Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1998-12-24
2001-04-10
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06213946
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to ultrasound imaging systems and, more particularly, to methods and apparatus for producing high quality medical ultrasound images using orthogonal pulse sequences.
BACKGROUND OF THE INVENTION
Ultrasound imaging is widely used in medical applications to non-invasively observe structures within the human body, such as cardiac structures, the vascular system, the fetus, the uterus, the abdominal organs and the eye. In a typical imaging system, short bursts of ultrasound energy are directed into a patient's body with a handheld transducer. The returning reflected energy is received by the same transducer. The signals representing the reflected energy are processed and formatted into a video image of the target region. Phased array scanning techniques are commonly used.
In real-time diagnostic medical ultrasound systems, two-dimensional image quality plays a key role in providing diagnostically significant information for accurate patient diagnosis. However, due to the physical properties governing the generation of an image based on acoustic propagation and reflection of ultrasound energy, state of the art images are affected by various artifacts and degradations. Specifically, clutter and speckle artifacts are observed in the images. Clutter appears as a more or less stationary area of cloudiness formed by small areas of excess, undesired ultrasound energy. Speckle is characterized by small grainy areas where, due to the coherent nature of acoustic image formation, no return signal energy is detected, despite the fact that there is a reflecting medium in the field of view.
Various techniques have been proposed in the prior art for reducing clutter and speckle artifacts in ultrasound images. These approaches generally involve combining images obtained at different times and/or in different frequency bands. See, for example, U.S. Pat. No. Re. 35,148 issued January 23, 1996 to Lizzi et al., which describes a frequency compounding technique. A broadband received signal is applied to separate bandpass filters. The outputs of the filters are detected and summed.
A two-dimensional median filter to reduce speckle artifact in ultrasound imaging is disclosed in U.S. Pat. No. 5,409,007 issued Apr. 25, 1995 to Saunders et al.
A technique for speckle reduction in ultrasound imaging using a two-dimensional array of transducer elements is disclosed in U.S. Pat. No. 5,653,235 issued Aug. 5, 1997 to Teo.
Spatial compounding is another prior art technique for reducing speckle in ultrasound images. In spatial compounding, an object is imaged from two or more points in space, and the resulting images are combined to form a single image. If the points in space are sufficiently distant from one another, the speckle patterns produced are not correlated with each other.
Prior art techniques for reducing clutter and speckle artifacts have exhibited limited improvement in image quality. Accordingly, there is a need for additional techniques for enhancing image quality, which may be used separately or in combination with known techniques.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, a method is provided for generating a medical ultrasound image. The method comprises the steps of transmitting ultrasound energy into a region of interest in a patient's body, the transmitted ultrasound energy producing ultrasound echoes from the region of interest, and acquiring, in response to the ultrasound echoes, first and second coherent signals that correspond to first and second pulse sequences, respectively. The second pulse sequence is orthogonal or nearly orthogonal to the first pulse sequence. The method further comprises combining the first and second coherent signals to provide a composite image signal that is representative of the region of interest.
In a first embodiment, the step of transmitting ultrasound energy comprises transmitting first and second sequences of ultrasound pulses at different times. The first and second sequences of ultrasound pulses correspond to the first and second pulse sequences, respectively.
In a second embodiment, a combined sequence of ultrasound pulses containing ultrasound pulses corresponding to the first and second pulse sequences is transmitted.
In a third embodiment, an ultrasound pulse having a pulse duration at least as long as the first and second pulse sequences is transmitted.
In the above embodiments, the step of acquiring a first coherent signal may include selecting the first coherent signal with a first matched filter having a characteristic matched to the first pulse sequence, and the step of acquiring a second coherent signal may include the step of selecting the second coherent signal with a second matched filter having a characteristic matched to the second pulse sequence.
In one example, the step of combining the first and second coherent signals may include processing the first and second coherent signals to remove phase information and form first and second noncoherent signals, respectively, and summing the first and second noncoherent signals. In another example, the step of combining the first and second coherent signals may include summing the first and second coherent signals to form a combined signal and processing the combined signal to remove phase information and form a noncoherent signal.
The steps of transmitting the first and second sequences of ultrasound pulses may comprise varying one or more parameters of the first and second sequences of ultrasound pulses during image data acquisition.
According to a further aspect of the invention, apparatus is provided for generating a medical ultrasound image. The apparatus comprises an ultrasound transducer, an ultrasound transmitter for transmitting ultrasound energy into a region of interest in a patient's body with the ultrasound transducer, the transmitted ultrasound energy producing ultrasound echoes from the region of interest, a receive beamformer for generating a beamformer signal representative of a receive beam in response to the ultrasound echoes, and a signal processor. The signal processor comprises first and second matched filters for acquiring first and second coherent signals, respectively, in response to the beamformer signal and a combining unit for combining the first and second coherent signals to provide a composite image signal that is representative of the region of interest. The first and second matched filters have characteristics that are matched to the first and second pulse sequences, respectively. The second pulse sequence is orthogonal or nearly orthogonal to the first pulse sequence.
According to another aspect of the invention, methods and apparatus are provided for generating an ultrasound image. An ultrasound transmitter transmits a first sequence of ultrasound pulses and a second sequence of ultrasound pulses into a region of interest in a patient's body with the ultrasound transducer. The transmitted ultrasound energy produces ultrasound echoes from the region of interest. The second sequence of ultrasound pulses is orthogonal or nearly orthogonal to the first sequence of ultrasound pulses. An ultrasound receiver acquires, in response to the ultrasound echoes, first and second coherent signals that correspond to the first and second sequences of ultrasound pulses, respectively, and combines the first and second coherent signals to provide a composite image signal that is representative of the region of interest.
REFERENCES:
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patent: Re. 35148 (1996-01-01), Lizzi et al.
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patent: 5187687 (1993-02-01), Burckhardt et al.
patent: 5255683
Agilent Technologie,s Inc.
Jaworski Francis J.
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