Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:…showed that imaging or therapeutic agents larger than the BBB’s exclusion threshold of 400 Da could be successfully delivered by FUS with microbubbles (Alonso et al. 2013; Choi et al. 2007; Choi et al. 2010a; Hynynen et al. 2001; Kinoshita et al. 2006; Marquet et al. 2014; Shen et al. 2014).
Category: