Possibilities of ultrasound catheters

Abstract

Ultrasound catheters offer the possibility of various non-imaging applications. Some of these applications, now in different stages of development, have been studied and will be described here in some detail.

Ultrasonically marked catheters have different applications. The catheter has a miniature marker transducer mounted at the tip or some other place of interest. Initially, this device was used to localize the pacing lead tip in the heart. Connected to a transponder of a passive type this device can generate a visible localization mark on the echograph screen. The basically same system can be used for early detection of cracking of the pacing lead insulation. In this case the marker transducer works as a high-frequency signal generator and detects characteristic capacitance changes better than other methods. The electronic circuit for measurement is built into the pacemaker. Other non-imaging applications have also been studied. The marker transducer can be used for echo ranging of the distance between a His bundle fulguration electrode and the structure to be destroyed. Such an automatic ‘proximity fuse’ can help to avoid the firing of energy at too great a distance from the His bundle. Technology of implantable defibrillators yields the possibility for a double transducer arrangement, one transducer mounted at the patch and the other being the marker transducer. Using on-line distance measurement this arrangement enables early detection of abnormal movement of the ventricle wall, thus detecting deterioration of the muscle function before it is electrically evident. The measurement was simulatedin-vitro. Measurement of axial blood flow using transit time methods, instead of Doppler, was also experimentally studied. The transit time method consists of measurement of the autocorrelation function of signals reflected from a perpendicular flow and gives a genuine measurement orthogonal to the Doppler method. Some of the feasibility studies will be discussed. Moreover, a waveguide catheter for uretral stone lithotripsy is reported which also has a potential for treatment of obstacles in other hollow organs. The system operates at 30 kHz/60W. Safety aspects of ultrasound catheters will be discussed, showing results of the measurement of shunt impedance at different frequencies, which sets the limit for the electrical safety design.