Apparatus and method for imaging small cavities

Claims

What is claimed is:

1. An imaging device for emitting ultrasonic acoustic waves and providing a useable image in response to detection of reflections of said ultrasonic acoustic waves, said imaging device comprising:

a body for insertion into a cavity;

an array of transducer elements mounted to said body for generating first electrical signals containing imaging information in response to said reflections of said ultrasonic acoustic waves;

a cable connecting said body to an environment external of said cavity and including at least one signal channel for transporting said first electrical signals;

means mounted on said body and proximate to said array of transducer elements for receiving said first electrical signals from said array of transducer elements and converting said first electrical signals to second electrical signals that may be transmitted along said at least one channel in said cable without significant loss of imaging information;

a processor responsive to said second electrical signals from said cable for providing imaging data; and

a display responsive to said imaging data for providing a visual image of said cavity and its surrounding structure.

2. An imaging device as set forth in claim 1 wherein said processor provides excitation signals to said array of transducer elements via at least one channel in said cable; and

said means being responsive to said processor for directing each of said excitation signals to an appropriate at least one of the transducer elements in said array of transducer elements.

3. An imaging device as set forth in claim 2 wherein said means includes means for providing low impedance paths through transducer elements in said array of transducer elements adjacent said at least one of the transducer elements that receives one of said excitation signals.

4. An imaging device as set forth in claim 2 wherein said processor incorporates further means for applying a plurality of successive excitation signals to said at least one of the transducer elements in said array of transducer elements; and

said processor including means (1) for averaging to one signal the plurality of successive second signals resulting from the acoustic reflections generated by said plurality of successive excitation signals.

5. An imaging device as set forth in claim 4 wherein said processor includes means (2) for accumulating and processing all of the averaged signals from said array of transducer elements with sufficient speed so as to maintain an apparent realtime image on said display.

6. An imaging device as set forth in claim 1 wherein said body is fitted to one end of a conventional catheter and includes means for accommodating conventional uses of said catheter.

7. An imaging device as set forth in claim 6 wherein said accommodating means includes a central bore through said body that is in direct communication with a guide wire lumen in said conventional catheter.

8. An imaging device as set forth in claim 1 wherein said means are transimpedance amplifiers and said first electrical signals are low current signals from said array of transducer elements where said array of transducer elements are composed of material that act as a high impedance source of said low current signals, said transimpedance amplifiers converting said low current signals to said second signals which are high voltage signals for transmission through said at least one channel in said cable to said processor.

9. A device as set forth in claim 1 wherein said array of transducer elements comprises a continuous piezoelectric material fitted over a plurality of conductive strips mounted on said body for receiving said first electrical signals from said piezoelectric material such that each conductive strip cooperates with an area of said continuous piezoelectric material overlying said strip to define one of said transducer elements in said array.

10. A device as set forth in claim 9 wherein said continuous piezoelectric material has a form of a ring and has an outside diameter of approximately four millimeters or less.

11. A device as set forth in claim 9 wherein the piezoelectric material of said array of transducer elements is a polymer selected from the group of PVDF, P(VDF-TrFE), P(VDF-TFE), a composite material consisting of a polymer and a ceramic such as PZT, or a depositable material such as ZnO.

12. A method of imaging characteristics of a small cavity and surrounding structure using a probe assembly provided with an array of transducer elements and located at the end of a transmission line, said method comprising the steps of:

inserting said probe assembly into small cavity,

emitting ultrasonic signals into said small cavity and surrounding structure by selectively exciting at least one of said transducer elements,

detecting reflections of said ultrasonic signals by receiving first electrical signals generated by said reflections impinging on at least one of said transducer elements,

converting said first electrical signals to second electrical signals suitable for transmission on said transmission line,

transmitting said second electrical signals on said transmission line to an area external from said small cavity,

processing said second electrical signals into image data, and

displaying said image data on a visual display.

13. A method as set forth in claim 12 wherein the elements in said array of transducer elements are excited in a predetermined sequence and repeating said predetermined sequence at a cycle frequency allowing the displaying of said image data to simulate a real-time performance.

14. A method as set forth in claim 12 wherein at least one element is excited at each step in said predetermined sequence and each step is repeated a plurality of times in one cycle of said predetermined sequence, said method including the additional step of, averaging a plurality of said second electrical signals resulting from the repetition of each step in said predetermined sequence and thereby providing an averaged electrical signal with a higher dynamic range than any one of said second electrical signals.

15. In a system for approximating real-time images of a structure including a probe assembly having an array of transducer elements for collecting image information and a remotely located high-speed memory for storing said image information, a method comprising the steps of:

a. exciting at least one of said array of transducer elements into mechanical vibration a plurality of times so as to generate ultrasonic waves for propagating through said structure;

b. detecting and averaging electrical signals derived from the excitation of at least one of said array of transducer elements in response to the reflections of said ultrasonic waves impinging on said array of transducer elements;

c. storing said average of said detected signals in said high-speed memory;

d. incrementing to a next at least one of said array of transducer elements in accordance with a predetermined sequence and repeating steps (a) through (c);

e. repeating steps (a) through (d) until end of said sequence;

f. finding values from the averages stored in said high-speed memory for a plurality of focal points spatially distributed about said probe assembly; and

g. mapping onto a display screen said focal points such that said values for all the focus points required for each frame of said display screen may be determined at a sufficiently high rate of speed so as to maintain the visual appearance of a real-time image.

16. The method of claim 15 wherein said array of transducer elements is in part composed of an acoustically continuous piezoelectric material of high electrical impedance characteristics, said method including the step of:

h. providing low impedance paths that effectively shunt the high impedance characteristics of those transducer elements adjacent said at least one transducer elements in steps (a) and (b) during the time said at least one transducer elements is active so as to provide the best beam pattern for determining the focal points of step (f).

17. The method of claim 15 wherein said structure includes a small cavity having approximately the size of a human coronary artery and said method includes the step of:

i. inserting said probe assembly into said small cavity.

18. The method of claim 15 wherein the step of detecting and averaging signals derived from the excitation of at least one of said array of transducer elements in response to the reflections of said ultrasonic waves impinging on said array of transducer elements includes the steps of:

j. detecting reflections of said ultrasonic signals by receiving first electrical signals generated by said at least one transducer element in response to said reflections impinging thereon;

k. converting said first electrical signals to second electrical signals suitable for transmission on a transmission line connecting said probe assembly to said remotely located high-speed memory; and

l. transmitting said second electrical signals on said transmission line to said remotely located high-speed memory.

19. The method of claim 18 wherein the averaging of the signals derived from said array of transducer elements occurs as the signals are generated so as to maintain a running average.

20. The method of claim 18 wherein said first electrical signals are low current signals and the material comprising said transducer elements serves as a high impedance source of said first electrical signals and said second electrical signals are high voltage signals suitable for transmission over said transmission line without significant loss of image information contained in said second electrical signals.

21. A system for providing images of the interior of a small cavity and surrounding structure comprising:

a probe assembly having an array of transducer elements for generating (1) ultrasonic waves in response to excitation signals and (2) imaging signals in response to reflections of said ultrasonic waves impinging on said array;

means (1) remote from said probe assembly for generating said excitation signals and control signals;

sequencing means (2) on-board said probe assembly responsive to said control signals for selectively and sequentially distributing said excitation signals to said array of transducer elements and providing a plurality of said excitation signals in succession to a same at least one transducer element in said array of transducer elements;

means (3) responsive to the imaging signals derived from the reflections of said plurality of successive excitation signals impinging on said array of transducer elements for averaging said imaging signals and providing an averaged imaging signal;

a processor responsive to said averaged imaging signal for providing display data; and

a display responsive to said display data for providing a visual image.

22. A system as set forth in claim 21 wherein said array of transducer elements comprises an acoustically continuous piezoelectric material, said system including:

means (4) for providing low impedance paths that effectively shunts those transducer elements that are at least adjacent to said at least one transducer element that receives said excitation signal.

23. A system as set forth in claim 21 wherein said processor includes means (5) for providing new display data to said display at sufficient speed so that the image provided by said display simulates a real-time image.

24. A system as set forth in claim 21 including:

means (6) on-board said probe assembly for receiving said imaging signals directly from said array of transducer elements and converting said imaging signals from a first form to a second form such that the converted imaging signals may travel over a transmission line in a cable without substantial loss of imaging information.

25. A system as set forth in claim 21 wherein said excitation signals are delivered to said array of transducer elements via a cable having a number of transmission channels that is less than the number of steps in a predetermined sequence of excitation of said elements that provides a full set of image data for said display, and wherein

said means (7) on-board said probe assembly distributes said excitation signals from said cable to said array of transducer elements in order to excite selected ones of said array of transducer elements in said predetermined sequence.

26. A system as set forth in claim 21 wherein said array of transducer elements comprises an acoustically continuous piezoelectric material fitted over a plurality of conductive strips mounted to a body portion of said probe assembly for delivering said excitation signals to said material and receiving said imaging signals from said material such that each conductive strip cooperates with an area of said material overlying said strip to define one of said transducer elements in said array.

27. In an ultrasonic imaging system, a probe assembly responsive to a source of excitation signals for insertion into a small cavity, said probe assembly comprising, in combination:

a body;

a transducer material forming an acoustically continuous surface and mounted to said body for generating ultrasonic acoustic waves in response to said excitation signals from said source and for generating imaging signals in response to the impinging of reflections of said ultrasonic acoustic waves;

a plurality of conductive traces on said body and underlying said transducer material;

a ground plane overlying said transducer material;

a plurality of elements forming an array, each of said elements comprising a conductive trace, a portion of said transducer material overlying said conductive trace and said ground plane such that application of said excitation signal from said source to at least one of said conductive traces causes an area of said transducer material proximate to or overlying said conductive trace to mechanically vibrate and generate said ultrasonic acoustic waves and reflections of said ultrasonic imaging devices impinging on said transducer material causes the generation of said imaging signals on at least one of said conductive traces; and

means (1) on-board said body for processing said excitation signals prior to their delivery to at least one of said conductive traces and processing said imaging signals prior to their delivery to a remote imaging device.

28. A probe assembly as set forth in claim 27 including:

means (2) on-board said body for providing a broad beam pattern by effectively shunting at least those elements immediately adjacent the element or elements including said at least one of said conductive traces receiving an excitation signal from said source.

29. A probe assembly as set forth in claim 28 wherein said means (2) also effectively shunts at least those elements immediately adjacent the element or elements generating an imaging signal in response to the impinging of reflections of said ultrasonic acoustic waves.

30. A probe assembly as set forth in claim 27 wherein the body is comprised of material having high acoustic impedance and said probe has a shape such that the resonant effects which occur due to energy reverberating through said body in response to the mechanical vibration of an element do not interfere with the acoustic behavior of said transducer material in the range of frequencies used to generate an ultrasonic image.

31. A probe assembly as set forth in claim 30 wherein said body is composed of material having high acoustic impedance and said transducer material is mounted on a hollow cylindrical portion of said body where the wall of said cylindrical portion has a thickness d that is equal to or less than V/2f, where f is the nominal frequency of the acoustic waves generated by said plurality of elements and V is the velocity of said acoustic waves through the material comprising said body.

32. A probe assembly as set forth in claim 27 wherein said ultrasonic imaging system provides said probe assembly with said excitation pulses in a serial format and said means (1) includes means (3) for distributing said excitation pulses to said plurality of elements in a predetermined sequence.

33. A probe assembly as set forth in claim 27 wherein said means (1) includes means (4) for converting said imaging signals to a format suitable for transmission over a cable without significant loss of imaging information.

34. A probe assembly as set forth in claim 33 wherein said transducer material has a high electrical impedance, said cable has a low electrical impedance and said means (4) is a transimpedance device in the range of frequencies used to generate an ultrasonic image.

35. A probe assembly as set forth in claim 27 wherein said body includes means (5) for attaching said probe assembly to an end of a conventional catheter such that whatever procedure and devices normally used with said conventional catheter are unaffected by the presence of said probe assembly.

36. An imaging device for emitting ultrasonic acoustic waves and providing a useable image in response to detection of reflections of said ultrasonic acoustic waves, said imaging device comprising:

a body for insertion into a small cavity;

an array of transducer elements mounted to said body for generating first electrical signals in response to said reflections of said ultrasonic acoustic waves and emitting said ultrasonic acoustic waves in response to second electrical signals;

a cable connecting said body to an environment external of said cavity and having a number of signal channels for transporting said first and second electrical signals where said number of signal channels is less than the number of elements in said array;

a signal processor for receiving said first electrical signals from said cable and transmitting to said cable said second electrical signals; and

distribution means mounted on said body for serially receiving said second electrical signals from said cable and applying said second electrical signals to said array of transducer elements in a predetermined sequence of selected elements, where the number of steps in the sequence is greater than the number of signal channels in said cable.

37. An imaging device as set forth in claim 36 where said array of transducer elements comprises a plurality of conductive traces underlying a continuous piezoelectric material, said distribution means delivering each of said second electrical signals to at least one of said conductive traces, thereby causing an area of said continuous piezoelectric material overlying said conductive trace to vibrate at ultrasonic frequencies.

38. An imaging device as set forth in claim 37 where said continuous piezoelectric material is characterized by a high electrical impedance and said imaging device includes means mounted to said body for effectively shunting at least those elements immediately adjacent the element or elements receiving one of said second signals from said signal processor.

FIELD OF THE INVENTION

The present invention relates generally to the field of ultrasonic imaging, and more particularly to ultrasonic imaging to determine various characteristics of relatively small cavities and surrounding structures.

BACKGROUND OF THE INVENTION

In the United States and many other countries, heart disease is the leading cause of death and disability. One particular kind of heart disease is atherosclerosis, which involves the deposition of fatty material on the inside of vessel walls throughout the body (commonly called "plaque"). As the plaque collects, the artery narrows and blood flow is restricted. If the artery narrows too much, the heart muscle nourished by the artery receives insufficient oxygen and a myocardial infarction or "heart attack" can occur. Atherosclerosis can occur throughout the human body, however, it is most life threatening within the coronary vasculature.

Physicians have a wide range of tools at their disposal to treat patients with coronary artery disease. Coronary artery bypass grafts or "open heart" surgery can be performed to bypass blocked artery segments. Other, less invasive procedures are available. For example, some blockages may be dissolved by chemical treatment. Alternatively, a procedure known as percutaneous transluminal coronary angioplasty (hereinafter "PTCA") may be performed in which a catheter with an expandable section on its end is placed within the narrowed artery and inflated to compact the plaque against the vessel wall, thereby relieving the blockage.

No matter what method is used to treat coronary artery disease, it is necessary for physicians to obtain quantitative information on the condition of the vasculature within the heart. Traditionally, coronary angiography has been the method of choice. Coronary angiography involves the placement of the end of a catheter at the beginning of the coronary vasculature. A small amount of radiopaque dye is injected, and a X-ray motion picture is taken while the dye is pumped through the vessels. The physician then examines the pictures and looks for any telltale narrowing of the blood flow opacified by the radiopaque dye. By the number and degree of such narrowing, the course of treatment can be determined.

Angiography has the extreme limitation of indicating only where the blood is within the vessel; it reveals nothing of the condition of the inside of the vessel and the vessel wall itself. Furthermore, most angiography machines present virtually only one-dimensional projections of where blood flow exists. Because of this imaging limitation, the complex structures within the coronary vasculature often exhibit quite ambiguous images.

Recently, imaging of soft tissue such as gross cardiac structures has provided physicians with diagnostic images having quality that is unavailable from conventional techniques using X-ray radiation. In particular, magnetic resonance imaging (MRI) and ultrasound have become important diagnostic tools for cardiac assessment. Although MRI has the ability to image blood vessels, the image resolution is not sufficient to allow assessment of the condition of the walls of the vessel. Conventional ultrasound scanning also suffers from lack of resolution. More recently, high frequency (hence, high resolution) ultrasound has been used during open heart surgery to access the coronary arteries. This method requires the opening of the chest cavity to expose the heart surface and is hence limited in its application.

In an even more recent development, in vivo ultrasonic imaging of the human body creates the potential for access to a wealth of information regarding the condition of a patient's vasculature that is currently only at best indirectly available from other sources. The information received from in vivo imaging may be used as a diagnostic tool to help determine patient treatment, or as a surgical tool, supplementing angiography in PCTA.

In vivo ultrasonic imaging from within the heart has been described in U.S. Pat. No. 3,958,502 to Bom. In order to provide for ultrasonic imaging inside the human body, the Bom patent provides an array of small transducer elements which may be introduced into the body by way of catherization. The array of elements is excited at ultrasonic frequencies and the reflections or echos of the generated ultrasonic acoustic waves are detected by the piezoelectric properties of the transducers. Unfortunately, due to the nature of the material used for the transducers, the array of elements cannot be made small enough to allow passage into small areas such as the coronary arteries. Therefore, use of the Bom device is limited to within the heart chambers and the associated great arteries.

An additional limitation of the Bom device is the poor resolution caused by a sparse distribution of transducer elements. Piezoelectric materials of the type used by Bom (e.g., ceramics) have a practical limitation in size reduction. Because of this size limitation and the fact that the maximum resolution of the transducer array is limited by the center-to-center spacing of adjacent elements, the Bom device is inherently limited in the quality of its image resolution.

A further limitation of the Bom device is the fixed delays it provides for focusing an image. Such fixed delays do not provide satisfactory images for identification of tissue structures. For a satisfactory image, a dynamic focusing feature is needed to provide an optimal focus at a plurality of points in the imaging plane. One approach to implementing such a dynamic focusing feature is a so-called "synthetic focus" or "synthetic-aperture" approach disclosed in U.S. Pat. No. 4,325,257 to Kino et al.

For many diagnostic and therapeutic purposes, in vivo ultrasonic imaging must simulate real-time performance. To achieve diagnostic or therapeutic quality images in small cavities while maintaining real-time performance is a formidable task and one which applicants believe has not previously been attained.

SUMMARY OF THE INVENTION

It is a general object of the invention to provide diagnostic quality, virtual real-time ultrasonic images of small cavities and their associated surrounding structures from within the cavities.

It is a further object of the invention to provide a method of providing diagnostic quality, virtual real-time ultrasonic images that is sufficiently flexible to accommodate a range of ultrasonic imaging requirements from within small cavities.

It is a further object of the invention to provide an array of transducer elements for generating ultrasonic imaging data that is small enough to enter small cavities, yet also exhibit controlled behavior and is manufacturable on a commercial basis. In this connection, it is a related object of the present invention to maintain a high degree of sensitivity to signals from weak reflectors of ultrasonic signals, such as human vascular tissues, while maintaining the small size of the array of transducers.

It is another object of the invention to provide the physician with the ability to accurately position the array of transducers within the imaging area.

It is yet another object of the invention to minimize the number of wires required to connect the in vivo portion of the ultrasonic imaging device of the invention to an in vitro processing stage. In this connection, it is a related object of the invention to distribute the control of the excitation of the array of transducer elements between in vivo and in vitro sites.

It is still another object of the invention to electrically isolate the in vivo portion of the imaging device of the invention in order that it is safe for use in human imaging applications. In this connection, it is a related object of the invention to provide operation of the imaging device without causing significant risk to humans from excessive localized heating or radiation.

A still further object of the present invention is to operate at very low power dissipation in vivo in order to prevent heating of surrounding tissue and expansion of parts.

It is a further object of the present invention to provide an imaging device whose in vivo portion may be mounted to a positioning device such as a catheter, which allows the use of, for example, conventional guiding catheters and guidewires. In this connection, it is a related object of the present invention that the imaging device be suitable for incorporation into recent catheter systems, and allowing for the continued use of, for example, guiding catheters and guidewires, in conjunction with catheter-based diagnostic and therapeutic procedures such as angioplasty, regional therapy for dissolving plaque and the like.

Briefly, the invention provides an in vivo imaging device for producing realtime images of small, moving or stationary cavities and surrounding tissue structure that is uniquely and advantageously constructed using a conventional catheter assembly fitted at its end with a probe assembly for transmitting and receiving ultrasonic signals from elements of an array of ultrasonic transducers incorporated into the probe assembly. The transducer elements are selected and controlled by an in vitro electronic signal processing and imaging unit wh