SU-F-303-06: Effect of Multiband RF Pulse Duration On Cine MRI Tracking Accuracy for MRI-GRT




Exception gating and dynamic MLC tracking on MRI-gRT systems require segmentation of high temporal resolution images. Multi-band excitation is an emerging MRI acceleration technique that simultaneously excites a short comb of 2D slices, thereby increasing effective temporal resolution. However, the design of multiband RF pulses imposes tradeoffs between slice profiles and the temporal duration of the pulse. The goal of this study was to determine whether slice profiles of shorter duration multiband RF pulses affect organ tracking accuracy for MRI-gRT.


Three-band Shinnar-Le Roux (SLR) RF pulses were designed with slice bandwidths of 1.5kHz and durations ranging from 750 to 3000 µs. The pulses were implemented into a golden angle radial pulse sequence of our own design. Time series of dynamic 2D sagittal cine images of an MRI-compatible dynamic phantom (Model 008M, CIRS) with 2.5 cm superior/inferior target motion were acquired at 3T using each of the RF pulse durations. Images were reconstructed offline in Matlab. The target was contoured on the last image of the time series and tracked through the time series using normalized cross-correlation with a Powell optimizer and B-spline interpolator. Peak-to-peak (P2P) centroid displacements along the superior/inferior dimension were estimated and compared against the phantom motion trajectory.


Transition band sizes of the multiband RF slice profiles ranged from 2.1 to 4.6 mm for intended 5 mm slices. P2P centroid displacements ranged from 2.3 to 2.6 cm, demonstrating good agreement with the 2.5 cm phantom displacement, and indicating that slice profile has minimal effect on tracking ability.


Slice profile does not affect tracking ability, permitting the use of shorter duration RF pulses to increase temporal resolution during MRI-gRT. Future work will investigate the advantages of multiband excitation for 2D/3D image registration and will combine multiband excitation with in-plane acceleration to further improve temporal resolution.

Elekta Instruments, AB