U.S. patent application number 10/194766 was filed with the patent office on 2004-01-15 for compression plate for diagnostic breast imaging.
Invention is credited to Change, Nicholas D. III, Entrekin, Robert R..
Application Number | 20040010193 10/194766 |
Document ID | / |
Family ID | 30114830 |
Filed Date | 2004-01-15 |
United States Patent
Application |
20040010193 |
Kind Code |
A1 |
Entrekin, Robert R. ; et
al. |
January 15, 2004 |
COMPRESSION PLATE FOR DIAGNOSTIC BREAST IMAGING
Abstract
A breast imaging system is described which includes compression
plates for retaining a breast during an examination. One of
compression plates is formed by a polymeric membrane stretched
under tension to provide a rigid but compliant retention surface
which is substantially acoustically transparent for the conduct of
a sonographic examination. The polymeric membrane may also be
substantially radiographically transparent for the conduct of a
mammographic examination. The thickness of the membrane is chosen
to be a function of one-half of the wavelength of the ultrasonic
frequency used for the sonographic examination so as to be
substantially transparent to ultrasonic energy at that
frequency.
Inventors: |
Entrekin, Robert R.;
(Kirkland, WA) ; Change, Nicholas D. III;
(Marysville, WA) |
Correspondence
Address: |
ATL ULTRASOUND
P.O. BOX 3003
22100 BOTHELL EVERETT HIGHWAY
BOTHELL
WA
98041-3003
US
|
Family ID: |
30114830 |
Appl. No.: |
10/194766 |
Filed: |
July 12, 2002 |
Current U.S.
Class: |
600/437 |
Current CPC
Class: |
A61B 8/0825
20130101 |
Class at
Publication: |
600/437 |
International
Class: |
A61B 008/00 |
Claims
What is claimed is:
1. An ultrasonic breast imaging system comprising: a first
compression plate; a second compression plate opposing the first
compression plate comprising a sheet of polymeric material
stretched under tension by a retention mechanism; and an ultrasonic
transducer oriented to transmit and receive ultrasonic energy
through the sheet of polymeric material, wherein the second
compression plate provides an acoustic window for the ultrasonic
transducer.
2. The ultrasonic breast imaging system of claim 1, wherein one of
the compression plates comprises a movable compression plate for
retaining a breast under compression between the compression
plates.
3. The ultrasonic breast imaging system of claim 2, wherein the
second compression plate exhibits a resilience which is a function
of the tension applied to the sheet by the retention mechanism.
4. The ultrasonic breast imaging system of claim 3, wherein the
second compression plate exhibits a resilience which is a function
of the thickness of the polymeric sheet and the tension applied to
the sheet by the retention mechanism.
5. The ultrasonic breast imaging system of claim 3, wherein the
retention mechanism applies a unidirectional tension force to the
sheet of polymeric material.
6. The ultrasonic breast imaging system of claim 3, wherein the
retention mechanism applies a biidirectional tension force to the
sheet of polymeric material.
7. The ultrasonic breast imaging system of claim 3, wherein the
retention mechanism applies a radial tension force to the sheet of
polymeric material.
8. The ultrasonic breast imaging system of claim 3, wherein the
tension applied by the retention mechanism is adjustable.
9. The ultrasonic breast imaging system of claim 3, wherein the
polymeric sheet has a thickness which is in the range of 50 to 300
microns.
10. The ultrasonic breast imaging system of claim 3, wherein the
retention mechanism retains the polymeric sheet under a tension
which does not exceed the material tensile strength.
11. The ultrasonic breast imaging system of claim 1, wherein the
second compression plate has a first surface which opposes the
first compression plate, and a second surface which is coupled to
an ultrasonic transducer for the transmission of ultrasonic energy
into the region between the compression plates.
12. The ultrasonic breast imaging system of claim 11, wherein the
ultrasonic transducer is coupled to the second surface by an
ultrasonic couplant.
13. The ultrasonic breast imaging system of claim 11, wherein the
second compression plate comprises a stationary compression plate
and the first compression plate comprises a movable compression
plate for applying a compressive force to a breast located between
the compression plates.
14. The ultrasonic breast imaging system of claim 13, further
comprising a scanning mechanism, connected to the ultrasonic
transducer which acts to scan ultrasonic beams in different
directions into the region between the compression plates.
15. The ultrasonic breast imaging system of claim 14, wherein the
scanning mechanism acts to scan ultrasonic beams over a volume in
the region between the compression plates.
16. The ultrasonic breast imaging system of claim 15, wherein the
scanning mechanism acts to articulate the ultrasonic transducer
over a plane which is substantially parallel to the second
surface.
17. The ultrasonic breast imaging system of claim 1, wherein the
second compression plate is substantially radiographically
transparent.
18. An ultrasonic breast imaging system comprising: a first
compression plate; a second compression plate opposing the first
compression plate comprising a sheet of polymeric material
exhibiting a thickness which is related to a frequency of interest
of an ultrasonic transducer; and an ultrasonic transducer oriented
to transmit ultrasonic energy through the sheet of polymeric
material at a transmit frequency, wherein the sheet of polymeric
material is substantially transparent to ultrasonic energy at the
frequency of interest.
19. The ultrasonic breast imaging system of claim 18, wherein the
thickness of the sheet of polymeric material is a function of the
wavelength of an ultrasonic wave at the transmit frequency.
20. The ultrasonic breast imaging system of claim 19, wherein the
thickness of the sheet of polymeric material is a function of
one-half of the wavelength of an ultrasonic wave at the transmit
frequency.
21. The ultrasonic breast imaging system of claim 20, wherein the
thickness of the sheet of polymeric material is substantially equal
to or less than one-half of the wavelength of an ultrasonic wave at
the transmit frequency.
22. The ultrasonic breast imaging system of claim 18, wherein the
second compression plate is substantially radiographically
transparent.
Description
[0001] This invention relates to medical diagnostic imaging systems
and, in particular, to diagnostic imaging systems designed for
imaging the breast.
[0002] The presently preferred imaging modality for breast disease
screening is mammography. During a mammographic examination the
breast is firmly held between two compression plates while exposed
to radiation. Adequate breast compression is fundamental in
mammography for optimal image quality. Compression provides breast
immobilization which minimizes exposure time and motion blur, and
minimizes breast thickness which minimizes geometric blur and
absorbed glandular dose. Compression forces can exceed 40 pounds of
force for a large breast, so the breast support table is ideally
constructed from a material that is both radiographically
transparent and mechanically rigid. Epoxy/carbon-fiber composite
materials generally satisfy these characteristics.
[0003] When a mammogram indicates a suspicious lesion, the next
diagnostic step is often to examine the lesion ultrasonically. For
this reason it is convenient for the diagnostic instrument to be
capable of performing both mammography and sonography. However, in
order to combine mammography and sonography into the same scanning
device, the breast support table must also be transparent to
high-frequency ultrasound. Unfortunately, carbon/epoxy composites
are poorly suited for this purpose, since their rigidity creates a
large mismatch in sound speed and bulk acoustic impedance compared
to human tissue. This mismatch results in high acoustic
transmission losses, focusing aberration that degrades spatial
resolution, and ring-down artifacts. Conversely, the known polymers
with acoustic properties that match human tissue are soft, flexible
rubbers or elastomers which readily deflect under large compression
forces and are thus poorly suited to act as compression plates.
Previous investigators claimed to rely on the intrinsic stiffness
of very thin (25 .mu.m) polymeric films such as Kapton.RTM.
polyimide (DuPont, Circleville, Ohio) to provide compression. In
practice, it is doubtful that a 25 .mu.m film of this (or any
known) polymer is sufficiently stiff to resist bending under the
large compressive forces needed for screening mammography.
Conversely, other investigators have recently studied the use of
thick polymer plates (-6 mm) to find the best compromise between
acoustic transmission loss and mechanical stiffness. However, the
large speed of sound mismatch in a thick plate requires corrections
to the beamformer delays to compensate for focusing errors due to
refraction. It is thus desirable to provide an acoustic window for
the breast support table which simultaneously provides adequate
rigidity while maintaining radiographic and acoustic
transparency.
[0004] In accordance with the principles of the present invention,
a compression plate suitable for providing an acoustic and
radiographic window for either sonography, mammography, or both is
provided by a thin polymer which is mounted under tension. By
choosing the appropriate tensile force, the desired rigidity can be
provided by a wide range of polymeric membranes. In accordance with
a further aspect of the present invention, the thickness of the
polymeric membrane is chosen in accordance with the nominal
ultrasonic imaging frequency. By choosing a thickness which is a
multiple of the ultrasonic wavelength or a fraction thereof such as
.lambda./2, the membrane can be made virtually completely
transmissive at the nominal imaging frequency. The suitability of a
wide range of polymeric membranes for the ultrasonic criteria mean
that a material with good radiographic transmissivity can be chosen
for the acoustic/radiographic window.
[0005] In the drawings:
[0006] FIG. 1 illustrates a breast scanning system constructed in
accordance with the principles of the present invention;
[0007] FIGS. 2 and 3 illustrate a breast imaging compression plate
in accordance with the principles of the present invention; and
[0008] FIGS. 4, 5, and 6 illustrate different ways by which the
membrane of an acoustic window of the present invention can be
placed under tension.
[0009] Referring first to FIG. 1, a breast scanning system 10 is
schematically illustrated. The breast 16 which is to be scanned is
retained between two compression plates 12 and 14. In this
embodiment the lower compression plate 14 is fixed in location and
the upper compression plate 12 is movable to apply a downward
compression force which retains the breast as indicated by the
compression force arrow 20. The compressed breast is scanned by an
ultrasound transducer 18 located below the lower compression plate
14. The transducer 18 scans the breast by articulation of the
transducer in two dimensions by a mechanical motion system 22. In a
constructed embodiment the transducer is driven by a motor along a
rail in either of two opposite directions in one dimension. The
rail is motor driven along two other rails which provides motion in
the opposing directions of a second, orthogonal dimension. One
dimension would be that of the motion system arrow 22 in FIG. 1,
and the other dimension would be into and out of the drawing plane,
for instance.
[0010] It will be appreciated that the entire system 10 of FIG. 1
could also be constructed in an inverted configuration. That is,
the ultrasound transducer could scan the breast from above an upper
compression plate and either of the compression plates could move
to apply the compressive force.
[0011] In accordance with the principles of the present invention,
the lower compression plate 14 in the embodiment of FIG. 1 is
formed by a thin polymeric sheet which is held under tension in at
least one dimension as indicated by the arrows directed in the
directions T. By using tension the lower compression plate can be
made of a very thin polymeric sheet which is highly transmissive to
ultrasound. The tension applied to the sheet provides significant
rigidity to the compression plate, a rigidity which, for a thin
sheet, can be virtually entirely determined by the amount of
tension applied to the sheet. The tension applied can be of any
force up to the tensile strength of the polymeric sheet.
[0012] In accordance with a further aspect of the present
invention, the thickness of the polymeric sheet can be chosen to be
highly transmissive to ultrasound. Since the desired rigidity of
the compression plate can be set by the tension applied to the
sheet, the thickness of the sheet can be chosen in accordance with
the wavelength of ultrasound being used. A thickness which is equal
to .lambda./2 where .lambda. is the wavelength of the nominal
operating frequency of the transducer will readily pass the
ultrasonic energy without reflections or reverberations. As the
thickness of the sheet exceeds .lambda./2 the problem of
reflections and reverberations becomes increasingly greater.
However as the thickness of the sheet decreases below the dimension
of .lambda./2 the problem of reflections and reverberations is
minimal. Thus, in a preferred embodiment, the thickness of the
sheet is chosen to be approximately .lambda./2 or less.
[0013] A broadband ultrasound transducer will transmit a range of
frequencies. This means that the frequency of interest will, for
such transducers, be a nominal frequency for the range used such as
the transducer center frequency. The thickness of the sheet can
also be chosen in consideration of the receive frequency of
interest. For example, for harmonic imaging the received harmonic
frequency is the frequency of interest and the thickness can be
chosen for the wavelength of the harmonic receive frequency rather
than the fundamental transmit frequency. This may mean that
reverberations and reflections are generated at the transmit
frequency due to a mismatched sheet thickness at the fundamental
transmit frequency. But these artifacts can be eliminated by
filtering or pulse inversion cancellation on reception as they are
outside of the received frequency of interest.
[0014] Table I lists a number of candidate materials which are
suitable for use as the polymeric sheet in an embodiment of the
present invention, together with mechanical and acoustic properties
of each material. The tensile strength of the various films range
from 4 Kpsi for polymethylpentene to 38 Kpsi for polyethylene
naphthalate. Half wavelength (.lambda./2) thicknesses range from 73
microns for polyimide at 15 MHz to 286 microns for polyethylene
naphthalate at 5 MHz. Other frequencies will yield other preferred
thicknesses. The chosen film is stretched to form the acoustic
window, applying a tension which preferably does not exceed the
tensile strength of the material. With the rigidity of the acoustic
window determined by the applied tension, the thickness of the
material is chosen to provide good transmissivity of ultrasound.
The acoustic transmission will be optimal when the acoustic window
thickness is an integer multiple of half of the ultrasound
wavelength. This objective is based on the observation that the
acoustic energy transmission coefficient T for a thin plate with
bulk acoustic impedance Z.sub.p immersed in a medium with impedance
Z.sub.0 is given [6] by the equation (1):
T={1+[(m.sup.2-1)/2m].sup.2 sin.sup.2(2.pi./.lambda.p)}.sup.-1
where m=Zp/Z.sub.0 (1)
[0015] which is premised upon a single ultrasound wavelength (CW
conditions) at normal incidence. By setting the material thickness
t equal to an integer multiple of .lambda./2, complete transmission
(T=1) occurs for a lossless material, regardless of the acoustic
impedance mismatch. For a polymer with a speed of 2,500 meters/sec
and a frequency of 7.5 MHz, .lambda./2 is equal to 166 .mu.m, for
instance. Exploiting this relationship will allow the use of
polymer films that are mechanically robust, while minimizing
acoustic transmission losses. Conversely, these films are still 10
to 30 times thinner than conventional thick windows for breast
imaging devices, which will minimize focal aberration due to speed
of sound mismatch. Adjusting focal delays in the beamformer could
mitigate these aberrations, but this is not expected to be
necessary with window materials this thin. Furthermore, at the
indicated thicknesses, all of these materials are expected to be
substantially radiographically transparent and have negligible
effect on x-ray image quality.
[0016] FIGS. 2 and 3 illustrate a compression plate 14 constructed
in accordance with the principles of the present invention. The
embodiment of these drawings comprises a frame 32 mounted on a
baseplate 34. The baseplate is seen to be flanged and perforated so
as to mount on a lower part of the breast scanning apparatus (not
shown) which contains the control electronics and power supply for
the transducer articulation mechanism and a lift mechanism which
raises or lowers the compression plate to a height which is
comfortable for the patient during the examination. A rectangular
opening 36 is formed on the top of the frame 32 and a Mylar
membrane 40 is stretched over the opening 36 to form the rigid
surface for the compression plate and also the acoustic window for
an ultrasound transducer. The ultrasound transducer 18 is located
below the membrane 40 and is acoustically coupled to the membrane
by an acoustic couplant such as a liquid, a gel, or an oil. In this
embodiment the membrane 40 is approximately cross-shaped, with a
forward extension 42, a rearward extension 44, and side extensions
46 and 48. The forward extension 42 extends down the front of the
frame 32 and underneath the front of the frame where the end of the
membrane is wrapped around a front bar 52 located inside the frame.
The front bar is held in place by screws 54. At the rear of the
frame 32 the rearward extension 44 extends down the back of the
frame and is wrapped around a rear bar 64 at the back of the frame
(see FIG. 3). The rear bar 64 is mounted on two end screws 66 which
can be adjusted to raise and lower the bar rear 64, the screws
being threaded into holes in the baseplate 34. As the rear bar is
raised or lowered the lateral front-to-back tension of the membrane
is decreased or increased correspondingly.
[0017] The side extensions 46 and 48 of the membrane 40 are wrapped
around side retention bars 56 on either side of the frame, one of
which is visible in the drawings. Each side retention bar 56 is
fastened to the side of the frame 32 by screws 58 to retain the
membrane 40 in a stretched condition across the opening 36. Above
each side retention bar 56 is a side tension bar 62 located beneath
the membrane 40. Each side tension bar has tabs extending into
holes in the side of the frame 32 and the inner surface of each
tension bar is contacted by screws threaded through the frame 32.
These screws are accessed from inside the frame and as they are
screwed through the frame 32 they push the side tension bars
outward against the membrane 40 which increases the tension of the
membrane across the opening 36.
[0018] Thus the side bars 62 and the rear bar 64 act to apply
controllable bilateral tension to the membrane 40 over the opening
36, the side bars 62 controlling the lateral (side to side) tension
force and the rear bar 64 controlling the front to back tension
force. The tension of the polymeric sheet 70 of a compression plate
of the present invention can be applied unidirectionally as
illustrated by arrows 72, 72' in FIG. 4. Alternatively the tension
force can be applied bidirectionally to a sheet 80 as indicated by
arrows 82, 82' and 84, 84' in FIG. 5 and as applied by the
tensioning bars in the embodiment of FIGS. 2 and 3. The tension
force can be applied to the sheet radially in the manner of a
drumhead as indicated by arrows 92 radially extending from the
sheet 90 in FIG. 6.
[0019] We have provided a breast imaging compression plate with a
mechanically stiff acoustic window which will not rely on the
intrinsic bending stiffness of the material. Instead, we pre-stress
a polymer membrane by applying tension to create a restoring force
that resists deflection, like a drumhead. The stiffness of the
acoustic window is thus limited only by the tensile strength of the
material, and can be made quite rigid for suitable high-strength
polymers such as polyimide, polyester, and others. Calculations
indicate, for example, that the maximum deflection of a 166 .mu.m
by 30 cm square polyester film with an edge preload of 15 pounds
per inch under a distributed compression load of 40 lbs which may
be needed for breast imaging will be about 5 mm. The membrane
preferably exhibits a thickness of about .lambda./2 or less at an
acoustic frequency of interest for good acoustic transmissivity and
is preferably radiographically transparent so as to be suitable for
x-ray imaging if desired.
1TABLE I Tensile Speed of Acoustic Thickness, Thickness, Thickness,
Strength Specific Sound, impedance microns microns microns Acoustic
Window Material Kpsi Gravity mm/.mu.sec Mrayl .lambda./2 at 5 MHz
.lambda./2 at 7.5 MHz .mu./2 at 15 MHz Polyethylene naphthalate 38
1.36 2.86 3.89 286 191 95 (PEN) Dupont Teonex Polyethylene
terephthalate 29 to 34 1.37 2.54 3.48 254 169 85 (PET) Dupont Mylar
Polyimide 24 1.42 2.2 3.12 220 147 73 (PI) Dupont Kapton
Polyetherimide 14 1.27 2.41 3.06 241 161 80 (PEI) GE Ultem
Polycarbonate 8.6 1.2 2.27 2.72 227 151 76 (PC) GE Lexan
Polyvinylidene Fluoride 7.5 1.78 2.3 4.09 230 153 77 (PVDF) Elf
Attochem Kynar Polymethylpentene 4 0.83 2.22 1.84 222 148 74 (PMP)
Mitsui TPX
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