Abstract
PurposeReal‐time magnetic resonance guided radiation therapy (MRgRT) uses 2D cine imaging for target tracking. This work evaluates the percent image uniformity (PIU) and spatial integrity of cine images in the presence of multileaf collimator (MLC) and gantry motion in order to simulate sliding window and volumetric modulated arc therapy (VMAT) conditions.MethodsPercent image uniformity and spatial integrity of cine images were measured (1) during MLC motion, (2) as a function of static gantry position, and (3) during gantry rotation. PIU was calculated according to the ACR MRI Quality Control Manual. Spatial integrity was evaluated by measuring the geometric distortion of 16 measured marker positions (10 cm or 15.225 cm from isocenter).ResultsThe PIU of cine images did not vary by more than 1% from static linac conditions during MLC motion and did not vary by more than 3% during gantry rotation. Banding artifacts were present during gantry rotation. The geometric distortion in the cine images was less than 0.88 mm for all points measured throughout MLC motion. For all static gantry positions, the geometric distortion was less than 0.88 mm at 10 cm from isocenter and less than 1.4 mm at 15.225 cm from isocenter. During gantry rotation, the geometric distortion remained less than 0.92 mm at 10 cm from isocenter and less than 1.60 mm at 15.225 cm from isocenter.ConclusionDuring MLC motion, cine images maintained adequate PIU, and the geometric distortion of points within 15.225 cm from isocenter was less than the 1 mm threshold necessary for real‐time target tracking and gating. During gantry rotation, PIU was negatively affected by banding artifacts, and spatial integrity was only maintained within 10 cm from isocenter. Future work should investigate the effects imaging artifacts have on real‐time target tracking during MRgRT.
Highlights
The ViewRay MRIdian (ViewRay Inc, Oakwood Village, OH, USA) system integrates a magnetic resonance imaging (MRI) unit with a linear accelerator to deliver MR-guided radiation therapy (MRgRT)
We evaluate the spatial integrity and percent image uniformity (PIU) of the 2D cine images used for tracking and gating during MRgRT in the presence of multileaf collimator (MLC) and gantry motion
Real-time 2D cine images obtained using the MRIdian 0.35 T MRI were characterized under the separate conditions of moving MLC and moving gantry
Summary
The ViewRay MRIdian (ViewRay Inc, Oakwood Village, OH, USA) system integrates a magnetic resonance imaging (MRI) unit with a linear accelerator (linac) to deliver MR-guided radiation therapy (MRgRT). Without MRgRT, an additional margin is added to the clinical target volume to account for internal motion during treatment This internal target volume (ITV) ensures that the tumor reliably receives the appropriate dose, and results in greater normal tissue irradiation. The motion of the multileaf collimator (MLC) and gantry produce eddy currents, field inhomogeneities, and nonlinear gradients Each of these can result in geometric distortions that affect the spatial integrity of the image.[2,3] eddy currents produce banding artifacts that interfere with the signal intensity. Deviations in signal intensity that are not related to patient anatomy can result in target tracking errors.[1,4,5,6] For these reasons, real-time MRgRT systems use step-and-shoot IMRT, in which the gantry and MLC are static during delivery, to maintain image quality during treatment. The advantages that sliding window IMRT and volumetric modulated arc therapy (VMAT) have over step-and-shoot delivery, plan quality and treatment time, are not currently available in MRgRT
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