RF coil efficacy for efficient spin control in low field magnetic resonance experiments✰

Abstract

At low magnetic fields (<1 T), the low loss of radio frequency (RF) coils can cause RF pulses, in magnetic resonance (MR) experiments, to suffer from long rise and fall times. When severe, long rise and fall times can result in RF pulses with undesirable and/or unexpected shapes, lengths, and amplitudes leading to inadequate spin control during an MR experiment. Experimentally, the lack of spin control becomes evident in the shape of the nutation curve and the inversion efficiency, the degree to which full inversion is accomplished following a 180° pulse. Lowering the quality factor (Q) of tuned coil is shown to reduce the duration of the rise and fall times. The effects of long rise and fall times on the spin behavior during an MR experiment is explored. It is shown that the inversion efficiency can be used to provide a threshold that ensures high fidelity pulses with adequate spin control.

Keywords

• Low field NMR
• Quantitative relaxation
• Inversion recovery
• Quality factor
• RF coils