MEASUREMENT SCIENCE REVIEW, Volume 14, No. 3, 2014
171
Compa ing Saddle, Slo ed- ube and Pa allel-pla e Coils
o Magne ic Resonance Imaging
D. Nespo
1
, K. Ba usek
2
, Z. Dokoupil
2
1
Depa men o Theo e ical and Expe imen al Elec ical Enginee ing,
B no Uni e si y o Technology, Kolejni 2906/4, 612 00 B no, Czech Republic
2
Ins i u e o Scien i ic Ins umen s, Academy o Sciences o he Czech Republic,
K alo opolska 147, 612 64 B no, Czech Republic
The pape is conce ned wi h a compa ison o he p ope ies o RF coils o h ee con igu a ions o MRI measu emen s on small
animals. In compa ison wi h he classical saddle coil he p oposed modi ica ion o slo ed- ube coil exhibi s iden ical homogenei y
o B
1
ield in a la ge space. The pa allel-pla e coil has a sa is ac o y homogenei y o B
1
ield o e he whole in e nal space. The
signal- o-noise a io measu ed o all h ee coils is oughly he same and is gi en by he magni ude o RF p e-ampli ie noise. As
he slo ed- ube and pa allel-pla e coils ha e a lowe induc ance compa ed wi h he saddle coil, hey can be uned o esonance on
he 200 MHz equency e en a la ge dimensions. The esul s show ha he pa allel-pla e coil has e y good p ope ies o he
measu emen o small animals.
Keywo ds: MRI measu emen s, RF coils, homogenei y o B
1
, nume ical analysis.
1.
I
NTRODUCTION
HE
MRI (magne ic esonance imaging) sys em, which
can p oduce high-quali y images in an a bi a y c oss
sec ion o he human body, has been ecognized as a
new powe ul echnique o medical diagnosis and has
g adually come o be employed in p ac ical si ua ions. In he
MRI sys em, an RF p obe is used o emi a uni o m RF
magne ic ield o e he human body and ecei e he
magne ic esonance signal om he body o imaging.
Se e al kinds o RF p obes ha e been de eloped. The
ans e se elec omagne ic bi dcage esona o is commonly
used o MRI. The cha ac e is ics o he bi dcage esona o ,
including he e ec s o a conduc ing shield, ha e been
analyzed by using he bounda y elemen me hod. In his
wo k a mul iconduc o ansmission line (MTL) model is
de eloped and success ully applied o analyze a 12-elemen
unloaded and loaded mic os ip line ans e se
elec omagne ic (TEM) esona o coil o animal s udies [1].
To p oduce a uni o m B
1
ield along he coil (z-di ec ion), a
mul i-sec ion al e na ing impedance mic os ip elemen was
in es iga ed in Akgun [2]. The wid hs o he mic os ip line
a e a ied o p oduce an al e na ing low-high impedance
con igu a ion. Simula ion and expe imen al esul s a e
compa ed o mic os ip esonan elemen s.
The slo ed- ube esona o (STR) is also used o MRI
applica ions [3]-[6]. Many e o s ha e been made o
analyze he EM pa ame e s o he slo ed- ube esona o in
o de o show he p ope ies o he p obe and design an
op imum s uc u e [3], [6]. In Ben Ahmed and Feham [7],
[8] igo ous analy ical exp essions o he EM pa ame e s o
he shielded STR wi h a ci cula c oss sec ion ha e been
ob ained using he ini e elemen me hod (FEM) and me hod
o momen s (MoM) in wo dimensions, bu wi hou aking
in o accoun he in luence o he hickness o he STR.
A saddle RF coil is used in MR sys ems. Single-channel
sys ems [9], [10] equi e an a angemen o coils ha co e s
he ull ield o iew (FOV) o he imaged objec . I is an
impo an conside a ion o ob ain a uni o m and comple e
image o he objec .
T adi ional clinical 1.5 T magne s a e being eplaced by
3 T, while esea ch magne s o 7 T, 8 T, and 9.4 T e e ed
o as high ield magne s a e being used o animal and
human expe imen s [7]-[14]. The g ea appeal o high ield
MRI is i s imp o ed image quali y and esolu ion [15]. The
signal- o-noise a io (SNR) co ela es in app oxima ely
linea ashion wi h ield s eng h, being oughly wice as
la ge a 3 T as a 1.5 T [16].
A pa allel-pla e wa eguide was buil and used o acqui e
images o a heal hy olun ee ’s leg a 3 T on a clinical MR
image [17]. Wa eguides ha e been success ully used o
gene a e magne ic esonance images a 7 T o whole-body
sys ems [18]. F om hese esul s, i has been es ablished ha
wa eguides a e only sui able o 7 T sys ems wi h wide
bo es o a leas 60 cm. This is mainly due o he cu -o
equency o he cylind ical wa eguides used. To o e come
his limi a ion, a pa allel-pla e wa eguide was employed
since i s cu -o equency depends on he sepa a ion o he
pla es. Zanche [19] has in es iga ed he possibili y o
inc easing he SNR and homogenei y o he RF magne ic
ield B
1
, o cha ac e ize mic o luidic low wi h di ec
de ec ion. To enable his, he used a pa allel-pla e RF
esona o wi h pu e phase spa ial encoding.
A uni o m B
1
and a s ong B
1
pe uni cu en a e c i ical
o magne ic esonance imaging. To design a modi ied RF
p obe esona o , nume ical simula ion in a wide- equency
band is o en equi ed. The pa allel pla e esona o has been
well in es iga ed in non-MR esea ch a eas [21], [22], and
he magne ic ield B
1
( ) be ween he pa allel pla es has been
shown o be homogeneous [20], [24].
2.
F
ORMULATION OF THE PROBLEM
The aim o his wo k is o modi y he slo ed- ube
esona o [6] and p opose a pa allel-pla e RF coil o MR
mic oscopy and measu emen o small animals o plan s.
T
10.2478/ms
-
2014
-
0023
MEASUREMENT SCIENCE REVIEW, Volume 14, No. 3, 2014
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Cha ac e is ics such as he homogenei y o he RF magne ic
ield and achie able signal- o-noise a io a e be e
compa ed wi h he classical saddle coil. Some heo e ical
esul s a e compa ed wi h he measu ed da a o con i m he
alidi y o he p esen simula ion.
To compa e he modi ied slo ed- ube esona o and
pa allel-pla e RF coil wi h he classical saddle coil, an
analysis o he RF magne ic ield was pe o med o all he
abo e coil con igu a ions, using he nume ical analysis. The
esul o he analysis was an op imum coil con igu a ion
wi h a minimum inhomogenei y o he ield B
1
, which is
shown in Fig.1. Fo he pu pose o expe imen al
compa ison, h ee MR p obes we e made wi h coils as pe
Fig.1.
Fig.1. Schema ic o RF coils unde examina ion: a) saddle coil,
b) slo ed- ube coil, and c) pa allel-pla e coil.
The saddle coil was made o sil e -pla ed coppe
conduc o o 1.4 mm in diame e . The slo ed- ube and
pa allel-pla e coils we e made o sil e -pla ed coppe oil
0.05 mm hick. The coil angle Φ was op imized ia
nume ical calcula ion o ob ain minimum inhomogenei y o
B
1
and is Φ = 70°. This alue is in ag eemen wi h he esul
gi en in [6].
Fo all he RF coils, he B
1
ield maps we e measu ed
using he MRI me hods, and compa ed wi h nume ical
calcula ion. Two main c i e ia we e used in he compa ison,
namely he magni ude/size o he space o ield
inhomogenei y below ∆B
1
< 10 % and 20 %, and he size o
he signal- o-noise a io in he image. Fo applica ions o RF
coils in MR expe imen s in which phase MR images a e
used he maximum phase de ia ion in he image was
e alua ed.
3.
M
ETHOD
The nume ical analysis o he EM ield o coils was sol ed
in wo ways: i s as an elec omagne os a ic model and hen
as an RF model. The concep o magne os a ic nume ical
analysis has been desc ibed in [23]. The ad an age o he
elec omagne os a ic model is i s simplici y and good
solu ion con e gence in he analysis. The
elec omagne os a ic model can only be used in cases when
he wa eleng h o he EM ield is much la ge han he coil
unde analysis. I he magni ude o he EM ield wa eleng h
comes close o he coil dimensions, he nume ical analysis
esul s will no longe co espond o he ac ual EM ield
dis ibu ion.
Fig.2. Compa ison o magne ic induc ion dis ibu ion nea ci cula
single- u n coil wi h cen e -line leng h 39 mm, calcula ed ia EM
and RF nume ical analyses o di e en wa eleng hs λ = 1.5, 0.6
and 0.3 m.
Fig.3. Geome y o nume ical RF analysis o plana single- u n
coil.
Fig.2. gi es he dis ibu ion o magne ic induc ion B o
he ci cula single- u n coil o cen e -line leng h 39 mm. The
dis ibu ion o magne ic induc ion is shown in a c oss-
sec ion pe pendicula o he coil plane. Fig.2. a) shows
MEASUREMENT SCIENCE REVIEW, Volume 14, No. 3, 2014
173
magne ic induc ion dis ibu ion sol ed using he magne o-
s a ic model. Figs.2. b), c) and d) show magne ic induc ion
dis ibu ion sol ed using he RF model o di e en
wa eleng hs. I is ob ious om Fig.2. ha o he
wa eleng h λ – 1.5 m (200 MHz) he dis ibu ion o
magne ic induc ion ma ches up wi h he esul s ob ained
using he s a is ical analysis. Fo he wa eleng hs λ = 0.6 m
(500 MHz) and λ = 0.3 m (1 GHz), howe e , he dis ibu ion
o magne ic induc ance is di e en .
The geome y o nume ical RF analysis o plana single-
u n coil is gi en in Fig.3. The geome ic con igu a ion o
he model is made up o a coil, which is o med by he
acuum. The bounda y condi ions o his coil a e de ined as
an impedance medium and a e ep esen ed by (1), whe e µ
is he pe meabili y, ε is he pe mi i i y, H is he magne ic
ield in ensi y, E is he elec ic ield in ensi y, γ is he
elec ic conduc i i y, ω is he angula equency, and n is he
no mal ec o .
0)(
0
0
=⋅−+×
−
nEnEHn
ω
γ
εε
µµ
j
(1)
EM ield exci a ion is p o ided using a po wi h lumped
pa ame e s. The coil is complemen ed wi h addi ional
capaci ance o uning he esonance equency
o he
equency o NMR sys em = 200 MHz. The coil is placed
in he compu a ion space and his space ends in pe ec ly
ma ched laye s (PML). PMLs p e en e lec ions and hus
simula e an open space.
In he i s pa o he RF model he Eigen- equency
analysis was used o de e mine he i s esonance equency
o he coil being modeled. Based on his analysis, he
magni ude o addi ional capaci ance was de ined o uning
he esonance equency
o he equency o he NMR
sys em. Addi ional capaci ance was de ined as elemen s
wi h de ined ela i e pe mi i i y ε
.
When sol ing he Eigen- equency analysis, equa ion (2)
was used, whe e he imagina y pa ep esen s Eigen-
equency, and he eal pa ep esen s a enua ion.
δ
ω
λ
+
−
=
j
(2)
In he second pa o he RF model he wa e equa ion (3),
whe e k is he wa e numbe , and he Maxwell equa ion (4)
we e used in he analysis o he high- equency
elec omagne ic ield o he coils de ined.
0)()(
0
2
0
1
=−−×∇×∇
−
EE
ωε
γ
εµ
jk
(3)
0
=
−
=
×
∇
HE
ωµ
j
(4)
The map o ield B
1
was measu ed using he spin-echo
echnique wi h he il angle α = 45°. The ield B
1
was
calcula ed acco ding o he ela ion
γτ
)a csin(
0
1
S
S
B=
(5)
Whe e S and S
0
a e he in ensi ies in he image o il
angles o 45° and 90°, espec i ely.
4.
E
XPERIMENT
The esul o calcula ing he map o magne ic induc ion
B
1
(x, y) o all he coils acco ding o Fig.1. and using he EM
me hod is gi en in Fig.4. The maps o B
1
(x, y) a e in he
cen e o he coil (z = 0) in a plane pe pendicula o he coil
axis. Fig.5. gi es he isolines o he ield B
1
, wi h he maps
being no malized o he alue B
1
in he cen e o he map.
The le els o ield B
1
a e 0.8 B
1
, 0.9 B
1
, B
1
, 1.1 B
1
, and 1.2
B
1
.
Fig.4. Map o magne ic induc ion B
1
(x, y) o es coils acco ding
o Fig.1. and using he EM me hod: a) saddle coil, b) slo ed- ube
coil, and c) pa allel-pla e coil.
The expe imen s we e conduc ed on a Magnex 4.7 T
200 mm i.d. ho izon al bo e supe conduc ing magne a
200 MHz a he Ins i u e o Scien i ic Ins umen s. A wa e -
cooled Magnex 150 mm i.d. g adien se was employed,
d i en by IECO GPA 200-350 ampli ie s, which can
p o ide 180 mT/m maximum g adien s. All expe imen s
we e pe o med wi h an MR Solu ion console. The da a
measu ed we e p ocessed using he MAREVISI and
MATLAB p og ams.
One sample was measu ed by all he coils, which was a
cylinde wi h a diame e o 20 mm and a leng h o 40 mm,
illed wi h a solu ion o deionized wa e o he ollowing
concen a ion: 1 li e o wa e , 1.2 g ams o NiSO
4
and 2.6
g ams o NaCl in o de o sho en he elaxa ion imes
o T
1
= T
2
= 130 ms. The sample akes up he whole space o
he RF coil because in o de o compa e he RF coils, we
wan o measu e he map o ield B
1
o e he whole c oss
sec ion inside he coil.
MEASUREMENT SCIENCE REVIEW, Volume 14, No. 3, 2014
174
Fig.5. Simula ion o he B
1
ield dis ibu ion depic ed as
equipo en ial le els, a) saddle coil, b) slo ed- ube coil, and
c) pa allel-pla e coil. Field le els o B
1
a e 0.8 B
1
, 0.9 B
1
, B
1
,
1.1 B
1
, and 1.2 B
1
.
Fig.6. Map o magne ic induc ion B
1
in a) saddle coil, b) slo ed-
ube coil, c) pa allel-pla e coil.
The spin-echo me hod (T
E
= 17 ms, T
R
= 1s) was used o
measu e 11 ans e sal slices 2 mm hick and 5 mm apa
om each o he . The FOV size/magni ude was 30x30 mm
(128 x 128 px). In he calcula ion o he map o B
1
acco ding
o (5), wo images wi h il angles α = 45° and 90° we e
measu ed. Fig.6. gi es B
1
ield maps o all he
con igu a ions o RF coils. Fig.7. shows he isolines o ield
B
1
, wi h he ield maps no malized o he alue o B
1
in he
cen e o he map. The le els o ield B
1
a e 0.8 B
1
, 0.9 B
1
,
B
1
, 1.1 B
1
, and 1.2 B
1
.
Fig.7. Equipo en ial le els o B
1
o : a) saddle coils, b) slo ed- ube
coils, c) pa allel-pla e coils.
E alua ed om he maps o ield B
1
we e he signal- o-
noise a io, size o he homogeneous ci cula and ec angula
a ea o 10 % inc ease and dec ease o B
1
, and o he
maximum phase change in he image. The esul s a e
summa ized in Table 1.
Table 1. Calcula ed pa ame e s o RF coils
Coil Saddle Slo ed- ube Pa allel-
pla e
Signal o noise
a io 57.2 64.4 49.7
Ac i e coil
esis ance
( = 200MHz) [Ω]
3.22 0.82 0.51
Induc ance L
( = 200MHz)
[µH]
0.23 0.051 0.048
Homogenei y
(±10 %):
ci cle wi h
diame e [mm]
8.9 9.8 20
Homogenei y
(±10 %):
ec angle
dimensions [mm]
17.7 x 4.5 11.5 x 10.4 20 x 20
∆Φ [
0
] 7.4 9.8 3.5
Using he pa allel-pla e coil he spin-densi y MR images
o biological issues (chicken wing and eupho bia) aking up
he whole space wi hin he coil we e measu ed. The SE
me hod was used, wi h echo ime T
E
= 17 ms, epea
ime T
R
= 3 s, slice hickness 3 mm, ield o
iew 40 x 40 mm, 256 x 256 px o he chicken wing and
30 x 30 mm, 128 x 128 px o eupho bia.
MEASUREMENT SCIENCE REVIEW, Volume 14, No. 3, 2014
175
Fig.8. Spin-densi y MR imaging o biological issue, a) chicken
wing (SE me hod, T
E
17 ms, 40 x 40 mm, 256 x 256 px),
b) eupho bia (SE me hod, T
E
= 17 ms, 30 x 30 mm, 128 x 128 px).
4.
R
ESULTS AND DISCUSSION
Signal- o-noise a io is he a io o he mean alue o
image in ensi y in a selec ed space o he s anda d de ia ion
o noise in he space ou side o he image. I ollows om
he measu emen ha he signal- o-noise a io ob ained is
oughly he same o all he coils. Table 1 gi es he ac i e
esis ances R and induc ances L o all coils wi h inse ed
sample, measu ed on an RLC analyze o he equency
= 200 MHz. F om he measu emen i ollows ha he
image noise is no due o he noise o he ac i e esis ance o
RF coil bu is gi en by he p ope ies o he MR p e-
ampli ie . The induc ances o slo ed- ube and pa allel-pla e
coils a e ma kedly lowe compa ed o he saddle coil. Bo h
hese coils can he e o e be uned a he 200 MHz equency
e en o la ge dimensions and hey can be used, o
example, o measu e small animals.
The homogeneous space o he pa allel-pla e coil on a
ci cle o 20 mm in diame e is smalle han + 10 % o B
1
.
The inhomogeneous phase in a ci cula space o 20 mm in
diame e is up o 10°. F om bo h he expe imen al
measu emen and nume ical modeling i is e iden ha o
he measu emen o small animals and biological objec s he
pa allel-pla e coil is o g ea e ad an age han he saddle o
slo ed- ube coil.
5.
C
ONCLUSION
The pape p esen s a compa ison o he p ope ies o RF
coils o h ee con igu a ions o MRI measu emen s on
small animals. The wo newly p oposed con igu a ions,
modi ied slo ed- ube and pa allel-pla e coils, ha e unde
iden ical homogenei y o ield B
1
a la ge exploi able
applica ion a ea ( ield o iew). The wo RF coils ha e he
same SNR, which is due o he noise o RF p e-ampli ie .
Since he slo ed- ube and pa allel-pla e coils ha e a lowe
induc ance compa ed wi h he saddle coil, hey can a la ge
dimensions be uned o esonance a he 200 MHz equency
e en i hey a e o la ge dimensions. Resul s o he
compa ison indica e ha he pa allel-pla e coil is o
ad an ageous p ope ies when measu ing small animals.
A
CKNOWLEDGMENT
The wo k desc ibed in he pape was inancially suppo ed
by he esea ch p ojec GACR 13-09086S, esea ch plan
MSM 0021630513 and p ojec o he BUT G an Agency
FEKTS-10-13.
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