Introduction

Although the knee is the most commonly MRI-imaged joint in the pediatric population, there is to date no dedicated pediatric knee coil commercially available. Thus, possible SNR gains and fast imaging with parallel acquisition methods are not fully realized due to the increased distance between the pediatric knee and receive arrays in the adult-size knee coils. Here we attempt to address this issue and present a dedicated pediatric knee coil array for use at 7 Tesla.

Materials and Methods

The coil combines 16 close fitting receive-only array elements with an 8-channel pTx stripline transceiver array for a combined 8-channel transmit / 24-channel receive 7 T Knee Array (Fig. 1). The eight-channel anterior receive array was built using a previously described vacuum-formed coil building technique1,2. Briefly, the conformal array was built by patterning traces on a flat polycarbonate sheet. The sheet was then vacuum formed onto a mold of a knee and the traces were made conductive with electroless copper plating (Fig. 2). After populating the traces with RF components, the form-fitting printed array (~ 12 cm x 7 cm ea.) was combined with an 8-channel posterior receive array which was integrated into the lower knee coil housing. The eight posterior receive coils were arranged in three rows along the Z-axis, (with 3-2-3 coil elements per row) to maximize the parallel imaging opportunities given the 8-channel limit of the system connector. Six of the posterior elements (3 in the superior, and 3 in the inferior rows) were circular loops with diameter of 5 cm, while the middle row consisted of two elliptically shaped loops with major diameter of ~7.6 cm, and a minor diameter of 5 cm. All receive array elements (both anterior and posterior) were connected to a low-input impedance preamplifier (WanTcom, Chanhassen, MN, USA) to take advantage of preamplifier decoupling, as well as being overlapped with neighboring loops for geometric decoupling. The 8-channel transmit array (which can also be used as a transceiver array) consisted of 8 independently-shielded stripline elements, equally split between the anterior and posterior housings. These 8 stripline elements were capacitively-shortened using fixed chip capacitors (KYOCERA AVX, Fountain Inn, SC, USA), and mounted on a 23.5 cm diameter former. Each element had a 12.5 mm wide central conductor separated from a copper shield by a polytetrafluoroethylene (PTFE) dielectric. Each of the stripline elements was tuned, matched, and decoupled with variable capacitors (Knowles/Voltronics, Denville , NJ, USA) to both a knee-shaped phantom load, as well as a human knee. The stripline elements were capacitively decoupled to -13.7 dB or better to the knee-shaped loading phantom, while the human knee presents a larger load to the coil and thus increases the transceiver element decoupling. The complete knee coil assembly was made up of three major subassemblies: the anterior, containing four stripline transceiver elements; the posterior, containing four stripline transceiver elements and 8 receiver elements; and the previously published 8-channel receiver insert [1,2]. The anterior and posterior housings were designed and 3D printed (Fusion3 F410, Greensboro, NC, USA) in-house using PETG filament, while the 8-channel receiver insert was vacuum formed [1,2]. The anterior and posterior housings formed a flared inner diameter of ~22, ~16.5, and ~17.5 cm as measured along the z-axis. The performance of the newly developed pediatric knee coil prototype was compared to the commercially available 16 cm i.d. 7T industry standard knee coil (birdcage transmitter and 28-channel receive array). To evaluate the expected SNR gains of the close-fitting receive arrays in the pediatric knee coil, a 3D-printed knee phantom was placed 7 mm above the bottom of the coil and scanned with both coils at 7T (Siemens Healthineers, Erlangen, Germany). The shape of the 11 cm long phantom was based on an MRI of a 17-year-old and the phantom was filled with 100 mM saline and 4 mM CuSO4. A fully-relaxed gradient echo image was reconstructed in SNR units [3] and corrected by flip angle maps[4] to simulate the SNR of a homogenous excitation[5]. The volunteer study was approved by the institutional review board and informed consent was obtained.

Results and Discussion

Contribution of individual stripline transceiver elements (S1 to S8) and of the vacuum formed receiver insert (RX1 to RX8) were acquired (Fig. 3). Since the phantom is located close to the bottom of the coils, both coils have good posterior SNR (Fig. 4). The close proximity of the vacuum-formed array significantly increases peripheral SNR and the nearly 100% filling-factor contributes additionally to a 39% increase in central SNR. Though we designed our initial prototype conformal receive array for older adolescents, we plan to build numerous sizes of the vacuum formed arrays to cover the full pediatric population. In future work we also plan to explore multi-row arrangements of the anterior array for further improved SNR and parallel imaging performance. Not discussed here, we observed extensive longitudinal coverage with our stripline transceivers and associated higher overall RF power requirements, thus we plan in the future to evaluate shielded loop transmitters or dipoles as alternative transceivers.

Acknowledgements

This research was funded by NIH, NIAMS RO1 AR070020, NIH BTRC P41 EB027061. The research was partially funded by NIH 1U01EB029427-01 and NIH 1U01EB025162-01. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No.(DGE 1752814). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

References

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