Purpose: MR simulation in the abdomen region is challenging because of respiratory motion. Multiple scans with different contrast weightings are ideally needed for better delineation of the tumor target and organs-at-risk (OARs). However, conventional diagnostic MR sequences often uses variable motion compensation strategies (e.g. respiratory gated or breath holds) and are susceptible to inter-scan misalignment. Emerging motion resolved volumetric MR (i.e. 4D-MR) techniques are limited to a single contrast weighting. In this study, we present a free-breathing MR sequence, namely multi-task MR (MT-MR), that produces multi-contrast, motion-resolved volumetric images for abdominal radiotherapy simulation.
Methods: Our technique builds upon the recently proposed MR Multitasking imaging framework that uses low-rank tensor modeling and exploit the correlation between images of different motion states and contrast weightings to achieve accelerated acquisitions. During each repetition time, a combination of saturation recovery pulses, idle gaps and T2 preparation pulses allows T1, proton density and T2 contrast weightings to be successively generated. MR data are collected using FLASH readouts with a Cartesian spiral-in sampling pattern, which allows for denser coverage of k-space center region and reduced eddy-current effect. This sequence was validated on an abdomen digital MR phantom as well as 7 healthy volunteers at 3.0T with a 1.6SI x 1.6LR x 3.2AP mm^3 resolution. Additionally, we acquired 2D real-time MR images as a motion reference in 4 of the 7 volunteers.
Results: The digital simulation confirmed the image accuracy of our MR technique. In vivo scans demonstrated our capabilities in producing motion-resolved, volumetric multi-contrast abdomen MR images from a free breathing scan under 10 mins. Differences between the respiratory motion range resolved by MT and the 2D real-time MR image reference are within 0.65mm.
Conclusion: MT-MR is a multi-contrast, motion resolved MR simulation sequence that may provide versatile, useful information for abdominal radiotherapy treatment planning.
Funding Support, Disclosures, and Conflict of Interest: This study is supported by NIH/NIBIB R01 EB029088