Introduction of a pseudo demons force to enhance deformation range for robust reconstruction of super‐resolution time‐resolved 4DMRI

Purpose The purpose of this study was to enhance the deformation range of demons‐based deformable image registration ( DIR ) for large respiration‐induced organ motion in the reconstruction of time‐resolved four‐dimensional magnetic resonance imaging ( TR ‐4 DMRI ) for multi‐breath motion simulation. Methods A demons‐based DIR algorithm was modified to enhance the deformation range for TR ‐4 DMRI reconstruction using the super‐resolution approach. A pseudo demons force was introduced to accelerate the coarse deformation in a multi‐resolution (n = 3) DIR approach. The intensity gradient of a voxel was applied to its neighboring (5 × 5 × 5) voxels with a weight of Gaussian probability profile ( σ = 1 voxel) to extend the demons force, especially on those voxels that have little intensity gradience but high‐intensity difference. A digital 4 DMRI phantom with 3–8 cm diaphragmatic motions was used for DIR comparison. Six volunteers were scanned with two high‐resolution (highR: 2 × 2 × 2 mm 3 ) breath‐hold ( BH ) 3 DMR images at full inhalation ( BHI ) and full exhalation ( BHE ) and low‐resolution (lowR: 5 × 5 × 5 mm 3 ) free‐breathing ( FB ) 3 DMR cine images (2 Hz) under an IRB ‐approved protocol. A cross‐consistency check ( CCC ) ( BHI → FB ← BHE ), with voxel intensity correlation ( VIC ) and inverse consistency error ( ICE ), was introduced for cross‐verification of TR ‐4 DMRI reconstruction. Results Using the digital phantom, the maximum deformable magnitude is doubled using the modified DIR from 3 to 6 cm at the diaphragm. In six human subjects, the first 15‐iteration DIR using the pseudo force deforms 200 ± 150% more than the original force, and succeeds in all 12 cases, whereas the original demons‐based DIR failed in 67% of tested cases. Using the pseudo force, high VIC (>0.9) and small ICE (1.6 ± 0.6 mm) values are observed for DIR of BHI & BHE , BHI → FB , and BHE → FB . The CCC identifies four questionable cases, in which two cases need further DIR refinement, without missing true negative. Conclusions The introduction of a pseudo demons force enhances the largest deformation magnitude up to 6 cm. The cross‐consistency check ensures the quality of TR ‐4 DMRI reconstruction. Further investigation is ongoing to fully characterize TR ‐4 DMRI for potential multi‐breathing‐cycle radiotherapy simulation.