WE‐G‐BRD‐02: Characterizing Information Loss in a Sparse‐Sampling‐Based Dynamic MRI Sequence (k‐T BLAST) for Lung Motion Monitoring

Abstract

Purpose:Rapid MRI is an attractive, non‐ionizing tool for soft‐tissue‐based monitoring of respiratory motion in thoracic and abdominal radiotherapy. One big challenge is to achieve high temporal resolution while maintaining adequate spatial resolution. K‐t BLAST, sparse‐sampling and reconstruction sequence based on a‐priori information represents a potential solution. In this work, we investigated how much “true” motion information is lost as a‐priori information is progressively added for faster imaging.Methods:Lung tumor motions in superior‐inferior direction obtained from ten individuals were replayed into an in‐house, MRI‐compatible, programmable motion platform (50Hz refresh and 100microns precision). Six water‐filled 1.5ml tubes were placed on it as fiducial markers. Dynamic marker motion within a coronal slice (FOV: 32×32cm2, resolution: 0.67×0.67mm2, slice‐thickness: 5mm) was collected on 3.0T body scanner (Ingenia, Philips). Balanced‐FFE (TE/TR: 1.3ms/2.5ms, flip‐angle: 40degrees) was used in conjunction with k‐t BLAST. Each motion was repeated four times as four k‐t acceleration factors 1, 2, 5, and 16 (corresponding frame rates were 2.5, 4.7, 9.8, and 19.1Hz, respectively) were compared. For each image set, one average motion trajectory was computed from six marker displacements. Root mean square error (RMS) was used as a metric of spatial accuracy where measured trajectories were compared to original data.Results:Tumor motion was approximately 10mm. The mean(standard deviation) of respiratory rates over ten patients was 0.28(0.06)Hz. Cumulative distributions of tumor motion frequency spectra (0–25Hz) obtained from the patients showed that 90% of motion fell on 3.88Hz or less. Therefore, the frame rate must be a double or higher for accurate monitoring. The RMS errors over patients for k‐t factors of 1, 2, 5, and 16 were.10(.04),.17(.04), .21(.06) and.26(.06)mm, respectively.Conclusions:K‐t factor of 5 or higher can cover the high frequency component of tumor respiratory motion, while the estimated error of spatial accuracy was approximately.2mm.