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This invention relates to improvements in ultrasonic diagnostic imaging
techniques, and in particular to ultrasonic scanning of the body to
acquire Doppler information for presentation in a three dimensional image
format.
Various methods and devices have been proposed for ultrasonically scanning
a volume within a subject for three dimensional analysis and display. Many
of these techniques involve the scanning of a number of spatially adjacent
image planes. The ultrasonic information from these associated planes can
be analyzed and displayed on the basis of spatial coordinates of the data
within a plane, and on the basis of the spatial relationship of each plane
to the others. The information can be displayed in a three dimensional
image format such as a perspective view of the volume being imaged.
A number of scanning techniques utilizing specially devised scanning
devices have been proposed for acquiring these spatially related image
planes. The article "Three-Dimensional Reconstruction of Echocardiograms
Based On Orthogonal Sections," by S. Tamura et al., Pattern Recognition,
vol. 18, no. 2, pp 115-24 (1985) discusses three such devices: a guide
rail to guide an ultrasonic probe while acquiring parallel image planes; a
jointed arm in which sensors in the arm joints provide spatial coordinates
for the transducer; and rotation of a transducer about the cardiac long
axis. A rotating transducer probe for the latter purpose is shown and
described in "Multidimensional Ultrasonic Imaging for Cardiology," by H.
McCann et al., Proceedings of the IEEE, vol. 76, no. 9, pp 1063-73 (Sept.
1988). It would be preferable, however, to be able to acquire multiple
image planes for three dimensional presentation without the need for
special scanning devices or apparatus.
Ultrasonic images are subject to image artifacts arising from a number of
sources such as reverberation, multipath echoes, and coherent wave
interference. These artifacts will manifest themselves in various ways in
the image which can be broadly described as image clutter. The image
clutter becomes particularly troublesome when images are presented in a
three dimensional format, as the three dimensional clutter can interfere
with and obscure pathology which the clinician is attempting to diagnose.
Accordingly it would be desirable to provide ultrasonic image information
in a format in which clutter does not significantly impair the pathology
being viewed.
In accordance with the principles of the present invention the present
inventors have addressed this problem of obscuring clutter through the use
of ultrasonic Doppler information signals. Doppler information has been
used to image the body in two distinct ways. One Doppler imaging technique
is commonly referred to as color Doppler velocity imaging. As is well
known, this technique involves the acquisition of Doppler data at
different locations called sample volumes over the image plane of an
ultrasonic image. The Doppler data is acquired over time and used to
estimate the Doppler phase shift or frequency at each discrete sample
volume. The Doppler phase shift or frequency corresponds to the velocity
of tissue motion or fluid flow in vessels within the body, with the
polarity of the shift indicating direction of motion or flow. This
information is color coded in accordance with the magnitude of the shift
(velocity) and its polarity, and overlaid over a structural image of the
tissue in the image plane to define the structure of the moving organs or
vessels in which fluids are flowing. The colors in the image thereby
provide an indication of the speed of blood flow and its direction in the
heart and blood vessels, for instance.
A second Doppler technique is known as color power Doppler. This technique
is unconcerned with estimations of the velocity of motion or fluid flow.
Rather, it focuses simply on the intensity of the received signals which
exhibit a Doppler shift. This Doppler signal intensity can be measured at
each sample volume in an image plane and displayed in a color variation.
Unlike color Doppler velocity imaging, color power Doppler does not
present the problems of directionality determination, aliasing, and low
sensitivity which are characteristic of velocity imaging. Color power
Doppler simply displays the Doppler signal intensity at a sample volume in
a coded color. Like color Doppler velocity imaging, the color power
Doppler display is overlaid with a structural B mode image to define the
organ or tissue structure in which motion is occurring. Since the value at
each sample volume can be averaged over time or based upon a peak value,
and is not subject to the constant changes of velocity and direction which
are characteristic of the pulsatility of Doppler velocity signals, the
color power Doppler display can be presented as a more stable display of
motion or flow conditions in the body.
In accordance with the principles of the present invention, a three
dimensional ultrasonic display technique is provided which utilizes power
Doppler signal information. The present inventors have utilized power
Doppler images in an unconventional way, which is in the absence of
structural (B mode) information. The present inventors have discovered
that utilizing power Doppler information alone in a three dimensional
display eliminates the substantial clutter contribution of the structural
information signals, eliminates pulsatility variation, provides excellent
sensitivity to low energy flow signals, reduces Doppler angle effects, and
provides a segmentation of the flow or motion characteristics in the three
dimensional image. The present inventors also present a technique for
acquiring diagnostic three dimensional ultrasonic images through manual
hand scanning of a patient, without the need for specially fabricated
scanning mechanisms or devices.
In the drawings:
FIG. 1 is a block diagram of an ultrasonic diagnostic imaging system
constructed in accordance with the principles of the present invention;
FIG. 2 illustrates the manual scanning of a bifurcation in the body of a
patient;
FIGS. 3a-3e illustrate a sequence of two dimensional Doppler power images
acquired from the bifurcation of FIG. 2;
FIG. 4 illustrates the relation of the image planes of FIGS. 3a-3e to the
structure of the bifurcation of FIG. 2;
FIGS. 5a and 5b are a comparison of the bifurcation of FIG. 2 to a three
dimensional Doppler power display of the blood flow of the bifurcation;
FIGS. 6a-6d illustrates the three dimensional relationship of manually
acquired two dimensional image planes;
FIG. 7 illustrates a scanning aid for manually acquiring uniformly spaced
image planes; and
FIG. 8 is a flow chart used to explain the preferred technique for
processing Doppler power images for three dimensional display.
Referring first to FIG. 1, a block diagram of an ultrasonic diagnostic
imaging system constructed in accordance with the principles of the
present invention is shown. An ultrasonic probe 10 includes a multielement
transducer 12 which transmits waves of ultrasonic energy into the body of
a patient and receives ultrasonic echoes returning from structures in the
body. In the case of ultrasonic wave transmission for Doppler
interrogation of the body, it is the echoes returning from moving tissue,
blood and other fluids in the body that are of interest. The ultrasonic
probe 10 is connected to a transmitter/receiver 14 which alternately
pulses individual elements of the transducer to shape and steer an
ultrasonic beam, and receives, amplifies and digitizes echo signals
received by the transducer elements following each pulse transmission.
The transmitter/receiver 14 is coupled to a beamformer 16 which controls
the times of activation of specific elements of the transducer 12 by the
transmitter/receiver. This timing enables the transducer 12 to transmit a
shaped and focused ultrasound beam in a desired direction. The beamformer
16 also receives the digitized echo signals produced by the
transmitter/receiver during echo reception and appropriately delays and
sums them to form coherent echo signals.
The echo signals produced by the beamformer 16 are coupled to a B mode
processor 30 and an I,Q demodulator 18. The B mode processor processes the
amplitude information of the echo signals on a spatial basis for the
formation of a structural image of the tissue in the area of the patient
being scanned. The I,Q demodulator 18 demodulates the received echo
signals into quadrature components for Doppler processing. The I,Q
components are filtered by a wall filter 20 to remove low frequency
artifacts stemming from the movement of vessel walls in applications where
it is only the motion of flowing fluids such as blood that is of interest.
The filtered I,Q components are then applied to a Doppler shift estimation
processor 22 and a Doppler power estimation processor 24.
The Doppler shift estimation processor 22 operates in the conventional
manner to estimate a Doppler phase or frequency shift from the I,Q
components at each sample volume location of the image field. The Doppler
shift estimation processor operates on a number of signal samples
resulting from the interrogation of each sample volume location by an
ensemble of Doppler interrogation pulses. The sample volume values are
applied to a velocity image processor 26 which maps the values to color
values for display. The color values are applied to a scan converter and
display processor 32 which spatially arranges the color values in the
desired image format. The color values are displayed as pixels on a
display 40, wherein each color represents a particular velocity of flow in
a particular direction at that pixel location. The color flow velocity
information is overlaid with a structural image of the interior of the
body utilizing the structural information provided by the B mode processor
30. This compound image shows both the direction and velocity of blood
flow, as well as the structure of the vessels or organs which contain the
flowing blood.
In accordance with the principles of the present invention the Doppler
system of FIG. 1 also includes a power Doppler imaging capability. The
power Doppler components include a Doppler power estimation processor 24
which estimates the Doppler signal power magnitude from the I,Q signal
components at each sample volume location using the expression (I.sup.2
+Q.sup.2).sup.1/2. The Doppler power estimates at each location can be
processed and displayed in real time or can be averaged with earlier
acquired power estimates for each sample volume location. In a preferred
embodiment, each sample volume location is interrogated by a number of
pulses and the estimation processor 24 utilizes the signals obtained from
all interrogations in the estimations of Doppler power at the sample
volume locations. These Doppler power estimates are mapped to display
intensity or color values by a power image processor 28. The display
values with their spatial coordinates are stored in separate planar images
in an image sequence memory 34 and are also applied to the scan converter
and display processor 32 which spatially arranges the Doppler power
display values in the desired image format, e.g., sector or rectangular.
The two dimensional Doppler power images may then be displayed on a
display 40 or recalled from the image sequence memory 34 for three
dimensional processing using a peak detector 36 for maximum Doppler power
intensity detection as discussed below. User operation of the system of
FIG. 1 is effected through various user controls 42 which enable the user
to select the type of imaging to be performed, i.e., B mode, color
velocity Doppler or Doppler power imaging, and to store and retrieve
images from the image sequence memory 34 for three dimensional display,
for example.
FIG. 2 illustrates the use of the ultrasonic probe 10 to manually acquire a
sequence of image planes for three dimensional display. A portion of the
probe cable 11 leading to the transmitter/receiver of the ultrasound
system is shown at the top of the probe. The transducer aperture of the
probe 10 is in contact with the skin of the patient over the region of the
body which is to be scanned. The skin of the patient is represented by a
layer 50 in the drawing. In this example the region of the patient being
scanned includes a blood vessel bifurcation 52 having a small vessel 54
branching out from a larger vessel 56. Blood is flowing inside the
structural walls of the vessels as indicated at 60 and 62.
The bifurcation 52 may be scanned by rocking or fanning the probe 10 while
it is in contact with the patient. In a preferred technique the probe
aperture slides over the skin 50 as indicated by arrow 58 to scan the
bifurcation region with a plurality of substantially parallel image
planes. One such image plane 64, here shown as a sector, is seen
projecting from the transducer aperture of the probe. The relation of the
image plane 64 to the probe is denoted by an image plane marker 13 on the
side of the probe case. The marker 13 is in the same plane as the image
plane 64, and denotes the upper left side of the image in its uninverted
display orientation.
In accordance with the present invention, the ultrasound system acquires
and processes power Doppler information from a plurality of image planes
as the probe slides over the bifurcation region of the patient as
indicated by the arrow 58. The duration of such a scan can typically last
about ten to twenty seconds, during which time 100 to 200 image planes of
power Doppler information are acquired, processed and stored in the image
sequence memory 34. This image information is processed to detect and
record the maximum Doppler intensity at a number of different viewing
angles over a range of such viewing angles as discussed below.
FIGS. 3a-3e shows a five image plane sequence which illustrates the
principles of the power Doppler three dimensional imaging technique of the
present invention. The five image planes of the sequence are referenced to
the structure of the bifurcation 52 in FIG. 4, which is a view of the top
of the two vessels. FIG. 3a is a power Doppler image taken along plane 3a
of FIG. 4, which is seen to intersect the upper edge of the blood flow of
the large vessel 56, just inside the vessel wall 56'. In FIG. 3b the image
plane intersects a greater cross section 72 of the blood flow of the large
vessel 56, and the edge 74 of the blood flow of the small vessel 54, just
inside the vessel wall 54' as plane 3b of FIG. 4 shows. The image plane of
FIG. 3c intersects the centers of both vessel as is seen by plane 3c in
FIG. 4. In FIG. 3d the image plane moves down to a lesser cross section of
both vessels and the plane 3e of FIG. 3e intersects only the peripheral
blood flow in the large vessel 56.
The images of FIGS. 3a-3e are processed and presented together in a three
dimensional presentation as illustrated in FIG. 5b. The three dimensional
image is seen to comprise the power Doppler information without any
structural image overlay. This is clearly seen by comparing the three
dimensional power Doppler image 80 of FIG. 5b with the similarly scaled
rendering of the bifurcation 52 in FIG. 5a. The rendering of FIG. 5a is
seen to include the structure of the vessel walls 54' and 56' which
contain flowing blood indicated at 60 and 62. The power Doppler image 80,
resulting from the Doppler detected movement of the flowing blood, is
displayed without any B mode structure of the vessel walls 54' and 56'. It
has been found that omitting the vessel walls from the three dimensional
display does not diminish the effectiveness of the display, as the
continuity of the blood flow intensity serves to define the paths in which
blood is flowing. In addition, the absence of B mode echos eliminates
considerable structural echo clutter from the image. The image is clearly
segmented by the flow selectivity, and the smoothly varying stability and
sensitivity of the maximum intensity power Doppler information.
FIG. 8 is a flowchart illustrating a preferred technique for processing a
sequence of planar Doppler power images for real time three dimensional
display. As described above, the Doppler power display values with their
spatial coordinates are stored in a sequence of planar images in the image
sequence memory 34, as shown by step 80 in FIG. 8. The images of FIGS.
3a-3e are illustrative of such a two dimensional image sequence. In step
82 the process receives processing parameters provided by the user
controls. One parameter is the range of viewing angles, .theta..sub.1
-.theta..sub.M, over which the three dimensional presentation is to be
viewed. The other parameter is the increment .DELTA..theta. between each
viewing angle in the range. For instance the user could input a range of
viewing angles of +60.degree. to -60.degree., referenced to a line of view
in a plane which is normal to the plane of the first image in the
sequence, and a range increment of 1.degree.. From these inputs the number
of three dimensional projections needed is computed in step 82. In this
example 121 projections are needed to display a 120.degree. range span in
one degree increments.
The process now begins to form the necessary sequence of 121 maximum
intensity projections. In step 84 the planar Doppler power images are
recalled from the image sequence memory for sequential processing by the
scan converter and display processor 32. In step 86 each planar image is
rotated to one of the viewing angles .theta..sub.n, then projected back to
the viewing plane. In step 88 the pixels of the projected planar images
are accumulated on a maximum intensity basis. Each projected planar image
is overlaid over the previously accumulated projected images but in a
transposed location in the image plane which is a function of the viewing
angle and the interplane spacing: the greater the viewing angle, the
greater the transposition displacement from one image to the next. The
display pixels chosen from the accumulated images are the maximum
intensity pixels taken at each point in the image planes from all of the
overlaid pixels accumulated at each point in the image. This effectively
presents the maximum intensity of Doppler power seen by the viewer along
every viewing line between the viewer and the three dimensional image. In
a preferred embodiment the relocation of image points after rotation about
the y axis, projection and transposition may be expressed as:
##EQU1##
and the relocation of image points after rotation about the x axis,
projection and transposition may be expressed as:
##EQU2##
where .theta. is the angle of rotation, (x, y, z) are the coordinates of a
point to be relocated, and (x', y') are the coordinates of a point in the
viewing plane after relocation.
After all of the planar images have been rotated, projected, transposed,
overlaid, and the maximum intensities at each pixel chosen, the resulting
three dimensional image for the viewing angle .theta..sub.n is stored in
the image sequence memory 34 as a brightness modulated monochrome image in
a three dimensional image sequence. In step 92 the process returns to step
84 and proceeds through steps 84-92 until the full three dimensional image
sequence has been stored in memory. In the present example this is a
sequence of 121 three dimensional images over the range of +60.degree. to
-60.degree..
The stored three dimensional sequence is now available for recall and
display in step 94 upon command of the user. As the sequence is recalled
and displayed in real time, the user sees a three dimensional presentation
of the motion or fluid flow occurring in the volumetric region over which
the planar images were acquired. The volumetric region is viewed three
dimensionally as if the user were moving around the region and viewing the
motion or flow from changing viewing angles. In this particular example
the user has the impression of moving over a range of viewing angles
spanning 120.degree. around the volumetric region. The viewer can sweep
back and forth through the sequence, giving the impression of moving
around the volumetric region in two directions.
FIGS. 6a-6d illustrate the effects of nonuniform spacing of image planes
which can arise from manual image plane scanning. FIG. 6a is a top view of
the large vessel 56, showing the blood flow 60 surrounded by the vessel
wall 56' for reference. FIG. 6b shows another sequence of five image
planes taken across the vessel but unlike the sequence of FIG. 4, these
image planes are unevenly spaced. Image planes 1 and 2 are seen to be more
widely spaced than the closer spacing of image planes 4 and 5. Such a
spacing will result for instance when the probe slides faster when
acquiring image planes 1 and 2 and slows down as it approaches the
positions of image planes 4 and 5. This sequence is acquired by manually
sliding the probe from left to right at a progressively slower speed
across the skin above the vessel 56.
In a constructed embodiment of the present invention the image planes are
assumed to be evenly spaced across the imaged volume and are processed and
displayed accordingly. FIG. 6c shows the five image planes of FIG. 6b from
above when they are evenly spaced for display. The result of this spacing
is more readily seen in FIG. 6d, in which the border of the blood flow and
the vessel wall 56" have been reconnected for ease of illustration. The
arrows at 78 illustrate the uniform image plane spacing, which is slightly
less than the spacing of image planes 1 and 2 in FIG. 6b and slightly
greater than the spacing of image planes 5 and 6 in that drawing. The
effect is to give the cross sectional area of the blood flow a slightly
oblong appearance in which the left side of the flow area is compressed
and the right side extended in relation to the actual proportions of the
blood flow area.
The present inventors have observed that this distortion of the aspect
ratio of the three dimensional image does not noticeably detract from the
effect of the overall three dimensional display. Even with such aspect
distortion the three dimensional image continues to show the relative
paths and orientations of blood vessels and the continuity or stenosis of
flow in vessels in a manner not achieved by two dimensional presentations.
The continuity of flow paths and display effectiveness is enhanced by
displaying the Doppler power on the basis of the maximum signal intensity.
When the image planes are acquired from a range of acquisition angles the
use of the maximum intensity display has the effect of diminishing
sensitivity variation resulting from Doppler angle effects. The image
planes may be concurrently displayed in the form of a surface rendering or
a transparency of the blood flow information, but a preferred presentation
is a monochrome display of the varying brightness of the maximum intensity
pixels of the combined images of a volumetric region as described above.
The flow and perfusion of the blood supply in an organ such as a kidney is
more completely displayed with a three dimensional power Doppler image
than can be accomplished with a two dimensional presentation. The
technique is well suited for assessing the success of organ transplants,
for instance.
Simple aids may be provided to improve the accuracy of manual three
dimensional scanning if desired. One such aid is shown in FIG. 7, and
comprises a ruler scale printed on a clear strip of surgical tape. The
tape is applied to the skin of the patient adjacent to the probe, and the
probe is moved along the scale with the marker 13 on the probe used as a
reference. Image planes can be acquired at each marker on the scale, or
the scale can be traversed in a given time such as twenty seconds. Other
aids may also be supplied by the ultrasound system such as audible signals
or lights telling the user when to start and stop movement of the probe,
and when the moving probe should be passing each marker on the scale.
The imaging techniques of the present invention including particularly that
of FIG. 8 can be applied to a sequence of planar images acquired with
position sensing of the image planes for display of anatomically precise
images. An advantageous Doppler technique for sensing the positions of the
image planes and lines in each plane in relation to each other is
described in U.S. Pat. No. 5,127,409. When the positions of the image
planes or lines are known in relation to each other the three dimensional
processor no longer has to assume uniform spacing between two dimensional
planes, but can utilize the measured spacing between three dimensional
display elements to form more geometrically accurate three dimensional
images.
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