Patent Description:
Ultrasound imaging is typically performed by sequential insonification of a medium using focused beams. Each focused beam allows the reconstruction of a single image line. A typical 2D image is typically made of few tens or hundreds of lines and is created by the sequential reconstruction of each line in the image, the time for reconstructing each line depending on the image depth. Therefore, the time to build an image (e.g., frame rate) is dependent on the image depth and spatial resolution (e.g., number of image lines).

Ultrafast Doppler imaging is an emerging imaging technique in which samples are acquired for each location in the field of view at a temporal rate that enables Doppler estimation without aliasing. Unlike conventional imaging architectures which reconstruct images sequentially single line at a time from several transmits, ultrafast imaging transmits one or more pulses or waves to insonify the entire area of interest, and then relies on parallel processing of all image lines to generate the image data from the insonified field of view. As a result, one is able to obtain an image with significantly less signal transmissions, resulting in increased frame rates. Ultrafast imaging may be advantageously used for shear wave elastography and Doppler flow analysis. However, B-mode images generated from the same insonifications as used in ultrafast imaging may suffer from low SNR, low resolution, and grating lobe artifacts.

<NPL> discloses a system for simultaneously acquiring both B-mode images and velocity data. In examples, an ultrasound sequence obtains a plurality of velocity samples, before obtaining a B-mode sample.

<CIT>, discloses an ultrasound imaging system that uses a Doppler flow imaging technique. In an embodiment, an acquisition sequence is used in which echographic imaging, color flow frame imaging and Doppler spectrum imaging can be performed using a single sequence.

<CIT> discloses an ultrasound imaging system configured to alternate Doppler pulses and B-mode pulses to obtain a combined image.

The invention proposes to improve ultrafast imaging acquisition in order to allow quality Doppler and B-mode images. This is accomplished by providing a pulse sequence that specifically interleaves continuous Doppler pulses with B-mode pulses in a manner that maintains the ultrafast paradigm while maximizing B-mode quality.

The method of the invention is described in claim <NUM>.

Any of the methods described herein may be embodied in executable instructions stored on non-transitory computer-readable medium according to claim <NUM>.

The ultrasound imaging system of the invention is configured for ultrafast imaging, and is described in claim <NUM>.

The following description of certain exemplary embodiments is merely exemplary in nature and is in no way intended to limit the invention or its applications or uses. In the following detailed description of embodiments of the present systems and methods, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustration specific embodiments in which the described systems and methods may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the presently disclosed systems and methods, and it is to be understood that other embodiments may be utilized and that structural and logical changes may be made without departing from the scope of the present system. Moreover, for the purpose of clarity, detailed descriptions of certain features will not be discussed when they would be apparent to those with skill in the art so as not to obscure the description of the present system. The following detailed description is therefore not to be taken in a limiting sense, and the scope of the present system is defined only by the appended claims.

As described, ultrafast imaging is an imaging technique in which a full image can be reconstructed from a single transmit pulse e.g., through the use of parallelized processing of all image lines in the field of view. For example, during ultrafast imaging, a single plane wave may be used to insonify a region of interest and an image of the whole region may be generated from this single insonification. However, B-mode images generated from the field of view insonifications of ultrafast imaging typically have reduced SNR, low resolution, and grating lobe artifacts. In some situation, multiple planes (e.g., pulsed plane waves) may be used to improve the quality of the image.

In existing implementations of ultrafast Doppler imaging (also referred to as continuous Doppler), the techniques do not employ specific B-mode pulses. Anatomical context is generally not provided (as shown in <FIG>). Rather, if the user wishes to visualize the anatomy as well as well as Doppler data, the same pulses that are used for the Doppler image are used to generate B-mode image data. This approach yields suboptimal B-mode images because the signal-to-noise ratio (SNR), resolution, and frame rate requirements are different for B-mode and for Doppler. Typically very few pulses are coherently compounded for Doppler in order to be fast enough to maintain the required temporal sampling. This can yield low SNR, low resolution and grating lobe artefacts that can be detrimental to the B-mode. The temporal sampling requirements for Doppler are generally greater than for B-mode. When Doppler pulses are used to produce B-mode images, temporal averaging is typically used to regain some SNR. However angular spectrum undersampling and/or reduced angular spectrum span typically used for ultrafast Doppler will hurt image quality in the form of grating lobes and loss in resolution, respectively. In addition, one may want to use different waveforms for Doppler and B-mode pulses. In conventional Doppler modes, pulses are typically more narrowband and slightly lower frequency than B-mode pulses to optimize for sensitivity and penetration.

To address one or more of these problems, the inventors have developed a system and method for configuring pulse sequences that interleave Doppler and B-mode pulses and thereby obtain improvements in duplex B-mode with ultrafast Doppler imaging. Before discussing specific examples of interleaved pulse sequences, some requirements for the Doppler and B-mode pulses will be reviewed.

For continuous Doppler imaging, several plane waves can be combined to enhance SNR and spatial resolution of the signal. Typically, the number of pulses that can be combined is limited by the motion. In other words, after a certain amount of time, beams stop interfering constructively and thus cannot be combined. The time interval ΔTxbr within which pulses must be sent to be able to combine them constructively is bound by: <MAT> where λd is the wavelength of the Doppler pulses and vd is the maximum expected axial velocity of the flow being measured. The time interval ΔTxbr defined by Equation <NUM>, which may interchangeably be referred to herein as the intra-sample time interval, is twice shorter than the temporal sampling requirement for acquiring Doppler data without aliasing, which is typically governed by: <MAT>.

The time interval ΔTd may be interchangeably referred to herein as inter-sample time interval. Note that if the system employs a nonaliasing displacement estimator (e.g., cross-correlation), then the only requirement is that the clutter filter is effective, which yields: <MAT>.

In summary, for continuous acquisition (e.g., ultrafast Doppler imaging), transmits (i.e., transmit pulses) to be coherently combined may be sent in an interval of time ΔTxbr (given by Equation <NUM>), yielding one Doppler sample. Consecutive Doppler samples, in turn, may be acquired within ΔTd of each other, given by Equation <NUM> or <NUM>. For simplicity, acquisition using Equation <NUM> is described herein, but it is understood that Equation <NUM> may be applied. These pulse sequence configuration parameters are summarized visually in <FIG>, which shows a first plurality <NUM>-<NUM> of Doppler transmit pulses <NUM>-<NUM> through <NUM>-<NUM> used to acquire a first Doppler sample <NUM>-<NUM> and second plurality <NUM>-<NUM> of Doppler transmit pulses <NUM>-<NUM> through <NUM>-<NUM> used to acquire a second Doppler sample <NUM>-<NUM>, as well as the intra-sample time interval <NUM> and the inter-sample time interval <NUM> illustrated in relation to a temporal reference frame (e.g., time axis T). <FIG> shows a first Doppler burst <NUM>-<NUM>, which includes a plurality of Doppler pulses <NUM>-<NUM> through <NUM>-<NUM> (shown as white stars) and a second Doppler burst <NUM>-<NUM> which also includes a plurality of Doppler pulses (e.g., Doppler pulses <NUM>-<NUM> - <NUM>-<NUM>, again shown as white stars). <FIG> also shows Doppler samples <NUM>-<NUM> and <NUM>-<NUM> (shown as diagonal line-filled stars), each of which includes the Doppler pulses within the bracket associated with the corresponding Doppler burst (e.g., <NUM>-<NUM> and <NUM>-<NUM>, respectively).

The B-mode pulses can include focused or unfocused pulses, for example unfocused pulses in the form of plane waves (to be coherently combined) or focused pulses or waves (to be coherently or not coherently combined). Similar to the concept described above, the maximum time interval between the first and last B-mode pulses to be coherently combined while maintaining constructive interference between the pulses may be defined by: <MAT> where λb is the wavelength of the B-mode pulses and vb is the maximum expected axial velocity of the tissue being imaged. Note that this number can be significantly higher than the number of Doppler pulses to be combined coherently, because the velocity vb is typically much lower than the flow velocity vd. Therefore, the sending of B-mode pulses to be combined can be interleaved with the sending of coherent Doppler packets. In fact, as many B-mode pulses or twice as many B-mode pulses can be interleaved while still being able to estimate velocities. It should be noted also that it is not necessary to combine B-mode transmits. In order to have a fully sampled transmit field (yielding optimal resolution and no grating lobes) the number of transmits per B-mode frame may be limited to D/(λF#) if transmit combination is used, twice that if the transmits are not coherently combined, where D is the lateral extent of the field of view, and λ is the imaging wavelength and F# is the F-number (transmit focal depth divided by probe aperture).

Systems and methods for duplex B-mode and ultrafast Doppler imaging in accordance with the present disclosure may operate to transmit sequences of pulses, in which Doppler pulses for continuous Doppler imaging are interleaved with B-mode pulses. The Doppler pulses for continuous Doppler imaging may be arranged such that they satisfy Equations <NUM> and <NUM>, discussed above. This can be achieved for example with the pulses sequences described further below with reference to <FIG> and <FIG>. Before discussing specific examples of pulse sequences, an exemplary ultrasound system in accordance with the present disclosure is described with reference to <FIG>.

<FIG> shows a block diagram of an ultrasound imaging system <NUM> constructed in accordance with the principles of the present disclosure. The ultrasound imaging system <NUM> may be configured to perform conventional B-mode and Doppler imaging, as well as imaging in ultrafast mode, including duplex B-mode with ultrafast Doppler imaging in accordance with the examples herein. In some embodiments, the ultrasound imaging system <NUM> may be configured to interleave Doppler and B-mode pulses in accordance with the examples herein.

The ultrasound imaging system <NUM> in the embodiment in <FIG> includes an ultrasound probe <NUM>, which includes a transducer array <NUM> for transmitting ultrasound waves (e.g., ultrasound pulses which may include focused and unfocused pulses) and receiving echoes responsive to the ultrasound waves. In some embodiments, the array may be incorporated into a transducer probe or it may be an ultrasound patch, e.g., of a flexible array, a large area array, or a multi-patch array. The array of the probe <NUM> may be configured to transmit any combination of pulsed waves including unfocused waves (e.g., plane or diverging waves), which may be tilted or angled, and focused waves, which may be steered. A variety of transducer arrays may be used, e.g., linear arrays, curved arrays, or phased arrays. The transducer array <NUM>, for example, can include a two dimensional array (as shown) of transducer elements capable of scanning in both elevation and azimuth dimensions for 2D and/or 3D imaging. As is generally known, the axial direction is the direction normal to the face of the array (in the case of a curved array the axial directions fan out), the azimuthal direction is defined generally by the longitudinal dimension of the array, and the elevation direction is transverse to the azimuthal direction. The transducer array <NUM> is coupled to a microbeamformer <NUM>, which may be located in the ultrasound probe <NUM> or other structure (e.g., in the case of an array which is not incorporated into a probe). The microbeamformer <NUM> controls transmission and reception of signals by the transducer elements in the array <NUM>. In some examples, the array <NUM> need not be incorporated in a probe but may be the array of a patch, e.g., a single or multi-patch array, which may be configured to at least partially conform to the subject and/or provide one, two or three degrees of freedom of positional adjustability of individual patches.

In some embodiments, the microbeamformer <NUM> may be coupled by a probe cable to a transmit/receive (T/R) switch <NUM>, which switches between transmission and reception and protects the main beamformer <NUM> from high energy transmit signals. In some embodiments, for example in portable ultrasound systems, the T/R switch <NUM> and other elements in the system can be included in the ultrasound probe <NUM> rather than in a separate ultrasound system base. The ultrasound system base typically includes software and hardware components including circuitry for signal processing and image data generation as well as executable instructions for providing a user interface.

The transmission of ultrasonic pulses from the transducer array <NUM> may be controlled by the microbeamformer <NUM>, which may be controlled by the transmit controller <NUM>. The transmit controller <NUM> may be coupled to the T/R switch <NUM> and the beamformer <NUM>. In some embodiments, the transmit controller <NUM> may be coupled to the beamformer <NUM> using a parallel data transfer link which is configured to transmit simultaneously data for multiple or all image lines in a field of view or from multiple or all points within the field of view of the array, such as during ultrafast scanning. In such embodiments, the array <NUM> may operate to simultaneously detect echoes along multiple axial lines or points, in some cases all axial lines or all points, within a field of view FOV of the array. In some embodiments, echoes may be detected along only a single or small number of axial lines or points and data transfer between the probe and system base may be performed in serialized fashion according to conventional techniques. The transmit controller <NUM> may also be coupled to the user interface <NUM> and receive input from the user's operation of a user controls. The user interface <NUM> may include one or more input devices such as a control panel <NUM>, which may include one or more mechanical controls (e.g., buttons, encoders, etc.), touch sensitive controls (e.g., a trackpad, a touchscreen, or the like), and other known input devices.

In some embodiments, the transmit controller <NUM> may also be coupled to a pulse sequence generator <NUM>. The pulse sequence generator <NUM> may be configured to generate, in part based on system parameters and/or user inputs, a pulse sequence for duplex B-mode with ultrafast Doppler imaging, as described herein. During duplex B-mode and ultrafast Doppler imaging, dedicated transmit pulses for B-mode and for Doppler imaging are transmitted in an interleaved manner, in accordance with a pulse sequence generated by the pulse sequence generator <NUM>, e.g., in accordance with the examples herein including the examples described further with reference to <FIG>. The pulse sequence generator <NUM> may be configured to generate a sequence of pulses and cause the transmit controller <NUM> to control the array <NUM> to fire the elements of the array in a sequence including at least one Doppler burst followed by at least one B-mode burst. As discussed, the time intervals for interleaving of the pulses may be dependent on parameters such as the wavelength and the velocity of tissue to be imaged, which in the case of B-mode imaging is significantly greater than that of moving tissue (e.g., blood or mechanically stimulated tissue in the case of elastography) and therefore the B-mode pulses can be transmitted over an extended time interval (e.g., over the span of multiple B-mode bursts) while Doppler samples may be generated more frequently.

In some embodiments, the B-mode transmit pulses may have different waveform than the Doppler transmit pulses. For example, relatively more narrowband and lower frequency pulses may be used for the Doppler transmit pulses as compared to the B-mode pulses. Accordingly, the pulse sequence generator <NUM> sends commands to the transmit controller <NUM> which then controls the voltage and sequence of firing of the elements of the array <NUM> as needed to produce the desired sequence of pulses. In some examples, the Doppler pulses may include unfocused pulses (e.g., several plane waves) transmitted within the time interval discussed above the echoes responsive to which may be combined constructively in a Doppler sample. The B-mode pulses may include one or more unfocused pulses (e.g., plane waves), which may be coherently combined, or one or more focused pulses (i.e., focused waves) which may be either coherently or non-coherently combined to form scan lines of the B-mode image. In some cases, a relatively larger number of unfocused pulses may be included in a Doppler burst (e.g., as compared to a B-mode burst) and echoes from multiple pulses may be used to generate a single Doppler sample. In some cases, a relatively fewer number of unfocused pulses may be included in a B-mode burst and echoes from one or more bursts may be used to improve the resolution of the B-mode image.

The user interface <NUM> may be configured to display an interface e.g., for selecting a duplex B-mode with ultrafast Doppler imaging mode as well as to display overlay images of B-mode and Doppler image data as is conventionally known. Upon selection of the duplex B-mode with ultrafast Doppler imaging mode, the system may automatically select and/or configure the pulse sequence as appropriate. In some embodiments, the user interface may provide one or more controls to enable the user to further tailor the pulse sequence for the duplex B-mode with ultrafast Doppler imaging mode.

Another function which may be controlled by the transmit controller <NUM> is the direction in which beams are steered. Beams may be steered straight ahead from (orthogonal to) the transducer array <NUM>, or at different angles for a wider field of view. In some embodiments, the partially beamformed signals produced by the microbeamformer <NUM> may be coupled to a main beamformer <NUM> where partially beamformed signals from individual patches of transducer elements may be combined into a fully beamformed signal. The beamformed signals are coupled to processing circuitry <NUM>, which may a signal processor <NUM>, a B-mode processor <NUM>, a Doppler processor <NUM>, or combinations thereof. In some embodiments, such as during ultrafast imaging, signals from the beamformer <NUM> are coupled to the processing circuitry <NUM> via a parallel communication data link (e.g., a parallel bus) for processing multiple or all image lines at the same time.

The signal processor <NUM> can process the received echo signals in various ways, such as bandpass filtering, decimation, I and Q component separation, and harmonic signal separation. The signal processor <NUM> may also perform additional signal enhancement such as speckle reduction, signal compounding, and noise elimination. The processed signals may be coupled to a B-mode processor <NUM> for producing B-mode image data. The B-mode processor can employ amplitude detection for the imaging of structures in the body. The signals produced by the B-mode processor <NUM> may be coupled to a scan converter <NUM> and a multiplanar reformatter <NUM>. The scan converter <NUM> is configured to arrange the echo signals in the spatial relationship from which they were received in a desired image format. For instance, the scan converter <NUM> may arrange the echo signal into a two dimensional (2D) sector-shaped format, or a pyramidal or otherwise shaped three dimensional (3D) format. The multiplanar reformatter <NUM> can convert echoes which are received from points in a common plane in a volumetric region of the body into an ultrasonic image (e.g., a B-mode image) of that plane, for example as described in <CIT>). A volume renderer <NUM> may generate an image of the 3D dataset as viewed from a given reference point, e.g., as described in <CIT>).

In some embodiments, the signals from the signal processor <NUM> may also be coupled to a Doppler processor <NUM>, which may be configured to estimate the Doppler shift and generate Doppler image data. The Doppler image data may include color data which is then overlaid with B-mode (i.e. grayscale) image data for display. The Doppler processor <NUM> may be configured to filter out unwanted signals (i.e., noise or clutter associated with non-moving tissue), for example using a wall filter. The Doppler processor <NUM> may be further configured to estimate velocity and power in accordance with known techniques. For example, the Doppler processor may include a Doppler estimator such as an auto-correlator, in which velocity (Doppler frequency) estimation is based on the argument of the lag-one autocorrelation function and Doppler power estimation is based on the magnitude of the lag-zero autocorrelation function. Motion can also be estimated by known phase-domain (for example, parametric frequency estimators such as MUSIC, ESPRIT, etc.) or time-domain (for example, cross-correlation) signal processing techniques. Other estimators related to the temporal or spatial distributions of velocity such as estimators of acceleration or temporal and/or spatial velocity derivatives can be used instead of or in addition to velocity estimators.

In some examples, the velocity and power estimates may undergo further threshold detection to further reduce noise, as well as segmentation and post-processing such as filling and smoothing. The velocity and power estimates are then mapped to a desired range of display colors in accordance with a color map. The color data, also referred to as Doppler image data, is then coupled the scan converter <NUM> where the Doppler image data is converted to the desired image format and overlaid on the B-mode image of the tissue structure containing the blood flow to form a color Doppler overlay image.

Output (e.g., B-mode images, Doppler images) from the scan converter <NUM>, the multiplanar reformatter <NUM>, and/or the volume renderer <NUM> may be coupled to an image processor <NUM> for further enhancement, buffering and temporary storage before being displayed on an image display <NUM>. A graphics processor <NUM> may generate graphic overlays for display with the images. These graphic overlays can contain, e.g., standard identifying information such as patient name, date and time of the image, imaging parameters, and the like. For these purposes the graphics processor may be configured to receive input from the user interface <NUM>, such as a typed patient name or other annotations. In some embodiments, one or more functions of at least one of the graphics processor, image processor, volume renderer, and multiplanar reformatter may be combined into an integrated image processing circuitry (the operations of which may be divided among multiple processor operating in parallel) rather than the specific functions described with reference to each of these components being performed by a discrete processing unit. Furthermore, while processing of the echo signals, e.g., for purposes of generating B-mode images or Doppler images are discussed with reference to a B-mode processor and a Doppler processor, it will be understood that the functions of these processors may be integrated into a single processor.

Examples of interleaved pulse sequences in accordance with the principles of the present invention are described further with reference to <FIG>.

<FIG> illustrates an example pulse sequence for interleaved B-mode and Doppler pulses which enable continuous Doppler acquisition in accordance with the principles of the present invention. <FIG> shows a first Doppler burst <NUM>-<NUM> (indicated by the dashed line), which includes a plurality of Doppler pulses <NUM>-<NUM>, <NUM>-<NUM> etc. (shown as white stars) and a second Doppler burst <NUM>-<NUM> (again indicated by the dashed line) and which also includes a plurality of Doppler pulses <NUM>-<NUM>, <NUM>-<NUM> etc. (shown as white stars). <FIG> also shows a plurality of B-mode pulses <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> and so on (shown as dot-filled stars) and interleaved with the Doppler bursts <NUM>-<NUM> and <NUM>-<NUM>. Specifically, in the example in <FIG>, five B-mode pulses are interleaved between each Doppler burst; however it will be understood that a different number of five B-mode pulses may be interleaved between the Doppler bursts in other examples. Sampling of the Doppler pulses are indicated as Doppler samples <NUM>-<NUM> and <NUM>-<NUM> (shown as diagonal line-filled stars), and include the Doppler pulses within the corresponding bracket. The process depicted in <FIG> is described in more detail hereinafter.

In the example in <FIG>, each burst of Doppler pulses is used to form an entire image. For example, each Doppler burst may include a plurality of unfocused pulses (e.g., plane or diverging waves), each of which may insonify the entire field of view (FOV) of the probe. Each pulse results in a set of echoes (e.g., along all axial lines in the FOV) being detected by the probe and the echoes from the sets associated with all of the Doppler pulses in the burst are combined to form a single Doppler sample and thus obtain a single Doppler frame. In contrast, a burst of B-mode pulses, for example when focused waves are used for the B-mode pulses, may form just part of the B-mode image. That is, the first B-mode pulse <NUM>-<NUM> may be configured to scan a first image line in the FOV and each successive pulse <NUM>-<NUM> through <NUM>-<NUM> and so on may be configured to scan another image line in the FOV until all image lines for generating a full B-mode frame have been acquired. As illustrated, multiple Doppler bursts <NUM>-<NUM>, <NUM>-<NUM>, and so on, are transmitted during the time interval necessary to scan all image lines with the above described B-mode pulses. The temporal separation between the B-mode pulses used to acquire a single B-mode frame is acceptable because the B-mode image data can be acquired more slowly than Doppler image data, as discussed above e.g., with respect to Equations <NUM> and <NUM>. In some embodiments, individual transmit pulses may have different tilt angles (in the case of plane wave imaging) or azimuthal direction (in the case of focused wave imaging).

In some embodiments, unfocused pulses (e.g., plane or diverging waves) may also be used for the individual B-mode pulses <NUM> in each B-mode burst. As will be appreciated, when unfocused pulses are used for B-mode imaging, the entire B-mode image may be formed responsive to a single pulse. In some embodiments, the echoes from multiple unfocused B-mode pulses may be combined in order to enhance the resolution (e.g., SNR) of the B-mode image.

As illustrated in <FIG>, a first example sequence may include the transmission of a first plurality of Doppler pulses followed by a first plurality of B-mode pulses, then a second plurality of Doppler pulses followed by a second plurality of B-mode pulses, and so on. Each grouping of Doppler pulses that is temporally spaced from another grouping of Doppler pulses by a grouping of one or more B-mode pulses is referred to as a Doppler burst and similarly, each grouping of one or more B-mode pulses that is temporally separated from another grouping of one or more B-mode pulses by a grouping of Doppler pulses is referred to as a B-mode burst. Each Doppler burst may include a plurality of pulses, each of which may be configured to insonify the full FOV of the probe. It is not necessary that each Doppler pulse cover the full FOV, but each Doppler burst must, since the Doppler sample is obtained from only the pulses of one burst, in this example. Individual ones of the B-mode pulses in a burst may, but need not, insonify the full FOV. Depending on the type of waveform used for B-mode imaging, such as when focused waves are used for the B-mode pulses, the sequence of a Doppler burst followed by a B-mode burst may be repeated until all B-mode image lines (i.e. the full B-mode image) have been acquired, which may require for example <NUM>-<NUM> lines to be scanned and thus <NUM>-<NUM> B-mode pulses <NUM> to be transmitted, depending on desired resolution and/or capability of the array and system. For example, if a scanner has receive beamforming capability for four lines in parallel, then it can form <NUM> A-lines out of <NUM> transmit events, as <NUM> lines are beamformed per transmit event. It will be appreciated that during the time it takes for a full B-mode image to be acquired, the system would continue to acquire Doppler data. Doppler data (e.g., spectral Doppler or color Doppler data) is generated from echoes responsive to the Doppler pulses and B-mode image data is generated from echoes responsive to the B-mode pulses. Thus because a greater number of Doppler samples are acquired during the time necessary to acquire a single B-mode frame (e.g., when imaging with focused B-mode pulses), the Doppler sampling rate, which in continuous (aka ultrafast) Doppler equals the frame rate (i.e., the frequency at which the Doppler frames are updated) may be higher than the B-mode frame rate. In other examples, B-mode imaging may occur in continuous or ultrafast mode such as when unfocused waves are used for the B-mode pulses. In such examples, the Doppler frame rates and B-mode frame rates may be similar, but this is not necessarily so.

For example, let us consider a "slow flow" situation where the goal is to be sensitive to slow-moving blood in small vessels to visualize the microvasculature. Say, the maximum velocity we want to measure is <NUM>/s and the imaging depth is <NUM>, and the imaging frequency is <NUM> (wavelength of <NUM>). Accordingly, the pulse repetition interval (PRI) is <NUM> (the time it takes for a wave to go the full depth of the field of view and back is 2d/c where c is speed of sound and d is depth. Here <NUM>*4e-<NUM>/<NUM>. 5e3 = <NUM>) and the Nyquist interval (longest PRI acceptable to avoid aliasing is) is <NUM> (per equation <NUM>). Thus <NUM> pulses are available (<NUM>/<NUM>) for each Doppler sample. No more than <NUM> plane waves can be coherently combined for each Doppler sample (equation <NUM>), so this leaves <NUM> pulses for building part of the B-mode frame with pulse characteristics optimized for B-mode. A high-quality B-mode frame can be built with <NUM> B-mode pulses in the time it takes to acquire <NUM> Doppler samples, for a B-mode refresh rate of <NUM>. <NUM>-angle spatial compounding could even be added for a B-mode frame rate of <NUM>. If no interleaving is used (current state of the art), only <NUM> pulses are available for each B-mode frame (the same <NUM> pulses that are used to build the Doppler samples), their characteristics may not be optimal for B-mode imaging (e.g.. no focusing), the B-mode frame rate is higher at <NUM>, but this is of no clinical interest.

Fast flow situations are such that very little interleaving can be allowed without aliasing, in these situations only one or two plane waves are available for each Doppler sample, and interleaving becomes complicated by the reverberation artefacts of B-mode pulses into Doppler ensembles.

Referring now also to <FIG>, which shows components of a system configured to interleave Doppler and B-mode pulses in accordance with the present disclosure, the description of the example sequence in <FIG> is continued. As shown in <FIG>, each Doppler burst may include a plurality of plane waves <NUM> (at the same or varying angles) transmitted toward a medium (e.g., tissue of a subject to be imaged). A first Doppler burst is initially transmitted responsive to control from the transmit controller, which is responsive to commands from the scanner <NUM>. The scanner <NUM> may include some or all of the components (e.g., pulse sequence generator <NUM>, processing circuitry <NUM>, etc. of imaging system <NUM>). For each pulse, the probe <NUM> is controlled to fire a specific combination of transducer elements of the array <NUM>, in some examples substantially the full array, to generate a plane wave <NUM> for insonifying the full field of view <NUM> of the array <NUM>.

As shown for example in <FIG>, multiple pulses <NUM> (each of which be a plane wave <NUM>) are transmitted for each Doppler burst (e.g., transmit bursts <NUM>-<NUM>, <NUM>-<NUM>), all or a subset of which are used to obtain a Doppler sample for generating a Doppler frame (e.g., Doppler frame <NUM>-<NUM>). Successive Doppler bursts are transmitted to obtain additional Doppler samples for generating additional Doppler frames (e.g., Doppler frames <NUM>-<NUM>, <NUM>-<NUM>). In real-time imaging, the Doppler frames may be updated on the display in real time at a frame rate determined by the specific sequence being used. All of the pulses in any given Doppler burst in the sequence are transmitted within an intra-sample time interval which is less than one eight of a wavelength of the unfocused pulses divided by a velocity of the medium, as described above with reference to Equation <NUM>. In some embodiments, one or more of the pulses in a burst, typically the first one or two pulses in the burst may be conditioning pulses and the echoes returning from these pulses may be ignored (e.g., not included in the Doppler analysis).

Following the first burst <NUM>-<NUM> of Doppler pulses <NUM> which is used to acquire a first Doppler sample <NUM>-<NUM>, the probe <NUM> is then controlled (e.g., responsive to the transmit controller, which is responsive to commands from the scanner <NUM>) to transmit a burst of B-mode pulses <NUM>. For the B-mode pulses, different settings may be applied to the array (e.g., different voltage and/or sequence of firing of individual array elements) in order to produce the desired B-mode wave form for each of the B-mode pulses <NUM>. For example, if focused waves <NUM> are used to scan individual lines <NUM>, subsets of the array elements may be fired during each pulse in sequential order (e.g., moving in the azimuthal direction of the array) to scan the lines associated with a given burst of B-mode pulses. That is, for the example sequence in <FIG>, for each of the pulses <NUM>-<NUM> through <NUM>-<NUM>, a different subset of elements of the array may be fired to scan five lines in the field of view of the array. The echoes for the scanned lines associated with a given burst is processed as conventionally known to obtain gray-scale image data, which is buffered in memory as is conventionally known until the full image is generated for display. Similar to the burst of Doppler pulses, one or more of the B-mode pulses (e.g., the first one or two pulses) may be conditioning pulses and may be ignored by the signal processing circuitry and thus not used for image generation. If conditioning pulses are used, the first non-conditioning pulse may re-scan the same line(s) as the conditioning pulse(s) (e.g., fire the same elements as the conditioning pulse) before moving to the next line in the sequence. Following the first set of B-mode pulses, which in this example produces only a portion of the image, the probe is automatically re-configured and controlled responsive to commands from the scanner <NUM> to fire the array in a manner suitable for producing the Doppler pulses. For example, the probe <NUM> again returns to the first configuration in which the probe is operable to transmit plane wave pulses (e.g., six waves in the example of <FIG>) to acquire the next Doppler sample <NUM>-<NUM>. Successive Doppler bursts are temporally spaced by an inter-sample time interval which is less than one quarter of the wavelength divided by the velocity, as described above with reference to Equation <NUM>. As shown, the inter-sample time interval which is about <NUM> times the intra-sample interval provides a time interval between the successive Doppler bursts during which B-mode pulses may be transmitted. In some examples, the number of B-mode pulses transmitted in a single burst is selected to be the maximum number of B-mode pulses that can be transmitted during any remaining time interval between successive Doppler bursts. The maximum number of pulses may depend on various pulse parameters (e.g., wavelength, transmit frequency, depth of imaging, etc.) and thus a different maximum number may be applicable depending on the specific B-mode pulses used.

Following acquisition of the second Doppler sample <NUM>-<NUM>, the probe <NUM> again transitions to the second configuration in which B-mode pulses (e.g., focused waves) are transmitted to now scan the next few image lines for the B-mode image frame <NUM>-<NUM>. All image lines for a single B-mode frame are thus acquired over a time interval during which multiple Doppler samples, in some cases <NUM>, <NUM> or more Doppler samples, and thus multiple frame have been acquired enabling continuous Doppler data to be overlaid on high resolution B-mode images. The overlay image <NUM>, which may include a single frame (e.g., single B-mode and Doppler frame) or a cineloop in which the Doppler data and B-mode data are updated at their respective rates, may be displayed as is conventionally known on a display of the scanner <NUM>.

In some examples, such as the example in <FIG>, only echoes responsive to a single Doppler burst are used in generating Doppler data for any given Doppler sample. In other examples, echoes from multiple Doppler bursts may be combined to form a single Doppler sample. As will be appreciated, the specific number of pulses shown in <FIG> (e.g., six pulses for each Doppler burst and five pulses for each B-mode burst) are illustrative only. In other example, a different number of pulses may be used in either of the Doppler and B-mode bursts as long as the Doppler pulses used to acquire any given sample are temporally spaced according to the sample interval (ΔTd). In some embodiments, fewer than six Doppler pulses (for example four pulses or five pulses) or greater number of Doppler pulses (<NUM> or more) may be used for each Doppler burst. In the case of the latter, fewer number of B-mode pulses may be used in each B-mode burst to maintain the required temporal spacing between the individual Doppler bursts. In yet further examples, different number of B-mode pulses may be used in each burst and/or different interleaving of B-mode pulses may be utilized, for example as described further below with reference to <FIG>.

<FIG> illustrates another example pulse sequence <NUM> for interleaved B-mode and Doppler pulses that enable continuous Doppler acquisition and which may provide higher resolution B-mode images as compared to conventional continuous Doppler imaging techniques. Similar to <FIG>, Doppler bursts <NUM>-<NUM>', <NUM>-<NUM>', <NUM>-<NUM>', etc., each include a plurality of Doppler pulses (shown by the white stars), in this example two pulses per burst as indicated by the dashed lines around each grouping of Doppler pulses. B-mode pulses (shown by the dot-filled stars) are interleaved with the Doppler bursts. In the illustrated example, each individual B-mode burst <NUM> includes a single B-mode pulse; however different number of B-mode pulses may be included in a B-mode burst in accordance with other examples. <FIG> also shows Doppler samples <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, which in this example are produced from Doppler pulses associated with multiple Doppler bursts (as indicated by the brackets above each of the diagonal line-filled stars. As shown in <FIG>, the B-mode pulses may preferably be more frequently interleaved, which may enable a greater number of Doppler samples to be acquired. As compared to the example in <FIG> where a total of <NUM> pulses were transmitted producing two Doppler samples, in this example for a total of <NUM> pulses, <NUM> Doppler samples can be acquired. Assuming, for purposes of this example, that each pulse is in the form of a plane wave, which insonifies the full field of view and thus can produce echoes which can be used to generate a full image, up to six Doppler pulses may be combined to acquire a single Doppler sample or frame. This is because any five adjacent Doppler pulses are transmitted within the time interval ΔTxbr that meets the requirements discussed herein. Specifically, echoes responsive to pulses <NUM> and <NUM> from the first Doppler burst <NUM>-<NUM>', pulses <NUM> and <NUM> from the second Doppler burst <NUM>-<NUM>', and pulse <NUM> and <NUM> from the third Doppler burst <NUM>-<NUM>' can be combined to form a first Doppler sample <NUM>-<NUM>. The echoes from pulse <NUM> from the first Doppler burst <NUM>-<NUM>', pulses <NUM> and <NUM> from the second Doppler burst <NUM>-<NUM>', pulses <NUM> and <NUM> from the third Doppler burst <NUM>-<NUM>', and pulse <NUM> from the fourth Doppler burst <NUM>-<NUM>', can be combined for the second Doppler sample <NUM>-<NUM>. Continuing in a similar manner, the pulses from the second and third Doppler bursts (<NUM> through <NUM>) and pulses <NUM> and <NUM> of the fourth Doppler burst (<NUM>-<NUM>') can be combined to form the third Doppler sample <NUM>-<NUM>, and so on. B-mode bursts <NUM> are interleaved between each Doppler burst (e.g., between each pair of Doppler pulses).

Here, Doppler samples are formed responsive to pulses associated with different Doppler bursts but because the bursts are more closely temporally spaced, pulses from multiple bursts are combined to form a single Doppler sample. Also, because the B-mode pulses are more frequently interleaved, a greater number of Doppler samples may be acquired and spaced closely together. The interleaving of B-mode pulses in this manner causes the Doppler samples to be irregularly spaced, but the missing samples can be recovered through interpolation. Similar as in the previous example, B-mode images are acquired much more slowly than Doppler samples, as their temporal sampling requirements are not as strict.

Similar to <FIG>, the sequence in <FIG> shows only part of the full sequence (e.g., covering only <NUM> lines of the B-mode image) as may be required to obtain a full B-mode image. Also, as discussed above with respect to <FIG>, the Doppler bursts and/or the B-mode bursts may include conditioning pulses that are not used for image data generation. In such examples, the interleaving may be modified. In some examples, each Doppler burst may include <NUM> pulses as shown, and each B-mode burst may include two pulses, the first pulse of each burst being a B-mode conditioning pulse. In another example, each Doppler burst may include <NUM> pulses, the first of which may be a Doppler conditioning pulse, and each B-mode burst may include <NUM> pulses, the first of which may be a B-mode conditioning pulse. In this example, four successive Doppler pulses can be used to form a Doppler sample, the first pulse being ignored and the remaining three pulses used for Doppler data analysis. For the B-mode image data generation each first pulse in each burst is ignored and each second pulse of each burst is used to scan a single line thus the sequence is repeated at least a number of times required to scan all image lines.

For example, let us consider again the "slow flow" situation where the goal is to be sensitive to slow-moving blood in small vessels to visualize the microvasculature. The maximum expected velocity is <NUM>/s, the imaging depth is <NUM>, and the imaging frequency is <NUM> (wavelength of <NUM>). Accordingly, the pulse repetition interval (PRI) is <NUM> as earlier (determined by the travel time of sound waves to the maximum imaged depth and back) and the Nyquist interval (longest PRI acceptable to avoid aliasing is) is <NUM> (per equation <NUM>). Thus <NUM> pulses are available (<NUM>/<NUM>) for each Doppler sample, and Doppler pulses to be combined into one sample should not be separated by more than <NUM> pulses (equation <NUM>). For example, the following sequence can be considered: angles <NUM>-<NUM> for a Doppler sample, followed by lines <NUM>-<NUM> of the B-mode image, angles <NUM>-<NUM> of Doppler, lines <NUM>-<NUM> of B-mode, angles <NUM>-<NUM> of Doppler, lines <NUM>-<NUM> of B-mode, angles <NUM>-<NUM> of Doppler, lines <NUM>-<NUM> of B-mode. and so on until <NUM> B-mode lines have been acquired. The advantage is a faster Doppler sampling rate which is beneficial for Doppler SNR (more samples are available per unit of time and can thus be averaged). The disadvantage is more back and forth between Doppler and B-mode pulses, and a slower B-mode refresh rate (<NUM> in this particular example, still largely acceptable clinically).

Again, fast flow situations are such that very little interleaving can be allowed without aliasing, in these situations only one or two plane waves are available for each Doppler sample, and interleaving becomes complicated by the reverberation artefacts of B-mode pulses into Doppler ensembles.

In cases where the interleaving of B-mode pulses leads to temporal undersampling of the Doppler signal, compressed or compressive sensing (also known as compressive sampling or sparse sampling) may be used for Doppler estimation. Compressive sensing for recovering Doppler samples in a sparse sequence may be performed in accordance with the technique described by <NPL>.

In accordance with further embodiments, ultrasound pulses may be transmitted in a sequence which includes a plurality of Doppler bursts temporally spaced by a time interval which is greater than one quarter of the wavelength of the transmit pulses divided by the maximum velocity of the medium. In such embodiments, the method may further include using a compressive sensing technique for generating the Doppler samples. For example, as shown in <FIG>, the requirement to satisfy Equation <NUM> may be relaxed by using compressed or compressive sensing for Doppler estimation. In this example, Doppler samples <NUM>-<NUM>, <NUM>-<NUM> are acquired at a rate slower than Nyquist, which allows the user to use longer B-mode packets or bursts <NUM> to improve the quality of the B-mode images. In the specific illustrated example, each repetition or burst of Doppler pulses <NUM>-<NUM>, <NUM>-<NUM> includes six pulses with a greater number of B-mode pulses <NUM> (e.g., six or more and in this particular case fifteen pulses) being transmitted between successive Doppler bursts. Compressive sensing may be used to retrieve the Doppler velocity. Although compressive sensing techniques are known and are in themselves outside of the scope of the present invention, a brief explanation is provided. Generally, compressive sensing Doppler estimation works by assuming that the Doppler signal (flow + clutter) is modelled by a sum of a few sinusoids (or wavelets). Mathematically, one wants to find the ak's and ωk's (i.e. the spectral decomposition) that minimize the difference between the sparsely measured signal s(t) and the sparse signal model, by solving: <MAT> where µ is the penalization term that adjusts the tradeoff between sparsity and fidelity of the reconstructed signals. This energy is typically minimized numerically, yielding the Doppler spectrum. From the Doppler spectrum, a 2D Doppler display can be derived, as well as retrospective spectral Doppler at each point in the image. Note that the reconstruction works best if the time interval between consecutive (sparse) Doppler samples is pseudorandom as a regular, sparse sampling introduces artifacts.

Conversely, if the B-mode pulses are spatially undersampled, compressive beamforming is used to restore the quality of the B-mode images. In some cases, one will be constrained both in Doppler acquisition speed and B-mode acquisition speed and only a small number of transmits can be used to reconstruct the B-mode images. In that case, compressive beamforming may be used to restore good image quality (see e.g., <FIG>, which shows cardiac <NUM>-chamber image acquired with a single diverging transmit beam and 7B, which shows the same data following compressive beamforming reconstruction). Compressive beamforming technique, which in themselves are outside of the scope of the present invention, are generally known and an example of such technique is described by <NPL>.

Alternatively to compressive sensing, in cases where Doppler sampling is slow and does not satisfy the Nyquist sampling criterion (equation <NUM>), other nonaliasing estimators known in the field can be used to estimate blood velocity. These include, but are not limited to, cross-correlation-based motion tracking, and spatio-temporal phase unwrapping of the Doppler angles.

Any pulse sequence which interleaves Doppler and B-mode pulses in accordance with the present invention, for example the sequences <NUM> and <NUM>, may be programmed in the pulse sequence generator <NUM> for generating commands which may cause the transmit controller <NUM> to control the firing of the array elements of probe <NUM>. In some examples, the pulse sequence generator <NUM> may additionally or alternatively be programmed with pulse generation logic which implements the requirements for interleaving additional pulses (e.g., B-mode pulses) with Doppler pulses for continuous Doppler imaging, as described with reference to <FIG>, and Equations <NUM>-<NUM>. In this manner, depending on the system parameters (e.g., pulse wavelength, pre-programmed velocity values or ranges for blood flow) and/or user specified parameters (e.g., user specified velocity for moving tissue to be imaged), the pulse sequence generator <NUM> may automatically generate an interleaved sequence which satisfies the requirements discussed herein and apply commands to the transmit controller when the user is operating the system in the duplex B-mode with ultrafast Doppler imaging mode.

<FIG> shows a flow diagram of a process <NUM> in accordance with the present disclosure. The process <NUM> may include transmitting, using an ultrasound probe, a plurality of ultrasound pulses toward a medium (e.g., tissue of a subject to be imaged). The plurality of ultrasound pulses may be transmitted in a sequence of a Doppler burst comprising a plurality of unfocused first pulses followed a B-mode burst comprising one or more second pulses, as shown in block <NUM>. All of the first pulses in a given Doppler burst are transmitted within an intra-sample time interval which is less than one eight of a wavelength of the unfocused pulses divided by a velocity of the medium. Successive Doppler bursts are temporally spaced by an inter-sample time interval which is less than one quarter of the wavelength divided by the velocity.

The method may further include detecting echoes responsive to the transmitted pulses, as shown in block <NUM>. Echoes may be detected responsive to each of the pulses in the Doppler and B-mode burst. Echoes are typically detected after each pulse is transmitted, thus each transmit pulse is typically followed by a receive (or listen) period during which a set of echoes responsive to a given pulse are detected and corresponding set of echo signals associated with the given pulse are generated. The echo signals are transmitted for signal processing (e.g., for generation of Doppler and B-mode data). In the case of serialized processing of the echo signals, the image data for each line may be buffered before the image is displayed. In the case of ultrafast imaging, the detecting of echoes may include simultaneously detecting a set of echoes from the entire insonified region (e.g., multiple echoes along multiple axial lines within a field of view (FOV)) and responsive to each pulse, and simultaneously transmitting the corresponding echo signals to the processing circuitry (e.g., signal processor <NUM>, B-mode processor <NUM>, Doppler processor <NUM>) for Doppler estimation and B-mode image data generation.

As shown in in blocks <NUM> and <NUM>, the method may continue by generating Doppler data from the echo signals associated with the Doppler bursts and generating B-mode image data from the echo signals associated with the B-mode bursts. In some embodiments, the sets of echo signals associated with a given Doppler burst may be used to generate one Doppler sample. In some embodiments, only sets of echo signals that are associated with a given Doppler burst may be used to generate a given Doppler sample. In some embodiments, sets of echo signals associated with a plurality of Doppler burst may be used to generate one Doppler sample. In some embodiments, a set of echo signals received responsive to a Doppler burst may be used multiple times (e.g., coherently combined) for generating a plurality of Doppler samples. In other words, in some embodiments, two or more Doppler samples may be based, at least in part, on echo signals received responsive to the same Doppler burst.

In some embodiments, the sequence of Doppler burst followed by B-mode burst may be repeated in order to acquire multiple Doppler samples for displaying Doppler data in continuous mode, as shown in block <NUM>. In some embodiments, in which the B-mode bursts include one or more focused pulses, individual ones of the focused pulses may be used for generating B-mode image data associated with a single image line in the FOV. In such embodiments, the transmitting of ultrasound pulses may include repeating the sequence until a sufficient number of pulses for generating B-mode image data for all image lines in the FOV have been transmitted. In other embodiments, the second pulses may include unfocused pulses, which may have the same or different properties (e.g., wavelength, frequency, intensity) than the unfocused pulses of the Doppler bursts. In some embodiments, each Doppler burst may include a greater number of transmit pulses than the B-mode burst, for example two times or greater than the transmit pulses in the B-mode burst.

In various embodiments where components, systems and/or methods are implemented using a programmable device, such as a computer-based system or programmable logic, it should be appreciated that the above-described systems and methods can be implemented using any of various known or later developed programming languages, such as "C", "C++", "FORTRAN", "Pascal", "VHDL" and the like. Accordingly, various storage media, such as magnetic computer disks, optical disks, electronic memories and the like, can be prepared that can contain information that can direct a device, such as a computer, to implement the above-described systems and/or methods. Once an appropriate device has access to the information and programs contained on the storage media, the storage media can provide the information and programs to the device, thus enabling the device to perform functions of the systems and/or methods described herein. For example, if a computer disk containing appropriate materials, such as a source file, an object file, an executable file or the like, were provided to a computer, the computer could receive the information, appropriately configure itself and perform the functions of the various systems and methods outlined in the diagrams and flowcharts above to implement the various functions. That is, the computer could receive various portions of information from the disk relating to different elements of the above-described systems and/or methods, implement the individual systems and/or methods and coordinate the functions of the individual systems and/or methods described above.

In view of this disclosure it is noted that the various methods and devices described herein can be implemented in hardware, software and firmware. Further, the various methods and parameters are included by way of example only and not in any limiting sense. In view of this disclosure, those of ordinary skill in the art can implement the present teachings in determining their own techniques and needed equipment to affect these techniques, while remaining within the scope of the invention. The functionality of one or more of the processors described herein may be incorporated into a fewer number or a single processing unit (e.g., a CPU) and may be implemented using application specific integrated circuits (ASICs) or general purpose processing circuits which are programmed responsive to executable instruction to perform the functions described herein.

Of course, it is to be appreciated that any one of the examples, embodiments or processes described herein may be combined with one or more other examples, embodiments and/or processes or be separated and/or performed amongst separate devices or device portions in accordance with the present systems, devices and methods.

Claim 1:
A method (<NUM>) of ultrafast imaging, the method comprising:
transmitting (<NUM>) a plurality of ultrasound pulses toward a medium from a transducer array, wherein the plurality of ultrasound pulses includes a regularly repeated sequence comprising a Doppler burst (<NUM>-<NUM>', <NUM>-<NUM>', <NUM>-<NUM>') of M unfocused first pulses (<NUM>) and a B-mode burst (<NUM>, <NUM>) of N second pulses (<NUM>), where M is <NUM> or more and N is <NUM> or more, and wherein the plurality of ultrasound pulses includes multiple repetitions of the sequence;
wherein the M first pulses in each Doppler burst are transmitted within an intra-sample time interval which is less than one eighth of the wavelength of the unfocused pulses divided by a maximum expected axial velocity of the medium,
wherein successive Doppler bursts are temporally spaced by an inter-sample time interval which is less than one quarter of their wavelength divided by the maximum expected axial velocity of the medium,
detecting (<NUM>) echoes responsive to the transmitted plurality of ultrasound pulses, wherein the detecting includes:
detecting, within a field of view, FOV, of the array, first sets of echoes, each first set of echoes being responsive to a respective unfocused first pulse in a Doppler burst, wherein the first sets of echoes include all detected echoes of at least one Doppler burst, and
detecting a B-mode image frame comprising the echoes of multiple B-mode bursts of the multiple repetitions of the sequence;
generating a Doppler sample from the first sets of echoes;
generating Doppler image data (<NUM>) from the Doppler sample;
generating (<NUM>) B-mode image data from signals representative of the B-mode image frame; and
simultaneously displaying (<NUM>) the Doppler image data and the B-mode image data;
wherein the echoes from more than M successive first pulses are used simultaneously to generate each Doppler sample, so that the first sets of echoes used to generate each Doppler sample include the echoes from two or more successive Doppler bursts.