NOTE

Human Brain Imaging at 9.4 T Using a Tunable PatchAntenna for Transmission

Jens Hoffmann,1 G. Shajan,1 Juliane Budde,1 Klaus Scheffler,1,2 and Rolf Pohmann1*

For human brain imaging at ultrahigh fields, the travelingwave concept can provide a more uniform B1

1 field over alarger field of view with improved patient comfort comparedto conventional volume coils. It suffers, however, from limitedtransmit efficiency and receive sensitivity and is not readilyapplicable in systems where the radiofrequency shield is toonarrow to allow for unattenuated wave propagation. Here, thenear field of a capacitively adjustable patch antenna for exci-tation is combined with a receive-only array at 9.4 T. Theantenna is designed in compact size and placed in closeproximity to the subject to improve the transmit efficiency innarrow bores. Experimental and numerical comparisons toconventional microstrip arrays reveal improved B1

1 homoge-neity and longitudinal coverage, but at the cost of elevatedlocal specific absorption rate. High-resolution functional andanatomical images demonstrate the use of this setup for invivo human brain imaging at 9.4 T. Magn Reson Med000:000–000, 2012. VC 2012 Wiley Periodicals, Inc.

Key words: traveling wave imaging; ultrahigh field; B1

inhomogeneity; receive array; patch antenna

Imaging the human brain at 9.4 T is affected by the highLarmor frequency, which causes an even more complexbehavior of the radiofrequency (RF) field than at 7 T.Because of the short wavelength and the increased con-ductivity of brain tissue, the B1

þ field of conventionalcircularly polarized volume coils becomes unacceptablyinhomogeneous (1). In addition, volume coils that arebased on a single row of half-wavelength resonators haveto be shorter and can no longer cover the whole brain.

Using static (2) or dynamic (3) parallel transmissionthrough an array of independent RF coils to aim for ho-mogeneous excitation (1,4) in localized brain regionsmakes the experiment more difficult due to the complex-ity of the hardware and the need for time consumingprescans, pulse calculations, and a reliable prediction ofthe local energy deposition.

At 7 T, traveling wave imaging (5–7) was presented asa simple RF concept that can potentially provide a morehomogeneous excitation over a larger field of view (FOV)compared to conventional volume coils (8) using the RFshield of a whole-body magnet as waveguide. It suffers,however, from relatively poor transmit efficiency (8–10)

and receive sensitivity (8,11). Dedicated receive-onlyarrays (9,10,12) can increase the sensitivity and open thepossibility of applying parallel reception techniques.

Lossless field propagation requires magnet bores withsufficiently large diameters to obtain a cutoff frequencybelow the Larmor frequency (7). Although the bore sizeof our 9.4 T scanner would easily support propagatingwaves, its narrow head gradient insert prevents travelingwave imaging in the proper sense, causing an exponen-tial decay of the field with increasing distance from theexcitation source. In addition, the wavelength becomesinfinite, so that the phase does no longer vary along thez-direction. The transverse pattern of the electromagneticfields, however, remains unchanged compared to thepropagating wave (13,14). This indicates that the favor-able B1

þ pattern of the TE11 mode can also be exploitedin narrow bores. The attenuation between antenna andsubject can be mitigated by reducing the distance, whilein the part of the bore where the body is present, travel-ing wave behavior may be observed due to the reducedcutoff frequency (7,15).

In this study, a simple RF setup was designed to com-bine the advantages of patch antenna excitation withthose of local, multichannel signal reception by con-structing a compact tunable antenna that can be quicklyadjusted to different loads and is placed close to thesubject (16). It was combined with a 24-channel receive-only array (17) that provides highly sensitive parallelsignal reception from the entire brain, as demonstratedby high-resolution functional and anatomical in vivohuman brain images at 9.4 T.

METHODS

Setup

Experiments were conducted on a 9.4 T whole-body MRsystem (Siemens Healthcare, Erlangen, Germany) with abore diameter of 82 cm, equipped with an AC84 head-only gradient insert with an RF shield diameter of 40cm. This corresponds to a cutoff frequency of roughly440 MHz, yielding a drop-off in RF magnitude of 3 dBover 18 cm in the empty RF shield (13,14).

Although a variety of patch antenna designs can befound in the literature, most traveling wave MR studiesuse circular patch antennas similar to the one describedin Ref. 18. This design does not allow reacting to differentloading conditions, as necessary due to load variationscaused by differences in the shape or position of the sub-jects if the near field of the antenna is to be used. Further-more, the diameter of the radiating patch has to be rela-tively large if standard, well machinable substrates (e.g.

1Max Planck Institute for Biological Cybernetics, High-Field MagneticResonance Center, Tubingen 72076, Germany.2Department for Biomedical Magnetic Resonance, University of Tubingen,Tubingen 72076, Germany.

*Correspondence to: Rolf Pohmann, Max Planck Institute for BiologicalCybernetics, High-Field Magnetic Resonance Center, Spemannstr. 41,72076 Tubingen, Germany. E-mail: Rolf.Pohmann@tuebingen.mpg.de

Received 21 November 2011; revised 8 May 2012; accepted 14 May 2012.

DOI 10.1002/mrm.24367Published online in Wiley Online Library (wileyonlinelibrary.com).

Magnetic Resonance in Medicine 000:000–000 (2012)

VC 2012 Wiley Periodicals, Inc. 1

Acrylic or Teflon) are used, to make the antenna resonantat the Larmor frequency. This results in a bulky antennathat may not fit into the RF shield of the gradient.

To resolve those issues, the radiating patch was short-ened by capacitive termination (19) for a more compactconstruction along with the possibility of quick adjust-ment to different loads (Fig. 1a). A low loss Teflon disc(er ¼ 2.1, thickness: 20 mm, diameter: 320 mm) with pro-visions for stable mounting on the patient table formsthe dielectric substrate. The radiating patch has a diame-ter of 210 mm, while the backside is completely coveredwith a thin copper layer (Novaclad G2202, Sheldahl)which acts as the ground. A netted structure was etchedon both copper surfaces to reduce eddy currents.

The equivalent circuit of one channel (Fig. 1b) consistsof a tuning/matching network which is realized on a FR4board, mounted on a slot in the substrate, using onefixed (2.12 pF at 400 MHz, 100C-Series, American Tech-nical Ceramics) and two variable capacitors CT1 and CM

(5610-Series, Johanson) as well as another trimmer ca-pacitor CT2 on the opposite side of the patch (NMNT15,Voltronics). This circuit is identical to that of a capaci-tively terminated microstrip element (20) that is tunedand matched using CT1, CT2, and CM.

To feed the antenna, the signal from the RF power am-plifier was split in two paths using a quadrature hybrid.A T/R switch with preamplifier was inserted in eachpath and connected to two semirigid coaxial cables(EZ250, Huber þ Suhner, Switzerland) which were

mounted on the antenna’s circuit boards. In transceivermode, the signals from the two channels were sampledseparately (16).

In transmit-only mode, a custom-built receive-onlyarray (17) was used for signal reception, consisting of 24loop elements that are arranged in three rows (Fig. 1c)on a close-fitting fiberglass helmet, extending over 17 cmfor whole brain coverage. During transmission with thepatch antenna, the receive array elements were activelydetuned using PIN diodes, which provided �34 dB ofdetuning on average. Passive blocking circuits wereimplemented on each receive element.

Numerical Simulations

Simulations of the RF fields were performed with CSTStudio Suite 2011 (CST, Darmstadt, Germany). Accuratemodels of two different 16-channel transmit arrays andthe antenna with the scanner’s RF shield and the loadwere constructed. All feed networks as well as the tun-ing and decoupling capacitors were replaced by 50 Vports, resulting in four ports for the antenna, and 32ports for each array. Models were meshed with about 30million cells, using mesh refinement in regions wherethe field gradients were expected to be large. A quarterof a wavelength of free space was kept between thestructure and the absorbing boundaries.

After transient excitation of all ports with a stop crite-rion of �50 dB, each coil element/channel was manuallytuned and decoupled from its nearest neighbors by co-simulation with CST Design Studio. Then, an alternatingcurrent signal simulation was applied with equal ampli-tudes for all ports and phase shifts between the singlecoil channels for circular polarization. The local specificabsorption rate (local SAR, averaged over 10 g) was cal-culated within the software. For the calculation of B1

þ

voxel statistics inside a volume of interest (VOI), datawas exported to MATLAB (The MathWorks, Natick).Losses between the scanner’s reference plane and thecoils were measured and considered in the simulations.

Experiments

Figure 1c shows the arrangement of antenna, receivearray, and preamplifier assembly as placed inside thescanner. The distance between antenna and subject wasbetween 9 and 12 cm for all subjects due to space limita-tions. To avoid interactions of the home-built preampli-fiers (20) with the transmit field, they were positionedbehind the antenna, connected by cables that wererouted around the Teflon disc and whose length wasadjusted for optimal preamplifier decoupling.

For comparison, additional measurements were con-ducted with two 16-element microstrip volume coils: Atight fitting elliptical transceiver array (20) and a larger,circular, actively detunable transmit-only array (17) withstrip lengths of 14 cm and 13 cm, respectively. Theywere driven in circularly polarized mode (consecutivephase offset of 22.5�) and the latter was combined withthe 24-channel receive array.

Imaging experiments were performed with three healthyadult volunteers as approved by the local authorities. The

FIG. 1. The modified patch antenna (a) and the equivalent circuitof one channel (b). The arrangement of antenna, receive array,

and preamplifier assembly as placed inside the scanner is shownin c. [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]

2 Hoffmann et al.

maximum allowed RF power was limited in the system’scoil file using the ratio of maximum local SAR to the totalpower that was absorbed in tissue, as proposed as a con-servative power limit by Siemens, to take uncertainties inour preliminary simulation data into account.

For bench measurements, a human head and shoulderphantom (Fig. 1c) filled with tissue equivalent solution(21) was used. Permittivity (er ¼ 58.6) and conductivity(s ¼ 0.64 S/m) of the phantom solution were determinedat room temperature using a dielectric probe kit (85070D,Agilent Technologies, Inc.). The same phantom was alsomodeled for field simulations.

RESULTS

Adjustments

As the S-parameter response of the antenna is sensitiveto load and position, initial adjustments were performedwith the antenna positioned inside the RF shield, 12 cmin front of the tissue-equivalent phantom and withoutthe receive array. The same situation was modeled forfield simulation (Fig. 2a).

Long tuning sticks were attached to the trimmer capaci-tors for adjustments. Although both tuning capacitors, CT1

and CT2, can be used to change the resonance frequencyof each channel, their balance determines the current dis-tribution on the patch and therefore the quality of decou-pling. A satisfying adjustment was found when the

capacitors for channel1/channel2 were set to the values:CT1 ¼ 1.70/1.74 pF, CT2 ¼ 4.70/4.88 pF, CM ¼ 7.25/9.20pF, similar to those found in simulations (CT1 ¼ 1.23 pF,CT2 ¼ 3.20 pF for both channels, CM ¼ 6.43/5.77 pF). Theresulting measured and simulated S-parameter response isshown in Fig. 2, together with simulated and measuredB1

þ maps. The broader S-parameter response and weakerB1

þ field in the measured data indicate additional losseswhich are not captured by the simulation.

For the in vivo scans with the receive array, the adjust-ment procedure was repeated. Capacitances found withthe phantom could be adopted for the in vivo scans withonly minor changes in CT1 and CM. The voltage necessaryfor a 180� flip angle with a 1-ms nonselective pulse wasdetermined in a circular region in the center of the brainand was around 400 V for all subjects. The presence ofthe receive array did not change the B1

þ magnitude inthe center of the phantom, but the field close to theantenna decreased by roughly 25%. This suggests someinfluence of the long receive cables on the transmit field.

Transmit Properties

The transmit properties of the antenna in comparison tothe two microstrip arrays were investigated by acquiringflip angle maps with all three setups (Fig. 3a) using aTurbo FLASH sequence with a 1 ms, 200 V nonselectivepresaturation pulse (22). All scans were obtained in cir-cularly polarized mode, with the same subject and

FIG. 2. Characterization of the patch antenna in transceiver mode, positioned inside the RF shield at a distance of 12 cm behind the

head phantom (a). Simulations are compared with experiment in terms of S-parameters (b) and B1þ field maps (c, d).

Human Brain Imaging at 9.4 T Using a Tunable Patch Antenna 3

identical sequence parameters. Simulated B1þ distribu-

tions for the different transmit coils are shown in Fig. 3busing the ‘‘Duke’’ model from the Virtual Family (23) fora distance of 12 cm between antenna and subject, notconsidering the influence of the receive array.

The transmit fields of antenna and both arrays showan increased field magnitude in the center of the brainand a drop-off due to destructive interference in the left-right direction, especially in the lower slices. For theantenna, however, the peak B1

þ field moves downwardinto the brainstem as observed previously (8), indicatingtraveling wave behavior despite the near-field applica-tion, while the arrays generated weak B1

þ in the lowertemporal lobes and the cerebellum. In total, the transmitefficiency of the patch antenna is comparable to theelliptic and even superior to the circular array. This dif-ference can be related to the high reflected power in thearrays (32% for the elliptic, 64% for the circular array)due to coupling compared to the antenna (accepted:98.40%, radiated: 33.68%, absorbed: 64.57%) accordingto the simulations. Results for the arrays should be

treated as approximate values as element decouplingwas found to be better in the experimental setup than insimulations. For all three setups, a similar percentage ofthe accepted power (65–70%) was absorbed in tissue.The sum of absorbed and radiated power differed by lessthan 2.7% from the accepted power in all simulations.

For a quantitative comparison of the RF field insidethe brain, two VOIs inside the head model were defined(Fig. 3c): The whole brain and the upper part of thebrain only, which is physically covered by the microstriparrays. For those volumes, we determined the B1

þ inho-mogeneity as the ratio of the |B1

þ| standard deviation toits mean value inside the VOI, and the safety excitationefficiency (24), defined as the mean |B1

þ| over the VOIdivided by the square root of the maximum local SARover the entire model. Both numbers are independentfrom input power and the latter describes the relativeB1

þ strength that can be safely generated inside the VOIwith respect to SAR limitations.

The results are summarized in Table 1: B1þ homoge-

neity was found to be best for the patch antenna in both

FIG. 3. Evaluation of the antenna’s transmit properties compared to an elliptic and a circular microstrip array. a: Measured B1þ maps

[nT/V]. b: Corresponding simulated B1þ data [nT/V]. c: Definition of the compartments for homogeneity and SAR comparisons. d: Distri-

bution of local SAR in a linear scale, with an input power to achieve an average B1þ field of 1 mT in the upper brain for all coils. All

planes traverse the position of peak local SAR, except for the sagittal slice in case of the elliptic array. [Color figure can be viewed inthe online issue, which is available at wileyonlinelibrary.com.]

4 Hoffmann et al.

VOIs, but especially for the whole brain due to the weakfield generated by the arrays in brainstem andcerebellum.

However, the antenna also produced the highest localSAR values (Fig. 3d), i.e., the lowest safety excitation ef-ficiency in both VOIs. Particularly for power calibrationto the upper brain, the level of allowed B1

þ magnitude isabout 25% lower than with the elliptic array.

Applications

To demonstrate the use of the presented setup for rou-tine human brain imaging at 9.4 T, we acquired func-tional and anatomical MR data.

Figure 4a shows FLASH images from a single volun-teer. The images demonstrate coverage of the entire brain

including cerebellum, brainstem, and parts of the spinalcord without major signal dropouts. The extent in z-direction is ultimately limited by the end of the headgradient’s linear range. In some slices, infolding artifactsappeared due to the limited extent of the gradients.

Figure 4b shows a T2*-weighted image, displaying ana-tomical features such as small venous structures andstrong variations in image contrast within white matter(10,25). The excellent gray/white matter contrast overlarge parts of the cortex deteriorates slightly in the poste-rior regions due to the smaller local flip angle.

Figure 4c displays the results of a functional MRIexperiment with a finger tapping task, using gradient-echo EPI with distortion correction (26). Strong activa-tion with high significance levels was found especiallyin the left motor cortex.

Table 1Average B1

þ Field, Inhomogeneity, and Safety Excitation Efficiency for Two Target Volumes and Two Antenna-Subject Distances

Obtained from Numerical Simulations

Average B1þ (nT/V)a B1

þ inhomogeneity (%)B1

þ safety excitationefficiency mT

ffiffiffiffiffiffikg

p� �=

ffiffiffiffiffiffiW

p

Whole brain Upper brain Whole brain Upper brain Whole brain Upper brain

Antenna Duke 9 cm/12 cm 27.09/27.18 26.97/27.37 35.49/34.49 36.07/34.59 0.33/0.34 0.32/0.34Antenna Ella 9 cm/12 cm 26.37/26.27 24.99/25.12 36.82/35.47 36.88/35.30 0.32/0.33 0.30/0.31

Circular array Duke 15.58 18.37 52.73 45.57 0.35 0.42Elliptical array Duke 25.95 31.27 53.73 43.38 0.37 0.45

The models ‘‘Duke’’ and ‘‘Ella’’ from the Virtual Family (23) were used with tissue properties valid at 400 MHz.aWithout cable losses between power source and coil.

FIG. 4. a: Two-dimensional FLASH images illustrate coverage, signal intensity distribution and image contrast. TR/TE ¼ 400/10 ms, twoaverages, BW ¼ 80 Hz/pixel, FA ¼ 25� in brain center. Sagittal images: FOV ¼ 215 � 215 mm2 (640 � 640 pixels), slice thickness (THK)¼ 1.5 mm. Coronal image: FOV ¼ 177 � 210 mm2 (540 � 640 pixels), THK ¼ 1.5 mm. Axial images: FOV ¼ 166 � 205 mm2 (468 �576 pixels), THK ¼ 1 mm. No intensity correction was applied; the same window level settings were used for all transversal images. b:T2*-weighted two-dimensional FLASH image. TR/TE ¼ 500/20 ms, FOV ¼ 170 � 210 mm2 (780 � 960 pixels), THK ¼ 1 mm, BW ¼ 30

Hz/pixel, FA ¼ 25� at brain center, duration ¼ 6 min 16 s. c: Gradient-echo EPI with 1 mm isotropic resolution acquired during righthanded finger tapping. Z-statistics between 6 and 20 are shown. [Color figure can be viewed in the online issue, which is available atwileyonlinelibrary.com.]

Human Brain Imaging at 9.4 T Using a Tunable Patch Antenna 5

DISCUSSION

Although the narrow head gradient of our 9.4 T scannerdoes not allow for proper traveling wave imaging, it waspossible to capitalize from the benefits of this techniqueusing the near field of the capacitively tunable patchantenna. This approach leaves free space around thehead, which improves subject comfort and is useful inexperiments where different body positions (10) or addi-tional hardware around the subject are required. Themain strength, however, is its simplicity together withthe capability to excite an extended FOV, from whichespecially applications may benefit that do not rely on aperfectly homogeneous transmit field.

The combination with a multichannel receive arrayyields superior sensitivity and provides effective parallelimaging. In addition, the large longitudinal coverage ofthe receive array made it possible to exploit the extendedexcitation volume of the patch antenna.

The existing setup could be further improved by opti-mizing the antenna parameters and the distance to thesubject. Miniaturized preamplifiers mounted directly onthe receive array elements would avoid cable losses andinteraction between cables and transmit field. Althoughcoupling between antenna and receive array elements wassmall due to the large spatial separation (S21 better than�20 dB), signal-to-noise ratio might be further improvedby actively detuning the antenna during signal reception.

The main disadvantages of the proposed setup com-pared to conventional volume coils are the higher localSAR values and the constricted parallel transmit capabil-ity. Although the available two channels may be used toimprove field homogeneity (27), this will not be able tocompete with B1 shimming using multichannel transmitcoils, which can reach similar longitudinal coveragewhen being designed in multiple rows (24,28).

The presented approach can also be beneficial forother scanners that do not support lossless propagationof the TE11 mode along the empty bore. Even in magnetswhere the Larmor frequency is significantly below thecutoff frequency, the near-field approach has beenshown to be a simple and efficient way of generating asufficiently homogeneous field (29).

In this study, we have shown that the benefits ofpatch antenna excitation and local, multichannel signalreception can be exploited even inside RF shieldswhose cutoff frequency is too high to allow travelingwave imaging in the proper sense. Although SAR limi-tations must be considered, this setup may find applica-tions in experiments where it is desired to investigateupper brain and cerebellum simultaneously or wherefree space around the subject is crucial but transmit ef-ficiency and highly sensitive parallel reception cannotbe relinquished.

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Human Brain Imaging at 9.4 T Using a Tunable Patch Antenna 7