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![[1]
Stark
DD.
Radiology,
1991;
179:333–5.
[2]
Anderson
LJ,
et.
al.
Eur
Heart
J.,
2001;
22(23):2171-‐9.
[3]
Kellman
PK,
et.
al.
J
Cardiovasc
Magn
Reson.,
2013;
15(1):
56.
The
esSmate
SD’s
turned
out
to
be
significantly
close
to
the
measured
SD’s.
Below
are
the
acquired
phantom
images
and
their
associated
T2*,
measured
SD,
and
esSmated
SD
maps.
Reliable
idenSficaSon
of
T2*
abnormaliSes
is
limited
by
measurement
noise
which
Reduces
the
precision
of
T2*
esSmaSon.
Knowing
how
precise
a
T2*
measurement
is
important
for
prototyping
new
techniques
or
if
a
clinician
is
deciding
how
aggressively
to
treat
a
paSent
with
a
border
line
case.
We
propose
a
method
for
evaluaSng
the
precision
of
T2*
mapping
techniques
by
esSmaSng
the
standard
deviaSon
(SD)
of
each
measurement
thereby
creaSng
an
SD
map.
Evaluating precision of T2* mapping methods
for MRI tissue characterization
Christopher Sandino1, Hui Xue2, Peter Kellman2
The
standard
deviaSon
for
each
T2*
can
be
esSmated
by
esSmaSng
the
parameters’
(A,
T2*)
covariance
matrix
[3]:
All
methods
were
implemented
in
C++
so
that
they
can
be
tested
and
ran
on
an
MRI
scanner
using
medical
image
reconstrucSon
framework
Gadgetron.
Experimental
Valida/on
In
order
to
validate
the
SD
map
formulaSon,
we
made
six
gel
phantoms
made
of
50
mL
soluSons
of
1.5%
agarose
gel,
cupric
sulfate
(CuSO4),
saline
and
varying
concentraSons
of
Feridex
I.V.,
an
intravenous
contrast
agent
containing
colloids
of
iron
oxide.
The
phantoms
were
scanned
N=64
Smes
to
measure
the
“true”
SD
and
compare
them
to
the
esSmated
SD
on
a
per-‐pixel
basis.
Measured
and
esSmated
SD’s
were
compared
for
two
methods
of
T2*
mapping
(convenSonal
and
truncated)
and
both
showed
>99%
correlaSon.
Note
that
higher
SDs
are
biased
upwards.
These
correspond
to
curves
with
longer
T2*
which
aren’t
sampled
at
long
enough
echo
Smes.
In-‐vivo
studies
showed
relaSvely
high
SD
in
the
bloodpool,
the
heart-‐lung
interface,
and
areas
where
moSon
occurred.
The
figure
above
shows
a
possible
clinical
applicaSon
where
the
SD
map
is
used
to
mask
unreliable
pixels.
T2*
mapping
is
a
widely
used,
non-‐invasive
magneSc
resonance
imaging
(MRI)
technique
used
to
detect
iron
overload
in
Sssue.
High
concentraSons
of
iron
change
the
paramagneSc
properSes
of
the
Sssue
around
it,
including
its
T2*
-‐
the
transverse
relaxaSon
Sme
aler
an
electromagneSc
excitaSon
[1].
The
T2*
relaxaSon
curve
is
measured
by
sampling
MR
signal
intensiSes
at
various
echo
Smes
(TE)
[2].
We
know
that
this
curve
is
of
the
form
Yi=A*exp(-‐TEi/T2*),
so
we
can
use
an
exponenSal
regression
to
esSmate
T2*
for
each
pixel
thereby
creaSng
a
map
such
as
the
one
shown
to
the
right.
This
parScular
paSent
has
low
T2*
in
the
liver,
a
sign
of
iron
overload
due
to
β-‐thalassemia.
Introduc/on
Objec/ve
Methodology
0 5 10 15 20
0
1
2
3
4
5
6
7
8
Echo Time (ms)
SNR
Observed
Truth
Fitted
0 5 10 15 20
0
1
2
3
4
5
6
7
Echo Time (ms)
SNR
Observed
Truth
Fitted
Low
SNR
Low
T2*
y
=
1.0446x
-‐
0.0004
R²
=
0.99959
0
1
2
3
0
1
2
3
Es/mated
SD
(ms)
Measured
SD
(ms)
T2*
meas.
SD
meas.
SD
est.
time
(ms)
RF
1.6 3.9 6.2 8.5 10.8 13.2 15.5 17.8
Mul -echo phantom images
Observed
Non-Linear
Regression
Estimated
TE
y
TE
y'
TE
y-y'
Residuals
Robust
Estimation
of variance
med(abs(residuals))
Compute
Covariance
Matrix
Results
Good
Breath-‐hold
Poor
Breath-‐hold
T2*
map
SD
map
T2*
map
w/
mask
• SD
esSmaSon
efficiently
and
accurately
provides
a
way
to
analyze
T2*
precision
• Useful
for
prototyping
new
T2*
esSmaSon
methods
• PotenSally
useful
for
clinical
use
and
re-‐
determining
thresholds
for
disease
management
Conclusions
References
1Department of Electrical Engineering, University of Southern California, Los Angeles, CA 2Laboratory of Cardiac Energetics, National Heart, Lung, and Blood Institute, Bethesda, MD](https://image.slidesharecdn.com/sacnasposter-200327214751/85/Evaluating-precision-of-T2-mapping-methods-for-MRI-tissue-characterization-1-320.jpg)
Evaluating precision of T2* mapping methods for MRI tissue characterization
- 1. [1] Stark DD. Radiology, 1991; 179:333–5. [2] Anderson LJ, et. al. Eur Heart J., 2001; 22(23):2171-‐9. [3] Kellman PK, et. al. J Cardiovasc Magn Reson., 2013; 15(1): 56. The esSmate SD’s turned out to be significantly close to the measured SD’s. Below are the acquired phantom images and their associated T2*, measured SD, and esSmated SD maps. Reliable idenSficaSon of T2* abnormaliSes is limited by measurement noise which Reduces the precision of T2* esSmaSon. Knowing how precise a T2* measurement is important for prototyping new techniques or if a clinician is deciding how aggressively to treat a paSent with a border line case. We propose a method for evaluaSng the precision of T2* mapping techniques by esSmaSng the standard deviaSon (SD) of each measurement thereby creaSng an SD map. Evaluating precision of T2* mapping methods for MRI tissue characterization Christopher Sandino1, Hui Xue2, Peter Kellman2 The standard deviaSon for each T2* can be esSmated by esSmaSng the parameters’ (A, T2*) covariance matrix [3]: All methods were implemented in C++ so that they can be tested and ran on an MRI scanner using medical image reconstrucSon framework Gadgetron. Experimental Valida/on In order to validate the SD map formulaSon, we made six gel phantoms made of 50 mL soluSons of 1.5% agarose gel, cupric sulfate (CuSO4), saline and varying concentraSons of Feridex I.V., an intravenous contrast agent containing colloids of iron oxide. The phantoms were scanned N=64 Smes to measure the “true” SD and compare them to the esSmated SD on a per-‐pixel basis. Measured and esSmated SD’s were compared for two methods of T2* mapping (convenSonal and truncated) and both showed >99% correlaSon. Note that higher SDs are biased upwards. These correspond to curves with longer T2* which aren’t sampled at long enough echo Smes. In-‐vivo studies showed relaSvely high SD in the bloodpool, the heart-‐lung interface, and areas where moSon occurred. The figure above shows a possible clinical applicaSon where the SD map is used to mask unreliable pixels. T2* mapping is a widely used, non-‐invasive magneSc resonance imaging (MRI) technique used to detect iron overload in Sssue. High concentraSons of iron change the paramagneSc properSes of the Sssue around it, including its T2* -‐ the transverse relaxaSon Sme aler an electromagneSc excitaSon [1]. The T2* relaxaSon curve is measured by sampling MR signal intensiSes at various echo Smes (TE) [2]. We know that this curve is of the form Yi=A*exp(-‐TEi/T2*), so we can use an exponenSal regression to esSmate T2* for each pixel thereby creaSng a map such as the one shown to the right. This parScular paSent has low T2* in the liver, a sign of iron overload due to β-‐thalassemia. Introduc/on Objec/ve Methodology 0 5 10 15 20 0 1 2 3 4 5 6 7 8 Echo Time (ms) SNR Observed Truth Fitted 0 5 10 15 20 0 1 2 3 4 5 6 7 Echo Time (ms) SNR Observed Truth Fitted Low SNR Low T2* y = 1.0446x -‐ 0.0004 R² = 0.99959 0 1 2 3 0 1 2 3 Es/mated SD (ms) Measured SD (ms) T2* meas. SD meas. SD est. time (ms) RF 1.6 3.9 6.2 8.5 10.8 13.2 15.5 17.8 Mul -echo phantom images Observed Non-Linear Regression Estimated TE y TE y' TE y-y' Residuals Robust Estimation of variance med(abs(residuals)) Compute Covariance Matrix Results Good Breath-‐hold Poor Breath-‐hold T2* map SD map T2* map w/ mask • SD esSmaSon efficiently and accurately provides a way to analyze T2* precision • Useful for prototyping new T2* esSmaSon methods • PotenSally useful for clinical use and re-‐ determining thresholds for disease management Conclusions References 1Department of Electrical Engineering, University of Southern California, Los Angeles, CA 2Laboratory of Cardiac Energetics, National Heart, Lung, and Blood Institute, Bethesda, MD