Browsing by Subject "Phase correction"

Now showing 1 - 2 of 2

Open Access
Phase-correcting non-local means filtering for diffusion-weighted imaging of the spinal cord
(John Wiley, 2018) Kafalı, Sevgi Gökçe; Çukur, Tolga; Sarıtaş, Emine Ülkü
Purpose: DWI suffers from low SNR when compared to anatomical MRI. To maintain reasonable SNR at relatively high spatial resolution, multiple acquisitions must be averaged. However, subject motion or involuntary physiological motion during diffusion-sensitizing gradients cause phase offsets among acquisitions. When the motion is localized to a small region, these phase offsets become particularly problematic. Complex averaging of acquisitions lead to cancellations from these phase offsets, whereas magnitude averaging results in noise amplification. Here, we propose an improved reconstruction for multi-acquisition DWI that effectively corrects for phase offsets while reducing noise. Theory and Methods: Each acquisition is processed with a refocusing reconstruction for global phase correction and a partial k-space reconstruction via projection-onto-convex-sets (POCS). The proposed reconstruction then embodies a new phase-correcting non-local means (PC-NLM) filter. PC-NLM is performed on the complex-valued outputs of the POCS algorithm aggregated across acquisitions. The PC-NLM filter leverages the shared structure among multiple acquisitions to simultaneously alleviate nuisance factors including phase offsets and noise. Results: Extensive simulations and in vivo DWI experiments of the cervical spinal cord are presented. The results demonstrate that the proposed reconstruction improves image quality by mitigating signal loss because of phase offsets and reducing noise. Importantly, these improvements are achieved while preserving the accuracy of apparent diffusion coefficient maps. Conclusion: An improved reconstruction incorporating a PC-NLM filter for multi-acquisition DWI is presented. This reconstruction can be particularly beneficial for high-resolution or high-b-value DWI acquisitions that suffer from low SNR and phase offsets from local motion.
Open Access
Simultaneous phase-correction and denoising for diffusion-weighted MRI
(IEEE, 2016) Kafalı, Sevgi Gökçe; Çukur, Tolga; Sarıtaş, Emine Ülkü
Diffusion-weighted imaging (DWI) suffers from low SNR when compared to regular MRI images. To attain reasonable SNR levels with high spatial resolution, multiple acquisitions of each slice have to be acquired. These repetitions are then averaged, with the goal of increasing the SNR. However, subject motion during diffusion-sensitizing gradients creates varying phases between repeated acquisitions. When direct complex averaging is performed, these phase offsets can cause unpredictable phase cancellations and local signal drops. Here, we propose a technique that simultaneously corrects phase and performs a non-local means filtering to reduce noise. The success of the proposed technique is demonstrated in vivo at 3T with DWI of the cervical spinal cord.