U.S. patent application number 10/134164 was filed with the patent office on 2003-10-30 for contrast-agent enhanced color-flow imaging.
This patent application is currently assigned to Koninklijke Philips Electronics N.V.. Invention is credited to Brock-Fisher, George A., Perry, Jodi L.T., Poland, Mckee Dunn, Rafter, Patrick G..
Application Number | 20030204142 10/134164 |
Document ID | / |
Family ID | 29249151 |
Filed Date | 2003-10-30 |
United States Patent
Application |
20030204142 |
Kind Code |
A1 |
Brock-Fisher, George A. ; et
al. |
October 30, 2003 |
CONTRAST-AGENT ENHANCED COLOR-FLOW IMAGING
Abstract
Systems and methods for enhanced color-flow imaging of
contrast-agent perfused blood vessels and other tissues within a
patient's body are disclosed. The method generally comprises,
introducing one or more contrast agents into the body,
power-modulating transmit pulses into the body, receiving echoes
from the body, processing the received echoes to reduce
tissue-generated echoes and echoes from stationary contrast agent,
using a color-flow processor to generate a color-encoded display
responsive to contrast-agent motion. The method may be implemented
by a system with a an excitation-signal source, a transducer, an
ultrasound-processing system having multiple image processors
including a color-flow processor, as well as, a clutter filter, and
an arbiter, and a display-processing system.
Inventors: |
Brock-Fisher, George A.;
(Andover, MA) ; Perry, Jodi L.T.; (Methuen,
MA) ; Rafter, Patrick G.; (Windham, NH) ;
Poland, Mckee Dunn; (Andover, MA) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P.O. BOX 3001
BRIARCLIFF MANOR
NY
10510
US
|
Assignee: |
Koninklijke Philips Electronics
N.V.
|
Family ID: |
29249151 |
Appl. No.: |
10/134164 |
Filed: |
April 26, 2002 |
Current U.S.
Class: |
600/458 |
Current CPC
Class: |
G01S 15/8981 20130101;
G01S 7/52038 20130101; G01S 7/52039 20130101; G01S 15/102 20130101;
A61B 8/13 20130101; A61B 8/481 20130101; A61B 8/06 20130101 |
Class at
Publication: |
600/458 |
International
Class: |
A61B 008/00 |
Claims
What is claimed is:
1. A method for imaging contrast agents, comprising: transmitting
power-modulated ultrasonic pulses comprising a predetermined
transmit sequence having a plurality of transmit lines into a
patient's body; receiving a plurality of ultrasonic echoes
comprising contrast-agent generated echoes and tissue-generated
echoes from the patient's body; processing the received ultrasonic
echoes to generate a plurality of ultrasonic-echo signals
responsive to both the contrast-agent generated and
tissue-generated echoes; processing the plurality of
ultrasonic-echo signals to suppress tissue-generated echoes;
processing the plurality of ultrasonic-echo signals to suppress
stationary contrast-agent generated echoes; applying the plurality
of contrast-agent generated echo signals to a color-flow algorithm
to generate a plurality of data points responsive to contrast-agent
motion; and displaying the plurality of data points over time.
2. The method of claim 1, wherein processing to suppress tissue
generated signals comprises applying a finite-impulse-response
(FIR) filter to the received ultrasonic echoes.
3. The method of claim 1, wherein processing to suppress stationary
contrast-agent generated echoes comprises applying a two-stage
clutter filter to the received ultrasonic echoes.
4. The method of claim 1, wherein the plurality of data points
responsive to contrast-agent motion contain information related to
direction of motion and relative velocity.
5. The method of claim 1, wherein the plurality of transmit lines
are generated with transmit signals having different voltage
amplitudes.
6. The method of claim 1, wherein the plurality of transmit lines
are generated with transmit signals having different phases.
7. The method of claim 1, wherein the plurality of transmit lines
are generated with transmit signals having different
polarities.
8. The method of claim 1, wherein the plurality of data points
responsive to contrast-agent motion contain information related to
direction of motion and relative velocity.
9. The method of claim 2, wherein a plurality of first coefficients
are applied to the received ultrasonic echoes.
10. The method of claim 4, wherein displaying is performed after a
determination that the intensity of the velocity information
exceeds a threshold.
11. The method of claim 4, wherein displaying is performed after
correcting the velocity information for tissue motion.
12. The method of claim 9, wherein a plurality of second
coefficients are applied to the received ultrasonic echoes.
13. The method of claim 10, wherein B-mode image data is displayed
after a determination that the intensity of the velocity
information fails to meet the threshold.
14. An ultrasound-imaging system, comprising: means for reducing
tissue-generated ultrasonic echo signals; means for reducing
stationary contrast-agent generated ultrasonic-echo signals; and
means for imaging moving contrast-agent generated ultrasonic-echo
signals.
15. The system of claim 14, wherein reducing tissue-generated
ultrasonic echo signals comprises a power-modulation technique that
uses multiple-transmit line subpackets.
16. The system of claim 14, wherein imaging comprises applying the
moving contrast-agent generated ultrasonic-echo signals to a
color-flow processor.
17. The system of claim 14, wherein reducing stationary
contrast-agent generated ultrasonic-echo signals comprises applying
a first clutter filter.
18. The system of claim 15, wherein the power-modulation technique
comprises repetitively firing the multiple-transmit line
subpackets.
19. The system of claim 16, wherein the color-flow processor
generates information responsive to the direction and the rate of
motion of moving contrast agent.
20. The system of claim 17, wherein the first clutter filter
comprises a one-zero filter.
21. The system of claim 20, wherein the one-zero filter is
time-shifted filter over multiple samples generated from a
plurality of ultrasonic-echo signals.
22. The system of claim 21, further comprising: means for
determining tissue velocity, and means for combining the tissue
velocity with the information responsive to the direction and the
rate of motion of moving-contrast agent.
23. The system of claim 22, wherein determining tissue velocity
comprises applying the received ultrasonic-echo signals to a second
clutter filter prior to the means for reducing tissue-generated
ultrasonic-echo signals.
24. An improved ultrasound-imaging system, comprising: an
excitation-signal source configured to generate a power-modulated
transmit-line sequence; a transducer coupled to the
excitation-signal source, the transducer configured to emit a
plurality of ultrasonic-pulses responsive to the power-modulated
transmit-line sequence into a medium and to convert a plurality of
received ultrasonic echoes responsive to both tissue and one or
more contrast agents within the medium to a plurality of echo
signals; an ultrasound-processing system coupled to the transducer,
the ultrasound-processing system configured to reduce
tissue-generated ultrasonic-echo signals and reduce stationary
contrast-agent generated ultrasonic-echo signals, while passing
ultrasonic-echo signals generated from moving contrast agent; and a
display-processing system coupled to the ultrasound-processing
system, the display-processing system configured to receive and
generate a graphic representation responsive to the ultrasonic-echo
signals generated from moving contrast agent.
25. The system of claim 24, wherein the power-modulated
transmit-line sequence is generated with transmit signals having
different voltage amplitudes.
26. The system of claim 24, wherein the power-modulated
transmit-line sequence is generated with transmit signals having
different polarities.
27. The system of claim 24, wherein the power-modulated
transmit-line sequence is generated with transmit signals having
different phases.
28. The system of claim 24, wherein the ultrasound-processing
system comprises a clutter filter.
29. The system of claim 28, wherein the ultrasound-processing
system comprises a plurality of two-dimensional imaging
processors.
30. The system of claim 29, wherein the ultrasound-processing
system comprises a color-flow processor.
31. The system of claim 28, wherein the clutter filter comprises a
multiple sample one-zero filter.
32. The system of claim 31, wherein the clutter filter time shifts
the zero between adjacent ultrasonic-echo signal samples.
33. The system of claim 32, further comprising: a tissue-velocity
processor coupled to the ultrasound-processing system, the
tissue-velocity processor configured to generate a first output
signal responsive to motion of tissue-generated ultrasonic-echo
signals; an arbiter coupled to a second output signal from the
color-flow processor and a third output signal from at least one of
the plurality of two-dimensional image processors, the arbiter
configured to forward the second output signal from the color-flow
processor when the intensity of the second output signal exceeds a
threshold; and an arithmetic junction coupled to an output of the
arbiter and the first output signal, the arithmetic junction
configured to perform a subtraction of the first output signal from
the second output signal.
34. The system of claim 33, wherein the arbiter is configured to
forward the third output signal from at least one of the plurality
of two-dimensional image processors when the intensity of the
second output signal fails to exceed a threshold.
Description
FIELD OF THE INVENTION
[0001] The present disclosure relates to ultrasonic imaging. More
particularly, a system and method for improved contrast-agent
enhanced-diagnostic evaluations are disclosed.
BACKGROUND OF THE INVENTION
[0002] Ultrasonic imaging has quickly replaced conventional X-rays
in many clinical applications because of its image quality, safety,
and low cost. Ultrasonic images are typically formed through the
use of phased or linear-array transducers which are capable of
transmitting and receiving pressure waves directed into a medium
such as the human body. These ultrasonic transducers may be further
assembled into a housing, which may contain control electronics,
the combination of which forms an ultrasonic probe. Ultrasonic
probes are used along with an ultrasonic transceiver to transmit
and receive pressure waves through the various tissues of the body.
The various ultrasonic responses are then processed by an
ultrasonic-imaging system to display the various structures and
tissues of the body.
[0003] Some ultrasound-imaging systems can create two-dimensional
B-mode images of tissue in which the brightness of a pixel is based
on the intensity of the received ultrasonic echoes. In another
common imaging modality, typically known as color-flow imaging, the
flow of blood or movement of tissue is observed. Color-flow imaging
modalities take advantage of the Doppler effect to color-encode
image displays. In color-flow imaging, the frequency shift of
backscattered ultrasound waves is used to measure the velocity of
the backscatterers from tissues or blood. The frequency of sound
waves reflecting from the inside of blood vessels, heart cavities,
etc. is shifted in proportion to the velocity of the blood cells.
The frequency of ultrasonic waves reflected from cells moving
towards the transducer is positively shifted. Conversely, the
frequency of ultrasonic reflections from cells moving away from the
transducer is negatively shifted. The Doppler shift may be
displayed using different colors to represent speed and direction
of flow. To assist diagnosticians and operators, the color-flow
image may be superimposed on the B-mode image.
[0004] Current color-flow imaging techniques have disadvantages in
that it is difficult to obtain diagnostic quality images from
patients that have a poor acoustic window (i.e., patients having a
relatively large volume of tissue between their skin and their rib
cage for heart related studies). In addition, it is often difficult
to separate desired blood-velocity signals from artifacts that
result from moving tissue. This problem is most severe when there
is not a relatively large difference in velocity between the tissue
and the blood contained therein.
[0005] Ultrasonic imaging can be particularly effective when used
in conjunction with contrast agents. In contrast-agent imaging, gas
filled micro-sphere contrast agents known as microbubbles are
typically injected into a medium, normally the bloodstream. Due to
their physical characteristics, contrast agents stand out in
ultrasound examinations and therefore can be used as markers that
identify the amount of blood flowing to or through the observed
tissue. In particular, the contrast agents resonate in the presence
of ultrasonic fields producing radial oscillations that can be
easily detected and imaged. Normally, this response is imaged at
the second harmonic of the transmit frequency, f.sub.o. By
observing anatomical structures after introducing contrast agents,
medical personnel can significantly enhance imaging capability for
diagnosing the health of blood-filled tissues and blood-flow
dynamics within a patient's circulatory system. For example,
contrast-agent imaging is especially effective in detecting
myocardial boundaries, assessing micro-vascular blood flow, and
detecting myocardial perfusion.
[0006] U.S. Pat. No. 5,410,516 to Uhlendorf et al. discloses
contrast-agent imaging along with single-pulse excitation
techniques such as harmonic imaging. Specifically, Uhlendorf
teaches that by choosing a radio frequency (RF) filter to
selectively observe any integer harmonic (2nd, 3rd, etc.),
subharmonic (e.g., 1/2 harmonic) or ultraharmonic (e.g., {fraction
(3/2)} harmonic) it is possible to improve the microbubble to
tissue ratio. The second harmonic has proven most useful due to the
large bubble response at this frequency as compared to higher-order
integer harmonics, subharmonics or ultrahanmonics. The second
harmonic also is most practical due to bandwidth limitations on the
transducer (i.e., <70% bandwidth, where percent bandwidth is
defined as the difference of the high corner frequency -6 dB point
from the low corner frequency 6 dB point, divided by the center
frequency.) However, single-pulse excitation techniques together
with harmonic imaging suffer from poor microbubble-to-tissue ratios
as large tissue generated integer-harmonic signals mask the signals
generated by the contrast agent.
[0007] As a result, of the discrimination problem associated with
single-pulse excitation techniques, many multiple-pulse
methodologies have been developed to suppress ultrasonic responses
from anatomical tissues. These multiple-pulse excitation techniques
result in diagnostic displays having an intensity that is
responsive to the concentration of the contrast agent within the
local insonified region.
[0008] Recently, it has been determined that tissue also produces
harmonic responses which influence the images produced during
contrast imaging. Several techniques have been developed which take
advantage of the primarily linear response behavior of tissue to
cancel or attenuate the linear-tissue signals. In several of these
techniques, multiple-transmit lines are fired along the same line
of sight into the body. The transmit waveform is modified (e.g., in
terms of power, phase, or polarity) from line-to-line to produce a
variation in the response received by the transducer. These data
points are then processed to remove the influence of their linear
components to yield data that primarily contains the non-linear
response of the contrast agents.
[0009] Although the above-described techniques work well in
removing the influence of stationary tissue, flash artifacts from
moving tissue can degrade the resultant images. In particular, this
movement causes decorrelation of the received echoes that is not
compensated by typical processing techniques. This degradation can
be substantial, particularly where the heart is being imaged due to
its frequent and rapid motions. Attempts have been made to reduce
the effects of such movement by applying two-zero filters to the
responses of the receive signals associated with the various
transmit lines. However, this technique assumes perfectly linear
tissue movement and therefore is not completely effective in
removing the moving tissue signals.
[0010] From the above, it can be appreciated that it would be
desirable to have a method for contrast imaging in which the
response of moving tissue is effectively suppressed so as to
enhance the imaging sensitivity of the contrast agents. It will be
further appreciated that it would be desirable to have a method for
contrast imaging in which the response of moving tissue is
effectively suppressed to permit quantitative assessment of flow
velocities.
SUMMARY OF THE INVENTION
[0011] The present disclosure relates to apparatus and methods for
imaging contrast agents within a patient's body. More specifically,
the disclosure relates to a system and method that improves
contrast-agent enhanced-diagnostic evaluations by applying a
color-flow processing algorithm to ultrasonic-response data
generated and processed by a power-modulation technique selected
for the ability to suppress tissue signals. The method generally
comprises introducing one or more contrast agents into the body,
transmitting power-modulated ultrasonic pulses into the body,
receiving echoed signals from the body, processing the received
data (in the received echo signals) to suppress tissue-response
signals, processing the received contrast-agent signals with a
color-flow algorithm, examining the color-flow processed
contrast-agent signals over time to generate a contrast-agent
velocity estimate, and generating a color-encoded display of
contrast-agent velocities.
[0012] In one embodiment, the ultrasound signal comprises a
plurality of signal lines that have been modulated to have
different transmit characteristics. The step of processing the
received data to suppress tissue-motion generated response signals
comprises repetitively applying a weighted finite-impulse-response
(FIR) filter to slow-time samples to substantially remove tissue
signals and applying the received sequence to a one-zero clutter
filter. This has the effect of removing signals from stationary
contrast-agent bubbles, while passing signals generated by moving
contrast-agent bubbles.
[0013] In an alternative embodiment, the velocity of the moving
tissue is measured to generate a secondary velocity signal. This
secondary velocity signal is then mathematically combined with the
velocity estimate from the power-modulated flow signal, producing a
corrected measurement of the blood-flow velocity relative to the
surrounding tissue, rather than relative to the ultrasonic probe
(e.g., the transducer).
[0014] Briefly described, in architecture, the system can be
implemented with an excitation signal source, a transducer, an
ultrasound-processing system having multiple-image processors
including a color-flow processor, as well as, a clutter-filter, and
an arbiter, and a display-processing system.
[0015] Other features and advantages of the system and method for
contrast-agent enhanced color-flow imaging will become apparent to
one skilled in the art upon examination of the following drawings
and detailed description. It is intended that all such additional
features and advantages be included herein as protected by the
accompanying claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The invention can be better understood with reference to the
following drawings. The components in the drawings are not
necessarily to scale, emphasis instead being placed upon clearly
illustrating the principles of the present invention. Moreover, in
the drawings, like reference numerals designate corresponding parts
throughout the several views.
[0017] FIG. 1 is a schematic diagram of an exemplar
diagnostic-imaging environment suited to the improved
ultrasound-imaging system.
[0018] FIG. 2 is a functional-block diagram of an embodiment of the
improved ultrasound-imaging system of FIG. 1.
[0019] FIG. 3 is a schematic diagram of an exemplar diagnostic
image that can be produced with the ultrasound-imaging system of
FIG. 2.
[0020] FIG. 4 is a schematic diagram illustrating power modulation
as may be practiced by the ultrasound-imaging system of FIG. 2.
[0021] FIG. 5 is a functional-block diagram further illustrating a
plurality of image processors within the ultrasound-imaging system
of FIG. 2.
[0022] FIG. 6 is a functional-block diagram illustrating an
embodiment of the improved color-flow processor introduced in FIG.
5.
[0023] FIG. 7 is a schematic diagram illustrating operation of the
clutter filter of FIG. 6.
[0024] FIG. 8 is a functional-block diagram illustrating an
alternative embodiment of the improved color-flow processor of FIG.
5.
[0025] FIG. 9 is a flow chart illustrating a method for improved
imaging of blood flow within moving tissue that may be practiced by
the ultrasound-imaging system of FIG. 2.
[0026] FIG. 10 is a flowchart illustrating a method for generating
an improved diagnostic imaging display that reduces the effect of
tissue movement to correct blood flow velocity assessments that may
be practiced by the ultrasound-imaging system of FIG. 2.
DETAILED DESCRIPTION
[0027] The present disclosure generally relates to contrast
imaging. According to one aspect of the invention, a contrast-agent
detection technique is used together with a tissue-signal
suppression technique to image contrast-agent concentrations within
blood vessels of contrast-agent perfused tissue. In another aspect
of the invention, a tissue-motion velocity signal is isolated and
used to correct blood-flow velocity information relative to the
tissue rather than relative to the transducer. In either case, a
color-flow processor is used together with a clutter filter to
generate a signal representing contrast-agent velocities. The
combination of the tissue-suppression feature of power modulation
with flow-estimation feature of color-flow processing makes it
possible to differentiate relatively slow-moving blood from moving
tissue. Some exemplar clinical applications may include
coronary-artery imaging, coronary-flow reserve assessment,
blood-perfusion imaging, and tumor detection by imaging blood
supply.
[0028] Referring now in more detail to the drawings, in which like
numerals indicate corresponding parts throughout the several views,
attention is now directed to FIG. 1, which illustrates the general
diagnostic environment where an improved ultrasound-imaging system
may practice the various methods enclosed herein to improve
color-flow ultrasound-imaging diagnostics. In this regard, the a
general diagnostic environment where the improved
ultrasound-imaging system may practice the various methods of
improved color-flow imaging is illustrated by way of a schematic
diagram in FIG. 1 and is generally denoted by reference numeral
100. As illustrated in FIG. 1, an ultrasound-imaging system 10 may
be disposed in a diagnostic environment 100 comprising a patient
under test 113, a transducer 18, and an interface cable 12.
[0029] As illustrated in FIG. 1, the transducer 18 may be placed
into position over a portion of the anatomy of a patient under test
113 by a user/operator (not shown) of the ultrasound-imaging system
10. As shown in FIG. 1, a plurality of transmit signals may be
generated within the ultrasound-electronics system 1 and conveyed
to the transducer 18 via the interface cable 12. The plurality of
transmit signals may be converted to a plurality of transmit pulses
115 that emanate from the transducer 18 in response to the applied
transmit signals.
[0030] When the transmit pulses (ultrasound energy) 115 encounter a
tissue layer of the patient under test 113 that is receptive to
ultrasound insonification, the multiple transmit pulses 115
penetrate the tissue layer 113. As long as the magnitude of the
multiple ultrasound pulses exceeds the attenuation affects of the
tissue layer 113, the multiple ultrasound pulses 115 will reach an
internal target 121. Those skilled in the art will appreciate that
tissue boundaries or intersections between tissues with different
ultrasonic impedances will develop ultrasonic responses at the
fundamental-transmit frequency of the plurality of ultrasound
pulses 115. Tissue insonified with ultrasonic pulses will develop
fundamental-ultrasonic responses that may be distinguished in time
from the transmit pulses to convey information from the various
tissue boundaries within a patient.
[0031] Those ultrasonic reflections 117a, 117b of a magnitude that
exceed that of the attenuation affects from traversing tissue layer
113 may be monitored and converted into an electrical signal by the
ultrasound-electronics system 1. As further illustrated in the
diagram of FIG. 1, the ultrasound-electronics system 1 and a
display-electronics system 5 may work together to produce an
ultrasound-display image 200 derived from the plurality of
ultrasonic echoes 117.
[0032] Those skilled in the art will appreciate that those tissue
boundaries or intersections between tissues with different
ultrasonic impedances will develop ultrasonic responses at both the
fundamental frequency, as well as at harmonics of the fundamental
frequency of the plurality of ultrasound pulses 115. Tissue
insonified with ultrasonic pulses 115 will develop both fundamental
117a and harmonic ultrasonic responses 117b that may be
distinguished in time from the transmit pulses 115 to convey
information from the various tissue boundaries within a patient. It
will be further appreciated that tissue insonified with ultrasonic
pulses 115 develops harmonic responses 117b because the
compressional portion of the insonified waveforms travels faster
than the rarefactional portions. The different rates of travel of
the compressional and the rarefactional portions of the waveform
causes the wave to distort producing a harmonic signal, which is
reflected or scattered back through the various tissue
boundaries.
[0033] In some embodiments, the ultrasound-imaging system 10 both
transmits and receives a plurality of ultrasound pulses 115 at a
fundamental frequency. Those skilled in the art will appreciate
that harmonic responses 117b may be received by a transducer 18
having an appropriately wide bandwidth to simultaneously transmit
at a fundamental frequency and receive associated responses at a
harmonic frequency thereof. While fundamental imaging is used, both
fundamental and harmonic imaging are contemplated and within the
scope of the present invention.
[0034] As further illustrated in FIG. 1, ultrasonic echoes 117a and
117b reflect fundamental responses and harmonic responses
respectively. It is significant to note that while FIG. 1
illustrates a second harmonic response to the incident multiple
ultrasound-transmit pulses 115 impinging the internal target 121
other harmonic responses may also observed. As by way of example,
it is known that subharmonic, harmonic, and ultraharmonic responses
may be created at the tissue boundary between a tissue layer 113
and the internal target 121, when the internal target has been
perfused with one or more contrast agents. The internal target 121
alone will produce harmonic responses at integer multiples of the
fundamental frequency. Various contrast agents on the other hand,
have been shown to produce subharmonic, harmonic, and ultraharmonic
responses to incident ultrasonic pulses. Those ultrasonic
reflections of a magnitude that exceed that of the attenuation
affects from traversing tissue layer 113 (e.g., fundamental,
subharmonic, harmonic, and ultraharmonic responses) may be
monitored and converted into an electrical signal by the
combination of the transducer 18, the interface cable 12, and the
ultrasound-electronics system 1 as will be explained in further
detail below.
[0035] System Architecture and Operation
[0036] The architecture of an ultrasound-imaging system 10 capable
of practicing the various contrast-agent imaging methods disclosed
below is illustrated by way of a functional-block diagram in FIG. 2
and is generally denoted by reference numeral 10. Note that many of
the functional blocks illustrated in FIG. 2 define a logical
function that can be implemented in hardware, software, or a
combination thereof. For purposes of achieving high speed, it is
preferred, at present, that most of the blocks be implemented in
hardware, unless specifically noted hereafter. It will be
appreciated that this figure does not necessarily illustrate every
component of the system, emphasis instead being placed upon the
components relevant to the methods disclosed herein.
[0037] Referring to FIG. 2, the ultrasound-imaging system 10 may
include an ultrasound-electronics system 1 in communication with a
transducer 18 and display-electronics system 5. As illustrated in
FIG. 2, the ultrasound-electronics system 1 may include a system
controller 21 designed to control the operation and timing of the
various elements and signal flow within the ultrasound-imaging
system 10 pursuant to suitable software. The ultrasound-electronics
system 1 may further comprise a transmit controller 14, a
radio-frequency (RF) switch 16, a plurality of preamps 20,
time-gain compensators (TGCs) 22, and analog-to-digital converters
(ADCs) 24. In addition, the ultrasound-electronics system 1 may
comprise a parallel beamformer 26, a power-modulation processor 27,
a RF filter 28, a mixer 30, an amplitude detector 32, a log
mechanism 34, a post-log filter 36, and one or more image
processors 300. As further illustrated in FIG. 2, the
display-electronics system 5 may comprise a video processor 40, a
video-memory device 42, and a display 44.
[0038] The transducer 18 may take the form of a phased-array
transducer having a plurality of elements both in the lateral and
elevation directions. The plurality of transducer elements may be
constructed of a piezoelectric material, for example but not
limited to, lead-zirconate-titanate (PZT). Each element may be
selectively supplied with an electrical pulse or other suitable
electrical waveform, causing the elements to collectively propagate
an ultrasound-pressure wave into the object-under-test. Moreover,
in response thereto, one or more echoes are reflected by the
object-under-test and are received by the transducer 18, which
transforms the echoes into an electrical signal for detection and
processing within the ultrasound-electronics system 1.
[0039] The array of elements associated with the transducer 18
enable a beam, emanating from the transducer array, to be steered
(during transmit and receive modes) through the object by delaying
the electrical pulses supplied to the separate elements. When a
transmit mode is active, an analog waveform is communicated to each
transducer element, thereby causing a pulse to be selectively
propagated in a particular direction, like a beam, through the
object.
[0040] When a receive mode is active, a waveform is sensed or
received at each transducer element at each beam position. Each
analog waveform essentially represents a succession of echoes
received by the transducer element over a period of time as echoes
are received along the single beam through the object. Time delays
are applied to the signals from each element to form a narrow
receive beam in the desired direction. The entire set of analog
waveforms formed by both transmit and receive mode manipulations
represents an acoustic line, and the entire set of acoustic lines
represents a single view, or image, of an object commonly referred
to as a frame.
[0041] As is known, a phased-array transducer may comprise a host
of internal-electronics responsive to one or more control signals
that may originate within the system controller 21 or alternatively
in the transmit controller 14. For example, the transducer
electronics may be configured to select a first subset of
transducer elements to apply an excitation signal to generate a
plurality of ultrasonic pulses. In a related manner, the transducer
electronics may be configured to select a second subset of
transducer elements to receive ultrasonic echoes related to the
transmitted-ultrasonic pulses. Each of the aforementioned
transducer-element selections may be made by the transducer 18 in
response to the one or more control signals originating in the
transmit controller 14 or the system controller 21.
[0042] As illustrated in FIG. 2, the transmit controller 14 may be
electrically connected to the transducer 18 via a RF switch 16. The
transmit controller 14 may be in further communication with the
system controller 21. The system controller 21 may be configured to
send one or more control signals to direct operation of the
transmit controller 14. In response, the transmit controller 14 may
generate a series of electrical pulses that may be periodically
communicated to a portion of the array of elements of the
transducer 18 via the RF switch 16, causing the transducer elements
to emit ultrasound signals into the object-under-test of the nature
described previously. The transmit controller 14 typically provides
separation between the pulsed transmissions to enable the
transducer 18 to receive echoes from the object during the period
between transmit pulses and forwards them onto a set of parallel
analog preamplifiers 20, herein labeled, "PREAMPs." The RF switch
16 maybe configured to direct the various transmit and receive
electrical signals to and from the transducer 18.
[0043] The plurality of preamplifiers 20 may receive a series of
analog electrical-echo waveforms from the transducer 18 that are
generated by echoes reflected from the object-under-test. More
specifically, each preamplifier 20 receives an analog
electrical-echo waveform from a corresponding set of transducer
elements for each acoustic line. Moreover, the set of preamplifiers
20 receives a series of waveform sets, one set for each separate
acoustic line, in succession over time and may process the
waveforms in a pipeline-processing manner. The set of preamplifiers
20 may be configured to amplify the echo waveforms to provide
amplified-echo waveforms to enable further signal processing, as
described hereafter. Because the ultrasound signals received by the
transducer 18 are of low power, the set of preamplifiers 20 should
be of sufficient quality that excessive noise is not generated in
the process.
[0044] Because the echo waveforms typically decay in amplitude as
they are received from progressively deeper depths in the
object-under-test, the plurality of analog preamplifiers 20 in the
ultrasound-electronics system 1 may be connected respectively to a
parallel plurality of TGCs 22, which are known in the art and are
designed to progressively increase the gain during each acoustic
line, thereby reducing the dynamic range requirements on subsequent
processing stages. Moreover, the set of TGCs 22 may receive a
series of waveform sets, one set for each separate acoustic line,
in succession over time and may process the waveforms in a
pipeline-processing manner.
[0045] A plurality of parallel analog-to-digital converters (ADCs)
24 may be in communication respectively with the plurality of TGCs
22, as shown in FIG. 2. Each of the ADCs 24 may be configured to
convert its respective analog-echo waveform into a digital-echo
waveform comprising a number of discrete-location points (hundreds
to thousands; corresponding with depth and may be a function of
ultrasound transmit frequency or time) with respective quantized
instantaneous-signal levels, as is well known in the art. In prior
art ultrasound-imaging systems, this conversion often occurred
later in the signal processing steps, but now, many of the logical
functions that are performed on the ultrasonic signals can be
digital, and hence, the conversion is preferred at an early stage
in the signal-processing process. Similar to the TGCs 22, the
plurality of ADCs 24 may receive a series of waveforms for separate
acoustic lines in succession over time and process the data in a
pipeline-processing manner. As an example, the system may process
signals at a clock rate of 40 MHz with a B-mode frame rate of 60
Hz.
[0046] A set of parallel beamformers 26 may be in communication
with the plurality of ADCs 24 and may be designed to receive the
multiple digital-echo waveforms (corresponding with each set of
transducer elements) from the ADCs 24 and combine them to form a
single acoustic line. To accomplish this task, each parallel
beamformer 26 may delay the separate echo waveforms by different
amounts of time and then may add the delayed waveforms together, to
create a composite digital RF-acoustic line. The foregoing delay
and sum beamforming process is well known in the art. Furthermore,
the parallel beamformer 26 may receive a series of data collections
for separate acoustic lines in succession over time and process the
data in a pipeline-processing manner.
[0047] A power-modulation processor 27 may be coupled to the output
of the parallel beamformers 26 and may be configured to receive and
process a plurality of digital-acoustic lines in succession. The
power-modulation processor 27 may be configured to work in concert
with the system controller 21 or the transmit controller 14 to
selectively process a plurality of digital-acoustic lines with
multiple levels of ultrasound insonification. An example of an
ultrasound-imaging system 100 for producing a series of ultrasonic
pulses with multiple excitation levels is disclosed in U.S. Pat.
No. 5,577,505 which shares a common assignee with the present
application and the contents of which are incorporated herein in
their entirety.
[0048] A RF filter 28 may be coupled to the output of the
power-modulation processor 27 as illustrated in FIG. 2. The RF
filter 28 may take the form of a bandpass filter configured to
receive each digital-acoustic line and to remove undesired out of
band noise. As further illustrated in FIG. 2, a mixer 30 may be
coupled at the output of the RF filter 28. The mixer 30 may be
designed to process a plurality of digital-acoustic lines in a
pipeline manner. The mixer 30 may be configured to combine the
filtered digital-acoustic lines from the RF filter 28 with a local
oscillator signal (not shown for simplicity) to ultimately produce
a plurality of baseband digital-acoustic lines.
[0049] Preferably, the local oscillator signal is a complex signal,
having an in-phase signal (real) and a quadrature-phase signal
(imaginary) that are ninety degrees out-of-phase. The mixing
operation may produce sum and difference frequency signals. The
sum-frequency signal may be filtered (removed), leaving the
difference-frequency signal, which is a complex signal at near zero
frequency. A complex signal is desired to follow direction of
movement of anatomical structures imaged in the object-under-test,
and to allow accurate, wide-bandwidth amplitude detection.
[0050] Up to this point in the ultrasound-echo receive process, all
operations can be considered substantially linear, so that the
order of operations may be rearranged while maintaining
substantially equivalent function. For example, in some systems it
may be desirable to mix to a lower intermediate frequency (IF) or
to baseband before beamforming or filtering. Such rearrangements of
substantially linear-processing functions are considered to be
within the scope of this invention.
[0051] An amplitude detector 32 may receive and process, in
pipeline manner, the complex baseband digital-acoustic lines from
the mixer 30. For each complex-baseband digital-acoustic line, the
amplitude detector 32 may analyze the envelope of the line to
determine the signal intensity at each point along the acoustic
line to produce an amplitude-detected digital-acoustic line.
Mathematically, this means that the amplitude detector 32
determines the magnitude of each phasor (distance to origin)
corresponding with each point along the acoustic line.
[0052] A log mechanism 34 may receive the amplitude-detected
digital-acoustic lines in a pipeline-processing manner, from the
amplitude detector 32. The log mechanism 34 may be configured to
compress the dynamic range of the data by computing the
mathematical logarithm (log) of each acoustic line to produce a
compressed digital-acoustic line for further processing.
Implementation of a log function enables a more realistic view,
ultimately on a display, of the change in brightness corresponding
to the ratio of echo intensities.
[0053] A post-log filter 36, usually in the form of a low-pass
filter, may be coupled to the output of the log mechanism 34 and
may be configured to receive the compressed digital-acoustic lines
in a pipeline fashion. The post-log filter 36 may remove or
suppress high frequencies associated with the compressed
digital-acoustic lines to enhance the quality of the display image.
Generally, the post-log filter 36 softens the speckle in the
displayed image. The low-pass post-log filter 36 can also be
configured to perform anti-aliasing. The low-pass post-log filter
36 can be designed to essentially trade spatial resolution for
gray-scale resolution.
[0054] One or more image processors 300 may be coupled to the
output of the low-pass post-log filter 36. Each of the image
processors 300 may further comprise a suitable species of
random-access memory (RAM) and may be configured to receive the
filtered digital-acoustic lines from the low-pass post-log filter
36. The acoustic lines can be defined within a two-dimensional
coordinate space. The image processors 300 may be configured to
mathematically manipulate image information within the received and
filtered digital-acoustic lines. In addition, each of the image
processors 300 may be configured to accumulate acoustic lines of
data over time for signal manipulation. In this regard, the image
processors 300 may further comprise a scan converter to convert the
data as stored in the RAM to produce pixels for display. Each scan
converter may process the data in the RAM once an entire data frame
(i.e., a set of all acoustic lines in a single view, or
image/picture to be displayed) has been accumulated by the RAM. For
example, if the received data is stored in RAM using polar
coordinates to define the relative location of the echo
information, the scan converter may convert the polar-coordinate
data into rectangular (orthogonal) data capable of raster scan via
a raster-scan capable processor. The ultrasound-electronics system
1, having completed the receiving, echo recovery, and
image-processing functions, to form a plurality of image frames
associated with the plurality of ultrasound-image planes, may
forward the echo-image data information associated with each image
frame to a display-electronics system 5 as illustrated in FIG.
2.
[0055] The display-electronics system 5 may receive the echo-image
data from the ultrasound-electronics system 1, where the echo-image
data may be forwarded to a video processor 40. The video processor
40 may be designed to receive the echo-image data information and
may be configured to raster scan the image information. The video
processor 40 outputs picture elements (e.g., pixels) for storage in
a video-memory device 42 and/or for display via a display 44. The
video-memory device 42 may take the form of a digital-videodisc
(DVD) player/recorder, a compact-disc (CD) player/recorder, a
video-cassette recorder (VCR), or other video-information storage
device. As is known in the art, the video-memory device 42 permits
viewing and or post-data collection image processing by a
user/operator in other than real-time.
[0056] A display device in the form of a display 44 may be in
communication with both the video processor 40 and the video memory
42 as illustrated in FIG. 2. The display 44 may be configured to
periodically receive the pixel data from either the video memory 42
and or the video processor 40 and drive a suitable screen or other
imaging device (e.g., a printer/plotter) for viewing of the
ultrasound image by a user/operator.
[0057] Contrast-Agent Imaging
[0058] As used herein, power level relates to insonification or
acoustic intensity. Mechanical index is one parameter used to
measure acoustic intensity. Mechanical index is a United States
Food and Drug Administration (FDA) regulated parameter defined as
peak-rarefactional pressure in megaPascal (Mpa) divided by the
square root of the center frequency in megahertz (MHz). Current FDA
regulations limit the mechanical index to a maximum of 1.9, after
allowing for tissue related frequency dependent attenuation.
[0059] It is important to note that different contrast agents
respond differently to various insonification and detection
techniques. It is theorized that these different responses can be
explained due to flexibility of the shell material used to encase
the agent, the size distribution within the body, and the
particular characteristics of the gas inside the shell. As a
result, determining an effective-mechanical index for a particular
application is somewhat patient and agent specific. The mechanical
index needs to be low enough to not destroy the contrast agent
while maintaining a linear response signal from insonified tissue.
On the other hand, the mechanical index needs to be high enough to
overcome the effects of tissue attenuation at the fundamental
frequency while initiating a non-linear response from the one or
more contrast agents. Generally, a mechanical index from 0.05 to
0.5 will meet these requirements for a broad range of contrast
agents starting from the most fragile to the more resilient.
[0060] As described earlier with regard to FIGS. 1 and 2 achieving
different power levels in each of two or more transmit events or
ultrasound lines 115 (see FIG. 1) may be accomplished in several
different ways. A method of achieving the different power settings
is by varying the transmit voltage. Varying transmit voltage has
the direct result of varying the pressure amplitude of the
resultant transmitted-ultrasound lines 115 (see FIG. 1).
Alternatively, different power levels may be accomplished by
controlling the size of the aperture of the transducer 18. The
aperture size may be varied in the lateral or elevation dimensions
by using a synthetic-aperture methodology. The aperture may be
divided into two or more groups with transmit-ultrasound lines 115
being separately fired from each group. The subsequent reflected
energy is then stored. The entire aperture is then used to transmit
a second incident pressure wave with an increased energy level. The
subsequent reflected energy is again stored. In this embodiment,
the scaling step includes beamforming the response from the two or
more smaller apertures and subtracting those results from the
response due to excitation from the entire aperture to determine
the non-linear response.
[0061] Another way of controlling transmitted-power levels is to
fire a subset of elements in the array and compare the
scaled-subset response to a response from the entire transducer
array. This method should be performed in a manner to reduce and or
minimize grating lobes that stem from under sampling the aperture
and steering errors that result from asymmetries about the center
of the aperture.
[0062] A non-limiting example of a multi-pulse technique that fires
three pulses is described below. Firing the "even" numbered
elements within transducer 18 may generate the first pulse. The
second pulse may be generated by controllably firing all elements
of the transducer 18. Firing the "odd" numbered elements may
generate the third pulse. The response signal-processing portion of
the ultrasound-electronics system 10 may be configured to
mathematically combine a response from the first and third pulses
for further mathematical manipulation with the second response
signal. It is important to note that the selection of elements to
form the various element subsets for the first and third pulses is
not limited to "even" and "odd" numbered elements of the transducer
element array. It will be appreciated by those skilled in the art
that more than three pulses may be generated and fired to further
extend a multi-pulse insonification and imaging technique.
[0063] The multi-pulse technique described above serves a couple of
purposes. First, adjusting the transmitted power by firing a subset
of elements reduces the transmit power while providing the same
voltage level to each transmission. If the transmit waveforms are
not properly scaled and inverted, or if the waveforms differ in
their frequency content, undesired residual artifacts from
imperfect tissue-response signal cancellations may be introduced by
the ultrasound-electronics system 10. By matching the voltage level
used to generate the various pulses, the ultrasound-electronics
system 10 reduces any undesired tissue signals introduced by
mathematically combining signal responses generated from ultrasonic
transmissions of varying power levels. Transmit-waveform
power-magnitude matching over a number of various levels of
comparison across a received bandwidth of interest will serve to
reduce residual-tissue response-signal artifacts that may result
from transmit-power mismatches.
[0064] A second important result from using the multi-pulse
technique is that by mathematically combining the first pulse
response with the third pulse response, motion of an
organ-of-interest (i.e., the heart) is averaged, so that when the
second pulse response is mathematically processed (i.e.,
subtracted) from the combination of the first and third pulse
responses, motion is suppressed between the various pulses.
[0065] Yet another way of suppressing the linear response of
tissues is to use a phase-inversion technique. Phase-inversion
techniques are well understood by those skilled in the art of
ultrasonic imaging. The description of an ultrasonic-imaging system
capable of producing, detecting, and image-processing ultrasonic
responses that use phase-inversion techniques need not be described
to understand the present invention and need not be described
herein. It is significant to note, however, that mathematical
post-processing of detected-response signals may vary based on the
desired effect of the processing and the phase of the transmitted
waveforms responsible for the response signals. By coordinating one
or more of the phase, intensity, and frequency content of multiple
transmitted pulses with the applicable response processing, motion
artifacts between pulses may be substantially reduced.
[0066] Another technique that may be used to vary the transmitted
levels would be to take advantage of the beam shape of a pressure
wave. Transmitted pressure waves have a reduced magnitude that
varies with angular distance. As by way of a nonlimiting example,
if a pressure wave is transmitted at 0 degrees (from the face of
the transducer-element array) and the ultrasound-electronics system
10 is configured to receive responses at 0.0 and at 0.25 degrees,
the power received at 0.25 degrees will be lower since it is off
the peak of the transmitted beam.
[0067] An exemplar diagnostic-imaging environment 100 suited to the
improved ultrasound-imaging system 10 having been described with
regard to FIGS. 1 and 2, reference is now directed to FIG. 3, which
illustrates a diagnostic image that can be produced with the
improved ultrasound-imaging system 10 of FIG. 2. In this regard,
ultrasound image 200 may comprise alphanumeric information in the
form of patient identifiers 202, date and time identifiers 204 and
scanning parameters 206. In addition to the one or more
alphanumeric identifiers, ultrasound image 200 may comprise a
real-time ultrasound image display 210 of structure in a body such
as a portion of the circulatory system such as a coronary-blood
vessel 212.
[0068] A clinical technician, to ascertain and locate an area of
interest (e.g., a portion of the myocardium of a patient's heart
muscle), may use a real-time image. Preferably the image is created
from echoes returned from the non-destructive ultrasonic imaging of
one or more contrast agents that have been introduced into the
bloodstream of the patient. It is important to note that real-time
contrast-agent images may be acquired at any phase of the heart
cycle, not just when the heart is predominately at rest. While the
aforementioned real-time imagery of the heart is especially useful
in cardiology, variations of this method may prove useful in
radiology where anatomical structures are more stationary as
well.
[0069] Irrespective of the particular transmit-signal
modulation-technique used, each transmit line normally comprises
repeated sequences of waveforms. By way of example, each waveform
comprises a Gaussian-modified sinusoid. The various transmit lines
are fired along the same line-of-sight into the body as indicated
in FIG. 4. Each group of lines fired in this direction is referred
to as a packet of lines. Normally, specific sequences of transmit
waveforms are used and repeated multiple times within each packet.
Each sequence of transmit waveforms is referred to as a
sub-packet.
[0070] After the multiple lines have been transmitted into the
body, the response echoes are received. Again, these received
signals are digitized so that the data contained therein can be
processed in the appropriate manner. Once digitized, these received
data may be stored in one or more of the image processors 300 (FIG.
2). Preferably, the data are organized in an array of data points
400 as shown in FIG. 4.
[0071] The array 400 may comprise as many columns 402 as there are
lines in the packet. Each column 402 contains a collection of
samples which correlate to a particular transmit line. There are as
many rows 404 in the array 400 as there are digitized-data samples
along any one of the received lines. Each successive sample along a
row 404 is representative of a particular imaging depth, but
acquired a full line-time after the previous sample. Normally, the
row 404 direction of the array 400 is referred to as slow-time.
Each successive data point down each column 402 is acquired
immediately after the previous data point in the line. Accordingly,
the column direction of the array 400 is referred to as
fast-time.
[0072] Once the various received data have been stored in the array
400, the first stage of processing can be conducted. First, a
correction function is applied to the data to compensate for the
variance of the transmit signals across the multiple transmit
lines. The nature of the correction function may depend upon the
particular modulation scheme used to vary the transmit signals. For
instance, if the transmit signals were varied according to
amplitude (i.e., power modulation), the correction function can
comprise a scaling factor which accounts for the amplitude variance
across the signal lines. If phase modulation was used in creating
the transmit signals, the correction function can comprise a phase
adjustment which accounts for the phase variance of the transmit
signal. Similarly, where the transmit signals were varied in
polarity, the correction can comprise inverting the receive data
for the positive or the negative transmit lines.
[0073] After the transmit variance has been accounted for in the
manner described above, the various lines of data can be subtracted
from the other, for instance with a contrast-imaging clutter
filter, to cancel the linear components of the data. However,
before this cancellation is effected, the response of moving tissue
is suppressed. As will be understood by persons having ordinary
skill in the art, if there is any appreciable motion of tissue
between the successive lines of the packets, the received-echo data
will not cancel precisely, and some residual signal due from moving
tissue will remain. Therefore, it is preferable to compensate for
this motion before attempting to cancel the linear signals of
moving tissue from the received data.
[0074] As also illustrated in FIG. 4, a power modulated multi-line
subpacket having the following transmit sequence: 0, L, H, L, 0 may
be applied by the ultrasound-electronics system 1 (FIG. 2). The
initial blank line allows time for reverberation from a previous
imaging line to die out. A FIR filter may then be applied to
combine the slow-time samples 404 with weighted values: 0, -1, 1,
-1, 1. This filtering results in the substantial reduction of
tissue-generated signals and reverberation signals, while having
little or no effect on signals from contrast-agent bubbles.
[0075] The data-point array 400 and the weighted FIR filtering of a
multiple transmit line sequence having been described with regard
to FIG. 4, reference is now directed to FIG. 5, which illustrates
some of the image processors 300 that may be provided in the
ultrasound-electronics system 10 of FIG. 2. In this regard, image
processors 300 may comprise a B-mode processor 310, a Doppler
processor 320, an improved color-flow processor 400, as well as
other image processors. As shown in the functional-block diagram of
FIG. 5, the image processors 300 may be inserted in the
architecture of the ultrasound-electronics system 10 generally
after beamforming (i.e., the parallel beamformers 26) and prior to
scan conversion and video processing (i.e., in the
display-electronics system 5). As previously described in
association with FIG. 2, it will be appreciated that each of the
image processors 300 may be configured with its own scan converter
(not shown). It will be further appreciated that one or more scan
converters may be provided in association with one or more of the
various image processors 310, 320, 400.
[0076] An improved color-flow processor 400 is illustrated via a
functional-block diagram in FIG. 6. In this regard, the improved
color-flow processor 400 may comprise a clutter filter 500 in
combination with a color-flow processor 410 known in the art. As
shown in the previous illustration, the improved color-flow
processor 400 may be introduced after parallel beamforming and
prior to scan conversion. FIG. 7 further illustrates the operation
of the clutter filter 500 introduced in FIG. 6.
[0077] In this regard, the power-modulation processor 27 of the
ultrasound-electronics system 10 (see FIG. 2) may be configured to
transmit the exemplar power-modulated transmit sequence illustrated
across the top of FIG. 7. More specifically, the power-modulated
transmit sequence may comprise the following 13 line packet: 0, L,
H, L, 0, L, H, L, 0, L, H, L, 0. As shown in FIG. 7, 0 indicates no
transmit pulse is sent; "L" indicates that a half-power transmit
pulse is sent; and "H" indicates that a fill-power transmit pulse
is applied to the tissue of interest. In accordance with the
improved color-flow processor 400, the clutter filter 500 for the
exemplar 13 line transmit sequence may provide two output samples
for color-flow processing with the weights as illustrated in FIG.
7. It can be seen that the clutter filter 500 applied to each
sample is a one-zero filter (with a sample spacing of four
slow-time samples). Convolved with the power-modulation FIR filter
described above, the cumulative effect of the two filters and the
power-modulation technique(s) is to reduce tissue generation
signals and stationary contrast-bubble signals, while passing
signals generated from moving contrast-agent bubbles.
[0078] In accordance with well known color-flow processing
techniques, the relative phase of the two data points from the
exemplar line sequence 13 of FIG. 7 can then be evaluated to
compute a velocity estimate of the contrast-agent bubbles. The
technique described above can also be applied to Doppler-imaging
modes including phased-array pulsed-wave (PW) Doppler.
[0079] An advantage that results from the combination of the
tissue-suppression feature of power modulation with the
flow-estimation technique of color flow as applied to
contrast-agent enriched blood, is it is now possible to
differentiate slowly-moving blood flows (e.g., in the small blood
vessels of the myocardium and vessels within organs other than the
heart) from moving tissue. Furthermore, it is now possible to
utilize contrast agents to enhance and improve the diagnostic
quality of a color-flow exam on patients with poor acoustic
windows.
[0080] Contrast-Agent Enhanced Color Flow With Tissue Signal
Velocity Adjustment
[0081] Reference is now directed to FIG. 8, which illustrates a
functional-block diagram of an alternative embodiment of an
improved color-flow processor. As illustrated in FIG. 8, an
improved color-flow processor 800 may comprise a clutter filter
500, a color-flow processor 410, a tissue-signal processor 810, a
mathematical junction 820, and an arbiter 830. As previously
described in association with FIG. 6, the improved color-flow
processor 800 may also be inserted in the ultrasound-electronics
system 10 of FIG. 2 generally after beamforming (i.e., the parallel
beamformers 26) and prior to scan conversion and video processing
(i.e., in the display-electronics system 5). As previously
described in association with FIG. 2, it will be appreciated that
the improved color-flow processor 800 may be configured with its
own scan converter (not shown).
[0082] As illustrated in the functional-block diagram of FIG. 8,
the improved color-flow processor 800 can be constructed by
providing a secondary-processing path, not entirely separate from
the path previously described with regard to the improved
color-flow processor of FIG. 6. More specifically, the
secondary-processing path may comprise a first branch that enters a
tissue-signal processor 810 before being forwarded to the
mathematical junction 820. A primary branch or
color-flow-processing path may be formed by the clutter filter 500,
the color-flow processor 410, and a signal from an image processor.
The signal from the image processor and the output of the
color-flow processor 410 are processed by the arbiter 830 before
being forwarded to the mathematical junction 820. The
secondary-processing path is designed to measure the velocity of
the tissue generated echo signals rather than the blood with the
contrast agent.
[0083] The tissue-generated echo signals can be applied to the
tissue-signal processor 810 to generate a tissue-velocity signal
formed from the same set of acoustic lines (i.e., the same
subpacket data) as the power-modulated color-flow signal. However,
the tissue-signal processor 810 will employ a different set of
coefficients in its own clutter filter (not shown). The
tissue-signal processor coefficients could, for example, select
equal power lines from each subpacket, such as the "H" transmit
lines and coefficients of 0 for the lower-power "L" transmit lines.
The tissue-signal processor 810 would then produce the same
output-sample rate as the clutter filter 500, and could be
processed by the same phase-detection steps as the color-flow
signal.
[0084] As illustrated in FIG. 8, and in accordance with standard
color-flow processing techniques, the signal that exits the
color-flow processor 410 (i.e., the color-flow velocity signal) is
processed along with the underlying image data from a
two-dimensional image processor (e.g., black and white image data
as supplied by a B-mode processor). Where color-flow velocity
samples are above a pre-defined (or user adjustable) intensity
threshold, the color-flow velocity samples are rendered instead of
the underlying image-data samples. In the improved color-flow
processor 800, the arbitration between the color-flow velocity
samples and the underlying image-data samples would remain
unchanged, but where color-flow velocity samples are selected for
display, the tissue-velocity signal would be subtracted. As a
result, the improved color-flow processor 800 would provide a
signal over time that suppresses the tissue "flash" artifact along
with providing information regarding the velocity of contrast
agents corrected for surrounding tissue motion.
[0085] Reference is now directed to FIG. 9, which illustrates a
flowchart describing a method for contrast-agent enhanced
color-flow imaging that may be implemented by the
ultrasound-electronics system 10 of FIG. 2. As illustrated in FIG.
9, the method for contrast-agent enhanced color-flow imaging 900
may begin with step 902, labeled "START." First, one or more
contrast agents may be introduced into a patient's bloodstream as
indicated in step 904. These contrast agents can comprise
microbubbles of a heavy gas, such as a perfluorocarbon-gas
encapsulated in an outer shell made of protein, lipid, or other
suitable material. Although the size of the agents may vary
depending upon the application, these microbubbles normally are in
the range of approximately 1.0 to 15 microns (,um) in diameter. As
the contrast agents are introduced into the bloodstream, they
travel throughout the cardiovascular system.
[0086] After having confirmed that tissues of interest (e.g.,
coronary blood vessels within the myocardium) contain sufficient
amounts of the contrast agent(s), the ultrasound-electronics system
10 may be configured to transmit a series of power-modulated
ultrasound signals into the body as shown in step 906.
[0087] As shown in step 908, the ultrasound-electronics system 10
is configured to receive the series of ultrasonic echoes induced by
the power-modulated transmit signals. Next, as illustrated in step
910, non-linear tissue responses can be suppressed using
power-modulation techniques as previously described hereinabove.
After the effects of the moving-tissue signals have been
suppressed, the various data can be processed using a
color-flow-processing algorithm as shown in step 912. It is
significant to note that the color-flow-processing will include
processing by the clutter filter 500 to reduce the effect of echo
signals generated by stationary contrast-agent bubbles, while
passing signals generated by moving bubbles. The relative phase of
the multiple data points generated in the clutter filter 500 are
then evaluated to generate a velocity estimate according to
well-known color-flow processing techniques. As illustrated in step
914 of the method for contrast-agent enhanced color-flow imaging
900, the color-flow processing may include generating a
color-encoded display of contrast-agent velocities. It will be
appreciated that the color-encoded display may be rendered along
with data generated in a B-mode processor to enable identification
of the tissue structures imaged. As illustrated in step 916, herein
labeled "END," the method for contrast-agent enhanced color-flow
imaging 900 may terminate.
[0088] It will be appreciated that steps 906 through 914 may be
repeated as desired to diagnose various blood vessels of various
sizes within the patient. It will be further appreciated that if
desired, step 904 may be repeated or continuously performed by
introducing the one or more contrast agents via an intravenous line
and commercially available infusers. Furthermore, imaging of the
one or more contrast agents can comprise simply imaging the
concentration of the contrast agents within human tissue, or can
comprise color-flow processing as described above to identify the
direction and velocity of flow of contrast agents within the
bloodstream or tissues.
[0089] The techniques described herein compensate for the response
of non-moving contrast-agent bubbles, as well as, non-moving
tissue. These adjustments are beneficial in that it is now possible
to differentiate slowly moving blood flows from surrounding tissue.
It will be appreciated that the velocity of contrast agent in the
blood is of particular clinical significance, especially when
imaging structures such as the heart. As described above, the
effects of moving tissue can be substantial, especially when areas
near or within the heart are being imaged. Notably, patient
breathing, coughing, or other such movements can also create tissue
movement. Regardless of the source of the movement, however, it is
preferable that this movement is reduced, or compensated for, such
that those flash artifacts that degrade the imaging of the contrast
agents are suppressed.
[0090] It is significant to note that the method for contrast-agent
enhanced color-flow imaging 900 is suited to any insonification
technique, which suppresses tissue-signal responses at the
fundamental frequency of a significant magnitude so that non-linear
responses from a contrast image can be detected and color-flow
processed to identify direction and velocity.
[0091] The method for contrast-agent enhanced color-flow imaging
900 having been described with regard to the flow chart of FIG. 9,
reference is now directed FIG. 10, which presents a flow chart
highlighting a method for contrast-agent enhanced color-flow
imaging with a tissue-velocity adjustment. As compared to the
method for contrast-agent enhanced color-flow imaging presented in
FIG. 9, the method for contrast-agent enhanced color-flow imaging
with correction for tissue velocity 1000 presented in FIG. 10
reflects the same steps of "start" 1002, introducing one or more
contrast agents 1004, transmitting a series of power modulated
ultrasound pulses 1006, and receiving/processing the ultrasound
echoes received from the body. Thereafter, and as illustrated in
FIG. 10, the method for contrast-agent enhanced color-flow imaging
with correction for tissue velocity 1000 branches. A first branch
is formed by steps 1010, 1012, and 1014. A second branch is formed
by steps 1011 and step 1020, where the first and second processing
branches combine.
[0092] Within the first processing branch, illustrated in the flow
chart of FIG. 10, a power-modulation technique is used to suppress
non-linear tissue responses as illustrated in step 1010. After the
tissue signals have been suppressed, the various contrast-agent
induced echo signals can be processed using a color-flow-processing
algorithm as shown in step 1012. The color-flow-processing may
include processing by the clutter filter 500 as described
hereinabove. As illustrated in step 1014 of the method for
contrast-agent enhanced color-flow imaging with correction for
tissue velocity 1000, processing may also include generating a
color-encoded display of contrast-agent velocities.
[0093] Within the second processing branch, a tissue-signal motion
processor and a related clutter filter with its own set of
coefficients may be used to determine a tissue velocity as shown in
step 1011. As illustrated in the flowchart, the tissue velocity
determined in step 1011 may be buffered for use later in step 1020
as described below.
[0094] Next, as shown in step 1016, the method for contrast-agent
enhanced color-flow imaging with correction for tissue velocity
1000 may be configured to compare the contrast-agent color-flow
processed velocities with a threshold value. If the velocity
samples exceed the threshold, processing may continue with step
1020, where the contrast-agent velocity value is corrected by
subtracting the tissue velocity determined in step 1011. Otherwise,
if the velocity samples fail to exceed the threshold value, the
method for contrast-agent enhanced color-flow imaging with
correction for tissue velocity may drop the color-flow processed
sample as indicated in step 1018. It will be appreciated that the
color-encoded display may be rendered along with data generated in
a B-mode processor to enable identification of the tissue
structures imaged. As illustrated in step 1022, herein labeled
"END," the method for contrast-agent enhanced color-flow imaging
with correction for tissue velocity 1000 may terminate. It will be
appreciated that steps 1006 through 1020 may be repeated as desired
to diagnose various blood vessels of various sizes within the
patient. It will be further appreciated that if desired, step 1004
may be repeated or continuously performed by introducing the one or
more contrast agents via an intravenous line as previously
described.
[0095] The techniques described herein compensate for the response
of non-moving contrast-agent bubbles, as well as, moving and
non-moving tissue generated echoes. These adjustments are
beneficial in that it is now possible to differentiate slowly
moving blood flows from surrounding tissue while compensating for
local tissue movement. Significantly, quantitative assessment of
blood flow velocities relative to the surrounding tissue, rather
than relative to the transducer 18 face are possible.
[0096] It will be appreciated by those having ordinary skill in the
art, the improved color-flow processors 400, 800 described above
can be implemented in software, hardware, or a combination thereof
within the ultrasound-electronics system 1 shown in FIGS. 1 and 2.
When implemented in software, the improved color-flow processors
400, 800 can be stored and transported on any computer-readable
medium for use by or in connection with an instruction-execution
system, apparatus, or device, such as a computer-based system,
processor-containing system, or other system that can fetch the
instructions from the instruction-execution system, apparatus, or
device and execute the instructions.
[0097] In the context of this disclosure, a "computer-readable
medium" can be any means that can contain, store, communicate,
propagate, or transport the program for use by or in connection
with the instruction-execution system, apparatus, or device. The
computer-readable medium can be, for example, an electronic,
magnetic, optical, electromagnetic, infrared, or semiconductor
system, apparatus, device, or propagation medium. More specific
examples of computer-readable media include the following: an
electrical connection having one or more wires, computer diskette,
random-access memory (RAM), read-only memory (ROM),
erasable-programmable read-only memory (EPROM or Flash memory), an
optical fiber, and a compact-disk read-only memory (CD ROM). It is
to be noted that the computer-readable medium can even be paper or
other suitable media upon which the program is printed as the
program can be electronically captured, via for instance optical
scanning of the paper or other media, then compiled, interpreted,
or otherwise processed and stored in a computer memory.
[0098] When implemented in hardware, the improved color-flow
processors 400, 800 can be implemented with any or a combination of
the following technologies, which are all well known in the art: a
discrete-logic circuit(s) having logic gates for implementing logic
functions upon data signals, an application-specific integrated
circuit (ASIC) having appropriate combinational logic gates, a
programmable-gate array(s) (PGA), a field-programmable gate array
(FPGA), etc.
[0099] It should be emphasized that the above-described embodiments
of the present invention are merely possible examples of
implementations, merely set forth for a clear understanding of the
principles of the invention. Many variations and modifications may
be made to the above-described embodiment(s) of the invention
without departing substantially from the spirit and principles of
the invention. All such modifications and variations are intended
to be included herein within the scope of this disclosure and the
present invention and protected by the following claims.
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