Author: Nebi Demez, Michael Kasper, Noufal Manthala Padannayil, Shyam Pokharel, Suresh Rana, Lauren A. Rigsby, Tino Romaguera, Nishan Shrestha, Somol Sunny 👨🔬
Affiliation: Lynn Cancer Institute, Boca Raton Regional Hospital, Baptist Health South Florida 🌍
Purpose: Accurate delineation of the urethra is critical for optimizing tumor control and minimizing urethral toxicity in prostate stereotactic body radiation therapy (SBRT). The purpose of this study is to evaluate the utility of post-void T2-weighted MRI acquired with a low-field 0.35T on ViewRay MRIdian machine for urethral identification in an MRI-guided radiation therapy (MRIgRT) workflow.
Methods: A total of 10 prostate SBRT patients previously treated on MRIdian were included in this study. Each patient underwent an treatment planning MRI sequence (TRUFI) with a full bladder, followed by CT imaging in the same setup. Subsequently, post-void T2-weighted images were acquired using the ViewRay A3i system after bladder voiding. Imaging protocols were divided into two groups: five patients were scanned with a limited field of view (FOV) focusing on the prostate, while the remaining five underwent full pelvic scans. Fusion of TRUFI and post-void T2-weighted MRI images facilitated urethral delineation.
Results: The radiation oncologist successfully delineated the urethra in all 10 cases using the TRUFI and post-void T2-weighted MRI images. Limited FOV scans with 2 mm isotropic slice thickness provided superior urethral visualization and were highly effective for delineation. In contrast, full pelvic scans offered complementary anatomical information but exhibited reduced urethral clarity compared to limited FOV imaging.
Conclusion: Low-field MRI at 0.35T presents challenges and advantages in clinical applications, particularly in prostate cancer imaging and treatment planning. The reduced spatial resolution and signal intensity compared to higher magnetic field systems can pose limitations in anatomical visualization. However, post-void T2-weighted imaging on a low field 0.35T MRI system can potentially enhance the signal due to the presence of residual urine within the urethra. Limited FOV imaging performed immediately after voiding in the treatment position provides superior urethral delineation, mitigating cross-modality registration errors and improving the accuracy of MRIgRT workflows.