Water Specific Magn
netization Tra
ansfer in Skeleetal Muscle ussing MT-IDEA
AL
Roberrt L. Janiczek1,2, Christopher D.J. Sin
nclair2, Giulio Gam
mbarota3, Xavier G
Golay2, Rexford D
D. Newbould4, and John S. Thornton2
1
Globaal Imaging Unit, GlaxoSmithKline,
G
London,
L
United Kiingdom, 2Institute of
o Neurology, Uniiversity College Loondon, London, Un
United Kingdom, 3P
Pharmaceutical and
Biologica
al Sciences, Univerrsité de Rennes 1, Rennes,
R
France, 4IImanova Ltd., Lonndon, United Kinggdom
Introoduction. A reduced magnetiization transferr (MT) effect has
h been obserrved in the muuscles of patiennts affected by neuromusculaar
diseasse1,2. This abno
ormal MT may
y be due to a combination
c
off lipid infiltratiion, edema, annd/or macromoolecular differences relative to
o
healthhy muscle. Ass lipids are kn
nown to have no MT effecct3, their preseence in a voxxel masks MT
T changes due to edema and
macroomolecular abnormalities. This work aimss to remove th
he MT dependdence on lipid concentration through the iintegration of a
chem
mical-species seeparation imag
ging technique,, Iterative Deccomposition off water and fatt with Echo A
Asymmetry andd Least-squarees
estim
mation (IDEAL)4, within an MT-weighted
M
im
maging acquissition. The new
w method, MT-IDEAL, calcuulates water-seeparated images
with aand without MT
M effects enabling the constrruction of an MT
M ratio (MTR)) map showingg only MT channges due to non-lipid sourcess.
Methods.
M
The measured signnal in a givenn voxel can bbe modeled as
a
∑
if
f
an
MT
pul
se
is
used,
or
o
,
if
no
MT
∑
p
pulse
is
used
d.
,
and
are
the
c
complex
water
r
signal
with
a
and
without
an
n
,
,
offf-resonance M
MT pulse, resppectively; is the complex fat signal; is
i
th
he echo-time; and Δ are tthe relative fatt peaks and freequency offsetss;
∗
an
nd
accounts ffor the local fiield off-resonaance, , and the
ap
pparent transveerse relaxationn rate4. The fatt signal, , andd off-resonance
term, , are uunaffected by the off-resonnance MT puulse. This MT
T
in
ndependence ennables the consstruction of a set of N coupledd equations,
0
∑
⋮
Figurre 1. MT-IDEAL in the calff of a patient with
w IBM.
The uupper panels show source images
i
withou
ut (a) and
with (b) MT con
ntrast. Panels (c) and (d) show the
calcuulated water image
and
,
,
,
respectively. In paanel (e) the resulting T2* generated
durinng the MT-IDE
EAL reconstrucction is shown alongside
the faat fraction map
p (f). In the lo
ower panels th
he MTR is
show
wn using a conv
ventional MT experiment
e
(g) and using
MT-IIDEAL (h). In
n the MT-IDE
EAL MTR map
p, a more
homoogenous MTR
R is calculated
d within the TA (gray
arrow
w) along with recovery
r
of thee MTR signal within the
gastroocnemius (w
white arrow) where seevere fat
replaccement has occcurred. The MTR
M
is only shown in
(g,h) for pixels witth greater than
n 15% water content
c
as
determ
mined by the fat
f fraction map
p.
Proc. Intl. Soc. Mag. Reson. Med. 20 (2012)
0
⋮
∑
⋮
0
,
⋮
x
,
,
∑
∑
0
where is the nnumber of colllected images. The set of couupled equation
ns
caan then be solvved using IDE
EAL reconstruuction5 and thee water isolated
MTR
M
calculatedd as MTR
.
,
,
,
All data were collecteed at 3T (Maagnetom TIM Trio, Siemen
ns
Healthcare, Gerrmany) using a 3D gradient echo sequencee. N=24 unique
ecchos ranging from 1.85 m
ms to 35.3 m
ms were colleccted over two
accquisitions, onee with and onee without an M
MT pulse. A coonventional MT
T
daataset was allso collected with a 1.855 ms echo-tiime. Both the
co
onventional M
MT and MT-ID
DEAL acquisiitions used a partial Fourieer
accquisition, 68 m
ms TR, 1.56 x 1.56 x 5 mm3 vvoxel size, GR
RAPPA factor =
2, and 20 partitioons for a total scan time of 2m
m 54s.
Results.
R
Repressentative MT-ID
DEAL results are shown in F
Figure 1 from a
paatient with thee neuromuscullar disease inclusion body m
myositis (IBM)).
Faat infiltration iin the gastrocnnemius results in a low MTR
R (white arrow
w,
Fiig. 1g) using a conventionnal MT acquuisition, while this effect is
i
markedly
m
reducced in the MT
T-IDEAL approach (Fig. 1hh). In healthy
yap
ppearing musccle, e.g. the tiibialis anteriorr (TA), similaar MTR valuees
were found by bboth sequences (grey arrow, F
Fig. 1g and 1h)).
Discussion. Thee specific absoorption rate lim
mits the minim
mum achievable
TR
R in MT imagging. The timee afforded by tthe longer TR has previously
y
beeen used to inccrease SNR thrrough averaginng of multiple ggradient echos6.
In
nstead, MT-ID
DEAL uses muultiple echoes to increase SNR as well as
a
caalculate the fatt fraction, T2*, and an MTR
R map that is uunbiased by the
prresence of fat with no scann time penaltyy. In combinaation with a T2
mapping
m
techniique, MT-IDE
EAL might bee used to diffferentiate fatty
y
in
nfiltration, fibroosis, and edem
ma in neuromuscular disorderss.
Reeferences.1McDanniel JD et al., JCA
AT 1999; 23(4): 609-14, 2Sinclair C
CDJ et al., J Neuro
ol
Neeurosurg Psychiattry 2011, 3Wolff SD et al., MRM 1989; 10(1):135--44, 4Reeder et al.,
5
6
MRM 2005;54 :6366-44, Yu et al., M
MRM 2007;26:1155-61, Gringel T. et al., JMRI 2009
9;
9(6): 1285-92
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