Effects of eddy currents on selective spectral editing experiments at 3T

Georg Oeltzschner; Karim Snoussi; Nicolaas A Puts; Mark Mikkelsen; Ashley D Harris; Subechhya Pradhan; Kyrana Tsapkini; Michael Schär; Peter B Barker; Richard A E Edden

doi:10.1002/jmri.25813

Effects of eddy currents on selective spectral editing experiments at 3T

J Magn Reson Imaging. 2018 Mar;47(3):673-681. doi: 10.1002/jmri.25813. Epub 2017 Jul 22.

Authors

Georg Oeltzschner^{1

2}, Karim Snoussi^{1

2}, Nicolaas A Puts^{1

2}, Mark Mikkelsen^{1

2}, Ashley D Harris^{3

4

5}, Subechhya Pradhan^{6

7}, Kyrana Tsapkini⁸, Michael Schär¹, Peter B Barker^{1

2}, Richard A E Edden^{1

2}

Affiliations

¹ Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
² F. M. Kirby Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, Maryland, USA.
³ CAIR Program, Alberta Children's Hospital Research Institute, University of Calgary, AB, Canada.
⁴ Department of Radiology, University of Calgary, AB, Canada.
⁵ Hotchkiss Brain Institute and Alberta Children's Hospital Research Institute, Calgary, AB, Canada.
⁶ Diagnostic Imaging and Radiology, Children's National Health System, Washington, DC, USA.
⁷ Department of Radiology, George Washington University, Washington, DC, USA.
⁸ Department of Neurology, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.

Abstract

Purpose: To investigate frequency-offset effects in edited magnetic resonance spectroscopy (MRS) experiments arising from B₀ eddy currents.

Materials and methods: Macromolecule-suppressed (MM-suppressed) γ-aminobutyric acid (GABA)-edited experiments were performed at 3T. Saturation-offset series of MEGA-PRESS experiments were performed in phantoms, in order to investigate different aspects of the relationship between the effective editing frequencies and eddy currents associated with gradient pulses in the sequence. Difference integrals were quantified for each series, and the offset dependence of the integrals was analyzed to quantify the difference in frequency (Δf) between the actual vs. nominal expected saturation frequency.

Results: Saturation-offset N-acetyl-aspartate-phantom experiments show that Δf varied with voxel orientation, ranging from 10.4 Hz (unrotated) to 6.4 Hz (45° rotation about the caudal-cranial axis) and 0.4 Hz (45° rotation about left-right axis), indicating that gradient-related B₀ eddy currents vary with crusher-gradient orientation. Fixing the crusher-gradient coordinate-frame substantially reduced the orientation dependence of Δf (to ∼2 Hz). Water-suppression crusher gradients also introduced a frequency offset, with Δf = 0.6 Hz ("excitation" water suppression), compared to 10.2 Hz (no water suppression). In vivo spectra showed a negative edited "GABA" signal, suggesting Δf on the order of 10 Hz; with fixed crusher-gradient coordinate-frame, the expected positive edited "GABA" signal was observed.

Conclusion: Eddy currents associated with pulsed field gradients may have a considerable impact on highly frequency-selective spectral-editing experiments, such as MM-suppressed GABA editing at 3T. Careful selection of crusher gradient orientation may ameliorate these effects.

Level of evidence: 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:673-681.

Keywords: GABA; MEGA-PRESS; eddy currents; edited MRS; macromolecules; offset.

Publication types

Research Support, N.I.H., Extramural
Research Support, Non-U.S. Gov't

MeSH terms

Aged
Algorithms
Brain / diagnostic imaging*
Brain / metabolism
Electromagnetic Fields
Humans
Macromolecular Substances / metabolism
Magnetic Resonance Spectroscopy / methods*
Magnetics
Male
Phantoms, Imaging
Signal Processing, Computer-Assisted*
gamma-Aminobutyric Acid / metabolism

Substances

Macromolecular Substances
gamma-Aminobutyric Acid

Abstract

Publication types

MeSH terms

Substances

Grants and funding