A system (2) comprising a balloon catheter (1) for determining the distance between a datum location (8) at which the balloon catheter (1) is inserted through an arterial system to a valve orifice (17) of an aortic valve (9) comprises a planimetry measuring system (19) in a balloon (7) of the balloon catheter (1), and a linear distance measuring element (23) slideable along the catheter (3) from a proximal end (4) thereof to the datum location (8) when the balloon catheter (1) has been inserted through the arterial system with the balloon (7) located in the valve orifice (17). A plurality of secondary optically detectable bands (30) are equi-spaced longitudinally along the catheter (3) and an optical encoder (32) in the linear distance measuring element (23) counts the secondary optically detectable elements (30) as the linear distance measuring element (23) is being moved from a reset position proximally of a primary optically detectable band (28) distally along the catheter (3) to the datum location (8). A signal processor (20) of the system (2) reads signals from the linear distance measuring element (23) and from the planimetry measuring system (19) for in turn determining the distance of the valve orifice (17) of the aortic valve (9) from the datum location (8).Un système (2) comprenant un cathéter (1) à ballonnet pour déterminer la distance entre une position (8) de référence au niveau de laquelle le cathéter (1) à ballonnet est inséré à travers un système artériel jusquà lorifice (17) de la valve dune valvule (9) aortique, comprend un système (19) de mesure de planimétrie dans un ballonnet (7) du cathéter (1) à ballonnet, et un élément (23) linéaire de mesure de distance coulissant le long du cathéter (3) depuis une extrémité (4) proximale de celui-ci jusquà la position (8) de référence lorsque le cathéter à ballonnet (1) a été inséré à travers le système artériel avec le ballonnet (7) situé dans lorifice (17) de la valve. Plusieurs bandes (30) secondaires op
