KR101388333B1 - Accurate time delay estimation method and system for use in ultrasound imaging - Google Patents

Abstract

A method of correcting the beamforming time delay in the ultrasound system 10 is provided. The method includes transmitting a beam of ultrasonic energy to an object by a transmission beamforming time delay. The method further includes receiving a plurality of echo signals by the receive beamforming time delay and estimating the beamforming time delay error for each echo signal and each imaging direction. The method further includes correcting the transmit and receive beamforming time delays and generating an ultrasound image of the object using the corrected transmit and receive beamforming time delays.

Description

ACCURATE TIME DELAY ESTIMATION METHOD AND SYSTEM FOR USE IN ULTRASOUND IMAGING

1 is a block diagram of one embodiment of an ultrasound system implemented in accordance with an aspect of the present invention.

2 is a block diagram of one embodiment of a beamformer system in accordance with an aspect of the present invention.

3 is a flow chart illustrating how arrival time errors are estimated and time delay corrections are generated.

4, 5 and 6 are graphs showing the comparison between the transducer element and the state of the individual complex correlation.

7 is a flow chart illustrating one method by which a complex correlation sum is labeled.

8 is a flow diagram illustrating one method in which image data is used to label correlations.

9 is an image of tissue showing the presence of blood vessels near the region of interest.

10 is a flow chart illustrating one method of detecting the presence of blood vessels near the region of interest.

FIG. 11 is an image of tissue showing the presence of shiny scatterers near the region of interest. FIG.

12, 13, 14 and 15 are graphs showing complex correlation states and corresponding data masks for transducer elements of a transducer array.

DESCRIPTION OF THE REFERENCE NUMERALS

10: ultrasound system 12: acquisition subsystem

14: Processing Subsystem 16: Imaging Object

18: Transducer Array 18A-18Z: Transducer Element

20: T / R switching circuit 22: transmitter

24: receiver 26: beamformer system

28: control processor 30: demodulator

32: imaging mode processor 34: scan converter

36: display processor 38: monitor

40: user interface 42: remote access subsystem

44: web server 46: interface

48: data storage 50: imaging workstation

62: beamformer delay 62A-62Z: beamformer delay element

64: adder 68: composite filter

70: Correlator Processor 70A-70Z: Correlator Processor

74: correlation processor 120: region of interest

122: blood vessel 124: tissue

126: Glowing Scatterer 128: Region of Interest

132: sidelobe energy

Reference to Related Application

This application includes the subject matter of US Patent Application No. 10/882910, entitled "TIME DELAY ESTIMATION METHOD AND SYSTEM FOR USE IN ULTRASOUND IMAGING," filed June 30, 2004, which is hereby incorporated by reference herein. .

FIELD OF THE INVENTION The present invention relates generally to imaging systems, and more particularly to methods and systems for estimating and correcting time delays in ultrasonic imaging systems.

The ultrasound system includes an array of transducer elements used to transmit a set of waveforms to an imaging object and receive a reflected set of ultrasound signals. Each waveform is emitted by a selected relative time delay to focus the total waveform transmitted in the desired direction and depth and in the desired shape. Likewise, each received signal is individually delayed to maximize the system's response with the reflected energy for the desired direction and depth and desired shape. The delayed received signals are summed and processed to generate and display an image of the imaged object.

Transmitted and received time delays, collectively known as beamforming time delays, are typically calculated assuming that sound propagates through the body at a known constant rate. If this assumption fails, the transmit and receive focusing will be degraded resulting in a loss of image resolution and contrast.

One way to reduce the loss of image quality is to adjust the beamforming time delay based on a measure of the relative time delay of the received signal. It is inconvenient to measure these relative time delays after the receive beamforming delay has been applied. If the estimation of the known fixed sound speed is correct, the delayed received signal will be well aligned in time, i.e., the arrival time error will be small. If this estimate is not correct, the delayed received signal will not be well aligned in time, ie there will be many arrival time errors. By correcting the beamforming delay for arrival time errors, focusing will be improved and image resolution and contrast will increase. The arrival time delay can be estimated using one of several methods well known in the art.

In medical ultrasound imaging, the estimation of the arrival time error must be fast, accurate and rigid. It is also highly desirable to minimize the surplus cost required to implement the estimated hardware. As used herein, time of arrival error is defined as the difference between two signals. The arrival time error is processed to obtain a time delay correction, which correction is applied to correct the beamforming time delay.

Calibration required as the operator moves the transducer relative to the imaging object due to the operator moving the transducer relative to the patient as part of a typical scanning procedure, or a slight movement of the operator's hand, or a movement or breathing movement of the patient. Because this will be variable, a fast estimation is desirable because the beamforming time delay needs to be updated quickly.

Adjustment of the beamforming time delay by inaccurate time delay correction requires accurate estimation to prevent undesired degradation of the image and to improve image resolution and contrast. The arrival time delay can be inaccurate for several reasons. For example, if the time of arrival error estimate is calculated using the phase of a complex correlation sum, the signal provided to the correlation sum may be poorly correlated, or the element of the transducer may fail and its output signal may be abnormally noisy. This may be a lot. The transducer element is obscured by the acoustically impermeable obstruction, such as ribs, to the imaging object, which can produce a noisy signal, resulting in inaccurate arrival time error estimation. It is not desirable to allow such unreliable or noisy phase estimates to be used to determine time delay correction, since the degradation of beamforming performance due to these incorrect values overwhelms the advantage of using more accurate values.

In addition, this error in arrival time error estimation introduces artifacts into the picture, which can lead to false diagnosis or long inspection time. The artifact generation rate should be low enough for most practitioners to routinely use the time delay correction feature to obtain the benefit of improved image resolution and contrast.

In many applications, it is necessary to project between the ribs of the human body (intercostal ribs), since the ribs can block the transmission and reception of ultrasound from the transducer part, especially if the desired imaging scan plate is generally perpendicular to the direction of the ribs. This is the case when it is required to be oriented. In addition, the muscle layers associated with the ribs are irregular in thickness and orientation, leading to time-of-arrival errors in the transducer. It is desirable to produce high quality images that allow more accurate diagnosis while projecting the intercostal spaces.

Therefore, what is needed is a method and system in an ultrasound system that accurately and robustly estimates and compensates for arrival time errors while minimizing system cost and size.

Briefly, in accordance with an aspect of the present invention, a method is provided for correcting a beamformer time delay of an ultrasound system. The method includes transmitting an ultrasonic energy beam to the object. An ultrasonic energy beam is generated using an array of transducer elements, each transducer element configured to transmit an ultrasonic energy pulse having a transmission beamforming time delay. The method also includes receiving a plurality of echo signals, wherein each transducer element is configured to receive an ultrasonic energy beam having a receive beamforming time delay and generate an arrival time error for each echo signal and each imaging direction. Estimate. The method also corrects the transmit and receive beamforming time delays and uses the corrected transmit and receive beamforming time delays to generate an ultrasound image of the object.

In another embodiment, an ultrasound system for estimating beamforming time delay is provided. The ultrasound system includes a transducer array having a set of array elements arranged in a pattern, each of which is configured to transmit an ultrasonic energy beam individually through the object during the transmission mode to the vibration energy impinging on the transducer during the reception mode. It can generate a responsive echo signal. The ultrasound system includes a transmitter connected to the transducer array and operable during a transmission mode to apply an individual transmit signal pulse having an individual transmit beamformer time delay to each of the array elements to produce an induced transmit beam. The receiver is connected to the transducer array and can sample the echo signals generated by each of the array elements during the receive mode and impose a receive beamformer time delay on each echo signal sample to generate a corresponding plurality of received signals. The system also includes a beamformer system for estimating arrival time errors for each echo signal and each imaging direction and correcting the transmit and receive beamforming time delays, and an image processor for generating ultrasound images.

These and other features, aspects, and advantages of the present invention will be better understood from the following detailed description of the invention with reference to the accompanying drawings, in which like numerals refer to similar parts.

1 is a block diagram of one embodiment of an ultrasound system 10 implemented in accordance with an aspect of the present invention. The ultrasound system consists of an acquisition subsystem 12 and a processing subsystem 14. Acquisition subsystem 12 includes transducer array 18 (including a plurality of transducer array elements 18A-18Z), transmit / receive switching circuit 20, transmitter 22, receiver 24, and beamformer. System 26. Processing subsystem 14 includes a control processor 28, a demodulator 30, an imaging mode processor 32, a scan converter 34, and a display processor 36. The display processor is also connected to the monitor for image display. User interface 40 interacts with control processor 28 and display monitor 38. The processing subsystem may also be connected to a remote connectivity subsystem 42 that includes a web server 44 and a remote connectivity interface 46. The processing subsystem may also be connected to the data store 48 for receiving ultrasound image data. The data store interacts with an image workstation.

As used herein, “operable to”, “configured to”, and the like, refer to a hardware or software connection between components such that the components provide the described effects, and these terms provide an output in response to a given input signal. Also refers to the operating characteristics of an electrical component, such as an analog or digital computer or an application specific device (such as an ASIC) that is programmed to perform a result.

The architecture and modules may be dedicated hardware components, such as circuit boards with digital signal processors, or software running on general-purpose computers or processors, such as off-the-shelf PCs. Various architectures and modules can be combined or separated in accordance with various embodiments of the invention.

In the acquisition subsystem 12, the transducer array 18 is in contact with the object 16. Transducer array 18 is coupled to a transmit / receive (T / R) switching circuit 20. The T / R switching circuit 20 is coupled to the output of the transmitter 22 and the input of the receiver 24. The output of the receiver 24 is an input to the beamformer 26. The beamformer 26 is also coupled to the input of the transmitter 22 and the input of the demodulator 30.

In the processing subsystem 14, the output of the demodulator 30 is also coupled to the input of the imaging mode processor 32. The control processor interfaces with the imaging mode processor 32, the scan converter 34 and the display processor 36. The output end of the imaging mode processor 32 is coupled to the input end of the scan converter 34. The output end of the scan converter 34 is coupled to the input end of the display processor 36. Display processor 36 is coupled to monitor 38.

The ultrasound system 10 delivers ultrasound energy to a selected area of the object 16 and receives and processes the backscattered echo signal from the object to generate and display an image.

To generate the transmitted ultrasonic energy beam, control processor 28 delivers the command data to beamformer 26 to produce the desired shape, which occurs at a desired steering angle from a specific point on the surface of transducer array 18. Compute the transmission parameters for generating the beam. This transmission parameter is passed from the beamformer 26 to the transmitter 22. The transmitter 22 uses the transmission parameters to properly encode the transmission signal transmitted to the transducer array 18 via the T / R switching circuit 20. The transmission signals are set to a specific level and time delay relative to each other and are provided to the individual transducer elements of the transducer array 18. The transmitted signal excites the transducer element, thereby emitting an ultrasonic wave having the same time delay and level relationship. Thus, when transducer array 18 is acoustically coupled to an object, for example by using an ultrasonic gel, the transmitted ultrasound energy beam is formed on the object in the scan plane along the scan line. This process is known as electronic scanning.

Transducer array 18 is a bidirectional transducer. Once the ultrasound is delivered to the subject, the ultrasound is scattered back onto the tissue and blood sample in the subject. Transducer array 18 generates the backscattered echo signal several times, depending on the distance to the tissue returned by the backscattered echo signal and the angle to the surface of the transducer array 18 on which the backscattered echo signal returns. Receive. The transducer element responds to the backscattered echo signal and converts the ultrasonic energy from the backscattered echo signal into an electrical signal.

The electrical received signal is routed to the receiver 24 via the T / R switching circuit 20. Receiver 24 amplifies and digitizes the received signal and provides other functions such as gain compensation. The digitized received signal corresponds to the backscattered wave received multiple times by each transducer element and preserves the amplitude and time of arrival information of the backscattered wave.

The digitized received signal is transmitted to the beamformer system 26. The control processor 28 passes the command data to the beamformer 26. The beamformer system 26 uses the command data to steer the received beam resulting from a point on the surface of the transducer array 18, which typically corresponds to the steered angle of the previous ultrasonic beam delivered along the scan line. To form.

The beamformer system 26 operates on an appropriate received signal by executing time delays, amplitude weights, and sums, in accordance with instructions of command data from the control processor 28, to operate on the sample volume along the scan line of the scan plane in the object. Generate a corresponding receive beam signal. The beamformer system may further include a deformation algorithm that adjusts the time delay and amplitude weighting to correct for errors introduced by the deformation tissue layer. The waveform itself changes to allow correction of distortion.

The received beam signal is sent to the processing subsystem 14. Demodulator 30 demodulates the received beam signal to produce demodulated I and Q pairs of data values corresponding to the volume of the sample in the scan plane. The demodulated data is sent to an imaging mode processor 32 configured to generate an image. The imaging mode processor 32 generates the imaging parameter value from the demodulated data in the scan sequence regime using parameter estimation techniques. Imaging parameters correspond to a number of possible imaging modes such as, for example, B-mode, M-mode, color velocity mode, spectral Doppler mode and tissue velocity imaging mode. It may include a parameter. The imaging parameter value is passed to the scan converter 34. The scan converter 34 processes the parameter data by performing the conversion from the scan sequence system to the display system. The transformation includes performing an interpolation operation on the parameter data to produce display pixel data in the display system.

The imaging processor also uses an image processing algorithm to detect the desired shape in the image. This form is detected by the image processing algorithm and then used to change how the beamforming time delay is calculated or implemented. For example, the image processor may correct any region or may be selected from different estimation techniques based on information derived from the image. In certain embodiments, an iterative aberration correction algorithm is implemented to quickly calculate the beamforming time delay. The image is divided into several regions and the first beam in the image is emitted at the beginning of the first region. Data is collected and processed from the first area and the next area is launched. This technique allows for processing time before the beam is fired in the next frame.

The scan-converted pixel data is sent to the display processor 36 to perform final spatial or temporal filtering of the scan-converted pixel data, apply grayscale or color to the scan-converted pixel data, and monitor 38) Convert the digital pixel data into analog data for display on. The user 40 interacts with the beamformer system 26 based on the data displayed on the monitor 38.

As described above, beamformer system 26 performs a time delay operation on the received signal. The method of estimating and correcting the beamforming time delay of the received signal by the beamformer system will be described in more detail below with reference to FIG. 2.

2 is a block diagram of one embodiment of an adaptive beamformer system 28. A beamformer system is shown that receives a received signal from transducer elements 18a-18z of transducer array 18 via multiplexer 27. The transducer element is also used to deliver ultrasonic energy to selected areas of the subject. Each block in the beamformer system is described in more detail below.

Beamforming delay 62 includes beamformer delay elements 62a-62z. Each delay element represents a delay of the received signal received from the corresponding transducer element 18a-18z. The time delayed received signal is provided to the adder 64 to generate a summed time delayed received signal.

The summed time delayed received signal is provided to the composite filter 68 to produce a complex beamsum signal. The composite beamsumming signal is provided to the correlator processor 70 as shown in FIG. Correlator processor 70 includes a plurality of correlator processors 70a-70z. Each correlator processor receives a beam sum signal and a delayed received signal from delay elements 62a-62z.

The output of each correlator processor is a complex number known as a complex correlation sum. The phase of the complex correlation sum is proportional to the estimated time delay between each received signal and the beam sum signal.

The correlation sum from each correlator processor corresponds to the beamforming channel and the imaging scan line beam. Correlation sums from each correlator processor are provided to the correlation processor 74. The correlation sum can be expressed by the following equation,

식 (1)

Equation (1)

Where B ^* (r) represents the complex conjugate of the beam sum signal and s (r) is the channel signal. Both 'B' and 's' may be baseband signals or interpreted signals, or 'B' may be baseband or interpreted signals and 's' may be real signals. The sum is calculated over the correlation range samples (r1) to (r2).

The correlation processor may also receive the other two signals as inputs used to normalize the correlation. In one embodiment, the input signal is the squared magnitude of the beam sum signal and the squared magnitude of the channel signal. The squared magnitude of the beam sum signal is summed over the correlation range samples and represented by the following equation.

식 (2)

Equation (2)

Similarly, the square of the summed channel signal over the correlation range sample is expressed by the following equation.

식 (3)

Equation (3)

In another embodiment, the summed magnitude of the beamsum signals and the summed magnitude of the channel signals are provided to the correlation sum processor 74. The summed magnitude of the beam sum signal is expressed by the following equation.

식 (4)

Equation (4)

Similarly, the summed magnitude of the channel signal is expressed by the following equation.

식 (5)

Equation (5)

The correlation processor generates a beamforming time delay correction set using the aforementioned input signals for each beamforming channel and beam. Then, time delay correction is applied to the beam forming time delay.

3 is a flow chart illustrating one method by which the processor 74 generates beamforming delays. Each step of this method is described in detail below.

In step 78, the correlation processor calculates a normalized correlation sum for some or all of the beam forming channels and the imaging scan line beam. The normalized correlation sum 'C' is represented by equation (6).

식(6)

Equation (6)

Equation (6) is applied when both 'B' and 's' are baseband signals, and 'B' and 's' are both analysis signals. The size of 'C' is between 0 and 1. The size of 'C' is 1 when 'B' is proportional to 's'. When 'B' is a baseband or analysis signal or 's' is a real signal, the magnitude of 'C' is between the inverse of the square root of zero and two. In another embodiment, the normalized correlation sum 'C' is represented by equation (7).

식(7)

Equation (7)

Where 'N' is the number of range samples for which to calculate the sum. Equation (7) is an approximation to the normalized correlation sum calculated using Equation (6), which may be easier to calculate in digital hardware. Equation (6) can be converted to Equation (7) using the definition of the standard deviation for _N samples (x ₁ , x ₂ , ..., x _N ), which can be represented by Equation (8): .

식(8)

Equation (8)

Where μ represents the average of N samples (x _i ).

는 주어진 통계 분산에 대해 상수이다. x=|s|이면, 수식 (8)은 수식 (6)을 수식 (7)로 변환하고, 상수 요소는 무시하는데, 이것은 실제 신호 또는 복합 얼룩형 신호의 크기를 나타내는 통계적 분산에 대해 1의 차수이며, 여기서는 후술하는 상관합 처리에서 상관합의 상대적인 놈(norm)에만 관심이 있으므로 무시될 수 있다.Element in formula (8)

Is a constant for a given statistical variance. If x = | s |, Equation (8) converts Equation (6) to Equation (7), ignoring the constant element, which is an order of one for a statistical variance representing the magnitude of the actual signal or complex specular signal. Here, in the correlation processing described later, only the relative norm of the correlation is interested and can be ignored.

In step 80, the normalized correlation sum is mapped to orders by the beam and transducer elements. In many systems, the number of elements in the transducer array is greater than the number of beam forming channels. For example, a 1D linear and curved transducer array may have 192 elements, but is typically connected to an ultrasound system with 128 beamforming channels. The connection between the channel and the elements is through a set of programmable multiplexing switches that select a subset of the transducer elements for each beam direction.

The elements of a multirow transducer array with rows of 'nRow' and columns of 'nCol' have a row index of 'row = 0,1 through (nRow-1)' and 'col = 0,1 through (nCol)' It is labeled by the column index indicated by. Alternatively, elements of transducer array 18 are labeled by element number 'el' (el is 'col' + 'row' x 'nCol').

In step 82, the mapped correlation is modified to minimize the impact of the unreliable time delay estimate. An unreliable time delay estimate is defined as an estimate that can be identified as being inaccurate and abnormal noise. The correlation processor builds a mask labeled 'mask [el.bm]' that labels the correlation for each element and beam as reliable and unreliable. Identifying an element whose phase of correlation sum is an unreliable estimate of arrival time error improves the certainty and accuracy of the finally estimated time of arrival error. The manner in which correlations are labeled as reliable or unreliable will be described in detail below with reference to FIG. 4.

In step 82 of FIG. 3, all entries in the mask are initially set to zero, where zero is any value selected to identify a reliable correlation. For each unreliable correlation, 'mask [el, bm]' is set to 1, which is an arbitrary second value. In addition, if the number of unreliable elimons for a given beam exceeds a specified threshold, then all entries in the 'mask' for that beam are set to 1, as shown in detail in FIG. Finally, when each entry in the 'mask' is 1, the corresponding correlation is corrected by setting its phase to 0 without changing the amplitude as shown in equation (9).

식 (9)

Equation (9)

In Equation (9), C is a complex correlation sum to be corrected, and C 'is a correlation sum after correction.

In step 84, the real part and the imaginary part of the modified correlation sum are filtered. In one embodiment, a one dimensional real symmetric low pass filter applied over an element index and a separate one dimensional real symmetric low pass filter applied over a beam index are used. The length selection of these filters is such that the phase change of the correlation sum is reduced without excessively suppressing the spatial change of the phase of the correlation sum. In one embodiment, a filter with triangular coefficients is used. A triangular filter is, for example, a filter whose frequency response never becomes negative. For the stable operation of the time delay correction algorithm, the spatial frequency response of the filter across the element and beam indexes need not change sign. The algorithm acts as a system with negative feedback, correcting the beamforming time delay so that the arrival time error is zero. If the spatial frequency response of any of the filters changes sign, the feedback will switch from negative to positive with the sign change, and by positive feedback the algorithm amplifies the arrival time error rather than suppressing it to some spatial frequency. Let's do it. In one embodiment, the width of the triangular filter is 13 beams for the beam filter and 5 elements for the element filter.

In step 86, the phase of the filtered correlation sum is calculated and converted to time delay correction. The time delay correction is obtained by dividing the phase of the correlation sum by the factor 2πf, where 'f' is the nominal center frequency of the received ultrasound signal.

Filtering the complex correlation sum, not the arrival time error, yields an arrival time error if the magnitude of the arrival time error estimate corresponds to a phase change greater than ± π, that is, if the arrival time error is greater than ± 1 / (2f). This has the advantage of greatly improving the accuracy of the. In such a case, the phase of the correlation sum within the range of -π to + π is wrapped from a value close to + π to a value close to -π and vice versa, as shown in FIG.

4, 5 and 6 are graphs displaying each transducer element (“x axis” phase) in a transducer array having a respective correlation phase (“y axis” phase). The solid line is the phase of the ideal correlation that corresponds to the smoothly varying arrival time error greater than 1 / (2f) for some elements as seen in the region near 18E and 18M-18Q. 4 is the phase of the ideal correlation sum after a small amount of noise is added. 5 is a result of low pass filtering the phase of the noise correlation sum. As can be seen in FIG. 5, a poor approximation was obtained for the desired phase (solid line) near the region where the phase was wrapped. 6 illustrates the result of calculating the phase of the correlation sum after lowpass filtering the noise correlation sum. As can be seen in Figure 6, the agreement with the actual phase (solid line) is even better.

With continued reference to FIG. 3, in step 88, beam steering terms relating to altitude and azimuth directions from the time delay compensation are estimated and then removed. Adjustment items may be removed to minimize the geographic distortion of the image, leading to misdiagnosis, for example when the size of some object in the image is important. The constraint that the average time delay correction should be zero to move the calibrated beam to the minimum within the range, or minimize the shift in the depth of focus of the transmitted beam and minimize the imposition of focus shift onto the dynamically focused receive beam. Other constraints may be imposed on the time delay, such as the constraint that no parabolic item should be present in the time delay correction.

In step 90, time delay correction is mapped from element to channel order. Time delay correction is provided to the beamforming delays 62A-62Z as shown in FIG. In one embodiment, time delay correction is applied to each acoustic frame.

As described in step 82 of FIG. 3, the calibration sum processor is configured to label each calibration sum as reliable or unreliable. 7 is a flow chart illustrating a manner in which the calibration sum processor determines the reliability of each calibration sum.

It is assumed that the source of arrival time error, that is, the aberrating layer, is changing smoothly in space. Therefore, the actual time delay correction (proportional to the observed calibration phase's phase) changes slowly across the transducer for a given beam and is assumed to change slowly along the beam direction for a given element in the transducer. Conversely, an unreliable element is an element whose phase is not slowly changing across the transducer or an element that is not slowly changing along the beam direction for a given transducer element. For two-dimensional transducers subdivided into elements spanning the height and azimuth dimensions of the transducer, the flatness of the phase will generally be estimated throughout both transducer dimensions in addition to the beam direction. In some situations, it may be appropriate to estimate the flatness of the phase for only one of the transducer dimensions. This has the advantage of reducing the complexity of the hardware or software that calculates the filters and derivatives described below. Next, the phrase “element direction” should be understood to mean either a single transducer dimension in a simplified embodiment or two dimensions in a general embodiment.

In step 92, the phase of the calibration sum is calculated. Approximate values for the derivatives of the phase in the element direction and the derivatives in the beam direction are calculated. In one embodiment, the approximation is a discrete derivative using the closest neighbor difference. In step 94, the discrete derivatives of the phases throughout the element index are calculated. In step 96, the discrete derivatives of the phases throughout the beam index are calculated. In step 98, the absolute value of the discrete derivative of the phase in the element direction and the absolute value of the discrete derivative of the phase in the beam direction are summed for each element and beam.

In step 100, the sum of the absolute values of the discrete derivatives is flattened by lowpass filtering in the beam direction. In step 102, the sum of the absolute values of the derivatives is flattened by lowpass filtering in the element direction. Filtering is performed to reduce the variance introduced by the processing to do the derivative which tends to magnify the noise. In one embodiment, pairs of adjacent values are added. The output of the two-point lowpass filter can be arranged to compensate for the half sample shift that the two-point discrete derivative produces.

In step 104, the filtered sum of the phase derivatives is compared with a user-specified first threshold. In one embodiment, the first threshold is about 5 radians. For each entry of the filtered sum greater than the first threshold, the corresponding entry in the mask is set to 1, indicating that correlation value is unreliable for that element and beam.

Similarly, the correlation sum for all elements in a given beam direction is marked unreliable if the correlation sum above the second threshold of correlation sum for the beam direction is marked unreliable in the previous step. The second threshold is also specified by the user. In one embodiment, the second threshold is one half of the number of beamforming channels. The identification beam, in which a significant number of estimated time delay corrections are unreliable, prevents the creation of artifacts when the image contains acoustic shadows due to obstructing ribs or due to insufficient contact between the transducer and the imaging object. The transmitted and received beams are actually degraded in this situation. The correlation sum of all elements for these beams is unreliable because the reference signal is a degraded or distorted beamsum signal. If most elements in the active opening are identified as having an unreliable correlation, it is observed that applying the remaining correlation as a correction results in image degradation instead of image enhancement.

Beamforming arrival time errors can also be marked as unreliable using an image processing algorithm. In one embodiment, the image is processed to detect "signatures" associated with defective calibration estimates. As used herein, the term “signature” refers to any discernible feature in the picture that may be associated with the reliability or accuracy of the associated beamforming time delay estimate. Such "signatures" can take many forms, such as local statistical parameters, tissue characteristics, or positions on anatomical structures.

8 is a flow diagram illustrating one method in which image data is used to identify an unreliable beamforming arrival time error. The illustrated method uses local statistics to estimate the reliability of the time delay correction. In step 105, statistical parameters for the region of interest in the image are estimated using beam sum or image data. Statistical parameters may include mean luminance, standard deviation, higher order moments, parameters related to the shape of the distribution, and figures of merit that quantitatively indicate how well a particular statistical distribution represents actual data, but here It is not limited to.

In step 106, the statistical parameters are filtered over the image or over time to improve the estimation of the statistical parameters. Alternatively, statistical parameters can be estimated over a larger area. The size or amount of filtering of the estimated area is calculated based on the need to locally limit the parameter and the resulting quality of the estimated value. In step 107, the filtered statistical parameters are used to indicate areas that do not have a set of statistical characteristics that contribute to estimating the time delay correction. In step 108, the indicated area is not used in the time delay estimation. In step 109, a check is made to see if there are no small isolated areas for which estimates have been rejected or maintained. If such regions exist, these regions are removed by local interpolation to avoid the creation of new artifacts. Indications of these areas may be incorporated into the mask described in step 104 of FIG.

It is known to those skilled in the art that certain time-delay estimation algorithms work best in the region of fully developed speckles. The magnitude of the complex signal from the speckle region has a statistical distribution that is well known as the Rayleigh distribution. It can be determined whether the signal magnitude set is well described by the Rayleigh distribution. As used herein, a region of interest (ROI) refers to a set of beam and range samples for which the correlation sum is calculated. If the signal amplitude in the ROI is not well described by the Rayleigh distribution, the ROI may be removed or the magnitude of the ROI may change until the statistical distribution is close to the Rayleigh distribution. If no region containing an approximate Rayleigh distribution of signal amplitudes is found for a small set of adjacent beams, then the arrival time error estimate can be used to interpolate the arrival time error estimate for that set. In addition to the size and position of the ROI, other parameters used for estimating the arrival time, for example phase induction thresholds, may be adjusted.

As described above, tissue types can be identified using various tissue characterization techniques. If a given tissue type has a characteristic that contributes to the estimation of the arrival time error, the region of interest that estimates the arrival time error may move to that region based on statistical information from the image. The liver is an example of this type of tissue because the liver usually contains large areas of speckle shaped scatterers. Likewise, if there are certain types of tissues that are known to be inappropriate for estimating arrival time errors, there is no need to detect such tissues in the image and apply a corresponding incorrect arrival time error estimate. The diaphragm is an example of a tissue type that is not speckle shaped and is therefore not appropriate for many methods of estimating arrival time errors.

Blood vessels or other anechoic regions are examples where distinct signatures exist in the image. 9 is an image region showing a region of interest and blood vessels. The size of the vessel 122 is such that all or most of the ROI 120 is inside the vessel. The echo from the blood is much smaller than the echo to the surrounding tissue 124. As a result, the acoustic energy from its ROI, which provides an estimate for the arrival time error, is relatively small. The surrounding tissue reflects the energy of the sidelobes of the transmitted beam and the resulting signal is greater than the signal reflected from the anechoic blood. Thus, the surrounding tissue acts as a “bright” target off-axis, and the estimate for time delay correction tends to direct the corrected beam towards the edge of the vessel or to grow side lobes of the transmitted and received beams. In this case, the image is used to determine if the ROI is located within the vessel or anechoic region. By using this information from the image, the algorithm can be configured to avoid artifacts caused by the growth of the sidelobe or adjustment towards the edge of the vessel.

10 is a flow diagram illustrating an algorithm for detecting whether a region of interest is located near or in a vessel. In step 110, the average signal amplitude 'Ai' is calculated over the region of interest for each beam. The average amplitude can be the average of log-compressed amplitude data or the average of linear amplitude data. The average can be calculated from the scan-converted data or from the raw data prior to the scan conversion. In step 111, the region average 'Li' across the 'M' beams may be calculated for each of the 'N' beams in the image. The mean of the region may be a simple mean of the 'Ai' values or may be a spatially weighted mean. The length of the average area 'M' may be a function of the image data or may be fixed or selected by the user.

In step 112, the region of interest for a given beam is considered to be the interior of the blood vessel if 'Ai' is at least a certain threshold less than Li. Such beams are labeled "dark" because noneochoic regions are typically displayed using black pixels. The threshold is set so that the beam placed in the center of the vessel is labeled "dark". However, on its own, such an implementation is often not labeled as "dark" in the part of the beam that is in the vessel but close to the edge, which results in a sharp transition from bright to dark in imaging, particularly in inaccurate imaging. Because you may not lose. In order to avoid artifacts in such a situation, neighboring beams near the labeled area are also labeled. In step 113, the 'n' beams before and after the identified "dark" beam are also labeled "dark". 'n' is chosen as a compromise between reducing the accuracy of calibration in areas that do not have an incorrect time of arrival estimate and avoiding artifacts caused by incorrect estimated time of arrival errors. In step 114, a beam labeled 'dark' is incorporated into a mask that labels the correlation as reliable or unreliable as described in step 104 of FIG.

Another example of a feature of imaging that tends to produce inaccurate arrival time error estimates is a very shiny target, a strong reflector of sound. 11 shows a luminous scatterer 126 located just outside of the region of interest 128. One example of a luminous scatterer is the diaphragm, which is typically much luminous than the surrounding tissue. When the luminous scatterer is located just outside of the region of interest, the sound waves combined with the sidelobes of the transmitted beam reflect the luminous scatterer and contribute significantly to the signal received from the region of interest. The sidelob sound wave energy 132 reflected from the shining target 126 is almost as large as the sound 130 reflected from the tissue in the central beam path 128. In some cases, the sidelobe energy is greater than the sound reflected from the region of interest.

In one embodiment, when a shining target is detected in imaging, the estimate of an aberrator near the shining scatterer is ignored and a correction for this area is inserted from the ambient flawless measurement. In another embodiment, the size of the region of interest increases within the range to reduce the effect of shiny scatterers.

An imaging processing algorithm can be used to detect discontinuities in imaging that appear after calibration is applied. Similarly, imaging processing algorithms can detect areas of imaging with improved brightness, sharp borders, and better statistical distribution and label such corrections as more accurate. By quantifying the reliability of the measurements in each region, weighted filters for correlation sums across each region can be used, which reduces the contribution of unreliable correlation sums to reliable correlation sums.

Imaging can be used to determine and quantify the movement of tissue relative to the transducer. As the transducer or tissue moves, the pattern of arrival time error also shifts across the transducer. If there is a large time interval between the measurement of the arrival time error pattern and the correction of the beamforming time delay, this shift will result in beamforming time delay correction being incorrectly applied to the beamforming channel, resulting in less than optimal improvement in imaging. . By measuring the arrival time error pattern shift, the shift can be compensated.

The present invention described above has several advantages, including improved accuracy obtained by normalizing the correlation before filtering. The amplitude of the complex correlation is a measure of the similarity between the two correlated signals. Normalizing the correlation before the filtering step places more weight on the reliable correlation compared to the unreliable correlation. For example, without normalization, a very loud but noisy element contributes more to an element-filtered correlation than a neighboring element with a small signal but a reliable signal. . Similarly, significantly bright but noisy beams contribute more to the filtered correlation than the neighboring beams whose time delay measurements are reliable.

The method minimizes the contribution of a "dead" or very noisy transducer element by normalizing the correlation, because the noisy channel signal has little correlation with the beamsum signal. Therefore, the additional processing or hardware required to detect and compensate for dead or noisy transducer elements and dead or noisy system channels is minimized.

The method described above also uses a simple noise-mixed phase meter to strongly ignore the region of the noise-mixed phase, minimizing the introduction of imaging artifacts without the need for complicated and expensive hardware for computation. By way of example, FIG. 12 shows a data mask that fails to adequately identify two regions in a noisy phase measurement and two elements in a noisy region between element 18I and element 18Q, where a data mask of zero is shown. Indicates that the measurement is reliable. FIG. 13 shows the result of replacing the correlation sum for an element marked as untrusted, i.e., with a data mask of 1 with the amplitude of the correlation sum and subsequently filtering the correlation sum. Although two of the noisy correlation sums have not been modified, the phase of the filtered and modified correlation sums is close to zero over the entire noise region as desired.

The present invention can be located between a beam that can be measured with a smoothly reliable arrival time and an unreliable beam so that unstable boundaries or discontinuities on the imaging between such areas can be avoided. 14 shows the correlation phase for one element as a function of beam number. The beams near 18I to 18Q are marked as unreliable in this embodiment, and the phase of the correlation sum is set to 0 to maintain the amplitude. The phase after filtering is shown in FIG. 15. The phase drops smoothly to zero near the left boundary of the noise region and increases smoothly near the right boundary of the noise region. The same smooth interpolation occurs in all elements for these beams, so that the shift in imaging between the corrected and uncorrected beams is smoothed.

Although only certain features of the invention are illustrated and described herein, many modifications and variations can be made by those skilled in the art. Accordingly, it should be understood that all modifications and changes belonging to the true spirit of the present invention are intended to be covered by the appended claims.

Claims (10)

A method of correcting beamforming time delays in an ultrasound system 10, Transmitting an ultrasonic energy beam through an object using an array of transducer elements 18A-18Z, each transducer element configured to transmit the ultrasonic energy beam with a transmission beamforming time delay. Wow, Receiving a plurality of echo signals, each transducer element configured to receive the ultrasonic energy beam with a receive beamforming time delay; Estimating a beamforming time delay error for each echo signal and each imaging direction; Correcting the transmit beamforming time delay and the receive beamforming time delay by generating a complex correlation sum representing a time delay between the sum of one or more received echo signals and each received echo signal; Generating an ultrasound image of the object using the calibrated transmit beamforming time delay and the calibrated receive beamforming time delay; The correcting step, Calculating a normalized correlation sum; Mapping the normalized correlation sum to an imaging direction and a transducer element direction; Modifying the mapped correlation sum; Filtering the modified correlation sum for an imaging direction and a transmitting element to produce a filtered correlation sum; Converting the phase of the filtered correlation sum into a corresponding time delay correction; Correcting the beamforming time delay using the time delay correction value; Beamforming time delay correction method. delete The method of claim 1, Wherein the modifying comprises: Labeling each mapped correlation as a reliable correlation sum or an unreliable correlation sum; Setting each correlation labeled as an unreliable correlation sum to an absolute value Beamforming time delay correction method. The method of claim 3, The labeling step, Calculating a phase of each mapped correlation sum corresponding to each transducer element and the imaging direction; Determining a continuously varying pattern in the calculated phase for each transducer element and imaging direction, wherein The mapped correlation sum is labeled as reliable if the continuously changing pattern is present. Beamforming time delay correction method. The method of claim 3, The labeling step, Comparing the sum of the absolute values of the derivatives of the phases in the element direction and the absolute values of the derivatives of the phases in the imaging direction with a first threshold value; Labeling the mapped correlation as unreliable if the calculated sum of the absolute values of the phase derivatives exceeds the first threshold; Labeling all of the mapped correlations as unreliable for the corresponding imaging direction if the amount of unreliable correlation in each imaging direction exceeds the second threshold. Beamforming time delay correction method. The method of claim 3, The labeling includes identifying signatures in the ultrasound image using an image processing algorithm and labeling the beamforming time delay error as reliable or unreliable based on the identified signature. Beamforming time delay correction method. The method according to claim 6, The image processing algorithm, Identify a plurality of regions of interest within the ultrasound image, Calculate statistical parameters for each identified region of interest, Apply the statistical parameter to label the estimated beamforming time delay error as reliable or unreliable. Beamforming time delay correction method. An ultrasound system 10 for estimating beamforming time delay, Transducer array 18 having a set of array elements arranged in a predetermined pattern, each of which transmits an ultrasonic energy beam into an object during a transmission mode and responds to vibrational energy that affects the transducer array during a reception mode. Individually to generate an echo signal A transmitter coupled to the transducer array and operative to apply a separate transmit signal pulse to each of the array elements with a respective transmit beamforming time delay during the transmit mode to produce a directed transmit beam With 22, Coupled to the transducer array, sampling the echo signal generated by each of the array elements during the receive mode and imposing a receive beamforming time delay on each sample of the echo signal to generate a corresponding plurality of received signals. A receiver 24 operable to operate, A beamformer system 26 configured to estimate the arrival time error for each imaging direction and each echo signal and correct the transmit beamforming time delay and the receive beamforming time delay; An image processor 32 configured to generate an ultrasound image, The beamformer system 26, Generate a correlation for each system beamforming channel and imaging direction, Calculate the normalized correlation sum, Map the normalized correlation to the imaging direction and transducer element order, Modify the mapped correlation sum, Filtering the modified correlation sum to generate a filtered correlation sum, Calculate a phase of the filtered correlation sum, Convert the phase of the filtered correlation sum into a calibration time delay, Configure the transmit beamforming time delay and the receive beamforming time delay using the calibration time delay. Ultrasound system. delete 9. The method of claim 8, The beamformer system is configured to label each mapped correlation sum with a reliable or unreliable correlation sum. Ultrasound system.

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