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. Author manuscript; available in PMC: 2012 Aug 1.
Published in final edited form as: Concepts Magn Reson Part B Magn Reson Eng. 2011 Aug;39(3):159–165. doi: 10.1002/cmr.b.20203

An Improved Element Design for 64-Channel Planar Imaging

Chieh-Wei Chang 1, Katherine Lynn Moody 1, Mary Preston McDougall 1,2
PMCID: PMC3259609  NIHMSID: NIHMS313980  PMID: 22267961

Abstract

Investigation of highly accelerated MRI has developed into a lively corner in the hardware and methodology arena in recent years. At the extreme of (one-dimensional) acceleration, our group introduced Single Echo Acquisition (SEA) imaging, in which the need to phase encode a 64×Nreadout image is eliminated and replaced with the well-localized spatial information obtained from an array of 64 very narrow, long, parallel coils. The narrow coil width (2mm) that facilitates this is accompanied by a concomitant constraint on the useful imaging depth. This note describes a 64-element planar array, constructed within the same 8×13cm total footprint as the original SEA array, still enabling full acceleration in one dimension, but with an element design modified to increase the imaging depth. This was accomplished by lowering the outer conducting legs of the planar pair with respect to the center conductor and adding a geometric decoupling configuration away from the imaging field of view. The element has been called a dual-plane pair in that the current carrying rungs in the imaging FOV function exactly as the planar pair, but are simply placed in two separate planes (sides of PCB in this case).

Keywords: SEA imaging, single echo acquisition imaging, rapid imaging, RF coils, array coils, parallel imaging

Introduction

Some of the earliest visions of accelerated MRI proposed parallelization to a high enough degree to allow for the complete elimination of phase encoding 1-3. Fittingly, investigation of increased channel counts has progressed in recent years far beyond what is considered “standard” in the clinic, with Sodickson even coining the term “massively parallel” 4. In-vivo implementations of parallel imaging with 32 channels 5, 96 channels 6, and 128 channels 7,8 even suggest the possibility of the eventual translation of massively parallel reception into select clinical applications.

Our group has maximized acceleration in one dimension, acquiring a complete 64×NReadout image in a single echo 9,10, using a 64 channel receiver 11 and a 64-channel array of long and narrow planar pair elements 12. The highly localized field patterns from the 64 planar pair elements are entirely responsible for providing the localization in one dimension, eliminating the need for phase encoding. This imaging technique is called Single Echo Acquisition (SEA) imaging, and has been applied to image flow 13, tag tracking 14, motion 10, and 2D RF pulse formation at up to 1000 frames per second 15. SEA imaging as currently reported, however, is a low resolution technique with shallow imaging depth – both on the order of millimeters – and it is desirable to investigate methods for improving both. Improved resolution can be achieved straightforwardly at the expense of imaging time by acquiring more than a single echo from each element, phase encoding even to the point that the planar pair array is used as an array of microcoils to accelerate microscopy imaging using PILS-like reconstruction methods 16,17. Increasing imaging depth is less a methodological issue, instead requiring a modification to the array element itself. The constrained sensitivity pattern of the original element accommodates the needs of SEA imaging well, providing the “resolution” of the image in what would be the phase encoding direction, but suffering from a concomitant limited imaging depth. This note describes a 64-element planar array, constructed within the same 8×13cm total footprint as the original SEA array, still enabling full acceleration in one dimension, but with an element design modified to increase the imaging depth. This was accomplished by lowering the outer conducting legs of the planar pair with respect to the center conductor and adding a geometric decoupling configuration away from the imaging field of view. The element has been called a dual-plane pair in that the current carrying rungs in the imaging FOV function exactly as the planar pair, but are simply placed in two separate planes (sides of PCB in this case).

Methods

Hardware and instrumentation

The primary objective of the new element design was to achieve a substantial increase in SNR at depth (distance from the surface of the planar array coil) as compared to the currently used planar pair element while maintaining the original 2mm × 8cm element footprint that afforded the highly localized imaging patterns necessary for SEA and PILS-like acceleration and reconstruction. To achieve this, the current return paths (two outer legs of the planar pair) were fabricated on the back side of 0.062″ FR-4 laminate PC board, reducing the field canceling interactions with the center current carrying conductor. For clarity, Fig. 1 shows the planar pair element and the dual-plane pair element diagrams, with their expected relative field sensitivity patterns as predicted by modeling.

Figure 1.

Figure 1

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Illustrations of the planar pair element (a, left) and the dual-plane pair element (a, right)and simulated relative field sensitivity patterns of both elements (b). The dual-plane pair element has higher sensitivity than the planar pair due to the dropped traces for current return and thus decreased field cancellation, enabling greater imaging depth

Each element of the 64-channel dual-plane pair array was matched and tuned at the coil by using a nonmagnetic single-sided varactor diode (BB639; Infineon, Milpitas, CA) for tuning and a variable capacitor (9702-1; Johanson Manufacturing Corp., Boonton, NJ) for matching. The use of varactor diodes for tuning provided very small component packaging in a space-limited situation, and also provided the ability to tune the 64 coils somewhat “in bulk” by controlling the main biasing voltage. The coils were biased over the RF lines by adding a 10KΩ resistor in parallel with the matching capacitor to prevent it from acting as a DC block. Four ultrasound cables, each containing twenty 50Ω coaxial lines (Precision Interconnect “Blue Ribbon,” Wilsonville, OR, USA), were used to connect the coil to two 32-channel “digital tuning boards” described in detail elsewhere 18. The boards allowed for individual control of the bias voltage on each line by using digital potentiometers with 7-bits of resolution on each channel (AD7376; Analog Devices, Inc., Norwood, MA) and controlled by a USB interface digital I/O board (USB-6501; National Instruments Corp., Austin, TX). The ribbon cables were preassembled with low-profile header connectors, and matching surface-mount receptacles (QSE/QTE series; Samtec, Inc., New Albany, IN) were installed on the array and digital tuning boards. A single 15pF fixed capacitor (A series; American Technical Ceramics, Huntington Station, NY) was used at the feed end of each element to electrically shorten the element and increase the homogeneity over its length.

With the increased sensitivity of the dual-plane pair came the need for a coil-to-coil decoupling mechanism that was unnecessary when using planar pair elements. In order to maintain the localization of the sensitivity patterns over each element, geometric decoupling was achieved through adding an overlapping region between coils at the end of the elements only, outside of the 8cm long imaging region. Overlapping the conductors located on two different sides of the PCB required a design implementation using three layer PCB fabrication in order to maintain straightforward, repeatable, and reliable production of the 64 elements. Fabrication of the multilayer board was outsourced to PCBExpress (PCBexpress/Sunstone Circuits., Mulino, OR, www.pcbexpress.com) by using the first (top), third, and fourth (bottom) layers of a standard 4-layer FR-4 board with a distance between layers of 0.012″. A photograph of the front and back of the populated coil and, more efficaciously, the PCB layout detailing the decoupling overlap are shown in Fig. 2. As detailed in the Figure, the current paths were connected through vias on the multi-layered board that allowed for geometric decoupling between adjacent coils. Coupling was evaluated by collecting the upper diagonal half of the 64×64 matrix of S21 values using an Agilent E5071 Network Analyzer and the full matrix was filled based on reciprocity.

Figure 2.

Figure 2

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(a) Front (imaging side) of the 64-channel dual-plane pair element array on a multi-layer board. The imaging side of the coil is free of all components and consists only of 64 signal (center conductor) traces. (b) Back (component side) of the 64-channel array. The geometric decoupling loops are fabricated at the end of the dual-plane pair element, opposite the feed point. A zoomed view of the decoupling loop is shown using circuit-board drawing software for clarity. The current path is denoted by yellow arrows. The center conductor of the dual-plane pair element on the top (imaging) layer is shown in red. It connects through a via to the bottom layer (shown in blue). At the end of the segment, the center conductor connects to the middle layer (shown in pink) through a via, and the coil is geometrically decoupled from its adjacent neighbors by overlap between the middle and bottom layers, spaced 0.012″ apart. On the far right inset, the zoomed match and tune region is shown with, from top to bottom, the fixed capacitor for full-wave effect compensation, the varactor diode for tuning, and the tunable match capacitor.

Imaging and Reconstruction

The 64-channel dual-plane pair array was used to acquire fully encoded high spatial resolution images, with retrospectively reconstructed SEA images, and the results were compared to the original 64-channel planar pair array performance. All imaging was performed in a 4.7 T/33cm magnet supported by a Varian Unity Inova console. The system was modified by interfacing a 64 channel receiver constructed in-house and described elsewhere 11. Sixty-four conventional 50Ω preamplifiers were used, constructed using commercial monolithic RF amplifiers (Model Gali-74+; Mini-Circuits., Brooklyn, NY). The array coil was placed inside a volume coil for transmitting, consisting of two parallel plates shorted at one end and impedance matched at the other end. The volume coil and array were placed on an acrylic positioning former with a flush fit to the magnet bore to eliminate variations in height when switching between the planar pair array and the dual-plane pair array. A resolution phantom built in-house was used for all 64-channel experiments. Fully encoded images were acquired from each channel simultaneously with the following imaging parameters: TR/TE=500/35ms, FOV=13×13cm, Nphase enc × Nreadout = 512×1024, rendering a total acquisition time of 4.26 minutes with a spatial resolution of 254×126 microns. Coronal images with a slice thickness of 1.5mm were taken (in the plane parallel to the plane of the arrays) at distances of 0.5 coil widths (1mm) and 3 coil widths (6mm) away from the array, and compared quantitatively with regard to SNR. The images were reconstructed using a sum-of-squares method, with the highly localized individual coil images (2mm wide coil in a 13cm wide FOV) first masked at six times their width before the sum-of-squares operation, at least partially indicative of the potential ability to perform accelerated higher resolution imaging with the array.

SEA images were created retrospectively from a single k-space line per coil as an evaluative process regarding imaging at the extreme of temporal resolution with the coil, not as a true exercise in the acquisition of accelerated images. Therefore, the effective SEA image acquisition time was TR (500 milliseconds), with TE=35ms, FOV=13×13cm, and Nphase enc × Nreadout = 1×1024 (for each of the 64 coils). SEA image reconstruction was performed by “stacking” the 1D Inverse Fourier Transformation of the single 1024 point echo from each coil into a 64 × 1024 matrix and interpolated to higher resolution for display.

Results and Discussion

The 64×64 matrix of S21 values between the elements of the geometrically decoupled dual-plane pair array is shown in Fig. 3. The average nearest neighbor S21 value was -19.9 dB, with maximum decoupling of S21 = -23 dB and minimum of S21 = -17 dB. The average decoupling between next nearest neighbors was measured as S21 = -25.7 dB, with maximum decoupling of S21 = -30.4 dB and minimum of S21 = -22.5 dB. The variability that exists in the measurements despite the repeatable PCB fabrication can likely be explained by coupling within the four blue ribbon cables, the effect of which is evident in the four “bulges” in the S21 matrix shown in Fig. 3. The decoupling achieved with the overlapped region is not without cost in this case. The elements are copper loss dominated by their “microcoil” nature; therefore, it must be noted that the addition of a geometric decoupling region approximately half the length of the imaging region undoubtedly incurred significant losses. Specifically, the additional 2.5 cm of copper trace used for decoupling in the 8 cm long elements brings a 15% loss in achievable SNR due simply to the increased resistance. As shown below, however, this loss was small compared to the improved SNR afforded by the new element design. The ability to easily fabricate a repeatable, integrated, decoupling configuration on a conventional PC board while achieving such significant improvements in imaging depth made the added loss (at least for the time being) insignificant compared to the benefit.

Figure 3.

Figure 3

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Decoupling matrix indicated by measured S21 values [dB] for the 64-channel dual-plane pair element array. The average nearest neighbor coupling was measured as S21 = 19.9dB, and average next-nearest neighbor coupling was measured at S21 = -25.7dB. The “bulges” visible in the matrix are most likely explained by coupling within the four ribbon cables used to interface to the

The images presented in Figs. 4 and 5 demonstrate the SNR improvement at depth offered by the dual-plane pair element. Figure 4 shows a reference transmit-receive image collected with the volume coil, with a 4.26 minute acquisition time, spatial resolution of 254×126 microns, and an SNR of 9.5. The white box outlines the area of the phantom zoomed to indicate the imaging capability and improvement in the SNR provided by the new array. Comparative sum-of-squares images are shown, with the dual-plane pair offering a five-fold improvement over the planar pair array at increasing depths and an SNR equivalent to the volume coil at up to three coil widths above the plane of the array. This improvement in SNR is slightly reduced from the ideal case represented by the relative sensitivities shown in Fig. 1b due to the additional copper loss in the decoupling network discussed above as well as the losses in the matching network and feed lines. Another notable benefit of the dual-plane pair element, at least for higher resolution (fully or partially encoding with each coil) applications, can be seen in the increased uniformity of the image acquired with the dual-plane pair elements. The highly constrained field sensitivity patterns of the original planar pair elements suit the needs of SEA imaging well, where the “resolution” of the image in what would be the phase encoding direction is instead determined by the width of the coil patterns. This narrow pattern, however, leads to a significant loss of sensitivity between elements, particularly close to the coil. This “striping artifact”, clearly evident in Fig. 4, is greatly reduced by the increased sensitivity (wider and deeper) of the dual-plane pair element.

Figure 4.

Figure 4

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Fully encoded transmit-receive reference image taken with the volume coil (left). The white rectangle outlines the zoomed region selected to demonstrate the relative capabilities of the planar pair and dual-plane pair element arrays. The sum-of-squares comparison images on the right show the dual-plane pair element to have a five-fold increase in SNR at depth over the planar pair element – equivalent to that of the volume coil. In addition, the wider and deeper sensitivity pattern of the dual-plane pair element improves the “striping artifact” seen when using the planar pair

Figure 5.

Figure 5

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Single echo acquisition (SEA) images reconstructed retrospectively from the fully-encoded data sets from the planar pair and the dual-plane pair arrays. The dual-plane pair array enables SEA imaging at depths not previously possible. The array also shows significant improvements in SNR close to the coil due to an increase in the effective voxel size due to the wider and deeper sensitivity pattern – a fact which can adversely affect the achievable resolution, as described in the text. The 2.5cm long decoupling loops located outside the 8×13cm main imaging region do detect signal, visible as the region of decreased intensity that “completes” the circular phantom in the dual-plane

SEA images were retrospectively reconstructed from the fully encoded sets, and the comparative SEA images are shown in Fig. 5. The decoupling loops exhibit noticeable signal pick-up from the portion of the phantom that extended beyond the 8×13cm imaging region of the array coils, visible as the lower intensity region that “completes” the top of the circular phantom in the two dual-plane pair images in the figure and is not present in the planar pair images. The SNR improvement offered by the dual plane pair is evident, particularly demonstrated by the fact that SEA imaging at a distance of three coil widths (6mm) from the array is achievable with the dual-plane pair elements and was not previously possible with the original planar pair elements. The SNR close to the array (1mm, 0.5 coil widths) when SEA imaging is notably increased to a degree not observed when fully encoding. This is explained by the same wider and deeper sensitivity of the dual-plane pair that mitigated the “striping artifact” in the higher spatial resolution sum of squares imaging. Since the “voxel size” when SEA imaging is determined in one direction by the width of the coil sensitivity pattern, the SNR of the SEA images acquired close to the dual-plane pair is higher than the planar pair with its highly constrained sensitivity pattern because the dual-plane pair element integrates a wider region into its “voxel”. This SNR increase close to the array was not seen in the sum of squares images in Fig. 4 because the voxel size of the two arrays was equivalent, determined in the standard manner by phase encoding. While higher SNR typically provides an unquestionable advantage, integrating a larger region into the signal received per coil, or voxel, will actually degrade the effective spatial resolution when SEA imaging. The effects of the wider element pattern are evidenced in the fact that the SEA image formed with the dual-plane pair elements at three coil widths from the array, where the sensitivity is broad compared to the element width, appears to be elongated and the features spread in what would be the phase encoding direction (vertical). This is due to the fact that the elements are actually integrating signal from a wide region above them, “smearing” the image in that direction. Therefore, the array element of choice is, to some extent, application-dependent, affected by the desired imaging depth and spatial and temporal resolutions.

In addition to the primary goal of obtaining improved sensitivity at depth achieved by the dual-plane pair element, other advantageous factors in the design are worth mentioning. The need to decouple only nearest neighbors straightforwardly with overlap allowed us to maintain a lack of complexity with regard to decoupling networks. This is of particular advantage as decoupling preamplifiers at our frequency of interest in the number that would be needed are expensive, not ubiquitous, and would require a philosophically undue amount of real estate compared to the 2mm × 8cm elements. In addition, the need for decoupling preamplifiers would complicate using the array design in future transmit-receive applications after switching the varactor diodes for tuning capacitors. Also of note, the unbalanced feed of the dual-plane pair (and the original planar pair) element reduces the need for a balun, and in this case of particularly small elements, baluns were not needed at all. This is not the case, for instance, with balanced-fed loop elements, even of this small size, with the metric being observational bench data regarding stability and tunability as the blue ribbon cables to the coils are moved.

In summary, this manuscript has presented an element design for 64-channel planar imaging that offers a significant improvement in sensitivity over previous designs, maintains the ability to fully accelerate in the manner of Single Echo Acquisition imaging by keeping overlapped areas for decoupling out of the imaging region, and maintains simplicity and repeatability of design with multilayer PC board fabrication of the element and decoupling overlaps.

Acknowledgments

The authors gratefully acknowledge support from the National Institutes of Health (1R21EB007649) and the American Heart Association (0930231N).

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