U.S. patent application number 10/013782 was filed with the patent office on 2002-04-11 for ultrasonic nonlinear imaging at fundamental frequencies.
Invention is credited to Averkiou, Michalakis.
Application Number | 20020042576 10/013782 |
Document ID | / |
Family ID | 24516677 |
Filed Date | 2002-04-11 |
United States Patent
Application |
20020042576 |
Kind Code |
A1 |
Averkiou, Michalakis |
April 11, 2002 |
Ultrasonic nonlinear imaging at fundamental frequencies
Abstract
Nonlinear tissue or contrast agent effects are detected by
combining echoes from multiple, differently modulated transmit
pulses below the second harmonic band. The received echoes may even
overlap the fundamental transmit frequency band. The modulation may
be amplitude modulation or phase or polarity modulation, and is
preferably both amplitude and phase or modulation. The present
invention affords the ability to utilize a majority of the
transducer passband for both transmission and reception, and to
transmit pulses which are less destructive to microbubble contrast
agents.
Inventors: |
Averkiou, Michalakis;
(Kirkland, WA) |
Correspondence
Address: |
ATL ULTRASOUND
P.O. BOX 3003
22100 BOTHELL EVERETT HIGHWAY
BOTHELL
WA
98041-3003
US
|
Family ID: |
24516677 |
Appl. No.: |
10/013782 |
Filed: |
December 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10013782 |
Dec 13, 2001 |
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09627918 |
Jul 28, 2000 |
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Current U.S.
Class: |
600/458 |
Current CPC
Class: |
G01S 7/52038 20130101;
G01S 15/895 20130101 |
Class at
Publication: |
600/458 |
International
Class: |
A61B 008/14 |
Claims
What is claimed is:
1. A method for nonlinear ultrasonic imaging comprising:
transmitting a first pulse exhibiting frequencies in a fundamental
frequency band to a target exhibiting a nonlinear response;
transmitting a second pulse exhibiting frequencies in the
fundamental frequency band to the target, the second pulse being
differently modulated in at least one of amplitude, phase or
polarity; receiving echoes from the target in response to each
pulse which include a frequency included in the fundamental
frequency band; combining the echoes received in response to the
pulses to produce signals embodying a nonlinear effect of the
target; and using the nonlinear effect signals to produce an
ultrasound image.
2. The method of claim 1, wherein transmitting comprises
transmitting pulses which are differently amplitude modulated.
3. The method of claim 1, wherein transmitting comprises
transmitting pulses which are differently modulated in phase or
polarity.
4. The method of claim 1, wherein transmitting comprises
transmitting pulses which are differently modulated in amplitude
and in phase or polarity.
5. The method of claim 2, further comprising normalizing the
received echoes to account for the amplitude modulation
difference.
6. The method of claim 1, wherein receiving comprises receiving
echoes from the target in a band of frequencies which include a
frequency which is within the fundamental frequency band and a
frequency which is above the fundamental frequency band.
7. The method of claim 1, wherein receiving comprises receiving
echoes from the target in a band of frequencies which include a
frequency which is within the fundamental frequency band and a
frequency which is below the fundamental frequency band.
8. The method of claim 1, wherein receiving comprises receiving
echoes from the target in a band of frequencies which declines in
frequency from a higher frequency band to a lower frequency band
with increasing depth, wherein said band includes a frequency which
is within the fundamental frequency band during at least a portion
of a period of echo reception.
9. The method of claim 1, wherein combining comprises canceling
linear components of the received echoes and enhancing nonlinear
components of the received echoes.
10. The method of claim 9, wherein combining further comprises
utilizing the modulation difference of the transmitted pulses to
cancel linear components of the received echoes.
11. The method of claim 1, wherein receiving comprises receiving
echoes along at least two spatially distinct scanline directions in
response to each act of transmitting.
12. The method of claim 2, wherein transmitting a first pulse
comprises transmitting a pulse of a given amplitude, and wherein
transmitting a second pulse comprises transmitting a pulse of an
amplitude which is less than the given amplitude.
13. A method for nonlinear ultrasonic imaging comprising:
transmitting a plurality of pulses exhibiting frequencies in a
fundamental frequency band to a target exhibiting a nonlinear
response; receiving echoes from the target in response to the
transmitted pulses in frequency bands which overlap the fundamental
frequency band; identifying the nonlinear components of the
received echoes; and using the identified nonlinear signal
components to produce an ultrasound image.
14. The method of claim 13, wherein transmitting further comprises
insonifying the target with differently modulated transmit pulses;
and wherein identifying comprises combining echoes received in
response to differently modulated transmit pulses.
15. The method of claim 13, wherein transmitting comprises
transmitting a plurality of pulses to a contrast agent; and wherein
receiving comprises receiving echoes from the contrast agent.
Description
[0001] This invention relates to ultrasonic diagnostic imaging
systems and, in particular, to ultrasonic diagnostic imaging
systems which image nonlinear signals in the fundamental frequency
band.
[0002] U.S. Pat. No. 5,879,303, of which I am a co-inventor,
describes methods and apparatus for doing harmonic ultrasound
imaging. As explained in my patent, an ultrasonic wave can be
transmitted at a fundamental frequency to give rise to harmonic
echo signals, in particular at the second harmonic, from two
distinct sources. One is the nonlinear behavior of microbubble
contrast agents. When these microbubble agents are insonified by
the transmit wave, they will oscillate or resonate nonlinearly,
returning a spectrum of echo signals including those at the second
harmonic of the transmit frequency. The strong harmonic echo
components uniquely distinguish echoes returning from the
microbubbles, which can be used to form B mode or Doppler images of
the bloodflow infused by the contrast agent. The other source of
harmonic echo signals is the nonlinear distortion which ultrasonic
waves undergo as they travel through tissue. The echoes returned
from these distorted waves manifest harmonic components developed
by this distortion.
[0003] My aforementioned U.S. patent describes two ways in which
the harmonic components of these echo signals may be detected. One
is by use of a highpass filter, which will pass signals in the
harmonic band while attenuating the stronger echo components in the
fundamental band. The other way is by transmitting two or more
pulses of opposite phase or polarity and combining the echoes
received in response from the two pulses. The fundamental
components, being of opposite phase or polarity by reason of that
characteristic of the transmit pulses, will cancel. The harmonic
components of the combined echoes, being quadratic in nature, will
additively combine, leaving the separated second harmonic
signals.
[0004] As discussed in my aforementioned patent, harmonic signals
are advantageous in many imaging situations because of the
distinctive way in which they identify echoes returned from
harmonic contrast agents. When used without contrast agents the
tissue harmonic signals are advantageous because their development
within the body eliminates much of the clutter caused by nearfield
effects. However, harmonic signals are of a significantly lower
amplitude than the fundamental signal echoes, providing lower
signal to noise ratios and requiring greater amplification. In
addition, harmonic signals require the use of relatively low
frequency transmit pulses so that the second harmonic echo signal
will be of a frequency which can be received within the
transducer's passband. Generally, the transmit signal will be
centered at the lower end of the transducer's passband so that the
second harmonic return signal will be below the upper cutoff of the
transducer passband. This can place the transmit and receive
signals at the extremes where broadband signals will experience
attenuating rolloff. It also mandates lower frequency transmit
signals, which can be more disruptive to microbubble contrast
agents than higher transmit frequencies would be. Accordingly it is
desirable to be able to overcome these deficiencies and limitations
of harmonic imaging.
[0005] In accordance with the principles of the present invention,
the nonlinear signals returned from tissue and contrast agents are
detected in the fundamental frequency band rather than at harmonic
frequencies. In a preferred embodiment the nonlinear signals are
detected by an amplitude modulated two (or more) pulse technique.
Preferably the transmit pulse waveforms are of opposite phase and
polarity and of different amplitudes. Upon reception the echoes are
normalized for the different transmit amplitudes and combined and
the signals within the fundamental band are used for imaging.
[0006] In the drawings:
[0007] FIG. 1 illustrates conventional transmit and harmonic
receive spectra within a transducer passband;
[0008] FIG. 2 illustrates the location of a harmonic spectrum at
the upper limit of the transducer passband;
[0009] FIG. 3 illustrates the fundamental transmit and second
harmonic receive spectra of an embodiment of the present
invention;
[0010] FIG. 4 illustrates the spectrum of fundamental nonlinear
echo signals resulting from the transmit spectrum of FIG. 3;
[0011] FIG. 5 illustrates receive passbands for the nonlinear echo
signals of FIG. 4;
[0012] FIGS. 6a-6g depict waveforms illustrating the principle of
pulse inversion harmonic separation;
[0013] FIGS. 7a-7g depict waveforms illustrating the principle of
nonlinear echo signal detection in the fundamental band in
accordance with the principles of the present invention;
[0014] FIG. 8 illustrates the transmit and receive passbands of one
embodiment of the present invention;
[0015] FIG. 9 illustrates the transmit and receive passbands of
another embodiment of the present invention; and
[0016] FIG. 10 illustrates an ultrasonic imaging system constructed
in accordance with the principles of the present invention.
[0017] Referring first to FIG. 1, typical fundamental and harmonic
spectra of an ultrasound system are shown. This drawing illustrates
a passband 10 of an ultrasonic transducer/beamformer which
transmits the fundamental frequency pulses or waves, and receives
the harmonic echo signals. In this example the transducer has a
passband extending from 1 to 3 MHz. When the same transducer is to
be used for both transmission and reception, both the fundamental
transmit pulse and the harmonic receive echoes must be encompassed
within the passband 10 of the transducer. In this example the
transmit pulse exhibits a passband 12 which is centered around 1.25
MHz. Second harmonic echo signals will be received in a passband 14
centered around 2.5 MHz. It is seen that because the transmit band
12 is at the lower end of the transducer passband 10, the harmonic
receive passband will fall in the upper portion of the passband 10
and thus both transmission and reception can be performed by this
particular transducer.
[0018] As FIG. 1 shows, in order to get both the fundamental band
12 and the harmonic band 14 within the same transducer passband it
is often necessary to fit one or the other or both of the transmit
or receive passbands at one of the cutoff extremes of the
transducer passband. FIG. 2 shows another example of this, in which
the fundamental transmit band 16 is centered about a frequency of
1.5 MHz and occupies the entire lower half of the transducer
passband 10. The transmit band 16 thus is for a more broadband
transmit signal than that which is transmitted by the passband 12
in FIG. 1, providing improved image detail and quality. However the
second harmonic receive passband 18 for this transmit pulse is
centered at 3 MHz at the upper extreme of the transducer passband
10. In this example the center of the harmonic band is at the upper
cutoff of the transducer band, resulting in significant attenuation
of signals in the upper portion of the band 18. Thus, broadband
harmonic imaging is limited by this transducer passband.
[0019] The passbands used in a first embodiment of the present
invention is shown in FIGS. 3-5 using the same transducer passband
10 as in the previous drawings. Two or more differently modulated
broadband transmit pulses having a passband 20 are transmitted
along each scanline in the image field as shown in FIG. 3. As this
drawing shows, the fundamental transmit pulses centered at 2 MHz
will elicit second harmonic echo signals in a band 22 centered at 4
MHz. These harmonic echo frequencies are outside the transducer
passband 10 and hence will be beyond the frequency range of the
transducer. However the transmit pulses will also elicit echoes in
the fundamental frequency band 20 and beyond, as shown by the
dashed line receive passband 24 in FIG. 4. As will be discussed
below, these fundamental frequencies can exhibit varying degrees of
linear and nonlinear characteristics depending upon the presence of
nonlinear reflectors such as contrast agents in the image field.
The nonlinear characteristics are extracted and the linear
fundamental components canceled by combining the echo signals in a
chosen receive passband such as receive bands 30, 32 or 34 as shown
in FIG. 5. The receive band can start at the higher frequency
location 32 at the initial reception of shallow depth echoes, then
be moved dynamically to the lower frequency position 34 during
reception to account for the effects of depth dependent frequency
attenuation. By virtue of the modulation of the transmit pulses and
the nonlinearity of the reflectors the correlated linear
characteristics in the fundamental band will cancel and the
uncorrelated nonlinear characteristics in the fundamental band will
not, leaving an echo signal component which is a measure of the
nonlinearity of the fundamental frequency echoes. Thus, nonlinear
components are detected which are below the second harmonic
frequency.
[0020] The use of these nonlinear components below the second
harmonic frequency provide several advantages. For one, most or
even all of the transducer passband can be used for transmission.
This enables the use of broadband transmit pulses which will result
in broadband echo signals for finer and more subtle image detail.
There is no need to constrain the transmit pulses to a narrow range
of the transducer passband. Another advantage is that the transmit
and receive passbands can both be more centered in the transducer
passband, away from the rolloff at the extremes of the transducer
passband. Higher transmit pulse frequencies and shorter duration
pulse bursts may also be used, providing advantages in the form of
reduced microbubble destruction.
[0021] FIG. 6 illustrates the principles of pulse inversion
transmission and reception. A first transmit pulse 40 (FIG. 6a) has
a first phase or polarity characteristic and a second transmit
pulse 50 has an second phase or polarity characteristic (FIG. 6b).
In this example both pulses are shown as a single cycle of a
waveform and the second pulse 50 is the inverse of the first. The
first transmit pulse 40 returns echo signals from a nonlinear
system such as a microbubble which have a fundamental frequency
component 42 (FIG. 6c) which follows the phase or polarity
modulation of the transmit pulse, and a second harmonic component
44 (FIG. 6e). The second transmit pulse 50 returns echo signals
from the nonlinear system which have a fundamental frequency
component 52 (FIG. 6d) which also follows the phase or polarity of
the transmit pulse, and a second harmonic component 54 (FIG. 6f).
When the echo signals from the two transmit pulses are combined the
fundamental frequency components will cancel each other and the
harmonic components will additively reinforce each other by reason
of the quadratic nature of harmonics, leaving a detectable second
harmonic component 60 (FIG. 6g). Thus, the second harmonic
components have been separated from the fundamental frequency
components of the echo signals.
[0022] FIG. 7 shows waveforms illustrating the principles of the
present invention. Like pulse inversion the technique of the
present invention uses multiple, differently modulated transmit
pulses to separate nonlinear signal components. FIGS. 7a and 7b
show two exemplary transmit pulses 70 and 80 which are of different
amplitudes. In this example the first transmit pulse 70 is twice
the amplitude of the second transmit pulse 80, although other
amplitude relationships may be employed. In this example the two
transmit pulses are also of opposite phase or polarity. Transmit
pulse 70 elicits different fundamental frequency echo signal
characteristics from nonlinear and linear targets. For instance, if
the echo is returned from a nonlinear contrast agent, the nonlinear
behavior of the microbubbles when insonified will return a
fundamental frequency echo waveform 72 as shown in FIG. 7e which is
nonlinearly related to the transmit waveform. If the echo is
returned from a linear reflector such as tissue, a fundamental
frequency echo 74 as shown in FIG. 7c results, which is seen to be
linearly related to the transmit pulse.
[0023] The second transmit pulse will elicit fundamental frequency
echo returns from nonlinear and linear reflectors as shown in FIGS.
7f and 7d. A nonlinear system such as a microbubble will return an
echo 82, which is nonlinearly related to the transmit waveform 80.
Since the transmit pulse 80 is of a lesser amplitude than the first
pulse, the microbubble will behave differently by reason of the
different level of insonification. A linear reflector returns an
echo 84 which is seen to be linearly related to the lesser
amplitude transmit pulse 80.
[0024] The first and second echo signals are normalized to account
for the different transmit pulse amplitudes. When the two transmit
pulses differ by a factor of two as they do in this example, the
echoes from the second pulse would be amplified by a factor of two,
for instance. When the corresponding echoes are combined after
normalization, it can be seen that the linear echoes will cancel as
shown by line 94 in FIG. 7g. The echoes returned from the nonlinear
reflectors will partially cancel but leave a difference which is a
manifestation of the different nonlinearities of the echoes as
shown by waveform 92 in FIG. 7h. That is because the nonlinear
characteristics of echoes 72 and 82 are not equalized by the linear
normalizing and will leave a residual signal after combining
because of the decorrelation of the two nonlinear echo signals. The
nonlinear effects in the echoes are not linearly related to the
difference in pulse amplitude of the two transmit pulses. This
means that the normalization, which will equalize the two linear
echoes 74 and 84 and result in cancellation, will not equalize the
echoes from the nonlinear reflectors. The oscillation of
microbubbles when insonified by pulses of different amplitudes is
nonlinear and more complex than just the amplitude difference.
Furthermore, the microbubbles can be disrupted by the first pulse
so that the microbubbles interrogated by the second pulse have a
different character than those encountered by the first pulse. The
combination of these different nonlinear and behavioral
characteristics of a nonlinear system provide the ability to
clearly distinguish echoes from nonlinear systems in the
fundamental frequency band.
[0025] FIG. 8 illustrates the passbands of another embodiment of
the present invention. The fundamental frequency transmit pulse
band is shown by the lined passband 200 centered at 2 MHz. The
receive band 202 is centered at 2.7 MHz and greatly overlaps the
transmit passband to receive echoes in a portion of the received
echo band 204. In the prior art great pains were taken to transmit
pulses in a band which did not overlap the receive band so that the
second harmonic signals could be cleanly separated from the
fundamental frequency signals. In the present invention, where it
is nonlinear components at and around the fundamental band which
are of interest, this is not a problem.
[0026] FIG. 9 illustrates yet another embodiment of the present
invention where the receive band 212 is below the transmit pulse
band 210. In this example the transmit band is a high frequency
band centered about 3.3 MHz. The high frequency transmit signals
result in better resolution in the echo signals. Locating the
transmit band at the upper end of the transducer passband 10
obviously cannot be done when trying to contain both the
fundamental and second harmonic bands in the transducer passband.
Thus, the present invention will afford better image resolution
than prior art harmonic systems. The receive band 212 encompasses
some of the echo signal frequencies in the echo signal band 214 and
is centered at 2.7 MHz in this embodiment.
[0027] One of the desires when performing contrast imaging is to
minimize the destruction of microbubbles causes by the ultrasonic
insonification, so that the period of contrast imaging can be
prolonged. Microbubbles can be disrupted and destroyed to a greater
extent by lower frequency pulses, higher amplitude pulses, and
longer pulse bursts. FIG. 9a compares four cycles of a 1.5 MHz
transmit pulse 220 which was the transmit frequency used in the
illustration of FIG. 2 with four cycles of a 3.3 MHz pulse 222 as
used in FIG. 9. As these drawings show, the duration of the 3.3 MHz
pulse is considerably less than that of the lower frequency burst,
causing less microbubble disruption. The 3.3 MHz pulse, being of a
higher frequency, will also cause less disruption than the lower
frequency pulse for this additional reason. Furthermore, the lower
amplitude transmit pulse (FIG. 7b) will cause less disruption than
the higher amplitude transmit pulse. While the different amplitude
pulses can be transmitted in either order, transmitting the lower
amplitude pulse as the first pulse will cause less disruption to
the microbubble field that is encountered by the second, higher
amplitude pulse. The two transmit pulses can also differ in
amplitude only or in phase or polarity only, but the combination of
the two modulation differences, both amplitude and phase or
polarity, provides better nonlinear decorrelation and thus better
nonlinear sensitivity.
[0028] FIG. 10 illustrates an ultrasound system in block diagram
form which is constructed in accordance with the principles of the
present invention. An ultrasound scanhead 100 including an array
transducer 102 is connected to a transmit/receive (T/R) switch 104.
A central controller 110 responsive to a user interface (not shown)
sets the frequency, amplitude, and phase or polarity of the
transmit pulse. A transmitter 106 transmits the pulses set by the
central controller by way of the T/R switch, exciting elements of
the transducer array in a timed sequence to transmit appropriately
steered and focused beams. The echoes received by the transducer
array 102 are coupled by the T/R switch to a receive beamformer
108. In this example the beamformer is shown as a multiline
beamformer which, under control of the central controller, produces
two spatially adjacent receive lines of coherent echo signals A and
B. The echo sequences produced in response to the first transmit
pulse are stored in line buffers 120 and 130 for the A and B
multilines, respectively. The echo sequences produced in response
to the second transmit pulse, in this example a lower amplitude
pulse, are multiplied by a factor of two in multiplier circuits 122
and 132 when the difference in amplitudes is a factor of two. The
multipliers can be easily implemented for multiplication by two in
a digital system by shifting the echo signal values one bit to the
left to multiply by two (or one bit to the right to halve a signal
which is twice the other signal amplitude). After this
normalization the echo sequences are combined in summers 124 and
134 respectively to separate the nonlinear fundamental components.
When these summers are set for subtraction the linear components
will be emphasized for oppositely phased or poled transmit pulses.
The echo signals are then filtered by filters 126 and 136. In a
preferred embodiment these filters are quadrature bandpass filters
as described in my aforementioned patent, to produce quadrature
signal components and also bandpass filtering for the receive
passband.
[0029] The echo signals may be processed by a B mode processor 140,
a Doppler processor 150, and/or a contrast signal processor 160.
The B mode processor will amplitude detect the echo signals in the
production of image signals, and the Doppler processor will process
ensembles of echo signals to produce image signals of tissue or
flow motion. The contrast signal processor is similar to the
previous processors, generally with a threshold which separates
contrast harmonic signals from tissue harmonic signals. Contrast
agents can be displayed in either Doppler or B mode format. Image
signals from the three processors are coupled over an image signal
bus 172 to a scan converter 170, which interpolates the image
signals and puts the scanlines in the desired image format. The
image information can be applied to a video processor 180 for
display of a two dimensional image on a display 190. The image
information can also be formed into three dimensional presentations
by 3D image rendering 182. Three dimensional images are stored in a
3D image memory 184 and displayed on the display 190 by way of the
video processor 180.
[0030] Other variations will be apparent. While the embodiment of
FIG. 10 provides the obvious benefit of 2x multiline, which is a
doubling of the scanline density or a halving of the framerate,
these factors can be further improved by interpolation. For
example, after transmitting two pulses of different modulation
characteristics to produce two A line sequences of different
characteristics and two B line sequences of different
characteristics, an A line sequence of one characteristic can be
combined with a B line sequence of the other characteristic to
interpolate a further line of nonlinear or linear signals between
the A and B lines. This could be done by combining the outputs of
multiplier circuit 122 and line buffer 130 in an additional summer,
for instance. By using more than two transmit pulses for a scanline
motional effects can be reduced as explained in U.S. patent
application Ser. No. 09/434,328 entitled "Ultrasonic Pulse
Inversion with Reduced Motional Effects", of which I am a
co-inventor. The system of FIG. 10 can be used for continuous
realtime contrast imaging as described in U.S. patent [application
Ser. No. 09/302,063] entitled "Realtime Ultrasonic Imaging of
Perfusion Using Ultrasonic Contrast Agents", of which I am a
co-inventor. Embodiments of my invention can be used for detecting
the nonlinear effects of numerous nonlinear reflectors, such a
microbubble contrast agents and nonlinear effects due to pulse
travel through tissue. The band selection performed by filters 126
and 136 can be omitted or performed by FIR filters or in the wall
filter of the Doppler processor 150.
* * * * *