A second-order and slice-specific linear shimming technique to improve spinal cord fMRI

D Tsivaka; S C R Williams; S Medina; O S Kowalczyk; J C W Brooks; M A Howard; D J Lythgoe; I Tsougos

doi:10.1016/j.mri.2023.06.012

A second-order and slice-specific linear shimming technique to improve spinal cord fMRI

Magn Reson Imaging. 2023 Oct:102:151-163. doi: 10.1016/j.mri.2023.06.012. Epub 2023 Jun 21.

Authors

D Tsivaka¹, S C R Williams², S Medina², O S Kowalczyk², J C W Brooks³, M A Howard², D J Lythgoe², I Tsougos⁴

Affiliations

¹ Medical Physics Department, Medical School, University of Thessaly, Larisa, Greece; Department of Neuroimaging, Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
² Department of Neuroimaging, Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
³ School of Psychology, University of East Anglia, Norwich, UK.
⁴ Medical Physics Department, Medical School, University of Thessaly, Larisa, Greece; Department of Neuroimaging, Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK. Electronic address: tsougos@uth.gr.

PMID: 37353180
DOI: 10.1016/j.mri.2023.06.012

Abstract

Purpose: To develop a second-order and slice-specific linear shimming technique and investigate its efficiency in the mitigation of signal loss and distortions, and the increase of temporal signal-to-noise ratio (tSNR) within the spinal cord during functional Magnetic Resonance Imaging (fMRI) of the human cervical spinal cord.

Methods: All scans were performed on a General Electric Discovery MR750 3 T scanner, using a head, neck and spine coil and a neurovascular array. To improve B₀ homogeneity, a field map was acquired, and second-order shims (SOS) were optimized over manually defined regions of interest (ROIs). Signal loss from dephasing by susceptibility-induced gradients was reduced by optimizing slice-specific x-, y- and z-shims to maximize signal within the spinal cord. Spectral-spatial excitation pulses were used in both the slice-specific linear shimming calibration scan and fMRI acquisitions. The shimming technique's efficiency was initially tested on eight healthy volunteers by comparing tSNR between images acquired with the manufacturer's standard linear shimming and with our SOS and xyz-shimming technique. Subsequently, using an increased spatial resolution as needed for fMRI of the spinal cord, tSNR measurements were performed on resting-state fMRI images from 14 healthy participants.

Results: Spinal fMRI images acquired with only the standard linear shimming suffered from severe signal loss below the C5 vertebral level. The developed shimming technique compensated for this loss especially at levels C6 and C7, while tSNR was significantly higher at all vertebral levels with SOS and xyz-shimming than without it.

Conclusion: A comprehensive shimming approach which includes the use of spectral-spatial excitation pulses along with both second-order and slice-specific linear shim optimization reduces regional signal loss and increases tSNR along the c-spine (C3-C7), improving the ability to record functional signals from the human spinal cord.

Keywords: Field-mapping; Linear shimming; Second-order shimming; Spinal cord; fMRI.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Brain*
Humans
Image Processing, Computer-Assisted / methods
Magnetic Resonance Imaging* / methods
Spinal Cord / diagnostic imaging

Substances

methyl 3-mercaptopropionimidate

Grants and funding

MR/N026969/1/MRC_/Medical Research Council/United Kingdom