A medical implant with a structure made of at least one biocompatible material, converted from a first spatially compact state into a second spatially deployed and flexible deformable state, has a modulus of elasticity corresponding to the wall of a blood vessel in an order of magnitude between 105 Nm.sup.2 and 107 N m.sup.2, has at least one region with an acoustic impedance of more than 1.63.Math.106 kg/m.sup.2 s, and in the second state has an effective operating surface that is flexibly deformable and at least partially reflects ultrasonic waves. Ultrasonic waves are generated by a sonographic device and which strike an effective operating surface of the intracorporeally applied medical implant and are partially reflected for detecting changes in the spatial distance between the operating surface and the sonography device.