Purpose: Currently, commercial MRI-Linacs only turn on the beam when the gantry and MLCs are stationary. However, treatment times could potentially be reduced by allowing the beam to be on during MLC motion (e.g., Sliding Window IMRT or VMAT). The purpose of this study was to quantify the effects of MLC motion on B₀ for a 0.35 T MRI-Linac (ViewRay, Mountain View) and determine their effect on image quality.
Methods: B₀ center frequencies and field homogeneities were measured in a 24 cm diameter spherical volume (DSV) water phantom doped with 5 mM NiSO4 (T₁ = 330 ms at 0.35 T) during MLC sweeps. Free induction decays (FIDs) were acquired every 0.29 s while the gantry was static at gantry angles 0-330° in increments of 30°. The spectra were calculated from the FIDs. The peak offset and full width at half maximum (FWHM) were calculated after fitting the spectra to a Lorentzian function. Measurements were conducted without gradient shimming. MRIs were also acquired during MLC sweeps using the 2D cine radial (8 fps) and Cartesian (4 fps) balanced steady state free precession (bSSFP) acquisitions normally used during treatment. MRIs were inspected for presence of artifacts.
Results: Variations in the mean center frequency and FWHM between gantry angles are within expected ranges. B₀ and field homogeneity standard deviations from MLC motion were <2.0 and <1.3 Hz, respectively. No conspicuous image artifacts were observed during the MLC sweeps.
Conclusion: MLC motion had a minimal effect on image quality and B₀. Therefore, dynamic beam shaping using MLCs could potentially be performed without an impact to image quality and target tracking.
Funding Support, Disclosures, and Conflict of Interest: ViewRay provides financial support and speaker fees to Washington University in St Louis and their faculty (unrelated to this abstract). Washington University has a Master Research Agreement with ViewRay.
Not Applicable / None Entered.
IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined Quality Assurance