Method of ultrasonic speckle reduction using wide-band frequency compounding with tissue-generated harmonics

The present invention relates to ultrasound imaging, and more particularly, to a method and apparatus for enhancing ultrasound images by reducing speckle.

Ultrasound imaging is an attractive modality for numerous diagnostic procedures because of its non-invasive nature, relatively low cost, and lack of radiation exposure. Medical ultrasound images are typically produced by generating an ultrasonic sound wave traveling in a known direction, known as a scan line or a vector, and observing the echoes created when the sound wave is scattered or bounces off of the boundaries between regions of differing density in the body. For any given direction of the ultrasound beam, the image pixels are generated by plotting a dot whose brightness is proportional to the echoes' amplitude at a coordinate whose location is a function of the time after a short ultrasound pulse is sent in the direction of the scan line being measured.

When forming images with coherent radiation, the desired distribution of image energy is subject to undesirable random modulation. This random distribution of energy is known as “speckle” and is manifested in visual images as flecks of random intensity and size distributed across the image. Speckle arises from constructive and destructive interference due to random phase cancellations and additions of the coherent field which is scattered by the coherently illuminated object. The power spectrum of speckle depends upon the spectrum of the coherent signal carrier, the texture or spatial distribution of scatterers in the field, the size of the irradiated object volume, and the transfer function of the receiving and imaging system.

The speckle caused by random tissue scattering and ultrasonic images may overshadow delicate tissue structures and degrade the image contrast. Frequency compounding is a well-known way to reduce speckles and hence enhance contrast resolution. In frequency compounding, images with different frequency characteristics are summed incoherently. In the experience of these inventors, existing frequency compounding methods for speckle reduction suffer from resolution degradation due to summation of detected narrow-band signals. These narrow-band signals are typically obtained by narrow-band filtering on receipt. What is needed is a high-resolution compounding method and apparatus.

Broadly, it is the object of the present invention to provide an improved method and apparatus for ultrasound imaging.

It is another object of the present invention to provide an improved ultrasonic imaging method and apparatus that can distinguish tissues while reducing the amount of speckle that overshadows delicate structures and degrades the image contrast. It is yet another object of the present invention to provide such a method and apparatus while using wide-band frequency compounding with tissue-generated harmonics. These and other objects of the present invention will become apparent to those skilled in the art from the follow detailed description of the invention and the accompanying drawings.

The present invention is a method and apparatus for smoothing the speckle pattern and increasing contrast resolution in ultrasound images. Compared to other frequency compounding techniques, wide-band harmonic frequency compounding reduces speckle noise without sacrificing the resolution. Compared to spatial compounding, wide-band harmonic frequency compounding is more robust against tissue motion because sequential vectors rather than frames are summed together for compounding. The method and apparatus of the present invention is implemented by transmitting two or more firings, combining two or more of the firings coherently to extract the tissue-generated harmonic components, detecting the outputs of the coherent sums and detecting one or more firings before coherent sum, and finally combining all detected outputs to form the compounded image.

The method and apparatus of the present invention sums wide-band fundamental and wide-band harmonic images after detection to form a compounded image. Unlike other frequency compounding methods, both transmit and receive signals are wide-band and no narrow-band filters are necessary. Multiple firings with two or more different transmit waveforms are transmitted to each focal zone.

FIG. 1 is a block diagram of an ultrasound imaging system according to the present invention.

FIG. 2 is a schematic diagram illustrating the two-firing configuration in the method of the present invention.

FIG. 3 is a schematic diagram illustrating the three-firing configuration in the method of the present invention

FIG. 4 is a spectrum diagram illustrating summation under the two-firing configuration shown in FIG. 2 .

FIG. 5 is a spectrum diagram illustrating summation under the three-firing configuration shown in FIG. 3 .

In general, an ultrasound beam is generated by using an array of transducers to form a transmit beam in a given direction with respect to the array. The ultrasound pulse is an amplitude modulated pulse at a predetermined frequency, having an amplitude specified by an envelope function. The echoes generated by this pulse are detected by the same, or a different, array of transducers that are used to form a receive beam corresponding to the transmit beam. The directional sensitivity of the microphone, or ultrasonic probe, reduces echoes generated by multiple reflections of the sound pulse.

The ultrasound imaging is generally accomplished by a system that utilizes certain essential components. Referring now to FIG. 1, which is a block diagram of an ultrasound imaging system according to the method of the present invention, the ultrasound imaging system is generally identified 10 . The system 10 includes an ultrasonic probe 12 , a beamformer 14 , a coherent vector summation means 16 , a detection means 18 , an incoherent vector summation means 20 , a post-processing means 22 and a scan converter and display means 24 . In general, the ultrasonic probe 12 is aligned with the object to be scanned and the operator uses the scan converter and display means 24 to visualize and observe the scan results as detected by the probe 12 .

This invention sums wide-band fundamental and wide-band harmonic images after detection to form a compounded image. Unlike other frequency compounding methods, both transmit and receive signals are wide-band and no narrow-band filters are necessary. Multiple firings with two or more different transmit waveforms are transmitted to each focal zone.

The method and apparatus of the present invention teaches compounding of wide-band images with different speckle patterns to suppress speckle without resolution loss. This is implemented by transmitting two or more firings, as shown by FIGS. 2 and 3, combining two or more of the firings coherently to extract the tissue-generated harmonic component, detecting output of the coherent sum and detecting one or more firings before coherent sum, and finally combining all detected outputs to form the compounded image. See also FIGS. 4 and 5.

Referring now to FIGS. 2 through 5, it will be seen that the transmit waveforms 62 , 64 , 110 generated by the beamformers 36 , 38 , 88 , respectively, are characterized by a peak power level near a fundamental frequency f o . The transmit waveforms 112 , 114 generated by the beamformers 90 , 92 , respectively, are characterized by a peak power level near another fundamental frequency f 1 . The ultrasound echo information generated by the beamformers includes information at the fundamental frequency and information generated non-linearly at the harmonic frequency. The echo information received by the beamformers is not filtered as in other prior methods to isolate the harmonic frequency information by substantially removing information at transmitted frequencies. In one embodiment, a wide-band harmonic image at 2f o is incoherently summed with a wide-band fundamental image at f o and 2f o to form the final compounded image. There may be two firings or three firings for each focal zone.

Referring now to FIGS. 2 and 4, a two-firing configuration, generally identified 30 , is illustrated in schematic form. In the two-firing configuration 30 , the wide-band transmit waveforms 62 , 64 of the two-firings 32 , 34 at frequency f o have the same amplitude but opposite phase. In other words, that situation would be represented by c(t)=−b(t). The received signals 66 of one of the firings 62 is detected 48 to form a wide-band fundamental vector, also at f o . The received signals 66 , 68 of the two-firings 32 , 34 are summed 46 with equal weighting, i.e. Gain 2 =Gain 3 42 , 44 , and are detected 50 to form a wide-band harmonic vector 70 at 2f o . The two vectors 66 , 70 are then summed 52 to form the compounded image 72 . The weighting Gain 1 40 is set to a certain value so that the speckle of the fundamental image and the harmonic image has approximately the same brightness.

In the three-firing setting, generally identified 80 , as shown in FIGS. 3 and 5, the first firing 82 with transmit waveform a(t) 110 is used to form a wide-band fundamental vector 116 . The other two firings 84 , 86 with transmit waveforms b(t) 112 and c(t) 114 with the same amplitude and opposite phase are combined 100 to form a wide-band harmonic vector 122 . The transmit waveform a(t) 110 is different from b(t) 112 and c(t) 114 . The two vectors 116 , 122 are then summed 106 to form the compounded image 124 . The gain setting 94 , 96 , 98 in this configuration 80 is the same as that in the two-firing case 30 . The two-firing configuration 30 gives higher frame rate while the three-firing configuration 80 ensures optimized quality for both fundamental and harmonic images.

In another embodiment, the wide-band harmonic image is obtained by firing two wide-band waveforms with the same phase but different amplitudes. The two-firings are then coherently summed with Gain 3 =−Gain 2/β to cancel the fundamental and extract the harmonic component. The same two-firing or three-firing settings stated above may be used in this embodiment.

In yet another embodiment, detection and log compression are performed prior to the summation of fundamental and harmonic vectors. This is equivalent to multiplying the detected vectors together for compounding.

It should also be understood that the foregoing embodiments could be extended to other multiple firing and wide-band compounding combinations that can be generally expressed as follows. First, two or more ultrasound firings would be made. These firings would be divided into several groups. In this way, each group can share firings with other groups. For groups with more than one firing, the firings would be coherently combined to form tissue-generated harmonic components. The harmonic components can include sub-harmonic, ultra-harmonics, 2 nd order harmonic, and higher order harmonic. The number of groups with more than one firing could be one or more. The number of groups with one firing could also be one or more. For each group, the output of the coherent sum is detected in the case of two or more firings and the firing is directly detected in the case of one firing. All detected outputs are combined to form the compounded image as described above.

Accordingly, it will be seen that the present invention is a method and apparatus for smoothing the speckle pattern and increasing contrast resolution in ultrasound images. Compared to other frequency compounding techniques, wide-band harmonic frequency compounding reduces speckle noise without sacrificing resolution. Compared to spatial compounding, wide-band harmonic frequency compounding is more robust against tissue motion because sequential vectors rather than frames are summed together for compounding. The method and apparatus of the present invention is implemented by transmitting two or more firings, combining two or more of the firings coherently to extract the tissue-generated harmonic component, detecting the output of the coherent sum and detecting one or more firings before coherent sum, and finally combining all detected outputs to form the compounded image.

The method and apparatus of the present invention sums wide-band fundamental and wide-band harmonic images after detection to form a compounded image. Unlike other frequency compounding methods, both transmit and receive signals are wide-band and no narrow-band filters are necessary. Multiple firings with two or more different transmit waveforms are transmitted to each focal zone. It is intended that the foregoing be regarded as illustrative of the method of the present invention, rather than limiting. It is the following claims that are intended to define the scope of this invention.

Parts List

10 ultrasound imaging system

12 ultrasonic probe

14 beamformer

16 coherent vector summation means

18 detection means

20 incoherent vector summation means

22 post-processing means

24 scan converter and display means

30 two-firing configuration

32 first firing

34 second firing

36 beamformer

38 beamformer

40 gain 1

42 gain 2

44 gain 3

46 summation

48 detector

50 detection

52 summation

54 rest of b-mode

62 transmit waveform generated by 36

64 transmit waveform generated by 38

66 received signal

68 received signal

70 vector

72 compounded image

80 three-firing configuration

82 first firing

84 second firing

86 third firing

88 beamformer

90 beamformer

92 beamformer

94 gain 1

96 gain 2

98 gain 3

100 summation

102 detection

104 detection

106 summation

108 rest of b-mode

110 transmit waveform generated by 88

112 transmit waveform generated by 90

114 transmit waveform generated by 92

116 wide-bond fundamental vector

118 receive vector

120 receive vector

122 wide-band harmonic vector

124 compounded image