Acoustically active injection catheter guided by ultrasound: navigation tests in acutely ischemic porcine hearts.Catheters are increasingly used therapeutically and investigatively.  An injection catheter navigated by ultrasound was designed and then tested in an open-chest model of acute ischemia in eight pigs.  Complete data were obtained from seven pigs, and the correlation between sonomicrometry and ultrasound measurements was excellent (p < 0.0001, ρ = 0.9820), as was the intraclass correlation coefficient (0.96) between two observers.  These initial experimental results suggest high accuracy of ultrasound navigation of the acoustically active catheter prototype located inside the beating left ventricle.
ultrasound
The catheter is made "acoustically active" by a piezo-electric crystal near its tip, electronically controlled, vibrating in the audio frequency range and uniquely identifiable using pulsed-wave Doppler. 
pulsed-wave Doppler
Sonomicrometry was used to measure distances between the two crystals and then compared with measurements from 2-D echocardiographic images.  Complete data were obtained from seven pigs, and the correlation between sonomicrometry and ultrasound measurements was excellent (p < 0.0001, ρ = 0.9820), as was the intraclass correlation coefficient (0.96) between two observers. 
sonomicrometry