The present invention provides medical devices with enhanced ultrasound visibility. The device uses real-time ultrasound monitoring of the position of the needle tip within the patient's body for topical drug delivery, probe positioning, fluid drainage, biopsy, or delivery of ultrasound pulses. The device controls the diffusion of drugs in solid tissues, injects particles into solid tissues, and delivers drugs to specific blood vessels. As the needle is inserted, a fluid with a different ultrasound visibility than the organ environment is injected into the patient. The fluid travels a short distance before being slowed and stopped by the patient's tissues, and the flow can be detected ultrasonically. During needle insertion, the needle position is monitored with ultrasound until the needle is at the ideal point of action. Therapeutic drugs are then delivered through the needle or probes are inserted to perform treatments such as removing tumors with radiofrequency heat. The flow rate can be adjusted during needle insertion to maintain a properly set image of the needle tip. At the point of application, the ultrasound imaging agent can be repeated and pulsed at different flow rates until a satisfactory fluid profile is obtained, after which the drug can be delivered. Ultrasound can also be sent through the needle with a detector mounted in the handheld assembly.