title: Noise robust spatially regularized myelin water fraction mapping with the intrinsic B1-error correction based on the linearized version of the extended phase graph model.
abstract: OBJECTIVE
  To improve the quantification accuracy of transverse relaxometry by accounting for B1 -error, after minimizing slice profile imperfections.
  
  
  METHODS
  The slice profile of refocusing pulses was optimized by setting refocusing slice thicknesses three times that of the excitation pulse. The first step of data processing combined the L-curve approach with the linearized version of the extended phase graph model to jointly estimate the temporal regularization constant map and the flip angle error (FAE)-map. The second step improved the noise robustness of the reconstruction by imposing a spatial smoothness constraint on T2 -distributions. The proposed method (spatial-regularization-with-FAE-correction) was evaluated against methods without FAE-correction (conventional-regularization-without-FAE-correction, spatial-regularization-without-FAE-correction) and conventional-regularization-with-FAE-correction using relevant statistics (simulated data: mean square myelin reconstruction error [MSMRE] and averaged-symmetric-Kullbeck-Leibler score [SKL] between returned distributions and ground truths; experimental data: median of mean square error [MMSE] of fitting across entire data-set and coefficient of variation [COV] in white-matter [WM] regions of interest [ROIs]).
  
  
  RESULTS
  In simulation, our method resulted in reduced MSMRE (at signal-to-noise ratio [SNR] = 200: MSMRESpatial-regularization-without-FAEC  = 0.057; MSMRESpatial-regularization-with-FAEC  = 0.0107) and reduced SKL scores (at SNR = 200: SKLSpatial-regularization-without-FAEC  = 0.061; SKLSpatial-regularization-with-FAEC  = 0.0143). In human volunteers, our method yielded a reduced MSE of fitting (MMSESpatial-regularization-without-FAEC  = (2.26 ± 0.60) × 10(-3) ; MMSESpatial-regularization-with-FAEC  = (1.57 ± 0.44) × 10(-4) )and also resulted in reduced COV (COVSpatial-regularization-without-FAEC  = 0.08-0.19; COVSpatial-regularization-with-FAEC  = 0.09-0.12). In a water-phantom, a good correlation between the absolute value of measured B1 -map and FAE-map was found (regression analysis: slope = 1.04; R(2)  = 0.66).
  
  
  CONCLUSIONS
  The proposed method resulted in more accurate and noise robust myelin water fraction maps with improved depiction of subcortical WM structures.
authors.affiliation: Department of Diagnostic and Interventional Neuroradiology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.|Institute of Neuroimmunology and Multiple Sclerosis, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.
authors.forename: Dushyant
authors.initials: D
authors.lastname: Kumar
authors.affiliation: Department of Diagnostic and Interventional Neuroradiology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.|Institute of Neuroimmunology and Multiple Sclerosis, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.
authors.forename: Susanne
authors.initials: S
authors.lastname: Siemonsen
authors.affiliation: Institute of Neuroimmunology and Multiple Sclerosis, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.|Department of Neurology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.
authors.forename: Christoph
authors.initials: C
authors.lastname: Heesen
authors.affiliation: Department of Diagnostic and Interventional Neuroradiology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.
authors.forename: Jens
authors.initials: J
authors.lastname: Fiehler
authors.affiliation: Department of Diagnostic and Interventional Neuroradiology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.
authors.forename: Jan
authors.initials: J
authors.lastname: Sedlacik
mesh_terms: D000328:Adult; D000465:Algorithms; D016477:Artifacts; D003198:Computer Simulation; D005260:Female; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D015233:Models, Statistical; D009186:Myelin Sheath / Q000737:chemistry*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D014867:Water / Q000737:chemistry*; D055815:Young Adult
chemical_list: D014867:Water
keywords: EPG; T2 relaxometry; demyelination; extended phase graph; multiexponential; myelin water fraction; spatial priors