U.S. patent application number 10/985865 was filed with the patent office on 2005-05-12 for ultrasonic bone testing with copolymer transducers.
Invention is credited to Cabral, Dick, Kubierschky, Klaus, MacGibbon, Glen, Park, Kyung, Stein, Jay, Thompson, Mitchell, Wang, Hong, Wilson, Kevin.
Application Number | 20050101862 10/985865 |
Document ID | / |
Family ID | 33518699 |
Filed Date | 2005-05-12 |
United States Patent
Application |
20050101862 |
Kind Code |
A1 |
Wilson, Kevin ; et
al. |
May 12, 2005 |
Ultrasonic bone testing with copolymer transducers
Abstract
Ultrasonic bone testing apparatus including a pair of spaced
piezoelectric copolymer transducers for transmitting and receiving
ultrasonic energy through a bone-containing portion of a human or
other animal disposed between the transducers, and circuitry for
detecting an electrical signal generated by the receiving
transducer in response to reception of ultrasonic energy. The
transducers are disks of the copolymer supported by rigid rings
spaced inwardly of their peripheries. A method of determining a
characteristic of bone in a bone-containing portion of an animal
includes positioning a pair of piezoelectric copolymer ultrasonic
transducers respectively on opposite sides of, and ultrasonically
coupling both transducers to, the animal portion, and transmitting
ultrasonic energy through the animal portion including the bone to
be tested from one transducer to the other. The animal portion may
be a human heel.
Inventors: |
Wilson, Kevin; (Cambridge,
MA) ; Wang, Hong; (Audubon, PA) ; Stein,
Jay; (Boston, MA) ; Thompson, Mitchell;
(Exton, PA) ; Kubierschky, Klaus; (North Reading,
MA) ; Park, Kyung; (Berwyn, PA) ; Cabral,
Dick; (Tewksbury, MA) ; MacGibbon, Glen;
(Berwyn, PA) |
Correspondence
Address: |
Christopher C. Dunham
c/o Cooper & Dunham LLP
1185 Avenue of the Americas
New York
NY
10036
US
|
Family ID: |
33518699 |
Appl. No.: |
10/985865 |
Filed: |
November 10, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10985865 |
Nov 10, 2004 |
|
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|
09595074 |
Jun 16, 2000 |
|
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6835178 |
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60140638 |
Jun 23, 1999 |
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Current U.S.
Class: |
600/442 ;
600/449 |
Current CPC
Class: |
A61B 8/0875 20130101;
A61B 8/4281 20130101; A61B 8/4483 20130101 |
Class at
Publication: |
600/442 ;
600/449 |
International
Class: |
A61B 008/00 |
Claims
1. Ultrasonic bone testing apparatus comprising a pair of
ultrasonic transducers at least one of which comprises a
piezoelectric copolymer; mounting structure supporting the
transducers in facing spaced relation to each other, so as to be
respectively positionable on opposite sides of and both coupled
ultrasonically to an animal portion containing a bone, for
respectively transmitting ultrasonic energy through and receiving
ultrasonic energy transmitted through said animal portion including
the bone; and electrical circuitry connected to the transducers to
energize one transducer to transmit ultrasonic energy and to detect
an electrical signal generated by the other transducer in response
to received ultrasonic energy.
2. Apparatus as defined in claim 1, wherein the mounting structure
includes a support for positioning the animal portion between the
transducers, and a device for coupling the transducers
ultrasonically to the animal portion; and wherein each of the
transducers comprises a piezoelectric copolymer transducer.
3. (canceled)
4. Apparatus as defined in claim 2, wherein said coupling device
comprises a pair of pads, respectively disposed in contact with
said transducers, and respectively engageable with opposed surface
regions of an animal portion positioned in said support as
aforesaid.
5. (canceled)
6. Apparatus as defined in claim 2, wherein said coupling device
includes a container for holding a body of a coupling fluid in
which the animal portion is immersed when positioned by the support
as aforesaid and with which said transducers are in ultrasonic
energy transmitting contact.
7. Apparatus as defined in claim 2, in which the animal portion is
a human heel containing a calcaneal bone, said support positioning
said heel, and said transducers and coupling device being disposed,
so that ultrasonic energy transmitted from said one transducer to
said other transducer passes through the calcaneal bone.
8. Apparatus as defined in claim 2, in which said electrical
circuitry is arranged to use the detected electrical signal for
deriving a value representative of the speed of sound through the
bone through which the ultrasonic energy is transmitted as
aforesaid.
9. Apparatus as defined in claim 2, in which said electrical
circuitry is arranged to use the detected electrical signal in
deriving a value representative of broadband ultrasonic attenuation
in the bone through which the ultrasonic energy is transmitted as
aforesaid.
10. A method of determining a characteristic of a bone in a
bone-containing portion of an animal comprising disposing a pair of
ultrasonic transducers at least one of which comprises a
piezoelectric copolymer respectively on opposite sides of, and
ultrasonically coupling both transducers to, a bone-containing
animal portion; electrically energizing one transducer to transmit
ultrasonic energy through the animal portion including the bone,
such that the transmitted ultrasonic energy is received and
converted to an electrical signal by the other transducer;
detecting the electrical signal; and using the detected signal to
derive a value representative of the bone characteristic to be
determined.
11. A method according to claim 10 wherein each of the transducers
is a disk of piezoelectric copolymer.
12. (canceled)
13. Apparatus as defined in claim 1, wherein at least one of said
transducers is a piezoelectric copolymer transducer having a curved
surface for focusing.
14. Apparatus as defined in claim 1, wherein at least one of said
transducers is a copolymer array transducer.
15. A method according to claim 10, wherein at least one of said
transducers is a copolymer array transducer.
16. A method according to claim 15, wherein the step of deriving a
value includes correcting for phase cancellation.
17. (canceled)
18. Ultrasonic bone testing apparatus comprising a pair of
ultrasonic transducers at least one of which comprises a
poly(vinylidene fluoride-trifluoroethylene) copolymer; mounting
structure supporting the transducers in facing spaced relation to
each other, so as to be respectively positionable on opposite sides
of and both coupled ultrasonically to an animal portion containing
a bone, for respectively transmitting ultra-sonic energy through
and receiving ultrasonic energy transmitted through said animal
portion including the bone; and electrical circuitry connected to
the transducers to energize one transducer to transmit ultrasonic
energy and to detect an electrical signal generated by the other
transducer in response to received ultrasonic energy.
19. Apparatus as defined in claim 18, wherein said one transducer
is an array transducer.
20. Apparatus as defined in claim 19, wherein said one transducer
is a transducer for transmitting ultrasonic energy as
aforesaid.
21. Apparatus as defined in claim 19, wherein said one transducer
is a transducer for receiving ultrasonic energy as aforesaid.
22. Apparatus as defined in claim 18, wherein each of said
transducers is an array transducer comprising a poly(vinylidene
fluoride-trifluoroethyle- ne) copolymer.
23. (canceled)
24. Apparatus as defined in claim 1, wherein the mounting structure
includes a support for positioning the animal portion between the
transducers, and a device for coupling the transducers
ultrasonically to the animal portion.
25-28. (canceled)
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the priority benefit, under 35
U.S.C. .sctn.119(e) (1), of applicants' copending U.S. provisional
application Ser. No. 60/140,638, filed Jun. 23, 1999, which is
incorporated herein in its entirety by this reference.
BACKGROUND OF THE INVENTION
[0002] This invention relates to apparatus and methods for testing
characteristics of animal bones, for example human bones, by
transmitting ultrasonic energy through bone tissue to be
tested.
[0003] The testing of bone tissue characteristics such as bone
mineral density (BMD) in living humans is widely utilized in
present-day medical practice to determine whether and to what
extent a patient has osteoporosis or other bone disease. One
convenient and effective way of testing bone tissue characteristics
is by transmitting ultrasonic energy through a bone in a patient's
limb or extremity, between ultrasonic transducers respectively
disposed on opposite sides of the limb or extremity, while the
transducers and bone-containing limb or extremity are stably held
in stationary, predetermined relation to each other in a support or
frame. It is currently preferred, in at least many instances, to
perform ultrasonic testing on the heel bone (os calcis, or
calcaneal bone), with the foot of the patient positioned in the
support and the ultrasonic transducers facing each other on
opposite sides of the heel.
[0004] For effective coupling of ultrasonic energy to the heel,
some testing systems (wet systems) include a tank or container of
liquid in which the foot is immersed, and which is in contact with
the transducers. Dry systems have also been developed, in which a
pair of pads constituted of a suitable polymer or filled with a
liquid coupling medium such as a gel (the term "liquid" herein
embracing gels) are respectively disposed between the heel and the
two transducers, each pad being in contact with and somewhat
compressed between its associated transducer and the heel.
[0005] The transducers employed are devices for converting
electrical energy to ultrasonic energy and vice versa. Thus, one of
the transducers is energized electrically to generate ultrasonic
energy which is transmitted through the heel or other body portion
containing the bone being tested and received by the other of the
transducers. The receiving transducer in turn generates an
electrical signal in response to the received ultrasonic energy
which has traversed the bone. This electrical signal is detected by
appropriate electrical circuitry and utilized in deriving a value
representing, for example, the speed of sound (SOS) through the
bone or broadband ultrasonic attenuation (BUA) by the bone. Such
values can be correlated with medically significant bone
characteristics such as BMD.
[0006] In conventional systems as heretofore known, each of the
ultrasonic transducers which produce and receive the ultrasonic
energy is typically a ceramic piezoelectric crystal heavily damped
with a composite backing and/or tuned to obtain the desired
broadband frequency response. These transducers are extremely
labor-intensive to manufacture, requiring an extensive amount of
machining, gluing and compressing in production. Because of the
many manufacturing steps, the nature of the ceramic crystal and the
damping backing, the final product is extremely variable in nature.
This variability is manifested in frequency response, amplitude and
ultrasonic wave shape.
SUMMARY OF THE INVENTION
[0007] The present invention, in a first aspect, broadly
contemplates the provision of ultrasonic bone testing apparatus
comprising a pair of ultrasonic transducers at least one of which
comprises a piezoelectric copolymer; mounting structure supporting
the transducers in facing spaced relation to each other, so as to
be respectively positionable on opposite sides of and both coupled
ultrasonically to an animal portion containing a bone, for
respectively transmitting ultrasonic energy through and receiving
ultrasonic energy transmitted through the animal portion including
the bone; and electrical circuitry connected to the transducers to
energize one transducer to transmit ultrasonic energy and to detect
an electrical signal generated by the other transducer in response
to received ultrasonic energy.
[0008] In this apparatus, the mounting structure may include a
support for positioning the animal portion between the transducers,
and a device for coupling the transducers ultrasonically to the
animal portion. Preferably, both transducers are piezoelectric
copolymer transducers. Each of the transducers typically or
conveniently comprises a plate of a piezoelectric copolymer, the
term "plate" being used herein to denote a generally flat element
such as a sheet, slab or film, whether self-sustaining or coated on
a substrate layer and/or itself provided with a protective or other
coating. Alternatively, since the copolymer of the transducer is
easily shaped, either or both of the copolymer transducers included
in the present invention may have a curved surface, to provide
focusing, and/or may be constituted as an array of multiple
discrete copolymer transducer elements, such an array having
advantages for correction for phase cancellation and/or for
imaging; in such an array, the individual elements of the array
could correspond to pixels and each pixel could have a different
BUA, SOS or BMD value.
[0009] A currently preferred embodiment of the invention includes
two copolymer transducers, respectively serving as a transmitter
and a receiver of ultrasonic energy, and each comprising a
copolymer disk having a periphery and two opposed major surfaces,
one of which is disposed to face the bone-containing animal
portion. In accordance with a particular feature of the invention,
in this embodiment each such transducer further includes a rigid
support structure, such as (for instance) a rigid ring, engaging
the other major surface of the disk inwardly of the periphery
thereof for supporting the disk against pressure exerted on the
first-mentioned major surface of the disk. For example, in
embodiments wherein the coupling device comprises a pair of polymer
pads, each disposed in contact with one major surface of one of the
transducers, and respectively engageable with opposed surface
regions of an animal portion positioned in the support as
aforesaid, the rigid support structure (e.g., ring) engaging the
other major surface of each transducer disk inwardly of the
periphery thereof supports the disk against pressure exerted on the
disk through the last-mentioned pad.
[0010] In other embodiments, the coupling device includes a
container for holding a body of a coupling fluid in which the
animal portion is immersed when positioned by the support as
aforesaid and with which the transducers are in ultrasonic energy
transmitting contact.
[0011] Any of these embodiments may be arranged for use in testing
procedures in which the animal portion is a human heel, the support
being configured and dimensioned to position the heel, and the
transducers and coupling device being disposed, so that ultrasonic
energy transmitted from one transducer to the other transducer
passes through the calcaneal bone of the heel.
[0012] The electrical circuitry may be arranged to use the detected
electrical signal in deriving a value representative of the speed
of sound through the bone through which the ultrasonic energy is
transmitted as aforesaid. Alternatively, or additionally, the
circuitry may be arranged to use the detected electrical signal in
deriving a value representative of broadband ultrasonic attenuation
(BUA) in the bone.
[0013] In a second aspect, the invention contemplates the provision
of a method of determining a characteristic of a bone in a
bone-containing portion of an animal comprising disposing a pair of
ultrasonic transducers at least one of which comprises a
piezoelectric copolymer respectively on opposite sides of, and
ultrasonically coupling both transducers to, a bone-containing
animal portion; electrically energizing one transducer to transmit
ultrasonic energy through the animal portion including the bone,
such that the transmitted ultrasonic energy is received and
converted to an electrical signal by the other transducer;
detecting the electrical signal; and using the detected signal to
derive a value representative of the bone characteristic to be
determined. Again, in embodiments wherein the transducers so used
are disks of a piezoelectric copolymer, each having a periphery and
opposed major surfaces, one of which is oriented to face the animal
portion, the method of the invention further includes the feature
of supporting the other major surface of each disk by disposing, in
contact therewith, rigid support structure spaced inwardly from the
disk periphery.
[0014] In the apparatus and method of the invention, the use of
piezoelectric copolymer disks as transducers affords significant
economies, as well as effective performance. The piezoelectric
copolymer material requires fewer manufacturing steps and is less
variable in ultrasonic properties than conventional ceramic
piezoelectric transducers; hence the use of this material enables a
more uniform product to be manufactured. The provision of the rigid
support structure, spaced inwardly of the periphery of the
copolymer disk, enables such use of these relatively deformable
and/or frangible thin disks without distortion or breakage.
[0015] Further features and advantages of the invention will be
apparent from the detailed description hereinbelow set forth,
together with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a side view of a foot restraint assembly of a
first dry system type of ultrasonic bone testing apparatus in which
the present invention may be embodied in an illustrative form;
[0017] FIG. 2 is a perspective view of a foot well assembly of the
apparatus of FIG. 1;
[0018] FIG. 3 is a sectional view of a transducer drive mechanism
of the apparatus of FIG. 1;
[0019] FIGS. 4A and 4B are front and side views of a position
encoder of the apparatus of FIG. 1;
[0020] FIG. 5 is a block diagram showing automatic positioning by a
transducer drive mechanism of the apparatus of FIG. 1;
[0021] FIG. 6 is a view of the pads and copolymer transducers of an
embodiment of the invention incorporated in the system of FIG.
1;
[0022] FIGS. 7A and 7B are diagrams of the transmitter circuit and
receiver circuit, respectively, of the last-mentioned embodiment of
the invention;
[0023] FIGS. 8A, 8B and 8C are graphs showing response curves for
the currently preferred embodiment of the receiver/transmitter pair
of transducers for the last-mentioned embodiment of the
invention;
[0024] FIG. 9A is an end view of the copolymer transmitter
transducer of FIG. 6;
[0025] FIG. 9B is a sectional view taken along line 9B-9B of FIG.
9A;
[0026] FIG. 9C is an enlarged sectional view of the portion
indicated by circle 9C in FIG. 9B;
[0027] FIG. 9D is a similarly enlarged sectional view of the
portion indicated by circle 9D in FIG. 9B;
[0028] FIG. 10A is an end view of the copolymer receiver transducer
of FIG. 6;
[0029] FIG. 10B is a sectional view taken along line 10B-10B of
FIG. 10A;
[0030] FIG. 10C is an enlarged sectional view of the portion
indicated by circle 10C in FIG. 10B;
[0031] FIG. 10D is a similarly enlarged sectional view of the
portion indicated by circle 10D in FIG. 10B;
[0032] FIG. 11 is a sectional view, similar to FIG. 9B, of a
modified and preferred structure of the copolymer transmitter
transducer of the invention;
[0033] FIG. 12 is a sectional view, similar to FIG. 10B, of a
modified and preferred structure of the copolymer receiver
transducer of the invention;
[0034] FIG. 13 is a plan view of the housing and support elements
incorporated in the transducers of FIG. 11 and FIG. 12, with the
copolymer transducer elements omitted;
[0035] FIG. 14 is a perspective view of another dry system type of
ultrasonic bone testing apparatus in which the present invention
may be embodied;
[0036] FIG. 15A is a sectional plan view of the apparatus of FIG.
14;
[0037] FIG. 15B is a sectional elevational view of the apparatus of
FIG. 14;
[0038] FIG. 16 is a perspective view of a wet system type of
ultrasonic bone testing apparatus in which the present invention
may be embodied;
[0039] FIGS. 17A and 17B are respectively a plan view and a
schematic side sectional view of a copolymer transducer receiver
arranged for use as an array of multiple discrete elements; and
[0040] FIGS. 18A, 18B, 18C, 18D, 18E and 18F of array detector
circuitry for use with the receiver of FIGS. 17A and 17B.
DETAILED DESCRIPTION
[0041] The invention will be described in detail, for purposes of
illustration, as embodied in apparatus and methods for testing the
calcaneal (heel) bone of living human subjects or patients,
including both dry and wet systems, such embodiments representing
currently especially preferred practice. In a broader sense,
however, the invention is not limited thereto, but may be employed
e.g. to measure SOS and/or BUA to test other bones in other
portions (e.g., limbs or extremities) of the human body, and to
test bones of living animals other than humans.
Dry System
[0042] An embodiment of the apparatus of the invention may be
incorporated in a dry system for ultrasonic bone testing, generally
of the type set forth in U.S. Pat. No. 5,755,228 and in U.S. patent
application Ser. No. 08/477,580 filed Jun. 7, 1995, Ser. No.
08/866,804 filed May 30, 1997 (allowed), and Ser. No. 09/277,838
filed Mar. 26, 1999 (which also describes wet systems for
ultrasonic bone testing), the entire disclosures of which are
incorporated herein by this reference.
[0043] Such a system, now to be described, is shown in FIGS. 1-5.
As set forth in the aforesaid application, with reference to FIG.
1, the dry-type ultrasonic bone analysis apparatus there
illustrated combines the mechanisms to position and restrain the
foot and lower leg into a single foot restraint device 1. The foot
restraint device 1 comprises two assemblies, a shin guide assembly
2 and a foot well assembly 3.
[0044] As seen in FIG. 2, the foot well assembly 3 comprises a box
cover 38 having a foot support 39, and foot well bottom 37. The
foot support 39 has an area slightly larger than a human foot such
that even a large foot can fit comfortably.
[0045] Transducer ports 36 are located on the sides of the foot
support 39, towards the rear.
[0046] Referring back to FIG. 1, the shin guide assembly 2 includes
a plastic molded form 20 lined with contoured foam lining 41. The
molded form 20 is a combination of restraints for the shin, instep,
and front of the foot into a single piece.
[0047] Referring now to FIG. 3, the transducer drive mechanism of
the apparatus of FIG. 1 includes a pair of transducer assemblies
110. The transducer assembly 110 includes transducer 101 and
acoustical coupling pad 150, as hereinafter further described.
[0048] In this structure, the repeatable coupling of the
transducers to the foot is accomplished by the specially shaped
delay lines constituted by the coupling pads 150 that conform to
the shape of the heel. The acoustical delay line also allows the
transducer's wavefronts to evolve from the granular near field
pattern to a smoother far field pattern before entering the foot.
The acoustical and mechanical properties of the elastomer coupling
pad are inherently critical to the operation of the described
apparatus.
[0049] The transducers 101 are mounted to respective carriages 103
that slide along a lateral-medial axis. Respective compression
springs 104 attached to the carriages 103 apply opposing lateral
forces towards the center of the foot. The carriage/spring assembly
is free floating and will center itself on the foot with
approximately equal pressure on both sides.
[0050] An extension spring 105 applies the initial pressure when
the coupling pads 150 reach the patient's foot. To adjust the
pressure in small increments, a stepper motor with rack and pinion
mechanism 106 will move a finite number of steps and compress the
compression springs 104 that are attached to the respective
carriages 103. The compression springs 104 will pull the respective
transducers 101 and pads 150 inward at a force proportional to the
spring rate and distance translated.
[0051] The distance between the transducers 101 is continuously
measured by means of a position encoder 120 that is mechanically
linked to the motion of the transducers 101. Referring to FIGS. 4A
and 4B, front and side views of the position encoder 120,
respectively, a preferred encoder uses a code strip 121 mounted
onto one of the carriages 103 along with an optical encoder reader
122 mounted on the other of the carriages 103. As the distance
between the transducers 101 changes, the code strip 121 moves
between the slot of the optical encoder reader 122, and the optical
reader 122 reads lines 123 of the code strip 121 as the lines 123
are traversed.
[0052] The transducer drive mechanism 100 automatically positions
transducer assemblies 110 against the patient's heel with
sufficient pressure to insure ultrasonic coupling. The controller
200 is preferably a microprocessor-based controller having memory
201 (e.g. RAM and ROM) for storing system and application software
and input/output circuitry. The controller 200 controls the
operations of the stepper motor 106 according to the program stored
in its memory and the positional data supplied by the position
encoder 120. Accordingly, the transducer drive mechanism 100 under
the control of the controller 200 provides automatic
positioning.
[0053] The transmitter circuit located in the controller 200
applies pulses to the transmitter transducer 101. Response signals
received by the receiving transducer 101 are amplified, digitized
and supplied to controller 200. Using these digitized signals and
information from the position encoder 120, the controller 200
determines parameters of interest, including broadband ultrasound
attenuation and bone velocity. Also, the controller 200 calculates
a speed of the ultrasonic signals through the foot using the
distance between the transducers determined by the position encoder
120. An apparatus for measuring bone characteristics by means of
ultrasound is well-known in the art. Such an apparatus is disclosed
for example in U.S. Pat. No. 4,774,959 issued to Palmer et al. on
Oct. 4, 1988 which is incorporated herein by this reference.
[0054] The controller 200 calculates a temperature from the
velocity of sound through the pad material. The temperature thus
calculated is used to correct for temperature dependent inaccuracy
in the ultrasound measurement. For example, the controller 200
applies a temperature dependent term to correct for the broadband
ultrasound attenuation through the coupling pads 150. Furthermore,
the controller 200 uses the temperature calculated to determine if
the apparatus is operating within the specified environmental range
allowed, and if not, the operator is informed that the apparatus is
not ready to be used.
[0055] In addition, guided by operator input 300, the following are
examples of additional selectable functions provided by the
transducer drive mechanism 100 under the control of controller 200
and its stored program: (1) separate the transducers 101 to allow
the foot to be moved to and from a position between the transducers
101 without interference from the transducers; (2) move the
position encoder 120 to a known transducer separation; (3) extend
the transducers 101 to a cleaning or standby position; and (4)
bring the transducer pads 150 into contact with one another for
initialization. The operator input 300 can be any one of the
conventional input devices such as pre-allocated buttons,
keyboard/keypad device, etc.
[0056] Several features of the coupling pads 150 are important to
the operation of the described invention. The acoustic impedance of
the material of the pads 150 is matched to the acoustic impedance
of human skin to provide a minimal loss of power and reduce
extraneous reflections. Further, the SOS of the pads is close to
the SOS of the heel so as to provide minimal error in the SOS
measurement. Preferably, the coupling pads are elastomer coupling
pads.
[0057] The coupling pads 150 also provide a waveguide function for
the acoustic beam providing a sufficient distance along the
propagation axis to allow the wavefronts to evolve into a more
uniform intensity pattern. To this end, the aforementioned
acoustical delay lines are provided by the pads 150 to allow the
wavefronts to evolve from the granular near field pattern to a
smoother far field pattern before entering the foot.
[0058] The pads 150 are chosen to have a durometer corresponding to
a sufficiently flexible waveguide that can partially conform to the
shape of a foot without discomfort to the patient. The shape of the
pads 150 conforms to the heel to eliminate any gaps between the
foot and pad.
[0059] The coupling pads 150 are illustrated in FIGS. 3 and 6. The
surface of the pad that contacts the patient's skin is shaped to
expel air bubbles from the contact area when pressure is applied.
In addition, in currently preferred embodiments of the unit 150 of
FIGS. 3 and 6, the shoulder for holding the pad is located at the
slightly convex end of unit 150 proximal to the transducer.
[0060] The material of the coupling pad is required to be
compatible with coupling gel and non-irritating to the skin. One
preferred material is Ciba-Geigy polyurethane (TDT 178-34) mixed
with an additive to provide a cured durometer of approximately 12
to 18 Shore A.
[0061] While the elastomer coupling pad is preferred, the coupling
pads may be a homogeneous material, a gel pad, or a liquid or
gel-filled bladder. The shape of the bladder may be conical whereby
air bubbles are expelled when the pad engages the heel.
[0062] In a known system, commercially available coupling gel is
commonly used between the skin and coupling pads. The commercially
available coupling gel is typically water-based. While such
water-based gels can be used, a non-aqueous jelly is preferred in
this invention. One implementation of the invention uses petroleum
jelly which has been processed to eliminate air bubbles as a
coupling gel.
[0063] In the operation of the apparatus, a method of calibration
may be employed which measures an ultrasonic signal transmitted
through the coupling pads while the pads are mutually in contact,
the measurement being relatively close in time to a measurement of
a signal passing through a heel or a phantom interposed between the
pads. The received signal passing through the mutually touching
pads may be used as a reference for a BUA measurement. A
measurement of a propagation time of the ultrasonic signal through
the mutually touching pads may be used as a reference time for
comparison to the signal passing through the heel, and thereby used
for calculating a time of propagation through the heel. Because
proximity in time is accompanied, presumptively, by proximity in a
in ambient temperatures for the respective measurements, no
correction for time or temperature drift between the measurements
is required.
[0064] In order to assess a bone characteristic such as BMD by
speed of sound (SOS) measurement in the apparatus of FIGS. 1-5, it
is necessary to measure accurately the SOS of the body part (heel)
through which ultrasonic energy is transmitted while obtaining
adequate ultrasonic contact between the heel and the transducer
pads, maintaining patient comfort, and allowing for the fact that
human body parts are irregularly shaped.
[0065] The SOS.sub.p of a body part (e.g., a human heel) is given
by
SOS.sub.p=w/t
[0066] where w is the width of the body part and t is the time it
takes the ultrasound to pass through the body part. Error in the
measurement of SOS.sub.p can come from incorrectly measuring w or
t. To measure t accurately, the procedure just described for
determining a reference time using mutually touching pads may be
employed, just before the heel is inserted between the pads. The
difference in time for the measurement with the pads touching and
the foot inserted between the transducer pads is used to estimate
t.
[0067] Measurement of w presents problems owing to considerations
of pad squish, i.e., the distance a pad is compressed when pressure
is applied to it. If the transducer pads are not very stiff, the
estimate of w may be significantly inaccurate. A typical heel is 33
mm wide; squishing of the pad by 1 mm, without compensation in the
calculations, will produce an error in SOS.sub.p of {fraction
(1/33)}=3.3%, which is a very large error, since the full
biological range of variation in SOS.sub.p is on the order of 10%,
and accuracies on the order of 0.3% are desired. Stiff transducer
pads, however, cannot conform very well to the human heel and do
not in general provide adequate ultrasonic coupling for BUA
measurements (which are commonly desired when evaluating bone).
Moreover, significant pressure is needed to provide ultrasonic
coupling, and a hard transducer pad can cause significant patient
discomfort with possible complications such as bruises.
[0068] Attempts to control the amount of squish by careful control
of the amount of pressure applied to the heel are hampered by the
irregularity of the heel shape and the difficulty of controlling
the durometer of any polymer to sufficient tolerance. Moreover, if
the durometer of the transducer pad changes with age and/or
temperature (as commonly occurs), the amount of squish will vary
notwithstanding control of applied pressure.
[0069] A procedure for circumventing these problems includes, as an
initial step, bringing the transducer pads into contact with each
other, transmitting ultrasound between the transducers, measuring
the propagation time t.sub.0, and at the same time recording the
distance d.sub.0 between the two transducer faces (not the distance
between the pads). Then, the patient's heel is inserted in the
apparatus, ultrasound is again transmitted, the propagation time
t.sub.1 of the ultrasound is measured, and the distance d.sub.1
between the transducer faces with the heel inserted is measured. A
value SOS' can then be determined from the relation
SOS'=w'/t'
[0070] where w' is the difference d.sub.1-d.sub.0, between the
inter-transducer distances measured when the heel is inserted and
when the pads are touching each other, and t' is the difference
t.sub.1-t.sub.0.
[0071] It will be appreciated that w' is not the same as the width
w of the foot, because it includes the different amount of squish
when the pads are touching each other and when they are against the
heel. Moreover, t' is not the same as the time t of ultrasound
propagation through the foot because the sound may travel a greater
or lesser distance if the amount of squish is different in the two
measurements. The relation of w' to w can be expressed by
w'=w(1+.delta.)
[0072] where .delta. is the difference between the amount of squish
when the pads touch each other and when the heel is between them,
divided by the width w of the heel. For all practical cases,
.delta.<<1. The relation of t' to t is given by
t'=t(1+.epsilon.)
[0073] where
[0074] .epsilon.=(w.delta./SOS.sub.pads)/t
[0075] The numerator (w.delta./SOS.sub.pads) is the difference in
squish of the two measurements (with the pads touching each other
and with the heel between them) divided by the speed of sound
through the transducer pad material; this gives the extra time
taken because of the greater or lesser squish of the two pads in
the two measurements. In all practical cases, the numerator is much
smaller than the denominator because the time of ultrasound travel
through the body portion (heel) is much greater than the time
through the difference in pad squish; hence .epsilon.<<1.
[0076] From the foregoing,
SOS'=w'/t'=(w/t)(1+.delta.)/(1+.epsilon.)
[0077] and since .epsilon.<<1, this equation can be expanded
as a power series, substituting SOS.sub.p (the quantity to be
measured) for (w/t):
SOS'=SOS.sub.p{1+.delta.}{1-.epsilon.+o.sup.2(.epsilon.)}=SOS.sub.p{1+.del-
ta.-.epsilon.+o.sup.2(.epsilon.)}
[0078] or, writing .epsilon. in terms of .delta., as defined
above,
SOS'=SOS.sub.p{1-(w.delta./SOS.sub.pads)/t)+.delta.+o.sup.2(.delta.)}=SOS.-
sub.p{1+.delta.(1-SOS.sub.p/SOS.sub.pads)+o.sup.2(.delta.)}
[0079] To estimate how accurately SOS' estimates SOS.sub.p, it is
only necessary to look at the first term (which is largest). As
noted, .delta. is very small because it is the difference between
the squish of the pads when they touch and when they contact the
heel, so that if the total squish of the pads is about 1 mm (as in
the example given above), the difference in squish is much less,
e.g., perhaps 0.5 mm, so that .delta.=0.5/33=0.015. There is a
self-adjustment in that if the durometer of the pads changes with
age or temperature, the pads will squish to a greater or lesser
extent on both measurements, hardly affecting the difference in
squish. Moreover, the error in SOS.sub.p estimate varies not as
.delta. but as .delta.(1-SOS.sub.p/SOS.sub.pads); thus, if the
transducer pads are made of a material chosen such that
SOS.sub.p.apprxeq.SOS.sub.pads, the error is very small. Of course,
since body parts have a range of SOS.sub.p (between about 1450 and
about 1670 m/s), it is impossible to select one
[0080] pad material that matches all SOS.sub.p, but if the pad
material is so chosen that SOS.sub.pads is approximately in the
middle of the biological range (an example is Ciba-Geigy elastomer
TDT 178-34), then (1-SOS.sub.p/SOS.sub.pads) is slightly more than
0.07 in the worst case, and will usually be much smaller.
Therefore, SOS' will differ from SOS.sub.p in the worst case by
only about 0.015.times.0.07 or 0.105%, which affords an acceptable
accuracy of estimation of SOS.sub.p by determination of SOS'.
Copolymer Transducers
[0081] The apparatus as thus far described is substantially the
same as that shown in the aforementioned U.S. Pat. No. 5,755,228
and U.S. patent application Ser. No. 08/477,580. Heretofore,
however, the transducers ordinarily employed in such apparatus have
utilized ceramic piezoelectric crystals as active transducer
elements. In contrast, as a particular feature of the present
invention, in the embodiment now to be further described with
reference to FIGS. 6-13, the transducers 101 are copolymer
transducers, viz., a copolymer transmitter transducer 10 and a
copolymer receiver transducer 11, respectively associated with two
integral pad/delay units 150 as illustrated in FIG. 6.
[0082] Illustrative, experimental circuits for the copolymer
transmitter transducer and the copolymer receiver transducer, in
the described embodiment of the invention, are respectively shown
in FIGS. 7A and 7B.
[0083] The following is a description of these experimental
circuits respectively used for driving a copolymer ultrasound
transmitter transducer and for conditioning the electrical signals
generated by an ultrasound receiver transducer.
[0084] TRANSMITTER CIRCUIT--The transmitter circuitry (FIG. 7A)
consists primarily of an inductor L1, resistors R1 and R2, a
switching transistor Q1, and a number of high speed controlled
avalanche diodes D1 through Dn. The number of diodes needed is
determined by the voltage it is desired to apply to the transducer
and by the voltage rating of the individual diodes. The circuit
requires a well regulated and well filtered DC supply voltage and
an electrical pulse signal suitable for switching on and off the
transistor Q1.
[0085] A preferred implementation of the circuit is shown
schematically in FIG. 7A. Immediately preceding the desired
application of a transmitter pulse a pulse is applied to the gate
to make transistor Q1 conductive for a period of approximately 12
microseconds. During this pulse the current through L1, R1 and Q1
increases and approaches asymptotically a current determined by the
supply voltage and the resistance of the circuit, primarily
resistor R1. At the conclusion of the 12 microsecond pulse
transistor Q1 suddenly ceases to conduct. Inductor L1 forces the
current through the inductor to continue to flow for a brief period
of time. Since the current cannot flow through transistor Q1 it
flows into the capacitance at the junction of resistor R1 and
inductor L1, primarily through diodes D1 and D2, to the capacitance
of the transmitter transducer. The current flowing into this
capacitance causes the voltage across the transducer to rise and
the voltage thus applied to the inductor causes the current in the
inductor to decrease. When this current has decreased to zero the
voltage across the transducer ceases to increase. Diodes D1 and D2
prevent the charge on the transducer from flowing back out through
the inductor. The transducer charge, instead, is drained gradually
through resistor R2 which is of sufficiently great resistance not
to affect the amplitude of the pulse significantly for several tens
of microseconds but eventually allows the transducer voltage to
decay to a negligible value before the next following pulse is
desired.
[0086] The voltage to which the transmitter transducer is charged
is determined by the transducer capacitance, the inductance L1, and
the current to which inductor L1 is charged. If the inductor is
charged to a current I.sub.L the energy stored in the inductor is
equal to one-half of L1 times the square of I.sub.L. In the course
of the pulse this energy is transferred fully (except for certain
small but inevitable losses due to the inefficiency of the circuit)
to the capacitance of the transducer, which is primarily a
capacitive device. The energy stored in a capacitor is equal to
one-half of the capacitance times the square of the capacitor
voltage. Therefore, the voltage across the transducer at the peak
of the pulse is equal to the inductor charging current times the
square root of the ratio between the inductance and the transducer
capacitance.
[0087] The time required for the voltage across the transducer to
increase from its baseline level to the peak is determined by the
resonant frequency of the inductance with the transducer
capacitance and is equal to one half the cycle time of this
frequency.
[0088] For the preferred circuit shown in the figure and a
transducer capacitance of 150 picofarad the peak transducer voltage
is calculated to be 128 volts. The time from the base to the peak
of the pulse is calculated to be 0.47 microsecond. This corresponds
to a conventional rise time (10% to 90%) of approximately 0.28
microsecond.
[0089] RECEIVER CIRCUIT--The receiver circuit conditions the charge
pulse generated by the receiver transducer in response to an
ultrasound signal. The preferred circuit shown in FIG. 7B consists
of an operational amplifier A1, a feedback capacitor C1, a feedback
resistor R1, an input resistor R2, and a bypass capacitor C2. The
operational amplifier requires a DC power supply of both a positive
and a negative voltage such as +/-8 volt or +/-15 volt. The DC
power supply leads of the amplifier should be bypassed to ground in
accordance with good engineering practice.
[0090] Matching the impedance of the transducer, which is
capacitive, the amplifier feedback is also primarily capacitive.
The 10 megohm feedback resistor provides a baseline at ground
potential. The feedback at the signal frequencies of interest is
provided primarily by the 10 picofarad feedback capacitor. The
transducer is connected to the inverting input of the amplifier.
The amplifier's non-inverting input is grounded through a 10 megohm
input resistor which compensates for the common mode input current.
This resistor is bypassed by a 0.01 microfarad bypass capacitor to
minimize noise in accordance with good engineering practice.
[0091] The amplifier output signal is proportional to the signal
charge. The effective gain of the circuit is equal to the ratio of
the transducer capacitance over the feedback capacitance. For
example, the circuit shown in the figure provides a gain of 15 if
the transducer capacitance is equal to 150 picofarad. The noise
contributed by the amplifier circuit is kept to a minimum by using
a high value (10 megohm) feedback resistor. If a signal
proportional to the signal current is preferred, a differentiator
(designed in accordance with good engineering practice) may be
provided in subsequent processing circuits (not shown).
[0092] The active transducer element of the transmitter transducer
10 is a plate in the shape of a disk 12 comprising one or more
layers of a piezoelectric copolymer. The disk 12 is mounted in a
shallow, cylindrical, cup-shaped, rigid molded plastic housing 14
so as to extend, in the manner of a diaphragm, across the open end
of the housing. Similarly, the active transducer element of the
receiver transducer 11 is a plate in the shape of a disk 16
comprising one or more layers of a piezoelectric polymer, mounted
in like manner at the open end of a shallow, cylindrical,
cup-shaped rigid molded plastic housing 18.
[0093] The two housings 14 and 18 may in turn be mounted in the
apparatus of FIGS. 1-6, in the manner illustrated in FIG. 3, with
the two disks 12 and 16 facing each other in spaced-apart relation
for interposition of a patient's heel therebetween. The slightly
convex proximal ends of the two pad units 150 are respectively in
contact with the transmitter and receiver disks 12 and 16, and, as
further shown in FIG. 6, the pads 150 project therefrom toward each
other for engaging opposite side portions of a patient's heel
inserted between them. Thereby, the pad units 150 couple the
transmitter and receiver transducers 10 and 11 ultrasonically to
the heel so that ultrasonic energy generated by the transmitter
transducer passes through the calcaneal bone of the heel and, after
such traverse, is received by the receiver transducer.
[0094] The slight convexity of the pad ends proximal to the
transducers serves to exclude bubbles of air, which would otherwise
affect the measurements made by the apparatus. A gel such as
petroleum jelly (pretreated by melting and resolidification for
removal of bubbles from the jelly) is placed between the transducer
and the slightly convex pad end.
[0095] The copolymers employed in the transducers are copolymers of
vinylidene fluoride and trifluoroethylene, i.e., poly(vinylidene
fluoride-trifluoroethylene) copolymers, hereinafter designated
P(VDF-TrFE), in sheet form, wherein the mole % ratio of vinylidene
fluoride to trifluoroethylene is between about 60/40 and about
90/10. Currently preferred are such copolymers having an
electromagnetic coupling coefficient k.sub.t of between about 0.20
and about 0.30. P(VDF-TrFE) copolymers can be used in transducers
in methods and apparatus within the broad scope of the invention,
not only as piezoelectric materials but also as electrostrictive
materials.
[0096] An illustrative but non-limiting specific example of a
transmitter transducer 10 is shown in FIGS. 9A-9D. The
piezoelectric polymer employed in this transducer is
poly(vinylidene fluoride-trifluoethylene) copolymer (in a weight
ratio of 75/25), hereinafter sometimes referred to as
"P(VDF-TrFE)(75/25) copolymer." In this transducer, the housing is
constituted of an outer cup member 14a and an inner sleeve 14b,
both of which are rigid molded plastic elements, with a strain
relief feature 14c. The periphery of the transducer disk 12
overlies the outer edge of the sleeve 14b and is secured by a
potted seal 14d to the rim of the cup member 14a.
[0097] The disk 12 is a laminate of two layers 26, 28 of the
P(VDF-TrFE) (75/25) copolymer, each 230 .mu.m (9 mils) thick,
bonded together by a 1 mil thick layer 34 of epoxy. On its
outwardly facing (front) surface, the disk also includes a 10 mil
front layer 44 of polycarbonate laminated by a 1 mil layer 46 of
epoxy to a 12 mil layer 48 of aluminum. A 5 mil shield electrode
layer 52 of silver ink overlies the outer surface of the outer
layer 26 of the copolymer and extends around the peripheral edge of
the two copolymer layers; the aluminum layer 48 is bonded to this
shield electrode ink layer by a 1 mil layer 54 of epoxy. On the
inner surface of the disk 12 is disposed a 40-mil-thick brass
electrode 56, which is the "hot" electrode of the transducer (i.e.,
the electrode through which electrical energy is supplied to the
copolymer transducer to cause the transducer to transmit ultrasonic
energy) and is bonded by a 1 mil epoxy layer 58 to the inner
surface of the inner copolymer layer 28.
[0098] The inner sleeve 14b of the housing is laterally surrounded,
on its outer surface, by a 10-mil-thick copper shield 62 which is
in electrical contact, at its outer extremity, with the silver ink
shield electrode layer 52. At the inner end of sleeve 14b there is
provided a 5 mil brass shield electrode 64 secured by 1-mil-thick
adhesive 66 to the inner edge of the sleeve and connected
electrically to the copper shield 62 by a 5-mil-thick ring 68 of
silver ink.
[0099] The hot brass electrode 56 is connected to external
circuitry by a shielded cable 70 which extends into the rear
surface of the cup member 14a through a potted seal 72 and includes
a shield braid wire 74 soldered to the brass shield electrode 64 as
well as a hot lead 76. The lead 76, extending within the inner
housing sleeve 14b, is bonded by silver epoxy to the brass
electrode 56.
[0100] FIGS. 10A-10D show an illustrative specific example of the
receiver transducer 11. The receiver transducer housing may be
essentially identical to that of the transmitter transducer
described above, including a rigid molded plastic outer cup member
18a and inner housing member 18b with a strain relief feature 18c;
the periphery of the receiver transducer disk 16 overlies the outer
edge of sleeve 18b and is secured by a potted seal 18d to the rim
of the cup member 18a.
[0101] In this receiver transducer, the disk 16 is a laminate of
four layers 78, 80, 82 and 84 of the P(VDF-TrFE)(75/25) copolymer,
each 230 .mu.m (9 mils) thick, bonded to each other by 1 mil layers
86, 88, 90 of epoxy. A 5-mil-thick silver ink shield electrode
layer 92 extends over the outer face of the outermost copolymer
layer 78 and around the peripheral side edge of the laminate. The
inner surface of the innermost copolymer layer 84 is secured around
its periphery by a 1 mil epoxy layer 94 to the edge of sleeve 18b;
and a 40-mil-thick hot brass electrode 96 is
[0102] bonded by a 1 mil epoxy layer 98 to the central portion of
the inner surface of copolymer layer 84.
[0103] A copper shield 108, 10 mils thick, laterally surrounds the
sleeve 18b and is in electrical contact with the silver ink shield
electrode layer 92. At the inner end of sleeve 18b there is
provided a 5 mil brass shield electrode 112 secured by 1-mil-thick
adhesive 114 to the inner edge of the sleeve and connected
electrically to the copper shield 108 by a 5-mil-thick ring 116 of
silver ink.
[0104] The hot brass electrode 96 is connected to external
circuitry by a shielded cable 118 which extends into the rear
surface of the cup member 18a through a potted seal 124 and
includes a shield braid wire 126 soldered to the brass shield
electrode 112 as well as a hot lead 128. The lead 128, extending
within the inner housing sleeve 18b, is bonded by silver epoxy to
the brass electrode 96.
[0105] As a further particular feature of the invention, shown in
FIGS. 11-13, but not in FIGS. 9A-10D discussed above, the inner
sleeves of the two transducer housings each include a support
structure for engaging the inwardly-facing surface of the
associated transducer disk, inwardly of the periphery of the disk,
for supporting the disk against pressure exerted on the
outwardly-facing surface of the disk. As will be apparent from FIG.
6, when a patient's heel is inserted between the two pad units 150
and the transducers are positionally adjusted as described above
with reference to FIG. 3, so that the transducers are
ultrasonically coupled to the heel, pressure is exerted by the pads
150 on the transducer disks 12, 16 which they respectively engage.
Such pressure could cause deformation of the disks, interfering
with proper operation of the apparatus and/or resulting in damage
such as disruption of bonding.
[0106] Specifically, in the transmitter transducer of FIG. 11, the
inner sleeve 14b of the housing shown in FIGS. 9A-9D is replaced by
a rigid molded plastic sleeve 14b' including a rigid central spider
portion 130, molded integrally with the outer wall of the sleeve,
and formed with a thin but rigid outwardly projecting central
annulus or ring 132 disposed to engage the inwardly facing surface
of the brass electrode 56 when the disk 12 including electrode 56
is in planar, unstressed condition. The ring 132 is concentric with
the cylindrical outer wall of the sleeve 14b' and with the disk
periphery, but spaced inwardly therefrom, so as to engage the
electrode surface in a narrow annular region. Within the ring 132,
the spider portion 130 has a central aperture 134, and has a
plurality of other apertures 136 regularly disposed around the
ring, radially outwardly thereof. A notch 138 may be provided in
one of the outer apertures 136 to accommodate the hot lead wire
76.
[0107] The receiver transducer of FIG. 12 has an inner housing
sleeve 18b' essentially identical to the sleeve 14b' of FIG. 11.
Thus, it includes an integrally molded rigid spider portion 140
providing a narrow, outwardly projecting ring 142 disposed in
concentric, inwardly spaced relation to the outer wall of sleeve
18b' for engaging the inwardly-facing electrode surface of the
receiver transducer disk 16 when the latter is in planar,
unstressed condition. In plan view, the spider portions of both
sleeves 14b' and 18b' have the appearance illustrated in FIG. 13,
which represents sleeve 14b'.
[0108] The rings 132 and 142 provide distributed rigid support
acting against the inner surfaces of disks 12 and 16, respectively,
so as to resist deformation of the disks by pressure exerted by the
pad units 150 during operation of the apparatus, yet without
impairing the transducer function of the disks. Such support has
not been necessary in the case of conventional ceramic
piezoelectric transducer elements, but avoids a potential problem
of distortion or deformation that might otherwise be encountered in
the use of plate-shaped piezoelectric copolymer transducers in
ultrasonic bone-testing apparatus.
[0109] Stated more generally, it is currently deemed convenient or
preferable that the piezoelectric copolymer transducers
(transmitter and receiver) utilized in the invention have the
following characteristics and properties:
1 center frequency 630 KHz .+-. 20% diameter of active 0.75" .+-.
0.1" element eccentricity of active 0.010" maximum concentric
element with respect to outer assembly to housing voltage rating
1000 volts (transmitter) capacitance (including 95 .+-. 10 pF
(transmitter) cable) 80 .+-. 10 pF (receiver) shunt resistance
>10 M.OMEGA. DC @1000 v. operating temp. 15-40.degree. C.
storage temp. -40-60.degree. C.
[0110] In addition to the examples of transducers described above
with reference to FIGS. 9A-9D and 10A-10D, further illustrative
examples of structures suitable for the transmitter disk 12 and
receiver disk 16 are as follows:
[0111] (a) a structure as shown in FIG. 9B, in which polycarbonate
layer 44 is 0.60 mm (about 24 mils) thick, aluminum layer 48 is
0.95 mm (about 38 mils) thick, and copolymer layers 26 and 28 with
the intervening epoxy layer 34 are replaced by a single layer of
the P(VDF-TrFE)(75/25) copolymer 100 .mu.m (about 4 mils)
thick;
[0112] (b) a structure as shown in FIG. 9B, in which polycarbonate
layer 44 is 0.60 mm (about 24 mils) thick, aluminum layer 48 is
omitted, and copolymer layers 26 and 28 with the intervening epoxy
layer 34 are replaced by a single layer of the P(VDF-TrFE)(75/25)
copolymer 230 .mu.m (about 9 mils) thick; and
[0113] (c) a structure as shown in FIG. 9B, in which polycarbonate
layer 44 is 0.55 mm (about 22 mils) thick, aluminum layer 48 is
0.60 mm (about 24 mils) thick, and copolymer layers 26 and 28 with
the intervening epoxy layer 34 are replaced by a single layer of
the P(VDF-TrFE)(75/25) copolymer 230 .mu.m (about 9 mils)
thick.
[0114] Each of these disk structures (a), (b) and (c) also included
a 1 mm (about 40 mil) thick brass electrode 56 arranged as shown in
FIG. 9B.
[0115] When tested, each of structures (a), (b) and (c) was found
to be slightly better than the structures of the disks 12 and 16
described above with reference to FIGS. 9A-9D and 10A-10D (two
samples of structure (c) were included in the tests). Structure (b)
had a wider bandwidth than the others (including the structures of
FIGS. 9A-9D and 10A-10D), but has more signal in the 1-3 MHz range,
which is not wanted or needed for bone testing operations; it has,
however, the advantage of a 3-layer design, which reduces the
number of manufacturing steps and bonding layers. Structure (a)
peaked at a higher frequency than the others (including the
structures of FIGS. 9A-9D and 10A-10D), but has the advantage of
using a thinner, hence less expensive and more easily manufactured,
copolymer transducer layer. Further, it was found that transducers
having the disk 12 or 16 structures (a), (b) and (c) described
above, and those of FIGS. 9A-9D and 10A-10D, exhibited no
difference in tested performance characteristics when used as
transmitters and as receivers.
[0116] The currently most preferred copolymer transducer for the
apparatus of the invention, primarily for reasons of cost, has a
backing of 1 mm brass, one layer of 230 .mu.m thick copolymer,
0.025 inch of aluminum and 0.020 inch of polycarbonate. Identical
transducers are used for the receiver and the transmitter. Response
curves for this transducer pair are shown in FIGS. 8A-8C.
[0117] It is currently considered desirable to use a low-pass
filter incorporated in the receive electronics to prevent aliasing,
and in such case, it is currently believed preferable to use a
transducer having disk structure (b) described above in combination
with a transducer having disk structure (a), (b) or (c) described
above.
[0118] The piezoelectric copolymer transducers of the invention
function, like the ceramic piezoelectric crystals heretofore used
in ultrasonic bone testing apparatus, to transmit ultrasonic energy
through a bone to be tested, e.g. the calcaneal bone of a human
heel (in the case of the apparatus embodiment shown in FIGS. 1-5),
and to receive the transmitted ultrasonic signal and produce, in
response thereto, an electrical signal which is detected and used
to derive a value representative of the bone characteristic (such
as bone mineral density, BMD) to be determined. Typically, in such
ultrasonic bone testing apparatus using the copolymer transducers,
BMD is determined by measuring the speed of sound (SOS) through the
heel and/or the broadband ultrasonic attenuation (BUA), in known
manner, as has heretofore been practiced with apparatus employing
conventional ceramic piezoelectric crystal transducers.
Alternative Dry System
[0119] Another example of dry-type apparatus for testing the
calcaneal (heel) bone of living human subjects or patients, in
which the present invention may be embodied, is shown in FIGS.
14-15B. In this embodiment, which is designed to be small and
highly compact, a base 144 is provided with an inclined surface 146
on which the foot of a human patient is placed. In the forward
(upper) portion of the inclined surface there is mounted a foot
positioner mechanism 148 comprising a pair of upward projections
152 carried on a mechanism 154 for moving the projections laterally
toward and away from each other, into and out of engagement with
the forward portion of a foot resting on the surface 146, to locate
and stably retain the foot on the surface. The base can be folded
about a pivot 156 for storage and transport; to permit such
folding, a button 158 releases a latch (not shown) that holds the
base open for use.
[0120] As shown in FIG. 15A, at the rear (low) end of the surface
146, a transmitter transducer 10' and receiver transducer 11' are
mounted, generally in the same relationship to each other as the
transducers 101 in FIG. 3, together with pad units 150a' for
ultrasonically coupling the transducers to opposite sides of a
patient's heel. The apparatus further includes a molded heel rest
160, positioned to locate the heel between the transducers. The
transducers are mounted on arms 162 carried on a telescoping
structure 164 operated by a gear and toothed track mechanism 166
under control of a manually rotatable wheel 168 so that the arms
162 are laterally movable toward and away from each other.
[0121] The transducers 10' and 11' may be piezoelectric copolymer
transducers having the same composition and structure as the
transducers 10 and 11 described above. In this highly simplified
machine, the circuitry and processing unit (not shown) are
contained within a housing 170 at the rear of the base, and a
numerical data measurement is displayed, for example, by an LED
device 172.
Wet System
[0122] In another embodiment of the invention, the above-described
copolymer transducers may be incorporated in "wet" apparatus
wherein the patient's heel is immersed in a liquid bath which
ultrasonically couples the heel to the transducers.
[0123] An example of such apparatus is illustrated in FIG. 16. A
base 174 includes a receptacle 176 for holding a body of liquid,
the receptacle being dimensioned to receive a human patient's foot
and having a rear inner wall portion 178 shaped to receive the
heel. At a forward location within the receptacle is a foot
positioner similar to the positioner 148 of FIG. 14 including
laterally movable projections 180 for engaging opposite sides of
the foot.
[0124] A transmitter transducer 10" and a receiver transducer 11"
are mounted in the rearward portion of the base, being respectively
positioned on opposite sides of the receptacle in the rearward
portion in which the patient's heel is placed. These transducers
are piezoelectric copolymer transducers again having essentially
the same composition and structure as the transducers 10 and 11
described above. They face each other through the body of liquid
contained in the receptacle, being ultrasonically coupled to the
liquid and aligned to transmit ultrasonic energy through the
calcaneal bone of a heel positioned in the receptacle, and to
receive the ultrasonic transmitted energy after it has passed
through the heel bone. Controls represented by buttons 182, an LED
display or other readout device 184, and electrical circuitry and a
processing unit (not shown) are housed in the base.
[0125] The performance of bone testing with this device, using the
piezoelectric copolymer transducers, may be performed in the same
manner as has heretofore been used, for example, to measure BMD in
wet-type ultrasonic apparatus with ceramic piezoelectric crystal
transducers. Very preferably, the liquid employed in the bath has a
speed of sound that is substantially invariant with changes of
temperature through a usual range of operating temperatures.
Examples of such a liquid are a mixture of water and ethyl alcohol
or water and isopropyl alcohol at approximately 17% alcohol by
volume.
[0126] More generally, in each of the dry and wet systems described
above, assessment of BMD may be performed using SOS measurements,
BUA measurements, or a combination of both. Measurement of SOS with
a particular dry system (that of FIGS. 1-5) is described in detail
above. Measurement of SOS in wet ultrasonic systems, and of BUA in
both wet and dry ultrasonic systems, is well known to persons
skilled in the art, and accordingly need not be further
described.
[0127] By way of explanation of the combined use of SOS and BUA
measurements, a system as shown in FIGS. 1-6 incorporating an
embodiment of the present invention can measure SOS (in m/s) and
BUA (in dB/MHz) of an ultrasound beam passed through the calcaneus
and combine these results linearly to obtain a Quantitative
Ultrasound Index (QUI) and an estimate of a patient's heel BMD.
While ultrasound parameters do not directly measure BMD, BUA and
SOS results are correlated (R=0.82-0.85) with heel BMD results
obtained by a standard dual energy x-ray absorptiometry (DXA)
technique, as are results for the combined QUI parameter. Thus an
estimate of heel BMD results is obtained by a simple linear
resealing of the QUI parameter into heel BMD units (in
g/cm.sup.2).
[0128] Particular relationships currently in use are as
follows:
QUI=0.41(BUA+SOS)-571
BMD.sub.est=0.006323-QUI-0.07632
[0129] where SOS is the measured value of speed of sound in m/s,
BUA is the measured value of broadband ultrasonic attenuation in
dB/MHz, and BMD.sub.est is the estimated value of BMD in
g/cm.sup.2.
[0130] Although the copolymer transducers included in the foregoing
embodiments of the invention have been shown and described as
comprising continuous flat layers or laminae of the copolymer
material, other arrangements are also embraced within the broad
scope of the invention. For instance, the copolymer material of one
or both of the (transmitter and receiver) transducers may have a
curved surface to achieve focusing, such curving being facilitated
by the ease with which the copolymer material is shaped.
[0131] Again, instead of being a continuous transducer, the
copolymer transmitter transducer and/or the copolymer receiver
transducer used in the system of the invention, for example, may
comprise an array of multiple discrete copolymer transducer
elements. Such a device is shown schematically in FIGS. 17A and
17B. In this embodiment, the receiver transducer 200 includes a
laminate of two continuous layers 202, 204 of the copolymer, with a
conducting layer 206 formed on the outer surface of the outer
copolymer layer 202 and matching layers 208, 210 overlying the
conducting layer 206. Bonding layers 212 and 214 are provided,
respectively, between the two copolymer layers and between the two
matching layers.
[0132] On the inner surface of the inner copolymer layer 204 are
provided an array of small, discrete, separated patches 216 of
conductive material, each e.g. about 3 mm.times.3 mm in area. These
conductive patches may be applied as by a silk-screen-like
operation. Inwardly of them is disposed a backing 218 of insulating
high density material such as lead oxide/epoxy. Each of the
conductive patches is connected by a separate wire 220 to suitable
circuitry, for example circuitry of the type hereinafter
described.
[0133] The circuitry for a copolymer array detector receiver
(having the transducer arrangement shown in FIGS. 17A and 17B) is
shown schematically in FIGS. 18A-18F.
[0134] The detector, as described above, is divided into a number
of segments that may be arranged to form a Cartesian array
(rectangular matrix) or any other pattern compatible with the
system in which the detector is used. These segments are created by
depositing mutually isolated elements of metal on a copolymer
substrate or by etching away parts of a metal layer deposited
continuously on such a substrate. The technology used for this
purpose is similar in many respects to the technology developed for
the purpose of manufacturing printed wiring assemblies and is well
known to those skilled in the art.
[0135] Alternatively, a copolymer array transducer may be produced
by providing an array of discrete conductive patches (metal
elements) on an extended surface of a circuit board (instead of
depositing the metal on a surface of the copolymer sheet or
substrate), and adhering the copolymer sheet or substrate in
overlying relation to the array-bearing circuit board surface with
a thin bonding layer such that there is capacitive coupling between
the copolymer substrate and the array of conductive patches. This
arrangement is advantageous, for piezoelectric sheet transducers
generally, from the standpoint of manufacturing convenience and
ease of providing electrical connections (through the circuit
board) to the individual patches of the array.
[0136] Corresponding to the detector array produced by one of the
techniques described above is an array of preamplifier input stages
consisting of an input transistor, a feedback resistor, a feedback
capacitor, a pulldown resistor, and a switch. The input transistor
is preferably a junction type field-effect transistor, either
P-channel or N-channel, with a very high transconductance, a low
pinch-off voltage, and a very low gate current. The feedback
resistor is preferably a resistor of high value, for example 10
megohm. The feedback capacitor is preferably a stable capacitor of
very low value, for example 5 to 10 picofarad. The pulldown
resistor is a resistor suitable for pulling the feedback network to
a voltage at which the input transistor is cut off when the switch
is open, for example 100 kilohm. The switch is a semiconductor
device capable of connecting or disconnecting the output of the
output stage described hereafter from the feedback resistor and
feedback capacitor in response to a digital control signal.
[0137] In addition to the array of preamplifier input stages the
system also include one or more preamplifier output stages. Each
preamplifier output stage has an input terminal biased to a voltage
appropriate for the collector or drain of the input stage input
transistor. Also connected to this input terminal is a current
source appropriate for providing the drain current of the input
stage input transistor. A number of the drains or collectors of the
input stage input transistors are connected together and to the
input terminal of one of the preamplifier output stages. If only
one output stage is provided, all input stage input transistor
drains or collectors are connected to the one output stage input
terminal. If more than one output stage is provided, some input
stage input transistor drains or collectors are connected to each
output stage input terminal but not in such a way that two or more
output stage input terminals are connected together.
[0138] The configuration of the output stage is such as to provide
an inverting configuration in combination with any one of the input
stages. The output of each output stage is connected to the free
terminal of the switches associated with the input stages connected
to the input of that output stage. Thus, by activating (closing)
one switch, a full preamplifier is created consisting of the input
stage corresponding to the activated switch and the corresponding
output stage. The output terminal of each output stage is connected
to external circuits for further processing in accordance with the
desired conventions.
[0139] The switches associated with each preamplifier input stage
(and, therefore, with each element in the sensor array) may be
controlled directly by external data processing equipment or within
the preamplifier system by an array of decoders operating on the
output of a counter or counters that may (but need not) be internal
to the preamplifier.
[0140] In the preferred implementation the preamplifier system is
fabricated in such a manner as to provide an integrated
detector/preamplifier module. Modern multi-layer printed wiring
technology is well suited for this purpose. The parts necessary can
be made available as subminiature "chip" components. Connections
and interconnections can be established using "vias" when
necessary. The number of interface connections is easily
manageable.
[0141] With such an array as illustratively exemplified by the
embodiment shown in FIGS. 17A and 17B and described above, it is
possible to correct for phase cancellation. The phenomenon of phase
cancellation, and its effect on accuracy of BUA measurements, are
discussed by Petley et al., Brit. J. Radiol. 68:1212-14 (1995). The
nature of the copolymer material, as contrasted with conventional
piezoelectric materials, facilitates production of an array
transducer.
[0142] Copolymer array transducers as just described may also be
employed for imaging, i.e., in use of the system of the present
invention to produce images. Each discrete patch or element of
copolymer may be considered to correspond to a pixel of an image to
be produced. Each pixel, in such case, could have a different BUA,
SOS or BMD value. Thus, in this broad sense, the invention embraces
apparatus in which at least one of the copolymer transducers
employed is an array of discrete copolymer transducer elements,
such a transducer being herein termed a copolymer array transducer.
Illustratively, the receiver transducer of the apparatus would be a
copolymer array transducer, used with a transmitter transducer
which might be a single unitary copolymer transducer or might be a
conventional (non-copolymer) transducer or, advantageously, is
itself a copolymer array transducer. Also, the method of the
invention embraces methods for producing an image wherein at least
one of the transducers employed is a copolymer array transducer as
defined above.
[0143] Copolymer arrays are discussed by Goldberg et al.,
Ultrasonic Imaging 14:234-48 (1992). Ultrasonic densitometers with
transducer arrays including a piezoelectric sheet of polyvinylidene
fluoride are discussed in U.S. Pat. No. 5,840,029.
Bladder System
[0144] A further example of apparatus for testing the calcaneal
(heel) bone of living human subjects or patients is described in
U.S. Pat. No. 5,772,596, the entire disclosure of which is
incorporated herein by this reference. As there set forth, an
osteoporosis apparatus for measuring ultrasonic characteristic (s)
of a patient's bone comprises two ultrasonic transducers spacedly
positioned in respective heads in the apparatus for ultrasonic
transmission from one to the other; circuitry for controlling
transmission from the one transducer, measuring the reception at
the other and providing an output indicative of the ultrasonic
characteristics(s); the apparatus including: two diaphragms
positioned in the respective heads; structural spacing means that
permits the diaphragms to be brought in contact with the patient's
bone so that there is a fluid path from each transducer to its
diaphragm and a gap between the diaphragms which is occupied in use
by the patient's bone, wherein the diaphragms are connected by a
fluid system which is adapted to be pressurized for adjustment of
the diaphragms by inflation against the patient's bone. The fluid
may, for example, be water.
[0145] In apparatus of the described type, a variety of specific
different forms and features may be provided. Thus, as further set
forth in U.S. Pat. No. 5,772,596, the apparatus may include a
sensor for sensing the temperature of the fluid, with the circuitry
being adapted to compensate the measurements for the temperature,
although preferably the apparatus uses a fluid (such as an
alcohol-water mixture) that has a speed of sound substantially
independent of temperature. The apparatus may have means for
adjustably spacing the transducers in the apparatus so as to
provide a standard length of fluid path. Again, the apparatus may
include means for fixedly spacing the transducers in the apparatus
and may further have means for adjusting the diaphragms relative to
each other so as to accommodate differing thicknesses of the
patient's bone. The diaphragms may be mounted on annular supports
with outer annular sleeves provided around the supports for
limiting radial inflation of the diaphragms, with the diaphragms
and annular supports being carried on respective tubes having
advancement means for adjustment of the apparatus to suit patients
having differing bone thicknesses, and the tubes being threaded and
adapted for diaphragm advance by screw action.
[0146] The tubes may be resilient for extension under fluid
pressure for diaphragm advance, having concertina formations for
their resilient extension. Each diaphragm and respective concertina
formations may be formed as a single moulding of resilient plastics
material and may include a peripheral bead engaging in a
circumferential groove in the respective head for securing of the
diaphragm to the head.
[0147] The fluid system may include a fluid interconnection between
the fluid paths to each transducer for diaphragm pressure
equalization, the apparatus having means for arranging the fluid
interconnection so as to avoid ultrasound transmission along the
fluid interconnection. Again, the fluid system may be divided into
separate portions, one for each transducer. The fluid system may be
open, the diaphragms being flexible and the apparatus including
hydraulic head biasing means for biasing the diaphragms into
contact with the patient's bone; or the fluid system may be closed
or closable and provided with means for pressurization, whereby the
diaphragms can be urged into contact with the patient. Such
pressurization means may be an air pump arranged to pump air into a
region of the fluid system higher than the transducers and the
diaphragms.
[0148] The present invention may be embodied in apparatus of the
type described in U.S. Pat. No. 5,772,596, by providing, as at
least one of the two ultrasonic transducers in such apparatus, a
transducer comprising a piezoelectric polymer. Preferably, in these
embodiments, at least one of the transducers is a copolymer array
transducer. More preferably both are copolymer array
transducers.
[0149] It is to be understood that the invention is not limited to
the features and embodiments hereinabove specifically set forth,
but may be carried out in other ways without departure from its
spirit.
* * * * *