Tractography validation Part 1: Foundations, numerical simulations, and phantom models

Tractography validation is the process of assessing and quantifying the relationship between anatomy and tractography. Validation allows us to evaluate the strengths and limitations of the fiber tractography process and aims to ultimately understand, improve, and refine this process. The focus of this chapter, along with the following two chapters, is to provide insights into different strategies, technologies, and models that can be used to validate tractography. The target audience ranges from beginners who need basic insights, practical experiences, tips, and tricks to advanced readers interested in the state-of-the-art cross-modality validation and reflections. In this first chapter on tractography validation, we provide the motivation for validating tractography across multiple anatomical length scales and at various steps in the tractography pipeline (Section 1). This is followed by one of two topics on validation strategies, presented as cookbooks containing detailed descriptions of validation methods, from the brain network level to the microstructure level. The first topic is validation using synthetic phantoms (numerical simulations) (Section 2) and physical phantoms (Section 3). The next chapter (Chapter 26) covers the second topic, including validation using anatomical model systems, i.e., application in real tissue, empirical validation (when there is no ground truth), and measures for validation (how to quantify validation). The final, third topic (Chapter 27) highlights what we have learned through validation studies, focusing on how these insights were gained and their implications for tractography, and provides perspectives on future challenges in tractography.