MRI distortion: considerations for MRI based radiotherapy treatment planning

Area of Science:

• Medical Imaging
• Radiotherapy Physics

Background:

• Geometric distortion in MRI can compromise accuracy in radiotherapy treatment planning (RTP).
• Understanding and quantifying these distortions is crucial for ensuring patient safety and treatment efficacy.

Purpose of the Study:

• To assess geometric distortion variations across different MRI scanners and sequences.
• To investigate the impact of acquisition parameters, such as bandwidth, on image distortion and signal-to-noise ratio (SNR).
• To evaluate the effectiveness of vendor-supplied correction algorithms.

Main Methods:

• A commercial MRI grid phantom was scanned on four MRI scanners using multiple sequences.
• Distortion maps were generated and analyzed across the field of view.
• The influence of varying bandwidth on distortion and SNR was examined.
• Distortions were compared to patient anatomy locations within the scanner bore.

Main Results:

• Significant variations in distortion magnitude and patterns were observed between MRI sequence protocols and scanners.
• Distortion increased with distance from the isocenter, reaching up to 4.1 mm with a turbo spin echo sequence at the phantom edge.
• Vendor-supplied correction algorithms reduced but did not eliminate distortions.
• Increasing acquisition bandwidth decreased distortion but also reduced SNR by 13.5%.

Conclusions:

• MRI geometric distortion is variable and influenced by scanner type, imaging sequence, and acquisition parameters (e.g., bandwidth, slice thickness, phase encoding direction).
• These distortions must be considered in RTP, and imaging protocol details should be documented.
• Distortion magnitudes differ across clinical sites, highlighting the need for site-specific evaluations.