U.S. patent application number 09/682358 was filed with the patent office on 2003-03-06 for method and apparatus for improved spatial and temporal resolution in ultrasound imaging.
Invention is credited to Bjaerum, Steinar, Kirkhorn, Johan, Kristoffersen, Kjell, Olstad, Bjorn, Saetre, Dagfinn, Steen, Erik N., Torp, Hans Garmann, Viggen, Kjetil.
Application Number | 20030045795 09/682358 |
Document ID | / |
Family ID | 24739335 |
Filed Date | 2003-03-06 |
United States Patent
Application |
20030045795 |
Kind Code |
A1 |
Bjaerum, Steinar ; et
al. |
March 6, 2003 |
METHOD AND APPARATUS FOR IMPROVED SPATIAL AND TEMPORAL RESOLUTION
IN ULTRASOUND IMAGING
Abstract
A method is provided to simultaneously acquire two ultrasound
images. A first set of ultrasound pulses are transmitted at a first
frame rate utilizing a first mode of operation. The echoes from the
first set of ultrasound pulses are received. A second set of
ultrasound pulses are transmitted at a second frame rate utilizing
a second mode of operation. The first and second frame rates are
different. The first set of ultrasound pulses defines an entire
image, while the second set of ultrasound pulses defines a partial
image. The echoes from the second set of ultrasound pulses are
received, and the echoes from the first and second sets of
ultrasound pulses are displayed as a single image.
Inventors: |
Bjaerum, Steinar; (Horten,
NO) ; Kirkhorn, Johan; (Horten, NO) ; Torp,
Hans Garmann; (Trondheim, NO) ; Viggen, Kjetil;
(Trondheim, NO) ; Olstad, Bjorn; (Stathelle,
NO) ; Kristoffersen, Kjell; (Olso, NO) ;
Steen, Erik N.; (Moss, NO) ; Saetre, Dagfinn;
(Horten, NO) |
Correspondence
Address: |
MCANDREWS HELD & MALLOY, LTD
500 WEST MADISON STREET
SUITE 3400
CHICAGO
IL
60661
|
Family ID: |
24739335 |
Appl. No.: |
09/682358 |
Filed: |
August 24, 2001 |
Current U.S.
Class: |
600/441 |
Current CPC
Class: |
G01S 7/52074 20130101;
G01S 7/52063 20130101; G01S 7/52085 20130101; G01S 7/52034
20130101; G01S 15/8979 20130101 |
Class at
Publication: |
600/441 |
International
Class: |
A61B 008/00 |
Claims
1. A method for obtaining diagnostic ultrasound images, the method
comprising: transmitting a first set of ultrasound pulses at a
first frame rate in accordance with a first mode of operation;
receiving echoes from the first set of ultrasound pulses;
transmitting a second set of ultrasound pulses at a second frame
rate in accordance with a second mode of operation, said first
frame rate differing from said second frame rate; receiving echoes
from the second set of ultrasound pulses; and displaying a single
image representative of echoes received from both of said first and
second sets of ultrasound pulses.
2. The method of claim 1, wherein the first set of ultrasound
pulses is transmitted at a higher frame rate than the second set of
ultrasound pulses.
3. The method of claim 1, wherein the first set of ultrasound
pulses defines a Doppler image, and the second set of ultrasound
pulses defines a B-mode image, said displaying step overlaying said
Doppler and B-mode images.
4. The method of claim 1, wherein the transmitting steps include
obtaining a single Doppler image frame from the first set of
ultrasound pulses and obtaining first and second portions of a
single B-mode image frame before and after respectively obtaining
the complete Doppler image frame.
5. The method of claim 1, wherein the first set of ultrasound
pulses defines a high resolution portion of a B-mode image, and the
second set of ultrasound pulses defines a low resolution portion of
a B-mode image, said high and low resolution portions forming said
single image that is displayed.
6. The method of claim 1, wherein the echoes from the second set of
ultrasound pulses define a partial image and the echoes from the
first set of ultrasound pulses define an entire image, said
displaying step overlaying the partial and entire images.
7. The method of claim 1, wherein the first set of ultrasound
pulses defines a high resolution image, and the second set of
ultrasound pulses defines a low resolution image.
8. The method of claim 1, further comprising: transmitting a series
of uninterrupted, successive pulses in a common direction relative
to an area being scanned; detecting a series of echoes from the
series of uninterrupted, successive pulses; and calculating a
portion of a Doppler image from the series of echoes, said portion
of the Doppler image corresponding to the common direction.
9. The method of claim 1, wherein the first set of ultrasound
pulses is interleaved with the second set of ultrasound pulses.
10. The method of claim 1, wherein transmitting the first set of
ultrasound pulses further comprises: transmitting at least one
ultrasound pulse in a first direction corresponding to a first
portion of the single image; transmitting an ultrasound pulse in a
second direction corresponding to a second portion of the single
image; transmitting a second ultrasound pulse in said first
direction; and transmitting a second ultrasound pulse in said
second direction.
11. A method for obtaining ultrasound images of an area of
interest, the method comprising: transmitting a set of Doppler
pulses in accordance with a Doppler mode of operation; receiving
Doppler echoes from the set of Doppler pulses; after receiving the
Doppler echoes, transmitting a set of non-Doppler pulses in
accordance with another mode of operation differing from the
Doppler mode of operation, wherein said set of non-Doppler pulses
corresponds to a sub-region of a displayed image; receiving
non-Doppler echoes from the set of non-Doppler pulses; and
displaying images based on the Doppler and non-Doppler echoes from
the Doppler and non-Doppler pulses.
12. The method of claim 11, wherein said transmitting step includes
transmitting a first packet of successive pulses directed in a
first direction relative to an area of interest followed by a
second packet of successive pulses directed in a second direction
relative to an area of interest.
13. The method of claim 11, wherein the first transmitting step
further comprises transmitting one pulse of a first packet of
pulses in a first direction relative to an area of interest
followed by transmitting one pulse of a second packet of pulses in
a second direction relative to an area of interest followed by
transmitting a second pulse of said first packet of pulses in said
first direction.
14. The method of claim 11, further comprising: transmitting at
least one Doppler pulse corresponding to a first sub-region of an
image; and immediately after receiving echoes from the Doppler
pulse, transmitting at least one Doppler pulse corresponding to a
second sub-region of the image.
15. The method of claim 11, wherein said transmitting step further
comprises: transmitting a first packet of successive Doppler pulses
directed in a first direction relative to an area of interest; and
after receiving the echoes from a subset of said set of non-Doppler
pulses, transmitting a second packet of successive Doppler pulses
in a second direction relative to an area of interest.
16. The method of claim 11, wherein the second transmitting step
includes transmitting at least one non-Doppler pulse corresponding
to a sub-region of an image, and wherein the echoes from the
non-Doppler pulses form a partial image.
17. The method of claim 11, wherein the set of Doppler pulses
defines N frames of Doppler images and the set of non-Doppler
pulses defines M frames of non-Doppler images, and wherein M is
less than N.
18. The method of claim 11, wherein the transmit and receive steps
form a scan sequence divided into scan intervals, each Doppler and
non-Doppler pulse being transmitted and received in a unique,
non-overlapping scan interval, further comprising: suspending the
transmitting and receiving steps during at least one scan interval
in a sequence of scan intervals, in which non-Doppler pulses are
being transmitted and received.
19. The method of claim 11, wherein the transmit and receive steps
form a scan sequence divided into scan intervals, wherein a subset
of non-Doppler pulses is transmitted and received during each scan
interval, further comprising: transmitting in a first scan interval
at least one non-Doppler pulse associated with a first sub-region
of a non-Doppler image; and transmitting in a second scan interval
at least one non-Doppler pulse associated with a second sub-region
of a non-Doppler image.
20. The method of claim 11, wherein the transmitting and receiving
steps form a scan sequence divided into scan intervals of equal
duration.
21. The method of claim 11, wherein said transmitting a set of
non-Doppler pulses includes transmitting at least one non-Doppler
pulse corresponding to a sub-region of the image, wherein the
received Doppler echoes form a complete image, and wherein the
received non-Doppler echoes form a partial image.
22. The method of claim 11, wherein said image based on the
received Doppler echoes is calculated by utilizing a sliding window
technique.
23. The method of claim 11, wherein said image based on the
received echoes from the Doppler pulses is comprised of a number of
transmit directions, and wherein the first transmitting step
further comprises: transmitting one Doppler pulse in each said
transmit direction.
24. The method of claim 11, wherein the Doppler and non-Doppler
pulses are transmitted at a predefined pulse repetition time,
further comprising the step of pausing during the transmitting
steps by a duration greater than the pulse repetition time.
Description
BACKGROUND OF INVENTION
[0001] Certain embodiments of the present invention relate to
ultrasound imaging of the human anatomy for the purpose of medical
diagnosis. In particular, certain embodiments of the present
invention relate to methods and apparatus for improving spatial and
temporal resolution in the ultrasound image.
[0002] Two dimensional (2D) ultrasound Doppler imaging is used for
blood flow visualization within the body, as well as for
visualization of muscular tissue movement and deformation, such as
forced compression of the human breast. A B-mode grayscale sector
and 2D Doppler information in a sector covering all or part of the
B-mode sector may be separately acquired. The Doppler information
is color-coded and overlaid onto the B-mode grayscale image to
visualize the velocity information of an area of interest. Because
the grayscale image is used to visualize tissue structures and the
Doppler image is used to represent velocity information, the B-mode
image is often referred to as the "tissue image".
[0003] Prior techniques combine a high resolution 2D B-mode image
with a lower resolution 2D Doppler image, acquired with the same
frame rate. For example, FIG. 2 illustrates the geometry for a
conventional sector scanned 2D Doppler acquisition. The geometry
image 202 shows a Doppler sector image overlaid on a B-mode sector
image. The B-mode sector image 204 is comprised of B-mode transmit
beams 206. The Doppler sector image 208 is comprised of Doppler
transmit beam directions 210. In this example, the number of B-mode
transmit beams (N.sub.B) is 12, and the number of Doppler transmit
beam directions (N.sub.D) is 4. Thus, the B-mode sector image 204
has a higher beam density and a higher resolution than the Doppler
sector image 208.
[0004] FIG. 3 illustrates a scan sequence of a conventional 2D
Doppler acquisition. Twelve Doppler pulses 302-324 and twelve
B-mode pulses 326-348 are illustrated. First, the Doppler pulses
302-324 are transmitted sequentially, starting with Doppler pulse
302. Then the B-mode pulses 326-348 are transmitted sequentially,
starting with B-mode pulse 326. The B-mode pulses 326-348 are
labeled to indicate transmission direction. For example, B-mode
pulse 326 labeled B.sub.1 indicates that the B-mode pulse is
transmitted in direction 1. B-mode pulse 346 labeled B.sub.11
indicates that the pulse is transmitted in direction 11. The
Doppler pulses 302-324 are labeled such that D.sub.ij indicates a
Doppler pulse transmission j in direction i. The Doppler pulses
302-324 are each part of a packet 350-356. Each Doppler pulse
302-324 transmitted in the same direction is part of the same
packet 350-356. The packet size (PS) is the number of Doppler
pulses 302-324 in each packet 350-356. For example, the PS of FIG.
3 is 3. Therefore, each packet 350-356 comprises the Doppler pulses
302-324 transmitted in one transmit beam direction, and each packet
350-356 is transmitted in a different direction.
[0005] The velocities for 2D Doppler are estimated at points along
each beam direction based on the received signals from the packets
350-356. For example, the Doppler pulses 302, 304, and 306 each may
be used to estimate a velocity measurement for packet 350 in beam
direction 1. The time between two Doppler pulses within a packet is
called the Doppler pulse repetition time (PRT.sub.D), thus the
Doppler pulse repetition frequency (PRF.sub.D) is
PRF.sub.D=1/PRT.sub.D.
[0006] The depth in the body of the item of interest will determine
the maximum PRF.sub.D (PRF.sub.DMAX). The transmitted Doppler pulse
302-324 must propagate to the deepest item of interest and back to
the transducer before a new pulse transmission can be made.
Additionally, hardware limitations and reverberations from deep
reflectors beyond the imaging depth may need to be considered. For
FIG. 3, PRF.sub.D>0.5*PRF.sub.DMA- X.
[0007] The time required to acquire one frame of Doppler and B-mode
data is T.sub.frame. The frame rate (FR) is calculated as
FR=1/T.sub.frame. Because one complete B-mode image frame is
acquired between each Doppler image frame, the frame rate for
Doppler (FR.sub.D) is equal to the frame rate for B-mode
(FR.sub.B). During the acquisition of one image frame, the Doppler
pulses 302-324 are each transmitted one time and the B-mode pulses
326-348 are each transmitted one time. The time to acquire one
frame (T.sub.frame) may be calculated as:
T.sub.frame=(N.sub.D.times.PS)/PRF.sub.D+N.sub.B/PRF.sub.B Equation
1
[0008] where T.sub.frame is the time to acquire one image frame,
N.sub.D is the number of Doppler transmit beam directions, PS is
the packet size, PRF.sub.D is the Doppler pulse repetition
frequency, N.sub.B is the number of B-mode transmit pulses per
frame, and PRF.sub.B is the B-mode pulse repetition frequency. In
FIG. 3, for example, N.sub.D=4, PS=3, and N.sub.B=12.
[0009] FIG. 4 illustrates a scan sequence of a conventional 2D
Doppler acquisition utilizing interleaving of the Doppler pulses
and the B-mode pulses. Twelve Doppler pulses 402-424 and twelve
B-mode pulses 426-448 are illustrated. The Doppler pulses 402-424
are each part of a packet 450-456. Each packet 450-456 comprises
the pulses transmitted in one beam direction, and each packet
450-456 is transmitted in a different direction.
[0010] As in FIG. 3, FIG. 4 has 4 Doppler transmit beam directions.
In FIG. 4, however, the transmit beam directions, each comprised of
three Doppler pulses 402-424, are interleaved with the B-mode
pulses 426-448. First, Doppler pulses 402-406 are transmitted in
direction 1. Next, B-mode pulses 426-430 are transmitted, then
Doppler pulses 408-412 are transmitted in direction 2, and so on.
By interleaving the B-mode pulses 426-448 into the Doppler pulse
402-424 sequence, the timing difference between acquiring the
Doppler image and acquiring the underlying B-mode image is reduced.
The acquisition time per frame has not changed however, thus the
time to acquire the Doppler scan image is the same as the time to
acquire the B-mode scan image. To put it another way, one B-mode
image is acquired for every Doppler image. Therefore, the FR.sub.D
is equal to the FR.sub.B.
[0011] When lower velocities are measured, the PRF.sub.D may be
decreased. If the PRF.sub.D decreases, the acquisition time per
frame may increase and the frame rate may decrease, as illustrated
by Equation 1. The frame rate may be maintained, however, by
utilizing Doppler beam interleaving. After transmitting a Doppler
pulse in a first direction, Doppler pulses are transmit in one or
more other directions before transmitting the second pulse in the
first direction. In Doppler beam interleaving, the Interleave Group
Size (IGS) indicates the number of Doppler beam directions that are
interleaved.
[0012] Therefore, for lower velocities, the same frame rate can be
maintained with the same number of transmit directions by using
Doppler beam interleaving where IGS is an integer.gtoreq.2, and
PRF.sub.D<PRF.sub.Dmax/IGS. If PRF.sub.Dmax=PRF.sub.D*IGS is
kept constant, the scanning time per frame remains constant when
PRF.sub.D is reduced. Thus, PRF.sub.Dmax may be kept constant by
increasing the IGS when the PRF.sub.D decreases, as illustrated in
the following relationship:
T.sub.frame=(N.sub.D.times.PS)/(PRF.sub.D.times.IGS)+N.sub.B/PRF.sub.B=(N.-
sub.D.times.PS)PRF.sub.Dmax+N.sub.B/PRF.sub.B
[0013] FIG. 5 illustrates a scan sequence of a conventional 2D
Doppler acquisition with 2 Doppler transmit directions interleaved.
Twelve Doppler pulses 502-524 followed in time by twelve B-mode
pulses 526-548 are illustrated.
[0014] As described in FIG. 3, Doppler pulses are each part of a
packet that comprises the pulse transmissions along one beam
direction in the image. In FIG. 3, all of the Doppler pulses that
comprise a packet are transmitted before transmitting a Doppler
pulse of a different packet. In FIG. 5, however, the Doppler pulses
502-524 utilize Doppler beam interleaving as discussed previously.
Doppler pulse 502 is transmitted in direction 1, then Doppler pulse
504 is transmitted in direction 2. Next, Doppler pulse 506 is
transmitted in direction 1, then Doppler pulse 508 is transmitted
in direction 2. The IGS of FIG. 5 is 2, because two Doppler
transmit beam directions are interleaved. Once the Doppler transmit
beams are completed, then the B-mode pulses are transmitted.
[0015] FIG. 6 illustrates a scan sequence of a conventional 2D
Doppler acquisition with 4 Doppler transmit directions interleaved.
Twelve Doppler pulses 602-624 and twelve B-mode pulses 626-648 are
illustrated. One Doppler pulse 602-624 is transmitted in each of
the 4 transmit directions before a second Doppler pulse 602-624 is
transmitted in any direction. The IGS of FIG. 6 is 4.
[0016] In addition to the techniques above, it is possible to
obtain several received beams for each transmitted pulse by
focusing in slightly different directions. This technique is called
parallel beamforming or Multi-Line Acquisition (MLA). The number of
parallel receive beams per B-mode transmit beam (MLA.sub.B) may be
different than the number of parallel receive beams per Doppler
transmit beam (MLA.sub.D).
[0017] Below is an example of the frame rate and beam densities
achieved with a conventional packet acquisition setup utilizing MLA
for cardiac imaging. In this example, the PRF.sub.B is lower than
the PRF.sub.D to minimize reverberation effects:
1 PRF.sub.B = 3 kHz PRF.sub.Dmax = PRF.sub.D * IGS = 4 kHz N.sub.B
= 36 N.sub.D = 8 MLA.sub.B = 2 MLA.sub.D = 4 PS = 3 Acquisition
time per T.sub.frame = N.sub.D * PS/PRF.sub.Dmax +
N.sub.B/PRF.sub.B = 18 ms frame: Frame rate: FR = 1/T.sub.frame =
55 Hz Receive beams Doppler: MLA.sub.D * N.sub.D = 32 Receive beams
B-mode: MLA.sub.B * N.sub.B = 72
[0018] For tissue Doppler techniques there is a desire for frame
rates considerably higher than what is achievable with the
conventional 2D Doppler acquisition techniques discussed
previously. Often the need to capture the details of flowjets or
rapid tissue accelerations requires a high frame rate for the
Doppler information, whereas the tissue B-mode image need not be
updated as often. However, in order to achieve a high resolution
B-mode image, the combined B-mode/Doppler frame rate becomes
relatively low. For example, when assessing the rapid movement in
the cardiac muscle during the relaxation phase of the cardiac
cycle, the main problem with the aforementioned acquisition
techniques is that the 2D Doppler frame rate can not be increased
without decreasing the spatial resolution of the B-mode image. But
in order to achieve the desired B-mode resolution in a sector
covering the whole myocardium, the frame rate has to be reduced.
For example, the frame rate of 55 Hz for conventional packet
acquisition utilizing MLA as indicated above is much lower than the
desired frame rate for cardiac imaging, which may be from 100
frames per second to as high as 300 frames per second for some
applications.
[0019] Additionally, during B-mode imaging, there may be different
demands on the resolution in different areas of a B-mode image. An
example is when studying a heart valve. In a small region
surrounding the valve, both high spatial and temporal resolution
are desired. The other parts of the image are mainly used for
orientation, and a lower resolution is acceptable.
[0020] Thus, a need has long existed in the industry for a method
and apparatus for acquiring ultrasound data that addresses the
problems noted above and previously experienced.
SUMMARY OF INVENTION
[0021] In accordance with at least one embodiment, a method is
provided to simultaneously acquire two ultrasound images. A first
set of ultrasound pulses is transmitted at a first frame rate in
accordance with a first mode of operation. The echoes from the
first set of ultrasound pulses are received. A second set of
ultrasound pulses is transmitted at a second frame rate different
from the first frame rate in accordance with a second mode of
operation. The echoes from the first and second set of ultrasound
pulses are displayed as one image.
[0022] In an alternative embodiment, the first set of ultrasound
pulses defines a Doppler image and the second set of ultrasound
pulses defines a portion of a B-mode image. A first portion of the
B-mode image may be obtained before the Doppler image is obtained.
Then a second portion of the B-mode image is obtained after the
Doppler image is obtained. The Doppler and B-mode images are
overlaid to display one image.
[0023] In another embodiment, the first set of ultrasound pulses
defines a high resolution B-mode image and the second set of
ultrasound pulses defines a low resolution B-mode image. The
portion of the low resolution B-mode image underlying the high
resolution B-mode image may be obtained using the ultrasound pulses
defining the high resolution B-mode image. The high and low
resolution B-mode images are overlaid to display one image.
[0024] In an alternative embodiment, a portion of a Doppler image
may be calculated by transmitting a series of uninterrupted,
successive pulses in a common direction and detecting the echoes
returned from the series of successive pulses. A first packet of
successive Doppler pulses directed in a first direction is
transmitted, followed by a second packet of successive Doppler
pulses directed in a second direction. In another embodiment, the
Doppler pulses may be interleaved, wherein one pulse of the first
packet is transmitted followed by one pulse of the second packet.
In an alternative embodiment, the first set of ultrasound pulses
and the second set of ultrasound pulses may be interleaved. The
first packet of successive Doppler pulses is transmitted in a first
direction. After the non-Doppler echoes are received, a second
packet of successive Doppler pulses is transmitted in a second
direction.
[0025] In an alternative embodiment, an image based on the received
echoes from the Doppler pulses is comprised of a number of transmit
directions, and one Doppler pulse is transmitted in each direction.
A Doppler image is calculated by utilizing a sliding window
technique based upon the received echoes.
[0026] In accordance with at least one embodiment, a method for
obtaining ultrasound images of an area of interest is provided. A
set of Doppler pulses is transmitted and the Doppler echoes are
received. A set of non-Doppler pulses corresponding to a sub-region
of a displayed image is transmitted and the non-Doppler echoes are
received. Images based on the Doppler and non-Doppler echoes are
displayed.
[0027] In one embodiment, the non-Doppler pulses correspond to a
sub-region of an image. In another embodiment, the Doppler echoes
form a complete image and the non-Doppler echoes form a partial
image. Therefore, the set of Doppler pulses defines more image
frames than the set of non-Doppler pulses.
[0028] In another embodiment, a scan sequence of transmitting and
receiving pulses is divided into scan intervals. The scan interval
in which non-Doppler pulses are transmitted and received may be
suspended. In another embodiment, non-Doppler pulses associated
with a first sub-region of a non-Doppler image are transmitted in a
first scan interval and non-Doppler pulses associated with a second
sub-region of a non-Doppler image are transmitted in a second scan
interval.
BRIEF DESCRIPTION OF DRAWINGS
[0029] FIG. 1 illustrates a block diagram of an ultrasound system
formed in accordance with an embodiment of the present
invention.
[0030] FIG. 2 illustrates the geometry for a conventional sector
scanned 2D Doppler acquisition.
[0031] FIG. 3 illustrates a scan sequence of a conventional 2D
Doppler acquisition.
[0032] FIG. 4 illustrates a scan sequence of a conventional 2D
Doppler acquisition utilizing interleaving of the Doppler pulses
and the B-mode pulses.
[0033] FIG. 5 illustrates a scan sequence of a conventional 2D
Doppler acquisition with 2 Doppler transmit directions
interleaved.
[0034] FIG. 6 illustrates a scan sequence of a conventional 2D
Doppler acquisition with 4 Doppler transmit directions
interleaved.
[0035] FIG. 7 illustrates a method of computing Doppler and B-mode
images from the same transmit pulses utilizing a sliding window
technique obtained in accordance with an embodiment of the present
invention.
[0036] FIG. 8 illustrates a scan sequence of a 2D Doppler
acquisition utilizing packet acquisition of 2D Doppler data with a
reduced B-mode frame rate obtained in accordance with an embodiment
of the present invention.
[0037] FIG. 9 illustrates a scan sequence of a 2D Doppler
acquisition utilizing packet acquisition of 2D Doppler data with
B-mode pulses interleaved between Doppler pulse packets and a
reduced B-mode frame rate obtained in accordance with an embodiment
of the present invention.
[0038] FIG. 10 illustrates a scan sequence of a 2D Doppler
acquisition utilizing packet acquisition of 2D Doppler data with
reduced B-mode frame rate and 2 interleaved Doppler transmit
directions obtained in accordance with an embodiment of the present
invention.
[0039] FIG. 11 illustrates a scan sequence of a 2D Doppler
acquisition utilizing packet acquisition of 2D Doppler data with
reduced B-mode frame rate and 4 interleaved Doppler transmit
directions obtained in accordance with an embodiment of the present
invention.
[0040] FIG. 12 illustrates a scan sequence of a 2D Doppler
acquisition utilizing packet acquisition of 2D Doppler data with
reduced B-mode frame rate and 4 interleaved Doppler transmit
directions obtained in accordance with an embodiment of the present
invention.
[0041] FIG. 13 illustrates a scan sequence of a 2D Doppler
acquisition utilizing packet acquisition of 2D Doppler data with a
reduced B-mode frame rate, 4 interleaved Doppler transmit
directions, and a pause inserted to achieve a constant Doppler
frame rate obtained in accordance with an embodiment of the present
invention.
[0042] FIG. 14 illustrates a scan sequence of a 2D Doppler
acquisition utilizing continuous 2D Doppler acquisition with a
reduced B-mode frame rate obtained in accordance with an embodiment
of the present invention.
[0043] FIG. 15 illustrates a scan sequence of a B-mode acquisition
utilizing continuous B-mode acquisition and a second B-mode
acquisition with a reduced frame rate obtained in accordance with
an embodiment of the present invention.
[0044] FIG. 16 illustrates a scan sequence of a 2D Doppler
acquisition utilizing continuous 2D Doppler acquisition with an
increased number of Doppler transmit directions and a reduced
B-mode frame rate obtained in accordance with an embodiment of the
present invention.
[0045] FIG. 17 illustrates a scan sequence of a 2D Doppler
acquisition utilizing continuous acquisition of Doppler data, with
B-mode pulses interleaved between the Doppler pulses obtained in
accordance with an embodiment of the present invention.
[0046] FIG. 18 illustrates a scan sequence of a 2D Doppler
acquisition utilizing continuous acquisition with a non-integer
ratio between the Doppler frame rate and the B-mode frame rate
obtained in accordance with an embodiment of the present
invention.
[0047] FIG. 19 illustrates a scan sequence of a 2D Doppler
acquisition utilizing continuous acquisition with a pause inserted
into the scan sequence to obtain a constant time interval between
Doppler pulses in the same direction obtained in accordance with an
embodiment of the present invention.
[0048] The foregoing summary, as well as the following detailed
description of the embodiments of the present invention, will be
better understood when read in conjunction with the appended
drawings. It should be understood, however, that the present
invention is not limited to the arrangements and instrumentality
shown in the attached drawings.
DETAILED DESCRIPTION
[0049] FIG. 1 illustrates a block diagram of an ultrasound system
100 formed in accordance with an embodiment of the present
invention. The ultrasound system 100 includes a transmitter 102
which drives transducers 104 within a probe 106 to emit pulsed
ultrasonic signals into a body. Any probe geometry may be used as
long as the probe 106 is capable of fast beam interleaving. The
ultrasonic signals are backscattered from structures in the body,
like blood cells or muscular tissue, to produce echoes which return
to the transducers 104. The echoes are received by a receiver 108.
The received echoes are passed through a beamformer 110, which
performs beamforming and outputs an RF signal. The RF signal then
passes through an RF processor 112. Alternatively, the RF processor
112 may include a complex demodulator (not shown) that demodulates
the RF signal to form IQ data pairs representative of the echo
signals. The RF or IQ signal data may then be routed directly to
RF/IQ buffer 114 for temporary storage.
[0050] The ultrasound system 100 also includes a signal processor
116 to process the acquired ultrasound information (i.e., RF signal
data or IQ data pairs) and prepare frames of ultrasound information
for display on display system 118. The signal processor 116 is
adapted to perform one or more processing operations according to a
plurality of selectable ultrasound modalities on the acquired
ultrasound information. Acquired ultrasound information may be
processed in real-time during a scanning session as the echo
signals are received. Additionally or alternatively, the ultrasound
information may be stored temporarily in RF/IQ buffer 114 during a
scanning session and processed in less than real-time in a live or
off-line operation.
[0051] The ultrasound system 100 often continuously acquires
ultrasound information at a frame rate that exceeds 50 frames per
second--the approximate perception rate of the human eye. The
acquired ultrasound information is displayed on the display system
118 at a slower frame-rate. An image buffer 122 is included for
storing processed frames of acquired ultrasound information that
are not scheduled to be displayed immediately. Preferably, the
image buffer 122 is of sufficient capacity to store at least
several seconds worth of frames of ultrasound information. The
frames of ultrasound information are stored in a manner to
facilitate retrieval thereof according to its order or time of
acquisition. The image buffer 122 may comprise any known data
storage medium.
[0052] FIG. 7 illustrates a method of computing Doppler and B-mode
images from the same transmit pulses utilizing a sliding window
technique. Doppler frames 702-706 and B-mode frames 708-716 are
illustrated. There are five sets of transmit pulses that are
transmitted in even time intervals.
[0053] Calculating the Doppler and B-mode images from the same
pulse transmissions can increase the frame rate. The Doppler data
are acquired with constant sampling intervals, and can be processed
using a sliding window technique as shown in FIG. 7. For example,
pulse transmissions 718, 720, and 722 are utilized to compute
Doppler data in one beam direction for Doppler frame 702. Pulse
transmissions 720, 722, and 724 are utilized to compute Doppler
data in one beam direction for Doppler frame 704. Below is an
example of the frame rate and beam densities achieved with this
technique for cardiac imaging:
2 PRF.sub.B = PRF.sub.D = 4 kHz N.sub.B = N.sub.D = 12 MLA.sub.B =
MLA.sub.D = 4 Acquisition time per frame: T.sub.frame =
N.sub.B/PRF.sub.B = 3 ms Frame rate B-mode and Doppler: FR.sub.B =
FR.sub.D = 1/T.sub.frame = 333 Hz Receive beams Doppler: MLA.sub.D
* N.sub.D = 48 Receive beams B-mode: MLA.sub.B * N.sub.B = 48
[0054] where T.sub.frame is the time required to acquire one image
frame, N.sub.D is the number of Doppler transmit beam directions,
N.sub.B is the number of B-mode transmit pulses per frame,
PRF.sub.D is the Doppler pulse repetition frequency, PRF.sub.B is
the B-mode pulse repetition frequency, MLA.sub.B is the number of
parallel receive beams per B-mode transmit beam, MLA.sub.D is the
number of parallel receive beams per Doppler transmit beam,
FR.sub.B is the B-mode frame rate, and FR.sub.D is the Doppler
frame rate.
[0055] With this acquisition method, PRF.sub.D equals the frame
rate. The frame rate of 333 Hz achieved for the system 100
utilizing MLA and the sliding window method to calculate the
Doppler frames is more desirable than the system 100 that achieved
a frame rate of 55 Hz utilizing MLA with conventional packet
acquisition. However, to achieve a sufficient PRF.sub.D and frame
rate, the number of transmitted pulses for each frame needs to be
relatively small. A large number of parallel receive beams is thus
necessary to image a wide sector. Another drawback with this
technique is that the B-mode and Doppler pulses can not be
differently optimized, for example in terms of frequency and pulse
length.
[0056] The aforementioned limitations may be overcome by scanning
the B-mode region with a high beam density (e.g. high spatial
resolution) and a low frame rate, and scanning the Doppler region,
which may be smaller than the B-mode region, with a lower beam
density (e.g. lower spatial resolution) and a higher frame rate.
The B-mode and Doppler images are acquired by fast beam
interleaving between the two modalities.
[0057] Separate pulses are used for the high resolution B-mode
image, and are transmitted in between the Doppler pulses. The mean
number of B-mode pulses and Doppler pulses per time unit can be
varied to obtain a requested beam density and frame rate for the
B-mode and Doppler images. The region of interest (ROI) of the
Doppler image may be different from the size of the tissue image,
and may be moved by the operator in order to focus on a different
area of anatomy.
[0058] For blood flow imaging, acquiring a low resolution Doppler
image and a high resolution B-mode image is also especially well
suited in combination with Blood Motion Imaging (BMI), which is a
method to enhance and display speckle pattern in the blood vessel
giving the operator a visual perception of the blood movement.
[0059] Alternatively, a multiple resolution B-mode image may be
acquired. Separate pulses are used for a high resolution B-mode
image inside a ROI, and are transmitted between pulses used to
acquire the surrounding lower resolution B-mode image. The ROI is
updated at a higher frame rate than the rest of the B-mode image,
and may be moved to view a different ROI similar to the Doppler
ROI.
[0060] The temporal resolution in B-mode imaging can be improved by
temporal interpolation. For stationary targets, a linear
interpolation in time between pixels is acceptable, but for moving
targets (i.e. cardiac walls) the spatial details will be smeared
out with linear interpolation. The interpolation can be improved by
tracking the positions of the objects in the B-mode image. The
acquisition technique described below provides Doppler data that
can be used for position tracking to get high quality temporal
interpolation of the B-mode image. Such a technique is especially
useful for 3-dimensional scanning where the temporal resolution
without any interpolation is relatively poor.
[0061] The high frame rate is a challenge for real-time display,
because the human eye is limited to approximately 50 frames/sec.
There are several possible options for real-time display:
[0062] 1. Only the B-mode images are displayed on the display
system 118, as a regular sequence of grayscale B-mode images. The
Doppler image data are stored for post-processing.
[0063] 2. Decimation in time of the Doppler images. The decimated
stream of Doppler images is displayed as an overlay to the B-mode
images on the display system 118. The Doppler image data are stored
for post-processing with full time resolution.
[0064] 3. Interpolation in time of the B-mode images. One B-mode
image is generated for each Doppler image to achieve full Doppler
frame rate. The time axis is divided into heart cycles. To be able
to display all the image frames within a heart cycle on the display
system 118, the frames from one or more of the subsequent heart
cycles are not displayed.
[0065] 4. Similar to 3, but the combined B-mode/Doppler images are
decimated in time, as described in 2. This means that fewer heart
cycles need to be discarded.
[0066] Several types of combined processing and display are of
interest. Especially, temporal displays like M-modes (e.g.
conventional, curved or anatomical) and curves of
velocity-parameters (e.g. velocity, displacement, strain) versus
time, where the high temporal resolution can be appreciated. Since
the Doppler data covers the whole myocardium, these temporal
displays can be shown simultaneously in several points of the
image, and with no degradation in quality compared to single
sample-volume methods.
[0067] The packet acquisition scan sequences discussed below divide
the B-mode region into M sub-regions. M is the ratio between the
Doppler frame rate and the B-mode frame rate, such that M=FR.sub.D/
FR.sub.B. If M is an integer, each M sub-region is equally sized
and comprises a number of B-mode pulses (.DELTA.N.sub.B). Scan
sequences in which M is not an integer will be further discussed
below.
[0068] For each scan of a B-mode sub-region, the Doppler region is
illuminated by D=N.sub.D*PS pulses. The number of pulses required
to cover one of the B-mode sub-regions and the Doppler region is
N=.DELTA.N.sub.B+D, and the N pulses are interleaved in a
predefined manner. One example of interleaving is shown below:
B.sub.1nD.sub.11D.sub.21D.sub.31B.sub.2nD.sub.12D.sub.22D.sub.32B.sub.3nD.-
sub.13D.sub.23D.sub.33B.sub.4nD.sub.14D.sub.24D.sub.34
[0069] where .DELTA.N.sub.B=4, N.sub.D=4, P.sub.S=3, and
B.sub.ij=B-mode pulse in direction i in sub-region j and
D.sub.ij=Doppler pulse number i in direction j.
[0070] The whole B-mode region is covered by repeating this pulse
sequence M times, changing the B-mode sub-region for each sequence
while keeping the Doppler region constant. The result is one B-mode
frame with N.sub.B=M*.DELTA.N.sub.B pulses, and M Doppler frames
with a frame rate M times higher than the B-mode frame rate. Thus,
in order to acquire one B-mode frame, the scan sequence is repeated
M times. Or, for every B-mode frame that is acquired, M Doppler
frames are acquired.
[0071] The time between two Doppler pulses within a packet is
called the Doppler pulse repetition time (PRT.sub.D), thus the
Doppler pulse repetition frequency (PRF.sub.D) is
PRF.sub.D=1/PRT.sub.D. In order to achieve the desired PRF.sub.D,
the Doppler pulses may be distributed in space and time in
different ways.
[0072] The technique described above, packet acquisition with
reduced B-mode frame rate, is illustrated in FIGS. 8 through 13.
The following parameters are utilized in FIGS. 8-11:
[0073] Number of Doppler transmit beam directions, N.sub.D=4.
[0074] Packet size, PS=3.
[0075] Number of Doppler pulse transmissions, D=N.sub.D*PS=12.
[0076] Number of B-mode sub regions, M=3.
[0077] Number of beams per B-mode sub region, .DELTA.N.sub.B=4.
[0078] FIG. 8 illustrates a scan sequence utilizing packet
acquisition of 2D Doppler data with a reduced B-mode frame rate.
Three Doppler frames 802-806, each comprised of 12 Doppler pulses
are illustrated. The Doppler pulses are arranged into four packets
(N.sub.D=4), with each packet containing three Doppler pulses
(PS=3). Each packet is directed towards a different beam direction,
as discussed previously. Three B-mode sub-regions 810-814, each
comprised of 4 B-mode pulses (.DELTA.N.sub.B=4) are illustrated. A
B-Mode frame 808 is comprised of three B-mode sub-regions 810-814.
For every Doppler frame 802-806 that is acquired, one third of the
B-mode frame 808 is acquired.
[0079] The direction and order of transmission within the packets
of each Doppler pulse is labeled. D.sub.ij indicates a Doppler
pulse transmission j in direction i. For example, the label
D.sub.12 of Doppler pulse 820 indicates that Doppler pulse 820 is
transmitted in direction 1, and is the second pulse (2) to be
transmitted in direction 1. The label D.sub.23 of Doppler pulse 822
indicates that Doppler pulse 822 is transmitted in direction 2, and
is the third pulse (3) to be transmitted in direction 2. The
aforementioned labeling, which indicates the transmission direction
and order of transmission in each packet direction for Doppler
pulses, is utilized by the remaining Figures.
[0080] Additionally, the direction and sub-region of each B-mode
pulse is indicated, such that B.sub.ij indicates a B-mode pulse in
direction i in sub-region j. For example, B-mode pulse 816
(B.sub.11) is the first (1) B-mode beam direction in sub-region 1.
B-mode pulse 818 (B.sub.12) is the first (1) B-mode direction in
sub-region 2.
[0081] The illustrated scan sequence of Doppler and B-mode pulses,
transmission of the pulse on the left of FIG. 8 first, then
sequentially transmitting the pulses moving to the right of the
FIG. 8 will be utilized by the remaining Figures. Although not
illustrated, any non-sequential scan sequence may be used.
[0082] The B-mode signal is received by the receiver 108 and
converted to 8 bit pixels after log-detection. The pixels are
displayed real-time on the display system 118, and stored in image
buffer 122. The Doppler data are stored as 32 bit IQ data.
Optionally, the IQ Doppler data may be converted to complex
autocorrelation coefficients in real-time and stored in the image
buffer 122. The IQ Doppler data may, for example, be used for 2D
tissue Doppler imaging, strain rate imaging, or for visualization
of integrated strain. The data may also be displayed on the display
system 118 in M-mode (i.e. conventional, anatomical or curved). In
addition, time/velocity curves, time/strain curves, and the like
may be generated and displayed either real-time or in post
processing.
[0083] FIG. 9 illustrates a scan sequence utilizing packet
acquisition of 2D Doppler data with B-mode pulses interleaved
between Doppler pulse packets and a reduced B-mode frame rate.
Doppler packets 902-908 and B-mode pulses 910-916 are illustrated,
and the B-mode pulses 910-91 6 are interleaved with the Doppler
packets 902-908. The scan sequences of FIGS. 8 and 9 are similar in
that Doppler packets 902-908 comprise a Doppler frame, similar to
Doppler frame 802, and the B-mode pulses 910-916 comprise a B-mode
sub-region, similar to B-mode sub-region 810. Also, for every
Doppler frame acquired in FIG. 9, one third of the B-mode frame is
acquired.
[0084] The scan sequences illustrated in FIGS. 8 and 9 may be
utilized when PRF.sub.D>0.5*PRF.sub.Dmax, because beam
interleaving within the Doppler transmit directions is not
possible. If a lower PRF.sub.D is used, scan sequences with
interleaved Doppler transmit directions are possible, such as the
scan sequences illustrated in FIGS. 10 and 11.
[0085] FIG. 10 illustrates a scan sequence utilizing packet
acquisition of 2D Doppler data with reduced B-mode frame rate and 2
interleaved Doppler transmit directions. Doppler pulses 1002-1024
and B-mode pulses 1026-1032 are illustrated. FIG. 11 illustrates a
scan sequence utilizing packet acquisition of 2D Doppler data with
reduced B-mode frame rate and 4 interleaved Doppler transmit
directions. Doppler pulses 1102-1124 and B-mode pulses 1126-1132
are illustrated.
[0086] The scan sequence of FIG. 10 may be compared to the scan
sequence of FIG. 5, and the scan sequence of FIG. 11 may be
compared to the scan sequence of FIG. 6. In FIGS. 10 and 11,
however, for every Doppler frame acquired, only one third of the
B-mode frame is acquired. The scan sequences of FIGS. 10 and 11 are
advantageous over the scan sequences of FIGS. 5 and 6 because the
Doppler frame rate is higher than the B-mode frame rate.
[0087] Below is an example of achievable frame rates and beam
densities with packet acquisition and different frame rates for
B-mode frames and Doppler frames. The parameters indicated are
suitable for cardiac imaging. In addition, the PRF.sub.B is lower
than the PRF.sub.D to minimize reverberation effects.
3 M = 3 N.sub.D = 8 .DELTA.N.sub.B = 4 PS = 3 PRF.sub.B = 3 kHz
PRF.sub.Dmax = PRF.sub.D * IGS = 4 kHz MLA.sub.B = 2 MLA.sub.D = 4
Frame rate Doppler: FR.sub.D = 1/(.DELTA.N.sub.B/PRF.sub.B +
PS*N.sub.D/PRF.sub.Dmax) = 100 Hz Frame rate B-mode: FR.sub.B =
FR.sub.D/M = 33 Hz Receive beams MLA.sub.D*N.sub.D = 32 Doppler:
Receive beams MLA.sub.B *.DELTA.N.sub.B*M = 72 B-mode:
[0088] Although the value of M in FIGS. 8-11 was an integer, any
fraction of M=FR.sub.B/FR.sub.D is possible. FIG. 12 illustrates a
scan sequence utilizing packet acquisition of 2D Doppler data with
reduced B-mode frame rate and 4 interleaved Doppler transmit
directions. Doppler pulses 1202-1224 and B-mode pulses 1226-1242
are illustrated. The scan sequence acquires 12 Doppler pulses for
every 4 B-mode pulses that are acquired. In FIG. 12,
.DELTA.N.sub.B=4, N.sub.B=10, and M={fraction (5/2)}. If N.sub.B is
fixed by other parts of the ultrasound system 100, it may not be
possible to find a suitable value of M that makes N.sub.b/M an
integer. It is not necessary, however, for M to be an integer, but
M may be a rational number. To achieve a uniform Doppler frame
rate, a pause is inserted into the scan sequence, as illustrated in
FIG. 13.
[0089] FIG. 13 illustrates a scan sequence utilizing packet
acquisition of 2D Doppler data with a reduced B-mode frame rate, 4
interleaved Doppler transmit directions, and a pause inserted to
achieve a constant Doppler frame rate. Doppler pulses 1302-1324,
B-mode pulses 1326-1344, and a pause 1346 are illustrated.
[0090] Without the inserted pause 1346, the value of M in FIG. 13
would be M={fraction (5/2)}, as illustrated in FIG. 12. By
inserting the pause 1346, the value of M is an integer, and M=3.
The length of the pause 1346 is calculated by
.DELTA.t=2/PRF.sub.B.
[0091] The packet acquisition scan sequences illustrated in FIGS. 8
through 13 enhance the B-mode image quality in high frame rate 2D
Doppler acquisition. Packet acquisition allows flexibility in
choosing PRF.sub.D, thus aliasing of the velocity estimates can be
avoided. In addition, it is not necessary for FR.sub.D to equal
FR.sub.B, as illustrated by scan sequences in FIGS. 3 through
6.
[0092] By further reducing the PRF, a number of B-mode pulses can
be interleaved between each Doppler scan as illustrated in FIG. 14.
FIG. 14 illustrates a scan sequence utilizing continuous 2D Doppler
acquisition with a reduced B-mode frame rate. Doppler pulses
1402-1424 and B-mode pulses 1426-1448 are illustrated. Continuous
acquisition is similar to the common Doppler and B-mode pulse
technique illustrated in FIG. 7, and differs from the packet
acquisition technique in that only one Doppler pulse is transmitted
in each beam direction before transmitting any B-mode pulses. Thus,
for each beam direction there is a continuous stream of Doppler
data with constant sampling intervals in the temporal
direction.
[0093] For example, FIG. 14 utilizes four transmit directions, or
N.sub.D=4. Doppler pulses 1402-1408 are each transmitted in a
different direction, then the B-mode pulses 1426-1432 are
transmitted. Doppler pulses 1410-1416 are then transmitted, one in
each of the four transmit directions, followed by B-mode pulses
1434-1440. If sliding window processing is utilized, as described
in reference to FIG. 7, a significant increase in the frame rate
compared to packet acquisition is possible.
[0094] Continuous acquisition can also be utilized to acquire a
high resolution B-mode image inside a ROI and a low resolution
B-mode image of the area surrounding the ROI. FIG. 15 illustrates a
scan sequence utilizing continuous B-mode acquisition and a second
B-mode acquisition with a reduced frame rate. In FIG. 15, a high
resolution B-mode ROI 1504 is illustrated surrounded by a low
resolution B-mode image 1502. Beam density is illustrated in the
low resolution B-mode image 1506 and the high resolution B-mode ROI
1508. The low resolution B-mode image 1506 is scanned with a lower
beam density than the high resolution B-mode ROI 1508. B-mode
pulses 1526-1540 scan the low resolution B-mode area 1502. B-mode
pulses 1510-1516 scan the high resolution ROI 1504 to form a first
acquired image of the high resolution ROI 1504, and B-mode pulses
1518-1524 scan the high resolution ROI 1504 to form a second
acquired image of the high resolution ROI 1504. In this example,
the high resolution B-mode ROI 1504 is scanned twice as often as
the low resolution B-mode image 1502. In one embodiment, the high
resolution B-mode pulses 1510-1524 may be used to form the
underlying low resolution B-mode image 1502. Therefore, it is not
necessary to scan with B-mode pulses 1526-1540 the portions of the
low resolution B-mode image 1502 angularly underlying the high
resolution B-mode ROI 1504. Thus, the number of pulse transmissions
can be reduced, and frame rates are increased. In the illustrated
example, B-mode pulses 1532 (B.sub.41) and 1534 (B.sub.52) may be
omitted as B-mode pulses 1532 and 1534 angularly underlie the high
resolution B-mode ROI 1504.
[0095] Alternatively, the Doppler frame rate may be kept constant
while increasing the number of Doppler transmit beam directions as
shown in FIG. 16. FIG. 16 illustrates a scan sequence utilizing
continuous 2D Doppler acquisition with an increased number of
Doppler transmit directions and a reduced B-mode frame rate.
Doppler pulses 1602-1624 and B-mode pulses 1626-1632 are
illustrated. As in FIG. 14, one Doppler pulse 1602-1624 is
transmitted in each beam direction before transmitting any B-mode
pulses 1626-1632. The Doppler data may be processed utilizing the
sliding window technique. Sliding window processing is possible
with any combination of B-mode and Doppler pulses, provided that
the Doppler pulses along each scanline direction are equally spaced
in time.
[0096] Below is an example of the performance achieved for
continuous acquisition with B-mode and Doppler frame rates that are
different. The parameters are suitable for cardiac imaging, and the
B-mode PRF.sub.B is lower than the Doppler PRF.sub.D to minimize
reverberation effects.
4 M = 10 .DELTA.N.sub.B = 4 N.sub.D = 10 PRF.sub.B = 3 kHz
PRF.sub.Dmax = PRF.sub.D * IGS = 4 kHz MLA.sub.B = 2 MLA.sub.D = 4
Frame rate FR.sub.D = PRF.sub.D = 1/(.DELTA.N.sub.B/PRF.sub.B +
N.sub.D/PRF.sub.Dmax) = 260 Hz Doppler: Frame rate B- FR.sub.B =
FR.sub.D/M = 26 Hz mode: Receive beams MLA.sub.D * N.sub.D = 40
Doppler: Receive beams B- MLA.sub.B * .DELTA.N.sub.B * M = 80
mode:
[0097] In general, the pulses in the B-mode sub-region can be
interleaved between Doppler pulses as shown in FIG. 17. FIG. 17
illustrates a scan sequence utilizing continuous acquisition of
Doppler data, with B-mode pulses interleaved between the Doppler
pulses. Doppler pulses 1702-1732 and B-mode pulses 1726-1632 are
illustrated. The interleaving of B-mode pulses between the Doppler
pulses reduces artifacts due to time differences between
neighboring beams in the B-mode images, but may also introduce
reverberation artifacts in the Doppler images.
[0098] The continuous acquisition has so far been illustrated by
integer values for M, where M represents the factor by which the
B-mode frame rate (FR.sub.B) is decimated relative to the Doppler
frame rate (FR.sub.D). As with packet acquisition, any fraction
M=FR.sub.D/FR.sub.B is possible in continuous acquisition.
[0099] FIG. 18 illustrates a scan sequence utilizing continuous
acquisition with a non-integer ratio between the Doppler frame rate
and the B-mode frame rate. Doppler pulses 1802-1808 and B-mode
pulses 1810-1828 are illustrated. FIG. 18 presents an example with
.DELTA.N.sub.B=4, N.sub.D=4 and M={fraction (5/2)}. Non-sequential
scanning of the B-mode sector is sometimes used to increase
PRF.sub.B. The non-sequential firing patterns may lead to an
N.sub.B which results in non-integer N.sub.B/M ratios for suitable
values of M. This can be resolved by inserting a pause in the scan
sequence to ensure constant intervals between Doppler pulses in the
same direction as shown in FIG. 19.
[0100] FIG. 19 illustrates a scan sequence utilizing continuous
acquisition with a pause inserted into the scan sequence to obtain
a constant time interval between Doppler pulses in the same
direction. Doppler pulses 1902-1908, B-mode pulses 1910-1928, and a
pause 1930 are illustrated.
[0101] Without the inserted pause 1930, the value of M in FIG. 19
would be M={fraction (5/2)}, as illustrated in FIG. 18. By
inserting the pause 1930, the value of M is an integer, and M=3.
The length of the pause 1930 is calculated by
.DELTA.t=2/PRF.sub.B.
[0102] Scan sequences utilizing continuous acquisition, such as
sequences illustrated in FIGS. 14-19, allow for a higher Doppler
frame rate than scan sequences utilizing packet acquisition. In
addition, the continuous stream of data with constant sampling
interval makes sliding window processing possible. This means that
spectrum Doppler and sound generation are possible at arbitrary
points in the 2D image. The spectrum Doppler display enables the
operator to visually differentiate moving objects and stationary
reverberations. This is a more robust technique than velocity
traces based on a mean velocity estimator. Additionally, more
efficient clutter filtering is possible, and tissue tracking is
easier when the data have constant sampling intervals. One
disadvantage to continuous acquisition is that the PRF is equal to
the frame rate, thus making correction for aliasing in velocity
estimates necessary. However, there are no severe aliasing
artifacts in strain rate imaging images calculated from data
acquired with the continuous acquisition method.
[0103] While the invention has been described with reference to at
least one embodiment, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted without departing from the scope of the invention. In
addition, many modifications may be made to adapt a particular
situation or material to the teachings of the invention without
departing from its scope. Therefore, it is intended that the
invention not be limited to the particular embodiment disclosed,
but that the invention will include all embodiments falling within
the scope of the appended claims.
* * * * *