Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-09-17
2004-04-27
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C029S025350, C310S334000
Reexamination Certificate
active
06726631
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to transducers and transducer arrays and, more particularly, to ultrasonic transducer arrays such as those used in medical imaging. Various aspects of the invention also relate to a method of manufacturing apodized transducers.
A transducer converts energy from one form into another form (for example, from mechanical energy to electrical energy or vice versa). Transducers in audio loudspeakers, for example, convert electrical signals into mechanical vibrations that in turn create audible sound waves. Similarly, transducers are often used to generate high frequency ultrasonic waves for various applications such as medical imaging, non-destructive evaluation (NDE), fluid flow sensing, non-invasive surgery, dentistry and the like. Transducers are widely used in the field of medicine for investigative purposes. For example, an ultrasound transducer makes it possible to observe the development of a baby in its mother's womb. This non-intrusive procedure assists doctors in estimating the date that the child will be born, and in verifying the proper development of the baby by noting, for example, details as tiny as the four chambers of the heart and the development of the lungs. This medical advance is facilitated by ultrasonic sound waves which are transmitted by the transducer and which are variably reflected off of varying types of tissue inside the body. The transducer receives these reflected ultrasonic signals and converts these ultrasonic signals into electrical signals which can be used to generate, for example, a two-dimensional picture of a baby or organs within the human body.
Ultrasonic technology has made large technological advances in recent years. For example, one kind of transducer that has experienced technological advances is a Brightness mode transducer (B-Mode). In a B-mode transducer, the amplitude of reflected pulses (i.e. the strength of a reflected ultrasonic signal) is indicated by the brightness of a dot. By scanning an entire area of interest, multiple dots are combined to map out an image for display. The area of interest can be scanned, for example, by moving the transducer linearly or in an arc like motion. Until the 1970′s, virtually all B-mode imaging systems required several seconds to produce an image. Consequently, these systems were limited to imaging non-moving targets. Since that time, rapid two-dimensional B-mode imaging, known as “real-time scanning”, has enabled visualization of moving targets within the body. In order to create a useful display of the moving targets within the body, methods were developed to rapidly move the acoustic beam throughout the area of interest inside the body. Three primary methods have been developed to rapidly move the acoustic beam: mechanical sector scanners, sequential linear arrays, and phased linear arrays. Mechanical sector scanners rapidly move the acoustic beam using one or more piston transducers which may be rocked or rotated about a fixed axis with, for example, an electric motor. Linear arrays generally consist-of a number of small individual transducers arranged side-by-side in a single assembly. Sequential linear arrays typically produce two-dimensional images in a rectangular format by transmitting on each of the array elements (or small groups of elements) and receiving the echo information with the same elements. Phased array scanners are the most sophisticated real-time systems. Phased array systems produce images by rapidly steering the acoustic beam through the target by electronic rather than mechanical means. The phased array scanners produce the pie-shaped image commonly seen in medical ultrasound applications, and popularly known as the “sector-scan”. These three systems have been generally described by Somer and Von Ramm.
Obviously, the ability to have a high quality resolution is important to producing accurate and readable images. There are three aspects of resolution which are relevant to ultrasound imaging: spatial resolution, contrast resolution, and temporal resolution. Spacial resolution generally refers to the ability to distinguish registrations in the displayed image of objects that are close together. Contrast resolution generally refers to the ability to produce distinguishable differences in the brightness of two different types of materials which would have slightly different echogenicities. For example, a tendon might reflect at a different brightness than a muscle. Temporal resolution refers to the ability to display an image when the object being imaged is moving.
One of the factors that interferes with achieving high resolution in these areas is the fact that the ultrasound signal undergoes attenuation and dispersion as it progresses deeper into tissue. This degradation is governed by the Kramer-Kronig relationships. See, M. O'Donnel, E. T. Jaynes, and J. G. Miller,
Kramer-Kronig, Relationship Between Ultrasonic Attenuation And Phase Velocity
, J. Acoust. Soc. Am. 69(3), March, 1981, pp. 696-701. One method of improving resolution is to frequency apodize the transducer aperture. A previous attempt to achieve this frequency apodization is described by U.S. Pat. No. 5,902,242. In this patent, the central zone of the array element is thin (elevation direction) and gradually thickens nearer the edges of the aperture. Two ultrasonic images are created using a first relatively high ultrasonic imaging bandwidth transmit pulse and a second narrower bandwidth transmit pulse. The first pulse activates the full aperture and creates an image that has relatively high axial resolution and relatively low elevational resolution. The second pulse activates the narrower width portion of the aperture and creates an image that has relatively lower axial resolution and a higher elevational resolution at ranges spaced from the geometric focus. Combining these two frames yields an image which has both enhanced spatial and contrast resolution. This method, however, offers some significant manufacturing challenges. The general functionality disclosed in U.S. Pat. Nos. 5,902,242 and 5,479,926 are incorporated herein by reference.
Another factor that interferes with achieving higher resolution is the existence of “side lobes” in the ultrasonic beam. When an ultrasonic beam passes through a human body or other medium, “blurring” occurs as the beam is defracted (i.e. bent) creating side portions (i.e. “side lobes”) which accompany the desired main lobe of the ultrasonic beam. The side lobes act as interference and tend to degrade the ability to achieve high resolution. Past attempts have been made to suppress the side lobes. One conventional method of suppressing side lobes is to apply an amplitude apodization function to the electrical signal, usually a Gaussian or Hanning function, to shape the electrical signals received by the array. (See, for example, Apodization of Ultrasound Transmission, U.S. Pat. No. 4,841,492 incorporated herein by reference.) An apodization function is applied to smoothly taper down to zero the edges of a sampled region of a signal. This electrical signal apodization has several undesirable aspects. For example, while in-plane (azimuth direction) electrical signal apodization is possible, out-of-plane (elevation direction) electrical signal apodization may not be possible in a 1D arrays because one signal connects across the whole elevation aperture. Although out-of-plane electrical signal apodization could possibly be done for 2D arrays, where the elevation aperture is discretized and can be electrically addressed individually, this may be quite difficult to achieve due to the electrical complexity, muxing, etc.
Another method of achieving amplitude apodization is to place a thin sheet of acoustic blocking layer over the front surface of the transducer to substantially block the ultrasonic wave emission from a portion of the front surface area, thus defining an inactive area. (See, generally, Ultrasonic Transducer Apodization Using Acoustic Blocking Layer, U.S. Pat. No. 5,285,789, incorporated
Chandran Sanjay
Hatangadi Ram
Pesque Patrick
Armstrong Teasdale LLP
GE Parallel Designs, Inc.
Jaworski Francis J.
Vogel Peter J.
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