Journal of Magnetic Resonance 246 (2014) 62–68

Contents lists available at ScienceDirect

Journal of Magnetic Resonance

journal homepage: www.elsevier .com/locate / jmr

An eight-channel T/R head coil for parallel transmit MRI at 3T usingultra-low output impedance amplifiers

http://dx.doi.org/10.1016/j.jmr.2014.06.0191090-7807/� 2014 Elsevier Inc. All rights reserved.

⇑ Corresponding author. Address: Department of Biomedical Engineering, 5045Emerging Technologies Building, 3120 TAMU, College Station, TX 77843-3120, USA.Fax: +1 979 845 4450.

E-mail address: mpmcdougall@tamu.edu (M.P. McDougall).

Katherine Lynn Moody a, Neal A. Hollingsworth b, Feng Zhao c, Jon-Fredrik Nielsen c, Douglas C. Noll c,Steven M. Wright b,a,d, Mary Preston McDougall a,b,⇑a Department of Biomedical Engineering, Texas A&M University, College Station, TX, USAb Department of Electrical and Computer Engineering, Texas A&M University, College Station, TX, USAc Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI, USAd Department of Radiology, Texas A&M Health Science Center, College Station, Texas, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 October 2013Revised 25 June 2014Available online 3 July 2014

Keywords:Parallel transmitArray coilsRF coilsUltra-low output impedance amplifiersAmplifier decouplingCoupling measurementsIsolation measurements

Parallel transmit is an emerging technology to address the technical challenges associated with MR imag-ing at high field strengths. When developing arrays for parallel transmit systems, one of the primary fac-tors to be considered is the mechanism to manage coupling and create independently operating channels.Recent work has demonstrated the use of amplifiers to provide some or all of the channel-to-channel iso-lation, reducing the need for on-coil decoupling networks in a manner analogous to the use of isolationpreamplifiers with receive coils. This paper discusses an eight-channel transmit/receive head array foruse with an ultra-low output impedance (ULOI) parallel transmit system. The ULOI amplifiers eliminatedthe need for a complex lumped element network to decouple the eight-rung array. The design and con-struction details of the array are discussed in addition to the measurement considerations required forappropriately characterizing an array when using ULOI amplifiers. B1 maps and coupling matrices areused to verify the performance of the system.

� 2014 Elsevier Inc. All rights reserved.

1. Introduction

The increase in signal-to-noise ratio (SNR) and spectral resolu-tion that comes with high field magnetic resonance imaging(MRI) can be traded for improvements in spatial and temporal res-olution [2]. In many cases, technical challenges prevent these ben-efits from being straightforwardly realized in practice. Onesignificant and well-known challenge is potential inhomogeneityin the transmit B1 field due to the higher frequencies (and shorterradiofrequency (RF) wavelengths) associated with higher magneticfield strengths [3–7]. In neuroimaging applications, this often man-ifests as a central brightening artifact due to constructive interfer-ence in the center of the head preventing uniform tip angles [8–9].

A number of research groups have used multiple transmit chan-nels to address this challenge. Approaches range from relativelystraightforward B1 shimming [10–13] to more complex transmitSENSE techniques in which separate RF excitation pulses are sentto each channel [14–16]. With either approach, however, the level

of independence between transmit channels is a concern. Any cur-rent applied to one element will induce a voltage (more accurately,an electromotive force or EMF) in any other elements if they haveany mutual impedance. In turn, this induced voltage can driveunwanted currents in the other coils, contaminating the desiredcoil pattern. This is commonly referred to as ‘‘coupling’’ betweencoils. One can preserve the patterns either by reducing the inducedvoltage by eliminating or cancelling the mutual impedance, or, byensuring that no additional currents are generated as a result of theinduced voltage. There are a number of possible ways to eliminatethe mutual impedance, such as geometrically overlapping the coilsor constructing a lumped element network on the coil to cancel themutual inductance [17]. To eliminate the currents driven by themutual impedance, one can introduce a high impedance acrossthe terminals of the coils as in the case of using isolating preampli-fiers [18]. A benefit of the latter approach is that it results in lessconcern for the mutual impedance between coils, just as withreceive arrays. To generalize, array coil design, specifically withrespect to the degree of on-coil decoupling required, depends onand operates in concert with the preamplifier regime in the receivecase and the amplifier regime in the transmit case [15].

Most transmit array coils are designed for use with standard RFpower amplifiers, which have been designed to produce maximum

K.L. Moody et al. / Journal of Magnetic Resonance 246 (2014) 62–68 63

output power assuming a 50 X load. To maintain expected perfor-mance, these ‘conventional’ amplifiers generally rely on decou-pling of the coil elements themselves, just as with receive arraysoperating with conventional preamplifiers, to ensure that theamplifier sees the expected load. Some combination of geometricoverlap and lumped element decoupling networks must beemployed on these arrays to enable independent operation of thechannels. Geometric overlap is limited in application to adjacentelements and imposes constraints on the array geometry. Lumpedelement networks require no overlap, but as the channel countsincrease, the network required to fully decouple all elementsincreases in complexity due to the increasing number of decou-pling capacitors required [17]. As an example, to decouple sevenelements with a lumped element ladder network, three decouplingcapacitors are required for each ladder stage [19], giving a total of21 capacitors needed for full decoupling. In extending theapproach to eight elements, four decoupling capacitors arerequired at each ladder stage for a total of 32 decoupling capaci-tors. The values for the decoupling capacitors are guided byclosed-form equations that require iterative non-linear andnumerical field solvers to compute [20], and further fine adjust-ments in element tuning are done iteratively and experimentally.The amount of available decoupling is also sensitive to loading.As indicated in one study [19], the decoupling between opposingelements of a four-element array was found to increase from�30 dB in the unloaded case to �10 dB in the loaded case, indicat-ing 10% of the power from one element being coupled into itsopposing element. Due to these complications involved with thevarious on-coil decoupling strategies, stripline elements andshielded loop elements have emerged as the two most commondesigns for parallel transmit arrays in neuroimaging due to theirinherent favorable coupling properties [11,21–25]. Even still, anadditional decoupling network is typically needed, adding com-plexity to the array design [11]. Recently, alternative approachesto amplifier design have been investigated that provide somedegree of isolation between channels, decreasing or even eliminat-ing the need for decoupling between the coil elements themselves.

One approach under investigation is current source amplifica-tion in which each series resonant element is driven with a pre-scribed current, insensitive to the effects of loading, includingelement-to-element coupling [21–25]. This allows for largeamounts of flexibility in the design of the array coils used with cur-rent sources, as in principle no on-coil decoupling techniques areneeded. Current source amplifiers provide high isolation by pre-senting a high impedance to the series tuned coil, but are notmatched for optimal power output, so they are limited in theamount of current they can produce as compared to standard RFpower amplifiers [26]. Recent work on current mode class-D(CMCD) amplifiers has shown to improve efficiency and may beable to produce higher peak output levels than linear currentsource amplifiers [24–25,27]; however, the CMCD design has othercomplications that must be addressed in the process [25].

Chu et al. introduced the ‘‘ultra-low output impedance (ULOI)amplifier’’, which, in contrast to the current source amplifier, pro-vides decoupling in a manner analogous to preamplifier decou-pling in parallel receive applications [28]. The array elements arematched to 50 O input impedance using a network that forms atrap when connected to the ULOI amplifier or preamplifier. ULOIamplifiers are power matched and present a low impedance tothe coil port. The power match enables peak output levels compa-rable to standard power amplifiers while the low impedance pro-vides isolation when combined with an appropriately designedmatching network on the coil. The isolation obtained with ULOIamplifiers is substantially lower than that of current source ampli-fiers, but the peak current delivered to the coil is higher. In addi-tion, as discussed in [28], the ULOI amplifier drain is biased into

saturation so that output current and parametric variation is min-imized in cases of non-passive loading such as the case with cou-pled transmit array elements. In practice, the ULOI amplifiersrepresent a ‘‘middle ground’’ between current source and standardpower amplifiers with respect to isolation and output power. Wechose to implement a parallel transmit system with ultra-low out-put impedance amplifiers to achieve greater power output overpreviously built current sources [23] and to achieve decouplingbenefits to simplify the on-coil decoupling network. An added ben-efit of this approach is that it allows for a straightforward transmit/receive configuration in that the matching network required forthe ULOI amplifier is identical to the one needed for low-inputimpedance preamplifiers. This paper discusses the constructionand characterization of an eight-channel transmit/receive headarray for use with an ultra-low output impedance parallel transmitsystem. The eight-element array was decoupled using a simpledecoupling network in combination with the isolation providedby the ULOI amplifiers. The characterization of the isolation pro-vided by the stages of the system, from amplifier to coil, is dis-cussed in detail.

2. Materials and methods

2.1. System overview

The eight-channel parallel transmit system was designed as aretrofit for a 3 Tesla GE clinical research scanner, requiring a rapidand transparent switchover from a standard single channel trans-mitter. Two inputs were required from the host GE scanner: theinput to the RF amplifier and the master exciter unblank (RF gate)signal. A single hard pulse played out from the scanner is dividedeight ways and then modulated by an in-house built vector modu-lator [29]. The control system employs a PXI-7853R FPGA-basedboard with programs written in LabVIEW (National Instruments,Austin, TX) to drive the hardware and provide the baseband in-phase and quadrature signals to the vector modulator from theuser-defined amplitude and phase information of each RF pulse[30]. The modulated waveforms pass through a first gain stageprior to the ultra-low output impedance amplifiers. The head arraycoil is connected to the amplifiers through transmit/receiveswitches. Low input impedance preamplifiers on the array coil pro-vide the first gain stage on the receive side prior to passing throughto the scanner receiver chain.

2.2. Array fabrication

The eight-channel head array was designed with shielded rungsfabricated using copper sheet metal mounted to a ½ inch wide,25 cm long acrylic piece with six breaks with 79 pF of capacitanceat each break (Passive Plus, 1111C Series, Huntington, NY). Themounting piece for all the element hardware was a 12-inch outerdiameter cylinder fabricated from white polycarbonate using afusion deposition modeling (FDM) rapid prototyping machine.

The shield consisted of two layers of single-sided ½ ounce cop-per Pyralux (AC182500E, DuPont, Research Triangle Park, NC)mounted to the 12 inch cylinder. Each layer was slotted longitudi-nally to mitigate eddy currents and the two layers were oriented toalternate the position of the longitudinal slits. Rectangular slotswere removed from the Pyralux shield and replaced with coppermesh to provide a view port through the coil for patient comfortand visual stimulus in functional imaging studies.

The elements were mounted on the inside of the cylinder withconnections to the shield at one end and the matching network atthe other. With the ½ inch thickness of the acrylic rung supportand the 1=4 inch thick cylinder, each rung was 3=4 inch from the

64 K.L. Moody et al. / Journal of Magnetic Resonance 246 (2014) 62–68

shield, creating an effective 10.5 inch diameter coil array. Shieldedcurrent probes were manufactured from 0.047 inch diameter semi-rigid (EZ 47-CU-SP, EZ Form Cable Corporation, Hamden, CT) tohave an approximate loop diameter of 1=4 inch. A small section ofthe outer conductor was removed to expose the inner conductorand enable B-field pick-up. The probes were placed adjacent toeach rung, and the probe cable dropped down and connected tothe RF shield along the length of the coil array. The probe signalwas pulled out on the match and tune board as shown in the dia-gram and photograph in Fig. 1. Baluns for both the element andcurrent probe were mounted to the match and tune boards.

A single-sided 1=4 oz Pyralux end piece (AC091200EV) connectedthe shield segments near the top of the head to prevent RF fieldsfrom interacting with the rest of the transmit and receive chains.The transmit/receive switches and low-input impedance(R = 1.5 O) preamplifiers (WMA3RA, WanTCom Inc., Chanhassen,MN), were mounted to an acrylic piece covered in a 1=4 oz Pyraluxground plane. All hardware components were contained within amodular polycarbonate housing manufactured in-house using a3D printer. A diagram of the array coil hardware with photos ofthe array and housing are shown in Fig. 2.

2.3. Coil matching network and transmit/receive configuration

The matching network used to match and tune the coil ele-ments to 50 O is diagrammed in Fig. 3 [1]. Because the ULOI ampli-fier operates as the analog on the transmit side to the low-inputimpedance preamplifier on the receive side with respect to provid-ing isolation, the matching network scheme is conveniently thesame in both transmit and receive modes. The values for thematching network components were theoretically determined bythe equations in [1] and then tuned to account for variations inpractice. For each element, the capacitor Cx was selected to resultin reasonable values for the remaining components that wouldnot result in high voltages (small capacitance) or large currents(large capacitance) through the capacitors in the matching net-work. Adjustments in capacitors C1 and C2 were made to fine tunethe coil. Tuning the trap was accomplished by short-circuiting thecoil port (located on the match and tune board at the coil feed) tomimic the presence of the ULOI amplifier or preamplifier, placingthe inductor L and the equivalent capacitance of the matching net-work in parallel to form a trap circuit. Slight adjustments to L weremade to minimize S21 between a pair of two decoupled loop probespositioned near the rung, corresponding to minimum power beingcoupled into the coil – that is, when induced currents were sup-pressed and the amplifier or preamplifier was providing decou-pling in the transmit and receive cases respectively. As seen inFig. 3, the amplifier and preamplifier for each channel are con-nected to the rung by a PIN diode transmit/receive switch(UM9415, Microsemi, Aliso Viejo, CA) with typical insertion lossof �.2 dB and electrical length of .34k in the transmit configuration.The total electrical length from the coil port to the preamplifier andamplifier accounted for a phase delay of a multiple of k/2 to ensure

Fig. 1. (a) Diagram of the shielded probe and its location along the rung. The probe is at tthe match and tune board at the far end of the coil. (b) Photograph of the shielded prob

the low impedance from the amplifier/preamplifier was presentedto the coil port.

2.4. Measurements overview

All measurements were conducted with a spherical phantom inplace that mimicked the loading of an average human head.Because the decoupling benefits of ULOI amplifiers are realizedby the impedance they present at the coil matching network port,it is not always appropriate to characterize or optimize with stan-dard 50 X port network analyzer measurements. If a measurementneeds to be acquired which involves the amplifiers being con-nected in an ‘‘operational’’ configuration, then this requires theuse of a more direct measurement of current on the rungs of thearray rather than a port measurement. The measurementsdescribed below that rely on the impedance the amplifiers presentwere acquired with current probes in one of two configurations.Disambiguation will be ensured by referring to them as ‘‘on-coilprobes’’ or ‘‘crossed-probes’’, representing respectively, the singlecurrent probe on each rung that is built into the coil (describedabove), or a pair of ‘‘free’’ crossed probes moveable to an individualrung. Three measurements are discussed in the sections thatfollow:

(1) Characterizing the ULOI amplifiers: measuring the isolationper channel provided by the ULOI amplifiers, requiring acrossed-probe measurement on each channel.

(2) Characterizing the coil: measuring the on-coil decouplingadded between elements, accomplished with a standardS21 port measurement under certain described conditions.

(3) Characterizing the system: measuring the total channel-to-channel coupling with the amplifiers connected, measuredwith the on-coil probes.

2.5. Isolation from the power amplifiers

Because the amount of isolation provided by the power ampli-fier depends on its output impedance, the architecture is particu-larly sensitive to losses in the transmit cables and in-linetransmit/receive switches. Ideally, the amplifiers might be placedat the coil and inside the bore [25], but to eliminate saturationand susceptibility effects from eight amplifiers with ferrite baluns,heat sinks, and MOSFET devices, as well as potential cooling prob-lems, we opted to locate the power amplifiers outside the bore.This required a transmit cable to span the distance between thetransmit/receive switch and the amplifier that maintained an elec-trical length of nk/2 between the coil port and amplifier matchingnetwork. As expected, the losses associated with the added cablelength and the transmit/receive switches reduce the achievableisolation from the amplifiers. Chu et al. discussed the effect of cablelosses and observed that with a 7k/2 cable (insertion loss of0.52 dB) at 3 Tesla, the isolation provided by the amplifierdecreased to �9 dB as opposed to �14 dB obtained with the

he center of the rung. The probe cabling is grounded to the RF shield and connects toe and location relative to the rung.

a

b c

Fig. 2. (a) Diagram of the array coil setup showing the rung elements with the decoupling capacitor at the end opposite the coil feed. The preamplifiers and t/r switches aremounted on the coil as indicated and shown in b. The elements are shielded and a shielded end piece (labeled and shaded) prevents stray RF from interacting with thehardware behind. (b) Outside view of the array coil showing the array hardware. (c) Fully assembled array coil shown in its completed housing.

Fig. 3. Block diagram of the transmit/receive configuration of the array andmatching network. During transmit and receive, the coil is presented with a lowinput impedance that places the inductor L in parallel with the equivalentcapacitance of the matching network, creating a trap and suppressing inducedcurrents. The electrical lengths between the coil port and the amplifier/preamplifierare a multiple of a half-wavelength to prevent an impedance transformation. Thematching network used was presented in [1].

K.L. Moody et al. / Journal of Magnetic Resonance 246 (2014) 62–68 65

amplifier at the coil element [28]. For our particular setup, a 5k/2transmit cable was required for each channel to place the amplifi-ers outside the bore.

Before measuring the isolation provided by the ULOI amplifiertransmit system, the trap tuning on the coil was verified by mea-suring the ideal case with a short circuit at the coil port. An S21

measurement between a pair of crossed-probes positioned alongthe rung was used, with one probe exciting the rung, and the othersensing the field produced by the current on the rung. The isolationwas calculated as the difference between S21 measurements col-lected with the coil port terminated in a 50 O load and terminatedin a short, in an analogous fashion to measuring the decouplingprovided by an isolation preamplifier. With a short at the coil port,the isolation provided by the traps was observed to be �30 dB orbetter in all cases. The isolation provided by each amplifier wasthen measured using the same method. During the measurementthe ULOI amplifier was connected to the coil and powered, withthe input to the amplifier terminated in 50 O. The isolation pro-vided by each amplifier was calculated as the difference in theS21 measurements between a 50 X load termination and termina-tion with the amplifier connected through the transmit/receiveswitch and transmit cable. Based on these measurements, the aver-age isolation provided by a well-tuned amplifier was found to be�11.5 dB with the 5k/2 transmit cable and transmit/receive switchin place. Using the same isolation measurement method, the coil

was instead terminated in the low-input impedance preamplifierwith the transmit/receive switch in place. The isolation providedin this scenario (the receive case) was measured to be �20 dB orbetter for all elements.

To evaluate if the isolation provided by the amplifier indepen-dently provided sufficient decoupling between channels, the ele-ments of the array coil were tuned individually with all otherelements open-circuited as described above. The tuning of one ele-ment then was observed using an S11 measurement while the otherelements were connected to the ULOI amplifiers. With the ULOIamplifiers, the element tuning shifted outside a standing-wave-ratio (SWR) of 2:1, indicating insufficient decoupling. While thereare other possible solutions, we decided to add a simple capacitivedecoupling network on the coil to augment the isolation providedby the amplifiers.

2.6. Providing additional decoupling between the elements

It is important to emphasize that the use of a conventional S21

port measurement to quantify coupling between elements is notalways appropriate when using ULOI amplifiers, depending onthe layout of any electrical connections between rungs. Specifi-cally, the current distribution between two connected rungs isnot always the same when a rung is terminated in the 50 X net-work analyzer (as with a conventional port measurement) andwhen a rung is terminated in the high impedance provided bythe trap circuit when the rung is connected to an ULOI amplifier(or preamplifier).

For clarification, Fig. 4 illustrates the effect of the differentplacement of the decoupling capacitor between elements of thisparticular array. Importantly, placing a decoupling capacitorbetween rungs at the feed end of the coil and performing a conven-tional S21 port measurement with a network analyzer could pro-vide an inaccurate indication of the coupling between thoserungs that would occur if they were fed by ULOI amplifiers. Thiscan be understood more clearly by comparing the current pathsin Fig. 4a and b, which illustrate the difference in resulting rungcurrents (and thus coupling) between termination in the 50 X net-work analyzer and termination in an infinite impedance. As shownin Fig. 4c, placing the decoupling capacitor between rungs at theopposite end of the coil in front of the last distributed capacitoravoided this difference between the two termination cases and

Fig. 4. Diagram showing the current paths for different decoupling capacitor placements to demonstrate appropriate coupling measurements when using ULOI amplifiers. (a)The decoupling capacitor is placed between the coil feeds and both coil ports are terminated in 50 X (as is the case for a conventional S21 port measurement). The current pathsplits at the juncture between the rung and decoupling network and splits again at the connection to the adjacent rung due to the 50 X termination. (b) Instead of a 50 X load,the adjacent rung is terminated in the ULOI amplifier, labeled with theoretical infinite impedance. The current paths do not split at the adjacent rung, and the ratio of rungcurrents is different from the case shown in (a) indicating a difference in coupling. Therefore, if a conventional S21 measurement were used to choose the decoupling capacitorvalue, the coupling provided by the network would change in the operating case with the ULOI amplifier (or preamplifier). (c) The decoupling capacitor is placed in front ofthe last distributed capacitor to avoid the difference in current paths for the two terminations cases allowing for direct S21 port measurements to set the decoupling capacitor.

66 K.L. Moody et al. / Journal of Magnetic Resonance 246 (2014) 62–68

allowed for use of a direct S21 port measurement for setting thedecoupling capacitor value between rungs. Placing a capacitorbetween the rungs changed the rung impedance slightly, requiringa second iteration to adjust matching network and trap tuning, asdescribed above. It is important to emphasize the simplicity, how-ever, of the single capacitor decoupling network that was enabledby the isolation provided by the ULOI amplifiers as compared toconventional on-coil decoupling networks.

2.7. Total channel-to-channel isolation

To accurately measure total channel-to-channel isolation withthe coil decoupling capacitors and amplifiers in place, the on-coilprobes were used to measure the relative currents on the rungs.Two measurements were made in order to ensure valid probe mea-surements: one to determine the limit of meaningful detection andone to calibrate for the varying sensitivities between probes. Tocharacterize the floor of meaningful measurements with the on-coil probes, a setup with two neighboring rungs and probes wasused. A network analyzer excited one rung, and the signal fromboth probes was measured with the adjacent rung open-circuitedto limit induced current. This measurement detected the signallevel sensed by both probes due to the field produced by theexcited rung. The measurement indicated a ‘‘detection limit’’ of

Fig. 5. Coupling matrices acquired using the on-coil current probes for the 50 X and amterminated in 50 X loads. The decoupling network adds an average of �11 dB of decoamplifiers. (b) The coupling matrix acquired with the coil terminated in the ultra-low ocoupling is an average of�23.6 dB. Minor differences in the reciprocal measurements in bindividually and were therefore subject to slight differences in probe sensitivities.

�24.5 dB between nearest neighbor probes. Therefore, nearestneighbor measurements made with the on-coil probes werecapped at �24.5 dB.

In addition, the probes were calibrated to compensate for differ-ences in probe sensitivity due to slight variations in loop size andorientation with respect to the rung. To do this, a test fixture wasfabricated to position a second probe at a repeatable fixed heightand orientation above each element. To collect a relative ‘‘sensitiv-ity measurement’’ for each on-coil probe, all other elements wereopen-circuited, and the second probe was positioned above therespective element. A network analyzer was used to collect twoS21 measurements: one between the element and the on-coil probeand one between the element and the second probe positionedabove the element. The difference between the two measurementscorresponded to the on-coil probe sensitivity and provided a set ofcalibration measurements for the eight probes. The measurementsof the total channel-to-channel isolation obtained using the probesthen were adjusted by this measurement.

To measure the isolation, one rung at a time was connected tothe network analyzer and S21 measurements were collectedbetween it and each of the eight on-coil probes. The measurementscorrelated to the relative currents on the rungs and thus to the ele-ment-to-element decoupling. In this manner 8 � 8 decouplingmatrices (64 individual S21 measurements) were collected for

plifier termination cases. (a) The coupling matrix acquired with the coil elementsupling between nearest neighbors to augment the isolation provided by the ULOIutput impedance amplifiers. With the amplifiers in line, overall channel-to-channeloth matrices (i.e. Sij – Sji) are due to the fact that all 64 measurements were acquired

Fig. 6. B1 maps in the flood region of the ACR phantom using the Bloch–Siegert method acquired using the eight-channel array and ULOI amplifiers interfaced to a GE 3 Teslaclinical research scanner. The maps are scaled individually to better show the coil patterns and are shown in Gauss.

K.L. Moody et al. / Journal of Magnetic Resonance 246 (2014) 62–68 67

two cases: (1) with the coil ports terminated in 50 X, indicating thedecoupling provided on the coil alone and (2) with the coil portsconnected to the transmit system, indicating the total channel-to-channel isolation.

Imaging experiments were performed on a 3 Tesla GE Signaclinical research scanner. Each transmit channel was calibratedto 300 W at full scale input. Overall system performance was thendemonstrated by acquiring B1 maps from each channel operatingat full scale. Mapping was done in the flood region of the AmericanCollege of Radiology (ACR) phantom using the Bloch–Siegertmethod with a TR of 200 ms and a TE of 18 ms [31–33].

3. Results and discussion

The 8 � 8 decoupling matrices acquired with the rungs termi-nated in 50 O and in the ULOI amplifiers are shown in Fig. 5. Whenthe elements were terminated in 50 O, the single capacitor decou-pling network on the coil provided an average of �11 dB (�17.4 dBmax, �6.9 dB min) of decoupling between nearest neighboring ele-ments, as seen in Fig. 5a. The decoupling matrix acquired with therungs terminated in the ULOI amplifiers is shown in Fig. 5b, indi-cating the added isolation provided by the amplifiers. The totalaverage decoupling between channels with the amplifiers in linewas �23.2 dB (�30.8 dB max, �13.9 min). Further iterative tuningof the amplifiers and/or of the decoupling between elements cancertainly improve this, but sufficient independence between chan-nels was considered to have been achieved based on observing aminimal shift in element tuning (within an SWR of 2:1) for all ele-ments as compared to the original tuning that was done with allother elements open-circuited.

In a few cases, the coupling between next-nearest neighbors ishigher than the coupling between nearest neighbors. This is due tothe addition of a current path between these elements via thedecoupling capacitor that was tuned for nearest-neighbor decou-pling only. There are minor differences in the reciprocal measure-ments of the coupling matrices (i.e. Sij – Sji) due to the fact that all64 measurements were acquired individually with the probes andtherefore were sensitive to small differences in calibration.

The B1 maps acquired from each element in the flood region ofthe ACR phantom are shown in Fig. 6. The patterns demonstrate aneffective transmit system, generating well-isolated sensitivitiesand corroborating the expected behavior based on the couplingmatrices.

4. Conclusion

In conclusion, we have discussed the design, construction, andcharacterization of the first eight-channel array coil for use withultra-low output impedance amplifiers. In particular, consider-ations when adding decoupling to the array coil with respect toappropriate measurements and placement of the decoupling

capacitor were detailed. The head array working with ultra-lowoutput impedance amplifiers provided effectively isolated chan-nels and avoided the need for complex on-coil decoupling, whichis typically load dependent. The approach made a transmit/receivearray configuration straightforward and demonstrates an option toconsider for parallel transmission applications.

Acknowledgments

The authors gratefully acknowledge support from the NationalInstitutes of Health (R01 NS058576).

References

[1] A. Reykowski, S.M. Wright, J.R. Porter, Design of matching networks for lownoise preamplifiers, Magn. Reson. Med. 33 (6) (1995) 848–852.

[2] W. Edelstein et al., The intrinsic signal-to-noise ratio in NMR imaging, Magn.Reson. Med. 3 (4) (2005) 604–618.

[3] T.S. Ibrahim et al., Effect of RF coil excitation on field inhomogeneity at ultrahigh fields: a field optimized TEM resonator, Magn. Reson. Imaging 19 (10)(2001) 1339–1347.

[4] D.I. Hoult, Sensitivity and power deposition in a high-field imagingexperiment, J. Magn. Reson. Imaging 12 (1) (2000) 46–67.

[5] R. Abraham, T.S. Ibrahim, Proposed radiofrequency phased-array excitationscheme for homogenous and localized 7-Tesla whole-body imaging based onfull-wave numerical simulations, Magn. Reson. Med. 57 (2) (2007) 235–242.

[6] T.S. Ibrahim et al., Dielectric resonances and< i> B</i>< sub> 1</sub> fieldinhomogeneity in UHFMRI: computational analysis and experimental findings,Magn. Reson. Imaging 19 (2) (2001) 219–226.

[7] A. Kangarlu et al., Dielectric resonance phenomena in ultra high field MRI, J.Comput. Assist. Tomogr. 23 (6) (1999) 821–831.

[8] Q.X. Yang et al., Analysis of wave behavior in Lossy dielectric samples at highfield, Magn. Reson. Med. 47 (5) (2002) 982–989.

[9] C.M. Collins et al., Central brightening due to constructive interference with,without, and despite dielectric resonance, J. Magn. Reson. Imaging 21 (2)(2005) 192–196.

[10] F. Seifert, H. Rinneberg, Adaptive coil control: SNR optimization of a TR volumecoil for single voxel MRS at 3 T, in: Proceedings of the 10th Annual Meeting ofISMRM, Honolulu, Hawaii, USA, 2002.

[11] G. Adriany et al., Transmit and receive transmission line arrays for 7 Teslaparallel imaging, Magn. Reson. Med. 53 (2) (2005) 434–445.

[12] Y. Zhu, R. Giaquinto, Improving flip angle uniformity with parallel excitation,Proc. Int. Soc. Magn. Reson. Med. (2005).

[13] P. Ullmann et al., Experimental analysis of parallel excitation using dedicatedcoil setups and simultaneous RF transmission on multiple channels, Magn.Reson. Med. 54 (4) (2005) 994–1001.

[14] U. Katscher et al., Transmit SENSE, Magn. Reson. Med. 49 (1) (2003) 144–150.[15] Y.D. Zhu, Parallel excitation with an array of transmit coils, Magn. Reson. Med.

51 (4) (2004) 775–784.[16] W. Grissom et al., Spatial domain method for the design of RF pulses in

multicoil parallel excitation, Magn. Reson. Med. 56 (3) (2006) 620–629.[17] R.F. Lee, R.O. Giaquinto, C.J. Hardy, Coupling and decoupling theory and its

application to the MRI phased array, Magn. Reson. Med. 48 (1) (2002) 203–213.

[18] P. Roemer et al., The NMR phased array, Magn. Reson. Med. 16 (2) (1990) 192–225.

[19] J. Jevtic, Ladder networks for capacitive decoupling in phased-array coils, in:Proceedings of the 9th Annual Meeting of ISMRM, Glasgow, Scotland, 2001.

[20] J. Jevtic, et al., Design guidelines for the capacitive decoupling networks, in:Proc 11th Annual Meeting ISMRM, Toronto, ON, Canada, 2003.

[21] K. Kurpad, E. Boskamp, S. Wright, Implementation of coil integrated RF powerMOSFET as voltage controlled current source in a transmit phased array coil,in: Proceedings of the 12th Annual Meeting of ISMRM, Kyoto, Japan, 2004.

68 K.L. Moody et al. / Journal of Magnetic Resonance 246 (2014) 62–68

[22] K. Kurpad, E. Boskamp, S. Wright, A parallel transmit volume coil withindependent control of currents on the array elements, in: Proceedings 13thScientific Meeting, International Society for Magnetic Resonance in Medicine,Miami Beach, 2005.

[23] K.N. Kurpad, S.M. Wright, E.B. Boskamp, RF current element design forindependent control of current amplitude and phase in transmit phased arrays,Concepts Magn. Reson. Part B – Magn. Reson. Eng. 29B (2) (2006) 75–83.

[24] N. Gudino, et al., 7T current-mode class-D (CMCD) RF power amplifier, in:Proceedings of the 17th Annual Meeting of ISMRM, Honolulu, HI, 2009.

[25] N. Gudino et al., On-coil multiple channel transmit system based on class-Damplification and pre-amplification with current amplitude feedback, Magn.Reson. Med. (2012) n/a.

[26] K.N. Kurpad, Transmit field pattern control for high field magnetic resonanceimaging with integrated RF current sources, Texas A&M University, 2012.

[27] J. Heilman et al., Preamp-like decoupling and amplitude modulation in CMCDamplifiers for transmit arrays, Proc. Int. Soc. Magn. Reson. Med. (2008) 1097.

[28] X. Chu et al., Ultra-low output impedance RF power amplifier for parallelexcitation, Magn. Reson. Med. 61 (4) (2009) 952–961.

[29] N. Hollingsworth, et al., An easily integrated multichannel modulator for allfield strengths, in: 21st Annual Meeting of the International Society forMagnetic Resonance in Medicine, Salt Lake City, Utah, 2013.

[30] K.L. Moody, et al., A LabVIEW-based operating system for parallel transmitsystems, in: Biomedical Imaging: From Nano to Macro, 2011 IEEE InternationalSymposium on: IEEE, 2011.

[31] L.I. Sacolick et al., B1 mapping by Bloch-Siegert shift, Magn. Reson. Med. 63 (5)(2010) 1315–1322.

[32] F. Zhao, et al., Optimized linear combinations of channels for complexmultiple-coil b1 field estimation with bloch-siegert b1 mapping in MRI.

[33] F. Zhao, et al., Regularized estimation of magnitude and phase of multiple-coilB1 field via Bloch-Siegert B1 mapping, in: 20th Scientific Meeting of theInternational Society for Magnetic Resonance in Medicine, Melbourne,Australia, 2012.