Image analysis – Applications – Biomedical applications
Reexamination Certificate
1998-07-30
2001-01-30
Black, Thomas G. (Department: 2771)
Image analysis
Applications
Biomedical applications
C382S260000, C382S134000, C600S437000
Reexamination Certificate
active
06181810
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to intravascular ultrasonic imaging, particularly to data processing techniques for improving image perception.
With the increasing frequency (above 40 MHz) ultrasonic signals, blood speckles appear more prominently in ultrasonic intravascular images. The speckles are sufficiently bright to lower the contrast between blood and tissue, making it harder for physicians to determine the true boundaries based on a single frame images.
Most blood speckle reduction algorithms use either spatial or temporal information only, which is insufficient to determine the characteristic. See, for example, B. Olstad, “Noise reduction in ultrasound images using multiple linear regression in a temporal context”, SPIE, vol. 1451, pp.269-281, 1991; Olstad, et al., “Analysis and measurement of temporal tissue variations”, U.S. Pat. No. 5476096, 1995; and Karaman, et al., “An adaptive speckle suppression filter for medical ultrasonic imaging”, IEEE Trans. Med. Imag., vol.14, pp.283-292, 1995.
Some algorithms have attempted to combine spatial and temporal filtering. However, they are so complex and cumbersome that processing cannot be realized in real-time with known technology. See, for example, Evans, et al., “Biased Motion-Adaptive Temporal Filtering for Speckle Reduction in Echocardiography”, IEEE Trans. Med. Imag., vol.15, pp.39-50, 1996.
Tissue tends to be static over short periods of time. Blood cells move rapidly so blood speckles are randomly scattered. However, due to the fast cardiac motion and speckling nature of the high frequency ultrasound signals, it is difficult to differential blood and tissue without the consideration of additional information, such as spatial properties. What is needed is a near real-time technique for suppression of spurious dynamic artifacts.
SUMMARY OF THE INVENTION
According to the invention, a method and an apparatus are provided for combining spatial and temporal filtering for blood speckle reduction in high frequency ultrasound images wherein a sequence of image frames is processed to produce a binary mask, preferably a two-dimensional binary mask. In this mask, a first value is assigned to regions of identified blood speckles and a second value is assigned to regions of the remainder. The images are then modulated with the mask, applying different filtering techniques to assigned blood regions and assigned nonblood (i.e., tissue) regions, the filtering techniques having been selected to optimize images for the type of feature to be highlighted. The preferred filtering technique is a spectral analysis of transformed data so that the energy content of features changing at higher frequencies can be weighed against features changing at lower frequencies. The images are preferably in polar coordinate format. The degree of blood speckle suppression can be determined based on the actual values of the pixels at the same spatial location in given frames.
In a specific embodiment, the process of generating the mask involves obtaining a vector in each frame for the given frames at the same spatial location to produce a two-dimensional matrix, with the matrix then being transformed to the frequency domain to determine its characteristics. A binary label is derived and given to the pixel that is located at the center of the vector in the current frame, indicating whether it is blood speckle. Assuming the blood clutter and tissue are not isolated, a morphologic operator is applied to the first mask and then isolated labels are removed.
Once the mask is generated, different operations are performed on the original input frames. This is done on a pixel-by-pixel basis. For a pixel which is labeled as tissue, an average with the previous output value is used. For a pixel which is labeled as blood speckle, a minimum value is derived from values of this pixel in the given image frames.
REFERENCES:
patent: 5224483 (1993-07-01), Lipschutz
patent: 5417215 (1995-05-01), Evans et al.
patent: 5476096 (1995-12-01), Olstad et al.
patent: 5520185 (1996-05-01), Soni et al.
patent: 5522392 (1996-06-01), Suorsa et al.
patent: 5876343 (1999-03-01), Teo
patent: 5885218 (1999-03-01), Teo et al.
patent: 0 571 084 A2 (1993-11-01), None
Teo Tat-Jin
Zhang Xiangmin
Black Thomas G.
Sci-Med Life Systems, Inc.
Townsend and Townsend / and Crew LLP
Veillard Jacques
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