title: Manifold learning based ECG-free free-breathing cardiac CINE MRI.
abstract: OBJECTIVE
  To present and validate a manifold learning (ML)-based method that can estimate both cardiac and respiratory navigator signals from electrocardiogram (ECG)-free free-breathing cardiac magnetic resonance imaging (MRI) data to achieve self-gated retrospective CINE reconstruction.
  
  
  METHODS
  In this work the use of the ML method is demonstrated for 2D cardiac CINE to achieve both cardiac and respiratory self-gating without the need of an external navigator or ECG signal. This is achieved by sequentially applying ML to two sets of retrospectively reconstructed real-time images with differing temporal resolutions. A 1D cardiac signal is estimated by applying ML to high temporal resolution real-time images reconstructed from the acquired data. Using the estimated cardiac signal, a 1D respiratory signal was obtained by applying the ML method to low temporal resolution images reconstructed from the same acquired data for each cardiac cycle. Data were acquired in five volunteers with a 2D golden angle radial trajectory in a balanced steady-state free precession (b-SSFP) acquisition. The accuracy of the estimated cardiac signal was calculated as the standard deviation of the temporal difference between the estimated signal and the recorded ECG. The correlation between the estimated respiratory signal and standard pencil beam navigator signal was evaluated. Gated CINE reconstructions (20 cardiac phases per cycle, temporal resolution ∼30 msec) using the estimated cardiac and respiratory signals were qualitatively compared against conventional ECG-gated breath-hold CINE acquisitions.
  
  
  RESULTS
  Accurate cardiac signals were estimated with the proposed method, with an error standard deviation in comparison to ECG lower than 20 msec. Respiratory signals estimated with the proposed method achieved a mean cross-correlation of 94% with respect to standard pencil beam navigator signals. Good quality visual scores of 2.80 ± 0.45 (scores from 0, bad, to 4, excellent quality) were observed for the proposed approach in comparison with the conventional ECG-gated breath-hold images (visual score: 3.00 ± 0.71).
  
  
  CONCLUSIONS
  Accurate respiratory and cardiac navigator signals can be estimated using the proposed framework from the acquired data itself, resulting in retrospective self-gated CINE reconstruction with high spatial and temporal quality.
authors.affiliation: King's College London, Division of Imaging Sciences and Biomedical Engineering, British Heart Foundation (BHF) Centre of Excellence, Medical Engineering Centre of Research Excellence, London, UK.
authors.forename: Muhammad
authors.initials: M
authors.lastname: Usman
authors.affiliation: University College London, Centre for Medical Imaging, London, UK.
authors.forename: David
authors.initials: D
authors.lastname: Atkinson
authors.affiliation: King's College London, Division of Imaging Sciences and Biomedical Engineering, British Heart Foundation (BHF) Centre of Excellence, Medical Engineering Centre of Research Excellence, London, UK.
authors.forename: Christoph
authors.initials: C
authors.lastname: Kolbitsch
authors.affiliation: King's College London, Division of Imaging Sciences and Biomedical Engineering, British Heart Foundation (BHF) Centre of Excellence, Medical Engineering Centre of Research Excellence, London, UK.
authors.forename: Tobias
authors.initials: T
authors.lastname: Schaeffter
authors.affiliation: King's College London, Division of Imaging Sciences and Biomedical Engineering, British Heart Foundation (BHF) Centre of Excellence, Medical Engineering Centre of Research Excellence, London, UK.|Pontificia Universidad Católica de Chile, Escuela de Ingeniería, Santiago, Chile.
authors.forename: Claudia
authors.initials: C
authors.lastname: Prieto
mesh_terms: D000328:Adult; D055424:Cardiac-Gated Imaging Techniques / Q000379:methods*; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D019028:Magnetic Resonance Imaging, Cine / Q000379:methods*; D008297:Male; D055425:Respiratory-Gated Imaging Techniques / Q000379:methods*
keywords: cardiac MRI; cardiac self-gating; manifold learning; self-navigation; undersampling