Geometric distortion in MRI for radiotherapy planning: A narrative review

Magnetic resonance imaging (MRI) provides superior soft-tissue contrast compared to computed tomography (CT), making it a valuable modality for target delineation in radiotherapy. However, geometric distortion inherent to MRI can compromise spatial accuracy and remains a critical consideration in treatment planning.

This narrative review summarizes current knowledge on geometric distortion in 1.5 T and 3.0 T MRI systems used for radiotherapy planning, focusing on distortion mechanisms, evaluation methods, distortion correction methods, and considerations for clinical implementation. Findings from phantom-based and clinical studies were reviewed.

Geometric distortion arises mainly from gradient nonlinearity, static magnetic field (B₀) inhomogeneity, and susceptibility effects, and tends to increase toward the periphery of the field of view. Phantom-based studies have reported spatial deviations of approximately 1-3 mm in clinically relevant regions, particularly in 3.0 T systems. Vendor-provided distortion correction, 3D distortion mapping, and periodic quality assurance (QA) can reduce distortion to clinically acceptable levels, although standardized evaluation protocols and cross-platform comparability remain limited.

MRI geometric distortion remains an important challenge in radiotherapy planning, especially in high-field 3.0 T systems. Although correction methods and QA procedures improve geometric reliability, differences in scanner characteristics, acquisition conditions, and evaluation methods continue to limit consistency across studies and institutions.

Practical clinical implementation of MRI in radiotherapy planning requires not only distortion correction but also standardized QA procedures to maintain geometric reliability and support future advances in image registration and MRI-guided radiotherapy.

Magnetic resonance imaging, or MRI, is often used in radiotherapy because it shows body tissues clearly, but it can slightly distort shapes. This study reviewed existing research to understand where these distortions come from, how they are measured, and how they can be corrected in clinical practice. This study found that distortions are usually small but can increase toward the edges of the image, and can be reduced with correction methods and regular quality checks. This matters because improving accuracy in imaging helps ensure that radiotherapy is planned and delivered safely and precisely.