Synopsis

3D multi-gradient echo MRI can be used to estimate T₁, T₂*, PD and the magnetization transfer (MT), which is increasingly used for multi-parametric mapping (MPM) of human brain. The increased polarization at 7T compared to lower B₀ allows for increased spatial resolution or reduced scan times. However, SAR restrictions imposed on the MT pulse and B₁ inhomogeneity pose challenges. In this work, we propose a protocol for MPM of human brain at 7T with special attention paid to eliminating bias when mapping MT_sat while obtaining submillimeter isotropic spatial resolution in under 12 minutes with acceptable SNR.

Introduction

3D multi-gradient echo MRI is a versatile and quick method to estimate maps of tissue parameters such as T₁, T₂*, proton density (PD) and magnetic susceptibility. At clinical field strengths, B₀, the method is also used to quantify magnetization transfer (MT)¹ or to calculate the MT saturation, MT_sat, using a dual flip angle (DFA) experiment². MT contrast is evoked by applying a high-energy off-resonance saturation pulse prior to excitation. The average RF power may be curtailed by SAR limits, especially at 7T.

At 3T, gradient-echo (GRE) multi-parametric mapping (MPM) for quantitative structural MRI of the human brain takes about 20 mins³, but requires accurate flip angle mapping to account for B₁⁺ inhomogeneity. The higher polarization at 7T will allow for increased spatial resolution or reduction of the scan time by parallel imaging. In addition, the T₂* contrast increases with B₀.

Based on an established 3T protocol, we optimized MPM of human brain at 7T with special attention paid to elimination of bias in MT_sat.

Methods

Healthy adult subjects were scanned on a 7T Philips Achieva MR system (Philips Healthcare, Best, NL), using a dual-channel transmit head coil with 32 receive elements (Nova Medical, Wilmington, MA) after giving informed written consent. Eight sagittal non-selective volumes were acquired from bipolar gradient echoes at 670 Hz/px bandwidth. The minimum T_E=1.97 ms and maximum T_E=15.76 ms (equidistant, fat water in-phase) yielded T_R=18 ms without the MT pulse, and 2x SENSE⁴ was applied in both phase encoding directions. The optimization procedure followed the following main steps:

1. The “block” excitation RF pulse was replaced by an asymmetric sinc pulse to reduce the sensitivity to the increased B₀-inhomogeneity at 7T. To determine the flip angles used in DFA, the flip angle, α, was varied from 2° to 27° (Figure 1).

2. A main-lobe sinc pulse of 4 ms duration was used for MT saturation. The SAR limit was reached at a saturation α of about 180° at T_R=28 ms. The frequency offset was varied from 0.75 kHz to 2.00 kHz (Figure 2).

3. The excitation α of the MT-weighted (MT-w) sequence was varied from 2° to 8° (Figure 3).

4. The maximum B₁⁺ amplitude of the excitation pulse was varied, using 5, 9, and 14 μT (Figure 4).

Flip angle mapping was performed using DREAM⁵ and B₁⁺ bias in the PD, T₁, and MT maps were corrected for post-hoc³. Pixel intensities were scaled to physical signal (in [a.u.]). Post-processing and evaluation were performed in MATLAB R2016a using the hMRI toolbox⁶.

Results

1. A linear plot of the VFA signal revealed the Ernst angle and incomplete spoiling at α>20° (Figure 1). Thus, α for DFA was chosen as 4° (PD-w) based on the median Ernst angle and 18° (T₁-w) to avoid spoiling bias^7,8.

2. Direct saturation was seen with MT pulses of lower frequency offsets below 2.00 kHz and the offset were thus set to 2.00 kHz (Figure 2).

3. Consistent MT maps were observed at flip angles below 8°, except for broadening of the CSF peak (Figure 3). The excitation flip angle was set to 4° to keep the free water saturation smaller than MT_sat analogous to ref., ², while still attaining acceptable SNR.

4. Peak B₁⁺ amplitude affected T₁ and MT_sat, with increasing inhomogeneity of T₁ and overestimation of MT_sat (Figure 4). Hence, a maximum B₁⁺ of 5 μT was chosen.

With this optimized 7T protocol, multi-parameter maps of 0.9 mm isotropic resolution are obtained in 10-12 mins depending on FoV ( Figure 5). MT_sat exhibits a high GM-WM contrast, especially in deep brain, but requires correction of residual B₁⁺ inhomogeneities².

Discussion

Conclusion

Acknowledgements

References

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