Simultaneous Phase Unwrapping and Removal of Chemical Shift (SPURS) Using Graph Cuts: Application in Quantitative Susceptibility Mapping

Abstract

Quantitative susceptibility mapping (QSM) is a magnetic resonance imaging technique that reveals tissue magnetic susceptibility. It relies on having a high quality field map, typically acquired with a relatively long echo spacing and long final TE. Applications of QSM outside the brain require the removal of fat contributions to the total signal phase. However, current water/fat separation methods applied on typical data acquired for QSM suffer from three issues: inadequacy when using large echo spacing, over-smoothing of the field maps and high computational cost. In this paper, the general phase wrap and chemical shift problem is formulated using a single species fitting and is solved using graph cuts with conditional jump moves. This method is referred as simultaneous phase unwrapping and removal of chemical shift (SPURS). The result from SPURS is then used as the initial guess for a voxel-wise iterative decomposition of water and fat with echo asymmetric and least-squares estimation (IDEAL). The estimated 3-D field maps are used to compute QSM in body regions outside of the brain, such as the liver. Experimental results show substantial improvements in field map estimation, water/fat separation and reconstructed QSM compared to two existing water/fat separation methods on 1.5T and 3T magnetic resonance human data with long echo spacing and rapid field map variation.

Age and sex related differences in subcortical brain iron concentrations among healthy adults

“…Background field removal was incorporated in the estimation of susceptibility (Dong et al, 2015; Li et al, 2004; Liu et al, 2014; Schweser et al, 2010). QSM values were referenced to CSF to allow comparison across subjects.…”

“…Dong et al 2015; Liu et al, 2012, 2014). The mask M was computed by filtering the gradient of the magnitude image of the first echo, with a threshold chosen such that 30% of the computed gradients (taken over all three spatial dimensions) were retained The energy minimization in equation 1 has a data fidelity term formulated in the complex plane to account for Gaussian noise and an L1 norm regularization term to encode the prior.…”

Clinical quantitative susceptibility mapping (QSM): Biometal imaging and its emerging roles in patient care

“…This can be achieved using breath-hold or navigator gating, and additional ECG triggering for the heart. Fat chemical shifts have to be accounted for in estimating the susceptibility field from the input complex data (45,46). A body QSM acquisition protocol of 20 sec breathhold (typically used in liver MRI) can be implemented with the following parameters on a 3T: # of echoes: 6; TE: TE1 minimal, ΔTE =3msec; TR: 15msec; R=2; Flip angle: 15; Bandwidth: 300Hz/pixel; FOV: 30 cm; Slice thickness: 3mm (further halved with ZIP); Matrix: 256×176×26.…”

“…Fortunately, QSM has a simple linear relationship with these intravoxel contents according to chemical decomposition, allowing linear extraction of iron content. Furthermore, except fat (quantifiable according to its chemical shifts and phase data (45)), other intravoxel contents, including fibrosis and edema, have a susceptibility that is virtually zero relative to water, making QSM ideally suited for determining the iron concentration of liver, heart, and other tissues (45,187) (Fig. 11).…”

Rapid automated liver quantitative susceptibility mapping

“…The graph cuts exact solution of the simultaneous phase unwrapping and removal of chemical shift (SPURS) problem under the assumption of no water-fat overlap has been found to provide a good initialization for robust liver QSM (6). Nonetheless, the graph cuts procedure is computationally expensive and leads to a processing time on the order of an hour, which is not feasible for routine clinical practice.…”