Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-08-08
2001-04-24
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S447000
Reexamination Certificate
active
06221018
ABSTRACT:
BACKGROUND
I. This invention generally relates to ultrasound imaging systems. In particular, the invention relates to improved systems and methods for imaging using harmonic frequency signals.
Ultrasound imaging systems generate and transmit ultrasound signals. The systems typically have several imaging modes, such as B-mode, color flow, and spectral Doppler.
The transmitted ultrasound signals have optimal characteristics set in response to the selected mode. The characteristics include frequency and bandwidth. As an example, B-mode imaging uses transmitted signals with a wide bandwidth and high frequency. As another example, color flow imaging uses transmitted signals with narrow bandwidth and lower frequency as compared to B-mode imaging.
Another type of imaging is harmonic imaging. Harmonic imaging is generally associated with imaging tissue or contrast agents at harmonic frequencies.
Typically, the transmitted ultrasound signal is a burst of sinusoidal waves associated with rectangular or sinusoidal transmit waveforms applied to the transducer. The transmitted signal has a center frequency within the 1 to 15 MHz range. The ultrasound signal propagates through a body. The ultrasound signal reflects off structures within the body, such as tissue boundaries. Some of the reflected signals, or echo signals, propagate back towards the transducer.
As the transmit signal propagates through and scatters within the body, additional frequency components are generated, such as at harmonics of the transmit frequency. These additional frequency components continue to propagate through and reflect off structures in the body. Echo signals having the same frequencies as the transmit signal and echo signals associated with the additional frequency components impinge on the transducer. The additional frequency components are caused by non-linear effects, such as non-linear propagation.
The harmonic signals may also be generated by ultrasound contrast agents. The contrast agents are typically gas or fluid filled micro-spheres which resonate at ultrasound frequencies. The contrast agents are infected in the blood stream and carried to various locations in the body. When insonified, harmonic echo signals are generated due to resonance within the contrast agents.
The echo signals are received, processed and detected by the ultrasound system. For harmonic imaging, energies associated with fundamental or transmit frequencies are removed by receive filtering. Thus, echo signals resulting from non-linear propagation and reflection are detected by the ultrasound system. However, the transmitted burst may include significant energy at the harmonic frequencies. The transmitted energy masks the non-linear response of the body and interferes with the harmonic signals from any contrast agents.
To improve harmonic imaging, it is known to reduce the energy at the harmonic in the transmit burst. The energy at the harmonic is reduced by generating a Gaussian envelope, complex sinusoidal waveform for each channel of a transducer. However, transmit beamformers capable of generating such a complex waveform require expensive components.
The present invention is directed to further improvements that enhance the imaging of the non-linear response of a body.
II. The present invention relates to medical diagnostic ultrasonic imaging systems, and in particular to transmit techniques that selectively suppress fundamental or harmonic energy in the backscattered signal.
Previous methods used to reject the fundamental signal component in tissue harmonic imaging are classical filtering and two-pulse methods. In the two-pulse method (Chapman U.S. Pat. No. 5,632,277, Hwang U.S. Pat. No. 5,706,819), two pulses are transmitted in sequence, the second being substantially identical to the first but inverted. The received signals are then added at some point in the receive signal path prior to amplitude detection. The result is a reduction in the level of the fundamental component of the signal and an increase in the second harmonic component.
Previous methods used to reject the tissue harmonic signal component in contrast agent imaging include waveform pre-distortion. In waveform pre-distortion, a second harmonic component is included in the signal that is launched from the transducer in order to cancel the tissue harmonic signal. Such an approach requires the functional form of the transmit signal to be dependent upon the transmit power level, the transmit aperture, and other parameters. In addition, the waveform pre-distortion method only suppresses the tissue harmonic signal over a limited range.
SUMMARY
I. The present invention is defined by the following claims. The preferred embodiments relate to improvements to a method for harmonic imaging, where the method comprises the steps of (a) transmitting ultrasonic energy at a fundamental frequency and (b) receiving reflected ultrasonic energy at a harmonic of the fundamental frequency.
According to a first aspect of these embodiments, the transmitting step includes the step of applying the plurality of waveforms to a respective plurality of transducer elements, a first waveform of the plurality of waveforms characterized by a first value of a harmonic power ratio, waveforms transmitted from the transducer elements and corresponding to the plurality of waveforms summing as an acoustic waveform substantially at the point, the acoustic waveform characterized by a second value of the harmonic power ratio less than the first value.
According to a second aspect of these embodiments, a method of generating waveforms in the acoustic domain for harmonic imaging is provided. The method includes the steps of: transmitting at a first start time at least a first waveform comprising a first number of cycles; transmitting at a second start time at least a second waveform comprising the first number of cycles, wherein the second start time corresponds to at least a one cycle delay from the first start time; generating at a point a third waveform responsive to the first and second waveforms, the third waveform comprising a shape rising gradually to a respective value and falling gradually from the respective value.
According to a third aspect of these embodiments, a method of generating waveforms in the acoustic domain for harmonic imaging is provided. The method includes the steps of transmitting at a first start time at least a first waveform comprising a first number of cycles; transmitting at a second start time at least a second waveform comprising a second number of cycles, wherein the second number of cycles comprises at least two cycles less than the first number and the second start time corresponds to at least a one cycle delay from the first start time; generating at a point a third waveform responsive to the first and second waveforms, the third waveform comprising a shape rising gradually to a respective maximum value and falling gradually from the respective maximum value.
According to a fourth aspect of these embodiments, a method of generating waveforms in the acoustic domain for harmonic imaging is provided. The method includes the steps of: transmitting at least a first waveform comprising a first amplitude; transmitting at least a second waveform comprising a second amplitude selected relative to the first amplitude in addition to apodization; generating at a point an acoustic waveform responsive to the first and second waveforms, the third waveform comprising a shape rising gradually to a respective maximum value and falling gradually from the respective maximum value and a number of amplitude levels more than a number of amplitude levels associated with each of the first and second waveforms.
According to a fifth aspect of these embodiments, various parameters of the waveforms relative to other waveforms are varied to alter the spectral response in the acoustic domain of the summed beam. The position, bandwidth and roll-off of a null or reduction in energy at harmonic frequencies may be altered.
According to a sixth aspect of these embodiments, an improved method for (a) t
Bradley Charles
Carp Stuart L.
Dodd Stirling S.
Hedberg David J.
Maslak Samuel H.
Acuson Corporation
Brinks Hofer Gilson & Lione
Imam Alt M.
Lateef Marvin M.
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