Synopsis

Due to high sensitivity to B₁⁺-inhomogeneities, Chemical Exchange Saturation Transfer MRI requires a correction or mitigation of the B₁⁺-inhomogeneity at ultra-high magnetic field strengths (B₀ ≥ 7 Tesla). A novel approach for mitigation of B₁⁺-inhomogeneity effects that affects the saturation process is presented. The method employs two interleaved excitation modes during the saturation pulse train. Simulations show a decrease of the relative difference of the MTRRex metric caused by B₁⁺ inhomogeneity. This “Multiple Interleaved Mode Saturation” scheme leads to improved homogeneity in both, phantom and in vivo measurements at 7 Tesla.

Introduction

Methods

Numerical Bloch-McConnell simulations^{6, 7} have been performed with the use of MATLAB (The MathWorks, Natick, MA, USA). Pulse sequences were implemented on a 7-Tesla whole-body MR system (MAGNETOM Terra, Siemens Healthcare GmbH, Erlangen, Germany). An 8Tx/32Rx head coil (Nova Medical, Wilmington, USA) was used for the measurements. Phantom measurements have been performed with an 18 cm diameter spherical phantom filled with 50 mM Creatine and 2% Agarose gel with a pH of 7.0. In vivo measurements have been performed on a healthy male volunteer.

For both, phantom and in vivo measurements images were acquired using a single slice GRE sequence with pulsed CEST presaturation^{8, 9}. In addition, B₁⁺ maps were acquired using pre-saturated 2D turbo-flash sequence sequence. For the MIMOSA acquisition, two sets of transmitter phases with constant amplitudes were used for the saturation pulse train. The first mode has 45° phase difference between two adjacent transmitter coil channels (“Circular Polarization - CP”). In the second set a 90° phase increment between the transmitter coil elements has been chosen (90° mode). Figure 1 exemplarily shows for the first three Tx channels how the two modes have been interleaved during the saturation pulse train. Each mode has a different B₁⁺ distribution. Thus, the interleaved saturation pulses will have two different effective B₁⁺ values in a single voxel, in the following denoted as B_1,A and B_1,B. The GRE image acquisition has been performed with the use of the CP mode. For comparison CEST images were also acquired using a CP mode in both, acquisition and saturation scheme, which is equivalent to the use of a single transmitter (1Tx) system .

Sequence parameters: CEST Saturation parameters for phantom measurements: T_p = 99.84 ms, T_D = 50 ms, n= 26, T_Recovery = 3 s, B_1,Nominal=0.4µT, Gaussian pulse shape; Saturation parameters for in vivo measurements: T_p = 15.32 ms, T_D = 10 ms, number of pulses n=150, T_Recovery = 0 s, B_1,Nominal=0.6µT Gaussian pulse shape; GRE acquisition parameters: T_R = 7.2 ms, T_E = 3.45 ms, Matrix Size = 128x128, FoV_phantom=190mm, FoV_invivo=210 mm;

Results and Discussion

Numeric simulations showing the influence of MIMOSA on the Z-spectrum and MTR_Rex metric are presented in figure 2 and 3. Figure 2 shows a rise of the MTR_Rex metric in comparison to a simulation without MIMOSA at B₁=0.3µT. Figure 3 shows that MIMOSA reduces the variation of the MTR_Rex metric caused by B₁⁺-inhomogeneity in comparison to the standard saturation using CP mode. Figure 3 a) shows also an increase of the MTR_Rex metric in comparison to the standard method with 0.3 µT.

Measurement results from both phantom and brain acquisitions without and with the use of the MIMOSA are presented in figure 4. A correlation between the loss of MTR_Rex magnitude and the B₁ map under saturation with CP mode in the phantom can be observed. The application of MIMOSA in phantom resulted in a more homogeneous image with still some slight loss of the MTR_Rex magnitude around areas affected by the B₁⁺ inhomogeneity. The in vivo measurements presented in figure 3 show similar MTR_Rex values in the lateral ventricles as in white matter in case of the use of CP saturation. In the MIMOSA acquisition a decrease of the MTRRex magnitude can be observed in the lateral ventricles resulting in a stronger contrast between the ventricles and white matter.

Conclusion

Acknowledgements

References

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