Abstract
Quantitative Susceptibility Mapping (QSM) is an MRI tool with the potential to reveal pathological changes from magnetic susceptibility measurements. Before phase data can be used to recover susceptibility (Δχ), the QSM process begins with two steps: data acquisition and phase estimation. We assess the performance of these steps, when applied without user intervention, on several variations of a phantom imaging task. We used a rotating-tube phantom with five tubes ranging from Δχ=0.05 ppm to Δχ=0.336 ppm. MRI data was acquired at nine angles of rotation for four different pulse sequences. The images were processed by 10 phase estimation algorithms including Laplacian, region-growing, branch-cut, temporal unwrapping, and maximum-likelihood methods, resulting in approximately 90 different combinations of data acquisition and phase estimation methods. We analyzed errors between measured and expected phases using the probability mass function and Cumulative Distribution Function. Repeatable acquisition and estimation methods were identified based on the probability of relative phase errors. For single-echo GRE and segmented EPI sequences, a region-growing method was most reliable with Pr (relative error <0.1) = 0.95 and 0.90, respectively. For multiecho sequences, a maximum-likelihood method was most reliable with Pr (relative error <0.1) = 0.97. The most repeatable multiecho methods outperformed the most repeatable single-echo methods. We found a wide range of repeatability and reproducibility for off-the-shelf MRI acquisition and phase estimation approaches, and this variability may prevent the techniques from being widely integrated in clinical workflows. The error was dominated in many cases by spatially discontinuous phase unwrapping errors. Any postprocessing applied on erroneous phase estimates, such as QSM's background field removal and dipole inversion, would suffer from error propagation. Our paradigm identifies methods that yield consistent and accurate phase estimates that would ultimately yield consistent and accurate Δχ estimates.
Highlights
Quantitative Susceptibility Mapping (QSM)[1,2,3] is a method to estimate magnetic susceptibility of tissue from the phase of the magnetic resonance (MR) signal
A summary of the Cumulative Distribution Function (CDF) is in the second-to-last column of Table 3 where we show the probability of observing relative errors
Fεr(10%) of SEGE + MEDI.RG was comparable to MEGE + MAGPI-unopt and MAGPI, despite the comparatively poorer μεr and σεr of SEGE + MEDI.RG. is is due to relative errors falling mostly within the chosen 10% threshold for these methods
Summary
Quantitative Susceptibility Mapping (QSM)[1,2,3] is a method to estimate magnetic susceptibility of tissue from the phase of the magnetic resonance (MR) signal. Repeatability and reproducibility of QSM have been assessed in phantoms and human subjects using different scanners, magnetic field strengths, and data processing methods. While some studies report high repeatability [11,12,13,14,15,16,17,18], both in vivo and in phantoms, recent in vivo studies report lower reproducibility across MRI scanners with the same data processing method [19] and across QSM algorithms using the same input data [20]. A typical QSM process requires four steps: data acquisition (Step 1), phase estimation (Step 2), background field removal (Step 3), and magnetic susceptibility reconstruction (Step 4). Proposed methods combine Steps 2–4 into fewer steps [21]
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