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.
In an effort to improve short range performance, some short range pulse-echo ranging systems have resorted to using separate transmitter and receiver devices. The amount of excitation from the transmit pulse resident in the receive channel is naturally reduced by the physical isolation of the two (often by physically spacing one a relatively large distance from the other) and, hence, the switching need not be so complete as is the case when a single device is used to transmit and receive. Additional improvements can be achieved by utilizing improved materials which exhibit reduced ringing. For example, as described hereinafter, certain plastic materials have been found particularly useful in medical imaging applications because of the reduced acoustic impedance of such materials relative to known crystalline materials. However, such systems are still range limited by the required dead space, and the physical separation of the two transmitter and receiver devices. Therefore, until the present invention the use of separate transmitter and receiver devices for pulse-echo ultrasonic acquisition of information regarding very close range targets, such as the walls of coronary arteries, has been impractical.
More specifically, the system disclosed in the Parent Application comprises a catheter probe that is adapted to be inserted into a part of a body, and is particularly good at providing relatively high resolution imaging data of an relatively small, predetermined portion of the body, such as a small section of a coronary artery (the catheter may also be adapted to deliver laser energy to the interior of the body part for modifying internal features thereof, e.g., removing plaque deposits from a coronary artery). A transducer assembly is attached to the distal end of the catheter for emitting and receiving acoustic pulses used in generating imaging information. Sets of imaging data are created by moving the catheter axially along and notationally about its axis, within the body section of interest, through a series of imaging locations, while the transducer assembly is actuated to emit a train of acoustic pulses and responsively receive a series of acoustic echoes at each imaging location. By also continuously sensing the location of the catheter, and relating the set of data to the respective location where it was generated, an image of the internal features of the body part may be generated.
It is clear that the target being imaged by the system disclosed in the Parent Application is extremely close to the transducer assembly. In fact the transducer assembly is frequently almost in contact with the targeted surfaces, such as the inner surface of the wall of an artery. At such close range, relying on standard radar and sonar techniques of allowing the required time of dead space to transpire after the transmit mode and before switching to a receiving mode is inadequate since the target ranges are well below the minimum range for the dead space required and imaging data will be lost during this time. Utilizing two transducer devices, one for transmitting and one for receiving, does not alone overcome the problem since the two devices must be mounted on the end of a catheter extremely close to one another. Because of the close proximity between the two devices, sufficient dead space still must be provided to prevent acoustic interference, such as acoustic cross talk, between the transmitting device and receiving device, as for example when the transmitting device is ringing following the generation of an acoustic pulse. In fact, because of the close proximity of the two, certain electrical interference problems can occur between the transmitting and receiving devices which do not occur with a single device, such as capacitive and inductive coupling between the two devices.