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Double Tuned 31P/1H Elliptical Transceiver Phased Array for the Human Brain Studies at 7T

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Avdievich, N., Pan, J., & Hetherington, H. (2011). Double Tuned 31P/1H Elliptical Transceiver Phased Array for the Human Brain Studies at 7T. Poster presented at 28th Annual Scientific Meeting ESMRMB 2011, Leipzig, Germany.

Introduction: The improved SNR at 7T provides advantages for both 1H and X nuclei. At 7T, transceiver phased arrays improve both B1 homogeneity and transmit efficiency compare to volume coils. Similarly, they improve SNR for brain periphery for studies of X nuclei. Therefore, double tuned transceiver arrays may provide advantages over volume head coils. We have developed a 16-element (8 at each frequency) split elliptical 31P/1H (120.7/298MHz) transceiver array with improved efficiency and homogeneity. Methods: The array consisted of two layers of surface coils circumscribing the head (2x8) with the inner array resonating at 31P frequency. The length of 31P and 1H arrays measured 10 and 9cm, respectively. All the adjacent surface coils were decoupled inductively. 31P and 1H surface coils were decoupled using resonant traps. Double tuned cable traps were used to suppress shield currents at both frequencies. The array was split in two sections with no electrical connection between them. To accommodate different head sizes we constructed two different array tops (Fig.1), which allowed varying the array’s height (21 and 23cm). The array width measured 19.5cm. We compared the array with a 31P/1H TEM volume coil (1H length - 13cm, 31P length - 16cm) and a single layer 31P/1H array utilizing LC-trap design. Results: For 1H transmission, the array achieved 1.00±0.09kHz of B1 at a power of 2.24kW over an axial slice near the array center. The 31P/1H TEM required 5.66kW to achieve 1.00±0.16kHz. The LC-trap array was also substantially worse and achieved 1kHz of B1 at 4.35kW. For 31P transmission, the double layer 31P/1H array required 35% less power than the TEM for the same B1. For 31P reception, the array achieved up to 400% increase in SNR over the TEM at the brain periphery, and up to 20% higher SNR from central brain regions. Fig.2 displays a scout image and 31P spectra from peripheral (A) and central (B) locations acquired with the TEM and the array. The spectra are plotted on the same vertical scale with identical processing. The 31P CSI images (Fig. 3) display a measure of the SNR of PCr resonance. Conclusion: The 31P/1H transceiver array increased both B1 homogeneity and efficiency for 1H transmission in comparison to the 31P/1H TEM. For 31P studies, the transceiver array provides similar transmission efficiency across the head (35% increase); similar SNR for central brain locations (20% increase) and substantially greater SNR (up to a 400%) for periphery.