Purpose: To develop a 4D MRI technique with high isotropic resolution and improved soft tissue contrast for respiratory motion assessment in radiotherapy treatment planning and monitoring of lung cancer.
Methods: T1-weighted ultrashort echo time (UTE) 3D radial acquisition with high isotropic resolution (1.1 mm) was performed on a 3T Philips MRI scanner on a female patient (55 years) with a pulmonary nodule (7.7x5.0 cm) referred for radiotherapy. A camera-based respiratory sensing technology (VitalEye, Philips Healthcare) was used to provide real-time respiratory motion signal for k-space data sorting and binning. 4D MRI with 10 motion states was reconstructed using the XD-GRASP framework. In addition, 4D CT with 10 respiratory phases was performed on the same day one hour before the MRI scan. Respiration-induced changes of the left and right lung volumes and the tumor center of mass shifts (X, Y, Z, and Euclidean distance) were compared between 4D MRI and 4D CT to evaluate the performance of the proposed 4D MRI method.
Results: Lung parenchyma, tumor, and surrounding organs at risk were well visualized and respiratory motion was well resolved on the high-resolution UTE 4D MRI images. The changes of the left lung volume were 92.16 mL (13.50%) for 4D MRI and 97.13 mL (13.03%) for 4D CT and the right lung volume were 103.23 mL (13.70%) for 4D MRI and 118.47 mL (13.46%) for 4D CT. The tumor center of mass shifts were 0.02 cm, 0.12 cm, and 0.47 cm in X, Y, and Z directions for 4D MRI in comparison to 0.04 cm, 0.18 cm, and 0.66 cm for 4D CT. The Euclidean distances were 0.49 cm for 4D MRI and 0.68 cm for 4D CT.
Conclusion: This work demonstrated the feasibility of a novel high-resolution UTE 4D MRI method for respiratory motion assessment and it can be utilized as an alternative technique to 4D CT for treatment planning and monitoring in radiotherapy of lung cancer.
Funding Support, Disclosures, and Conflict of Interest: The work is supported by Philips Healthcare.