The National Football League has backed out of an ambitious Boston University brain study meant to assess the dangers related to head trauma caused by football, ESPN reported Tuesday morning. The league, which spent years criticizing researchers who warned about the dangers of such trauma, has now severed ties with the seven-year, $16 million initiative. The National Institutes of Health awarded the project to a group led by a prominent Boston University researcher who has been critical of the league, and the NFL reportedly balked at that decision.