4D‐MRI at 0.55T for internal target volume delineation in liver radiation therapy planning

Abstract Background Low‐field MRI provides superior soft‐tissue contrast compared to CT while costing significantly less than high‐field MRI, which makes it a more accessible option for MRI‐guided radiation therapy planning. Four‐dimensional MRI (4D‐MRI) is a technique that has been increasingly adopted clinically for internal target volume (ITV) delineation in free‐breathing liver radiotherapy planning, and it requires high spatial resolution and accurate respiratory phase differentiation to enable precise dose planning. The feasibility of 4D‐MRI at low‐field strength, specifically at 0.55T, has not been evaluated. Purpose This study aims to investigate the feasibility of 4D‐MRI for ITV delineation in liver radiation therapy planning using a commercial 0.55T MRI scanner. Methods A 3D stack‐of‐stars T1‐weighted sequence was implemented with two respiratory self‐navigation methods: (1) k‐space center point tracking (“k‐center”) and (2) superior‐inferior one‐dimensional projection‐based tracking (“SI‐projection”). These methods were evaluated using ten phantom scans simulating diverse respiratory motion patterns and five liver tumor patient scans. Results Both self‐navigation approaches demonstrated strong correlation between the extracted self‐gating signals (SGS) and ground‐truth motion traces across four breathing patterns: sinusoidal waveform, typical respiration, drifting motion, and irregular breathing. For sinusoidal motion, measured ITV deviations were within 1.1% of the true ITV for both methods. In non‐sinusoidal cases, ITV deviations remained within 2% except for two drifting motion cases where k‐center SGS based reconstructions showed deviations of 6.0% and 2.4%. In liver tumor patient scans, both self‐navigation techniques produced images with sufficient tumor delineation for treatment planning, with SI‐projection SGS‐based reconstructions yielding sharper images than k‐center SGS‐based reconstructions. ITV volumes contoured by two radiation oncologists showed strong and comparable inter‐observer agreement across both techniques. Conclusions This study demonstrates that 4D‐MRI at 0.55T is feasible and provides adequate image quality for ITV delineation. Self‐navigation techniques play an important role in improving the sharpness of tumor boundaries, with SI‐projection based self‐navigation offering superior performance in cases of irregular respiratory motion.