T1- and T2-weighted images were registered to CEST data. â¢ Gradient field mapping for shifting Z-spectrum after interpolation [3,4]. Evaluation: â¢ MTR asym.
Investigation of correlation between APT-CEST MRI and 31P-MRSI Jan-Ruediger Schuere 1, Stella Breuer 1, Manoj Shrestha 2, Ralf Deichmann 2, Marlies Wagner 1, Ulrich Pilatus 1 1 Department of Neuroradiology, University Hospital Frankfurt, Frankfurt/Main, Germany 2 Brain Imaging Center (BIC), Goethe University Frankfurt, Frankfurt/Main, Germany
Materials and Methods
Changes in brain tissue pH have been observed for several pathologies, such as tumor, stroke or inflammation. The tissue pH can be determined from the spectral distance of inorganic phosphate (Pi) to phosphocreatine (PCr) using phosphorus (31P) MRSI. Amid proton transfer (APT) in chemical exchange saturation transfer (CEST) imaging offers an alternative way of pH quantification, with the advantage of not requiring special hardware. However, CEST imaging is still challenging in clinical routine . In this study we compare pH determined by 31P-MRSI with the asymmetric magnetization transfer ratio (MTRasym) from APT-CEST MRI to evaluate the feasibility of CEST contrast.
Equipment: 3T whole-body MRI-scanner CEST-MRI: • 8 channel phased array head receive coil • Sequence: two-dimensional (2D) CEST-EPI • TR/TE /spatial resolution = 4000ms/22ms/3x3x5mm3 Saturation Module: • 8 rectangular pules (pulse duration/inter-pulse delay/B1= 250ms/250ms/1µT • Frequency offsets (ω): • -8 to 8 ppm , Δω =0.5 ppm, 2 measurement repetition • 3 to 4 ppm and -3 to -4 ppm, Δω =0.1 ppm, 8 measurement repetition 31P -MRSI: • 1H/ 31P double-tuned head coil • Sequence: 3D CSI FID • TR/FA/spatial resolution = 2000ms/60°/ 17.5x17.5x25mm3 • Acquisition matrix = 8x8x8, extrapolated to 16x16x16
In vitro: • MTRasym at 3.5ppm and chemical shift of Pi increase with rising pH
Discussion/Conclusion Highly significant differences could be observed in MTRasym comparing tumor tissue and normal appearing WM. Outliers in MTRasym above -0.025 could be identified as tumor infiltrated tissue. A ROI based correlation between pH as measured with 31P and MTRasym as measured by CEST was found in vitro and in vivo. Results in pH are consistant with previous observations on increased intracellular pH e.g. . Spectroscopy data might be affected by partial volume effects, while MTRasym includes MT effects from other tissue compartments. Both factors may affect the correlation.
In conclusion we could show a correlation between pH and MTRasym in human brain tumor supporting the hypothesis that increased APT due to increased pH at least in part contributes to the measured contrast.
References  Jones et al. Amide Proton Transfer Imaging of Human Brain Tumors at 3T  Windschuh J et al. Charakterisierung der CEST Signale von Gelantine.19 Jahrestagung der ISMRM-DS.2016.157-158  Hua J. , Jones C.K. , Quantitative Description oft he Asymmetrie in Magnetization Transfer Effects arround Water Resonance in the Human Brain  Zaiss M. et al. Relaxation-compensated CEST-MRI of the human brain at 7T. Unbiased insight into NOE and amide signal changes in human glioblastoma  Petroff et al. Cerebral Intracellular pH by 31P nuclear magnetic resonance spectroscopy, Neurology June 1985  Hattingen et al. Bevacizumab impairs oxidative energy metabolism and shows antitumoral effects in recurrent glioblastomas: a 31P/1H MRSI and quantitative magnetic resonance imaging study., Neuro Oncol. 2011
In vivo: • Differences in MTRasym and pH between tumor and WM regions (contralateral , ipsilateral, diagonal) • Cerebrospinal fluid and tumor infitltrated tissue shows higher MTRasym values • pHtumor = 7.05 ± 0.03 vs. pHcontralateral = 7 ± 0.02 • MTRasym_tumor = -0.0043 ± 0.01 vs. MTRasym_contralateral = -0.03 ± 0.01 • Correlation between pH and MTRasym for tumor (r=0.52,p=0.08) • Standard error (SE) < 0.0001 for each MTRasym value • T-test shows differences in MTRasym between tumor and remaining ROI (p