title: Intrinsic correction of system delays for radial magnetic resonance imaging.
abstract: BACKGROUND
  When using radial MR image acquisition techniques gradient or sampling delays due to hardware imperfections can cause mismatch between the expected and the actual k-space trajectory along the readout direction. To provide a robust and simple correction of such system delays we developed a new calibration method which is independent of using any reference data or applying sequence modifications.
  
  
  METHODS
  Radial data obtained with 180°, 360° and golden-angle radial ordering schemes were deliberately shifted along the readout direction for a discrete range of gradient delays. Following 2D regridding, images were reconstructed and analyzed in image space for all applied shifts to estimate the optimal system delay. Phantom and in vivo measurements were performed to test the robustness of the algorithm.
  
  
  RESULTS
  Using the 360° and golden-angle radial ordering schemes system delays in the range of 3.3μs to 6.3μs were estimated and corrected for several imaging applications and different conditions, including cardiac and real-time MRI as well as multiple acquisitions using different imaging parameters and slice orientations. When using the standard 180° radial acquisition scheme no automated correction was possible. With a mean computation time of 23.2±14.0s for the delay estimation computational demands were moderate allowing implementation of the algorithm on the image reconstruction system of any modern MR system.
  
  
  CONCLUSIONS
  We have demonstrated that radial data acquired with a 360° or golden-angle ordering scheme can be used for reliable intrinsic correction of system delays. The proposed technique enables a per-scan correction of system delays without the need for additional calibration data or modifications of the radial imaging sequence.
authors.affiliation: Medical Physics Group, Institute of Diagnostic and Interventional Radiology, Jena University Hospital-Friedrich Schiller University Jena. Electronic address: MartinKraemer84@gmail.com.
authors.forename: M
authors.initials: M
authors.lastname: Krämer
authors.affiliation: Medical Physics Group, Institute of Diagnostic and Interventional Radiology, Jena University Hospital-Friedrich Schiller University Jena.
authors.forename: J
authors.initials: J
authors.lastname: Biermann
authors.affiliation: Medical Physics Group, Institute of Diagnostic and Interventional Radiology, Jena University Hospital-Friedrich Schiller University Jena.
authors.forename: J R
authors.initials: JR
authors.lastname: Reichenbach
mesh_terms: D000465:Algorithms; D016477:Artifacts; D001921:Brain / Q000033:anatomy & histology*; D002138:Calibration; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
keywords: Gradient delays; MRI; Projection reconstruction; Radial calibration