Phase-sensitive sodium B1 mapping

Abstract

Quantitative sodium MRI requires accurate knowledge of factors affecting the sodium signal. One important determinant of sodium signal level is the transmit B(1) field strength. However, the low signal-to-noise ratio typical of sodium MRI makes accurate B(1) mapping in reasonable scan times challenging. A new phase-sensitive B(1) mapping technique has recently been shown to work better than the widely used dual-angle method in low-signal-to-noise ratio situations and over a broader range of flip angles. In this work, the phase-sensitive B(1) mapping technique is applied to sodium, and its performance compared to the dual-angle method through both simulation and phantom studies. The phase-sensitive method is shown to yield higher quality B(1) maps at low signal-to-noise ratio and greater consistency of measurement than the dual-angle method. An in vivo sodium B(1) map of the human breast is also shown, demonstrating the phase-sensitive method's feasibility for human studies.

FIG. 1

Monte Carlo simulation of dual-angle and phase-sensitive B₁ mapping methods. The mean bias (upper panels) and standard deviation (lower panels) of the flip angle estimate obtained by each method over a range of actual flip angles are shown for values of system SNR ranging from 5 to 15 (panels a–c). The phase-sensitive method is seen to have lower flip angle estimate mean bias and lower standard deviation over most of the measurable range of flip angles.

FIG. 2

Monte Carlo simulation of effect of off-resonance on the phase-sensitive method. The left panels (a) show the effect of off-resonance on the mean bias and standard deviation of the phase-sensitive flip angle estimate when B₀ correction is performed, as described in Ref. . The right panels (b) show the effect of off-resonance if no B₀ correction is performed. Off-resonance phase accrual during the RF excitation causes mean bias, but has little effect on the standard deviation of the estimate for the values of off-resonance precession of 7° and 14° shown. Note that these values were informed by the in vivo study shown in Fig. 5, where the maximum off-resonance precession angle across the breast was measured to be about 7°.

FIG. 3

Dual-angle sodium B₁ maps for a single acquisition (a) and 20 averaged acquisitions (b). Phase-sensitive sodium B₁ maps for a single acquisition (c) and 20 averaged acquisitions (d). As seen, the phase-sensitive method yields much more robust sodium B₁ maps than the dual-angle method. Note that the phase-sensitive maps are centered around a nominal flip angle of 90° vs. 60° for the dual-angle method, reflecting the inherently wider dynamic range of the phase-sensitive method.

FIG. 4

Standard deviation of the sodium B₁ maps for both the dual-angle method (a) and phase-sensitive method (b) as measured across 20 acquisitions. The phase-sensitive B₁ mapping method yields a much more consistent measurement in the low-SNR environment typical of sodium imaging.

FIG. 5

Proton three-point Dixon water (a) and fat (b) image of the breast of a healthy volunteer. The corresponding sodium magnitude image is shown in (c), showing good correlation with breast anatomy (high sodium concentration in fibroglandular tissue and lower concentration in fatty tissue). A 3D sodium B₁ map using the phase-sensitive method is shown in (d). The phase-sensitive method yields a high-quality B₁ map in vivo despite low sodium image SNR.