title: Estimating the apparent transverse relaxation time (R2(*)) from images with different contrasts (ESTATICS) reduces motion artifacts.
abstract: Relaxation rates provide important information about tissue microstructure. Multi-parameter mapping (MPM) estimates multiple relaxation parameters from multi-echo FLASH acquisitions with different basic contrasts, i.e., proton density (PD), T1 or magnetization transfer (MT) weighting. Motion can particularly affect maps of the apparent transverse relaxation rate R2(*), which are derived from the signal of PD-weighted images acquired at different echo times. To address the motion artifacts, we introduce ESTATICS, which robustly estimates R2(*) from images even when acquired with different basic contrasts. ESTATICS extends the fitted signal model to account for inherent contrast differences in the PDw, T1w and MTw images. The fit was implemented as a conventional ordinary least squares optimization and as a robust fit with a small or large confidence interval. These three different implementations of ESTATICS were tested on data affected by severe motion artifacts and data with no prominent motion artifacts as determined by visual assessment or fast optical motion tracking. ESTATICS improved the quality of the R2(*) maps and reduced the coefficient of variation for both types of data-with average reductions of 30% when severe motion artifacts were present. ESTATICS can be applied to any protocol comprised of multiple 2D/3D multi-echo FLASH acquisitions as used in the general research and clinical setting.
authors.affiliation: Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, University College London London, UK.
authors.forename: Nikolaus
authors.initials: N
authors.lastname: Weiskopf
authors.affiliation: Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, University College London London, UK.
authors.forename: Martina F
authors.initials: MF
authors.lastname: Callaghan
authors.affiliation: Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, University College London London, UK ; Birkbeck-UCL Centre for NeuroImaging London, UK.
authors.forename: Oliver
authors.initials: O
authors.lastname: Josephs
authors.affiliation: Laboratoire de Recherche en Neuroimagerie, Department of Clinical Neurosciences, Centre Hospitalier Universitaire Vaudois Lausanne, Switzerland.
authors.forename: Antoine
authors.initials: A
authors.lastname: Lutti
authors.affiliation: Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, University College London London, UK.
authors.forename: Siawoosh
authors.initials: S
authors.lastname: Mohammadi
keywords: T2*; apparent transverse relaxation; artifacts; motion; multiple contrasts; quantitative imaging; robust fit; transverse relaxation