3055

Optimization of fast Quantitative Multiparameter Mapping (MPM) at 7T using parallel transmission

Difei Wang¹, Rüdiger Stirnberg¹, Eberhard Pracht¹, and Tony Stöcker^1,2
¹German Centre for Neurodegenerative Diseases (DZNE), Bonn, Germany, ²Department of Physics and Astronomy, University of Bonn, Bonn, Germany

Synopsis

We propose a fast MPM protocol at 7T using skipped-CAIPI 3D-EPI with simple PTx water-excitation based on 3 k_T-points. By comparison to corresponding CP mode scans, the 3 k_T-points excitation mainly improves the B₁⁺ field homogeneity in the Cerebellum. Using MPM B₁⁺ field correction, this simple improvement is sufficient to achieve good and homogeneous T₁, PD and T₂* estimates throughout the brain. However, the lack of MT homogenization still results in the inadequate MT_sat CNR. By combining MPM with EPI and PTx, we obtained quantitative whole-brain parameter maps of high quality, except for Cerebellar MTsat within 3 minutes scan time.

Introduction

Recently, following a fast multiparameter mapping (MPM) implementation at 3T¹, a corresponding protocol at 7T was proposed² using 3D-EPI with segmented CAIPIRINHA (skipped-CAIPI) sampling³. In this work we propose to overcome a critical part of the remaining challenges of the previous work by utilizing water-selective k_T-points pulses⁴.

Methods

All data were acquired on a Siemens MAGNETOM 7T Plus scanner using 32-channel head receive coil and 8-channel transmit array. One set of 3D-EPI T₁w, PDw and MTw images was acquired at 4 equidistant TEs between 4.64 and 19.64 ms at 1 mm isotropic resolution. A 20.2x2_z1 skipped-CAIPI sampling was employed, resulting in an EPI factor of 4. Repetition time/flip angle were chosen as follows: 27 ms/20° for T₁w, 27 ms/3° for PDw,and 45 ms/4° for MTw. A single rectangular RF pulse with long duration (1.0 ms) was used for simple fat suppression using traditional circularly polarized excitation⁵. The corresponding PTx scans were acquired using 3 k_T-points RF excitation pulses with the same nominal flip angles. Each sub-pulse also had 1.0 ms duration for fat suppression as previously used for fMRI⁴. Both CP and PTx MTw scans used the same Gaussian-shaped CP mode RF pulse applied prior to each excitation.
In order to keep the specific absorption rate (SAR) within the safety limits, the nominal MT flip angle had to be reduced from 320° in the CP mode MTw scan to 260° in the PTx-MTw scan. A second PTx-MTw scan was acquired with prolonged TR (74 ms) to achieve the same 320° nominal MT flip angle within SAR limits. The total acquisition time of the CP mode MPM scans and the PTx MPM scans (incl. both MTw scans) were 2:59 and 5:05, respectively. All imaging parameters are summarized in Tab.1.
Two B₁ maps were acquired using the same CP mode and PTx pulses by modifying the 3D-EPI sequence to include two subsequent TRs according to the Actual Flip angle Imaging (AFI) method⁶ with TR₂ = 100 ms/TR₁ = 20 ms, nominal flip angle = 50°, 5 mm isotropic resolution, 4.2x2_z1 skipped-CAIPI sampling (EPI factor 5), TA = 0:10 for both.
All parameter maps are calculated with the hMRI toolbox⁷.

Results

Fig.1 shows representative sagittal, coronal and axial slices of all scans at TE = 4.64 ms. In the PDw images, it can be observed that PTx excitation is more homogeneous in the Cerebellum (red arrow). In the T₁w images, this shows as an improved WM/GM contrast compared to the CP scan. However the PTx scans have slightly reduced signals above the sphenoid sinus (blue arrow). The different nominal saturation flip angles and TRs resulted in nearly 100% SAR estimation (in “1st level safety mode“).
Fig.2 demonstrates the example sagittal slices of distortion corrected MTw image along with MTw scan with uncorrected AP and PA phase encoding. The difference is neglectable, therefore the remaining results are presented for AP phase encoding only.
Fig.3 shows the quantitative parameter maps acquired using CP mode and PTx pulses together with the corresponding B₁⁺ scale maps.
Fig.4 shows the (a) MT_sat histogram and bar plots of (b) T₂*, (c) PD*, (d) T₁ estimates in different regions of interest (ROI) from CP and PTx scans. The smaller error bars of T₁ in WM and GM in PTx scans show that the transmit field inhomogeneities could be compensated for in the parameter maps. However, low MT saturation flip angles of all three scans are not adequate to provide enough MT contrast: the low CNR is not sufficient to create clearly separated modes in histogram⁸. In some ROIs, the T₁ estimates of PTx scans are slightly higher than that of CP scans and the literature values.

Discussion

Our study shows that MPM protocol using skipped-CAIPI 3D-EPI with PTx pulses is feasible. The simplest form of water-selective k_T-points PTx pulses is utilized in order to reduce the B₁⁺ field inhomogeneity with reasonable pulse prolongation and minimal SAR increase. The PTx pulses achieve improved excitation in specific regions like Cerebellum, which CP mode pulses could not excite sufficiently. With an additionally acquired B₁⁺ map, the remaining transmit field variation was successfully counterbalanced, which was not possible using CP mode pulses. Thus more homogeneous quantitative parameter maps of high-quality can be obtained.
Although PTx pulses provide the expected benefits, the acquisition currently suffers from SAR limitations. The MT saturation flip angle had to be reduced at the price of lower CNR. Currently, if MT_sat is not required, PTx scans can provide quantitative T₁, T₂* and PD* maps at 1mm isotropic resolution of high quality within 2 minutes scan time.
In a future study, we will investigate the use of parallel transmit MT pulses, or different CP mode MT saturation, like TIAMO¹³.

Conclusion

We have presented a fast MPM protocol at 7T using skipped-CAIPI 3D-EPI with simple PTx pulses. This sufficiently improved the CNR of T₁w and PDw scans in the Cerebellum, while the MTw scans still suffered from CP mode saturation in the current implementation. With an additional B₁⁺ map acquired, more homogeneous T₁, PD* and T₂* maps of high-resolution and high-quality can be obtained throughout the brain.

Acknowledgements

No acknowledgement found.

References

1. Wang D, Stöcker T, Stirnberg R. Fast Quantitative Multiparametric Mapping using 3D-EPI with Segmented CAIPIRINHA Sampling at 3T. In: Proceedings of the international society of magnetic resonance in medicine.; 2020. pp. 4–7.

2. Wang D, Stirnberg R, Stöcker T. Fast high-quality and -resolution multi parameter mapping (MPM) at 7 Tesla using skippedCAIPI 3D-EPI. ESMRMB 2020. pp. 2–4.

3. Stirnberg R, Stöcker T. Segmented K‐space blipped‐controlled aliasing in parallel imaging for high spatiotemporal resolution EPI. Magnetic Resonance in Medicine 2021;85:1540–155.

4. Le Ster C, Moreno A, Mauconduit F, et al. Comparison of SMS-EPI and 3D-EPI at 7T in an fMRI localizer study with matched spatiotemporal resolution and homogenized excitation profiles. PLOS ONE 2019;14:e0225286.

5. Stirnberg R, Brenner D, Stöcker T, Shah NJ. Rapid fat suppression for three-dimensional echo planar imaging with minimized specific absorption rate. Magn. Reson. Med. 2016;76:1517–1523.

6. Yarnykh VL. Actual flip-angle imaging in the pulsed steady state: A method for rapid three-dimensional mapping of the transmitted radiofrequency field. Magnetic Resonance in Medicine 2007;57:192–200.

7. Tabelow K, Balteau E, Ashburner J, et al. hMRI – A toolbox for quantitative MRI in neuroscience and clinical research. Neuroimage 2019;194:191–210.

8. Helms G, Dathe H, Kallenberg K, Dechent P. High-resolution maps of magnetization transfer with inherent correction for RF inhomogeneity and T₁ relaxation obtained from 3D FLASH MRI. Magn. Reson. Med. 2008;60:1396–1407.

9. Andersson JLR, Skare S, Ashburner J. How to correct susceptibility distortions in spin-echo echo-planar images: application to diffusion tensor imaging. NeuroImage 2003;20:870–888.

10. Smith SM, Jenkinson M, Woolrich MW, et al. Advances in functional and structural MR image analysis and implementation as FSL. NeuroImage 2004;23:208–219.

11. Peters AM, Brookes MJ, Hoogenraad FG, et al. T2* measurements in human brain at 1.5, 3 and 7 t. Magnetic Resonance Imaging 2007;25:748–753.

12. Rooney WD, Johnson G, Li X, et al. Magnetic field and tissue dependencies of human brain longitudinal¹H₂O relaxation in vivo. Magnetic Resonance in Medicine 2007;57:308–318.

13. Orzada S, Maderwald S, Poser BA, Bitz AK, Quick HH, Ladd ME. RF excitation using time interleaved acquisition of modes (TIAMO) to address B₁ inhomogeneity in high‐field MRI. Magnetic Resonance in Medicine 2010;64:327–333.

Figures

Tab. 1 Sequence parameters of T₁w, PDw, MTw scans and B₁⁺ mapping using CP mode and PTx pulses.

Fig. 1 Sagittal, coronal and axial view of T₁w, PDw and MTw images acquired using CP mode and PTx pulses. The last column shows the MTw scan with prolonged TR and the same nominal MT flip angle as the CP mode. The acquisition times are listed for each scan. All three MTw scans share the same CP mode MT pulse with different nominal saturation flip angles and TRs in order to match the SAR limits, listed respectively. The ones with a higher FA show slightly better soft tissue contrast.

Fig. 2 Sagittal slices of (A) distortion corrected MTw image using TOPUP ^9,10 as well as MTw image with (B) uncorrected AP and (C) uncorrected PA phase-encoding direction. The CSF mask is generated from the corrected MTw scan and overlaid in red. With a high segmentation factor of 20 (EPI factor of 4), the geometric distortions along the AP direction are neglectable, while imaging speed is much higher than the conventional FLASH for MPM (“EPI factor“ 1).

Fig. 3 A sagittal view of T₁, PD* , MT_sat and T₂* maps using the data acquired with the CP mode and PTx pulses along with the corresponding B₁⁺ scale map. The TR is 45 ms for both MTw scans. The nominal MT flip angle is 260° for PTx image and 320° for CP. Both MTsat maps have low CNR due to the insufficient MT saturation, especially in the Cerebellum. The PTx B₁⁺ scale map shows improved excitation in the Cerebellum compared to the CP map. The PTx T₁ map has higher values than the CP T₁ map and robust voxel estimates throughout the brain.

Fig. 4 (a) Whole-brain MT_sat histogram with Cerebellum excluded and (b) T₂*, (c) PD*, (d) T₁ estimates in different ROIs. The T₂* literature values are taken from (11) and the T₁ literature values are taken from (12). The bars of CSF in (b) and (d) refer to the second vertical axis. The TR/nominal MT saturation flip angle were 45 ms/260° (red line) and 74 ms/320° (blue line) in the histogram. The low MT saturation flip angle (260°) is not sufficient to provide enough saturation effect to clearly separate different modes for CSF, WM and GM.

Proc. Intl. Soc. Mag. Reson. Med. 29 (2021)

3055