Image analysis – Image compression or coding – Adaptive coding
Reexamination Certificate
1995-11-06
2001-09-04
Johnson, Timothy M. (Department: 2623)
Image analysis
Image compression or coding
Adaptive coding
C375S240030, C382S251000
Reexamination Certificate
active
06285793
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to angiography, and more particularly relates to method and apparatus used to record angiographic image sequences. In its most immediate sense, the invention relates to lossy JPEG compression of angiographic image sequences such as are generated by, and stored in, angiographic apparatus.
Angiographic apparatus acquires and archives angiographic image sequences, and, advantageously, communicates such image sequences and makes them available over networks. Diagnostic and interventional angiographic procedures produce such vast quantities of data that image compression is required to carry out these functions properly. For example, during embolization of cerebral arteriovenous malfunctions, as much as 2.4 Gb of data may be acquired, at rates exceeding 5 Mb/S. Without compression of image data, the performance of angiographic imaging apparatus would suffer (e.g. storage space would be rapidly depleted and speed of operation would decrease) and the cost of such apparatus would increase. Thus, all angiography apparatus provides for compression of image data. Lossy JPEG compression is one image compression technique that is widely used in such apparatus.
The JPEG algorithm is an image compression algorithm that is defined by the International Organization for Standardization (ISO), the International Telegraph and Telephone Consultative Committee (CCITT) and the International Electrotechnical Commission (IEC). The JPEG algorithm works by dividing up the image into blocks and then transforming each block to the frequency domain using a two-dimensional discrete cosine transform. More specifically, JPEG compression first divides the image into 8 by 8 pixel blocks and then replaces the 64 original pixels with a block of 64 coefficients. The block contains one DC coefficient and 63 AC coefficients; the DC coefficient equals the average value of the 64 pixel block and the 63 AC coefficients express how the image information in the pixel block is distributed in the frequency domain. After the original image has been compressed, it can be reconstructed from the AC and DC coefficients using an inverse discrete cosine transform.
In lossy JPEG image compression, all these coefficients are quantized, i.e. divided by a visibility threshold that degrades their precision. Because the human eye is less sensitive to higher frequencies, the quantization may be coarser at higher frequencies and finer at lower frequencies. By using such a frequency-dependent quantization scheme (known as a quantization table) information that is visually more important is preserved, and information that is visually less important is discarded. It may therefore be understood that selection of the quantization table determines the degree to which image information is compressed, because such selection determines the quantity of information that is discarded.
In lossy JPEG image compression, the image quantization table is generated by multiplying a predetermined table by a scale factor. This scale factor is known as a quantization factor, or Q factor. Consequently, the degree of image compression depends upon the value chosen for the Q factor. The higher the value of the Q factor, the greater the degree of image compression.
Selection of the desired degree of image compression is of great significance to the performance of angiography equipment. If the image compression ratio is too high, important detail may be lost and the medical value of the angiographic image sequences may suffer. If image compression is too low, the equipment will not perform adequately, i.e. will be too slow or will cause overflow and truncation errors. For these reasons, it is advantageous to optimize the degree of image compression, i.e. to so operate the angiography apparatus as to use e.g. an average 12:1 image compression ratio for all incoming image information, and to thereby optimize the tradeoff between image detail and system performance. (It should be noted that the image compression ratio is only meaningful on an average basis; if a sequence of images is compressed on an average 12:1 basis, this means that the entire sequence will be so compressed; the individual images will likely be more or less compressed.)
Unfortunately, such optimization is difficult to do. The JPEG compression algorithm was developed to compress still images rather than image sequences. Different angiographic sequences compress differently even when JPEG-compressed using the same value for the Q factor. In other words, it is impossible to correlate the value of the Q factor and image compression on an a priori basis. This is because the image content of the incoming information is not known in advance. Patient size, organs of interest, collimation setting and acquisition mode (fluoro or cine) all affect this image content. As a result, to arrive at a desired optimal image compression, it is presently necessary to experiment with different values for the Q factor. This is unsatisfactory, because such an approach requires a capable operator and does not permit real time operation.
One object of the invention is to provide angiographic method and apparatus for automatically, i.e. without an operator, determining an appropriate value for the quantization factor Q for a desired average compression ratio C.
Another object of the invention is to provide such method and apparatus that will operate in real time.
Still another object is, in general, to improve on known image compression methods and apparatus of this general type.
SUMMARY OF THE INVENTION
The invention proceeds from the realization that angiographic image sequences fall into a special category. Angiographic image sequences are highly correlated with respect to time. As a result, a sample sequence from an angiographic image sequence can be used to establish, for the angiographic sequence as a whole, a correlation between average compression ratio and Q factor. Furthermore, within a certain domain that includes a predetermined range of values for the quantization factor, the average compression ratio varies linearly with respect to quantization factor. Therefore, by determining the average compression ratio resulting from use of a value for the Q factor at or near one end of the range and the average compression ratio resulting from use of another value for the Q factor at or near the other end of the range, the value of the Q factor corresponding to the desired average compression ratio can be determined by linear interpolation.
While the invention is therefore specifically applicable to angiographic image sequences, the invention also applies to image sequences that have the same temporal correlation and linearity characteristics. Furthermore, the invention encompasses mathematical equivalents; using a variable other than the quantization factor is within the scope of the invention if its relationship with compression ratio can be derived from the relationship between the quantization factor and the compression ratio.
REFERENCES:
patent: 3908081 (1975-09-01), Greenberg
patent: 4922273 (1990-05-01), Yonekawa et al.
patent: 5146324 (1992-09-01), Miller et al.
patent: 5241383 (1993-08-01), Chen et al.
patent: 5327254 (1994-07-01), Daher
patent: 5543844 (1996-08-01), Mita et al.
patent: 5594598 (1997-01-01), Shikakura
patent: 0 444 918 A2 (1991-09-01), None
patent: 0 514 663 A2 (1992-11-01), None
Wallace, The JPEG Still Picture Compression Standard, Feb. 1992, pp. xviii-xxxiv, IEEE.*
Tsai et al., Motion Estimation and Wavelet Transform in Angiogram Video Coding, Nov. 1993, pp. 1121-1125.*
XP 000479218, “A Video Compression Algorithm With Adaptive Bit Allocation and Quantization”, Eric Viscito et al., Publication Date: Nov. 11, 1991, SPIE vol. 1605 Visual Communications and Image Processing '91: Visual Communication, pp. 58-72.
XP 000613973, “A Model for JPEG Quantization”, B. G. Sherlock et al., 1994 International Symposium on Speech, Image Processing and Neural Networks, Apr. 13-16, 1994, Hong Kong, pp. 176-179.
XP 000613902, “Simulated Annealing Applied
Jay Mark H.
Johnson Timothy M.
Siemens Medical Systems Inc.
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