Spinal cord MRI at 7T

Abstract

Magnetic resonance imaging (MRI) of the human spinal cord at 7T has been demonstrated by a handful of research sites worldwide, and the spinal cord remains one of the areas in which higher fields and resolution could have high impact. The small diameter of the cord (∼1 cm) necessitates high spatial resolution to minimize partial volume effects between gray and white matter, and so MRI of the cord can greatly benefit from increased signal-to-noise ratio and contrasts at ultra-high field (UHF). Herein we review the current state of UHF spinal cord imaging. Technical challenges to successful UHF spinal cord MRI include radiofrequency (B) nonuniformities and a general lack of optimized radiofrequency coils, amplified physiological noise, and an absence of methods for robust B shimming along the cord to mitigate image distortions and signal losses. Numerous solutions to address these challenges have been and are continuing to be explored, and include novel approaches for signal excitation and acquisition, dynamic shimming and specialized shim coils, and acquisitions with increased coverage or optimal slice angulations.

A method for correcting breathing‐induced field fluctuations in T2*‐weighted spinal cord imaging using a respiratory trace

“…In this work, we have presented a correction for breathing‐induced field fluctuations in spinal cord T2*‐weighted imaging. Dynamic B 0 fields are a considerable challenge in spinal cord imaging, especially at ultrahigh field . We have here demonstrated that appropriate correction is crucial for both image quality and quantitative measures in multi‐shot structural and functional acquisitions.…”

Real‐time correction of respiration‐induced distortions in the human spinal cord using a 24‐channel shim array

“…Hence, the fmincon function of MatLab (MathWorks) was used to find a constrained minimum to [8]: limiting the minimum and maximum current for each of the N s ¼ 24 channels to 6 1 A (27). These optimal current distributions were used to solve for the shim field offset and coupling coefficients in Equations [6] and [7] via the relation Ai s ¼ b s .…”

“…To train the real-time shim updates, GRE field maps were acquired in inspired and expired respiratory states with acquisition and phase processing parameters similar to those described in our previous work (20). Pressure measurements were recorded during each of the field map acquisitions, with the difference in their respective median values used as Dp in Equations [6] and [7]. The shim VOI M was generally prescribed as the full set of GRE field map voxels for which reliable field measurements could be obtained: This was defined empirically as the intersection of regions over which the absolute field measurements jb vjin j and jb vjex j were both less than 600 Hz, and where their maximum absolute difference jb vjin À b vjex j was less than 250 Hz, with the latter threshold loosely informed by previous RIRO measurements observed near the spinal cord at 3T (2).…”

“…The optimal shim field offset b sjo and coupling coefficients c s were determined via Equations [6] and [7]. Real-time shim updates were computed according to Equation [11] and issued every 250 ms based on the respiratory trace sampled at 100 Hz.…”

“…As noted previously (6), the need for high resolution motivates a push to ultrahigh field strengths ( ! 7T) for the advantages afforded in terms of SNR and susceptibility contrast (e.g., for BOLD-fMRI and susceptibility-weighted imaging); however, these advantages are undermined insofar that the field distortions and their adverse effects on the images also scale with the strength of the main field (7). Given the extent of these artifacts, along with the inherent subtlety of the signal changes associated with BOLD activation (typically on the order of 1%), fMRI of the spinal cord remains technically challenging.…”