title: Motion detection improvement of a pencil-beam navigator echo using a gradient reversal technique.
abstract: OBJECTIVE
  To develop and demonstrate the feasibility of a pencil-beam navigator using a gradient reversal technique for reducing signal contamination from undesired excitation for precise motion detection and correction.
  
  
  METHODS
  The navigator echo was obtained using normal and reversed gradient waveforms sequentially in three-dimensional spoiled gradient-recalled echo imaging. These two signals were combined in the complex domain for generating the final navigator data, which were used for detecting the diaphragm motion to nullify the side lobe effects with the smallest radius from the beam center. The navigator signals were compared with and without the proposed technique in phantom and human scans. In addition, navigator-gated imaging was performed in the human scans.
  
  
  RESULTS
  In a phantom experiment, the proposed technique diminished signals from a phantom placed outside the beam's area. In human scans, the proposed technique reduced undesired signals in the navigator data for all subjects. The resultant images had fewer motion-induced ghosts than the images from the conventional technique for 8 subjects out of 10.
  
  
  CONCLUSIONS
  We have demonstrated that the gradient reversal technique reduced undesired signals in a pencil-beam navigator. This technique can be an alternative for free-breathing abdominal scan when the conventional navigator technique cannot detect the patient's respiratory motion precisely.
authors.affiliation: Global MR Applications and Workflow, GE Healthcare Japan, 4-7-127 Asahigaoka, Hino, Tokyo, 193-0825, Japan. Electronic address: yuji.iwadate@ge.com.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Iwadate
authors.affiliation: MR Engineering, GE Healthcare Japan, 4-7-127 Asahigaoka, Hino, Tokyo, 193-0825, Japan.
authors.forename: Kunihiro
authors.initials: K
authors.lastname: Miyoshi
authors.affiliation: Global MR Applications and Workflow, GE Healthcare Japan, 4-7-127 Asahigaoka, Hino, Tokyo, 193-0825, Japan.
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Kabasawa
mesh_terms: D003964:Diaphragm / Q000033:anatomy & histology; D005240:Feasibility Studies; D006801:Humans; D021621:Imaging, Three-Dimensional; D008279:Magnetic Resonance Imaging / Q000379:methods*; D009038:Motion; D019047:Phantoms, Imaging
keywords: Gradient reversal; Magnetic resonance imaging; Motion correction; Navigator; Pencil beam excitation