A balloon catheter (60) comprises an elongated catheter member (63) and an inflatable balloon (65) located adjacent a distal end (5) of the catheter member (63). Band type measuring electrodes (7) located on the catheter member (63) within the balloon (65) comprise a pair of outer stimulating electrodes (8) and sensing electrodes (9) for measuring the cross-sectional area and volume of the balloon (65) by impedance planimetry measuring. A light emitting diode (20) is located on each measuring electrode (7) for emitting light so that the location of the balloon catheter, and in particular, the measuring electrodes (7) can be detected in a vessel (91) with a translucent wall within the body of a human subject. By capturing an image of the vessel and the light from the light emitting diodes (20) emanating through a translucent wall of the vessel by an imaging device (92) located within the human body externally of the vessel (91), the location of the balloon catheter and the electrodes (7) within the vessel can be determined.
