Optimized parallel transmit and receive radiofrequency coil for ultrahigh-field MRI of monkeys

Kyle M Gilbert; Joseph S Gati; Kevin Barker; Stefan Everling; Ravi S Menon

doi:10.1016/j.neuroimage.2015.10.048

Optimized parallel transmit and receive radiofrequency coil for ultrahigh-field MRI of monkeys

Neuroimage. 2016 Jan 15:125:153-161. doi: 10.1016/j.neuroimage.2015.10.048. Epub 2015 Oct 20.

Authors

Kyle M Gilbert¹, Joseph S Gati², Kevin Barker³, Stefan Everling⁴, Ravi S Menon⁵

Affiliations

¹ Centre for Functional and Metabolic Mapping, The University of Western Ontario, London, ON, Canada. Electronic address: kgilbert@robarts.ca.
² Centre for Functional and Metabolic Mapping, The University of Western Ontario, London, ON, Canada.
³ Centre for Functional and Metabolic Mapping, The University of Western Ontario, London, ON, Canada; Neuronitek, London, ON, Canada.
⁴ Centre for Functional and Metabolic Mapping, The University of Western Ontario, London, ON, Canada; Department of Physiology and Pharmacology, The University of Western Ontario, London, ON, Canada.
⁵ Centre for Functional and Metabolic Mapping, The University of Western Ontario, London, ON, Canada; Department of Medical Biophysics, The University of Western Ontario, London, ON, Canada.

PMID: 26497267
DOI: 10.1016/j.neuroimage.2015.10.048

Abstract

Monkeys are a valuable model for investigating the structure and function of the brain. To attain the requisite resolution to resolve fine anatomical detail and map localized brain activation requires radiofrequency (RF) coils that produce high signal-to-noise ratios (SNRs) both spatially (image SNR) and temporally. Increasing the strength of the static magnetic field is an effective method to improve SNR, yet this comes with commensurate challenges in RF coil design. First, at ultrahigh field strengths, the magnetic field produced by a surface coil in a dielectric medium is asymmetric. In neuroimaging of rhesus macaques, this complex field pattern is compounded by the heterogeneous structure of the head. The confluence of these effects results in a non-uniform flip angle, but more markedly, a suboptimal circularly polarized mode with reduced transmit efficiency. Secondly, susceptibility-induced geometric distortions are exacerbated when performing echo-planar imaging (EPI), which is a standard technique in functional studies. This requires receive coils capable of parallel imaging with low noise amplification during image reconstruction. To address these challenges at 7T, this study presents a parallel (8-channel) transmit coil developed for monkey imaging, along with a highly parallel (24-channel) receive coil. RF shimming with the parallel-transmit coil produced significant advantages-the transmit field was 38% more uniform than a traditional circularly polarized mode and 54% more power-efficient, demonstrating that parallel-transmit coils should be used for monkey imaging at ultrahigh field strengths. The receive coil had the ability to accelerate along an arbitrary axis with at least a three-fold reduction factor, thereby reducing geometric distortions in whole-brain EPI.

Keywords: Echo-planar imaging; Monkey; Parallel transmit; Receive coil; Transmit coil.

Publication types

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

MeSH terms

Animals
Brain Mapping / instrumentation*
Brain Mapping / methods
Echo-Planar Imaging / instrumentation*
Echo-Planar Imaging / methods
Macaca mulatta
Neuroimaging / instrumentation*
Neuroimaging / methods

Abstract

Publication types

MeSH terms

Grants and funding