Room: AAPM ePoster Library
Purpose: The purpose of this study was to quantify the effects of gantry rotation on B0 for a 0.35 T MRI-Linac (ViewRay, Mountain View) and understand the causes of the resulting image artifacts. The ViewRay MRI-Linac system has six 227 kg ferromagnetic steel shields that hold the linear accelerator components and are spaced 60° about the rotating gantry. The gantry is driven by a three-phase synchronous motor (B&R Model 8LS, Austria) that is located at 225° (clockwise) relative to the top of the system.
Methods: B0 center frequencies and field homogeneities were measured in a 24 cm diameter spherical volume (DSV) water phantom doped with 8 mM NiSO4 (T1 = 330 ms at 0.35 T) during gantry rotation. Free induction decays (FIDs) were acquired every 0.29 s while the gantry was rotated clockwise (40°-30°) and counterclockwise (30°-40°) with 0° corresponding to the Linac at the top (12:00) position. The resulting B0 off-resonances were fit to a sinusoidal function to model and understand the phenomena.
Results: B0 fluctuations were sinusoidal with an amplitude >7 µT (causing changes in the Larmor frequency of >300 Hz). When the gantry was stationary, the magnitude of the off-resonance dropped to <40 Hz. Null bands in TrueFISP were expected and observed at ±238 Hz off resonance [±n/(2·TR), where TR=2.1 ms]. The polarity of the off-resonance depended on the direction of the rotation (clockwise vs. counterclockwise). The peaks and valleys depended on the gantry position of the steel shield buckets and their proximity to the gantry motor.
Conclusion: B0 fluctuations associated with gantry rotation on the ViewRay MRI-Linac were consistent with field inhomogeneities that cause banding artifacts for steady state free precession acquisitions commonly used in target tracking (e.g., 2D cine TrueFISP sequences).
Funding Support, Disclosures, and Conflict of Interest: Dr. Mutic has previously consulted for ViewRay. He currently consults for Varian Medical Systems. Dr. Green has received speaking fees from ViewRay. Washington University in St. Louis has a Master Research Agreement with, and may receive financial support from, Viewray.