Bernhard Gruber1,2 and Elmar Laistler1
1Division MR Physics - Center for Medical Physics and Biomedical Engineering, Medical University Vienna, Vienna, Austria, 2Massachusetts General Hospital, Harvard Medical School, A. A. Martinos Center for Biomedical Imaging, Charlestown, MA, United States
Synopsis
Coaxial coils as highly flexible elements of RF
coil arrays have been proposed, and superior inter-element decoupling over
standard loops has been reported, but so far without convincing explanation of
the underlying mechanism. In an attempt for an explanation measurements of the
magnetic and electric coupling coefficients for coaxial and standard loop coils
were performed, and showed differences in the S21
findings and the coupling parameters, which are topic of further
investigations.
Introduction
MRI phased arrays suffer from
inter-element coupling, restricting coil design to fixed array layouts and
limit the patient variability to be investigated. Coaxial coil elements [1] [2] offer higher
flexibility and, therefore, increased filling factor compared to conventional
coils, directly affecting SNR. Coaxial coil elements have been reported to show
less interaction between adjacent elements [1] [2]. The mechanism why
coaxial coils would (if at all) exhibit inherently lower inter-element coupling
has not yet been explained satisfactorily. Reduced electric coupling due to a
shielding effect of the outer shield of the coaxial cable [2] and more efficient
preamp decoupling for coaxial coils [1] were suggested. To add to the discussion and
investigate whether coaxial coils exhibit a different behavior w.r.t. magnetic
and electric coupling leading to a similar effect to that presented for “self-decoupled”
coils [3] we present measurements
of the magnetic and electric coupling coefficients for coaxial and standard
loop pairs.Methods
Magnetic and electric coupling
coexist between adjacent resonators and a transmission zero at frequency fM is measured where the two contributions
cancel. The dominant type of coupling can be derived from the position of fM relative to the resonance
frequency f0 (fM < f0: magnetic
coupling dominant, fM > f0:
electric coupling dominant). By using the two-mode-frequency method, the resonant-mode-splitting
frequencies fodd and feven can be observed, as
soon as the two loops are over-coupled [4]
[5].
The total coupling coefficient $$k_{total} = (k_m - k_e) / (1-k_m k_e)$$, magnetic coupling $$k_m = 1/2 * (((f^2_{odd} - f^2_0) / (f^2_{odd} - f^2_m)) + ((f^2_{even} - f^2_0) / (f^2_m - f^2_{even})))$$ and electric
coupling $$k_e = (f^2_m / 2f^2_0) * (((f^2_0 - f^2_{odd}) / (f^2_m - f^2_{odd})) + ((f^2_0 - f^2_{even}) / (f^2_{even} - f^2_m)))$$
, can
be derived from fodd, feven, fM, and f0 (see Fig. 3) [5].
Two coaxial and two
standard loops with diameters of 40 mm and a resonance frequency of 242.6 MHz
(self-resonance of the coaxial elements (U =
2πr = 125 mm); see
Fig. 2) and
matched to 50 ohm. The coaxial loops used a 3.4 pF ceramic SMD capacitor Cm. The standard loops
were tuned using a 1.4-3.0 pF tuneable capacitor Ct and matched
using a 1.7 pF capacitor Cm. The four loops were used to observe coupling (ktotal, kE, kM) between them. Qloaded and Qunloaded values were
acquired using a decoupled (-75 dB) double loop probe at 2 cm distance to the loop
elements. A 4 mm polyurethane foam piece was placed between the loops and a loading
phantom (5 liter H20 + NaCl, 1 mL/L Gd, DC conductivity = 0.2 S/m).
Measurements were performed on a VNA (E5071C, Keysight Technology) with the
coil interfaces on opposite sides of the coil elements, or parallel to each
other (Fig. 1).Results
The S21 decoupling of the standard loops is in general
slightly better compared to the coaxial loops (see Fig. 1). The optimal overlap
is achieved at the same relative position for both coil types. The Q-ratio = Qunloaded / Qloaded
was 58 / 23 = 2.5 for coaxial elements and 109 / 27 = 4.0 for standard
loops.
The orientation of the
interfaces for the coaxial loops changes km
and ke by 37 to 76% whereas
only little change is found for the standard loops. For the total coupling
coefficient ktotal, this pronounced difference is not observed. Standard coils show an overall higher ktotal of 37 to 70% in all
investigated configurations (See Fig. 3).Discussion and Conclusion
The findings for the coupling coefficients would
indicate lower coupling for coaxial coils, however, the measurement of S21 shows the exact opposite.
The reason for this contradictory result is still unclear and will be further
investigated. The higher Q-ratio
for standard loops could
be explained by increased coil noise for the coaxial elements. Given that these
measurements have only been performed at one coil size and frequency, and at
the self-resonance of the coaxial coils, in addition to the contradictory
results for k and S21, more
in-depth experiments are required.Acknowledgements
This work was supported by OENB grant #17980.References
[1]
B. Zhang, D. K. Sodickson and M. A. Cloos, "A
high-impedance detector-array glove for magnetic resonance imaging of the
hand," Nature Biomed. Eng., 2018.
[2]
T. Ruytenberg, A. Webb and I. Zivkovic,
"Shielded-coaxial-cable coils as receive and transceive array elements
for 7T human MRI," Magn. Reson. Med., pp. 1-12, 2019.
[3]
X. Yan, J. C. Gore and W. A. Grissom, "Self-decoupled
radiofrequency coils for magnetic resonance imaging," Nat. Commun., no.
9, p. 3481, 2018.
[4]
J. S. Hong and M. J. Lancaster, "Couplings of
Microstrip square open-loop resonators for cross-couple planar microwave
filters," IEEE Transactions on Microwave theory and techniques, no.
44(12), pp. 2099-2109, 1996.
[5]
Q. X. Chu and H. Wang, "A compact open-loop filter
with mixed electric and magnetic coupling," IEEE Transactions on
Microwave theory and techniques, no. 56(2), pp. 431-439, 2008.