GluCEST Shows Improved Signal in MRI-Negative Patients

Peter N. Hadar

Single-slice glutamate chemical exchange saturation transfer (GluCEST) has been shown to lateralize and localize epileptogenic regions of the hippocampus on 7 Tesla magnetic resonance imaging (MRI) in MRI-negative patients.

New data have shown that in 4 nonlesional—or MRI-negative—patients with left-sided temporal lobe epilepsy (TLE), there were statistically significantly increased GluCEST signals in the ipsilateral total hippocampus, relative to the contralateral total hippocampus (P = .048; 1-tailed 2-sample t test). GluCEST values in the ipsilateral hippocampus were 7.15% to 9.22%, compared with 6.57% to 7.85% in the contralateral hippocampus.

“As we recruit more patients, we will further investigate GluCEST signals in hippocampal subfields and correlate these findings to electrophysiological and clinical outcomes, eventually leading to a superior understanding of epilepsy excitatory networks and improvement of surgical resection outcomes,” Peter N. Hadar, a medical student from the University of Pennsylvania’s Litt Laboratory, and colleagues said.

Patients underwent a protocol that included a localizer scan, followed by a T1w magnetization-prepared rapid acquisition gradient echo scan, which found repetition time/inversion time/echo time (TR/TI/TE) = 2800/1500/4.4 ms, fractional anisotropy = 70, generalized autocalibrating partial parallel acquisition (GRAPPA) = 2, 170 sagittal slices, and voxel size 0.8 mm³.

Patients then underwent T2w MRI for subfield segmentation (TR/TE = 3000/388 ms, matrix = 448 x 428, in-plane resolution = 0.4 mm x 0.4 mm, slice thickness = 1.0 mm, 224 oblique coronal slices perpendicular to the hippocampal long axis), followed by B0 field map and B1 field map.

Finally, after the field mapping, patients went through the 3-D GluCEST scan (TR/TE = 5.9/2.83 ms, matrix = 240 x 192, in-plane resolution = 1 mm², slice thickness = 1.0 mm, GRAPPA = 2, 60 axial slices). CEST raw images were obtained by varying saturation offset frequencies from ±1.8 to ±4.2 parts per million (ppm) with a step size of 0.3 ppm and saturation B1 + root-mean-scale values of 3 mT, 1.5 mT, and 0.75 mT.

B0 inhomogeneity was <1.2 ppm with global shimming, although relative B1 inhomogeneity varied from 0.5 to 1.4 ppm.

“To acquire whole brain B0 and B1 corrected GluCEST, a segmented elliptical center encoding strategy was used for the phase encode(ky)—slice encode(kz) plane, with ASHS [automatic segmentation of hippocampal subfields] segmentation to measure hippocampal GluCEST,” the authors wrote. “The total scan session took about 1 hour.”

Currently, the parameters for high-fidelity acquisition optimization are still being put together, although the results found were consistent with those of the previous investigation of single-slice GluCEST in MRI-negative patients with TLE.

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