Patent Application
20050273010
The present invention relates generally to ultrasound imaging systems. More particularly, the present invention relates to methods and systems for contrast-imaging in ultrasound imaging systems.
Medical ultrasound systems may be used to study, for example, anatomical structures, detect anomalies in tissues and measure blood flow within the body. Ultrasound systems typically comprise a transducer, which is used for transmitting pulses of ultrasound waves, known as imaging pulses, into the body. Acoustic echo signals are generated at interfaces in the body in response to these waves. These echo signals are received by the transducer and transformed into an electrical signal that can be used to produce an image of the body part under examination. This image may be displayed on a display device.
In typical ultrasound systems, the amplitude of reflected ultrasound waves is detected and measured as a function of time to construct dynamic or static images of structures within a body. These ultrasound systems, however, have a limited ability to image blood flow within small and deep lying blood vessels in the body.
The use of administrable ultrasound contrast agents has improved the imaging of blood flow, particularly within small and deep lying blood vessels and capillaries. Thus, improved ultrasound images of organs and surrounding tissue are obtained by introducing contrast agents into the bloodstream and organs to be investigated. The contrast agents typically contain microbubbles stabilized with additional material (e.g., as Albumin, Polymer, Phospholipid, Liposomes, Galactose, etc.) on their surface. Such microbubbles show nonlinear behavior upon interaction with ultrasound waves.
When contrast agent microbubbles are irradiated with ultrasound waves, the microbubbles oscillate or resonate nonlinearly, returning a spectrum of echo signals including those at the second harmonic of the transmitted frequency. The strong harmonic echo components uniquely distinguish echoes returning from the microbubbles. Therefore, an improved contrast in the image may be achieved. The amount of ultrasound energy that is scattered and returned by contrast agents is mainly a function of microbubble size and surface properties (e.g., surface elasticity and viscosity) and the frequency and pressure of the ultrasound wave. For a given ultrasound frequency, there is a corresponding microbubble resonance size that is unique for each contrast agent. The effective scattering strength of microbubbles reaches a peak when microbubbles resonant at the incident ultrasound frequency. A significant difference in the harmonic component strength of the reflected signals due to the microbubbles and tissue makes it possible to obtain an image with higher contrast.
Although administration of contrast agents generally improves imaging, it may reduce contrast between the region of interest and its surrounding anatomical structures. Microbubbles that scatter ultrasound can greatly dampen the ultrasound waves, thereby affecting echo intensity and the image produced by targeted anatomical structures. High intensity ultrasound pulses may be transmitted to ensure that microbubble size, after modification, has reached a state where the microbubble can resonate, thereby improving contrast. However, the microbubbles may get destroyed and, thus, real-time imaging may not be possible.
Thus, known methods for providing ultrasound imaging employing contrast agents have several limitations that may affect the acquired and displayed image.
In one exemplary embodiment, a method for controlling an ultrasound system is provided. The method includes transmitting a nonlinear contrast imaging pulse sequence and transmitting at least one modification pulse between successive pulses in the nonlinear contrast imaging pulse sequence. The modification pulse is configured to change a contrast agent within an object being imaged by an ultrasound system.
In another exemplary embodiment, an ultrasound system is provided. The ultrasound system includes an ultrasound probe having at least one transducer for transmitting pulses to an object and a controller configured to transmit at least one modification pulse between successive pulses in a nonlinear contrast imaging pulse sequence. The modification pulse changes a contrast agent within an object being imaged by the ultrasound probe.
Various embodiments of the present invention provide methods and systems for improving the contrast harmonic signals generated in response to nonlinear imaging pulses in an ultrasound imaging system. In one embodiment, a modification pulse is provided with the nonlinear imaging pulses. Various embodiments of the present invention generate enhanced images of anatomical structures and blood flow in a body when using ultrasound contrast agents including microbubbles by modifying, for example, the physical and acoustic properties of microbubbles using one or more modification pulses.
In various embodiments of the present invention, the modification pulse is configured to change the acoustic and/or physical properties of the contrast agent microbubbles without affecting the surrounding environment (e.g., tissue in a body). In an embodiment, the modification pulse is used to make the contrast agent microbubbles more flexible. In another embodiment, the encapsulation of the contrast agent microbubbles is removed by utilizing the modification pulse, thereby providing free microbubbles. The increase in flexibility and providing of free microbubbles increases scattering capabilities of the contrast agent microbubbles. The size of the contrast agent microbubbles, before modification, ranges generally from sub-micron size to about ten microns. In various embodiments, the size of the contrast agent microbubbles is increased to a resonant size that is in the range of one to ten microns, which is similar to or smaller than the size of blood cells. The modification pulse is also configured in various embodiments to increase the intensity of the second harmonic signal from the contrast agent microbubbles. It should be noted that the modification pulse does not change the acoustic properties of, for example, tissues in a human body being imaged.
In another embodiment, the modification pulse is configured to change the resonant frequency of the contrast agent microbubbles rather than changing the size of the microbubbles. The change in resonant frequency of the contrast agent microbubbles is performed such that the resonant frequency matches the imaging pulse sequence frequency. Essentially, the modification pulse changes (e.g., shifts) the resonant frequency of the microbubbles to an imaging frequency. This increases the contrast achieved while using the echo response of the imaging pulse sequences.
It should be noted that the modification pulse also may be configured to be transmitted, for example, between successive pulses of an imaging pulse sequence. In one embodiment, transmission of the imaging pulses increases the size of the microbubbles. This is a direct result of the compression and rarefaction occurring because of the nonlinear imaging pulses. This decrease and increase to a resonant size of the microbubbles is not symmetric (e.g., microbubbles once expanded do not return to their original size after compression). A harmonic signal generation in response to the incident nonlinear imaging pulses results. Transmission of the modification pulse at 104 increases the size of the microbubbles to a resonance size, thereby improving the imaging performance of the ultrasound system. In other embodiments, transmission of the modification pulse at 104 may change other properties of the microbubbles including, for example, the surface properties of the microbubbles (e.g., elasticity). Based on the transmitted pulses, an image of a body part under examination is generated. The process for generation of images is further described in detail in connection with
An analysis of echo signals is performed at 206. In various embodiments, the analysis includes separating harmonic and linear components, enhancing the harmonic component and generating an ultrasound image by utilizing the enhanced harmonic component. Separation of harmonic components from the received signals is performed using suitable filters.
In various other embodiments of the present invention, the separation of harmonic components may be performed by utilizing a processor. In these embodiments, a transducer within the ultrasound system converts the echo signals to electrical signals. These electrical signals are then converted to a digital form by employing an analog-to-digital converter. The processor processes the digital data for separating harmonic components using a suitable algorithm as is known. The separated harmonic signals, received in response to the imaging and modification pulse, are then added for enhancing the harmonic component of the signal for generating enhanced contrast images using the ultrasound imaging system.
In an exemplary embodiment of the present invention, echoes received in response to the imaging pulses are used for generating images using an ultrasound imaging system. The modification pulse is used for generating a weighted image (e.g., for enhancing the intensity of reflected echo signals by changing the acoustic and physical properties of the contrast agent microbubbles) and the response generated as a result of this pulse is thus not used in generating the images. In another exemplary embodiment of the present invention, echoes received in response to the imaging pulses and the modification pulse are used for generating images using the ultrasound imaging system.
Modification pulse 404 changes contrast agent microbubbles, for example, changes the physical and/or acoustic property of the microbubble. For example, modification pulse 404 may increase the contrast agent microbubble size to about equal to and or above a resonance size of the microbubble. The introduction of modification pulse 404 causes an expansion of the contrast agent microbubbles and thereby increases the effective scattering capability of the contrast agent microbubbles. The increase in scattering capability results because the ultrasound scattering is a function of the contrast agent microbubble size. The scattering increases when the contrast agent microbubbles are about equal and/or above the resonance size. Hence, an increase in the level of the second harmonic signal from the contrast agent microbubbles is achieved by the introduction of modification pulse 404 between the first imaging pulse 402 and second nonlinear imaging pulse 406. It should be noted that in various embodiments a plurality of modification pulses 404 may be transmitted between each of a plurality of imaging pulses 402 and 406.
Modification pulse 404 is configured to change any acoustic and/or physical properties of the microbubbles and is not limited to the changes described above. For example, the modification pulse 404 may be configured to change one or more surface properties of the microbubbles. However, again, modification pulse 404 does not change, for example, the acoustic properties of the tissue in a human body. In the complete process of transmitting imaging and modification pulses and receiving echo signals, two images are generated by the two echo signals in response to the two nonlinear imaging pulses. One of the differences in these two obtained images is in the area where contrast agent microbubbles are present.
Various embodiments of the present invention may be used to generate ultrasound images of the body parts using a modification pulse enhanced technique that allows for enhancement of the pulse-echo ultrasound energy returned by targeted tissue or blood flow containing microbubbles of the ultrasound contrast agents. Backscattering of signals from the contrast agent microbubbles is also improved with minimal disturbance to the tissue, thereby providing improved contrast. Further, the imaging techniques described herein enhance the sensitivity of contrast imaging. An increase in the contrast-to-tissue ratio is provided. Thus, various embodiments of the present invention use the difference in the intensities of harmonics generated and reflected by tissue and microbubbles in response to a sequence of nonlinear imaging pulses to improve imaging. It is noted that the imaging and modification pulse sequences may be transmitted along the same vector or different vectors.
The various embodiments of systems as described herein and any of their components, may be embodied in the form of a computer system. Typical examples of a computer system include a general-purpose computer, a programmed microprocessor, a micro-controller, a peripheral integrated circuit element, and other devices or arrangements of devices that are capable of implementing the process described herein.
The computer system may include, for example, a computer, an input device, a display unit and a communication interface, for example, for communicating with the Internet. The computer includes a microprocessor, with the microprocessor connected to a communication bus. The computer also includes a memory. The memory may include Random Access Memory (RAM) and Read Only Memory (ROM). The computer system further includes one or more storage devices. The storage devices may be a hard disk drive or a removable storage drive, such as, for example, a floppy disk drive, optical disk drive, and the like. The storage devices also may be other similar means for loading computer programs or other instructions into the computer system.
The computer system executes a set of instructions that are stored in one or more storage elements in order to process input data. The storage elements also may hold data or other information as desired or needed. The storage element may be in the form of an information source or a physical memory element present in the processing machine.
The set of instructions may include various commands that instruct the processing machine to perform specific tasks such as the processes or the various embodiments described herein. The set of instructions may be in the form of a software program. The software may be in various forms such as, for example, system software or application software. Further, the software may be in the form of a collection of separate programs, a program module within a larger program or a portion of a program module. The software also may include modular programming in the form of object-oriented programming. The processing of input data by the processing machine may be in response to user commands, or in response to results of previous processing or in response to a request made by another processing machine.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
