R mapping in the presence of macroscopic B₀ field variations

Abstract

R*2 mapping has important applications in MRI, including functional imaging, tracking of super-paramagnetic particles, and measurement of tissue iron levels. However, R*2 measurements can be confounded by several effects, particularly the presence of fat and macroscopic B0 field variations. Fat introduces additional modulations in the signal. Macroscopic field variations introduce additional dephasing that results in accelerated signal decay. These effects produce systematic errors in the resulting R*2 maps and make the estimated R*2 values dependent on the acquisition parameters. In this study, we develop a complex-reconstruction, confounder-corrected R*2 mapping technique, which addresses the presence of fat and macroscopic field variations for both 2D and 3D acquisitions. This technique extends previous chemical shift-encoded methods for R*2, fat and water mapping by measuring and correcting for the effect of macroscopic field variations in the acquired signal. The proposed method is tested on several 2D and 3D phantom and in vivo liver, cardiac, and brain datasets.

Single-scan R2⁎ measurement with macroscopic field inhomogeneity correction

“…However, given the long echo needed for R 2 ⁎ quantification, it too will require some form of shimming. Previous studies have presented such a z-shimming method for 3D imaging (Glover, 1999;Hernando et al, 2012). A preliminary extension to 3D z-shimming has also been presented by Han et al (2012).…”

Improved estimation of myelin water fraction using complex model fitting

“…Multiple methods have been proposed to compensate for the macroscopic B 0 field inhomogeneity in mGRE (Han et al, 2015;Hernando et al, 2012;Nam et al, 2012;Oh et al, 2014;Yablonskiy et al, 2013). However, many of them require a longer minimum TE and may lose a significant portion of fast decaying myelin water signal.…”

Comparison of brown and white adipose tissues in infants and children with chemical‐shift‐encoded water‐fat MRI

“…First, the estimation of T 2 * implemented in this work did not explicitly account for the contributions of microscopic and macroscopic B0 field map variations. Both effects can lead to additional signal dephasing and cause a shorter apparent estimated T 2 * value (38,39). However, we believe that macroscopic susceptibility effects were minimized through the use of small voxels, a high readout bandwidth, and thin slices during data acquisition.…”