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:All experiments were performed using a 3T scanner (Tim-Trio System, Siemens Medical Solutions, Erlangen, Germany) using a four-channel head coil with a spinecho spiral developed using the algorithm presented in (13) (maximum gradient: 22.
Category: