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:In this study, the Brix model [12, 13], a linear two-compartment open model where in the peripheral compartment has negligible effects on the central compartment, was used because it did not need T10 mapping or arterial input function measurements.
Category: