The invention relates to a method for marker-free DNA expression cassette exchange in the genome of cells or parts of cells by using the FLP recombinase mediated cassette exchange. In particular, a stent delivery system configured to carry one or more of a pair of dissimilar stents. The pulse width ratio of the square wave produced by the varying current is compared to a pulse width ratio function derived from a calibration procedure to determine the ion concentration of the measured fluid. The image can be determined based upon an iteratively adjusted movement of the patient.