Magnetic resonance imaging with ultra-short echo time sequence for head and neck radiotherapy planning.

Background: Radiotherapy treatments are usually planned on computed tomography (CT) images. For head and neck localizations, magnetic resonance imaging (MRI) is also increasingly used for delineation as it provides better soft-tissue contrast.

Purpose: Treatment planning exclusively based on MRI is currently not straightforward, as there is no direct link between MRI signal intensity and electron density. This study aims to generate a treatment planning using a UTE sequence, for regions with high anatomical variability.

Methods: An ultra-short echo time pulse sequence (1H MRI UTE) was performed on 25 patients with head and neck cancers, treatable by radiotherapy (protocol number R201-004-314), without exclusion due to dental induced artifacts. The hydrogen tissue content, achievable with this sequence can be linked to the electron density of tissues. Generated synthetic CT (sCT) images were compared with reference CT using mean absolute error (MAE) computation. Patient dose calculations were performed on CT and sCT and compared using dose differences, Bland-Altman analysis and global gamma pass rate computation.

Results: The mean MAE was 210.9 HU for all patients. The mean 3D global gamma pass rates were 93.1 %, 89.2 % and 80.9 %, for 3 %/3mm, 2 %/2mm and 1 %/1mm criteria respectively. The mean of the median dose difference for the planning tumor volume (PTV) was 0.87 % of 70 Gray.

Conclusions: The UTE sequence enables a direct physical-based method suitable for radiotherapy planning. The proposed method, based on a dedicated acquisition sequence compatible with clinical duration, provided dosimetry results similar to the reference CT, in a region with high anatomical variability.