Semester of Graduation

Spring 2025

Degree

Master of Science (MS)

Department

Physics & Astronomy

Document Type

Thesis

Abstract

Purpose: In radiotherapy, tumor displacement measurement throughout the breathing cycle is essential to ensure proper treatment planning and delivery. For soft-tissue abdominal tumors, a 4D-MR may provide improved tumor tracking over the clinical standard 4D-CT. This project aims to improve a clinically available 4D-MR pulse sequence to achieve similar motion displacement accuracy as 4D-CT for slightly hyperintense objects with movement in the superior-inferior direction.

Methods: Waveforms passed through a Quasar MRI4D Motion Phantom to oscillate sphere. Two different sinusoidal amplitudes with three different breathing periods were played out. The number of respiratory phase bins, slice thickness, radial views, and acquisition direction were individually varied from the clinical protocol. Scans were acquired for four repetitions each on a 1.5T MR machine (Siemens Magnetom Sola) using a 3D radial “stack-of-stars" (StarVIBE) sequence, and CT images were acquired for comparison. Displacement values were measured, and ANOVA calculations determined best parameters based on accuracy. The combination of best parameters was tested using sinusoidal waveforms and patient breathing waveforms acquired from a Varian RPM system. Nine patient breathing waveforms were tested and categorized into three groups based on phase and amplitude changes: regular, semiregular, and irregular. The adjusted protocol was compared against the clinical MR protocol, CT protocol, and the waveform average input amplitude. Average amplitudes were determined using a MATLAB algorithm. Results were analyzed using a linear mixed model. For verification, all three protocols were compared using three patients with abdominal tumors, with displacement measurement performed by a qualified medical physicist.

Results: The adjusted MR protocol improved the displacement measurements by more than 1 mm for all tested sinusoidal periods and amplitudes. Using patient breathing waveforms, the modified MR protocol had displacement measurements 0.9mm greater than 4D-CT and 1.2mm below the mean waveform displacement.

Conclusion: 4D-MR shows promise for motion management in radiotherapy, offering potential advantages over 4D-CT in abdominal tumor motion estimation. Future work will involve analyzing interplay effects using a factorization approach.

Date

3-18-2025

Committee Chair

Kirby, Krystal

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