Room: Karl Dean Ballroom B1
Purpose: Develop and validate model-interpolated gating (MI-gating), a method for radiotherapy gating using low frame rate (less than 1 frame per second) magnetic resonance imaging (MRI) and a motion model.
Methods: MI-gating was validated by acquiring high-frame rate (approximately 3 frames per second) planar MRI images, using a periodic low-frame rate sample (e.g. every 10th image) to build the motion model. The images omitted from model building were used as a ground truth to evaluate MI-gating. Eight volunteers were imaged using a balanced steady state free precession sequence (bSSFP) on a commercially available 0.35T MRI-guided radiotherapy unit. The motion model was built and continuously updated using motion information obtained from deformable image registration. A pneumatic bellows was used to acquire a respiratory surrogate for the model. MI-gating performance was evaluated in terms of accuracy (fraction of agreement between model and ground-truth gating decisions), beam-on positive predictive value (proportion of model gating beam-on decisions that are correct), and the distance between modeled and ground-truth target centroids. A proof-of-concept T2-weighted MI-gating study was performed using a transverse relaxation (T2)-weighted half-Fourier single shot turbo spin echo sequence acquired for 6 of the volunteers.
Results: The average model gating accuracy, positive predictive value, and median target centroid distances were 93.7%, 92.1% and 0.86 mm, respectively, for the bSSFP imaging studies. The same values obtained from the T2 modeling studies were 94.3%, 92.5% and 0.86 mm respectively.
Conclusion: MI-gating was found to be potentially sufficiently accurate for clinical use. This technique could enable the use of new pulse sequences that require significant delays between image acquisitions to preserve contrast to be used for radiotherapy gating. The technique could also enable additional imaging studies to be performed during gated radiotherapy treatment (e.g. functional imaging), reducing the time patients spend on the table.
Funding Support, Disclosures, and Conflict of Interest: A provisional patent application related to this work has been submitted. Some authors have previously received speaking and consulting fees from ViewRay.