Combining phase images from multi‐channel RF coils using 3D phase offset maps derived from a dual‐echo scan

Abstract

A method is presented for the combination of phase images from multi‐channel RF coils in the absence of a volume reference coil. It is based on the subtraction of 3D phase offset maps from the phase data from each coil. Phase offset maps are weighted combinations of phase measurements at two echo times. Multi‐Channel Phase Combination using measured 3D phase offsets (MCPC‐3D) offers a conceptually and computationally simple solution to the calculation of combined phase images. The dual‐echo data required for the phase maps can be intrinsic to the high‐resorlution gradient‐echo scan to be reconstructed (MCPC‐3D‐I). Alternatively, a separate, fast, low‐resolution dual‐echo scan can be used (MCPC‐3D‐II). Both variants are shown to give near perfect phase matching, yielding images with high SNR throughout and high GM‐WM contrast. MCPC‐3D is compared with other reference‐free phase image crombination methods; high‐pass phase filtering, phase difference imaging, and matching using constant offsets (MCPC‐C). Multi‐Channel Phase Combination using measured 3D phase offsets method does not need an overlap between the signals from individual coils and can be used with parallel imaging, making it ideally suited to multi‐channel coils with a large number of elements, and to high and ultra‐high field systems. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.

Iron accumulation in the basal ganglia in Huntington's disease: cross-sectional data from the IMAGE-HD study

“…In-depth analysis of the different phase processing methods and their effects on QSM results has not been undertaken and is outside the scope of this investigation. However, the susceptibility values reported for controls were within the range of variation previously reported in other QSM studies,16 17 35 38 reflected the same rank ordering by brain structure (from highest to lowest: pallidum, putamen, caudate and thalamus) and obtained good to excellent linear approximation not only with measures from other QSM16 17 35 40 but also biochemical studies 20 34. This was also the case for pre-HD and symp-HD groups, with reference to iron content measured with MRI techniques other than QSM 3 7 9.…”

“…Ideally, a dual echo acquisition would produce reliable coil phase offset correction 38. QSM processing therefore followed a previously published method,31 39 adapted for retrospective analysis of single-echo GRE data.…”

Combining phase images from array coils using a short echo time reference scan (COMPOSER)

“…COMPOSER reconstructions were compared with the reconstructions from the following methods: 1) the complex sum of coil signals with no phase correction applied; 2) the Hammond method (an image‐based phase‐matching approach based on the subtraction of a channel‐dependent constant 22); 3) the Adaptive Combined method implemented on the scanner console (Syngo baseline version N4_VB17A_LATEST_20090307); 4) MCPC‐3D‐II 26, which calculates the phase offsets from a dual‐echo reference scan (using PRELUDE in 2D 38 to spatially unwrap separate channel data and interslice phase‐jump corrected as described in 26); 5) the Roemer method 9 (one subject only); and 6) the phase difference method (ie, the voxel‐by‐voxel Hermitian inner product (Eq. (3) in 39; see also 40; applied to the multi‐echo data from one subject).…”

“…The phase offset consists of inhomogeneous B 1 + and B 1 − phase 26, 28, a linear gradient in the readout direction introduced by timing errors in the acquisition 27 and a constant term reflecting eg, the length of the receiver chain. In contrast to the phase arising from local field effects, the phase offset is constant over time.…”

“…When the assumption of linearity holds, a multi‐echo acquisition allows ${\theta }_{0,l}$ to be calculated from two or more measurements of ${\theta }_l$, although these need to be unwrapped 26.…”