Indirect 1H-[13C] MRS of the human brain at 7 T using a 13C-birdcage coil and eight transmit-receive 1H-dipole antennas with a 32-channel 1H-receive array.

The neuronal tricarboxylic acid and glutamate/glutamine (Glu/Gln) cycles play important roles in brain function. These processes can be measured in vivo using dynamic 1H-[13C] MRS during administration of 13C-labeled glucose. Proton-observed carbon-edited (POCE) MRS enhances the signal-to-noise ratio (SNR) compared with direct 13C-MRS. Ultra-high field further boosts the SNR and increases spectral dispersion; however, even at 7 T, Glu and Gln 1H-resonances may overlap. Further gain can be obtained with selective POCE (selPOCE). Our aim was to create a setup for indirect dynamic 1H-[13C] MRS in the human brain at 7 T. A home-built non-shielded transmit-receive 13C-birdcage head coil with eight transmit-receive 1H-dipole antennas was used together with a 32-channel 1H-receive array. Electromagnetic simulations were carried out to ensure that acquisitions remained within local and global head SAR limits. POCE-MRS was performed using slice-selective excitation with semi-localization by adiabatic selective refocusing (sLASER) and stimulated echo acquisition mode (STEAM) localization, and selPOCE-MRS using STEAM. Sequences were tested in a phantom containing non-enriched Glu and Gln, and in three healthy volunteers during uniformly labeled 13C-glucose infusions. In one subject the voxel position was alternated between bi-frontal and bi-occipital placement within one session. [4-13C]Glu-H4 and [4-13C]Gln-H4 signals could be separately detected using both STEAM-POCE and STEAM-selPOCE in the phantom. In vivo, [4,5-13C]Glx could be detected using both sLASER-POCE and STEAM-POCE, with similar sensitivities, but [4,5-13C]Glu and [4,5-13C]Gln signals could not be completely resolved. STEAM-POCE was alternately performed bi-frontal and bi-occipital within a single session without repositioning of the subject, yielding similar results. With STEAM-selPOCE, [4,5-13C]Glu and [4,5-13C]Gln could be clearly separated. We have shown that with our setup indirect dynamic 1H-[13C] MRS at 7 T is feasible in different locations in the brain within one session, and by using STEAM-selPOCE it is possible to separate Glu from Gln in vivo while obtaining high quality spectra.

Amide proton transfer weighted imaging in pediatric neuro-oncology: initial experience.

Amide proton transfer weighted (APTw) imaging enables in vivo assessment of tissue-bound mobile proteins and peptides through the detection of chemical exchange saturation transfer. Promising applications of APTw imaging have been shown in adult brain tumors. As pediatric brain tumors differ from their adult counterparts, we investigate the radiological appearance of pediatric brain tumors on APTw imaging. APTw imaging was conducted at 3 T. APTw maps were calculated using magnetization transfer ratio asymmetry at 3.5 ppm. First, the repeatability of APTw imaging was assessed in a phantom and in five healthy volunteers by calculating the within-subject coefficient of variation (wCV). APTw images of pediatric brain tumor patients were analyzed retrospectively. APTw levels were compared between solid tumor tissue and normal-appearing white matter (NAWM) and between pediatric high-grade glioma (pHGG) and pediatric low-grade glioma (pLGG) using t-tests. APTw maps were repeatable in supratentorial and infratentorial brain regions (wCV ranged from 11% to 39%), except those from the pontine region (wCV between 39% and 50%). APTw images of 23 children with brain tumor were analyzed (mean age 12 years ± 5, 12 male). Significantly higher APTw values are present in tumor compared with NAWM for both pHGG and pLGG (p < 0.05). APTw values were higher in pLGG subtype pilocytic astrocytoma compared with other pLGG subtypes (p < 0.05). Non-invasive characterization of pediatric brain tumor biology with APTw imaging could aid the radiologist in clinical decision-making.

In vivo three-dimensional reconstruction of human median nerves by diffusion tensor imaging.

The in vivo assessment of axonal projections of the peripheral nervous system has been severely limited by the lack of noninvasive techniques. We examined whether MR diffusion tensor imaging with fiber tracking of the human median nerve is feasible. The median nerve was examined with a 3-T MRI scanner in wrists of three healthy volunteers and the wrist of a patient after median nerve repair. In vivo three-dimensional (3D) reconstruction of axonal projections of the median nerve could be achieved in healthy volunteers with intact median nerves and a patient with a transected median nerve using diffusion tensor imaging combined with fiber tracking. The median nerve could be visualized and correlated well to the expected location on T1-weighted images in the patient and all volunteers. The transected median nerve in the patient could be detected up to the site of repair one month postoperatively, whereas the nerve could not be detected more distally immediately after repair. Two months postoperatively, it could clearly be tracked more distally, indicating nerve regeneration. Diffusion tensor imaging can detect the median nerve in healthy individuals as well as up to the lesion site in a patient with a complete transection of the median nerve.