Deviation from
from random
random pathspaths“Fractional
“Fractional Anisotropy”
Anisotropy”
-Measure
-Measure of
of WM
WM integrity
integrity
Practices of DWI: Artifacts
Artifacts in EPI DWI can be serious
Artifacts arise from susceptibility issues
Signal drop-outs/pile-up
Aspect ratio distortions
Sensitivity to shims/metal
Eddy currents effects
Aspect ratio distortions/smearing
Use auto-shimming prior to exam
Avoid overly high resolutions (<192)
New correction algorithms
New Sequences for DWI
Can DWI Be Improved?
“Perhaps the biggest immediate improvement affecting
all of MR today is the development of parallel imaging…”
Parallel Imaging (SENSE)
SE EPI
Improve Temporal Resolution
Increased Volume Coverage/ Time
Reduction of Motion Artifacts
Reduce T2/T2*-Blurring
SE EPI
Diminish Off-resonance Artifacts
conventional DWI
DWI-SENSE-EPI.
Decrease SAR Problems
Reduction of Total Acquisition Time
SE EPI
less susceptibility artifacts
less image blurring
less T2 decay (shorter TE)
The MR PROPELLER Sequence
“Periodically rotated overlapping parallel lines with enhanced reconstruction”
Multi-shot, multi-echo FSE (blades)
Each blade passes thru k-space center
Robust motion correction
No distortions, no susceptibility
Slower, marketed by GEMS
DWI
DWI “Mimics”
“Mimics” Beyond
Beyond Clinical
Clinical Stroke
Stroke
Reduced
Reduced ADC
ADC “cytoxic”
“cytoxic” vs.
vs. reduced
reduced mobility.
mobility.
Screen
Screen for
for “non-stroke”
“non-stroke” events
events (headaches,
(headaches, TIA).
TIA).
Secondary
Secondary ischemia
ischemia (acute
(acute trauma,
trauma, venous
venous occlusion)
occlusion)
ADC
ADC reduced
reduced in
in “cytotoxic”
“cytotoxic” edema:
edema: CJD,
CJD, pediatric
pediatric diseases.
diseases.
ADC
ADC reduced
reduced in
in abscess
abscess **
** vs.
vs. highly
highly cellular
cellular tumor
tumor cores.
cores.
Elevated
Elevated ADC
ADC “vasogenic”
“vasogenic” (usually
(usually seen
seen also
also as
as elevated
elevated T2).
T2).
MS
MS plaques**
plaques** (older),
(older), edema,
edema, inflammation,
inflammation, necrosis.
necrosis.
T2 FSE
4 y.o. male, 3 days s/p MVA,
decreasing mental status,
UE weakness
Motion Correction With PROPELLER MRI
J. Pipe, MRM 42:963-969 (1999)
PROPELLER blades, Arfanakis, et al. ISMRM, Kyoto 2004
Diffusion vs. Perfusion
(Self)-Diffusion
Perfusion
… has zero mean, but r.m.s.
displacement characterizes diffusion
coefficient !
Start
Capillary microcirculation in
organs. Rate of nutrient supply.
G. Sorensen,
MGH
DWI
Why does ADC of water decrease in acute stroke ?
perfusion deficit
Eventual lysis/edema
energy deficit
DWI hyperintensity
failure of
Na+/K+ pumps
End
Mobility hindered =
ADC drop
intracellular
accumulation of Na+
influx
of water
large diffusion coefficient
small diffusion coefficient
Single-shot EPI is Ideal for Diffusion MR
180 rf
90 rf
glial and
neuronal swelling
reduction in
extracellular space
The
The Principles
Principles of
of DWI:
DWI: The
The Protocol
Protocol
Acquire
Acquire all
all gradient
gradient axes.
axes.
δ=25 msec
∆= 40 msec
G
EPI Readout
G
xyz
xyz
Diffusion-Weighted images
T2-wt
DWI
(b=0)
(b=800)
** Don’t
Don’t be
be fooled
fooled by
by
WM
WM anisotropy
anisotropy effects.
effects.
** X,
X, Y,
Y, and
and ZZ (“trace”)
(“trace”)
** Off-axis
Off-axis directions
directions
also
also hot
hot (“tensor”)
(“tensor”)
** Note
Note b0,
b0, DWI,
DWI, AND
AND ADC!
ADC!
DWI
ADC
ADC map
=
2 2 2
e (-γγ δ G [D-δ
δ/3] ADC)
“b”
reduced ADC
due to anisotropy
Trace ADC
X
Y
Z
ADC
ADC vs
vs T2-wt
T2-wt Shine-Thru
Shine-Thru
••
••
hyperacute
hyperacute(<
(<6hrs)
6hrs)
acute
(6hrs
to
acute (6hrs to44d)
d)
••
••
subacute
subacute(4
(4to
to10
10d)
d)
chronic
(>10
d)
chronic (>10 d)
1.4
T2
T2
iso
iso
high
high
high
high
high
high
0.8
ADC
ADC
low
low
low
low
iso
iso
high
high
T2
T2
ADC
1.2
1
DWI
DWI
high
high
high
high
iso/h
iso/h
iso/l
iso/l
ADC
ADC
0.6
10
100
The
The Principles
Principles of
of DWI:
DWI: The
The Protocol
Protocol 2004
2004
The
The impact
impact of
of new
new MR
MR technology
technology
Better
Better SE-EPI
SE-EPI
Image
Image unwarping
unwarping (postprocessing)
(postprocessing)
Dual-echo
Dual-echo (TRSE)
(TRSE)
Parallel
Parallel Imaging
Imaging for
for DWI
DWI
SENSE-DWI
SENSE-DWI
FSE-based
FSE-based DWI
DWI
PROPELLER
PROPELLER (and
(and Turbo-PROP)
Turbo-PROP)
Spiral-based
Spiral-based DWI
DWI
SNAILS
SNAILS
1000 Time After Onset of Stroke (h)
The
The Principles
Principles of
of DWI:
DWI: The
The Protocol
Protocol 2004
2004
SENSE-EPI
SENSE-EPI for
for DWI
DWI
The
The Principles
Principles of
of DWI:
DWI: The
The Protocol
Protocol 2004
2004
PROPELLER
PROPELLER (and
(and Turbo-PROP)
Turbo-PROP)
Conventional DWI (TE107)
EPI with SENSE (r=2)
Advantages:
Image distortions
Short TE’s
Less blurring
Increased conspicuity
Hi-res!
The
The Principles
Principles of
of DWI:
DWI: The
The Protocol
Protocol 2004
2004
T2wT2w-SNAIL (b0)
(b0)
isoDWIisoDWI-SNAIL
(b1000)
FAFA-SNAIL
512x512, 8 averages, 6464-leaf, b800, 23 minutes
Integrated
Integrated MRI
MRI for
for Acute
Acute Stroke
Stroke
What
What is
is PWI?
PWI?
Collect
Collect serial
serial EPI
EPI images
images
Inject
Inject contrast
contrast agent:
agent:
Perfused
Perfused brain
brain -- dark
dark
Ischemic
Ischemic brain
brain -- bright
bright
Hemodynamics
Hemodynamics tracked
tracked
Time-resolved
transit
Time-resolved transit
Correlate
Correlate with
with DWI,
DWI, T2
T2
Perfusion
Perfusiondeficit
deficitpresent:
present:
(low
(lowrCBF,
rCBF,long
longrMTT)
rMTT)
DWI
rMTT
rCBF
rMTT
Why Do We Call PWI “Perfusion”?
PWI Protocol
Gd shortens T1…
However…
Gd also shortens T2*.
Brain cell
On a GRE-EPI image,
a bolus of
0.1mmol/kg Gd
can decrease T2*
by 50%-80%
Gd centers present big
gradient = T2* loss
Gd
small gradient
across cell...
GRE
EPI
TE60
Gd
Gd
Gd
Maps
Maps of
of rCBV,
rCBV, rMTT,
rMTT, rCBF
rCBF from
from transit.
transit.
At bolus
peak
Arterial
Arterial input
input needed.
needed.
DWI
2. From the SI behavior map, model the transit dynamics
by a “gamma variate” analysis. This yields parametric
maps of the vascular volume, transit, and flow...
Artery
6
area= rCBV
Input
T2-wt
Early stroke
not seen
width = rMTT
Blood Volume
Lesion has
reduced CBV
raw
EPI
area
rCBV
Width
rMTT
flow
rCBF
Diffusion-wt
Clear depiction of
lesion
Mean Transit
large perfusion
deficit
Apparent Diffusion
Acute stroke has low
ADC
Blood Flow
Reduced flow
around lesion
flow = rCBF
4
2
seconds
0
0
20
40
60
80
Proton
Proton Motions
Motions are
are Ordered
Ordered by
by WM
WM
Is There a Need for CBF Quantitation?
DWI
DWI White
White Matter
Matter “Anisotropy”
“Anisotropy” is
is Altered
Altered with
with
Growth,
Growth, Aging,
Aging, Disease…
Disease…
DWI
Vol DWI (7 days)/
Vol DWI (5 hrs)
CBF
11
ml/100g/min
53
PWI
Tissue
10
R2*8
Bolus
transit
The Diffusion - Perfusion Mismatch
How Does PWI Map Hemodynamics?
12
GRE-EPI
GRE-EPI images
images acquired
acquired
during
during bolus
bolus injection.
injection.
12
12 slices
slices every
every 22 sec.
sec.
40
phases
40 phases over
over 80
80 sec.
sec.
0.1
0.1 mmol/kg
mmol/kg Gad
Gad -- Power
Power injector.
injector.
1.5mm
1.5mm per
per pixel
pixel resolution.
resolution.
as
as for
for T2,
T2, FLAIR,
FLAIR, DWI,
DWI, PWI.
PWI.
31
20
MTT
57
(AIC-Ostergaard, et al)
73yof - Stenosis
MRI at 6hr. - NIHSSS 13
Motion of water hindered in a restricted
environment (inside neuronal fibers).
Complex motion described by a “tensor”.
Number, integrity, myelination of fibers
alters the measured tensor.
Cognitive performance appears to be
correlated with anisotropy.
How will DTI impact neuroimaging?
DTI is Not all Myelin!
In DTI, We Measure ADC Along All Axes
ADC vs. Direction = ADC Tensor
Y
Fiber
Fiberdiameter
diameterand
and neuronal
neuronaldensity,
density,
Status
Status of
ofthe
the myelination
myelination in
inneurogenesis,
neurogenesis,
Degree
Degreeof
of(de)myelination
(de)myelination in
in aging/disease,
aging/disease,
Macrostructural
Macrostructuralfeatures
features such
such as
as
intravoxel
intravoxelfiber-tract
fiber-tractcoherence/crossings
coherence/crossings
Diffusion Tensor Imaging DTI: The EPI Sequence
180 rf
90 rf
δ=20
δ=
20 ms
Diffusion-Weighted images
Trace ADC
b=800
FA
-yz
-yz
x-z
x-z
0.05 (GM)< FA < 0.8 (splenium)
) + (λ
2
2
ADC yy
ADC yz
ADC yz
ADC zz
DW Images to ADC Maps to Eigenvalues
xy
-xy
3
3
i =1 j =1
yz
-yz
b ijADC ij
ADCxx
ADCxy
ADCxz
λ1
0
ADC = ADCxy
ADCyy
ADC yz
ADC = 0
λ2
0
0
ADCxz
ADC yz
ADC zz
0
0
λ3
xz
λ1
x-z
− D
) + (λ
2
3
− D
D : D = λ12 + λ22 + λ23
Isotropic, 128x128, 25FOV, 2mm slice
37 slices, 13 min, 6NEX
λ2
b=0
0 (sphere) < FA < 1 (cylinder)
D : D = (λ1 − D
ADC = ADCxy
ADC xz
λ3
Tensor
calc
Fractional Anisotropy = FA = Created from λ1, λ2, λ3
3 D:D
2 D:D
ADCxz
xz
xz
Eigenvalues to FA Maps
FA =
ADC xy
EPI Readout
G2
(Gx,Gy,Gz) = {(1,1,0), (0,1,1), (1,0,1), (-1,1,0), (0,-1,1), (1,0,-1)}
SE-EPI TR6000, TE107, 128x128, b0-1000, 6 axes, 3.5min/NEX
-xy
-xy
Y
ADC xx
S( b ) = S 0 exp −
= 40 ms
G2
yz
yz
X
Z
G1
G1
Gz
xy
xy
Fiber axis
Z
X
DTI
DTI is
is related
related to…
to…
b=0
Magnet axis
)
2
DTI Protocol
Parameters (CV-i Signa 1.5T)
δ = 20ms; ∆ ≈ 34ms; Gd = 1.4G/cm
Gradients (Gx,Gy,Gz) =
{(1,1,0), (0,1,1), (1,0,1), (-1,1,0),
(0,-1,1), (1,0,-1)}
TE/TR/TI = 106/6000/2100ms
FOV = 24cm; slice 2/0
matrix 128x128.
For each slice
2 images with b=0 and IR
for CSF suppression (unwarping)
4-6 averages with b=800s/mm2
Clinical DTI: Protocols and Parameters
FA “Atlas” of the Brain
TR 10-20 RR (if gated), TR10000
TE min full (107)
FOV24, 96x96
Slice 2.5-5mm
δ (duration) 18ms
∆ (separation) 49ms
G (grad amp) 40 mT/m
b max 1300 sec/mm2
NEX (b0) 2-4
NEX (b1300) 64 (64x1)
Slices 39
Scan time 14-17minutes
2 2 2
∆-δ
δ/3] ADC)
“b” = e (-γγ δ G [∆
Jones, et al.
Neuroimage 17, 2002
FA Mapping Applications
Tumor Displacment, Infiltration, or Degradation of WM
S. Mori, JHU
FA Maps, Vectors and Fiber Tracking:
Concepts and Applications
Anderson, et al.
Clinical DTI: Visualization
Streamtubes overlaid onto FA
Stochastic trajectories
DTI and Reading
Torkel Klingberg, Maj Hedehus*, Russ Poldrack,
Mike Moseley*, John Gabrieli, Gayle Deutsch,
Bob Dougherty, WT Siok, Roland Bammer*, Brian Wandell.
*Center for Advanced MR Technology at Stanford
http://www-radiology.stanford.edu/research/RR.html
Stanford University
Jones, et al.
Neuroimage 17, 2002
Tench, et al.
MRM 48, 2002
School of Medicine
DTI and Reading, 2000
“Significant differences were found between adult dyslexics and age-matched
controls in the temporo-parietal region…”
DTI and Reading, 2003
“The results support the hypothesis that the microstructural integrity of left temporo-parietal neural
pathways is a significant component of the neural system needed to develop fluent reading.”
Reading Score vs. FA
Dyslexics (DD) – Controls (CT)
Reading Score vs. FA
Red indicates FA; Blue indicates CI
Klingberg et al. (2000) Neuron 25:493
High Resolution DTI and fMRI @ 1.5T:
Reading Score vs. FA
Significant regions projected to cortex surface
Summary
DWI
DWI ideal
ideal for
for all
all neuro
neuro screening.
screening.
DTI
DTI is
is aa subset
subset of
of DWI
DWI -- opens
opens new
new doors.
doors.
FA
FA sensitive
sensitive to
to WM
WM development
development /status.
/status.
Does
Does WM
WM FA
FA reflect
reflect status
status of
of regional
regional
GM
GM metabolism
metabolism -”Use
-”Use it
it or
or lose
lose it”?
it”?
Would
Would learning
learning increase
increase local
local FA?
FA?
MRGray visualization
courtesy B. Dougherty
Why
Why does
does DTI
DTI correlate
correlate with
with performance?
performance?
“Integrity”
“Integrity” ala
ala FA
FA means
means what?
what?
Effective
Effective neural
neural density?
density?
Structural
Structural defects
defects or
or potholes?
potholes?