Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2002-06-13
2004-08-17
Imam, Ali (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C029S025350, C310S334000
Reexamination Certificate
active
06776762
ABSTRACT:
FIELD OF INVENTION
This invention relates to the design and construction of piezocomposite ultrasound arrays in conjunction with integrated circuits, and in particular to improvements in thermal and crosstalk performance in piezocomposite ultrasound array and integrated circuit assemblies.
BACKGROUND OF THE INVENTION
Diagnostic ultrasound is an established and growing medical imaging modality. The configuration of a typical system includes a handheld probe connected to a host computer and image display unit by an umbilical cable carrying power, control signals and image data. Currently handheld probes using one-dimensional acoustical transducer arrays with up to 128 elements are the standard in the industry, although partial and full two dimensional arrays are well into development with several vendors.
Medical ultrasound systems transmit a short pulse of ultrasound and receive echoes from structures within the body. The handheld probes are most often applied to the skin using a coupling gel. Specialty probes are available for endocavity, endoluminal and intraoperative scanning. Almost all systems on the market today produce real-time, grayscale, B-scan images. Many systems include colorflow imaging.
Improved image quality requires the use of matrix (n×m) arrays with a thousand or more elements. As element numbers increase and their dimensions grow smaller, limitations to present fabrication technologies arise. Cost, ergonomics, produceability and reliability are important issues. Connecting an integrated circuit directly to the array elements alleviates these problems.
All linear arrays currently on the market use piezoelectric materials as the transducing mechanism from electrical signals to ultrasound (transmitter) and ultrasound back to electrical signals (receiver). The signals are generally in the form of short pulses or tone bursts.
Referring to prior art
FIG. 1
, most high performance acoustical arrays use a piezocomposite material, which is fabricated by a “dice and fill” technique as is suggested, for example, in Smith et al's U.S. Pat. No. 5,744,898. The piezocomposite transducer array structure
9
, having a planar array of piezocomposite transducer elements
10
, provides improved bandwidth and efficiency as well as reduced crosstalk or interference between adjacent elements
10
, relative to older designs.
Pieces of native ceramic such as Type 3203HD made by CTS Piezoelectric Products, or PZT-5H made by Morgan-Matroc, are diamond sawed in a crosscut pattern to yield pillars
18
, there being multiple pillars in each piezocomposite element
10
. The intra-element spaces
28
between the pillars are filled with a polymer, such as DER332 epoxy made by Dow Chemical. The inter-element spaces
24
are often left air-filled or are filled with a sound-absorbing polymer. Top electrode
30
is the common electrical connection between elements
10
. Bottom electrodes
22
are delineated for each array element
10
and are used as the electrical connections to the piezocomposite material.
In a beamsteered transducer array, the dimensions of elements
10
are typically less than a wavelength (in water or tissue) in the steering dimension. For example, in a 3.5 MHz (1×128) array the element width is between 0.2 to 0.4 mm with a center to center spacing of 0.5 mm for a total array length of approximately 64 mm. In the other dimension, the element dimensions are a tradeoff between resolution and depth of focus. For a 3.5 MHz array, this dimension is typically 12 to 15 mm.
As the frequency of the array increases, element size decreases, as does element thickness, however, the aspect ratio remains constant. Other methods of fabrication such as laser milling or scribing, etching or deposition are under development. At present, they are not well accepted.
As is more fully described in the parent applications, in a fully assembled transducer scanner head, there is a backing behind the array and its supporting ASIC that provides mechanical support and acoustical attenuation. When a piezoelectric transducer is electrically pulsed, two acoustical pulses are generated that travel in opposite directions. The pulse traveling out of the scanhead is desired, while the pulse propagating into the backing is unwanted and is absorbed by the backing.
One or more matching layers are placed in the scanner head in the path of the desired pulse, to improve the coupling of energy from the piezocomposite into the body of the subject by matching the higher acoustical impedance piezocomposite to the lower acoustical impedance of the body. This matching layer functions in the same way as the anti-reflection coating on an optical lens.
The system electronics focus the pulse in the scanning plane dimension. A simple convex lens forms the front surface of the scanner head that contacts the patient's skin. It provides a fixed focus to the sound pulse in the “out-of plane” dimension, which is perpendicular to the scanning plane.
Modern systems impose increasingly stringent requirements on the acoustical arrays. Parameters that characterize typical medical ultrasound arrays are described in more detail in the parent applications, but with regard to this disclosure include in particular; crosstalk. Crosstalk is the interference of signals between array elements
10
. The interference may be electrical, mechanical or acoustical. It is expressed in dB relative to the nearest neighbor element. Crosstalk in a well-constructed array is better than −30 dB.
Extension of the several inherent technologies to matrix transducer arrays is underway at most transducer and system manufacturers. As the number of elements increases and their size decreases, however, the existing approaches may no longer be feasible or practical. Processing time, touch labor, yield, reliability and cost become limiting issues and new processes are required.
In
FIG. 2
, a cross-section of a prior art piezocomposite integrated array, the array elements
10
are electrically and mechanically connected to integrated circuit (IC) substrate
32
with electrically conductive bumps
34
using metallized pads
36
on IC
32
to form a complete electrical circuit. Integrated circuit substrate
32
is typically composed of silicon, although other semiconductors may be used. Conductive bumps
34
may be composed of solder or a conductive polymer such as silver epoxy.
Placing an integrated circuit (IC) directly behind an ultrasound array is a well-known solution to the problem of many long cables connecting the array elements in a scanhead to electronics in a separate electronics console. The preferred method places an IC with unit cells of similar dimensions to the array elements
10
immediately behind the array elements. The corresponding elements and unit cells are bonded electrically and mechanically to the IC using micro-solder balls. The space between the IC and the array can be filled with a material such as epoxy for improved mechanical strength. However, this leads to excessive crosstalk (signal interference) between the array elements. Crosstalk leads to poor dynamic range and loss of contrast in the images.
One solution to this problem is simply to leave an air space between the array and IC. The air gap effectively prevents sound transmission into the IC. By limiting the contact area to the micro solder balls alone, which area is much smaller than the wavelength of sounds, crosstalk is effectively eliminated. Use of an air gap, however, results in a relatively narrow bandwidth transducer array, typically with 30 to 60% fractional bandwidth. Modern medical ultrasound systems require bandwidths of 100 to 120%.
In a United States government agency funded program, a real-time, 3D, ultrasound camera intended for Army medic use on the front lines was designed and its feasibility proven. In this camera, an acoustical lens was used to image a volume onto a 128×128 (16,384 element), 5 MHz matrix array. Each element of the piezocomposite array had a custom integrated circuit bump-bonded directly behind it using micro-so
Erikson Kenneth R.
Lewis George K.
White Timothy E.
BAE Systems Information and Electronic Systems Intergration Inc.
Imam Ali
Maine & Asmus
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