Alias-free spectral Doppler envelope tracing in medical ultrasound.

Abstract

To provide a method for spectral Doppler imaging to better separate bands so that aliasing can be avoided.SOLUTION: For spectral Doppler imaging, the search range for tracing a spectral envelope is set dynamically. The limit for searching for the envelope is established by spectrum-by-spectrum placement between bands. This search may be aided by plotting the spectra from 0 to 2 π. The limit varies over time to better separate bands so that subsequent tracing can avoid aliasing.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present embodiment relates to Doppler mode (eg, spectral) imaging. [Background technology]

[0002] By transmitting multiple pulses (e.g., pulsed waves (PW)) at a location, a Doppler response is generated. In spectral Doppler, the frequency spectrum of an object's motion or flow over a spatial region is estimated and displayed as a function of time. Spectral Doppler ultrasound imaging provides an image of the spectrum as energy-scaled velocity values ​​(vertical axis) as a function of time (horizontal axis) relative to a gate. The spectrum can be used to study fluid flow or tissue motion within a patient. Summary of the Invention [Problem to be solved by the invention]

[0003] Spectral Doppler can suffer from aliasing. Spectral aliasing can cause the envelope to appear incorrect. In spectral envelope tracings with spectral aliasing, the trace does not follow the peaks correctly. Instead, the trace is clipped at the Nyquist limit, the baseline, or somewhere in between, depending on the spectral bandwidth and the signal-to-noise ratio (SNR). While aliasing can sometimes be corrected by baseline shifting, it cannot always be corrected even if the sampling frequency is smaller than the maximum Doppler shift. This is because the search range of the tracing is typically fixed at the Nyquist limit, which is determined by the position of the baseline. [Means for solving the problem]

[0004] To begin with, the preferred embodiments described below include a method, system, computer-readable medium, and instructions for spectral Doppler imaging. The search range for tracing the spectral envelope is dynamically set. The limits for searching the envelope are determined by the spectral placement between bands. This search may be aided by plotting the spectrum from 0 to 2π. The limits are varied over time to better separate the bands so that subsequent traces can avoid aliasing.

[0005] In a first aspect, a method for spectral Doppler imaging is provided. An ultrasound system acquires samples representing responses from a range gate. A Doppler estimator estimates the spectrum of the range gate over time from the samples. The spectra are plotted over time from 0 to 2π. For each spectrum, points in the noise region between the first signal and the second signal, 0 or 2π, are identified. A spectral envelope is traced over time. The trace is limited by the points of the spectrum. The spectral envelope, or information derived from the spectral envelope, is displayed in an image.

[0006] In one embodiment, the samples are acquired by transmitting at a pulse repetition frequency and acquiring one sample in response to each repetition. In another embodiment, a Fourier transform is applied to the samples to estimate the energy of each spectrum as a function of frequency for a period.

[0007] According to one embodiment, the plotting includes plotting each spectrum starting at 0 and ending at 2π, such that negative signals extend from 2π to 0 and positive signals extend from 0 to 2π.

[0008] In various embodiments, the point is identified by locating the point midway between the first signal and the second signal, 0, or 2π; searching the spectrum for a band containing the first signal and a band containing the second signal, and locating a point in a noise region between the first signal and (1) the second signal if the search finds the second signal, or (2) 0 or 2π if the second signal is not found; A noise level is determined from at least a portion of the spectrum, and the noise level is used to search for the first and second signals and / or to locate a boundary over time, the boundary being formed by points of the spectrum and varying between 0 and 2π.

[0009] In another embodiment, the tracing step includes locating edges of the noise region and the first signal and locating edges of the noise region and the second signal, where the locating of edges is over time and the edges are spectral envelopes. In yet another embodiment, the tracing includes using 0 or 2π for a spectrum where the noise region extends to 0 or 2π and a second or third signal where the noise region does not extend to 0 or 2π. In another embodiment, the tracing step includes tracing the first signal as a positive signal between 0 and a point on the spectrum and tracing the second signal as a negative signal between 2π and a point on the spectrum.

[0010] According to one embodiment, the displaying step includes displaying the spectral envelope as positive above the baseline and negative below the baseline, hi another embodiment, the displaying step includes displaying information, the information being based on peak velocities of the spectral envelope.

[0011] In a second aspect, a method for spectral Doppler imaging is provided. An ultrasound system acquires samples representing a response from a range gate. A Doppler estimator estimates a spectrum of the range gate over time from the samples. The spectrum is plotted over time from 0 to 2π. Based on the spectrum plotted from 0 to 2π, positive and / or negative spectral envelopes are detected over time. The spectral envelope, or information derived from the spectral envelope, is displayed as an image.

[0012] In one embodiment, plotting includes plotting each spectrum starting from 0 and ending at 2π, such that negative signals extend from 2π to 0 and positive signals extend from 0 to 2π. In one embodiment, detecting includes setting a boundary that varies along the spectrum from 0 to 2π for each spectrum, and searching for a positive spectral envelope on one side of the boundary and a negative spectral envelope on another side of the boundary. According to another embodiment, setting a boundary includes setting a boundary halfway between signals in different bands, where 0 or 2π is used in place of signals in the spectrum where there is no signal in the band.

[0013] In a third aspect, a system for spectral Doppler imaging is provided. The beamformer is configured to sample a gate at a pulse repetition interval established in response to a velocity scale. The Doppler estimator is configured to generate a plurality of spectra from the gated sampling. The signal processor is configured to identify a line in two bands or one of the bands and a noise region between 0 and 2π, the line varying in time for each spectrum of the spectrum, and the line limits a search of the spectral envelope of the spectrum. The display is configured to display the spectral envelope or information derived from the spectral envelope.

[0014] In one embodiment, the signal processor is configured to plot the spectrum from 0 to 2π, such that one of the two bands is 2π and the other of the two bands is 0, and identify lines from the plotted spectrum in the noise region. In yet another embodiment, the signal processor is configured to place the line for each spectrum halfway between the two bands.

[0015] The present invention is defined by the following claims, and nothing in this section should be construed as limiting those claims. Further aspects and advantages of the present invention are described below in conjunction with the preferred embodiments.

[0016] The components and figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals indicate corresponding parts throughout the different views. [Brief explanation of the drawings]

[0017] [Figure 1] Figure 1 illustrates aliasing in spectral Doppler imaging. [Figure 2] Figure 2 is a flow chart diagram of one embodiment of a method for spectral Doppler imaging with improved constraints on the trace to avoid aliasing. [Figure 3] FIG. 3 is a diagram showing an example of a plot of a spectrum from 0 to 2π. [Figure 4] FIG. 4 shows an example of dynamic boundaries or search limits identified for the plot of FIG. [Figure 5] Figure 5 is a block diagram of one embodiment of a system for spectral Doppler imaging with reduced aliasing. DETAILED DESCRIPTION OF THE INVENTION

[0018] Figure 1 shows three spectral Doppler images with different baseline settings. Each column in each image represents a spectrum. The spectrum over time is plotted on a velocity scale based on the Nyquist criterion, which takes into account the sampling rate used to form the spectrum. This results in aliasing 100, where a portion of the positive signal is placed in the negative part of the strip (i.e., below the baseline). The middle strip shifts the baseline by -0.25 (a 25% negative shift). This results in the negative signal being aliased into the positive region. When trace 110 is detected, this negative signal is traced as a positive peak. The bottom strip shows that shifting the baseline by an additional -0.5 (50% negative shift) results in further aliasing. Similarly, shifting the baseline to the positive side increases the aliasing of the positive signal to the negative side.

[0019] This approach avoids aliasing. Even when baseline shifting fails, the spectral Doppler envelope is traced without aliasing. The search range for positive and negative flow peaks is dynamically adjusted rather than fixed by the baseline position. The boundary between positive and negative flow is dynamically determined for each spectral train and used as the Nyquist limit for subsequent positive and negative searches. The search limit is dynamically determined based on spectral characteristics such as bandwidth and signal-to-noise ratio (SNR). In this way, existing peak searches function correctly and generate envelopes consistent with the spectral content. By dynamically adjusting the search range, true peak velocities in either direction can be correctly detected, even in the presence of spectral aliasing. Similarly, envelopes can be displayed correctly. Both positive and negative flow envelopes are correctly traced, even in the presence of spectral aliasing. This handles situations where the envelope becomes confused due to aliasing of the Doppler signal itself, improving the ability to extract diagnostically useful information compared to envelope tracing using existing techniques. If the maximum Doppler shift is less than the sampling rate, both positive and negative envelopes are displayed correctly without aliasing artifacts.

[0020] Figure 2 shows one embodiment of a flowchart for the spectral Doppler imaging method. By plotting the spectrum in a way that distinguishes between positive and negative signals, a variable limit between two bands of the spectrum is identified. This limit is used to trace the positive and / or negative signals and avoid aliasing.

[0021] The method may be performed by system 500 of FIG. 5 or another system. For example, an ultrasound system, such as a beamformer and transducer, or another ultrasound scanner, acquires samples. A processor, such as a Doppler estimator, estimates, plots, detects / identifies, and traces a spectrum from the spectrum. The ultrasound system or scanner displays an image of the traced spectral envelope or information from the spectral envelope. One or more operations may be performed through user interaction (e.g., gate placement for sampling). Other or all operations may occur automatically without user input other than initial activation or gate location.

[0022] The operations are performed in the order shown, although other orders are possible. Additional, different, or fewer operations may be performed. For example, operation 250 is not performed. As another example, operations 230 and / or 232 are not performed. As another example, operation 220 is not performed. As yet another example, operations for gate placement, gate sizing, velocity scale setting, pulse repetition frequency setting, filtering, processing, maximum velocity over time determination, or other Doppler functions are performed.

[0023] This method has been implemented for pulsed wave (PW) spectral Doppler. In PW, gate locations are sampled using pulsed wave transmission (e.g., each pulse using a waveform of 1 to 50 cycles) interleaved with echo reception. PW may be interleaved with other imaging modes, such as B-mode or flow mode. Alternatively, continuous wave Doppler is used. In continuous wave Doppler, samples are created and then transformed to create a spectrum.

[0024] In spectral Doppler imaging, a sample gate, a range gate, and a spectral Doppler gate are placed. For example, a B-mode or flow-mode scan is performed. The user indicates the location of the gate on the resulting image. In other cases, the gate is automatically placed, such as at a maximum Doppler velocity or energy determined from flow-mode data. The user or an automated process also determines the size of the gate, such as its depth or range length. The lateral extent and / or range extent can be default, data-driven, user-defined, or selected by other methods.

[0025] In operation 200, the ultrasound system acquires samples representing the position of the range gate. These samples are beamformed samples, but may also be other raw data (e.g., channel data). In alternative embodiments, the samples are acquired by loading from memory or transferring from other devices.

[0026] For scanning acquisition, the transducer transmits multiple beams of acoustic energy in succession. The acoustic or ultrasonic energy of each transmit is focused near or at a gate. A sequence of transmits is performed. This repetition allows for the reception of enough samples to perform spectral or other Doppler analysis. Any ensemble number of transmit beams, such as 3 to 512, can be transmitted to estimate the spectrum of the response from the Doppler gate.

[0027] Additional transmissions provide additional information for estimating the spectrum at other times or time periods. A given response for a given beam may be used for different spectra (i.e., different times), such that a moving window of received responses or samples is used to generate each spectrum.

[0028] A sufficient number of samples are acquired for the velocity scale set for the spectral Doppler image. The ultrasound system uses this velocity scale for image processing. The velocity scale defines the frequency range over which the spectrum or velocity is estimated and displayed. The velocity scale may be selected by the user, may be a system default or predetermined value, and / or may be adaptively determined by the ultrasound system. Based on the velocity scale, transmissions are performed at a pulse repetition frequency or interval that samples the motion or flow signal at the gate location.

[0029] In response to the transmission, the transducer receives acoustic echoes. A receive beamformer samples the echoes to obtain receive signals for the gate. The receive beams are formed by focusing the receive signals to coherently combine data representing the gate. This combined data representing the gate is the beamformed signal or sample.

[0030] Receive operations occur repeatedly in response to repeated transmissions. Beamformed samples are received from gate locations at different times. A sample is received in response to each transmission. Samples from the same location are taken in ensembles over time. Doppler analysis involves taking an ensemble of samples from the same location. Samples can be taken in a continuous manner, such that a moving window (e.g., ensemble or flow sample number) with any step size (e.g., every sample or every third sample) is used to estimate the spectrum.

[0031] In operation 210, the Doppler estimator estimates the spectrum of the Doppler gate from the samples of the ensemble or number of flow samples. By repeating for other ensembles or number of flow samples, multiple spectra are estimated for corresponding multiple times or time periods (i.e., over time).

[0032] Each spectrum represents energy as a function of frequency or velocity for the same period. Frequency has a known relationship to velocity, so frequency equals velocity and velocity equals frequency.

[0033] The spectrum is estimated by applying a Fourier transform, a wavelet transform, or a Wigner-Ville distribution to a series of ultrasonic responses or samples. Any transform may be applied to determine each spectrum.

[0034] The spectrum is estimated using a velocity scale. Signals from fluids and tissues are in the positive and negative velocity range. The range used for estimation is the velocity scale. Velocities above the velocity scale wrap around or alias. The spectrum provides energy as a function of frequency over a frequency range set by the velocity scale. The baseline determines the band range within the velocity scale.

[0035] A spectrum is estimated for a Doppler gate. A spectrum is estimated from ultrasound samples in a series of samples from the Doppler gate. Each spectrum corresponds to a period during which the samples were acquired.

[0036] Operations 200 and 210 are repeated for different times or periods. To create a spectral strip, a spectrum is determined for each different time. The spectrum for a given time (period) of the spectral strip is mapped with energy adjusting speed and intensity on the vertical axis. The spectrum is distributed over time along the horizontal axis. Other mappings may be used.

[0037] This operation is repeated to acquire spectra at different times. As more samples are acquired, they are added to the group and older samples are removed. The series of spectra represents the Doppler gates at different times. Other spectra can be estimated for other periods or times corresponding to different periods or ensembles of acquisition. The periods can be overlapping or unique, such as when using a moving window with a step size smaller than the ensemble period. The acquisition of samples and estimation of different periods is repeated to obtain spectra over time. For spectral strips, the process and its corresponding repetitions can occur continuously or multiple times.

[0038] In operation 220, the signal processor plots the spectrum over time from 0 to 2π. Rather than using -π to π at baseline 0 or a shift of less than 100% (e.g., 25% or -50%) of the baseline (e.g., -0.75π to 1.25π), the spectrum is plotted from 0 to 2π. The spectrum may be plotted substantially from 0 to 2π, where substantially means a shift of 10% or less.

[0039] An example is shown in Figure 3. In Figure 3, "0" on the y-axis is 2π, and "256" on the y-axis is 0. Black or dark areas represent noise, and white or gray areas represent signal.

[0040] By plotting each spectrum starting at 0 and ending at 2π, negative signals are in the upper region (i.e., extending along 2π or from 2π to 0) and positive signals are in the lower region (i.e., extending along 0 or from 0 to 2π). In an alternative embodiment, the plot is from 2π to 0, with positives in the upper region and negatives in the lower region. Either 0 to 2π or 2π to 0 is plotted over time from 0 to 2π.

[0041] In a typical spectral strip, this plot is not used. Spectral strips display positive signals in the upper half and negative signals in the lower half, and separations other than half may be used with baseline shifts to avoid aliasing. Unlike this typical plot, the plot of operation 220 provides better separation between positive and negative signals, with noise in between. This 0 to 2π or 2π to 0 plot places the noise region 340 between the signals, avoiding aliasing.

[0042] Plotting substantially from 0 to 2π or 2π to 0 aids in band separation without aliasing. In another embodiment, the velocity scale is plotted with a baseline at any position. Machine learning models or signal processing are applied to distinguish the bands, establishing boundaries no matter how they are plotted. The boundaries can be varied over time or over spectra to separate the bands.

[0043] In operation 230 of Figure 2, the signal processor detects positive and / or negative spectral envelopes over time based on the spectrum plotted from 0 to 2π. The edges of the noisy signal are detected. In the example of Figure 3, there are two bands (positive and negative). The edges of each band are detected separately. If the spectrum or spectra show only one band, the envelope is detected for that one band (e.g., only positive or only negative). More than one band may be detected.

[0044] Operation 232 represents one approach to individually detecting the spectral envelope or bands: each spectrum is divided into band regions bounded by the limits of the noise region 340. In other embodiments, noise-based thresholding is used to ensure that the spectrum is divided into bands without noise.

[0045] In operation 232, the signal processor identifies, for each spectrum of the spectra, a point in the noise region between one signal and another, either 0 or 2π. The points across the spectrum (over time) define boundaries over time. The boundaries are formed by the points of the spectrum. As the points are positioned across the spectrum, the boundaries formed by the points of the spectrum vary between 0 and 2π.

[0046] A signal processor searches each spectrum to identify boundary points. The signal processor searches one or more bands, such as a positive signal band and a negative signal band. To find the band edges, a noise level is determined to distinguish between noise and signal. The amplitude of the energy modulation is used to define the noise level. Statistical analysis is used to find the noise floor. For example, the energy is sorted by magnitude. From the sorted energy, a magnitude transition is determined, for example by slope. Strong and weak signals are identified, and a noise level or threshold is set that distinguishes between signal and noise.

[0047] Next, find the edge that separates the strong signal from the noise signal (i.e., threshold noise level). To find the edge, the number of aligned energy above noise can be used. The edge separates into separate regions. In another approach, the cluster or full spectrum of energy is low-pass filtered to remove outliers. A threshold is then applied to identify the signal or noise regions 340. Other approaches that use the noise level to search for the signal or different bands can also be used.

[0048] Once the spectral edges are identified, points are placed in the signal, band, or noise region 340 between the edges. An example is shown in FIG. 4. The spectral edges of the positive and negative signals 420, 410 are used to set points per spectrum that form boundaries 400 in the noise region 340. In this example, each point and the resulting boundaries 400 are placed midway between the edges or envelopes of two bands. For example, the point at time 400 is approximately 57, based on the negative signal edge 410 being 10 and the positive signal edge 420 being 125. As another example, if the negative signal edge 410 is 100 and the positive signal edge is 256, the point at time 1 would be 178. Divisions other than half may also be used. Multiple boundaries may be used, for example, by setting points at a fixed distance or percentage apart in each band (e.g., 1 / 3 of the noise band from edge 410, 1 / 3 of the noise band from edge 420, with the 1 / 3 of the noise in between).

[0049] If there is no positive or negative signal for a spectrum, 0 or 2π is used as the envelope or edge 410, 420. In the example of Figure 4, 0 is used for the positive edge 420 from approximately times 0-75, 125-375, 525-630, 700-950, 1075-1225, and 1275-1500, and 2π is used for the negative edge 410 from 75-125, 700-725, 950-1050, and 1225-1275. For each spectrum, a point is located in the noise region 340 between the positive or negative signal and either (1) the other of the negative or positive signal if the other signal is found in the search, or (2) 0 or 2π if the other signal is not found.

[0050] Boundary 400 is the limit used to search the trace. Rather than using a set baseline to separate bands, the search for the spectral envelope uses the limit or speed defined by boundary 400. Boundary 400 acts as a dynamic or variable Nyquist limit, with different values ​​(points) for different times, to avoid aliasing.

[0051] In operation 240 of FIG. 2, the signal processor traces the spectral envelope over time. The same approach as detecting the spectral envelope or otherwise locating band or signal edges in operation 230 may be used. Different approaches may also be used. For example, a more processing-efficient but less accurate band edge or signal-to-noise boundary detection may be performed in operation 230, the boundaries may be found in operation 232, and a more processing-intensive but more accurate band edge or signal-to-noise boundary detection and tracing may be performed in operation 240. As another example, the detection in operation 230 may be performed without any constraints other than 0 and 2π, and the tracing in operation 240 may constrain the locations where the search for edges occurs. Any now known or later developed spectral envelope tracing may be used.

[0052] In another embodiment, a peak velocity search is performed. Tracing may be performed for all of the time or only for a portion of the time (one or a few spectra). Tracing may be performed for less than all of the spectra.

[0053] The tracing identifies the edges of the signal in time or at a given time. For example, the maximum positive and / or maximum negative signal over one or more cardiac cycles is determined. A noise threshold is found from the spectrum and used as the threshold to identify the signal. The trace is placed where the signal is dominant on one side of the trace and the noise is dominant on the other side of the trace. The trace identifies the edges of the signal on the spectrum. The maximum of this trace is then found.

[0054] The trace in act 240 is constrained by the points that define the band separation. Rather than relying on the positioning of a baseline to define where the positive and negative are separated, the dynamic or variable boundary identified in act 232 is used. This boundary varies with the spectrum so that only positive signals are fed to the positive signal side of the boundary and only negative signals are fed to the negative signal side of the boundary. A search for the positive spectral envelope occurs on one side of the boundary and a search for the negative spectral envelope occurs on the other side of the boundary. Aliasing is avoided.

[0055] The signal processor identifies the edges of each band (e.g., positive and negative signals) in or within the noise region. The edges extend across time or the spectrum. The identified edges are the spectral envelope. Using Figure 4 as an example of traces, rather than identifying boundary 400, traces 410 and 420 are located as spectral edges. The spectrum may be replotted, for example, based on the baseline position (e.g., replotted from -π to π, since baseline 0 is the baseline). Boundary 400 is used to constrain the trace, preventing aliasing when replotting. Aliased signals are not included as false signals because boundary 400 indicates actual band membership. Positive signals are traced from 0 to the point or boundary 400, and negative signals are traced from 2π to the point or boundary 400, resulting in an alias-free spectral envelope regardless of baseline position.

[0056] In operation 250 of Figure 2, the signal processor generates an image on a display, which image is an image of the spectral envelope or an image of information related to a function of the spectral envelope.

[0057] In one embodiment, a spectral strip or spectral Doppler image for a Doppler gate is generated. Filtering may be applied across the time and / or frequency dimensions or energy to obtain a smooth spectrum. The spectral strip shows frequency modulated by energy as a function of time. Any currently known or later developed spectral strip mapping may be used, such as a grayscale mapping of intensity representing energy. The energy modulates the pixels. Grayscale or color is mapped from the energy values.

[0058] The traced spectral envelope is also displayed. The traced spectral envelope is included in the image, for example, enhanced by coloring or overlaid graphics (e.g., curves or lines along the edges). Different colors may be used for different bands. Additional information may be included, for example, annotations including alphanumeric text indicating one or more maximum values ​​in the band (e.g., maximum positive velocity).

[0059] The velocity scale defines the vertical range on the spectral strip. The baseline defines the separation between positive and negative values ​​within the velocity scale. The spectral envelope is displayed with positive values ​​above the baseline and negative values ​​below the baseline. As additional samples are acquired, the resulting spectra at different times are added to the spectral strip. For example, a spectrum is added to the right side of the strip, the remaining spectra are shifted one time step to the left, and the leftmost spectral strip is removed. Other spectral strip updates or scrolling may be used.

[0060] The spectral strip may or may not be displayed with a spatial image, such as a 1D M-mode, 2D B-mode, 2D F-mode (flow mode), or a combination thereof. The gate location may be indicated graphically within the image, such as by a circle, double line, or other graphic within the display area.

[0061] In another or alternative embodiment, information derived from the spectral envelope is displayed on the image. For example, the spectral envelope is used to detect maximum positive and / or negative velocities. The maxima are displayed as alphanumeric text within any image or are represented graphically (e.g., patient maxima for disease grading and / or population charts). As another example, maximum energy and / or velocity values ​​and / or other statistical information (e.g., standard deviation) are determined and displayed. In yet another example, statistical information from the spectral envelope is used for disease grading or other diagnostic or prognostic assessment, e.g., strandness criteria. The grading, diagnostic, or prognostic results are displayed.

[0062] 5 illustrates a system 500 for spectral Doppler imaging. The system 500 traces the spectral envelope using dynamically determined boundaries, such as boundaries based on mapping or plotting the spectrum from approximately 0 to 2π. Aliasing may be avoided by using dynamically determined boundaries and / or plotting from approximately 0 to 2π.

[0063] System 500 is a medical diagnostic ultrasound imaging system. Other imaging systems may be used, such as a workstation that loads samples from memory or other sources.

[0064] The system 500 includes a transmit beamformer 510, a transducer 520, a receive beamformer 530, a Doppler processor 540, a display 550, a signal processor 560, and a memory 570. Additional, different, or fewer components may be provided, such as a system 500 without the front-end beamformers 510, 530, and / or the transducer 520, or a system 500 with a scan converter. The Doppler processor 540 and the signal processor 560 may be combined into a single device that functions as both processors, or additional processors for serial or parallel processing may be used. User input may be provided for setting gate placement and / or gate size.

[0065] System 500 implements the method of FIG. 1. Beamformers 510, 530 and transducer 520 are used to acquire samples. Doppler processor 540 estimates a gated spectrum from the samples. Doppler processor 540 and / or signal processor 560 plot substantially from 0 to 2π and / or determine and trace the boundaries between bands, and display the traced spectral envelope or information derived therefrom. Other methods may be implemented. Doppler processing may be performed before or after CINE.

[0066] The transducer 520 is an array of multiple elements. The transmit beamformer 510 is shown separately from the receive beamformer 530. Alternatively, the transmit and receive beamformers 510, 530 may be provided with common components. Operating alone or together, the transmit and receive beamformers 510, 530 form a beam 524 of acoustic energy for sampling a range gate 522 and / or scanning a one-, two-, or three-dimensional region.

[0067] The transmit beamformer 510 is configured to transmit a sequence of transmit beams 524 of ultrasound energy. The acoustic energy is directed toward the Doppler gate 522, but may be focused at other locations (e.g., a location where the Doppler gate is along the scan line but not at the focal point). The beams 524 of acoustic energy are transmitted toward the Doppler gate 522 or other locations.

[0068] An ongoing sequence of transmit beams 524 is generated at a PRF. The PRF determines the spacing between temporally adjacent transmits or transmit beams 524. The PRF may be set low enough to have periods of non-transmission that are not necessary for propagation time, which can be utilized for interleaving with other imaging modes or for reverberation reduction. In one embodiment, the PRF is set based on a velocity scale, propagation time, interleaving, and reverberation reduction. In another embodiment, the PRF is set based on a velocity scale and a Nyquist criterion.

[0069] The receive beamformer 530 forms one or more receive beams 526 corresponding to each transmit beam 524. Although shown parallel, the receive beams 526 may be coaxial with the transmit beam 524 or at an angle relative to the transmit beam 524. Acoustic echoes are received by the transducer 520 in response to the transmitted acoustic energy. The echoes are converted into electrical signals by the transducer 520, and the receive beamformer 530 forms the receive beams 526 from the electrical signals and generates samples representing a position or positions within the range gate 522.

[0070] Given an ongoing transmit beam 524 at a PRF or PRI (pulse repetition interval) at each location, samples are also generated in an ongoing manner. Responses are acquired over time. These responses generate an ensemble or accumulation of samples that are used to estimate a spectrum representing the same time period.

[0071] The Doppler processor 540 is a spectral Doppler estimator. Other imaging detectors, such as a B-mode detector, may also be included. In one embodiment, the Doppler processor 540 is a digital signal processor or other device for applying a transform to the receive beam sample data. A sequence of transmit and receive events is performed over a period of time. A buffer (e.g., a corner turning memory) or memory 570 stores the receive beamformed data from each transmit and receive event. A wall filter, such as a programmable filter for distinguishing between tissue and fluid motion, may filter the samples before applying the transform. Any number of transmit and receive events may be used to determine the spectrum, e.g., three or more. The Doppler processor 540 estimates the spectrum of the gate. A spectrum representing the response from the gate is determined by applying a discrete or fast Fourier transform or other transform to the ultrasound samples for the same gate. A histogram or data representing the energy levels at different frequencies for acquiring samples over a period of time is obtained. Velocity can be determined from the frequency, or frequency can be used without a velocity conversion.

[0072] By repeating this process, the Doppler processor 540 can obtain different spectra for a given gate at different times. Overlapping data may be used, for example, to calculate each spectrum over a moving window of selected ultrasound samples. Alternatively, each ultrasound sample may be used for a single time period and corresponding spectrum.

[0073] The Doppler processor 540 applies the transform over a range of frequencies. The frequency range or velocity scale limits the positive and negative velocities resulting from the estimation. Various velocity scales may be used, including up to a velocity scale equal to the transmit PRF. The spectrum is estimated using the given velocity scale. Similarly, a baseline or center for the velocity scale may be set.

[0074] Signal processor 560 may be part of Doppler processor 540 or may be a separate processor. Signal processor 560 may be a general processor, a control processor, a digital signal processor, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a graphics processing unit (GPU), an analog circuit, a digital circuit, a combination thereof, or any now known or later developed processing device. Signal processor 560 may be implemented using software, firmware, and / or hardware.

[0075] The signal processor 560 is configured to identify lines in the noise region between two bands, or between one of the bands and 0 and 2π. Other lines may be identified between other bands, or between the limits of the velocity scale. Lines may also be determined between other plots, for example, between -π and π.

[0076] The signal processor 560 identifies the lines as curved (i.e., the lines vary over time and in the velocity scale). The lines are continuous, but can also be discontinuous. Line fitting may be performed. Line filtering may be used. The lines vary over time for each spectrum, but can also vary for groups of spectra. To avoid aliasing, the lines are placed between noise bands or between a band and the velocity scale limits (e.g., 0 or 2π). The lines are placed halfway between two bands (between the band edges) or between a band and the velocity scale limits (e.g., 0 or 2π). Other locations are also possible, such as spacing closer to one band than the other.

[0077] The signal processor 560 can plot the spectrum from 0 to 2π, with one of the two bands located at 2π and the other at 0. This may help identify the line separating the bands. The plotted spectrum is used to locate the line in the noise region.

[0078] The signal processor 560 is configured to limit the search of the spectral envelope of the spectrum by the lines that define or separate different bands. Rather than tracing based on the baseline position for a given velocity scale, dynamic lines are used to define the search region. Rather than searching for one band above the baseline and another below, the identified lines or boundaries are used to limit the search of each band. As a result, the search or tracing is free of aliasing.

[0079] The display device 18 may be a CRT, monitor, LCD, plasma screen, projector, printer, or other display device now known or later developed for displaying an image of the spectral envelope or information derived from the spectral envelope. In a spectral Doppler image, a range of velocities is provided as a function of time, with each velocity scaled as a function of energy.

[0080] The spectral envelope is displayed as a trace or graphic on the spectral Doppler image. Peak velocity or other information derived from the spectral envelope may be displayed on the spectral Doppler image (e.g., as an annotation) or as a separate image.

[0081] Other image configurations may also be provided, including colorized spectral Doppler images. Color or flow mode images may be generated, for example, showing average velocity as a function of position within the region of interest in grayscale B-mode.

[0082] The memory 570 stores ultrasound samples, estimated spectra, settings, image data, spectral plots, dividing lines or points that form dynamic lines, signal edges, traces, annotations, and / or other information. The memory 570 can store information for any stage of processing or information used for display generation.

[0083] In one embodiment, memory 570 is a non-transitory computer-readable storage medium on which data representing instructions for Doppler imaging executable by Doppler processor 540 and / or signal processor 560 is stored. Instructions for implementing the processes, methods, and / or techniques discussed herein are provided on a computer-readable storage medium or memory, such as a cache, buffer, RAM, removable media, a hard drive, or other computer-readable storage medium. Computer-readable storage media include various types of volatile and non-volatile storage media. The functions, acts, or tasks illustrated in the figures or described herein are performed in response to one or more sets of instructions stored in or on a computer-readable storage medium. These functions, acts, or tasks do not depend on a particular type of instruction set, storage medium, processor, or processing strategy and may be performed by software, hardware, integrated circuits, firmware, microcode, etc., alone or in combination. Similarly, processing strategies may include multi-processing, multitasking, parallel processing, etc.

[0084] In some embodiments, the instructions are stored on a removable media device for reading by a local or remote system, in other embodiments, the instructions are stored at a remote location for transfer over a computer network or over telephone lines, and in still other embodiments, the instructions are stored within a particular computer, CPU, GPU, or system.

[0085] While the invention has been described above based on various embodiments, it should be understood that many changes and modifications are possible without departing from the scope of the invention. Accordingly, the foregoing detailed description is to be considered illustrative rather than restrictive, and it should be understood that it is the following claims, including all equivalents, that define the spirit and scope of the invention.

Claims

1. 1. A method for spectral Doppler imaging, comprising: The method comprises: acquiring samples with an ultrasound system representing a response from a range gate; estimating a spectrum over time for said range gate from said samples with a Doppler estimator; Plotting the spectrum over time from 0 to 2π; for each of the spectra, identifying points within a noise region; searching the spectrum for a first signal band and a second signal band; and Locating the point in the noise region between the first signal and (1) the second signal if the search finds the second signal, or (2) 0 or 2π if the search does not find the second signal. and tracing a spectral envelope over time, said trace being bounded by said points on said spectrum; and and displaying the spectral envelope or information derived from the spectral envelope in an image.

2. 2. The method of claim 1, wherein the acquiring step comprises transmitting at a pulse repetition frequency and acquiring one of the samples in response to each repetition.

3. 2. The method of claim 1, wherein the estimating step comprises applying a Fourier transform to the samples, each spectrum being energy as a function of frequency over a period of time.

4. 2. The method of claim 1, wherein the plotting step includes plotting each spectrum starting at 0 and ending at 2π, such that negative signals extend from 2π to 0 and positive signals extend from 0 to 2π.

5. The method of claim 1 , wherein the identifying step includes locating the point midway between the first signal and the second signal, 0, or 2π.

6. 2. The method of claim 1, wherein the identifying step includes determining a noise level from at least a portion of the spectrum and using the noise level to search for the first and second signals.

7. 2. The method of claim 1, wherein the identifying step includes locating a boundary over time, the boundary formed by the points of the spectrum varying about 0 and 2π.

8. 2. The method of claim 1, wherein the tracing step includes locating edges of the first signal having the noise region and locating edges of the second signal having the noise region, the locating edges occurring over time, and the edges constituting the spectral envelope.

9. 2. The method of claim 1, wherein the tracing step includes using 0 or 2π for the spectrum where the noise region extends to 0 or 2π, and using 0 or 2π when no positive or negative signal is present for the spectrum.

10. 2. The method of claim 1 , wherein the tracing step includes tracing the first signal as a positive signal between 0 and the point on the spectrum, and tracing the second signal as a negative signal between 2π and the point on the spectrum.

11. The method of claim 1 , wherein the step of displaying comprises displaying the spectral envelope positive above a baseline and negative below the baseline.

12. The method of claim 1 , wherein the displaying step comprises displaying the information, the information being based on peak velocities of the spectral envelope.

13. 1. A method for spectral Doppler imaging, comprising: The method comprises: acquiring samples representing a response from a range gate using an ultrasound system; estimating a spectrum over time for the range gate from the samples using a Doppler estimator; Plotting the spectrum over time from 0 to 2π; detecting a positive or negative spectral envelope over time based on the spectrum plotted from 0 to 2π; displaying the spectral envelope or information derived from the spectral envelope in an image; A method comprising:

14. 14. The method of claim 13, wherein the plotting step includes plotting each spectrum starting at 0 and ending at 2π, such that the negative signals extend from 2π to 0 and the positive signals extend from 0 to 2π.

15. The detecting step includes setting a boundary that varies along the spectrum from 0 to 2π for each spectrum, and searching for the positive spectral envelope on one side of the boundary and the negative spectral envelope on the other side of the boundary. The method of claim 13.

16. Setting the boundaries includes setting the boundaries midway between signals of different bands, where 0 or 2π is used in place of the signal for spectra without a signal in the band.

16. The method of claim 15.

17. 1. A system for spectral Doppler imaging, comprising: The system comprises: a beamformer configured to sample the gate at a pulse repetition interval established in response to a velocity scale; a Doppler estimator configured to generate a plurality of spectra from the sampling of the gate; a signal processor configured to identify a line in two bands or in one of said bands and in a noise region between 0 and 2π, said line varying over time for each spectrum of said spectrum, and configured to limit a search of the spectral envelope of said spectrum by said line; and a display configured to display the spectral envelope or information derived from the spectral envelope; The signal processor is configured to spectrally position the line midway between the two bands. A system characterized by:

18. 20. The system of claim 17, wherein the signal processor is configured to plot the spectrum from 0 to 2π, with one of the two bands close to 2π and the other of the two bands close to 0, and identify the line from the plotted spectrum in a noise domain.

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