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Description  |
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This invention relates to methods and apparatus for imaging breast tissue
employing both X-ray and ultrasound technology to provide enhanced
diagnostic capability, and enhanced X-ray imaging. In particular, the
present invention provides methods and apparatus for augmenting
conventional mammography equipment with an ultrasonic imaging system that
provides geometrically registered X-ray and ultrasonic fields, and
associated equipment which may be used to enhance imaging in conventional
X-ray equipment.
BACKGROUND OF THE INVENTION
The use of X-ray technology for providing two-dimensional images of breast
tissue for diagnosis of carcinoma or other abnormalities is well known.
X-ray imaging has a number of limitations which are universally recognized
by radiologists. In particular, X-ray imaging of breast tissue has the
inherent limitation that a mammogram provides only a two-dimensional image
of a three-dimensional object. Thus, although a potential area of concern
may be indicated on a mammogram, the elevation of the subject area within
the breast may be uncertain, leading to a biopsy of broader scope than
would otherwise be necessary.
In addition to conventional mammograms, apparatus has been developed that
employs ultrasound technology for breast tissue imaging. Ultrasound
imaging devices display echoes received from a piezoelectric transducer as
brightness levels proportional to the backscattered echo amplitude. The
brightness levels are displayed at the appropriate echo range and
transducer position or orientation, resulting in cross-sectional images of
the object in a plane perpendicular to the transducer emitting face.
Previously known ultrasound equipment, in the form of dedicated ultrasound
breast imaging apparatus, have met with limited acceptance by the medical
community. For example, Brenden U.S. Pat. No. 3,765,403 describes the use
of ultrasound technology to provide direct and holographic imaging of
breast tissue. That device requires the patient to lie prone on a patient
supporting surface while her breast is immersed in a water-filled tank.
Taenzer U.S. Pat. No. 4,434,799 describes an alternative device wherein
the patient's breast is immobilized between an ultrasonic transducer and
ultrasonic receiving transducer. Both of the systems described in those
patents are dedicated ultrasound systems.
In addition to dedicated apparatus, hand-held ultrasound devices have found
application in performing free-hand examinations. Free-hand examination
using a hand-held ultrasound transducer is described, for example,
Mendelson, "Ultrasound Secures Place In Breast Ca Management", Diagnostic
Imaging, April 1991, pp. 120-129. A drawback of such freehand
examinations, when used to supplement mammography, is the inability to
provide geometric registration between the mammogram and ultrasound
images. This lack of registration may result in the freehand ultrasound
examination being directed at a different portion of the breast tissue
than would otherwise have been indicated were geometric registration
possible.
For example, recent studies have shown that over 10% of the masses detected
with free-hand ultrasound and initially believed to be the
mammographically detected mass, were subsequently found to represent
different areas of the breast. Because ultrasound can depict 2-3 times
more cysts than mammography, the possibility of characterizing a malignant
lesion as benign is real.
In addition, the three dimensional shape of the lesions", as reported in
Homer, "Imaging Features And Management Of Characteristically Benign And
Probably Benign Lesions, Rad. Clin. N. Am., 25:939-951 (1987) and the
increased vascularity associated with carcinoma, as reported in Cosgrove
et al., "Color Doppler Signals From Breast Tumors", Radiology, 176:175-180
(1990), have been suggested to be added to the diagnostic criteria. Such
volumetric spatial registration of the ultrasonic data with a mammogram
cannot be accomplished with previously known ultrasound devices.
While there is recognition within the medical community of the advantages
offered by ultrasound technology, the construction of conventional
mammography and sonography equipment has prevented combination of these
two technologies. In particular, polycarbonates such as Lexan.RTM., are
typically used in mammography because of their tensile strength and
transparency to X-ray. These materials are acoustically opaque.
On the other hand, the compression plates used in the conventional breast
ultrasound devices, for example, Brenden U.S. Pat. No. 3,765,403, are
composed of materials such as polystyrene or polyurethane, which have
insufficient tensile strength for use in mammography equipment.
Because of their high densities, all of the materials potentially useful
for the compression plates in mammography equipment have relatively high
attenuation and reflection coefficients (table 1, below). These
characteristics limit the use of ultrasound to low frequencies (3 MHz or
below as described in Taenzer U.S. Pat. No. 4,434,799) and shallow depths.
At 10 MHz and a 0.5 to 1 cm round trip path through a typical compression
plate, the attenuation with most polymers would be 20-50 dB.
For any interface thicker than a quarter wavelength (several hundred
microns, depending on the nominal frequency and acoustic velocity within
the material) transmission loss must also be taken into account (which
could exceed 50 dB). In addition, the impedance mismatch between the
biological tissues, the compression plate and the transducer results in at
least a 6 dB loss at each interface, or an additional total loss of 24 dB
round trip. Since the total dynamic range is no greater than 100 dB for a
typical ultrasound system, ultrasound imaging through previously known
mammographic compression plates would be impossible.
In addition, since the acoustic propagation within the compression plate is
substantially different than water or the coupling gel, refraction effects
on each of the emitted waves from the elements of a phased array, would
severely corrupt the beam forming process that assumes a constant velocity
of 1540 m/sec.
TABLE 1
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Attenuation Coefficient
Impedance
Material (dB/MHz/cm) (Pa s/m)
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Polyvinylchloride
11.1 3.4
Polybutane 6.1 3.2
Polyacetyl, 2.5-3.3 2.2
Polyethylene,
Polypropylene
Polyamid (Nylon)
1.1 2.9
Polystyrene 1 2.5
Water 0.02 1.5
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The lower frequencies used in the previously known ultrasonic devices would
be inadequate for the diagnostic applications, which currently require
7-10 MHz transducers, yet this higher frequency requirement would increase
the transmission loss by at least threefold (in dB). While it is possible
to generate larger pulses in the transducer in the water bath approach,
the low electro-mechanical efficiency results in heat generation. Placing
the transducer directly upon the compression plate, and as a result in
close proximity to the biological tissue, would require even higher energy
pulses from each element. The resulting heat generation would cause damage
and should be avoided.
Conway, "Occult Breast Masses: Use Of A Mammographic Localizing Grid For US
Evaluation", Radiology, 181:143-146 (1991) and Brem and Gatewood,
"Template Guided Breast Ultrasound", Radiology, 184:872-874 (1992),
describe attempts to achieve spatial registration between a mammogram and
an ultrasound image by cutting a hole in the compression plate of the
mammography device to insert an ultrasound transducer. In Conway et al., a
cut-open compression plate with a localization grid was used to allow
acoustic transmission. Using the identical ultrasound device, the
ultrasound study was performed in free-hand and through the localizing
grid. Several additional X-ray exposures were needed to detect the lesion,
replace the compression plate with the cut-out grid compression plate,
then place the cut-out over the coordinates of the lesion. The grid
positioned ultrasound detected 24% more lesions than free-hand. Ten
percent were misidentified using freehand ultrasound. None of the lesions
were misidentified with the grid-guided compression.
The approach described in the foregoing articles has several practical
drawbacks. For example, in Conway the patient's breast is marked with an
indelible pen to assist the mammographer in repositioning the patient's
breast on the localization grid after the compression plate is replaced by
the cut-open compression plate used with the ultrasound transducer. As
noted in that article, even the use of indelible markings on the patients
skin does not absolutely guard against movement of the underlying breast
tissue. In addition, the mammographer had to be present during the exam to
ensure correct positioning, and the procedure length was significantly
increased.
A cut-open compression plate with a localization grid suffers from the
problem that the ultrasonic field is interrupted by the shadow of the
compression plate, in all regions but the cut-out hole, thereby requiring
prior knowledge of the interrogated lesion. As a result, in order to
obtain a complete ultrasonic diagnostic image of the desired region of
interest, it would be necessary to carry out a complex and burdensome
manipulation of the mammographic compression procedure, and expose the
patient to additional ionizing radiation.
In addition to the foregoing, compression plates used in conventional X-ray
mammography typically compress most of the breast mass to a uniform
thickness. The amount of X-ray exposure needed for imaging is then
determined by the uniform thickness of the tissue between the plates. The
nipple region and the outer edges of the breast under compression have
thicknesses that vary widely from the uniform thickness. Thus, the amount
of radiation required to properly expose the tissue of uniform thickness
causes the nipple region and outer edges of the breast to be highly
overexposed. To obtain an acceptable image of the outer edges and nipple
region of the breast, it is typical for the radiologist to perform a
second, lower dose, X-ray exposure.
Yet another drawback associated with previously known compression plates is
the patient discomfort resulting from the force applied to the breast
tissue to compress the tissue to a uniform thickness.
In view of the drawbacks of previously known breast imaging apparatus and
methods, it would be desirable to provide an apparatus and methods for
providing geometrically registered X-ray and ultrasound images of breast
tissue.
It would further be desirable to provide a compression plate that is both
radiolucent and sonolucent, so that both a mammogram and ultrasound images
of a patient's breast tissue may be obtained without moving the breast
between the X-ray exposure and ultrasound imaging.
It also would be desirable to provide an apparatus for moving an ultrasound
transducer through a predetermined path to generate a plurality of
ultrasound images of breast tissue at preselected intervals.
It would also be desirable to provide an apparatus for maintaining a
lubricating and acoustically coupling fluid film between an ultrasound
transducer and a compression plate to minimize attenuation and reflection
of acoustic energy.
It would be still further desirable to provide an apparatus capable of
correlating geometrically registered X-ray and ultrasound images to
provide holographic views of a patient's breast tissue.
It would further be desirable to provide an apparatus for use with
conventional mammography equipment which would enhance imaging of the
nipple region and outer edges of the breast so that a high quality image
could be obtained with a single X-ray exposure.
It would further be desirable to provide apparatus for use in conventional
mammography equipment which would reduce patient discomfort caused when
compressing the patient's tissue to a uniform thickness.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to
provide an apparatus and methods for providing geometrically registered
X-ray and ultrasound images of breast tissue.
It is another object of the invention to provide a compression plate for
use in combination mammography/ultrasound (hereinafter "sonomammography")
apparatus that is both radiolucent and sonolucent, so that both a
mammogram and ultrasound images of a patient's breast tissue may be
obtained without moving the breast between the X-ray exposure and
ultrasound imaging.
It is a further object of the present invention to provide an apparatus for
contacting an ultrasound transducer to a compression plate for providing
ultrasound images of breast tissue at preselected intervals.
It is a further object of the present invention to provide an apparatus for
maintaining a lubricating and coupling film between an ultrasonic
transducer and a compression plate.
It is a further object of the invention to provide radiolucent ultrasound
transducer apparatus for use in sonomammography apparatus, to provide a
plurality of ultrasound images of breast tissue that are in geometric
registration with a mammogram obtained by the equipment.
It is a further object of the invention to provide methods for digitally
manipulating ultrasound images of breast tissue, both individually and in
conjunction with mammographic views, to isolate and diagnose potential
tissue abnormalities.
It is a still further object of the invention to provide an apparatus
capable of correlating geometrically registered X-ray and ultrasound
images to provide holographic views of a patient's breast tissue.
It is a still further object of the present invention to provide an
apparatus for use with conventional mammography equipment that enhances
imaging of the nipple region and outer edges of the breast to provide a
high quality image using a single X-ray exposure.
It is yet another object of the present invention to provide apparatus for
use in conventional mammography equipment that reduces patient discomfort
caused when compressing the patient's tissue to a uniform thickness.
These and other objects of the invention are accomplished in accordance
with the principles of a first embodiment of the invention by providing a
radiolucent and sonolucent compression plate that enables sonography
apparatus to be combined with conventional mammography equipment. Either
before or after the X-ray exposure, a carriage mounted ultrasound
transducer is translated in increments across the compression plate to
generate a plurality of sectional views of the breast tissue. The X-ray
and ultrasound images produced by the sonomammography apparatus of the
present invention are therefore in geometric registration. Those images
may in turn be processed by a conventional microprocessor-based
workstation to provide holographic views of the internal features of a
patient's breast.
The compression plate in accordance with the present invention may include
a gel pad for acoustically coupling the outer edges of the breast and
nipple region with the transducer. This gel pad may also be advantageously
individually used in conjunction with conventional X-ray mammography
equipment to provide enhanced X-ray imaging by attenuating the incident
X-ray radiation proportionally to the tissue thickness being imaged and by
reducing the scattering of the X-ray radiation. The gel pad of the present
invention also advantageously enhances breast positioning and reduces
patient discomfort relative to conventional compression plates.
In a second embodiment of the present invention, a radiolucent ultrasound
transducer is provided which is adapted to conventional mammography
equipment. The transducer of the present invention, which may be a phased
array, serves as both the sending and receiving ultrasound transducer, and
is positioned beneath the diffraction grid typically found in mammography
equipment for reducing exposure of the X-ray film by scattered radiation.
The diffraction grid is modified to function as the component of the
acoustic circuit in this embodiment.
In yet a third embodiment of the present invention, an ultrasound
transducer is mounted on a movable carriage positioned between the
compression plate and the diffraction grid of conventional mammography
equipment. For this embodiment, neither the sonolucent compression plate
of the first embodiment, nor the radiolucent ultrasound transducer of the
second embodiment, is required.
The present invention also includes methods of imaging a patient's breast
tissue using mammography and sonography equipment to provide geometrically
registered images. The methods further include processing of those images
using a conventional microprocessor based workstation to permit
image-guided biopsy of the patient tissue. Alternatively, the medical
practitioner can perform detailed review of the processed and stored
images in an off-line setting.
The present invention further includes methods of manipulating ultrasound
images, either individually or in conjunction with mammographic views, to
assist the practitioner in identifying and diagnosing potential tissue
abnormalities. For example, applicants have discovered that tissue
abnormalities are less compressible than healthy tissue. Consequently,
applicants have discovered that by performing multiple ultrasound scans of
a tissue mass under different compressive loads and then digitally
subtracting the images, the tissue abnormalities can be readily detected.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features of the invention, its nature and various advantages will
be more apparent from the accompanying drawings and the following detailed
description of the preferred embodiments, in which:
FIG. 1 is a perspective view of a first embodiment of the sonomammography
apparatus of the present invention;
FIG. 2 is a partial elevation side view of the sonomammography apparatus of
FIG. 1;
FIGS. 3A and 3B are, respectively, a side view of a breast compressed in
conventional mammography apparatus and an X-ray image obtained with such
apparatus;
FIGS. 4A and 4B are, respectively, a side view of a breast compressed in
mammographic apparatus including the gel pad of the present invention and
an X-ray image obtained with such apparatus;
FIG. 5 is a detailed perspective view of one embodiment of a compression
plate in accordance with the present invention;
FIGS. 6A and 6B are, respectively, a perspective view of an illustrative
ultrasonic transducer lubricating/coupling device of the invention and a
cross-sectional view of the device of FIG. 6A taken along view line
6B--6B;
FIG. 7 is a schematic view of an illustrative embodiment of the drive means
employed in the sonomammography apparatus of FIG. 1;
FIG. 8 is a perspective view of a workstation and digitizing tablet adapted
for use with the present invention;
FIG. 9 is a perspective view of an alternative embodiment of the
sonomammography apparatus of the present invention;
FIG. 10 is a cross-sectional view taken along view line 10--10 of FIG. 9;
FIG. 11 is a perspective view of the diffraction grid and ultrasonic
transducer apparatus of the present invention;
FIG. 12 is a cross-sectional view of another alternative embodiment of the
present invention;
FIG. 13 is a block diagram of the elements of an ultrasonic imaging system
in accordance with the present invention;
FIG. 14 is a perspective view of the ultrasonic images and X-ray image
generated with the apparatus of FIG. 1 in accordance with the methods of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIGS. 1 and 2, a first illustrative embodiment of
sonomammography apparatus 10 constructed in accordance with the present
invention is described. Sonomammography apparatus 10 comprises base 11,
vertical column 12, X-ray tube 13 suspended from arm 14, compression plate
15, ultrasound transducer 16 supported from gantry 17, gantry support 18,
diffraction grid 19, film holder 20 and biopsy needle guide 21.
The mammography components of sonomammography apparatus 10, that is, base
11, column 12, X-ray tube 13, arm 14, diffraction grid 19 and film holder
20 may include the features hereinafter described, but otherwise may be
conventional. As in previously known mammography equipment, the vertical
elevation of arm 14 in column 12 may be selectively and movably determined
either manually or using a motorized arrangement which is per se known.
X-ray film 22 is disposed beneath diffraction grid 19 in film holder 20
through a door in the end face of the film holder.
While the illustrative embodiments provided herein refer to mammography
equipment that generates X-ray films, it will of course be understood by
one familiar with radiology that digital (filmless) X-ray systems or
digitized X-ray film could be employed as well. It is sufficient for
purposes of practicing the present invention that X-ray radiation emitted
from an X-ray source pass through biological tissue and form an image in a
receptor, whether an X-ray film or a digital X-ray receptor. Commercially
available mammography equipment that may be augmented in accordance with
the present invention includes, for example, the Contour system by Bennett
X-Ray Technologies, Inc., Copiague, N.Y., the AVIVA system available from
Kramex, Saddle Brook, N.J., and the LORAD DSM system, available from
Lorad, Danbury Conn.
In addition to the above-described components of sonomammography apparatus
10 that are common to previously known mammography systems, the apparatus
of the present invention includes compression plate 15 and ultrasonic
transducer 16 movably supported on gantry 17. As shown in FIGS. 1 and 2,
compression plate 15 includes gel pad 23 disposed from the underside of
the compression plate, for example, by polyethylene bag 24. Compression
plate 15 may include fenestrations (not shown) for conducting biopsies of
the patient's tissue. Alternatively, depending upon the composition of the
gel pad, gel pad 23 may be used without polyethylene bag 24 and may
include a tacky or adherent surface to assist in positioning the breast.
Gel pad 23 contacts the frontal area of the patient's breast, i.e., the
nipple area, to ensure proper transmission of acoustic waves from
transducer 16 to the distal-most portion of breast tissue 100 with a
minimum of impedance mismatch. As seen in FIGS. 1 and 2, gel pad 23
conforms to the distal-most portion of the breast to minimize impedance
mismatch and acoustic reflectance at the gel pad/breast interface.
Accordingly, gel pad may comprise an agar gelatin and water composition or
other suitable rheostatic material, for example, the gelatinous
elastomeric compositions described in U.S. Pat. Nos. 4,369,284, 4,618,213
and 5,262,468. For sanitary purposes, gel pad 23 (and polyethylene bag 24,
if used) may be disposable, and therefore removably attached to
compression plate 15.
Referring now to FIGS. 3 and 4, another advantage of the gel pad of the
present invention, when used in conjunction with a conventional
mammography system, is described. Referring to FIG. 3A, a portion of a
previously known X-ray mammography system compresses breast 104 between
standard compression plate 91 and bottom plate 92 to a uniform thickness
105. As is per se known, the X-ray exposure is set to provide proper
exposure of thickness 105. By properly exposing the uniform thickness,
however, the outer edges of breast 104, including the nipple region, are
typically overexposed, as reflected at region 106 in illustrative X-ray
image 93. To compensate for this effect, it is typical for a radiologist
to take a second exposure of breast 104 at a lower X-ray dosage, thus
providing an X-ray image wherein the outer edges of the breast are
properly exposed, but the uniform thickness 105 is then underexposed.
Referring now to FIGS. 4A and 4B, an important advantage of the present
invention is illustrated. Applicants have determined that gel pad 23 not
only provides acoustic coupling when used in a sonomammographic system as
described hereinbelow, but that gel pad 23 provides an X-ray attenuation
ability as well. In FIG. 4A, the portion of the system of FIG. 3A is
shown, but also including gel pad 23 of the present invention. Gel pad 23
conforms or adjusts itself to those thinner parts of the breast at the
outer edges where standard radiation doses cause over-exposure.
When gel pad 23 is constructed of a material having an X-ray attenuation
close to that of human tissue, the gel pad attenuates X-rays as if it were
part of the uniform thickness of breast tissue. As shown in FIG. 4B, the
outer edges of the breast, including the nipple region, in the resulting
X-ray image 94 are no longer over-exposed. Gel pad 23 can therefore be
used to enhance conventional mammography equipment by enabling the
radiologist to obtain an X-ray image having properly exposed detail at the
peripheral edges of the subject anatomy with an overall reduction in X-ray
dosage to the patient.
In addition, because gel pad 23 conforms to the shape of the patient's
tissue, it distributes the force applied by compression plate 15 over a
larger surface area, thus reducing the compressive stress applied to the
tissue and reducing patient discomfort. Moreover, if gel pad 23 includes a
slightly tacky or adherent surface, it will better grip the patient's
tissue and reduce difficulties in positioning the tissue.
Referring again to FIGS. 1, 2 and 5, in the first embodiment of the present
invention, compression plate 15 comprises a high performance acoustically
transparent ("sonolucent") and X-ray transparent ("radiolucent") film
which is sufficiently rigid to serve as a compression plate at a thickness
of about 25 micron (1 mil). In particular, it is preferred that
compression plate 15 have sufficient rigidity so that the local slope of
the plate, under load, does not exceed one degree from the horizontal
within the scan area. For further rigidity, compression plate 15 may
include metal reinforcing bars 15 along its lateral end faces.
Kapton.RTM. manufactured by E. I. Du Pont de Nemours and Company,
Wilmington, Del., is a suitable material for practicing the present
invention, as it provides both the needed sonolucent/radiolucent qualities
as well as the needed rigidity to provide satisfactorily as a compression
plate. In particular, a 1 mil (25 micron) thickness of Kapton.RTM., when
used as a compression plate, is expected to cause less than 3 dB
transmission loss in acoustic energy, while providing a tensile strength
equivalent to that of a 2 mm thick polycarbonate plate. In addition,
Kapton.RTM. is unaffected by exposure to X-ray radiation.
Other materials suitable for use in making a radiolucent and sonolucent
compression plate include Surlyn.RTM. ionomers, such as Surlyn.RTM. 8940,
available from E. I. Du Pont de Nemours and Company, Wilmington, Del., and
polymethyl pentenes, such as TPX.RTM. MX-002 and MX-004, available from
Mitsui & Co., Tokyo, Japan. Plates of these materials approximately 6.4 mm
(0.25 inch) thick are expected to be sufficiently rigid to meet the
above-defined deflection criterion if properly supported by a stiffening
frame around their periphery. In FIG. 5 compression plate 15 is shown
comprising a 6.4 mm (0.25 inch) thick sheet of TPX.RTM. 95 fastened to a
metal frame 96. Three sides of the TPX.RTM. sheet 95 are fastened to the
metal frame 96 by suitable fasteners, such as staggered screws 97, and the
fourth side is bonded into a groove 98 in the frame 96. Of the two
materials, the polymethyl pentenes, and TPX.RTM. in particular, are
preferred due to their lower acoustic attenuation and impedance and higher
strength. A sheet made of a Surlyn.RTM. ionomer can also be used in a
similar fashion although it is softer and the acoustic losses are
approximately double that of TPX.RTM..
Referring now to FIGS. 6A and 6B, ultrasonic transducer 16 may comprise a
single piston, annular or phased array imaging device of conventional
design. Such array devices may permit beam-focussing of ultrasonic energy
to provide high resolution images of the internal structures of a
patient's tissue. Ultrasound transducer 16 combines both transmit and
receive functions that are switched, respectively, between transmitting
and receiving operational modes at selected times by control circuitry.
Because the internal structure and operation of ultrasonic apparatus is per
se known, the specific internal configuration of that apparatus forms no
part of the present invention. Transducer 16 preferably operates in a
range of about 2 to 15 MHz. More preferably, the signal produced by the
transducer in the transmit mode is a 10 MHz burst having a 100% bandwidth.
To improve the transfer of acoustic energy, transducer 16 may in addition
be acoustically coupled to the upper surface of compression plate 15 using
an appropriate coupling agent such as, for example, glycerol, or an
additional thin gel pad disposed atop compression plate 15 (omitted for
clarity from FIG. 1).
With respect to the illustrative embodiment of transducer 16 shown in FIGS.
6A and 6B, apparatus for applying a lubricating/coupling agent between
ultrasonic transducer 16 and compression plate 15 is described. Transducer
16 is surrounded by a skirt or cover 110 that includes a spacer 111 formed
along its lower edge. Spacer 111 lifts the contact surface of transducer
16 about 0.06 mm (2.5 mils) above the surface of compression plate 15, and
is shaped to optimize lubrication and acoustic coupling. A sponge-like
material 112 dampened with a suitable lubricating/coupling fluid, for
example, a water-based solution of surfactant and detergent, is disposed
around the transducer 16 such that the sponge-like material 112 and the
spacer 111 are in contact with compression plate 15 at substantially the
same time. Thus, as the transducer assembly moves along the surface of
compression plate 15 a thin film 113 of the lubricating/coupling fluid is
deposited on the plate. Cover 110 also permits the transducer assembly to
be handled without contacting material 112.
Referring again to FIGS. 1 and 2, gantry support 18 is vertically
positioned along column 12 using a motorized or manually adjustable
mechanism. Gantry support 18 includes arms 18' disposed above the lateral
edges of compression plate 15. Gantry support 18 movably supports gantry
17 for movement in distal and proximal directions "A" and "B", using a
motorized track or cable arrangement 25. Gantry support 18 moves gantry 17
in precise increments in the distal and proximal directions. During X-ray
exposure of the patient's tissue, gantry 17 is moved to a distal-most
position in direction "A" so that it does not interfere with the mammogram
exposure. Alternatively, gantry 17 and gantry support 18 may be hinged to
swing away from the compression plate, thus providing clear access for an
X-ray exposure.
Gantry 17 (shown by dotted lines in FIG. 7) in turn comprises carriage 26
that supports ultrasonic transducer 16. Gantry 17 includes its own
motorized drive means 27 for moving carriage 26 laterally in directions
"C" and "D".
Illustrative embodiments of drive means 25 and 27 are described with
respect to FIG. 7. Drive means 25 of gantry support arm 18 may comprise
cables 30 that extend through arms 18' of gantry support 18. Cables 30 are
captured on pulleys 31 and drive wheels 32 to form upper and lower flights
30A and 30B, respectively. Drive wheels 32 are synchronously driven by
motor 33. Gantry 17 is fixedly connected to the upper flights of cables 30
at points 34, so that when the upper flights of cables 30 move in
directions "A" and "B", gantry 17 translates in the corresponding
direction. Motor 33 is of a type that enables exact positioning of gantry
17, for example, so that the gantry 17 can be moved in the proximal and
distal directions in precise increments, such as 1 to 10 mm.
Still referring to FIG. 7, gantry 17 includes its own cable arrangement 27
for precisely positioning carriage 26 and transducer 16. In particular, in
the illustrative embodiment shown, cable 35 runs on drive wheel 36 and
pulley 37 to form upper and lower flights 35A and 35B, respectively.
Carriage 26 is fixed to lower flight 35B of cable 35 at point 38 so that
carriage 26 moves in directions "C" and "D" in response to movement of
lower flight 35B. Motor 38, which is supported on gantry 17, enables
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