Patent Application
20100256502
Catheter-based ultrasound imaging techniques are interventional procedures that generally involve inserting a probe, such as an imaging catheter, into a vein, such as the femoral vein, or an artery. The probes are specially designed to provide two-dimensional or real-time three-dimensional imaging. Such applications are demanding and may require very small transducer packages that are nevertheless capable of collecting large amount of information. In some circumstances, it may be desirable to provide some form of acoustic coupling between the transducer assembly and the surrounding ultrasound probe housing to provide an effective or suitable acoustic transition between the transducer and the housing
The presence of acoustic fluid however, may degrade image quality if bubbles form in the fluid, due to mechanical rotation of the transducer, and the bubbles interfere with the imaging. The acoustic coupling fluid may also cause undesirable focusing effects if the sound velocity in the coupling fluid is different than the sound velocity in the imaged medium (i.e., blood or tissue). Therefore a need exists for the development of a method to minimize the risk of bubble formation and interference of the acoustic fluid. One approach may be to design an ultrasound probe using acoustically neutral material to occupy the space between the transducer and the probe housing.
This invention describes materials and processes for manufacture of an acoustically neutral material that may be used in an ultrasound probe housing. Choice of material, based on acoustic properties, and process to manufacture a part, which is readily mountable to other components of an ultrasound probe, is described.
In one embodiment, the present invention provides an ultrasound probe comprising a probe housing defining a distal end, an ultrasonic transducer array disposed within the probe housing and rotatable within said probe housing, an acoustically neutral structure bonded to a surface of the ultrasonic transducer array by an adhesive, a motor coupled to the ultrasonic transducer array, said motor being configured to rotate the ultrasonic transducer array in order to image a three-dimensional volume; and an acoustic coupling fluid disposed within free volume of the probe housing.
In another embodiment, the present invention provides a method of manufacturing an ultrasound probe. The ultrasound probe comprises a probe housing defining a distal end, an ultrasonic transducer array disposed within the probe housing and rotatable within the probe housing, an acoustically neutral structure bonded to a surface of the ultrasonic transducer array by an adhesive and wherein the acoustically neutral structure comprises a polymer cap bonded to a polymer film base. The said method of manufacturing comprises molding the polymer cap to the polymer film base using injection molding, compression molding, or a combination thereof.
Various other features, objects, and advantages of the invention will be made apparent to those skilled in the art from the accompanying drawings and detailed description thereof.
This invention describes materials and processes for manufacture of an acoustically neutral structure, for use at ultrasonic frequencies in an ultrasound probe. Ultrasound probes include, but are not limited to, endoscopes, intraoperative or intracavity ultrasound probes, and ultrasound catheters. Ultrasound catheters, which may incorporate an embodiment of the invention include, but are not limited to, transesophageal catheters, transnasal catheters, transthoracic catheters, intracavity catheters, intracardiac catheters, intravascular catheters, and intraoperative catheters.
An ultrasound probe may be configured to image a three-dimensional volume and comprise a probe housing having a transducer array disposed within the housing, and a motor coupled to the transducer array. The motor is configured to rotate the transducer array in order to image a three-dimensional volume. Free space within the ultrasound probe housing may be filled with an acoustic coupling fluid such as water, propylene glycol, saline, mineral oil, ethylene glycol, castor oil, or a combination thereof. Typically the coupling fluid would have acoustic impedance and sound velocity near those of the imaging medium such as blood and tissue (Z˜1.5 M Rayl, V˜1540 m/sec). While the coupling fluid assists in imaging, a problem with the acoustic fluid is the tendency of the fluid to form bubbles during operation of the probe. The bubbles may form due to incomplete filling of the probe housing leaving air voids in the chamber and may also form during operation of the motor. The bubbles may interfere with image acquisition if they are within the acoustic path. Reducing the amount of free volume in the probe housing and the spacing between the housing and the transducer may alleviate image quality degradation due to the presence of bubbles in the acoustic path. A solid material may be used as a filler.
Transmission of ultrasound through a material may result in modification of the transmitted beam profile and reflection or absorption of energy. This also applies to a solid material used in a probe housing as a filler. An acoustic impedance mismatch between an ultrasound probe component material and another material may cause reflection of energy at the material interfaces and lead to reverberation and a loss of axial resolution in an image. If the acoustic path length through the probe housing material is not uniform, then an acoustic velocity mismatch between the material and adjacent components may cause a lens effect, which can focus, de-focus, or distort the ultrasound beam, substantially reducing resolution and contrast in an image. Therefore a material which is acoustically neutral would be the preferred as filler. Acoustically neutral materials for use in the application include, but are not limited to, thermoplastic elastomers, polyurethanes, polymethylpentene, low density polyethylene, ethylene vinyl acetate (EVA), and filled silicones. One example of a thermoplastic elastomer is a polyether-amide block copolymer.
In certain embodiments the acoustically neutral material may comprise a two-layered structure such as a polymer cap bonded to a polymer film base. In one embodiment, a polyether-amide block copolymer having a controllable ratio of soft to hard blocks may be used as a polymer cap. By varying the molecular weight of the relatively low modulus, soft ether block relative to that of relatively high modulus, hard amide block, the elastic modulus of the copolymer and the acoustic properties dependent on modulus can be varied, more or less continuously, across a wide range. This degree of freedom allows selection of an ether-to-amide block ratio such that the material has sound velocity and acoustic impedance similar to those of water, tissue, or acoustically equivalent coupling fluids. The result is an acoustically neutral material relative to water, tissue or coupling fluids, which is transparent to the ultrasonic radiation and may have minimal effect on an ultrasonic beam passing through other than reduction of transmitted intensity by absorption. An example of such a material series are the PEBAX resins offered by Arkema, of which PEBAX 2533 grade is especially suitable.
However such materials, being both elastomeric and partially amide-based, may be difficult to bond to other components of the ultrasound system. The amide block imparts crystallinity and a consequent degree of chemical resistance to the composite, so that it does not readily enter into bond-forming reactions with commonly used adhesives, such as epoxies, acrylates, or silicones.
In one embodiment, a low speed injection-compression process in which the block copolymer is molded against a polymer film base, the polymer film base having better adhesion facilitates bonding. An example of a polymer film base with better adhesion is a polyimide such as polyimide Kapton film available from Dupont. Bonding between the copolymer and the polyimide film may occur through interfacial adhesion. Interfacial adhesion results from molding the copolymer to the polyimide using injection molding, compression molding, or a combination thereof. The resulting two-layered copolymer/film composite, which is used as the acoustically neutral structure, may then be bonded to adjacent components using conventional techniques.
Referring to
One embodiment of a molding process is illustrated in the flow diagram of
Referring again to
The polyimide film assists demolding as well as serving as an attachable base for the copolymer. The combination of extended drying in vacuum and exposure to temperature well above the melting point of the copolymer during molding serves to eliminate or reduce contaminants in the copolymer, which may otherwise inhibit adhesion.
Referring to
The ICE catheter 40 shown in
As shown in the depicted embodiment in
The catheter housing 60, or at least the portion that intersects the ultrasound imaging volume, is acoustically transparent, e.g. low attenuation and scattering, acoustic impedance near that of blood and tissue. The space between the transducer and the housing may be filled with an acoustic coupling fluid (not shown), e.g., water, also with acoustic impedance and sound velocity near those of blood and tissue (Z equal to approximately 1.5 M Rayl, V equal to approximately 1540 m/sec). In one embodiment, the acoustically neutral material may have a sound velocity in the range 1.0 to 3.0 millimeters per microsecond, and acoustic impedance in the range of 1.0 to 3.0 MegaRayls (MRayls).
An additional advantage of incorporating an acoustically neutral solid filler material between the transducer and the catheter housing is that the shape of the filler material can be specifically designed to conform to the inside of the catheter housing, minus a small uniform gap. This has the effect of somewhat relaxing the sound velocity requirement on the acoustic coupling fluid. Since the coupling fluid would only occupy the small uniform gap between the solid filler material and the catheter housing, detrimental focusing effects due to a mismatched sound velocity of the coupling fluid will be minimized.
A cross section of the ICE catheter 40 depicted in
The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects as illustrative rather than limiting on the invention described herein. The scope of the invention is thus indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
