Double‐tuned ³¹P/¹H human head array with high performance at both frequencies for spectroscopic imaging at 9.4T

Abstract

Purpose:To develop a robust design of a human head double-tuned 31 P/ 1 H array, which provides good performance at both 31 P and 1 H frequencies for MR spectroscopic imaging at 9.4T. Methods: Increasing the number of surface loops in a human head array improves the peripheral signal-to-noise ratio (SNR), while the central SNR doesn't substantially change. High peripheral SNR can contaminate MR spectroscopic imaging data at both 1 H and 31 P frequency. To minimize this effect, we limited the number of elements in the 31 P array to 10, i.e., 8 transceiver surface loops circumscribing the head and 2 receive "vertical" loops placed at the superior location. The 1 H-portion of the array also consists of 10 elements, i.e., 8 transceiver surface loops circumscribing the head and 2 transceiver "vertical" loops at the superior location of the head. Both the 31 P array and 1 H array are placed in a single layer at the same distance to the head, which provides high loading and, thus, a good performance for both arrays. Results: Transmit efficiency of the 1 H-portion of the double-tuned array was very similar to that of the single-tuned arrays of similar size. Also, addition of the crossloops substantially improved the brain coverage. Conclusion: We developed a novel 31 P/ 1 H double-tuned array for MR spectroscopic imaging of a human brain at 9.4T. Placing both 31 P and 1 H loops in a single layer provides for high transmit efficiency at both frequencies without compromising SNR near the brain center at the 31 P-frequency. Addition of the cross-loops at the superior location improves the brain coverage.

Comparison of four 31P single‐voxel MRS sequences in the human brain at 9.4 T

“…However, the SNR of the coil is higher the closer to the periphery (see Figure 1C). 38 Moreover, the inhomogeneity of the surface coil mode might influence pulse performance further inside the brain. These two reasons reduce the effective voxel size of the 5 × 5 × 5 cm 3 VOI, which results in a lower SNR difference than theoretically calculated.…”

“…All experiments were conducted on a 9.4T whole‐body MRI scanner (Siemens, Erlangen, Germany) using a home‐built double‐tuned 20‐loop 31 P/ 1 H head array 38 . The array consisted of 16 transceiver surface loops (eight loops at each frequency) circumscribing the head.…”

“…B, Simulated transversal map obtained using the 31 P surface coil mode of the array (only three surface loops for transmission) loaded by the Duke voxel model. C, Measured SNR map obtained using the 31 P array with all 10 receive loops loaded by the elliptical phantom, as described in detail in Avdievich et al 38 Dotted lines present the chosen voxel positions of the two different voxel sizes 3 × 3 × 3 cm 3 and 5 × 5 × 5 cm 3…”

“…All experiments were conducted on a 9.4T whole-body MRI scanner (Siemens, Erlangen, Germany) using a home-built double-tuned 20-loop 31 P/ 1 H head array. 38 The array consisted of 16 transceiver surface loops (eight loops at each frequency) circumscribing the head. All 16 loops were placed on the surface of the same tight-fit holder at the same distance to the sample, ensuring high transmit and receive performance at both frequencies.…”

Electromagnetic simulation of a 16‐channel head transceiver at 7 T using circuit‐spatial optimization

“…However, more complex features, such as the Q factors and isolated resonant frequencies of the TD circuits, are not well modeled and optimized in the electromagnetic simulation or circuit analysis software. 33 In this work, we establish methods to facilitate the design and optimization of a decoupled transceiver array for the human head. Based on the double row transceiver array as described by Avdievich, [34][35][36] we introduce a closed-form S-parameter matrix of the transceiver that accounts for the matching circuits, decoupling circuits, and lumped capacitors.…”