Exploring pathways: MRI insights into a child’s brain in the operating theatre

Abstract

This thesis explores the feasibility of integrating advanced MRI acquisition techniques, particularly diffusion MRI, in the intraoperative setting of pediatric posterior fossa tumor (pPFT) patients. The potential value of this project was to provide new morphological information of the relevant connecting pathways, such as the dentato-rubro-thalamic tract (DRTT), during this type of surgery in relation to speech disturbance symptoms. We addressed technical challenges unique to intraoperative imaging and developed methods for accurate reconstruction of the DRTT from intraoperative data. Finally, we investigated how the DRTT that is located in proximity to the pPFT tumor resection site is impacted by the surgical intervention. Chapter 2 explored literature on the added value of integrating advanced MRI techniques into pediatric intraoperative MRI (ioMRI) for brain tumor surgery, as compared to conventional imaging. Our systematic literature review revealed that advanced sequences have been used in selective cases to visualize eloquent fiber bundles, assess cerebral perfusion, or detect metabolic abnormalities. However, their application remains uncommon in routine practice. In Chapter 3, we addressed the practical challenges involved in developing a pediatric ioMRI protocol optimized to increase the sensitivity of the advanced MRI techniques described in Chapter 2. Quantitative ASL-derived cerebral blood flow (CBF) measurements showed no difference between the surgical prone position and the standard supine position. Therefore, ASL was incorporated into the ioMRI protocol alongside a T1-weighted, T2-weighted, and diffusion MRI. The image quality of these sequences was clinically acceptable in all cases. No differences were found between images acquired with two or four radiofrequency (RF) coils. Finally, the setup with four RF coils resulted in improved signal-to-noise and contrast-to-noise ratios (SNR and CNR), which helped reduce susceptibility artefacts and decreased variation in metrics derived from diffusion MRI (e.g., FA and MD) and perfusion MRI (e.g., CBF). In particular, the reduced artefacts led to improved quality of corticospinal tract (CST) reconstruction based on diffusion MRI. Despite the benefits of ioMRI acquisition with four RF coils described in Chapter 3, in practice, we used data acquired with two RF coils for further analysis. This decision was driven by difficulties with the positioning of the four RF coils, which compromised consistent image quality and lengthened the transition time between the operating theatre and MRI. Based on the findings in Chapter 3, an improved intraoperative diffusion MRI protocol was used to perform fiber tractography in our patient group, as described in Chapter 4, with a particular focus on the DRTT that is potentially affected in pPFT patients. These techniques were compared in healthy volunteers, where the diffusion MRI data supported reliable DRTT reconstruction. We found that reconstruction of the DRTT in the intraoperative setting of children undergoing PFT surgery with the use of two RF coils was feasible. The nondecussating DRTT (nd-DRTT) was more robustly reconstructed than the decussating component (d-DRTT), with greater tract volume and more coherent streamlines. In contrast, CST reconstructions were consistent across pre- and intraoperative settings, with no differences in volume or coherence in streamlines. In Chapter 5, we applied the optimized fiber tractography settings found in Chapter 4 to investigate the effect of pPFT surgery on the DRTT. We analyzed its structural changes between preoperative and intraoperative measurements, compared them to those of a distant control tract, and examined their relation to postoperative speech disturbances. PPFT surgery affected the variability of DRTT microstructure near the resection cavity, whereas the microstructure of the arcuate fasciculus remained stable. Subsequently, these intraoperative microstructural alterations seemed to be more pronounced in patients who developed postoperative speech disturbances. These findings indicate a potential effect of DRTT disruption, as measured with mean diffusivity (MD), on postoperative speech disturbances.