Ultimate intrinsic signal‐to‐noise ratio for parallel MRI: Electromagnetic field considerations

Abstract

A method is described for establishing an upper bound on the spatial encoding capabilities of coil arrays in parallel MRI. Ultimate intrinsic signal-to-noise ratio (SNR), independent of any particular conductor arrangement, is calculated by expressing arbitrary coil sensitivities in terms of a complete set of basis functions that satisfy Maxwell's equations within the sample and performing parallel imaging reconstructions using these basis functions. The dependence of the ultimate intrinsic SNR on a variety of experimental conditions is explored and a physically intuitive explanation for the observed behavior is provided based on a comparison between the electromagnetic wavelength and the distance between aliasing points. Imaging at high field strength, with correspondingly short wavelength, is shown to offer advantages for parallel imaging beyond those already expected due to the larger available spin polarization. One-dimensional undersampling of k-space yields a steep drop in attainable SNR for more than a 5-fold reduction of scan time, while 2D undersampling permits access to much higher degrees of acceleration. Increased tissue conductivity decreases baseline SNR, but improves parallel imaging performance. A procedure is also provided for generating the optimal coil sensitivity pattern for a given acceleration, which will serve as a useful guide for future coil designs. Magn Reson Med 50: 1018 -1030, 2003.

Optimizing signal‐to‐noise ratio of high‐resolution parallel single‐shot diffusion‐weighted echo‐planar imaging at ultrahigh field strengths

“…2b. As theoretically expected, PI performance at 7 T outperforms that at 3 T and the g-factor for a given PI reduction factor decreases with increasing coil array size (12,13,17). The latter emphasizes the importance of sufficiently large coil arrays when performing PI since the theoretically feasible SNR is inversely proportional to the g-factor.…”

“…However, PI acceleration is limited by the inherent geometryfactor (g-factor) penalty that leads to spatially inhomogeneous noise amplification at high PI reduction factors (11). Theoretical investigations of the ultimate SNR of PI have shown that PI performance significantly improves from onsetting far-field wave behavior at ultrahigh field strengths (12,13) allowing for a greater range of feasible PI reduction factors which could compensate for increased image distortions and T 2 * blurring while achieving a SNR gain. The feasibility has been demonstrated experimentally by Wiesinger et al (14) but the extent of the benefit depends largely on the coil array size and geometry.…”

Event-related single-shot volumetric functional magnetic resonance inverse imaging of visual processing

“…First, increasing the number of channels in a coil array can provide more independent spatial information. However, the benefit of increasing channels will reach a plateau as the consequence of electromagnetic theoretical limitations (Ohliger et al, 2003;Wiesinger et al, 2004). In addition, at high field more independent spatial information can be obtained from the same geometry of a coil array as the consequence of a shorter wavelength.…”

Development and initial evaluation of 7-T q-ball imaging of the human brain

“…It is noteworthy that 7T has an inherent advantage over lower field strengths in this regime of highly parallel imaging, because the shorter resonance wavelength allows better radiofrequency energy focusing. In this manner, larger reduction factors can theoretically be achieved without as much loss of SNR at 7T as at 3T [34][35][36].…”

“…With regard to the former, current whole-body gradient systems are already reaching the limits of gradient amplitude and slew rate imposed by the need to avoid peripheral nerve stimulation. Higher density phased array head coils may indeed have an impact on 3T HARDI; however, the electrodynamic advantages of the higher resonance frequency at ultra-high field [34][35][36] mean that they will likely provide an even greater benefit for 7T HARDI.…”