1. Field of the Invention
The present invention relates to an ultrasonic imaging system, and more particularly, to a filtering method for removing a low-frequency Doppler signal from an ultrasound image.
2. Description of the Related Art
Medical imaging systems are used as diagnostic tools for viewing internal areas of the patient's body. Imaging systems based on ultrasound technology are desirable because such systems are non-invasive, portable and generally easy to use. A Doppler ultrasound system displays a frequency deviation amount together with a scattering strength of the signal as image on a screen to enable evaluation of a dynamic function of a living body. In particular, the Doppler imaging system is capable of displaying color images and demodulates a received signal and digitizes the demodulated signal, thereby imaging a flow of blood flowing through a heart or blood vessel into a real-time two-dimensional image. Such color Doppler imaging systems can display cross-sectional images and blood flow information simultaneously. To discriminate the layer image and the blood flow information, the cross-sectional image is indicated as white and black and the blood flow information is indicated as color.
Although an ultrasound beam is focused, some part of the Doppler sample volume typically lies outside the blood vessel. This results in the ultrasound instrument receiving a signal from the vessel wall and surrounding tissue (referred to as the clutter component), as well as from the blood flowing in the vessel (referred to as the flow component). The clutter component returned from the wall and tissue has an amplitude that is generally many times greater than the flow component. Because of respiration and cardiac motion, the tissue and vessel tend to move at slower velocities than the flowing blood. The Doppler signal also contains noise produced by the Doppler instrument's electronics. As such, the Doppler signal is a composite signal composed of a clutter component, a flow component and a noise component. The flow component tends to have a low amplitude and high-frequency, while the clutter component tends to have a large amplitude and low-frequency. In order to ensure clinical efficiency, Doppler instruments must be able to determine the velocity of moving blood accurately in the presence of clutter and noise signal components. Accordingly, the clutter component is generally removed from the composite Doppler signal via filtering prior to velocity estimation. If the clutter component is not filtered, the velocity measurement returns to the velocity of the moving tissue and not of the moving blood. Typically, a high-pass filter referred to as a wall filter is used to remove the clutter component of the signal while leaving the flow component intact.
For example, conventional clutter processing uses a mean frequency method to estimate the clutter frequency. The clutter frequency is then mixed with the original clutter data in an attempt to position the clutter at a predetermined DC level. The conventional method assumes that flow has little effect on the mean frequency estimation. However, this assumption is not valid when the flow component is comparable to the clutter component in amplitude which may occur, for example, in the middle of a big vessel where the effect of clutter is much smaller than at the vessel boundary and/or where contrast agents are used to boost flow amplitude. This may cause the mean frequency clutter frequency to shift towards the blood flow frequency. When flow velocity is high, the mean frequency shifts toward the flow frequency even more. When this frequency is mixed with the clutter, a combination of the following results: (1) clutter is not positioned at the predetermined DC level resulting in incomplete clutter cancellation; (2) clutter may be even further from the predetermined DC level than it would have been without mixing; and (3) strong blood flow can be misinterpreted as clutter and thus shifted erroneously close to the predetermined DC level, resulting in either incorrect velocity estimation or cancellation by a subsequent wall filter. Conventionally, a combination of clutter mean frequency, power and variance may be used in an attempt to identify such situations and disable mixing accordingly. Although this may mitigate the effect of the strong blood flow being misinterpreted as clutter, clutter residue may still be excessive due to the inaccurate positioning of the clutter away from the predetermined DC level.
Accordingly, there is a need for an improved method for eliminating clutter components from Doppler ultrasound signals. There is a further need for a method for removing clutter components from Doppler ultrasound signals when the flow component is comparable in amplitude to the clutter component. There is further a need to remove the clutter component from the flow component when flow velocity is relatively high.