A novel phase‐unwrapping method based on pixel clustering and local surface fitting with application to Dixon water–fat MRI

Abstract

Purpose: To develop and evaluate a novel 2D phaseunwrapping method that works robustly in the presence of severe noise, rapid phase changes, and disconnected regions. Theory and Methods: The MR phase map usually varies rapidly in regions adjacent to wraps. In contrast, the phasors can vary slowly, especially in regions distant from tissue boundaries. Based on this observation, this paper develops a phaseunwrapping method by using a pixel clustering and local surface fitting (CLOSE) approach to exploit different local variation characteristics between the phase and phasor data. The CLOSE approach classifies pixels into easy-to-unwrap blocks and difficult-to-unwrap residual pixels first, and then sequentially performs intrablock, interblock, and residual-pixel phase unwrapping by a region-growing surface-fitting method. The CLOSE method was evaluated on simulation and in vivo waterfat Dixon data, and was compared with phase region expanding labeler for unwrapping discrete estimates (PRELUDE). Results: In the simulation experiment, the mean error ratio by CLOSE was less than 1.50%, even in areas with signal-to-noise ratio equal to 0.5, phase changes larger than p, and disconnected regions. For 350 in vivo knee and ankle images, the water-fat swap ratio of CLOSE was 4.29%, whereas that of PRELUDE was 25.71%. Conclusions: The CLOSE approach can correctly unwrap phase with high robustness, and benefit MRI applications that require phase unwrapping. Magn Reson Med 79:515-528,

Seed prioritized unwrapping (SPUN) for MR phase imaging

“…In water-fat separation Dixon scans, 5 out-of-phase echo times can lead to signal cancellation in fat-containing tissues. To date, methods in this category generally suffer from certain restrictions, such as the need for tissue masking or segmentation to exclude low SNR regions and boundaries, 5,9,10,21,22 or using short echo spacing to achieve aliasing-free phase difference maps as a reference, 12,13 and/or requires a long calculation time. 10 Therefore, the scope of applicable scenarios are limited.…”

“…Phase aliasing is a frequent imaging problem . In magnetic resonance imaging (MRI), phase information is needed for many applications, such as field shimming, chemical shift imaging (CSI), susceptibility‐weighted imaging (SWI), and flow quantification . As the phase value returned by inversed trigonometric functions falls within a restricted 2π range, it may differ from its true value by multitudes of 2π and creates phase aliasing artifacts.…”

“…The problem lies in that when a voxel is unwrapped incorrectly, the error will propagate along the growing paths and generate substantial artifacts . Therefore, tissue masking or segmentation is usually needed to exclude low SNR regions and abrupt phase changes, such as cavities, air backgrounds, and scalps . However, imposing tissue masking or segmentation will give rise to other issues, such as suboptimal boundary determination, or excluding normal or pathologic tissues (eg, hemorrhage) in areas of low signals.…”

A dual-VENC 4D Flow MRI Framework for Analysis of Subject-Specific Heterogeneous non-linear Vessel Deformation

“…In an attempt to overcome this issue, previous studies have proposed phase unwrapping algorithms to estimate velocities higher than VENC. However, significant errors have been reported for such techniques, in addition to increased post-processing time [36]- [38].…”

A novel phase‐unwrapping method by using phase‐jump detection and local surface fitting: application to Dixon water–fat MRI

“…The PRELUDE method has been shown to be robust and fast; however, it cannot obtain correct unwrapped results when the difference of true phase between adjacent pixels exceeds π . Recently, we proposed a phase‐unwrapping algorithm based on pixel clustering and local surface fitting (CLOSE) . The local difference of phasors, which are defined as complex signals divided by their magnitudes, reflects the local variations of true phase (LDTP) and are theoretically less than or equal to the corresponding local difference of wrapped phase (LDWP).…”