1. Field of the Invention
The present invention relates to apparatuses that are adapted for use in determining information relating to the flow of blood within a blood vessel, such as blood velocity information and information reflected from the wall of the blood vessel.
2. Description of the Related Art
Vascular disease and, in particular, cardiovascular disease is a major health care problem in the United States that results in over one million deaths per year. Over the last two decades a tremendous amount of research has been conducted on vascular diseases in an effort to identify the causes of such diseases as well as to diagnose and treat such diseases.
Characteristic of many apparatuses used in the diagnosis of vascular diseases and/or in guiding various therapeutic devices used for the treatment of vascular diseases is the use of an ultrasonic device. The ultrasonic device is typically attached to a guide wire or catheter that allows the ultrasonic device to be placed inside a blood vessel or attached to an endoscope, for example, that permits the ultrasonic device to be positioned in a cavity adjacent to the blood vessel of interest. When the apparatus is in operation, the ultrasonic device either generates an ultrasonic signal that interacts with the blood or blood vessel, or receives an ultrasonic signal after it has interacted with the blood or blood vessel and, as a consequence, now possesses information relating to the blood or blood vessel. Among the information that the ultrasonic device can be used to obtain are information on the vessel lumen or wall as well as hemodynamic measurements, such as blood velocity. To obtain blood velocity information and other related information, the ultrasonic device must be capable of generating or receiving an ultrasonic signal where at least one component of a vector representative of the direction of propagation of the ultrasonic signal is substantially parallel to the direction in which the blood is flowing. The vector representative of the direction of propagation of a plane ultrasonic wave that is generated or received by the ultrasonic device is a vector that is substantially perpendicular to the wave fronts of the plane ultrasonic wave. For non-planar ultrasonic waves that are generated by the ultrasonic device, such as concentric waves, the vector representative of the direction of propagation of the non-planar ultrasonic wave is a vector that is substantially perpendicular to the wave fronts as well as substantially centered in the angular extent over which the wave propagates or indicative of the direction in which the greatest signal strength lies. The vector representative of the direction of propagation of a non-planar ultrasonic wave that is received by the ultrasonic device is a vector that is substantially perpendicular to the wave fronts. Hereinafter, the vector representative of the direction of propagation of an ultrasonic signal will be referred to as the directional vector.
One known apparatus that has been proposed for transmitting or receiving an ultrasonic signal where at least one component of its directional vector is substantially parallel to the direction of blood flow includes a guide wire or catheter with a piezoelectric transducer positioned on the tip of the guide wire or catheter such that the area vector of the transducer, a vector that is perpendicular to the surface of the transducer from which an ultrasonic wave emanates or upon which an ultrasonic wave impinges, is substantially parallel to the direction of blood flow when the apparatus is in use. In operation, the piezoelectric transducer transmits or receives ultrasonic signals having directional vectors that are substantially parallel to the area vector of the transducer and, as a consequence, substantially parallel to the direction of blood flow. An example of such an apparatus is shown in U.S. Pat. No. 4,920,967, which issued on May 1, 1990, to Cottonaro et al., and is entitled "Doppler Tip Wire Guide". Under ideal conditions, the apparatus is inserted into a blood vessel and the piezoelectric transducer is pulsed to produce an ultrasonic signal having a directional vector that is substantially parallel to the area vector of the piezoelectric transducer and, as a consequence, the direction of blood flow. The blood reflects the ultrasonic signal and in so doing Doppler shifts the frequency of the ultrasonic signal by an amount that is indicative of the velocity at which the blood is flowing. The portion of the reflected signal that has a directional vector that is substantially parallel to the area vector of the transducer is then detected by the piezoelectric transducer and converted into an electrical signal that can be processed to determine the blood velocity. There are several drawbacks associated with this apparatus. Namely, the tip of the catheter or guide wire and the piezoelectric transducer in many instances tend to rest against the wall of the blood vessel. As a consequence, the ultrasonic signal transmitted or received by the piezoelectric transducer is corrupted by the vessel wall and therefore does not result in a very reliable indication of the blood flow velocity or related parameter. Moreover, in order to produce an ultrasonic signal having sufficient power to obtain reliable blood flow information, the frontal surface area of the piezoelectric transducer must be relatively large. This large surface area, however, inhibits the placement of the piezoelectric transducer in small diameter vessels, some of which, and especially in the case of the myocardium, are quite important.
Another known apparatus that has been proposed for transmitting or receiving ultrasonic signals having directional vectors with a component parallel to the direction in which the blood is flowing includes a catheter or guide wire with a piezoelectric transducer located on the side of the catheter or guide wire and at an angle to the longitudinal axis of the catheter or guide wire. An example of such an apparatus is shown in U.S. Pat. No. 4,770,185 ('185), which issued on Sept. 13, 1988, to Silverstein et al., and is entitled "Method and Apparatus for Endoscopic Blood Flow Detection By The Use of Ultrasonic Energy." Another example of such a device is shown in U.S. Pat. No. 4,947,852 ('852), which issued on Aug. 14, 1990, to Nassi et al., is entitled "Apparatus and Method for Continuously Measuring Volumetric Blood Flow Using Multiple Transducers and Catheter for Use Therewith". The piezoelectric transducer can be planar, as shown in FIG. 3 of the '852 patent, or frusto-conical, as shown in FIG. 14 of the '185 patent. In either case, when the apparatus is in use, the piezoelectric transducer generates or receives ultrasonic signals having directional vectors that are substantially parallel to the area vector of the piezoelectric transducer. Since the transducer is angled with respect to the longitudinal axis of the catheter or guide wire and the longitudinal axis is substantially parallel to the direction of blood flow when the apparatus is in use, the directional vector of the ultrasonic signals generated by the piezoelectric transducer are at an angle to the direction of blood flow. Similarly, the directional vector of the ultrasonic signals that the piezoelectric transducer can receive are at angle to the direction of blood flow. Since the directional vectors of the transmitted or received ultrasonic signals are at an angle to the direction of blood flow, a component of the directional vectors is also substantially parallel to the direction of blood flow and, as such, can be used to determine blood velocity and related information. This type of apparatus also has several drawbacks. Specifically, the overall diameter or thickness of the apparatus increases as the angle of the transducer with respect to the longitudinal axis of the guide wire or catheter increases. This, in turn, reduces the ability of the device to be placed in small diameter vessels, many of which, as previously mentioned, can be quite important. Moreover, placing the piezoelectric transducer at an angle to the longitudinal axis of the catheter or guide wire requires that an appropriately angled mounting surface be fabricated or in some other way machined on the catheter or guide wire. This, in turn, adds to the complexity and, in all likelihood, the cost of producing such an apparatus.
Yet another known apparatus that has been proposed for use in determining blood velocity and related information using an ultrasonic signal includes a catheter or guide wire with a piezoelectric transducer located on the side of the catheter or guide wire with the area vector of the piezoelectric transducer oriented such that it is substantially parallel to the longitudinal axis of the catheter or guide wire. Positioned adjacent to the piezoelectric transducer is a reflector that reflects any ultrasonic signal produced by the piezoelectric transducer such that the directional vector of the reflected ultrasonic signal is at an angle to the longitudinal axis of the catheter or guide wire, which is also at an angle to the direction of blood flow when the apparatus is in use. Conversely, the reflector reflects ultrasonic signals that impinge upon it at an angle to the longitudinal axis of the catheter, or at an angle the direction of blood flow when in use, so that the directional vector of the reflected ultrasonic signal impinges upon the piezoelectric transducer from a direction that is substantially parallel to the area vector of the piezoelectric transducer. Since the directional vectors of the ultrasonic signals that the reflector receives from the blood or reflects into the blood are at an angle to the direction of blood flow, there is a component of the ultrasonic signals that is substantially parallel to the direction of blood flow that can be used to determine blood velocity and related information. An example of such a device is shown in U.S. Pat. No. 4,757,821, which issued on Jul. 19, 1988, to Snyder and is entitled "Omnidirectional Ultrasonic Probe". Due to the orientation of the piezoelectric transducers and reflective structures, this type of apparatus possesses a relatively large diameter that prevents this type of apparatus from being maneuvered into small diameter blood vessels. Moreover, the need to fabricate reflective surfaces and properly orient the piezoelectric transducer with respect to such surfaces makes this type of apparatus difficult to build and, as a consequence, more expensive.
Another known device that has been proposed for use in determining blood flow information using an ultrasonic signal is shown in FIG. 11 of the '852 patent. The device includes a catheter or guidewire with an ultrasonic transducer that, due to either its curved surface, narrow dimension, or its coupling with an acoustic lens is apparently capable of receiving ultrasonic signals having directional vectors that extend over a range of angles. The use of a transducer with a curved surface, another example of which is shown in FIGS. 10A and 10B of the '185 reference, is apparently capable of receiving ultrasonic signals having directional vectors that extend over a range of angles with respect to the longitudinal axis of the catheter or guidewire. This ability is apparently due to the curved surface having a plurality of surface area vectors that extend over the relevant range. Consequently, if the directional vector of an ultrasonic signal that impinges upon the transducer is substantially parallel with the surface area vector of the transducer at the point where it impinges upon the transducer, then the transducer will convert the ultrasonic signal into an electrical signal. This type of transducer is not sensitive to signals that impinge upon it at an angle to the surface area vector at the point of impact. A drawback associated with using a transducer with a curved surface is that the curved surface increases the overall diameter of the device in much the same manner as the devices that utilize a transducer set at an angle to the longitudinal axis of the guide wire or catheter. The relatively large diameter of such a device, as previously mentioned, is undesirable in many instances.
The narrow dimension transducer, at least functionally, also has a plurality of surface area vectors and, as a consequence, is apparently capable of receiving ultrasonic signals over a range of angles. However, such a transducer does not appear to be able to receive an ultrasonic signal that impinges upon it that has a directional vector that is at an angle to one of the plurality of surface area vectors. Such sensitivity is achieved by using a narrow dimension transducer with a plurality of surface area vectors going in different directions. Consequently, even though the transducer is of narrow dimension, it must be oriented to capture ultrasonic signals over the range of interest. This, in turn, results in the ultrasonic transducer contributing to the overall diameter of the device and thereby reducing the ability of the device to be used in small diameter blood vessels.
The apparent purpose of the acoustic lens is to bend or divert ultrasonic signals with directional vectors that impinge upon it over a range of angles with respect to the surface area vector of the ultrasonic transducer such that the directional vector of the ultrasonic signal when it departs the acoustic lens is substantially perpendicular to the surface area vector of the transducer. This, in turn, allows the transducer to detect the ultrasonic signal and convert the ultrasonic signal into an electrical signal. Moreover, the use of a lens, if placed on the side of a catheter or guide wire, increases the overall diameter of the device which, in turn, limits the diameter of blood vessels within which such a device can be positioned. In addition, the use of an acoustic lens, as with the devices that use a reflector, makes the device more difficult to build and, in all likelihood, more expensive.
The '852 patent also shows in FIG. 10 a pair of transducers that are used to measure the distance between the catheter and the wall of the blood vessel. The transducers used in the '852 patent appear to generate or receive ultrasonic waves that have directional vectors that are parallel to the surface area vector of the ultrasonic transducer. When a wave pattern is generated where the wave fronts are non-parallel, as in a wave pattern where the wave fronts are concentric, and form a fan-like pattern similar to lobe of the signal associated with an antenna, the directional vector is considered herein to be the vector that is substantially centered in the wave pattern. Consequently, the direction of wave propagation of the ultrasonic signal generated by the ultrasonic transducer T.sub.3 that is shown in FIG. 10 of the '852 patent is not believed to be indicative of the directional vector of the ultrasonic wave as defined herein, but merely to indicate that the transducer generates ultrasonic signals having non-parallel or concentric wave fronts that can be used to perform the required distance measurement. Similar statements can be made with respect to the ultrasonic signal received by the ultrasonic transducer T.sub.4. Consequently, the transducers T.sub.3 and T.sub.4 shown in the '852 reference are not believed to be capable of generating or receiving, respectively, ultrasonic signals with directional vectors that are at angles to the surface area vectors of the transducers T.sub.3 and T.sub.4, respectively.
Based on the foregoing, there is a need for an ultrasonic apparatus for obtaining hemodynamic information, such as blood flow velocity, that addresses the drawbacks associated with the presently known ultrasonic apparatuses for obtaining such information. More specifically, there is a need for a device for obtaining hemodynamic information that is capable of transmitting or receiving ultrasonic signals at an angle to the surface area vector of the ultrasonic transducer employed in the device. This, in turn, would allow a device having a relatively small thickness or diameter to be realized that would, in turn, allow the device to be placed in blood vessels or orifices adjacent to a blood vessel having correspondingly small diameters. In addition, there is a need for an ultrasonic apparatus for obtaining hemodynamic information that has a reduced number of components relative to many of the known apparatuses for obtaining hemodynamic information, and, as a consequence, is easier and less expensive to manufacture.