An internal probe device for insertion into the body of a patient, comprises an elongate body with a plurality of EAP actuators mounted at the surface of the body. The EAP actuators are made to vibrate so that their position becomes visible in a Doppler ultrasound image. The use of EAP actuators to provide vibrations enables individual locations to be identified. In particular, the movement of the EAP actuator may be largely isolated from the main body of the probe. Furthermore, EAP actuators can be thin, lightweight and have a small form factor suitable for application to or within the surface of a probe, such as a catheter, needle or endoscope.