title: Effects of RF inhomogeneity at 3.0T on ramped RF excitation: application to 3D time-of-flight MR angiography of the intracranial arteries.
abstract: OBJECTIVE
  To demonstrate the effects of inherent RF inhomogeneity on ramped RF excitation at 3.0T, and to introduce a simple correction for improving visualization of distal intracranial arteries in three-dimensional time-of-flight MR angiography (3D-TOF-MRA).
  
  
  METHODS
  At 3.0T, the effects of RF inhomogeneity arising from RF interference were demonstrated for ramped RF excitation in intracranial 3D-TOF-MRA. Computer simulations and experiments on phantoms and eight normal volunteers were performed. Four different ramp shapes were tested as a possible means of countering the reduced RF field that affects the distal intracranial arteries.
  
  
  RESULTS
  RF destructive interference alters the ramp pulse shape, which is problematic for vessels that proceed from the center to the edge of the brain. Increasing the ramp pulse slope was shown to be an effective yet simple correction to counter the falling-off of the RF field toward the periphery of the head. With this approach, circle-of-Willis 3D-TOF-MRA studies had improved distal visibility.
  
  
  CONCLUSIONS
  Ramped RF excitation is severely affected by RF interference at 3.0T, which makes the ramp profile suboptimal for distal intracranial blood vessels. A simple correction of the ramp slope can make a marked improvement.
authors.affiliation: Department of Biomedical Engineering, University of Alberta, Edmonton, Alberta, Canada.
authors.forename: Amir M
authors.initials: AM
authors.lastname: Eissa
authors.forename: Alan H
authors.initials: AH
authors.lastname: Wilman
mesh_terms: D000328:Adult; D002536:Cerebral Arteries / Q000033:anatomy & histology*; D002941:Circle of Willis / Q000033:anatomy & histology; D003198:Computer Simulation; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D021621:Imaging, Three-Dimensional / Q000379:methods*; D018810:Magnetic Resonance Angiography / Q000295:instrumentation / Q000379:methods*; D008280:Magnetics; D008297:Male; D019047:Phantoms, Imaging; D012016:Reference Values; D012815:Signal Processing, Computer-Assisted