title: Nonrigid PET motion compensation in the lower abdomen using simultaneous tagged-MRI and PET imaging.
abstract: OBJECTIVE
  We propose a novel approach for PET respiratory motion correction using tagged-MRI and simultaneous PET-MRI acquisitions.
  
  
  METHODS
  We use a tagged-MRI acquisition followed by motion tracking in the phase domain to estimate the nonrigid deformation of biological tissues during breathing. In order to accurately estimate motion even in the presence of noise and susceptibility artifacts, we regularize the traditional HARP tracking strategy using a quadratic roughness penalty on neighboring displacement vectors (R-HARP). We then incorporate the motion fields estimated with R-HARP in the system matrix of an MLEM PET reconstruction algorithm formulated both for sinogram and list-mode data representations. This approach allows reconstruction of all detected coincidences in a single image while modeling the effect of motion both in the emission and the attenuation maps. At present, tagged-MRI does not allow estimation of motion in the lungs and our approach is therefore limited to motion correction in soft tissues. Since it is difficult to assess the accuracy of motion correction approaches in vivo, we evaluated the proposed approach in numerical simulations of simultaneous PET-MRI acquisitions using the NCAT phantom. We also assessed its practical feasibility in PET-MRI acquisitions of a small deformable phantom that mimics the complex deformation pattern of a lung that we imaged on a combined PET-MRI brain scanner.
  
  
  RESULTS
  Simulations showed that the R-HARP tracking strategy accurately estimated realistic respiratory motion fields for different levels of noise in the tagged-MRI simulation. In simulations of tumors exhibiting increased uptake, contrast estimation was 20% more accurate with motion correction than without. Signal-to-noise ratio (SNR) was more than 100% greater when performing motion-corrected reconstruction which included all counts, compared to when reconstructing only coincidences detected in the first of eight gated frames. These results were confirmed in our proof-of-principle PET-MRI acquisitions, indicating that our motion correction strategy is accurate, practically feasible, and is therefore ready to be tested in vivo.
  
  
  CONCLUSIONS
  This work shows that PET motion correction using motion fields measured with tagged-MRI in simultaneous PET-MRI acquisitions can be made practical for clinical application and that doing so has the potential to remove motion blur in whole-body PET studies of the torso.
authors.affiliation: Division of Nuclear Medicine and Molecular Imaging, Department of Radiology, Massachusetts General Hospital, 55 Fruit Street, Boston, Massachusetts 02114, USA.
authors.forename: B
authors.initials: B
authors.lastname: Guérin
authors.forename: S
authors.initials: S
authors.lastname: Cho
authors.forename: S Y
authors.initials: SY
authors.lastname: Chun
authors.forename: X
authors.initials: X
authors.lastname: Zhu
authors.forename: N M
authors.initials: NM
authors.lastname: Alpert
authors.forename: G
authors.initials: G
authors.lastname: El Fakhri
authors.forename: T
authors.initials: T
authors.lastname: Reese
authors.forename: C
authors.initials: C
authors.lastname: Catana
mesh_terms: D000005:Abdomen / Q000000981:diagnostic imaging*; D000465:Algorithms; D001921:Brain / Q000000981:diagnostic imaging; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008168:Lung / Q000502:physiology; D008279:Magnetic Resonance Imaging / Q000379:methods*; D009068:Movement; D019047:Phantoms, Imaging; D049268:Positron-Emission Tomography / Q000379:methods*; D012119:Respiration; D012542:Scattering, Radiation; D013997:Time Factors