Purpose: To quantify imaging isocenter shifts due to fluctuations in the primary magnetic field (B₀) as a result of gantry rotation in a commercial 0.35T magnetic resonance image guided radiotherapy (MR-IGRT) system and to correct those shifts within the imaging sequence in real time.
Methods: A hard pulse navigator (FA:90, dwell time = 8 µs, complex points = 64) was added to a sagittal 2D cartesian spoiled gradient echo (GRE) sequence (TR/TE: 20/3.7ms, FA: 20, rBW: 260 Hz/pixel) following each image on a commercial 0.35T MR-IGRT system. The gantry was rotated from 33° to 153° during imaging and measurements were acquired using a spherical 24 cm DSV phantom (T1/T2: 330/260 ms) using the system body coil. The resonant frequency offset was calculated by comparing the phase data from the first (reference) hard pulse navigator with each subsequent navigator. The calculated frequency offset was then fed back into the sequence from the image reconstruction computer as a real-time feedback object. This data was then used to modify the imaging pulse and receiver frequencies for the next image. Phantom images were acquired with and without the correction. Shimming was done at 33° for both image sets. Images were time aligned and the center of the phantom was calculated for each image using MATLAB. The phantom shifts in the frequency encoding direction were then compared.
Results: RMS isocenter shifts in the frequency encoding direction were reduced using the resonance frequency correction (RMS = 2.70mm/1.14mm). Peak displacement in the frequency encoding direction was additionally reduced from 5.7mm without real time correction to 2.5mm with correction.
Conclusion: This work provides a method for real-time reduction of imaging isocenter shifts due to B₀ fluctuations during gantry rotation in GRE sequences. More precise gantry rotation data could yield further reductions.