qualitative covariate of non interest labelling

Dear Alfonso and Conn users,

I am running a resting state experiment with a set of 23 patients obtained from two different scans (1.5T Siemens scan:8 patients and 3T Siemens scan:15 patients). In my second level analyses I would like to take into account that bias and specify the MR scan used for each patient as a covariate of non interest.
How should I specify that covariate in the covariates setup? I tried with 1 for 1.5T and 0 for 3T, and the other way also, but I get different results.
Is there another way of expressing a qualitative covariate than 1 and 0? Or should I specify two different covariates, one for each scan?

Thank you very much for your help

best regards

Jerome