Method of making a thin film acoustic array

Metal working – Piezoelectric device making

Reexamination Certificate

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C310S336000, C600S459000

Reexamination Certificate

active

06305060

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to devices which are used for measuring the density of members, such as bones, and more particularly to devices which utilize ultrasonic acoustic signals to measure the physical properties and integrity of the members.
2. Description of the Prior Art
Various devices presently exist which may be used to measure the physical properties and integrity of a member such as a bone. Non-invasive density measuring devices can be used to determine cumulative internal damage caused by micro-crushing and micro-fracturing occurring in the bones of humans or animals such as race horses. Additionally, osteoporosis, or loss of bone mineralization, detection in humans and its cure or prevention are increasingly becoming areas of intense medical and biological interest. As the average age of the human population increases, a greater number of patients are developing complications due to rapid trabecular bone loss.
U.S. Pat. No. 3,847,141 to Hoop discloses a device for measuring the density of a bone structure, such as a finger bone or heel bone, to monitor the calcium content thereof. The device includes a pair of opposed spaced ultrasonic transducers which are held within a clamping device clamped on the bone being analyzed. A pulse generator is coupled to one of the transducers to generate an ultrasonic sound wave which is directed through the bone to the other transducer. An electric circuit couples the signals from the receive transducer back to the pulse generator for retriggering the pulse generator in response to those signals. The pulses therefore are produced at a frequency proportional to the transit time that the ultrasonic wave takes to travel through the bone structure, which is directly proportional to the speed of the sound through the bone. The speed of sound through a bone has been found to be proportional to the density of the bone. Thus the frequency at which the pulse generator is retriggered is proportional to the density of the bone.
Another device and method for establishing, in vivo the strength of a bone is disclosed in U.S. Pat. Nos. 4,361,154 and 4,421,119 to Pratt, Jr. The device includes a launching transducer and a receiving transducer which are connected by a graduated vernier and which determine the speed of sound through the bone to determine its strength. The vernier is used to measure the total transit distance between the surfaces of the two transducers.
Lees (Lees, S. (1986) Sonic Properties of Mineralized Tissue,
Tissue Characterization With Ultrasound,
CRC publication 2, pp. 207-226) discusses various studies involving attenuation and speed of sound measurements in both cortical and spongy (cancellous or trabecular) bone. The results of these studies reveal a linear relationship between the wet sonic velocity and wet cortical density, and between the dry sonic velocity and the dry cortical density. The transit times of an acoustic signal through a bone member therefore are proportional to the bone density. Langton, et al. (Langton, C. M., Palmer, S. D., and Porter, S. W., (1984), The Measurement of Broad Band Ultrasonic Attenuation in Cancellous Bone,
Eng. Med.,
13, 89-91) published the results of a study of ultrasonic attenuation versus frequency in the os calcis (heel bone) that utilized through transmission techniques. These authors suggested that attenuation differences observed in different subjects were due to changes in the mineral content of the os calcis. They also suggested that low frequency ultrasonic attenuation may be a parameter useful in the diagnosis of osteoporosis or as a predictor of possible fracture risk.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a low cost acoustic transducer array suitable for use in producing bone density images of the human heel or in other similar applications.
Specifically, the acoustic transducer array includes a piezoelectric film having multiple electrically independent electrodes positioned over a first surface of the film, each electrode opposed by a second electrode on a second surface of the film opposite the first surface of the film. An acoustic signal passing through a particular location of the film produces a corresponding voltage across one of the first electrodes and second electrode. The film may be a polymer such as polyvinylidene fluoride.
Thus it is one object of the invention to provide a simple acoustic transducer array. The electrodes may be applied by low cost metallization techniques providing multiple independent sensors with similar characteristics at low cost.
The transducer array may include a contact plate having multiple conductive terminals affixed to a support surface and positioned adjacent to the first surface of the film with different terminals electrically connected to different portions of different first electrodes.
Thus it is another object of the invention to provide a simple method of making electrical connections to the array of the present invention.
The support surface may be spaced away from the piezoelectric film. Further, the piezoelectric film may be polarized only at the regions near the first electrodes and not at regions between the first electrodes. The conductive terminals may be attached to the first electrodes by means of an acoustically transparent conductor such as metalized mylar.
Thus it is another object of the invention to permit the use of an acoustically transparent transducer array while reducing artifacts caused by reflections of acoustic signals off of the supporting structure of the array. By positioning the conductive terminals and support surface away from the regions of sensitivity, using acoustically transparent conductors and decreasing the piezoelectric effect at the point of contact between the terminals and the film, such artifacts are minimized.
The foregoing and other objects and advantages of the invention will appear from the following description. In this description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention.


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