1. Method for navigating a flexible medical device within a flexible organ of the body of a patient, toward a target region of interest of an inner wall of the flexible organ, by employing a previously acquired global three-dimensional (3D) model of the flexible organ, the method comprising the procedures of: acquiring an initial stereoscopic image of an initial region of said flexible organ, and reconstructing an initial local 3D model of said initial region, according to said initial stereoscopic image; identifying and registering said initial local 3D model with an initial segment of said previously acquired global 3D model; acquiring a successive stereoscopic image of a successive region of said flexible organ, and reconstructing a successive local 3D model of said successive region, according to said successive stereoscopic image; identifying and registering said successive local 3D model, with a successive segment of said previously acquired global 3D model; detecting said target region of interest by identifying a topography of said target region of interest according to said previously acquired global 3D model, in said successive local 3D model; and producing a notification to indicate that said target region of interest is detected, when said target region of interest is detected.