Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-03-28
2001-11-06
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S447000
Reexamination Certificate
active
06312384
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to ultrasound imaging systems, and more particularly, to methods and apparatus for imaging moving fluid and tissue.
BACKGROUND OF THE INVENTION
Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which brightness of a pixel is based on intensity of the echo return. In a so-called “color flow” mode, the flow of blood or movement of tissue can be imaged. Conventional ultrasound flow imaging methods use either the Doppler principle or a time-domain cross-correlation method to estimate average flow velocity, which is then displayed in color overlaid on a B-mode image.
Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The frequency shift of backscattered ultrasound waves may be used to measure velocity of tissue or blood. The change or shift in backscattered frequency increases when blood flows toward the transducer and decreases when blood flows away from the transducer. The Doppler shift may be processed to estimate the average flow velocity, which is displayed using different colors to represent speed and direction of flow. The color flow velocity mode displays hundreds of adjacent sample volumes simultaneously, all color-coded to represent velocity of each individual sample volume.
Conventional ultrasound flow imaging displays either average Doppler power (“power Doppler imaging”) or average flow velocity (“color flow velocity imaging”) as a color overlay on a B-mode image. The transmitted pulses are typically more narrow-band than B-mode pulses in order to gain Doppler sensitivity. Operating on a packet of as many as
16
transmits, a high-pass wall filter first rejects echoes from slower-moving tissue or vessel walls to reduce the signal dynamic range. The number of wall filter output samples per packet is given by (N−W+1), where N is the packet size and W is wall filter length. Subsequently, instantaneous Doppler power is computed as the magnitude squared of each wall filter quadrature output signal, and the average of all output signals yields the average Doppler power. Alternatively, the average velocity is computed from the wall filter quadrature output signal based on the Doppler principle (phase change) or time delay between firings. The Kasai autocorrelation algorithm or a time-domain cross-correlation algorithm can be used to estimate the average flow velocity.
Although conventional color-flow imaging has very good flow sensitivity, the ability to see physical flow is limited by its limited dynamic range (which is partially dependent on the compression curve), limited resolution (due to narrow-band pulses), limited frame rate (due to large packet sizes), and axial-only flow sensitivity (which is dictated by the reliance on the Doppler effect). In addition, conventional color-flow imaging suffers from artifacts such as aliasing, color blooming and bleeding.
In medical diagnostic ultrasound imaging, it is also desirable to optimize the signal-to-noise ratio (SNR). Increased SNR can be used to obtain increased penetration at a given imaging frequency or to improve resolution by facilitating ultrasonic imaging at a higher frequency. Coded excitation is a well-known radar technique used to increase signal-to-noise ratio in situations where the peak power of a transmitted signal cannot be increased but the average power can. This is often true in medical ultrasound imaging, where system design limitations dictate the peak amplitude of the signal driving the transducer. In this situation, longer signals, such as chirps, can be used to deliver higher average power values, and temporal resolution can be restored by correlating the return signal with a matched filter. Chirps, however, are expensive to implement on a phased array ultrasound system due to the complexity of the electronics, so binary codes, or codes that can be easily represented digitally as a series of digits equal to +1, −1 or 0, are much more practical. Binary codes are also preferred because they contain the most energy for a given peak amplitude and pulse duration.
A method for imaging moving blood reflectors using binary codes and displaying a combination of the flow image and the tissue image without overlay has been disclosed in the parent (Ser. No. 09/299,034) of the present application. One method of flow imaging disclosed uses single-transmit (e.g., Barker) codes. However, single-transmit codes have range lobes and require a long mismatched decoding filter. Consequently, single-transmit codes cannot be used on lower-frequency probes if the decoding filter length in the hardware is insufficient.
There is a need for a way of achieving flow imaging which will alleviate the limitations of the single-transmit codes and which can be employed with all types of probes.
SUMMARY OF THE INVENTION
Tissue and blood flow are imaged simultaneously, with improved sensitivity, by using Golay codes. Golay codes can achieve higher SNR gain and lower sidelobes than single-transmit (e.g., Barker) codes for a given transmit duration and receive-filter length. Since Golay codes use a pair of firings to achieve sidelobe cancellation, such codes were previously thought to be unsuitable for flow imaging. This is because the sidelobe cancellation property of Golay codes relies on the reflectors being invariant between the pair of firings. Thus, reflector motion between the two firings results in imperfect sidelobe cancellation and potentially high sidelobes.
In accordance with the preferred embodiments, multiple pairs of Golay codes are transmitted, and filtered on reception, to achieve SNR gain, sidelobe suppression, and equalization of tissue echoes such that the tissue and blood flow may be displayed together without overlay. By using matched filtering, a much shorter receive filter can be used than for mismatched filtering, as required for single-transmit (e.g., Barker) codes.
In a preferred embodiment, a pair of preferred Golay codes {A, B} is first selected, based on the autocorrelation sidelobes of the code. For a given code length, a large set of Golay code pairs may be generated. The preferred Golay pair is selected to minimize sidelobe energy and to maximize the concentration of the remaining sidelobe energy close to the mainlobe. Many different Golay pairs may have the same autocorrelation, so the preferred Golay pair is not unique.
The selected Golay code pair is then used to encode a base sequence into a pair of encoded transmit sequences {A*, B*}. The encoded transmit sequences are transmitted multiple times to a given focal position, with matched filtering performed on the received echoes. In accordance with a preferred embodiment, the matched filtering is performed with a constant scalar multiplier f (close to unity) that changes from one Golay pair to the next, but remains the same for each Golay pair. The output signals that result from the matched receive signal filtering for all of the firings are vector-summed to form a compressed and high-pass-filtered signal which is detected, log-compressed, and displayed in the conventional B-mode (i.e., as a gray-scale image). The high-pass filtering suppresses the strong tissue signal, thereby enabling visualization of the weaker blood signal with or without the tissue background.
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Breedlove Jill M.
Cabou Christian G.
General Electric Company
Imam Ali M.
Lateef Marvin M.
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