A patient-friendly 16-channel transmit/64-channel receive coil array for combined head-neck MRI at 7 Tesla

Abstract

Purpose: To extend the coverage of brain coil arrays to the neck and cervical-spine region to enable combined head and neck imaging at 7 Tesla (T) ultra-high field MRI.

Methods: The coil array structures of a 64-channel receive coil and a 16-channel transmit coil were merged into one anatomically shaped close-fitting housing. Transmit characteristics were evaluated in a B₁⁺ -field mapping study and an electromagnetic model. Receive SNR and the encoding capability for accelerated imaging were evaluated and compared with a commercially available 7 T brain array coil. The performance of the head-neck array coil was demonstrated in human volunteers using high-resolution accelerated imaging.

Results: In the brain, the SNR matches the commercially available 32-channel brain array and showed improvements in accelerated imaging capabilities. More importantly, the constructed coil array improved the SNR in the face area, neck area, and cervical spine by a factor of 1.5, 3.4, and 5.2, respectively, in regions not covered by 32-channel brain arrays at 7 T. The interelement coupling of the 16-channel transmit coil ranged from -14 to -44 dB (mean = -19 dB, adjacent elements <-18 dB). The parallel 16-channel transmit coil greatly facilitates B₁⁺ field shaping required for large FOV neuroimaging at 7 T.

Conclusion: This new head-neck array coil is the first demonstration of a device of this nature used for combined full-brain, head-neck, and cervical-spine imaging at 7 T. The array coil is well suited to provide large FOV images, which potentially improves ultrahigh field neuroimaging applications for clinical settings.

Keywords: 7 Tesla (7T); MRI; array coil; head and neck; neuroimaging; ultrahigh field (UHF).

Figure 1:

CAD model of the constructed 16ch^Tx/ 64ch^Rx head-neck coil. The coil is divided into an anterior and posterior part (A, B). The green highlighted rail structure shows the mounting framing of the anatomically shaped 16-channel Tx array. The assembled head and neck coil (C) features cutouts for the eyes and mouth to facilitate visual stimulation and free breathing, respectively.

Figure 2:

Fully assembled 16ch^Tx/ 64ch^Rx head-neck coil without the covers on top of the stand for bench top measurements. The Tx and Rx coil structures were merged into one anatomical shaped close-fitting housing. The coil housing is splittable to allow an easy entry for the patient.

Figure 3:

Un-wrapped loop configuration of the constructed 16ch^Tx/ 64ch^Rx head-neck coil (A). The anterior Rx coil comprises 24 elements (black), and the posterior segment consists of 40 Rx elements (blue). The Tx array is made of 16 elements, arranged in two z-stacked rows (red). The Tx loop elements are anatomically shaped and radially encompass the head and neck regions (B).

Figure 4:

Circuit schematics of the Rx loop element (A) and Tx coil (B). Rx loop elements consist of tuning capacitors (C_T1, C_T2, C_T3, C_T4, C_T5), an active and passive detuning circuitry (D_1, D_X, C_D, L_RFC1), and a fast-switching RF-fuse (F). The Tx elements consist of series tuning capacitors (C_T6, C_T7, C_T8), a PIN diode (D₂) for active tuning, a balanced drive port with two matching capacitors (C_M2), and a “bazooka balun” to eliminate common mode RF cable currents. The RF choke L_RFC1 is needed to provide a stable DC voltage potential between the X-diode and the capacitor C_D during the bias switching transient.

Figure 5:

Six representative S-matrices of the 16-channel Tx array for different coil loads obtained from bench measurements. Matching (S₁₁, diagonal elements of each triangled matrix) and interelement coupling (S₂₁, off diagonal of each triangled matrix) were obtained from a head neck phantom and five volunteers. The 16-channel Tx coil shows only modest variation in coupling across different loading sizes.

Figure 6:

Noise correlation comparison between the constructed 16ch^Tx/ 64ch^Rx head-neck coil and the commercial 8ch^Tx/ 32ch^Rx head-only coil. The average noise correlations of the head-neck and the commercial coils were measured to be 11.7% (range 0.1% – 58%) and 5.1% (0.08% – 47%), respectively.

Figure 7:

In vivo SNR map for the 64-channel head-neck array and the commercially available 32-channel head-only coil. In the brain area, the SNR maps show roughly equivalent performance for both coils. However, in the face, lower brain stem, and C-spine region, the 64-channel head-neck array outperforms the 32-channel vender coil by a factor of 1.5, 3.4, and 5.2, respectively.

Figure 8:

Inverse G-factors for representative slice obtained from the 64-channel head-neck and the 32-channel coils. In the brain region, the 64-channel head-neck coil shows slightly lower noise amplifications at all acceleration stages when compared to the 32-channel head coil. The green and red numbers in brackets indicate the mean and maximum G-factors, respectively (non-inverted values). Information for slices in the neck area can be found in Supporting Information Figures S3.

Figure 9:

Simulated and measured B₁⁺ field maps for each individual channel in two representative transverse slices of the brain and neck region. Qualitatively, the spatial distributions of the B₁⁺ fields between the simulation and the measurement showed good correlation.

Figure 10:

Combined head and neck image obtained from a gradient echo sequence, acquired with the constructed 16ch^Tx/ 64ch^Rx head-neck coil to demonstrate the extended coil coverage (gradient echo sequence: TR/TE/α = 40 ms / 5 ms / 20°, M: 344 × 344, resolution: 0.3 × 0.3 × 2 mm, BW: 320 Px/Hz).