“Squashing peanuts and smashing pumpkins”: How noise distorts diffusion‐weighted MR data

TLDR

Diffusion‑weighted imaging techniques such as high‑b, q‑space, and high‑angular‑resolution MRI aim to reveal non‑Gaussian diffusion and fine anatomical structures like white‑matter fasciculi beyond what low‑b or diffusion‑tensor MRI provide. The study reports noise‑induced artifacts in high‑b DWI, including orientational deviations and anisotropy underestimation, and proposes a rectification strategy to mitigate these effects. Artifacts arise when increased diffusion weighting or reduced voxel size lowers signal‑to‑noise ratio, and the authors counteract this by applying a rectification method to high‑b data. The authors find orientationally dependent ADC deviations, anisotropy underestimation, mean‑diffusivity–anisotropy correlation, heightened gray/white contrast, and apparent anisotropy elevation in acute ischemia, all of which are negligible at the typical b ≈ 1000 s mm⁻² used in DT‑MRI.

Abstract

New diffusion-weighted imaging (DWI) methods, including high-b, q-space, and high angular resolution MRI methods, attempt to extract information about non-Gaussian diffusion in tissue that is not provided by low-b-value (b approximately 1000 s mm(-2)) diffusion or diffusion tensor magnetic resonance imaging (DT-MRI). Additionally, DWI data with higher spatial resolution are being acquired to resolve fine anatomic structures, such as white matter fasciculi. Increasing diffusion-weighting or decreasing voxel size can reduce the signal-to-noise ratio so that some DWI signals are close to the background noise level. Here we report several new artifacts that can be explained by considering how background noise affects the peanut-shaped angular apparent diffusion coefficient (ADC) profile. These include an orientationally dependent deviation from Gaussian behavior of the ADC profile, an underestimation of indices of diffusion anisotropy, and a correlation between estimates of mean diffusivity and diffusion anisotropy. We also discuss how noise can cause increased gray/white matter DWI contrast at higher b values and an apparent elevation of diffusion anisotropy in acute ischemia. Importantly, all of these artifacts are negligible in the b-value range typically used in DT-MRI of brain (b approximately 1000 s mm(-2)). Finally, we demonstrate a strategy for ameliorating the rectified noise artifact in data collected at higher b values.