Estimating the apparent transverse relaxation time (R2(*)) from images with different contrasts (ESTATICS) reduces motion artifacts.

Nikolaus Weiskopf,Martina F Callaghan,Siawoosh Mohammadi,Oliver Josephs,Antoine Lutti

doi:10.3389/fnins.2014.00278

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.

Highlights

Quantitative magnetic resonance imaging provides standardized information about the tissue microstructure that can be compared across time points and imaging sites (e.g., Tofts, 2003; Schmierer et al, 2004; Dick et al, 2012; Weiskopf et al, 2013)
We propose a novel method for correcting motion artifacts and increasing signal-to-noise ratio (SNR) in the R2* maps estimated from 3D multi-parameter mapping (MPM) modulus image data or similar protocols with multiple acquisitions: ESTimating the Apparent Transverse relaxation time (R2∗) from Images with different ContrastS (ESTATICS)
If several contrasts were affected by motion, ESTATICS did not achieve a complete correction of artifacts as can be seen in a case where the MTw and PDw images were affected and some artifacts in the frontal cortex remained (Figure 2)

Summary

Introduction

Quantitative magnetic resonance imaging (qMRI) provides standardized information about the tissue microstructure that can be compared across time points and imaging sites (e.g., Tofts, 2003; Schmierer et al, 2004; Dick et al, 2012; Weiskopf et al, 2013). Quantitative multi-parameter mapping (MPM) is a fast method for mapping the longitudinal relaxation rate R1, effective proton density PD∗ (Lin et al, 1997; Weiskopf et al, 2013), magnetization transfer saturation (MT) and apparent transverse relaxation rate R2∗ across the entire brain. These parameters are estimated from three differently weighted datasets acquired with a 3D fast low angle shot (FLASH) sequence using established physical models (Helms et al, 2008a,b; Weiskopf et al, 2013). The other quantitative maps (R1, MT, PD∗) generated from the MPM protocol are significantly less affected by motion, since they are estimated from averages across multiple echo times (including short echo times, Weiskopf et al, 2013)

Methods

Results

Conclusion