Method and apparatus for improved spatial and temporal...

Details Method and apparatus for improved spatial and temporal... Method and apparatus for improved spatial and temporal...

2001-08-24

2003-03-25

Jaworski, Francis J. (Department: 3737)

Surgery

Diagnostic testing

Detecting nuclear, electromagnetic, or ultrasonic radiation

C600S440000, C600S447000, C600S453000, C600S454000, C600S455000

Reexamination Certificate

active

06537217

ABSTRACT:

BACKGROUND OF INVENTION
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.
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”.
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
B
) is 12, and the number of Doppler transmit beam directions (N
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
.
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
1
indicates that the B-mode pulse is transmitted in direction
1
. B-mode pulse
346
labeled B
11
indicates that the pulse is transmitted in direction
11
. The Doppler pulses
302
-
324
are labeled such that D 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.
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
D
), thus the Doppler pulse repetition frequency (PRF
D
) is PRF
D
=1/PRT
D
.
The depth in the body of the item of interest will determine the maximum PRF
D
(PRF
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
D
>0.5*PRF
DMAX
.
The time required to acquire one frame of Doppler and B-mode data is T
frame
. The frame rate (FR) is calculated as FR=1/T
frame
. Because one complete B-mode image frame is acquired between each Doppler image frame, the frame rate for Doppler (FR
D
) is equal to the frame rate for B-mode (FR
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
frame
) may be calculated as:
T
frame
=(
N
D
×PS
)/
PRF
D
+N
B
/PRF
B
  Equation 1
where T
frame
is the time to acquire one image frame, N
D
is the number of Doppler transmit beam directions, PS is the packet size, PRF
D
is the Doppler pulse repetition frequency, N
B
is the number of B-mode transmit pulses per frame, and PRF
B
is the B-mode pulse repetition frequency. In
FIG. 3
, for example, N
D
=4, PS=3, and N
B
=12.
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.
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
D
is equal to the FR
B
.
When lower velocities are measured, the PRF
D
may be decreased. If the PRF
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.
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 ≧2, and PRF
D
≦PRF
Dmax
/IGS. If PRF
Dmax
=PRF
D
*IGS is kept constant, the scanning time per frame remains constant when PRF
D
is reduced. Thus, PRF
Dmax
may be kept constant by increasing the IGS when the PRF
D
decreases, as illustrated in the following relationship:
T
frame
=(
N
D
×PS
)/(
PRF
D
×IGS
)+
N
B
/PRF
B
=(
N
D
×PS
)/
PRF
Dmax
+N
B
/PRF
B
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.
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

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