Glutamate Chemical Exchange Saturation Transfer (GluCEST) Magnetic Resonance Imaging in Pre-clinical and Clinical Applications for Encephalitis

Abstract

Background: Encephalitis is a common central nervous system inflammatory disease that seriously endangers human health owing to the lack of effective diagnostic methods, which leads to a high rate of misdiagnosis and mortality. Glutamate is implicated closely in microglial activation, and activated microglia are key players in encephalitis. Hence, using glutamate chemical exchange saturation transfer (GluCEST) imaging for the early diagnosis of encephalitis holds promise.

Methods: The sensitivity of GluCEST imaging with different concentrations of glutamate and other major metabolites in the brain was validated in phantoms. Twenty-seven Sprague-Dawley (SD) rats with encephalitis induced by Staphylococcus aureus infection were used for preclinical research of GluCEST imaging in a 7.0-Tesla scanner. For the clinical study, six patients with encephalitis, six patients with lacunar infarction, and six healthy volunteers underwent GluCEST imaging in a 3.0-Tesla scanner.

Results: The number of amine protons on glutamate that had a chemical shift of 3.0 ppm away from bulk water and the signal intensity of GluCEST were concentration-dependent. Under physiological conditions, glutamate is the main contributor to the GluCEST signal. Compared with normal tissue, in both rats and patients with encephalitis, the encephalitis areas demonstrated a hyper-intense GluCEST signal, while the lacunar infarction had a decreased GluCEST signal intensity. After intravenous immunoglobulin therapy, patients with encephalitis lesions showed a decrease in GluCEST signal, and the results were significantly different from the pre-treatment signal (1.34 ± 0.31 vs 5.0 ± 0.27%, respectively; p = 0.000).

Conclusion: Glutamate plays a role in encephalitis, and the GluCEST imaging signal has potential as an in vivo imaging biomarker for the early diagnosis of encephalitis. GluCEST will provide new insight into encephalitis and help improve the differential diagnosis of brain disorders.

Keywords: Staphylococcus aureus; chemical exchange saturation transfer; encephalitis; glutamate; magnetic resonance imaging.

FIGURE 1

Phantom studies performed on 7.0 T scanner. (A) Z-spectra show that the amine protons on glutamate (Glu) generated a chemical exchange saturation transfer (CEST) effect with a chemical shift of 3.0 ppm away from bulk water. (B) GluCEST map showing that the GluCEST signal was concentration-dependent. (C) GluCEST signal increased as the Glu concentrations increase, which appears to have a good linear relationship, indicating that the GluCEST effect increases by nearly 0.215% with every 1 mM Glu added. (D) Under physiological conditions, Glu is the main contributor to the GluCEST signal and γ-aminobutyric acid (GABA) and creatine (Cr) have a small contribution to the GluCEST effect, while the contribution of other metabolites [N-acetylaspartate (NAA), myo-inositol (MI), glutamine (Gln)] to the GluCEST effect was negligible.

FIGURE 2

Glutamate chemical exchange saturation transfer imaging in vivo in encephalitis rats. (A) T2_W imaging and GluCEST map of encephalitis rats with Staphylococcus aureus infections at different time points (0, 3, and 7 days), suggesting that the GluCEST signal was increased with increasing time; statistically significant differences were found among the different time points [0 days vs 3 days (B), 0 days vs 7 days (C), and 3 days vs 7 days (D), n = 27, Student’s t test, two-tailed, unpaired, ****p < 0.0001].

FIGURE 3

Glutamate chemical exchange saturation transfer imaging was feasible in human brain using the 3.0 T MR system. (A,B) In a healthy volunteer, GluCEST magnetic resonance image (MRI) could clearly distinguish white matter and gray matter, similar to T2_W imaging, and the gray matter signal intensity of GluCEST was higher than that of white matter. (C,D) In a patient with encephalitis, GluCEST imaging also clearly showed the left parietal cortex and subcortex lesion.

FIGURE 4

Glutamate chemical exchange saturation transfer imaging in vivo of patients with encephalitis and lacunar infarction. (A) Conventional T2w imaging, T2Flair, and diffusion-weighted images (DWI) could not discriminate encephalitis and lacunar infarction (LI), but GluCEST MRI could distinguish between them. (B) Z-spectrum demonstrated that a small signal dip at 3.0 ppm (green circle) was observed in the inflamed regions but not observed in the LI regions, indicating that inflammation is a cause of the elevated local glutamate concentration. (C) GluCEST signal was elevated in patients with encephalitis and decreased in patients with LI (n = 6, Student’s t test, two-tailed, unpaired, ****p < 0.0001).

FIGURE 5

Glutamate chemical exchange saturation transfer imaging was used for efficacy evaluation. (A) T2_W imaging, T2Flair, and GluCEST maps of a patient with encephalitis before and after intravenous immunoglobulin therapy. (B) Compared with pre-treatment, the magnetization transfer ratio (MTR_asym) value (GluCEST signal) at post-treatment was significantly decreased (n = 6, Student’s t test, two-tailed, paired).