Abstract
Fiber tractography is widely used to non-invasively map white-matter bundles in vivo using diffusion-weighted magnetic resonance imaging (dMRI). As it is the case for all scientific methods, proper validation is a key prerequisite for the successful application of fiber tractography, be it in the area of basic neuroscience or in a clinical setting. It is well-known that the indirect estimation of the fiber tracts from the local diffusion signal is highly ambiguous and extremely challenging. Furthermore, the validation of fiber tractography methods is hampered by the lack of a real ground truth, which is caused by the extremely complex brain microstructure that is not directly observable non-invasively and that is the basis of the huge network of long-range fiber connections in the brain that are the actual target of fiber tractography methods. As a substitute for in vivo data with a real ground truth that could be used for validation, a widely and successfully employed approach is the use of synthetic phantoms. In this work, we are providing an overview of the state-of-the-art in the area of physical and digital phantoms, answering the following guiding questions: “What are dMRI phantoms and what are they good for?”, “What would the ideal phantom for validation fiber tractography look like?” and “What phantoms, phantom datasets and tools used for their creation are available to the research community?”. We will further discuss the limitations and opportunities that come with the use of dMRI phantoms, and what future direction this field of research might take.
Highlights
Diffusion-weighted magnetic resonance imaging can noninvasively reveal microstructural features of the brain by exploiting the signal attenuating effect of water molecules diffusing in the tissue
Tractography largely relies on the data quality of the Diffusion-weighted magnetic resonance imaging (dMRI) which is prone to many issues and artifacts including head-motion, low signal-tonoise ratio (SNR), thermal noise, eddy-current artifacts, etc
Phantoms are a necessary tool for creation, optimization and validation of every Diffusion MR imaging pipeline, in particular tractography
Summary
Diffusion-weighted magnetic resonance imaging (dMRI) can noninvasively reveal microstructural features of the brain by exploiting the signal attenuating effect of water molecules diffusing in the tissue. An ideal phantom is expected to possess the true axonal characteristics, microstructural properties, an ideal model of signal generation, various fiber structures (short, long, deep and superficial fibers) and anatomical connections and true complexity of the human brain (crossing fibers) generated under a wide range of acquisition parameters and artifacts with quantitative quality metrics that could be used for validating the tractography algorithms. It should have realistic magnetic properties such as surface relaxivity (relevant to material wettability) or magnetic susceptibility (relevant to phantom materials). The readers are referred to (Fieremans and Lee, 2018) for phantoms to study the brain microstructures with MRI
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