Aliasing artifact reduction in spiral real‐time MRI

Abstract

Purpose: To mitigate a common artifact in spiral real-time MRI, caused by aliasing of signal outside the desired FOV. This artifact frequently occurs in midsagittal speech real-time MRI. Methods: Simulations were performed to determine the likely origin of the artifact.Two methods to mitigate the artifact are proposed. The first approach, denoted as "large FOV" (LF), keeps an FOV that is large enough to include the artifact signal source during reconstruction. The second approach, denoted as "estimationsubtraction" (ES), estimates the artifact signal source before subtracting a synthetic signal representing that source in multicoil k-space raw data. Twenty-five midsagittal speech-production real-time MRI data sets were used to evaluate both of the proposed methods. Reconstructions without and with corrections were evaluated by two expert readers using a 5-level Likert scale assessing artifact severity. Reconstruction time was also compared. Results: The origin of the artifact was found to be a combination of gradient nonlinearity and imperfect anti-aliasing in spiral sampling. The LF and ES methods were both able to substantially reduce the artifact, with an averaged qualitative score improvement of 1.25 and 1.35 Likert levels for LF correction and ES correction, respectively. Average reconstruction time without correction, with LF correction, and with ES correction were 160.69 ± 1.56, 526.43 ± 5.17, and 171.47 ± 1.71 ms/frame. Conclusion: Both proposed methods were able to reduce the spiral aliasing artifacts, with the ES-reduction method being more effective and more time efficient. K E Y W O R D Sgradient nonlinearity, low-pass filter, real-time MRI, speech production, spiral trajectory | INTRODUCTIONReal-time MRI is an established tool to study the dynamics of vocal tract shaping during human speech production. Compared with other imaging modalities, such as CT, 1 electromagnetic articulography 2 and ultrasound, 3 MRI 4-9 involves no ionizing radiation, can provide arbitrary imaging planes, and provides excellent tissue-boundary delineation. Real-time MRI (RT-MRI) for speech production is usually designed to achieve high enough temporal resolution

Real‐time water/fat imaging at 0.55T with spiral out‐in‐out‐in sampling

“…During RT‐MRI acquisition, each spiral OIOI trajectory is rotated with golden angle (111.25°) after each TR, which benefits constrained reconstruction 25 and allows for flexible selection of the reconstruction FOV and temporal resolution 26,27 . Reconstruction FOV was selected to include all signal area to avoid spiral aliasing artifact 28 . We report both the FOV during the single spiral‐out trajectory design and the FOV used for image reconstruction in Table S1.…”

“…26,27 Reconstruction FOV was selected to include all signal area to avoid spiral aliasing artifact. 28 We report both the FOV during the single spiral-out trajectory design and the FOV used for image reconstruction in Table S1.…”

“…Sensitivity maps were estimated from temporally averaged first spiral-out data using the Walsh method. 32 The FOV during iterative reconstruction was kept large enough to include all areas that have signal to avoid aliasing, 28 as reported in Table S1. Temporal resolution and regularization parameters were selected through a parameter sweep.…”

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