An 8-channel Tx dipole and 20-channel Rx loop coil array for MRI of the cervical spinal cord at 7 Tesla

Abstract

The quality of cervical spinal cord images can be improved by the use of tailored radiofrequency (RF) coil solutions for ultrahigh field imaging; however, very few commercial and research 7-T RF coils currently exist for the spinal cord, and in particular, those with parallel transmission (pTx) capabilities. This work presents the design, testing, and validation of a pTx/Rx coil for the human neck and cervical/upper thoracic spinal cord. The pTx portion is composed of eight dipoles to ensure high homogeneity over this large region of the spinal cord. The Rx portion is made up of twenty semiadaptable overlapping loops to produce high signal-to-noise ratio (SNR) across the patient population. The coil housing is designed to facilitate patient positioning and comfort, while also being tight fitting to ensure high sensitivity. We demonstrate RF shimming capabilities to optimize B₁ ⁺ uniformity, power efficiency, and/or specific absorption rate efficiency. B₁ ⁺ homogeneity, SNR, and g-factor were evaluated in adult volunteers and demonstrated excellent performance from the occipital lobe down to the T4-T5 level. We compared the proposed coil with two state-of-the-art head and head/neck coils, confirming its superiority in the cervical and upper thoracic regions of the spinal cord. This coil solution therefore provides a convincing platform for producing the high image quality necessary for clinical and research scanning of the upper spinal cord.

Keywords: 7 T; MRI; dipole; radiofrequency coil; spinal cord; transmit/receive coil; ultrahigh field.

Figure 1.

Views of the assembled Tx/Rx coil in its imaging mode (A) and with the hinged assembly (Tx-8/Rx-16–20) completely open to allow subject placement (B). The head-neck phantom built for coil adjustment and evaluation is shown in (C).

Figure 2

Posterior views of the coil and element layout. (A) Rx subarray comprising elements 1 to 15; lateral Tx elements can be seen on the sides: 1, 2 on the left and 6, 7 on the right. (B) Full view of Tx elements 3, 4 and 5. (C) Distribution of Rx loops on the posterior (1–15) and anterior (16–20) Rx coil sections. (D) Posterior-right perspective view of the phantom showing the arrangement of Tx dipoles.

Figure 3

Electrical schematic of the Rx (A) and Tx (B) elements.

Figure 4.

Sensitivity profile of the posterior (A) and anterior (B) Rx subarrays obtained with a FLASH sequence having an FOV of 320 × 320 mm and a 512 × 512 matrix. The intensity colormap scaling was kept the same across the 20 panels.

Figure 5.

SNR maps along the sagittal midline (A) and the C3-C4 level (B). SNR along the spinal cord (C) ranges from 80 at the T2-T3 disc, increasing to 200 at the top of the C1 vertebra.

Figure 6.

Maps of the inverse g-factor (1/g), show at the sagittal midline with acceleration in the superior-inferior and right-left directions (top), at the sagittal midline with acceleration in the anterior-posterior and right-left directions (middle), and axially at the C3-C4 level with acceleration factors in the right-left and superior-inferior directions (bottom).

Figure 7.

S-parameter matrices of the Tx coil measured on the bench inside a mock RF shield (A) and obtained from the scanner (B), showing a fair resemblance.

Figure 8.

Simulated B₁⁺ efficiency, 10-g-averaged SAR efficiency, and B₁⁺ efficiency per square root of the maximum local SAR for four CST body models (shown on the left)—Hugo, Gustav, Laura, and Donna—when driven in the nominal CP mode. The maximum local SAR occurs in the nose for a large body model like Hugo (due to close proximity to the anterior transmit dipole) and in the neck for small-to-average-sized body models. All four body models were incorporated into online SAR matrices—the diversity of which ensures a conservative estimate of local SAR.

Figure 9.

Example of the effect of RF shimming for optimal uniformity on the B₁⁺ map (A), GRE scan (B) and T1 maps from the MP2RAGE sequence (C).

Figure 10.

SNR assessment and comparison with two different coils displaying SNR maps (A) and profiles (B). Each point on the profiles is the average SNR within the region of interest shown on the corresponding maps for each slice.