title: Respiration-based sorting of dynamic MRI to derive representative 4D-MRI for radiotherapy planning.
abstract: OBJECTIVE
  Current pretreatment, 4D imaging techniques are suboptimal in that they sample breathing motion over a very limited "snap-shot" in time. To potentially address this, the authors have developed a longer-duration MRI and postprocessing technique to derive the average or most-probable state of mobile anatomy and meanwhile capture and convey the observed motion variability.
  
  
  METHODS
  Sagittal and coronal multislice, 2D dynamic MRI was acquired in a sequential fashion over extended durations in two abdominal and four lung studies involving healthy volunteers. Two sequences, readily available on a commercial system, were employed. Respiratory interval-correlated, or 4D-MRI, volumes were retrospectively derived using a two-pass approach. In a first pass, a respiratory trace acquired simultaneous with imaging was processed and slice stacking was used to derive a set of MRI volumes, each representing an equal time or proportion of respiration. Herein, all raw 2D frames mapping to the given respiratory interval, per slice location, were averaged. In a second-pass, this prior reconstruction provided a set of template images and a similarity metric was employed to discern the subset of best-matching raw 2D frames for secondary averaging (per slice location and respiratory interval). Breathing variability (per respiratory interval and slice location) was depicted by computing both a maximum intensity projection as well as a pixelwise standard deviation image.
  
  
  RESULTS
  These methods were successfully demonstrated in both the lung and abdomen for both applicable sequences, performing reconstructions with ten respiratory intervals. The first-pass (average) resulted in motion-induced blurring, especially for irregular breathing. The authors have demonstrated qualitatively that the second-pass result can mitigate this blurring.
  
  
  CONCLUSIONS
  They have presented a novel methodology employing dMRI to derive representative 4D-MRI. This set of techniques are practical in that (1) they employ MRI sequences that are standard across commercial vendors; (2) the 2D imaging planes can be oriented onto an arbitrary axis (e.g., sagittal, coronal, axial[ellipsis (horizontal)]); (3) the image processing techniques are relatively simple. Systematically applying this and similar dMRI-based techniques in patients is a crucial next step to demonstrate efficacy beyond CT-only based practice.
authors: Department of Radiation Oncology and Molecular Radiation Sciences, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21231, USA. Tryggestad.Erik@mayo.edu
authors: Erik
authors: E
authors: Tryggestad
authors: Aaron
authors: A
authors: Flammang
authors: Sarah
authors: S
authors: Han-Oh
authors: Russell
authors: R
authors: Hales
authors: Joseph
authors: J
authors: Herman
authors: Todd
authors: T
authors: McNutt
authors: Teboh
authors: T
authors: Roland
authors: Steven M
authors: SM
authors: Shea
authors: John
authors: J
authors: Wong
mesh_terms: D003964:Diaphragm / Q000000981:diagnostic imaging; D056973:Four-Dimensional Computed Tomography; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy; D008279:Magnetic Resonance Imaging; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012119:Respiration; D013997:Time Factors