Joint estimation of chemical shift and quantitative susceptibility mapping (chemical QSM)

Abstract

Purpose The purpose of this work is to address the unsolved problem of quantitative susceptibility mapping (QSM) of tissue with fat where both fat and susceptibility change the MR signal phase. Theory and Methods The chemical shift of fat was treated as an additional unknown and was estimated jointly with susceptibility to provide the best data fitting using an automated and iterative algorithm. A simplified susceptibility model was used to calculate an updated value of the chemical shift based on the local magnetic field in each iteration. Numerical simulation, phantom experiments and in vivo imaging were performed. Artifacts were assessed by measuring the susceptibility variance in uniform regions. Accuracy was assessed by comparison with ground truth in simulation, and using a susceptibility matching approach in phantom. Results Using the proposed method, artifacts on the QSM image were markedly suppressed in all tested datasets compared with results generated using fixed chemical shifts. Accuracy of the estimated susceptibility was also improved in numerical simulation and phantom experiments. Conclusion A joint estimation of fat content and magnetic susceptibility using an iterative chemical shift update was shown to improve image quality and accuracy on QSM images. Magn Reson Med 73:2100–2110, 2015. © 2014 Wiley Periodicals, Inc.

Preconditioned total field inversion (TFI) method for quantitative susceptibility mapping

“…Example ROIs were delineated for sinus air, skull, and fat, as in Figure , and the mean susceptibility was calculated to be 7.38 ppm for the sphenoidal sinus, ‐1.36 ppm for the skull, and 0.64 ppm for fat. These susceptibility values are consistent with prior literature . When comparing Differential QSM and whole‐head TFI, a similar susceptibility map was observed within the brain, while Differential QSM did not depict the susceptibilities of the skull and sinus air.…”

Phase-corrected bipolar gradients in multi-echo gradient-echo sequences for quantitative susceptibility mapping

“…Phase corrections with higher-order errors may further improve the accuracy of the field map estimation, which has been demonstrated to be important to fat quantification using multiecho sequences with bipolar gradients [36,38,47]. The application of the presented technique may be extended from brain tissue to include venous blood by including flow compensation gradients and additional phase modeling [19],, and may be further extended to imaging organs outside the brain by including the fat component in the signal model[48,49]. …”

The effects of concomitant gradients on chemical shift encoded MRI

“…Then, representative CG phase shifts were calculated using Equation [] for several common CSE‐MRI acquisitions (as shown in Fig. a): a single echo train with flyback readout; a single echo train with bipolar readout; and two interleaved echo trains with flyback readout. A single echo train with flyback readout is commonly used for water‐fat separation at 1.5T, whereas bipolar acquisitions are being investigated for many CSE‐MRI applications at both 1.5T and 3T, and two interleaved trains are commonly used to obtain the shorter echo spacing required for water‐fat separation at 3T.…”

“…In recent years, there has been tremendous progress in the technical development and validation of chemical shift encoded (CSE) magnetic resonance imaging (MRI) techniques. These techniques include: mapping B 0 field inhomogeneity (estimating the B 0 field map) as required for quantitative susceptibility mapping (QSM) and many other applications; water‐fat separation for mapping of proton density fat fraction (PDFF) as a quantitative biomarker of tissue triglyceride concentration ; and mapping as a quantitative biomarker of tissue iron concentration .…”

“…Additionally, complex‐based techniques enable the measurement of B 0 field inhomogeneities. B 0 field inhomogeneity mapping has many important applications, including shimming , off resonance correction techniques , and emerging QSM techniques . For all of these reasons, complex‐based CSE‐MRI techniques have important advantages over magnitude‐based techniques.…”