title: Accelerating sequences in the presence of metal by exploiting the spatial distribution of off-resonance.
abstract: OBJECTIVE
  To demonstrate feasibility of exploiting the spatial distribution of off-resonance surrounding metallic implants for accelerating multispectral imaging techniques.
  
  
  METHODS
  Multispectral imaging (MSI) techniques perform time-consuming independent three-dimensional acquisitions with varying radio frequency offsets to address the extreme off-resonance from metallic implants. Each off-resonance bin provides a unique spatial sensitivity that is analogous to the sensitivity of a receiver coil and, therefore, provides a unique opportunity for acceleration. Fully sampled MSI was performed to demonstrate retrospective acceleration. A uniform sampling pattern across off-resonance bins was compared with several adaptive sampling strategies using a total hip replacement phantom. Monte Carlo simulations were performed to compare noise propagation of two of these strategies. With a total knee replacement phantom, positive and negative off-resonance bins were strategically sampled with respect to the B0 field to minimize aliasing. Reconstructions were performed with a parallel imaging framework to demonstrate retrospective acceleration.
  
  
  RESULTS
  An adaptive sampling scheme dramatically improved reconstruction quality, which was supported by the noise propagation analysis. Independent acceleration of negative and positive off-resonance bins demonstrated reduced overlapping of aliased signal to improve the reconstruction.
  
  
  CONCLUSIONS
  This work presents the feasibility of acceleration in the presence of metal by exploiting the spatial sensitivities of off-resonance bins.
authors.affiliation: Department of Radiology, University of Wisconsin, Madison, Wisconsin, USA.
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Smith
authors.forename: Nathan S
authors.initials: NS
authors.lastname: Artz
authors.forename: Kevin M
authors.initials: KM
authors.lastname: Koch
authors.forename: Alexey
authors.initials: A
authors.lastname: Samsonov
authors.forename: Scott B
authors.initials: SB
authors.lastname: Reeder
mesh_terms: D000465:Algorithms; D006621:Hip Joint / Q000033:anatomy & histology* / Q000601:surgery; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D007719:Knee Joint / Q000033:anatomy & histology* / Q000601:surgery; D007720:Knee Prosthesis; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008670:Metals; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted
chemical_list: D008670:Metals
keywords: acceleration; imaging near metal; multispectral imaging; off-resonance