4013

Relaxation-compensated APT and rNOE CEST-MRI of human brain tumors at 3 T
Steffen Goerke1, Yannick Soehngen1, Anagha Deshmane2, Moritz Zaiss2, Johannes Breitling1, Philip S Boyd1, Kai Herz2, Ferdinand Zimmermann1, Karel D Klika3, Mark E Ladd1, and Peter Bachert1

1Divsion of Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany, 2Department of High-field Magnetic Resonance, Max-Planck-Institute for Biological Cybernetics, Tübingen, Germany, 3Molecular Structure Analysis, German Cancer Research Center (DKFZ), Heidelberg, Germany

Synopsis

In this study, the established acquisition protocol for relaxation-compensated APT and rNOE CEST-MRI at 7 T has been successfully transferred to a clinically relevant magnetic field strength of 3 T. This opens up the door to clinical trials with a large number of participants, thus enabling a comprehensive assessment of the clinical relevance of relaxation-compensation in CEST-MRI. The presented CEST sequence is currently part of the clinical routine acquisition protocol for brain tumor patients at our institute.

Introduction

Relaxation-compensated CEST-MRI (i.e. the AREX contrast1) has already been shown to provide valuable information for brain tumor diagnosis at ultra-high magnetic field strengths2,3. This study aims at transferring the established acquisition protocol at 7 T2 to a clinically relevant magnetic field strength of 3 T. Insights gained from analyses of a protein model solution at multiple B0 were utilized to assess the spectral widths of the APT and rNOE signals at 3 T where clear peaks are no longer resolved in the Z-spectra (Fig. 1).

Methods

In vivo 3D-CEST-MRI (1.7×1.7×3 mm3, 12 slices) was performed on a 3 T MR scanner (Siemens Prisma) using the snapshot-CEST4 approach. Image-readout parameters were adapted from ref. 5 and found to be optimal for: 340 Hz/pix BW, Grappa 2, 7° FA. Pre-saturation (tsat = 3.7 s) was achieved by a train of Gaussian-shaped pulses (tp = 20 ms, 80% DC). Z-spectra were sampled at 57 frequency offsets, corrected for B0-inhomogenities, de-noised using a principal component analysis, and fitted pixel-wise with a Lorentzian 4-pool model. Relaxation-compensated CEST contrasts1 were calculated according to: MTRRex = 1/Z – 1/Zref and AREX = MTRRex/T1. Contrasts were acquired for two different B1 = 0.6 & 0.9 µT, and corrected for B1-inhomogeneities (reconstructed B1 = 0.7 µT). The overall acquisition time, including the two CEST scans as well as B0-, B1-, and T1-mapping, was approx. 20 min.

Examinations were approved by the local ethics committee of the Medical Faculty of the University of Heidelberg.

Results & Discussion

Investigation of a protein model solution at different B0 revealed a substantial increase in the spectral range of CEST signals at low B0 (Fig. 1). The signal broadening is in line with common observations in MR spectroscopy. This prior knowledge of the spectral range of CEST signals (±15 ppm at 3 T) was utilized to correctly assess the magnitude of the APT and rNOE signals at 3 T even though clear peaks are no longer resolved in the Z-spectra (Fig. 2a). The acquired MTRRex-maps (Fig. 2b) of a healthy volunteer are in line with previous results at 7 T, showing a hyperintense APT signal in gray matter and a hyperintense rNOE signal in white matter. AREX-maps of the APT and rNOE signals are similar, but interestingly show substantial differences in the putamen (Fig. 2b, white arrows). Examination of a patient with glioblastoma (Fig. 3) demonstrates the applicability of this acquisition protocol in a clinical setting. Interpretation of contrast changes in the tumor region (Fig. 3, pink arrows) will be evaluated after examination of a sufficiently large cohort of patients.

Conclusion

The presented acquisition protocol allows relaxation-compensated APT and rNOE CEST-MRI at 3 T with a 3D coverage of the human brain. The transfer to a clinically relevant magnetic field strength of 3 T is of particular interest with respect to clinical trials with a large number of participants, and thus to assess the clinical relevance of relaxation-compensation in CEST-MRI. A pilot study with brain tumor patients is currently under investigation.

Acknowledgements

No acknowledgement found.

References

1. Zaiss M, Xu J, Goerke S, et al. Inverse Z-spectrum analysis for spillover-, MT-, and T1-corrected steady-state pulsed CEST-MRI – application to pH-weighted MRI of acute stroke. NMR Biomed 2014; 27(3):240-252.

2. Zaiss M, Windschuh J, Paech D, et al. Relaxation-compensated CEST-MRI of the human brain at 7 T: Unbiased insight into NOE and amide signal changes in human glioblastoma. NeuroImage 2015; 112:180-188.

3. Paech D, Windschuh J, Oberhollenzer J, et al. Assessing the predictability of IDH mutation and MGMT methylation status in glioma patients using relaxation-compensated multipool CEST MRI at 7.0 T. Neuro Oncol 2018; doi:10.1093/neuonc/noy073.

4. Zaiss M, Ehses P, and Scheffler K. Snapshot-CEST: Optimizing spiral-centric-reordered gradient echo acquisition for fast and robust 3D CEST MRI at 9.4 T. NMR Biomed 2018; 31:e3879.

5. Deshmane A, Zaiss M, Lindig T, et al. 3D GRE snapshot CEST MRI with low power saturation for clinical studies at 3 T. Magn Reson Med 2018; doi:10.1002/mrm.27569.

Figures

Fig. 1: Z- and AREX-spectra (B1 = 0.75 µT) of 10 %(w/v) bovine serum albumin (BSA) at various B0. As B0 decreases, the spectral range of CEST signals broadens substantially (colored arrows).

Fig. 2: (a) Z- and AREX-spectra (B1 = 0.7 µT) of a representative voxel in gray matter. The spectral range of CEST signals (orange arrow) is the same as in protein model solutions (Fig. 1). (b) Relaxation-compensated APT and rNOE images (slice 4/12) of a healthy volunteer.

Fig. 3: Relaxation-compensated APT and rNOE images (slice 8-10/12) of a patient with glioblastoma (WHO grade IV).

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
4013