Strongly coupled versus uncoupled spin response to radio frequency interference effects: application to glutamate and glutamine in spectroscopic imaging

Abstract

It is well known that comparable radio frequency (RF) wavelengths and human head dimensions at high fields can lead to an inhomogeneous RF field when using standard RF transmission. However, the impact of RF inhomogeneity on potential differences in quantification between coupled and uncoupled spins at longer echo times has not been investigated thoroughly. The consequence of this RF interference on metabolite quantification in spectroscopic imaging at 4.7 T was investigated for the strongly coupled spin systems of glutamate and glutamine at an echo time of 120 ms, and compared with the singlet response of choline. These effects were studied using a single-voxel PRESS sequence (alpha-2alpha-2alpha) with varying flip angle (alpha) from 90 degrees to 65 degrees in simulation, phantom, and in vivo experiments. Phantom metabolite yield decreased to 57% for choline and 27% for glutamate/glutamine in agreement with the simulations. Even a minor reduction from alpha = 85 degrees to 80 degrees produced a large difference between coupled and uncoupled yields, with a reduction of 7% for choline and 17% for glutamate/glutamine. Anecdotal in vivo spectroscopic imaging studies show similar trends, with large differences between choline and glutamate/glutamine yield over a small, 2.2 cm, region. These results demonstrate severe effects on metabolite yield due to RF variation between strongly coupled and uncoupled spin systems at long echo time, which complicates metabolite quantification.

MR spectroscopic imaging of glutathione in the white and gray matter at 7 T with an application to multiple sclerosis

“…As a result of this inhomogeneity, the excitation across the region of interest is variable and can affect quantification of spectra since the gain of acquired spectra is not at the prescribed nutation angle. Nonuniform B1 has been shown to affect the yield of coupled resonances [22] relative to singlet resonances. Adiabatic RF pulse design offers a possible solution to mitigate the effects of B1 inhomogeneity.…”

Measurement of N‐acetylaspartylglutamate in the human frontal brain by 1H‐MRS at 7 T

“…Although editing may provide complete differentiation between NAAG and NAA, the reliability is limited by low signal‐to‐noise ratio. B 1 inhomogeneity effects may be detrimental in editing, given that the effects of flip angle variations differ between coupled and uncoupled spin signals (13). The artifacts can be reduced using adiabatic pulses for volume localization (14).…”

Direct in vivo measurement of glycine and the neurochemical profile in the rat medulla oblongata

“…As a result of the use of short TE, the effect of an inhomogeneous B 1 within the voxel (approximately 15–30% variation) on the evolution of J‐coupled resonances, reported by Snyder et al . 16, was considered to be negligible. Therefore, nominal flip angles were used in the simulation of the spectra.…”