Diagnostic ultrasound imaging based on rate subtraction...

Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation

Reexamination Certificate

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Reexamination Certificate

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06673019

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to diagnostic ultrasound imaging by which imaging is performed based on a contrast echo technique for an object into which an ultrasound contrast agent of which main constituent is microbubbles is administered. In particular, the present invention relates to diagnostic ultrasound imaging capable of separating transient signals caused due to the microbubbles of the ultrasound contrast agent on the basis of a rate subtraction imaging (RSI) technique.
2. Description of the Related Art
Ultrasound signals have been clinically used in various fields, and one field is an application to diagnostic ultrasound apparatus. A diagnostic ultrasound apparatus acquirers an image signal through transmission and reception of an ultrasound signal toward and from an object and is used in a variety of modes utilizing non-invasiveness of the signal.
One typical type of diagnostic ultrasound apparatus produces tomographic images of a soft tissue of a living body by adopting ultrasound pulse reflection imaging. This imaging method is noninvasive and produces tomographic images of the tissue. Compared with other medical modalities such as diagnostic X-ray imaging, X-ray CT imaging, MRI, or diagnostic nuclear medicine imaging, this pulse reflection imaging has many advantages: real-time display is possible, a compact and relatively inexpensive apparatus can be constructed, patient's exposure to X-rays or others will not occur, and blood imaging is possible thanks to ultrasound Doppler imaging. This imaging is therefore widely used for diagnosis of the heart, abdomen, mammary gland, and urinary organs, and for diagnosis in obstetrics and gynecology. In particular, pulsation of the heart or motion of a fetus can be observed in real time through manipulation that is as simple as just placing an ultrasound probe on a patient's body surface. Moreover, since patient's X-ray exposure need not be cared about, screening can be carried out many times repeatedly. There is also the advantage that the apparatus can be moved to a bedside position for ready screening.
In the field of the ultrasound diagnosis, a trans-venous injection type of ultrasound contrast agent has been commercially available. This agent, which is injected into an object through the vein in screening the heart or organs in the abdomen, is used to enhance echo signals emanated from flows of blood so as to evaluate blood flow kinetics. This imaging is known as contrast echo imaging. Since the trans-venous injection of the contrast agent is less expensive than its trans-arterial injection (i.e., trans-arterial injection type of contrast echo imaging), diagnosis based on the trans-venous injection has been spotlighted. The main constituent of the contrast agent is microbubbles that act as sources to reflect ultrasound waves. The larger the amount or concentration of an injected contrast agent is, the larger the contrast effect is. However, the microbubbles are delicate substances, so they are characteristic of collapse due to irradiated ultrasound waves. It has been found that various conditions, such as, an extremely shortened duration of the contrast effect resulted from a certain condition of ultrasound waves, will occur. Although a contrast agent of high persistency and high durability has been developed recently, the collapse phenomenon of the contrast agent cannot be avoided fundamentally, because its main constituent is microbubbles. Meanwhile, the long-term persistence of the contrast agent in a human body may raise the problem of invasiveness.
In the contrast echo imaging, a contrast agent (i.e., microbubbles) is successively supplied into a region of interest of an object through blood flow. Hence, it is assumed that, even when once bubbles existing within the region collapsed by irradiated ultrasound waves, the contrast effect will be maintained as long as new microbubbles will inflow into the region of interest at the next timing of ultrasound irradiation. However, ultrasound waves are normally transmitted and received as many times as a few thousands per second, there is organ parenchyma whose blood flow speed is rather slow, and there is blood kinetics in relatively thin blood vessels. Considering these conditions, microbubbles will collapse in turn before observing on a diagnostic image intensified intensity of data due to a contrast agent, thus the contrast effect being lessened instantaneously. Various reports concerning this collapse phenomenon of microbubbles have already been published and flush echo imaging (FEI) which will be described later belongs to the imaging techniques based on the collapse phenomenon.
Of diagnostic techniques using the contrast agent, the most fundamental diagnostic technique is to detect whether or not there is blood flow in a region to be diagnosed by examining the existence of intensified intensity data depending on the contrast agent. More advanced diagnostic techniques include a technique of detecting temporal changes in spatial distributions of the contrast agent from spreads of changes in intensity or from degrees of intensified intensity data in a diagnostic region. Also included is a technique of acquiring an interval from the start of injection of a contrast agent to its arrival at a region of interest (ROI) and temporal changes in intensity data (Time Intensity Curve: TIC) or a maximum of intensity data, both of which is due to the contrast agent, within the ROI.
The contrast echo imaging can also be performed effectively with harmonic imaging (HI) using a non-fundamental component of ultrasound waves. The harmonic imaging is based on separation and detection of only a non-fundamental component derived due to nonlinear behaviors of ultrasound-excited microbubbles, which are main constituents of a contrast agent. Since internal organs of a living body are relatively difficult to cause nonlinear behaviors, the harmonic imaging can give contrast agent images with preferable contrast ratios.
It has been known that echo signals emanated from tissue such as internal organs include non-fundamental components (mainly, harmonic components), though it is lower in level than that emanated from the contrast agent, and the non-fundamental components are mixed with the entire received echo signal. Such harmonics from tissue are normally called as tissue harmonic signals, which give a basis to imaging, called tissue harmonic imaging (THI).
As practical techniques of detecting and imaging non-linear signals (mainly harmonic signals), there have been known two techniques. One technique is that a high-pass type of echo filter is used to cut off a fundamental component, providing only a filtered harmonic component used for recombining images. The other is that a technique referred to as an inverted phase pulse adding technique (pulse inversion technique) is used to extract a harmonic component that is involved in recombining images.
Of these techniques, the inverted phase pulse adding technique is disclosed, for example, by U.S. Pat. No. 5,632,277. Practically, in this imaging, two ultrasound pulses of which phase difference is 180 degrees from each other are transmitted along each raster (two times of transmission in total), resultant echo signals are received, respectively, and then the echo signals are added to produce added signals for imaging. This addition causes linear components included in the mutual echo signals to be cancelled out, because one of the echo signals is inversed in the waveform and both echo signals are added. Accordingly, a doubled harmonic component remains (180 degrees×2=360 degrees). This technique requires two pulses to be transmitted and received for each raster, thus a frame rate is reduced. However, design of filters is easy, even if the high-pass type of filter is used. In addition, a transmission pulse of which bandwidth is wide can be used, increasing spatial resolution.
By the way, it has been reported that the phenomenon that the microbubbles are vanished by irrad

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