CEST-weighted MRI at 21.1 T: application to glioma and ischemic rat model
Tangi Roussel1,2, Jens T. Rosenberg3, Samuel C. Grant 3,4, and Lucio Frydman2

1NeuroSpin, Commisariat à l'Energie Atomique et aux Energies Alternatives, Gif-sur-Yvette, France, 2Department of Chemical Physics, Weizmann Institute of Science, Rehovot, Israel, 3National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL, United States, 4Department of Chemical & Biomedical Engineering, The Florida State University, Tallahassee, FL, United States


This study explores and demonstrates the opportunities opened by ultrahigh fields for in vivo CEST-weighted imaging. CEST-weighted fast spin-echo imaging was performed on two neurological models: hypoxic ischemic and glioblastoma. A remarkably strong CEST contrast (≈15%) was observed for the tumors at 3.32 ppm originating mainly from an APT increase and a strong decrease in NOE and MT. Ischemic lesions were still more robustly detected with standard T2-weighted images than with CEST. Potential explanations and dependence with high magnetic field are discussed.


CEST methodologies are receiving growing interest in the MRI community1,2 and are involved in many preclinical and clinical studies. CEST measures the losses in water signal intensity caused by the saturation of exchangeable protons. Amide Proton Transfer3 (APT) is the most commonly used endogenous CEST-weighted imaging approach, and has been used to quantitatively map tissues in brain tumors4 and ischemic tissue.5 However, the origins of the CEST contrast –especially in tumors– are still debated and can be influenced by several physical and physiological parameters including temperature, pH5 and specific metabolite concentration such as glucose6 or glutamate.7 Recent preclinical8 and clinical studies9 at higher magnetic fields have shown that NOE can contribute to the CEST contrast. This paper explores for the first use of endogenous CEST-weighted imaging to detect strokes (a) and glioblastoma tumors (b) in rats at 21.1T.


Brain ischemia model. To mimic cerebral hypoxic ischemia, a middle cerebral artery occlusion (MCAO) was performed on seven Sprague-Dawley for 1.5h followed by re-perfusion. The animals were imaged 24h following the occlusion.

Glioblastoma model. 9L glioma cells were cultured; 100,000 9L glioma cells were injected at 2mm anterior, 2.5mm lateral and 3.5mm deep with respect to Bregma. Seven male Sprague-Dawley rats were used. Tumor bearing animals were imaged at 7 and 11 days post injection.

MRI acquisitions. All experiments were performed at 21.1T using a Bruker Avance III console and a home-built transmit/receive quadrature surface coil. CEST-weighted 1H RARE images (TE/TR=9/5000ms) were acquired from a 32x32-mm field of view and 1-mm slice thickness (250x250µm in-plane resolution). The CEST preparation achieve a B1 of 3.5µT using a train of 400 10-ms gaussian pulses with a frequency offset varied between –10 and +10ppm (with 23 increments).

Data processing. CEST-weighted images were processed and quantified with a customized MATLAB code using three methods: (a) the commonly used Magnetization Transfer Ratio (MTR) method, (b) the Lorentzian Fit Difference (LFD) method10 which consisted in adjusting a double Lorentzian model to fit the direct saturation of the Z-spectrum and analyzing the fit residue and (c) an APT and NOE three-offset area integration technique.8 Healthy and diseased tissue regions of interest (ROI) were selected and their quantification results were compared.


MTR maps from the glioma rat model showed an intense negative contrast highlighting the tumor tissue (Fig. 1 and 2) whereas very low contrast was observed on ischemic tissues. Glioma tissue was observed with a maximum CEST contrast for an average frequency of 3.32±0.57ppm while no statistically significant saturation frequency was found for ischemic tissue (6.35±2.17ppm). Quantitative CEST maps obtained using the LFD method are shown in Figures 3 and 4 respectively for a day-11 glioma rat and a MCAO rat. Figure 5 shows a summary of the quantitative results obtained using the LFD method (Fig. 5a) and the three-offset technique (Fig. 5b) for normal brain, glioma and ischemic tissue.

Discussion & Conclusion

A remarkably strong CEST contrast was observed for the glioma tissues at 21.1 T and originates from a decrease in MTR which can reach -15% at +3.3 ppm (Fig. 1c and 2). Using the LFD and the three-offset integration techniques, we observed that three different pools are contributing to the MTR decrease: (i) a significant +1% increase in APT at +3.5ppm (Fig. 3 and 5b), (ii) a very significant -1 to -2% decrease in NOE at -3.5ppm (Fig. 3 and 5c) and (iii) a global -5% drop in MT observable at -10 ppm (Fig. 3 and 5a). Our findings are in agreement with literature where both APT increase8 and NOE decrease1,8,9 – only at high fields – have been reported in glioma tissue. On the contrary, CEST-weighted imaging provided poor contrast when applied to the ischemic rat model. The LFD analysis did not reveal any CEST contributions specific to ischemic tissue (Fig. 4) and APT/NOE levels were not significantly different from normal brain tissue (Fig. 5b and 5c). Ischemic tissue was reported in literature to show a slight decrease in APT11 which can however depend on the measurement time point.12 To conclude, this impressive CEST-weighted enhancement in glioma tissue is probably magnified at ultrahigh fields by the longer T1, the larger chemical shift separation and the faster exchange rates, which potentiate one another to magnify the degree of contrast.


We thank Cathy Levenson for providing 9L glioma cells and advice. This work was performed at the NHMFL which is supported by NSF DMR-1157490; support also came from the Israel Science Foundation (grant 795/13) and the Kimmel Institute of Magnetic Resonance (Weizmann Institute).


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Fig. 1: Results obtained on a glioma rat for two different points in time: (top) seven days and (bottom) eleven days after 9L glioma cell injection; (from left to right) localized Z-spectra extracted from normal brain (green) and glioma (red) tissues (a), SE RARE images (b), corresponding MTR spectra (c) and final MTR maps (d).

Fig. 2: Normalized MTR maps obtained on a glioma rat for different saturation frequencies and for two different points in time: 7 days (a/top) and 11 (b/bottom) days after 9L glioma cell injection. For each map, a blue-to-red color scale was used to display the minimum-to-maximum MTR intensities.

Fig. 3: Quantitative CEST maps obtained for different saturation frequencies using the Lorentzian fit difference method on a rat bearing a day-11 glioma. A strong negative contrast is observed upfield (-3.5 ppm) originating from NOE and MT effects while a positive contrast appears at +3.5 ppm suggesting an increase in APT.

Fig. 4: Quantitative CEST maps obtained for different saturation frequencies using the Lorentzian fit difference method on a MCAO rat. No specific contrast that could highlight the ischemic brain tissue was observed.

Fig. 5: Average quantification results obtained using the Lorentzian fit difference method (a) and the three-offset integration technique (b,c) for the whole study (12 rats). APT increase (+3.5 ppm) and NOE decrease (-4 ppm) in glioma tissues were quantified successfully with both techniques. No significant changes were detected in ischemic tissue.

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)