Synopsis

To perform Chemical Exchange Saturation Transfer MRI of the whole brain a homogeneous saturation and fast readout are required. To achieve a fast and robust 3D acquisition a spiral-centric-reordered GRE readout was used. In addition, a Multiple Interleaved Mode Saturation scheme was applied to mitigate B₁⁺-inhomogeneity effects of the CEST saturation. Combination of these two methods allows acquiring a homogenous CEST contrast in a volume of approximately 220x220x45mm³ within an acquisition time of 7 min 26s.

Introduction

Methods

Pulse sequences were implemented on a 7-Tesla whole-body MR system (MAGNETOM Terra, Siemens Healthineers, Erlangen, Germany). An 8Tx/32Rx head coil (Nova Medical, Wilmington, USA) was used for the measurements. Phantom measurements were performed with an 18 cm diameter sphere filled with egg-white. In vivo measurements were performed on a healthy female volunteer. The images were acquired using a pulsed CEST spiral-centric-reordered -GRE sequence^{[1, 8]}.

For the MIMOSA acquisition, two sets of complimentary transmitter phases with constant amplitudes (‘modes’) were used for the saturation pulse train as described previously^[7]. The mode A had a 45° phase difference between two adjacent transmitter coil channels (“Circular Polarization - CP”). In mode B a 90° phase increment between the channels was chosen (also called B²⁺)^[9]. Each mode has a distinct effective relative B₁⁺ value distribution. Through interleaving of the two modes during saturation, (c.f. Figure 1) a more homogenous saturation can be achieved. During the GRE multi-slice acquisition the excitation pulses are employed in the CP mode. CEST images were also acquired using the CP mode during both acquisition and saturation. MTR_Rex(NOE) contrast was calculated for both standard CEST acquisition and MIMOSA acquisition based on the work by Windschuh et al.^[4]. B₁⁺ maps of both CP and B²⁺ mode were acquired using a multi flip angle (FA) sequence. A two-point B₁ correction was applied on the CP mode acquisition as presented by Windschuh et al.^[4]. The B₁⁺ distribution of MIMOSA was calculated based on the B₁ continuous wave power equivalent (CWPE) as described by Zu et al^[10]:

$$ B_{1,CWPE} (r,t)=\sqrt{\frac{1}{(\tau_p+\tau_d)}\int_0^{\tau_p} \! B^2_P(r,t) \, \mathrm{d}t} $$

Sequence parameters: CEST Saturation parameters: τ_p=46.08ms, τ_D=30 ms, n=50, Trec=1s, B_1,Nominal=0.6µT and 1.0µT in CP mode, Gaussian pulse; GRE acquisition parameters: FA=6°, TR=4.5ms, TE=2.3ms, Matrix Size=128x128x16, FoV=220x220x80 mm³, GRAPPA 3, Elongation Factor=0.6,

Results and Discussion

In comparison to the standard saturation using CP mode, MIMOSA reduces the intra slice variation of the B₁⁺-inhomogeneity in all of the acquired slices (c.f. phantom measurement, Figure 2). An influence of the higher rB1 value of the CP mode can be observed in the central slices of the phantoms B_1,CWPE maps. Figure 3 shows that homogeneity of the MTR_Rex(NOE) contrast is comparable to B₁ corrected images in most of the slices with MIMOSA saturation. The fitting routine fails in the outermost slices which might be caused by imperfect slab selection as well as the lower B₁⁺ or B₀ homogeneity.

In vivo results show similarities to the phantom measurements. MIMOSA reduces the intra slice variation of the B₁⁺-inhomogeneity in all of the acquired slices (c.f. Figure 4). In cranial slices, similarly to the outermost slices of the phantom, a decrease of the rB1 value in the MIMOSA B_1,CWPE maps can be observed.

In Figure 5, MTR_Rex(NOE) values of a healthy human brain derived both without and with the use of MIMOSA are presented. In CP mode without B₁ correction, a correlation between the loss of MTR_Rex(NOE) magnitude and the B₁ map can be observed (c.f. Figure 4A). The application of MIMOSA resulted in a more homogeneous image in central slices of the brain (c.f. Figure 5C), which is comparable to B₁ corrected images (c.f. Figure 5B). Cranial slices acquired with MIMOSA show reduced MTR_Rex(NOE) values and slightly reduced homogeneity in comparison to images acquired in CP mode both with and without B₁ correction. This is caused by low CWPE B₁⁺ values in these slices (c.f. Figure 4C).

Conclusion

Acknowledgements

References

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