Violent criminal behavior causes much human suffering and is a costly social problem, accounting for 6.5% and 11.9% of the gross domestic product of Germany and the United States, respectively.¹ Consequently, furthering our understanding of the small group of men who commit most of the violent crimes is a priority.^2,3 These men display antisocial and aggressive behavior that emerges in childhood and persists through adulthood. Prior to the age of 15 years, they meet criteria for conduct disorder, and as adults, they meet criteria for antisocial personality disorder (ASPD). Results from behavioral and molecular genetic studies^4-6 and from prospective, longitudinal investigations^2,7 have consistently indicated that complex interactions of biological, psychological, and social factors during the course of development initiate and maintain this pattern of behavior. Neuropsychological test results² from as early as 3 years of age and results from recent neuroimaging studies^8-11 suggest abnormal neural development. Yet, much remains to be learned about the heterogeneity of these men who exhibit violent behavior (hereafter referred to as violent offenders) and the etiological mechanisms driving their persistent violence. Studies of the brain morphology of violent offenders can provide clues to causal mechanisms (hypotheses to be tested in prospective investigations of children) and can provide information about mechanisms that contribute to maintaining the violent behavior and its associated features. Such information may be useful for increasing the effectiveness of learning-based programs aimed at reducing violent behavior.¹²

Free water elimination (FWE) in brain diffusion MRI has been shown to improve tissue specificity in human white matter characterization both in health and in disease. Relative to the classical diffusion tensor imaging (DTI) model, FWE is also expected to increase sensitivity to microstructural changes in longitudinal studies. However, it is not clear if these two models differ in their test–retest reproducibility. This study compares a bi‐tensor model for FWE with DTI by extending a previous longitudinal‐reproducibility 3T multisite study (10 sites, 7 different scanner models) of 50 healthy elderly participants (55–80 years old) scanned in two sessions at least 1 week apart. We computed the reproducibility of commonly used DTI metrics (FA: fractional anisotropy, MD: mean diffusivity, RD: radial diffusivity, and AXD: axial diffusivity), derived either using a DTI model or a FWE model. The DTI metrics were evaluated over 48 white‐matter regions of the JHU‐ICBM‐DTI‐81 white‐matter labels atlas, and reproducibility errors were assessed. We found that relative to the DTI model, FWE significantly reduced reproducibility errors in most areas tested. In particular, for the FA and MD metrics, there was an average reduction of approximately 1% in the reproducibility error. The reproducibility scores did not significantly differ across sites. This study shows that FWE improves sensitivity and is thus promising for clinical applications, with the potential to identify more subtle changes. The increased reproducibility allows for smaller sample size or shorter trials in studies evaluating biomarkers of disease progression or treatment effects.Hum Brain Mapp 38:12–26, 2017. ©2016 Wiley Periodicals, Inc.