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 a body and receiving ultrasonic echoes backscattered from the body. 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 shift, 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, 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.
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 estimate will be a combination of the velocities from the blood flow and the surrounding, non-flowing tissue. The backscatter component from non-flowing tissue is many times larger than that from blood, so the velocity estimate will most likely be more representative of the non-flowing tissue, rather than the blood flow. In order to obtain the flow velocity, the non-flowing tissue signal must be filtered out.
In a conventional ultrasound imaging system operating in the color flow mode, an ultrasound transducer array is activated to transmit a series of multi-cycle (typically 4-8 cycles) tone bursts which are focused at a common transmit focal position with common transmit characteristics. These tone bursts are fired at a pulse repetition frequency (PRF) that is typically in the kilohertz range. A series of transmit firings focused at a common transmit focal position with common transmit characteristics is 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 return signals are detected by the elements of the transducer array and 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 a common position, producing the respective receive signals: EQU F.sub.1 F.sub.2 F.sub.3 F.sub.4 . . . F.sub.N
where F.sub.i is the receive signal for the i-th firing and N 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 downrange position across firings, i.e., in "slow time". In the simplest case of a (1, -1) wall filter, each range point is filtered to produce the respective difference signals: EQU (F.sub.1 -F.sub.2) (F.sub.2 -F.sub.3) (F.sub.3 -F.sub.4) . . . (FN.sub.N-1 -F.sub.N)
and these differences are supplied to a color flow velocity estimator.
One of the advantages of Doppler ultrasound is that it can provide noninvasive and quantitative measurements of blood flow in vessels. Given the angle .theta. between the insonifying beam and the flow axis, the magnitude of the velocity vector can be determined by the standard Doppler equation: EQU .nu.=c.function..sub.d /(2.function..sub.0 cos .theta.) (1)
where c is the speed of sound in blood, .function..sub.0 is the transmit frequency and .function..sub.d is the motion-induced Doppler frequency shift in the backscattered ultrasound.
A conventional ultrasound imaging system collects B-mode or color flow mode images in a cine memory on a continuous basis. The cine memory provides resident digital image storage for single image review and multiple image loop review, and various control functions. The region of interest displayed during single-image cine replay is that used during the image acquisition.
If an ultrasound probe is swept over an area of interest, two-dimensional images may be accumulated to form a three-dimensional data volume. The data in this volume may be manipulated in a number of ways, including volume rendering and surface projections. In particular, three-dimensional images of power and velocity data have been formed by projecting the maximum pixel values onto an imaging plane. In this process, noise is introduced due to inherent error in estimating the velocity of power level because of a multiplicity of small scatterers present in most body fluids (e.g., red cells in blood). Noise present in the velocity and power signals causes errors in selection of the maximum value and affects uniformity within the projected image and sharpness of edges. Thus, there is need for a method for improving the uniformity and edge sharpness of three-dimensional images of velocity and power data.
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