Ultrasonic transducer

#The present invention relates generally to electro-acoustical devices for converting electrical energy into acoustical energy and vice versa, and more particularly to a miniature transducer, an improved system utilizing such transducers for imaging internal features of various parts of a body and, a method of making such transducers.

#Devices are known which use pulse-echo ultrasound to provide information regarding the thickness, configuration, composition and other characteristics of internal features of various parts of a body. Such a device is described, for instance, in U.S. Pat. No. 4,576,177 to Webster. However, specific problems are encountered when utilizing pulse-echo ultrasound to acquire information with sufficiently high resolution in extremely close range applications, such as acquiring information relating to the walls of coronary arteries.

#More specifically, in known long range ranging systems, such as pulse-echo radar or sonar, a single electro-acoustical device is typically used for transmitting the sounding pulses and receiving the echoes. The device is switched between transmit and receive modes using a TR switch. The latter is designed to operate so as to minimize any interference during the receive mode caused by the excitation of the electro-acoustic device during the transmit mode. For example, the instant the device stops transmitting it may continue to ring. Accordingly, in order to receive any echo signals it is important that the device be allowed to stop ringing before the device is switched to the receiving mode. The TR switch can be operated quickly to quench the device at the termination of the transmit mode in order to switch the device to the receive mode. However, a finite time is nevertheless required to expire before the electro-acoustic device is sufficiently quiescent in order to operate in the receive mode without residual interference from the transmit mode. This finite time, called "dead space", thus creates a minimum time the receiver must be turned off following the end of the transmit mode. In other words, the target distance must be sufficiently large so that echoes are not received by the electro-acoustic device until after the time of the dead space has elapsed. The dead space therefore creates a minimum target distance at which the ranging device will operate without cross interference between the two modes and without a loss of information. This is usually not a problem for typical radar and sonar applications where target distances are well beyond the minimum distance required.

#FIG. 1 is a perspective view of the preferred embodiment of the transducer of the present invention, partially broken away to expose internal features thereof;

#FIGS. 2-8 are schematic, perspective views of successive intermediate products provided in successive steps of the preferred process of fabricating the transducer shown in FIG. 1;

#FIG. 9 is a perspective view of the tip portion of the distal end of the catheter of the type used in the imaging system described in the Parent Application, and incorporating the present invention;

#The preferred embodiment of the transducer of the present invention is illustrated in FIG. 1. The transducer comprises an inner core or substrate 20 and a piezoelectric device 32 disposed on the substrate. The transducer is constructed so that it is electrically shielded such that two such transducers are capable of operating adjacent one another, with one transducer operating as a transmitting device and the other as a receiver device without significant electrical interference. Further, as will be more evident hereinafter, the two transducers can be suitably mounted on a catheter 100 (as described in greater detail hereinafter with respect to FIGS. 9 and 10) so that the two transducers are substantially acoustically isolated from one another.

#More specifically, substrate 20 is preferably made from a suitable ceramic material, such as alumina, i.e., aluminum oxide. The substrate is shown in the drawings as a rectangular block or brick comprising a front surface 22 (FIGS. 1 and 2), a top surface 24 (FIG. 2), a left side surface 26 (FIG. 3), a right side surface 28 (FIG. 3), a bottom surface (not shown) opposite top surface 24, and a rear surface (not shown) opposite front surface 22. It should be appreciated, however, that the exact configuration of the substrate can vary depending upon the application for which the transducer is intended to be used.

#Since the transducer is preferably designed for use on a catheter of an imaging system of the type described in the Parent Application, the size of the transducer must be relatively small. Where the imaging system is designed to image selected portions of coronary arteries, the substrate 20 is dimensioned so that it is thick enough to function as an acoustic reflector for the piezoelectric device 32. In the preferred embodiment where the piezoelectric device is a polymer piezoelectric device (as described hereinafter), the substrate is dimensioned on the order of about 0.028 inches wide, 0.038 inches long, and 0.012 inches thick, although I believe, at least in the case of alumina, the substrate can be made even thinner. The minimum thickness of the substrate is limited by the acoustic reflectivity of the substrate material for the piezoelectric device 32 and the acoustic impedance mismatch between the two. For alumina at the acoustic frequency of interest (in the megahertz region), the minimum thickness is believed to be about 0.006 inches thick at which dimension the substrate still adequately functions as a reflector for acoustic energy generated by the piezoelectric device 32 disposed on the substrate. Decreasing the thickness to less than 0.006 inches reduces the ability of the substrate to reflect the acoustic energy to the point where a substrate of 0.003 inch thickness will cease to function as an adequate reflector so that substantially all of the acoustic energy transmitted by the device 32 downwardly will be transmitted through the substrate. As discussed hereinafter, the relatively small size of the transducer is made possible because of the material from which the piezoelectric device 32 is made, and more particularly, its relatively close impedance match to that of body fluids.