Method and apparatus for improved spatial and temporal resolution in ultrasound imaging

[0002] 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.

[0003] 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”.

[0004] 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.

[0005] 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 B₁indicates that the B-mode pulse is transmitted in direction 1. B-mode pulse 346 labeled B₁₁indicates 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.

[0006] 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.

[0007] 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.

[0008] 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:

[0009] T_frame=(N_D×PS)/PRF_D+N_B/PRF_B  Equation 1

[0010] where T_frameis the time to acquire one image frame, N_Dis the number of Doppler transmit beam directions, PS is the packet size, PRF_Dis the Doppler pulse repetition frequency, N_Bis the number of B-mode transmit pulses per frame, and PRF_Bis the B-mode pulse repetition frequency. In FIG. 3, for example, N_D=4, PS=3, and N_B=12.

[0011] 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.

[0012] 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_Dis equal to the FR_B.

[0013] When lower velocities are measured, the PRF_Dmay be decreased. If the PRF_Ddecreases, 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.

[0014] 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_Dis reduced. Thus, PRF_Dmaxmay be kept constant by increasing the IGS when the PRF_Ddecreases, as illustrated in the following relationship:

[0015] T_frame=(N_D×PS)/(PRF_D×IGS)+N_B/PRF_B=(N_D×PS)/PRF_Dmax+N_B/PRF_B

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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_B) may be different than the number of parallel receive beams per Doppler transmit beam (MLA_D).

[0020] 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_Bis lower than the PRF_Dto minimize reverberation effects:

[0021] 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 flow jets 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] FIG. 1 illustrates a block diagram of an ultrasound system formed in accordance with an embodiment of the present invention.

[0033] FIG. 2 illustrates the geometry for a conventional sector scanned 2D Doppler acquisition.

[0034] FIG. 3 illustrates a scan sequence of a conventional 2D Doppler acquisition.

[0035] FIG. 4 illustrates a scan sequence of a conventional 2D Doppler acquisition utilizing interleaving of the Doppler pulses and the B-mode pulses.

[0036] FIG. 5 illustrates a scan sequence of a conventional 2D Doppler acquisition with 2 Doppler transmit directions interleaved.

[0037] FIG. 6 illustrates a scan sequence of a conventional 2D Doppler acquisition with 4 Doppler transmit directions interleaved.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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:

[0057] where T_frameis the time required to acquire one image frame, N_Dis the number of Doppler transmit beam directions, N_Bis the number of B-mode transmit pulses per frame, PRF_Dis the Doppler pulse repetition frequency, PRF_Bis the B-mode pulse repetition frequency, MLA_Bis the number of parallel receive beams per B-mode transmit beam, MLA_Dis the number of parallel receive beams per Doppler transmit beam, FR_Bis the B-mode frame rate, and FR_Dis the Doppler frame rate.

[0058] With this acquisition method, PRF_Dequals 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 PRFD and frame rate, the number of transmitted pulses for each achieve a sufficient PRF_Dand frame rate, the number of transmitted pulses for each 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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:

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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_D/FR_B. If M is an integer, each M sub-region is equally sized and comprises a number of B-mode pulses (ΔN_B). Scan sequences in which M is not an integer will be further discussed below.

[0071] For each scan of a B-mode sub-region, the Doppler region is illuminated by D=N_D*PS pulses. The number of pulses required to cover one of the B-mode sub-regions and the Doppler region is N=ΔN_B+D, and the N pulses are interleaved in a predefined manner. One example of interleaving is shown below:

[0072] B_1nD₁₁D₂₁D₃₁B_2nD₁₂D₂₂D₃₂B_3nD₁₃D₂₃D₃₃B_4nD₁₄D₂₄D₃₄

[0073] where ΔN_B=4, N_D=4, PS=3, and B_ij=B-mode pulse in direction i in sub-region j and D_ij=Doppler pulse number i in direction j.

[0074] 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_B=M*ΔN_Bpulses, 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.

[0075] 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. In order to achieve the desired PRF_D, the Doppler pulses may be distributed in space and time in different ways.

[0076] 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:

[0077] Number of Doppler transmit beam directions, N_D=4.

[0078] Packet size, PS=3.

[0079] Number of Doppler pulse trarmssions, D=N_D*PS=12.

[0080] Number of B-mode sub regions, M=3.

[0081] Number of beams per B-mode sub region, ΔN_B=4.

[0082] 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_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 (ΔN_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.

[0083] The direction and order of transmission within the packets of each Doppler pulse is labeled. D_ijindicates a Doppler pulse transmission j in direction i. For example, the label D₁₂of 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 D₂₃of 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.

[0084] Additionally, the direction and sub-region of each B-mode pulse is indicated, such that B_ijindicates a B-mode pulse in direction i in sub-region j. For example, B-mode pulse 816 (B₁₁) is the first (1) B-mode beam direction in sub-region 1. B-mode pulse 818 (B₁₂) is the first (1) B-mode direction in sub-region 2.

[0085] 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.

[0086] 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.

[0087] 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 910-916 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.

[0088] The scan sequences illustrated in FIGS. 8 and 9 may be utilized when PRF_D>0.5*PRF_Dmax, because beam interleaving within the Doppler transmit directions is not possible. If a lower PRF_Dis used, scan sequences with interleaved Doppler transmit directions are possible, such as the scan sequences illustrated in FIGS. 10 and 11.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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_Bis lower than the PRF_Dto minimize reverberation effects.

[0093] Although the value of M in FIGS. 8-11 was an integer, any fraction of M=FR_B/FR_Dis 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.

[0094] The scan sequence acquires 12 Doppler pulses for every 4 B-mode pulses that are acquired. In FIG. 12, ΔN_B=4, N_B=10, and M=5/2. If N_Bis fixed by other parts of the ultrasound system 100, it may not be possible to find a suitable value of M that makes N_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.

[0095] 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.

[0096] Without the inserted pause 1346, the value of M in FIG. 13 would be M=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 Δt=2/PRF_B.

[0097] 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_D, thus aliasing of the velocity estimates can be avoided. In addition, it is not necessary for FR_Dto equal FR_B, as illustrated by scan sequences in FIGS. 3 through 6.

[0098] 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.

[0099] For example, FIG. 14 utilizes four transmit directions, or N_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.

[0100] 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 (B₄₁) and 1534 (B₅₂) may be omitted as B-mode pulses 1532 and 1534 angularly underlie the high resolution B-mode ROI 1504.

[0101] 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.

[0102] 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_Bis lower than the Doppler PRF_Dto minimize reverberation effects.

[0103] 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 artifacts due to time differences between neighboring beams in the B-mode images, but may also introduce reverberation artifacts in the Doppler images.

[0104] 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_B) is decimated relative to the Doppler frame rate (FR_D). As with packet acquisition, any fraction M=FR_D/FR_Bis possible in continuous acquisition.

[0105] 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 ΔN_B=4, N_D=4 and M=5/2. Non-sequential scanning of the B-mode sector is sometimes used to increase PRF_B. The non-sequential firing patterns may lead to an N_Bwhich results in non-integer N_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.

[0106] 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.

[0107] Without the inserted pause 1930, the value of M in FIG. 19 would be M=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 Δt=2/PRF_B.

[0108] 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.

[0109] 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.