Nikolai I. Avdievich1, Ioannis A. Giapitzakis1, and Anke Henning1,2
1Max Planck Institute for Biological Cybernetics, Tübingen, Germany, 2Institute of Physics, Ernst-Moritz-Arndt University, Greifswald, Germany
Synopsis
At ultra-high fields
(UHF, >7T) a simple increase of the length of a single-row human head
transmit (Tx) phased array cannot provide an adequate longitudinal coverage for
the whole brain imaging. Multi-row (>2) arrays together with RF
shimming have to be used instead. In this work, we constructed a 9.4T
(400 MHz) 16-loop double-row transceiver array based on the analytical modeling.
We demonstrated that simply by overlapping a very good decoupling can be obtained
without additional decoupling strategies. This provides a recipe of a simple,
robust, and very Tx-efficient design for parallel transmission and whole brain
imaging at UHFs.
Purpose:
To optimize the transmit (Tx)
performance of a human head transceiver (TxRx) phased array and provide for the
longitudinal coverage suitable for the whole brain imaging at 9.4T.Introduction:
It
has been previously demonstrated for ultra-high field (UHF, >7T)
human brain imaging that a simple increase of the length of a single-row
Tx-array cannot provide an adequate longitudinal coverage (along the magnet
axis) of a whole brain (1-4). Multi-row (2 and above) arrays together with RF
shimming have to be used instead (2,3,5). Also tight-fit surface loop
TxRx-phased arrays (2,4) improve Tx-efficiency in comparison to Tx-only arrays
(5), which are larger to fit smaller multi-channel Rx-only arrays inside. Previously,
based on the analytical modeling (6) we demonstrated that at 9.4T both the magnetic and electric
coupling between two heavily loaded loops can be compensated at the same time
simply by overlapping, and excellent decoupling can be obtained between adjacent
loops of a single-row 8-element human head tight-fit TxRx-array without
additional decoupling strategies (7). In this work, we
extended this idea by constructing a 9.4T
tight-fit human head 16-loop double-row (2x8) TxRx-array decoupled entirely by
overlapping. The developed transceiver array provides efficient transmission and the whole
brain coverage at the same time.Methods:
Loop
size (10.5 cm x 10 cm) was first evaluated analytically (Fig.1) and then
adjusted on a bench. The array (Fig.2) measures 20 cm in width (left-right), 23
cm in height (anterior-posterior), and 17.5 cm in length. Overlapping provides
very good decoupling (Fig.2D). To decrease radiation losses the array is
shielded with the cylindrical shield located at 4-cm distance from the coil
elements. Loaded Q-factor, QL, measured between
17 (bottom) and 30 (top), which corresponds to a QU/QL
ratio between 8 and 14. We compared the performance of the tight-fit 16-channel
TxRx-array with a larger 16-element Tx-only/ 30-element Rx-only (ToRo) surface loop
phased array (28 cm – diameter, 19 cm - length) described previously for human
head application at 9.4T (5). Electromagnetic (EM) simulations of the transmit B1+ and the
local specific absorption rate (SAR) were performed using CST Studio Suite 2015
(CST, Darmstadt, Germany) and the time-domain solver based
on the finite-integration technique (FIT). Three
voxel models were used, i.e. a head/shoulder (HS) phantom (5), which was constructed to match tissue properties at 400 MHz (ε = 58.6, σ =0.64 S/m),
and two virtual family multi-tissue models, “Duke” and “Ella”. Experimental
B1+ maps were obtained using the AFI sequence (8). All
data were acquired on a Siemens Magnetom 9.4T human imaging system.Results and Discussion:
After performing safety evaluation of the array,
which included EM modeling (B1+,
SAR) and phantom imaging, we conducted in-vivo experiments (Fig.3). The
16-channel tight-fit TxRx-array provided ~40% improvement in B1+ efficiency
(B1+/√P) compared to the larger 16-channel To- /
30-channel Ro-array, which agrees well with results
obtained from EM simulations that indicate a ~2 fold increase in the RF power
deposited into the tissue. Circular polarized (CP) mode (45º phase
shift between adjacent loops in each row; 22.5º - between the rows)
doesn’t provide a very homogeneous B1+ field at UHFs
(Figs3A, 3B). However, introduction of a 70º-phase shift between the rows
(Fig.3C) provides an improvement of the homogeneity (9). In-vivo images (Fig.4A)
demonstrate coverage of the whole brain. Figs 4B and 4C show a comparison of
the SNR maps obtained in-vivo using both arrays. While the 30-element Ro-array
still has a better peripheral SNR, SNRs in the center of the head are similar. We
also evaluated the dependence of the maximum local SAR10g
(10g-averaged) on the phased shift, φ, introduced between the rows for the homogeneity
improvement (Fig.5) (9). At φ~70º the maximum SAR10g
value decreased by 13% as compared to the CP Mode. Additional drop (~30% in total) of
the maximum SAR10g can be obtained by unbalanced driving of the array
(Fig.5C) as suggested in (9). Conclusion:
We constructed a 9.4T
(400 MHz) 16-loop (2x8) overlapped transceiver head array based on the results
of the analytical modeling. We demonstrated that simply by overlapping a very
good decoupling can be obtained without additional decoupling strategies. This
provides a recipe for a simple, robust as well as very Tx-efficient design suitable
for parallel transmission (pTx) and whole brain imaging at UHFs.Acknowledgements
No acknowledgement found.References
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