Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1998-12-24
2001-01-23
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06176828
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to ultrasound power Doppler imaging of fluid flow fields. In particular, the invention relates to methods and apparatus for imaging blood flowing in the human body by detecting the power in the ultrasonic echoes reflected from the flowing blood.
BACKGROUND OF THE INVENTION
Ultrasonic scanners for detecting blood flow based on the Doppler effect are well known. Such systems operate by actuating an ultrasonic transducer array to transmit ultrasonic waves into the object and receiving ultrasonic echoes backscattered from the object. In the measurement of blood flow characteristics, returning ultrasonic waves are compared to a frequency reference to determine the frequency shift imparted to the returning waves by flowing scatterers such as blood cells. This frequency, i.e., phase, shift translates into the velocity of the blood flow. The blood velocity is calculated by measuring the phase shift from firing to firing at a specific range gate.
The change or shift in backscattered frequency increases when blood flows toward the transducer and decreases when blood flows away from the transducer. Color flow images are produced by superimposing a color image of the velocity of moving material, such as blood, over a black and white anatomical B-mode image. Typically, the color flow mode displays hundreds of adjacent sample volumes simultaneously, all laid over a B-mode image and color-coded to represent each sample volume's velocity. The power Doppler mode also displays sample volumes laid over a B-mode image, but the displayed sample volumes are color-coded to represent the power or energy of the signals reflected from each sample volume.
In standard color flow processing, a high pass filter known as a wall filter is applied to the data before a color flow estimate is made. The purpose of this filter is to remove signal components produced by tissue surrounding the blood flow of interest. If these signal components are not removed, the resulting velocity or power estimate will be a combination of the velocity or power of the signal returned from the blood flow and the velocity or power of the signal returned from the surrounding tissue. The backscatter component from tissue is many times larger than that from blood, so the parameter estimate will most likely be more representative of the tissue, rather than the blood flow. In order to get the flow velocity or power, the tissue signal must be filtered out.
In the color flow mode of a conventional ultrasound imaging system, an ultrasound transducer array is activated to transmit a series of multi-cycle (typically 4-8 cycles) tone bursts which are focused at the same transmit focal position with the same transmit characteristics. These tone bursts are fired at a pulse repetition frequency (PRF). The PRF is typically in the kilohertz range. A series of transmit firings focused at the same transmit focal position are referred to as a “packet”. Each transmit beam propagates through the object being scanned and is reflected by ultrasound scatterers such as blood cells. The returned signals are detected by the elements of the transducer array and then formed into a receive beam by a beamformer.
For example, the traditional color firing sequence is a series of firings (e.g., tone bursts) along the same position, which firings produce the respective receive signals:
F
1
F
2
F
3
F
4
. . . F
M
where F
i
is the receive signal for the i-th firing and M is the number of firings in a packet. These receive signals are loaded into a corner turner memory, and a high pass filter (wall filter) is applied to each down range position across firings, i.e., in “slow time”. In the simplest case of a (1, −1) wall filter, each range point will be filtered to produce the respective difference signals:
(F
1
−F
2
)(F
2
−F
3
)(F
3
−F
4
) . . . (F
M−1
−F
M
)
and these differences are input to a color flow power estimator.
Power Doppler imaging maps the power or energy in blood flow over a two-dimensional image. The transfer function of the resultant displayed power Doppler image is the product of the transfer function of the underlying compression curve and a mapping function. State-of-the-art ultrasound systems typically give the user a series of mapping functions from which to choose. These mapping functions provide for an arbitrary selection of displayed colors, but do not allow the user or the system to optimize the image information based either on user settings or imaging data itself.
SUMMARY OF THE INVENTION
The present invention is a method and an apparatus for color mapping of flow power data in which the flow states containing information of most interest to the user are enhanced while flow states not containing information of interest are suppressed. This is accomplished using a color mapping having an optimal form. In accordance with that optimal form, the color mapping function comprises three segments which are connected at upper and lower knee points, which are settable either automatically or via operator inputs. The color intensity values of the first, second and third mapping segments are applied to the power Doppler imaging data in low, middle and high ranges respectively. The lower knee point is positioned based on the system noise floor of the power Doppler imaging data and the upper knee point is positioned x units higher than the lower knee point along the power Doppler imaging data axis of the mapping function, where x is defined as the useful flow dynamic range and signal-to-noise ratio.
In accordance with one preferred embodiment, the upper and lower knee points can be adaptively determined from the power Doppler imaging data available. In particular, this function can be performed automatically by the host computer. For example, the host computer can perform an image analysis algorithm on one or more frames of power Doppler imaging data retrieved from cine memory, determine the optimal location of the upper and lower knee points based on the analyzed power Doppler imaging data, construct an optimal color mapping function having the optimally located knee points, and then load the optimal color mapping function into the video processor. In particular, the positions of the knee points can be determined as a function of the signal-to-noise ratio or the dynamic range, which can be estimated from the power Doppler imaging data during image analysis.
Alternatively, the host computer can be programmed to adaptively determine the positions of the knee points of the color mapping function based on user-selected inputs, e.g., the system color flow gain setting or the dynamic range setting.
In accordance with a further preferred embodiment, the position(s) of one or both knee points of the color mapping function can be moved by the user through the operator interface. In one mode, the user can move the positions of the upper and lower knee points in tandem. This will allow the user to shift the entire curve up or down to trade off between display states used for low versus high flow states. In another mode, the user can move position of the lower knee point relative to the position of the upper knee point. This change in the position of the lower knee point changes the slope of the curve in the useful flow dynamic range area (x) and the effective dynamic range in the display space. In response to operator inputs changing the position(s) of one or both knee points, the host computer constructs a color mapping function having the knee point positions requested by the user and loads that color mapping into the video processor.
REFERENCES:
patent: 5471990 (1995-12-01), Thirsk
patent: 5882306 (1999-03-01), Ramamurthy et al.
patent: 5961460 (1999-10-01), Guracar et al.
Becker David D.
Seyed-Bolorforosh Mir Said
Washburn Michael Joseph
Cabou Christian G.
Flaherty Dennis M.
General Electric Company
Lateef Marvin M.
Patel Maulin
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