Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1999-04-29
2001-01-09
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06171246
ABSTRACT:
This invention relates to methods and apparatus for performing ultrasonic diagnostic imaging and, in particular, to the imaging of perfused tissue in realtime using ultrasonic contrast agents.
Ultrasonic contrast agents are becoming increasingly available to enhance the ultrasonic imaging of bloodflow and tissue perfusion. These contrast agents are generally composed of encapsulated microbubbles of various gases which are highly reflective of ultrasound. When the microbubbles are infused into the body, the bloodflow carrying the microbubbles through blood vessels or the capillary bed of perfused tissue can be delineated by detecting the significant echo reflections from the microbubbles in the bloodstream. Microbubbles have also been found to have another characteristic which is useful which is that, under certain conditions, the microbubbles will resonate and return ultrasonic echoes at frequencies which are harmonics of the transmit frequency of an insonifying acoustic wave. This means that the echoes from microbubbles can be distinguished or segmented from echoes from other structures on the basis of their higher frequency content. This has led to the desire to enhance the imaging of bloodflow and particularly perfused tissue such as the myocardium with contrast agents, and most desirably to be able to do so in realtime.
There have been several impediments to doing so, however. One is the fragile nature of the microbubbles in the presence of acoustic energy. When insonified with acoustic energy at the higher diagnostic levels, microbubbles have been found to become disrupted and break up or dissolve. Often this disruption is instantaneous in effect, causing the generation of a distinct acoustic signature by the microbubbles. This appreciable return energy can be imaged to great effect as described in U.S. Pat. No. 5,456,257 which uses the energy released by “bursting” microbubbles to image the locations of these microbubble destruction events. But since these events are instantaneous and not repeatable, a drawback of this technique is that only a very few or even only a single image can be acquired at the time of microbubble destruction. A subsequent image at the same location cannot be acquired until the location is reperfused with a new flow of microbubbles.
Faced with this dilemma, a number of techniques have been developed to deal with the problem. These techniques all revolve around the concept of intermittently transmitting ultrasonic energy which disrupts the microbubbles. During the intervening times either low energy transmission or no transmission at all takes place. U.S. Pat. Nos. 5,560,364, 5,740,807, and 5,735,281 all describe variations of this basic premise of acquiring a high energy image of the contrast agent intermittently, while low energy imaging or no imaging is done during the intervening time periods while waiting for the bloodflow to reinfuse the imaged region with a new supply of microbubbles. Such a technique allows imaging of the structure of blood vessels and vasculature in realtime, with periodic filling in of the bloodflow inside those vessels when the high energy contrast agent image is acquired. While the structure of the tissue is shown in realtime, the contrast agent is not.
A second impediment to contrast agent imaging is the contamination of the received microbubble echoes with echoes from tissue. This occurs at the fundamental transmit frequency and at the harmonic frequencies. Harmonic contamination is caused by the development of harmonic components in the transmit wave as it passes through tissue, and the reflection of these tissue harmonic components by the microbubbles in addition to their own harmonic resonant energy. The tissue harmonic signal components should be eliminated from the received signal so that only the harmonic components of the microbubbles are used to form the contrast image.
In accordance with the principles of the present invention, the bloodflow in organs, tissue and vessels is imaged in realtime with the enhancement of a microbubble contrast agent. The ultrasound pulses which are transmitted to scan the imaged area are transmitted at a low power level which is insufficient to cause substantial destruction of the microbubbles yet high enough in power to elicit a harmonic response from the microbubbles. Additionally, at a low power level tissue is not generating a significant harmonic component. Upon reception of the echoes, nonlinear signal components are separated from the linear (fundamental frequency) components, preferably by a multiple pulse inversion technique, and used to create the image. The contrast agent is imaged by processing the nonlinear echo components received from multiple echoes from each sample volume of the imaged bloodflow. The inclusion of tissue harmonic echo components in the processed signals is suppressed by the use of low power, with any residual effects removed by a thresholding technique. In a preferred embodiment the echo signals are filtered to further attenuate fundamental frequency components. The processed signals from the microbubbles are preferably displayed in combination with a structural display of the surrounding tissues and with a distinctive appearance as by display of the microbubble locations in color.
REFERENCES:
patent: 5255683 (1993-10-01), Monaghan
patent: 5694937 (1997-12-01), Kamiyama
patent: 5735281 (1998-04-01), Rafter et al.
patent: 5740807 (1998-04-01), Porter
patent: 5848968 (1998-12-01), Takeuchi
patent: 5873829 (1999-02-01), Kamiyama et al.
Miller, Ultrasonic detection of resonant cavitation bubbles in a flow tube by their second-harmonic emissions, Ultrasonics, Sep. 1981, at pp. 217-224.
Burns et al., Harmonic power mode Doppler using microbubble contrast agents, JEMU, 1994, vol. 16, No. 4, at pp. 132-142, Masson, Paris, 1995.
Burns et al., Harmonic Imaging Principles and Preliminary Results, Angiology, Jul. 1996, vol. 47, No. 7, Pt. 2, at pp. S63-S74, Westminster Publications, Inc. Glen Head, NY.
Averkiou Michalakis
Bruce Matthew
Powers Jeffry E.
Skyba Danny M.
Imam Ali M.
Lateef Marvin M.
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