Multi-center benchmarking of cervical spinal cord RF coils for 7 T MRI: A traveling spines study

Abstract

Purpose: The depth within the body, small diameter, long length, and varying tissue surrounding the spinal cord impose specific considerations when designing RF coils. The optimal coil configuration for 7 T cervical spinal cord MRI is unknown and currently there are very few coil options. The purpose of this work was (1) to establish a quality control protocol for evaluating 7 T cervical spinal cord coils, and (2) to use that protocol to evaluate the performance of four different coil designs.

Methods: Three healthy volunteers and a custom anthropomorphic phantom (the traveling spines cohort) were scanned at seven 7 T imaging centers using a common protocol and each center's specific cervical spinal cord coil. Four different coil designs were tested (two in-house, one Rapid Biomedical, and one MRI.TOOLS design).

Results: The Rapid Biomedical coil was found to have the highest $B_1^{+}$ efficiency, whereas one of the in-house designs (NeuroPoly Lab) had the highest SNR and the largest spinal cord coverage. The MRI.TOOLS coil had the most uniform $B_1^{+}$ profile along the cervical spinal cord; however, it was limited in its ability to provide the requested flip angles (especially for larger individuals). The latter was also the case for the second in-house coil (MSSM).

Conclusion: The results of this study serve as a guide for the spinal cord MRI community in selecting the most suitable coil based on specific requirements and offer a standardized protocol for assessing future coils.

Keywords: B 1 + $$ {\mathrm{B}}_1^{+} $$ ; MRI; SNR; g‐factor; hardware; spinal cord.

FIGURE 1

(A) Coil element layout for the four tested designs. Coil 1: Rapid Biomedical, coil 2: NeuroPoly (in‐house), coil 3: MRI.TOOLS, coil 4: MSSM (in‐house). Tx‐only coils are shown in red, Rx‐only in blue, and Tx/Rx in purple. The site(s) possessing a coil design that was included in this comparative study is indicated below the coil description, as well as the corresponding 7 T system. (B) Spinoza V6 phantom. Rx, receive; Tx, transmit.

FIGURE 2

Phantom SC mask creation workflow. The leftmost image is the CRMBM phantom scan that was coregistered onto a representative human anatomical scan. The red dots represent SC labels that were joined (red ROI) to form a path that was dilated, resulting in a phantom SC mask (yellow ROI), covering roughly from brainstem to T5–T6 vertebral level. CRMBM, Centre de Résonance Magnétique Biologique et Médicale; SC, spinal cord.

FIGURE 3

Co‐registration of phantom scans across sites.

FIGURE 4

(A) Spinoza TFL ${\mathrm{B}}_1^{+}$ maps at each participating site and measured ${\mathrm{B}}_1^{+}$ along the SC mask (white outline) for each site. (B) In vivo TFL ${\mathrm{B}}_1^{+}$ maps at each participating site for the same representative subject (subject 3). TFL, turboFLASH.

FIGURE 5

(A) Spinoza DREAM ${\mathrm{B}}_1^{+}$ maps at each participating site and measured ${\mathrm{B}}_1^{+}$ along the SC mask (white outline) for each site. (B) In vivo DREAM ${\mathrm{B}}_1^{+}$ maps at each participating site for the same representative subject (subject 3). DREAM, dual refocusing echo acquisition mode.

FIGURE 6

(A) TFL ${\mathrm{B}}_1^{+}$ along the SC from C1‐T2 across sites for each subject. (B) DREAM ${\mathrm{B}}_1^{+}$ along the SC from C1‐T2 across sites for each subject. TFL, turboFLASH; C1, cervical level 1; T2, thoracic level 2.

FIGURE 7

(A) CoV across C1‐T2 in the spinal cord of TFL ${\mathrm{B}}_1^{+}$ for each subject, across sites. (B) SNR₉₀ averaged across vertebral levels in the spinal cord. The average ${\mathrm{B}}_1^{+}$ CoV and SNR₉₀ across subjects is shown as a black horizontal line. CoV, coefficient of variation.

FIGURE 8

(A) SNR maps measured with the Spinoza phantom at each participating site and measured SNR₉₀ along the SC mask (white outline in the ${\mathrm{B}}_1^{+}$ maps) for each site. (B) SNR for a single representative subject across all sites (subject 3).

FIGURE 9

SNR₉₀ along the spinal cord from C1 to T2 across sites for each subject.

FIGURE 10

(A) 1/g sagittal maps with R_F/H × R_A/p  = 2 × 2 of Spinoza at each site, except for MSSM. (B) 1/g‐factor maps with R_F/H × R_A/p  = 2 × 2 for a single representative subject across all sites (subject 2).