EP0605756A2

EP0605756A2 - Interface element for medical ultrasound transducer

Info

Publication number: EP0605756A2
Application number: EP93111871A
Authority: EP
Inventors: Rodney J. Solomon; Gregory G. Vogel
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1993-01-08
Filing date: 1993-07-23
Publication date: 1994-07-13
Also published as: EP0605756A3; JPH06277219A

Abstract

An ultrasound transducer assembly includes an ultrasound transducer for transmitting and receiving ultrasound energy 10 and one or more interface elements for conducting transmitted ultrasound energy from the transducer 10 to a patient's body and for conducting received ultrasound energy from the patient's body to the transducer. At least one ultrasound-transmissive interface element 20 is fabricated of a rigid, low sound speed material. The material preferably has a sound speed that approximately matches the speed of sound in human tissue. The rigid, low sound speed material is preferably a material that is suitable for use on a human eye as a contact lens. The ultrasound-transmissive element fabricated of a rigid, low sound speed material can be a protective lens cover 20.

Description

Field of the Invention

This invention relates to transducer assemblies for medical ultrasound systems and, more particularly, to an interface element for coupling ultrasound energy between a transducer and a patient's body.

Background of the Invention

Ultrasound transducers are frequently utilized in a variety of medical applications. The transducer may include a single element or an array of transducer elements. The transducer is typically part of an ultrasound imaging system for generating an image of a region of interest within a patient's body. For many applications, the transducer is mounted in a hand-held probe which is positioned adjacent to a selected external area of the patient's body, for example, adjacent to the chest wall to scan the heart. In other instances, the transducer is mounted in a probe that can be positioned in an internal body cavity or passage. The transducer frequently includes an ultrasound lens for focusing the ultrasound energy.
When an ultrasound transducer is used for medical imaging, it is extremely important to ensure that any materials between the transducer and the region of the patient's body being imaged do not distort or otherwise interfere with the image. In particular, when the ultrasound energy encounters an interface between materials having different sound speeds, the energy can be partially reflected and refracted. Since the speed of sound in air (about 332 meters/second) is much different from the speed of sound in the human body (about 1540 meters/second) it is important to eliminate air between the ultrasound transducer and the patient's body. For this reason, it is common practice to employ an acoustic gel between the transducer and the patient's body.
Single piece ultrasound lenses for flat piezoelectric crystals typically have an outer structure that is convex. This permits the transducer to better contact the body portion being imaged. In order to provide a lens for focusing ultrasound energy that is planar on the inside surface that contacts the transducer and convex on the outside, the sound speed of the material must be lower than the sound speed within the body. A typical material that has such a low sound speed is silicone rubber, which is relatively soft, is not durable, is quite attenuative and must be cast in place over the ultrasound transducer. It would be desirable to provide a protective cover over the silicone rubber lens. However, the cover must not significantly distort the ultrasound image or attenuate the ultrasound energy.
Another ultrasound transducer configuration involves the use of a rotating transducer and lens in a transesophageal probe as described in U.S. Patent No. 5,127,410 issued July 7, 1992. The transducer and lens are positioned behind a sealed window and rotate relative to the window. The lens includes a silicone rubber inner element and a urethane rubber outer element. An acoustic lubricant fills a gap between the surfaces of the lens and the window. The urethane rubber lens element is relatively soft and may not provide adequate mechanical support for the window in the event that an object is pressed or impacts against the window.

Summary of the Invention

According to the present invention, a medical ultrasound transducer assembly is provided. The transducer assembly comprises an ultrasound transducer for transmitting and receiving ultrasound energy and interface means for conducting transmitted ultrasound energy from the transducer to a patient's body and for conducting received ultrasound energy from the patient's body to the transducer. The interface means comprises at least one ultrasound-transmissive element which is fabricated of a rigid, low sound speed material. The material preferably has a sound speed that approximately matches the speed of sound in human tissue. The ultrasound transducer can comprise a single transducer element or an array of transducer elements.
The rigid, low sound speed material is preferably a material that is suitable for use in a human eye as a contact lens. The rigid, low sound speed materials of the invention are durable and can be machined or cast into desired shapes. The sound speed in the rigid, low sound speed materials approximately matches the speed of sound in the human body so that distortion of the ultrasound image and reflection of ultrasound energy are minimized.
In a first embodiment of the invention, the interface means includes an ultrasound lens, and the ultrasound-transmissive element comprises a protective cover on the ultrasound lens. The protective cover is in contact with the patient's body during use of the transducer.
In a second embodiment, the transducer assembly includes a fixed window and means for rotating the transducer relative to the fixed window. Anultrasound lens is affixed to and rotates with the transducer. The ultrasound-transmissive element comprises a protective cover fabricated of a rigid, low sound speed material affixed to the ultrasound lens and located between the ultrasound lens and the window.
In a third embodiment, the transducer assembly includes a fixed window and means for rotating the transducer relative to the fixed window. The window is fabricated of rigid, low sound speed material.
In a fourth embodiment, the transducer assembly includes a fixed window and means for rotating the transducer relative to the fixed window. An ultrasound lens is affixed to and rotates with the transducer. The ultrasound-transmissive element comprises an outer element of the ultrasound lens fabricated of a rigid, low sound speed material.
In a fifth embodiment, the ultrasound-transmissive element fabricated of a rigid, low sound speed material comprises a sound pipe for coupling ultrasound energy between the transducer and the patient's body. The sound pipe can comprise a standoff for spacing the transducer from the patient's body. Alternatively, the sound pipe can include a surface for changing the direction of propagation of ultrasound energy by total internal reflection within the sound pipe.
According to another aspect of the invention, an element for coupling ultrasound energy to and between an ultrasound transducer and a patient's body is provided. The element comprises a member fabricated of a rigid, low sound speed material.

Brief Description of the Drawings

For better understanding of the present invention, reference is made to the accompanying drawings which are incorporated herein by reference and in which:

FIG. 1 is a partial cross-sectional view of a transducer assembly including a soft ultrasound lens and a protective cover of a rigid, low sound speed material in accordance with the invention;
FIGS. 2A and 2B are cross-sectional views of transducer assemblies that employ a rotating transducer and lens; and
FIG. 3 is a schematic diagram of a transducer assembly that employs a sound pipe for coupling ultrasound energy between a transducer and a patient's body.

Detailed Description

The present invention provides a novel interface element for use between an ultrasound transducer and a patient's body. The invention is based on the discovery that rigid, gas permeable materials, which are typically used on the human eye as contact lenses, have properties that are highly desirable for transmission of ultrasound energy. The rigid, gas permeable materials are used to fabricate various ultrasound-transmissive elements that are positioned between an ultrasound transducer and a patient's body.
The most important requirement for such an ultrasound-transmissive element is the speed of sound in the material. The speed of sound within the material preferably approximately matches the speed of sound within the human body (approximately 1540 meters/second). This ensures that the ultrasound image will not be significantly distorted or otherwise degraded when the transmitted or received ultrasound energy passes through the element. It is desired that the sound speed within the material match the sound speed within the human body as closely as possible. However, sound speeds within a range of about 950 to 2000 meters/second are considered acceptable. Most materials having sound speeds within this range are soft and thus lack durability.
A further requirement is that the acoustic impedance of the rigid material should be approximately matched to the acoustic impedance of the human body (1.54 Mrayls). Acoustic impedances in a range of about 1.0 to 2.1 Mrayls are considered acceptable.
Applicants have found that rigid, gas permeable materials normally used for contact lenses have sound speeds that closely match the speed of sound in the human body. Furthermore, these materials are relatively hard and durable and can be machined orcast into desired shapes. An additional advantage of these materials is that they may have lower acoustic attenuation (typically 20dB/cm) than the soft materials used in the prior art (typically 30dB/cm). Thus, the rigid, gas permeable materials can be used to fabricate a variety of ultrasound-transmissive elements that can be positioned between an ultrasound transducer and a patient's body. The gas permeable property required by contact lenses is not required for ultrasound-transmissive elements. Thus, rigid materials having sound speeds within the range of 950 to 2000 meters/second (hereinafter called "rigid, low sound speed materials") are considered acceptable for fabrication of ultrasound-transmissive elements in accordance with the present invention.
A preferred rigid, low sound speed material is sold under the trade name Equalens II by Polymer Technology Corporation of Wilmington, Massachusetts. This material has a measured sound speed within the range specified above. Other gas permeable contact lens materials are described generally in U.S. Patent No. 4,152,508 issued May 1, 1979 to Ellis et al.; U.S. Patent No. 4,424,328 issued January 3, 1984 to Ellis; U.S. Patent No. 4,463,149 issued July 31, 1984 to Ellis; U.S. Patent No. 4,604,479 issued August 5, 1986 to Ellis; U.S. Patent No. 4,625,007 issued November 25, 1986 to Ellis et al; U.S. Patent No. 4,686,267 issued August 11, 1987 to Ellis et al.; U.S. Patent No. 4,826,889 issued May 2, 1989 to Ellis et al.; U.S. Patent No. 4,826,936 issued May 2, 1989 to Ellis; U.S. Patent No. 4,996,275 issued February 26, 1991 to Ellis et al.; and U.S. Patent No. 5,032,658 issued July 16, 1991 to Baron et al. In general, the rigid, low sound speed materials have a composition including polymethylmethacrylate and silicones or fluorosilicones.
A first embodiment of an of the ultrasound-transmissive element fabricated of a rigid, low sound speed material is illustrated in FIG. 1. An ultrasound transducer 10 is mounted within a probe housing 12. The transducer 10 includes an array of transducer elements in a direction perpendicular to the plane of FIG. 1. An ultrasound lens 14 has a flat surface attached to transducer 10 and a convex outer surface. The convex outer surface of lens 14 is cylindrical in a direction perpendicular to the plane of FIG. 1. The lens 14 is typically fabricated of a soft material having a low sound speed, such as silicone rubber. A protective cover 20 fabricated of a rigid, low sound speed material as described above covers the convex outer surface of lens 14. The cover 20 has a shape that matches the outer surface of lens 14 to avoid any air gaps between these elements. The cover 20 may typically have a thickness of about 0.5 millimeter. However, it will be understood that other thicknesses can be utilized. The cover 20 prevents damage to ultrasound lens 14 and does not distort or otherwise interfere with the ultrasound image obtained. The protective cover 20 is typically placed in contact with a patient's body using acoustic gel.
Further embodiments of the ultrasound-transmissive element fabricated of rigid, low sound speed material are described with reference to FIG. 2A. An ultrasound transducer probe 30 is located in contact with a patient's body 32. The probe 30 includes a phased array ultrasound transducer 36 formed of piezoelectric material. Transducer 36 is rotated by a mechanism 38 which either directly or indirectly rotates the transducer utilizing a reciprocating motor or other suitable means. A compound lens 40 includes a convex cylindrical lens element 46 and a concave element 48 which mates to convex element 46. Lens element 46 is typically a silicone rubber such as RTV. The lens element 48 is typically fabricated of urethane rubber. A window assembly is mounted in a housing 41 covered by an epoxy seal 42. The window assembly includes a thin polyester film window 45 and a backing layer 47. The backing layer 47 may be fabricated of urethane rubber. The backing layer 47 may include an RFI screen 49. An acoustic lubricant 51, such as a fluorosilicone oil, is located between lens 40 and backing layer 47 to permit rotation of the transducer 36 and lens 40 relative to the window assembly. The probe assembly is described in more detail in U.S. Patent No. 5,127,410, which is hereby incorporated by reference.
In accordance with a second embodiment of the invention, a protective cover 60 fabricated of a rigid, low sound speed material is affixed to the outer surface of lens element 48. The protective cover 60 prevents physical damage to the soft urethane lens element 48 as the transducer rotates. Furthermore, the cover 60 protects the urethane lens element 48 against degradation by the acoustic lubricant 51. Finally, the cover 60 provides a mechanical backing for the window assembly, thus reducing the possibility of damage to the window by pressure or impact from an external object.
Preferably, the cover 60 is compatible with the acoustic lubricant 51 to ensure that the acoustic lubricant 51 remains in place and does not evaporate or form air pockets during rotation of the lens 40. It has been found that this requirement is met by the rigid, low sound speed material, possibly because of the gas permeable nature of the material.
In a third embodiment of the invention, the window 45 of transducer probe 30 is fabricated of a rigid, low sound speed material. In the prior art transducer assembly, the window 45 was very thin to reduce refraction and reflection of ultrasound energy, and the backing layer 47 was fabricated of urethane rubber. Thus the window assembly was subject to damage by an external object. By contrast, when the window 45 is fabricated of a rigid, low sound speed material, it can be made relatively thick since the acoustic properties are closely matched to those of the human body. Therefore, the potential for damage to the probe assembly is reduced without adversely affecting the ultrasound image. It will be understood that the protective cover 60 and the rigid, low sound speed window 45 can be used separately or in combination in the transducer assembly of FIG. 2.
A fourth embodiment of the invention is shown in FIG. 2B. The ultrasound transducer probe 30 of FIG. 2B has a construction similar to the probe shown in FIG. 2A. Like elements in FIGS. 2A and 2B have the same reference numerals. In the embodiment of FIG. 2B, a lens element 62 which mates to convex lens element 46 is fabricated of a rigid, low sound speed material in accordance with the present invention. The lens element 62 provides similar advantages to the protective cover 60 shown in FIG. 2A and described above. The lens element 62 prevents physical damage to the convex element 46 and is not degraded by the acoustic lubricant 51. In addition, the lens element 62 of a rigid, low sound speed material provides a mechanical backing for the window assembly, thus reducing the possibility of damage to the window.
A fifth embodiment of the invention is illustrated in FIG. 3. A sound pipe 70 is used to transmit ultrasound energy between an ultrasound transducer 72 and a patient's body 74. The sound pipe is fabricated of a rigid, low sound speed material and is configured to change the direction of the ultrasound energy transmitted and received by transducer 72. A surface 75 of the sound pipe 70 is oriented at an angle of 45° angle with respect to the direction of received and transmitted ultrasound energy. The surface 75 is in contact with air or another material 76 of substantially different acoustic impedance than the rigid, low sound speed material of sound pipe 70. This causes ultrasound energy to be reflected from the surface 75 by total internal reflection and to remain coherent.
A simpler version of the sound pipe is a straight section of rigid, low sound speed material that functions as a standoff for spacing an ultrasound transducer from a patient's body. The rigid nature of the sound pipe permits construction of a clip-on unit for imaging in tight quarters, e.g. a finger-tip transducer. The rigid, low sound speed material can be machined to conform to the curvature of the organ being imaged.
Several embodiments of the ultrasound-transmissive element fabricated of rigid, low sound speed material have been shown and described above. It will be understood that the present invention encompasses any ultrasound-transmissive element fabricated of a rigid, low sound speed material. Such elements provide structural rigidity and have a sound speed that facilitates transmission of ultrasound energy to and from the human body with minimal reflection and refraction of ultrasound energy.
While there have been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims

A medical ultrasound transducer assembly, comprising:
an ultrasound transducer (10, 36, 72) for transmitting and receiving ultrasound energy; and
interface means for conducting transmitted ultrasound energy from said transducer (10, 36, 72) to a patient's body and for conducting received ultrasound energy from the patient's body to said transducer, said interface means comprising at least one ultrasound-transmissive element (20, 45, 60, 61, 70) which is fabricated of a rigid, low sound speed material.
A medical ultrasound transducer assembly as defined in claim 1 wherein said interface means further comprises an ultrasound lens and said ultrasound-transmissive element comprises a protective cover on said ultrasound lens.
A medical ultrasound transducer assembly as defined in claim 1 or 2 further including means for rotating said transducer, wherein said interface means further comprises an ultrasound lens affixed to and rotating with said transducer and a fixed window positioned between said ultrasound lens and the patient's body, and wherein said ultrasound-transmissive element comprises a protective cover affixed to said ultrasound lens and extending from said ultrasound lens to said window.
A medical ultrasound transducer assembly as defined in claim 1 or 2 further including means for rotating said transducer, wherein said interface means further comprises an ultrasound lens affixed to and rotating with said transducer and wherein said ultrasound-transmissive element comprises a fixed window positioned between said ultrasound lens and the patient's body.
A medical ultrasound transducer assembly as defined in one of the preceding claims wherein said ultrasound-transmissive element comprises a sound pipe for coupling ultrasound energy between said transducer and the patient's body.
A medical ultrasound transducer assembly as defined in claim 5 wherein said sound pipe includes a surface for changing the direction of propagation of ultrasound energy by total internal reflection within said sound pipe.
A medical ultrasound transducer assembly as defined in one of the preceding claims wherein the speed of sound within said rigid, low sound speed material is in a range of about 950 to 2000 meters/second.
A medical ultrasound transducer assembly as defined in one of the preceding claims further including means for rotating said transducer, wherein said interface means comprises an ultrasound lens affixed to and rotating with said transducer and a fixed window positioned between said ultrasound lens and the patient's body, and wherein said ultrasound-transmissive element comprises an element of said ultrasound lens.
A medical ultrasound transducer assembly as defined in one of the preceding claims wherein said ultrasound-transmissive element comprises an ultrasound lens or lens element.
An element for coupling ultrasound energy to and between an ultrasound transducer and a patient's body comprising a member 20, 45, 60, 61, 70 fabricated of a rigid, low sound speed material.