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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. 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.
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 roundtrip 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
roundtrip. 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 beamforming 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,
Polyethylene, 2.5-3.3 2.2
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 free-hand 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 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 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.
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 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 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.
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.
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.
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;
FIG. 3 is a schematic view of an illustrative embodiment of the drive means
employed in the sonomammography apparatus of FIG. 1;
FIG. 4 is a perspective view of a workstation and digitizing tablet adapted
for use with the present invention;
FIG. 5 is a perspective view of an alternative embodiment of the
sonomammography apparatus of the present invention;
FIG. 6 is a cross-sectional view taken along view line 6--6 of FIG. 5;
FIG. 7 is a perspective view of the diffraction grid and ultrasonic
transducer apparatus of the present invention;
FIG. 8 is a cross-sectional view of another alternative embodiment of the
present invention;
FIG. 9 is a block diagram of the elements of an ultrasonic imaging system
in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIGS. 1 and 2, an illustrative embodiment of a first
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 endface 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 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. 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 and polyethylene bag 24 conform 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
sanitary purposes, gel pad 23 and polyethylene bag 24 may be disposable,
and therefore removably attached to compression plate 15.
In accordance with 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. Suitable materials
should have mechanical properties, for a 1 mil (25 micron) thickness, such
as a tensile strength of about 24,000 psi, a dielectric strength of about
6000 AC volts/mil, and a volume resistivity of about 10.sup.12 ohm-cm at
200.degree.. For further rigidity, compression plate 15 may include metal
reinforcing bars 15' along its lateral endfaces.
Kapton.TM. manufactured by E.I. Du Pont de Nemours and Company, Wilmington,
Del., is an ideal 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, 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 is unaffected by
exposure to X-ray radiation.
Referring still to FIGS. 1 and 2, 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.
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).
Referring to FIGS. 1-3, 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.
Gantry 17 (shown by dotted lines in FIG. 3) 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. 3. 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. 3, 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 35' 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
precise control of carriage 26 and thus transducer 16.
Alternatively, a toothed belt and gear arrangement may be substituted for
the cables, pulleys and drive wheels of the above-described illustrative
embodiment. As further alternatives, drive means 25 and 27 may employ, for
example, a conventional motorized track, a threaded block carried on a
threaded drive rod controlled by an encoder and stepper motor, or any
other suitable means.
It is to be understood that appropriately programmed control circuitry is
provided for use with any of the foregoing drive means 25 and 27 so that
the drive means pauses at predetermined locations during transit for a
period sufficient to obtain an ultrasound image of the breast tissue at
that location. In addition, gantry 17 and gantry support 18 may provide
release mechanisms that enable transducer 16 to be manually positioned by
the operator.
Referring again to FIG. 2, arm 18' of gantry support 18 includes slot 39,
through which an extension of gantry 17 projects to engage biopsy needle
guide 21. Thus, as gantry 17 moves in distal and proximal directions "A"
and "B", biopsy needle guide 21 remains in alignment with ultrasonic
transducer 16. Biopsy needle guide 21 includes a needle support element 40
having an aperture through which a biopsy needle may be inserted to
perform an ultrasound image-guided biopsy of the patient's tissue. Needle
support element 40 may be positioned at any desired position by the
medical practitioner and then engaged with biopsy needle support 21 for
performing image-guided biopsy.
Lateral alignment of the biopsy needle in accordance with this aspect of
the present invention provides important psychological benefits to the
patient. Since the biopsy needle is laterally inserted into the patient's
breast, rather than through the upper surface, it produces no scarring on
the upper surface of the breast. Accordingly, the patient will not be
discouraged from wearing clothing, e.g., an evening gown which exposes the
upper surface of the breasts, out of concern that unsightly scar tissue
from a biopsy puncture will be visible.
Ultrasound transducer 16 generates an image corresponding to the internal
structure of the tissue located in the plane perpendicular to transducer
at each of the locations where carriage 26 stops during its transit across
compression plate 15. The images or frames generated at each of these
locations is stored on a microprocessor based workstation 41, such as
shown in FIG. 4, for later postprocessing and manipulation.
Referring now to FIG. 4, for an embodiment of the present invention for use
with conventional mammography apparatus that generates an X-ray film, an
X-ray film 42 is positioned on digitizing tablet 43 so that index marks 44
and 44' on the X-ray film coincide with positioning marks on digitizing
tablet 43. Digitizing tablet 43 includes pen 45 and is connected to
workstation 41 having monitor 46. Workstation 41 includes suitable
software for interpreting movement of pen 45 with respect to digitizing
tablet 43.
When X-ray film 42 is aligned on digitizing pad 43, pen 45 of the
digitizing tablet enables the medical practitioner to display on monitor
46 the orthogonal ultrasound image corresponding to a location on X-ray
film 42 by touching pen 45 to digitizing tablet 43. Thus, the position of
the contact of pen 45 to digitizing tablet 43 automatically brings up the
corresponding orthogonal ultrasound frame at that location, providing the
medical practitioner with a holographic, i.e, three-dimensional, view of
the internal structure of the tissue. Moreover, the precise geometric
registration of the ultrasound image frames and the X-ray film provided by
the present invention enables the medical practitioner to manipulate the
ultrasound images, to perform, for example, digital subtraction, thereby
enhancing breast lesion detection capability.
The PowerPC.TM. commercially available from Apple Computer, Cupertino,
Calif., provides a suitable workstation for use as described above, while
the HiSketch series of digitizing tablets, available from Kye
International Corp., Ontario, Calif., provide suitable digitizing tablets
for use in conjunction with the sonomammography apparatus of the present
invention.
Referring now to FIGS. 5-7, an alternative embodiment of a sonomammography
apparatus 50 constructed in accordance with the principles of the present
invention is described. Sonomammography apparatus 50 includes base 51,
upright vertical column 52, X-ray tube 53 supported on vertical movable
arm 54, compression plate 55, diffraction grid 56, ultrasound transducer
57 and film holder 58. Components 50-54 may constitute the elements of a
conventional mammography system as described hereinabove. X-ray sensitive
film 59 is disposed in film holder 58 beneath ultrasound transducer 57.
Sonomammography apparatus 50 differs from apparatus 10 described
hereinabove principally in that the sonolucent compression plate 15,
transducer 16, gantry 17 and gantry support 18 are replaced by modified
diffraction grid 56 and ultrasound transducer 57. Compression plate 55 may
be fenestrated to enable the medical practitioner to perform
ultrasound-image guided biopsies.
Referring now to FIG. 7, diffraction grid 56 comprises an array of an X-ray
absorptive material 61, such as lead, having its interspaces f | | |
