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:CBF images (SCBF) with intensity in units of milliters per gram per minute were calculated (4,9) pixel by pixel using SCBF /T1 (Sc SL)/(SL (2 1)Sc), where SC and SL are signal intensities of the control and labeled images, respectively.
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