title: Central brightening due to constructive interference with, without, and despite dielectric resonance.
abstract: OBJECTIVE
  To aid in discussion about the mechanism for central brightening in high field magnetic resonance imaging (MRI), especially regarding the appropriateness of using the term dielectric resonance to describe the central brightening seen in images of the human head.
  
  
  METHODS
  We present both numerical calculations and experimental images at 3 T of a 35-cm-diameter spherical phantom of varying salinity both with one surface coil and with two surface coils on opposite sides, and further numerical calculations at frequencies corresponding to dielectric resonances for the sphere.
  
  
  RESULTS
  With two strategically placed surface coils it is possible to create central brightening even when one coil alone excites an image intensity pattern either bright on one side only or bright on both sides with central darkening. This central brightening can be created with strategic coil placement even when the resonant pattern would favor central darkening. Results in a conductive sample show that central brightening can similarly be achieved in weakly conductive dielectric materials where any true resonances would be heavily damped, such as in human tissues.
  
  
  CONCLUSIONS
  Constructive interference and wavelength effects are likely bigger contributors to central brightening in MR images of weakly conductive biological samples than is true dielectric resonance.
authors.affiliation: Center for NMR Research, Department of Radiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA. cmcollins@psu.edu
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Collins
authors.forename: Wanzhan
authors.initials: W
authors.lastname: Liu
authors.forename: Weston
authors.initials: W
authors.lastname: Schreiber
authors.forename: Qing X
authors.initials: QX
authors.lastname: Yang
authors.forename: Michael B
authors.initials: MB
authors.lastname: Smith
mesh_terms: D004574:Electromagnetic Fields; D004578:Electron Spin Resonance Spectroscopy; D006257:Head / Q000033:anatomy & histology; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D008954:Models, Biological; D019047:Phantoms, Imaging; D012965:Sodium Chloride
chemical_list: D012965:Sodium Chloride