A system and method use the frequency spectrum of a radio frequency (RF) signal backscattered from vascular tissue to identify at least one border on a vascular image. A data gathering device is connected to a computing device and a transducer via a catheter. RF data generated via the transducer is provided to the computing device via the data-gathering device. The computing device includes: at least one data storage device for storing a plurality of tissue types and parameters related thereto, and at least one application (e.g., a characterization application, a gradient-border application, a frequency-border application and/or an active-contour application). The characterization application converts the RF data into the frequency domain and identifies a plurality of associate parameters. The identified parameters are compared to the parameters stored in the data storage device to identify the corresponding tissue type and thereafter determine at least one border on a vascular image.