Image analysis – Applications – Biomedical applications
Reexamination Certificate
1998-10-30
2002-06-25
Boudreau, Leo (Department: 2621)
Image analysis
Applications
Biomedical applications
C382S128000, 36, C128S126100, C600S407000
Reexamination Certificate
active
06411729
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for estimating aspects of the bone quality or skeletal status of a vertebrate on the basis of two-dimensional image data comprising information relating to the trabecular structure of at least a part of a bone, typically a long or tubular bone. Especially bones comprising a tube (diaphysis) and at least at one end an epiphysis comprising trabecular bone are suitable for use in the present method.
In the present context, skeletal status is taken to relate to e.g. biomechanical competence, trabecular homogeneity, cortical thickness of the bone or a quantification of sensitivity, specificity or accuracy of diagnosis of osteoporosis.
DESCRIPTION OF RELATED ART
Age, or more likely co-variates with age, such as e.g. risk of falling, the type of falls and skeletal status, significantly affects fracture risk. Hence, any type of BMD must be viewed only as an index of risk. However, when age-matched individuals with and without fractures are compared, an overlap in BMD measurements is apparent. It is contemplated that this, in part, is due to the fact that important skeletal properties, such as trabecular connectivity (or homogeneity), is largely ignored by BMD measurements, since small losses in cancellous bone mass may lead to a disproportinately large loss in biomechanical competence.
The surface area of cancellous bone is greater than that of cortical bone, even though cancellous bone may occupy only 25% of the skeletal mass of a healthy individual and as little as 10% of that of an osteoporotic patient. Because of the high surface-to-volume ratio of cancellous bone tissue, osteoporosis is thought to affect trabecular sites early in the disease process. In postmenopausal osteoporosis, there is a marked loss of trabecular elements, but little evidence of actual trabecular thinning. The loss of elements is thought to be a result of cavities that transect or perforate structures. This is thought to have important implications for skeletal strength, and hence for those components of fracture risk related to skeletal strength.
Currently, the connectivity of cancellous bone can be measured reliably only by invasive techniques or by the technique described in the Applicants co-pending International application No. PCT/DK95/00338.
In U.S. Pat. No. 4,903,203, a method of positioning an ROI in a bone is disclosed. In this reference, only a single ROI is determined, mined, and no variance of PTD is mentioned.
In EP-A-0 648 467, different methods are disclosed wherein BMD is evaluated in a distal radius. In order to evaluate the method herein, BMD was investigated at different positions along the length of the bone. However, the actual method of this reference relates to a single BMD value averaged over a number of measurements. Thus, no variation of PTD has been investigated or is pointed to.
Presently, a new method is proposed that by quantifying trabecular homogeneity, as it appears on e.g. a radiograph of the distal forearm, an index of skeletal status may be provided.
Researchers have proposed a number of different ways to measure variations in radiographs that can be attributed to cancellous bone. Geraets et. al. (W. G. Geraets, P. F. Van der Stelt, C. J. Netelenbos and P. J. Elders, “A new method for automatic recognition of the radiographic trabecular pattern”, J. Bone Miner. Res. 5(3), pp 227-233, March, 1990) proposed different textural measures estimated from a 10 mm×10 mm Region Of Interest (ROI), extracted close to the border of the former epiphysal growth plate. The extracted measures were correlated to wrist BMD, and moderate correlations were obtained. The inventor has not been able to obtain statistically significant correlations with biomechanical competence (of the hip), using this method. Weinstein et. al. (R. S. Weinstein and S. Majumdar, “Fractal geometry and vertebral compression fractures”, J. Bone Miner.. Res., 9(11), Nov. 1994) and, Majumdar et. al. (S. Majumdar, R. S. Weinstein and R. R. Prasad, “Application of fractal geometry techniques to the study of trabecularbone”, Med. Phys., 20(6), pp 1611-1619, November 1993) proposed applying a box counting algorithm for fractal analysis of the trabecular bone matrix.
Chung (H. W. Chung. C. C. Chung, M. Underweiser and F. W. Wehrli, “On the fractal nature of trabecular structure”, Med. Phys., 21(10), pp 1535-1540, October 1994) investigated this method and concluded that the box counting algorithm is not suitable for fractal analysis of cancellous bone and continues to suggest that the fractal appearance of the trabecular network is artifactual. Benhamou et al. (C. L. Benhamou et al., “Fractal organization of trabecular bone images on calcaneous radiographs”, J. Bone Miner. Res., 9(12), pp 1909-1918, December 1994) suggests that the surface may, in fact, be fractal, if it is not binarized. It is proposed to use a fractional gaussian noise model to obtain an estimate of the fractal dimension for a number of calcaneous images, and a high degree of reproducibility of this method is reported, although correlations to specific biomechanical properties are not reported in this paper.
Trippi et al. (D. Trippi, M. Chimenti and R. Bozzi, “A computer-assisted method for the study of the trabecular . . . ”, J. Digit. Imaging, 6(2), pp 140-147, May 1993) propose measuring a coarseness index, based on image variance, on the forearm, and find a decrease in coarseness in the distal-proximal direction. The paper aims at defining a systematic way of selecting an ROI, and recommends measuring 6 to 12 mm from the former epiphysal growth plate.
SUMMARY OF THE INVENTION
According to the invention, it has been found that it is of limited use to look at one particular Region Of Interest. Instead, changes in the local variations, attributable to cancellous bone, must be quantified in order to obtain meaningful information about trabecular homogeneity. These findings also suggest that very strong correlations between BMD and measures of trabecular homogeneity may not exist, due to very fundamental differences in the nature of such measures. Finally, trabecular homogeneity measured on eg the forearm offers important independent information about biomechanical competence of the hip, that is not offered by any densitometric measures obtained on the appendicular skeleton.
Consequently, the present invention relates to a method for estimating the skeletal status of a vertebrate on the basis of two-dimensional image data comprising information relating to the trabecular structure of at least a part of a bone of the vertebrate, the image data being data obtained by exposing at least the part of the bone to electromagnetic radiation, the method comprising subjecting the image data to a statistical analysis comprising:
a background correction procedure in which low frequency intensity variations not related to the trabecular structure of the bone is reduced relative to image data related to the trabecular structure of the part of the bone,
a feature extraction procedure comprising
(a) determining values reflecting the projected trabecular density in the image data, caused by the X-ray attenuating properties of cancellous bone in the part of the bone, for each of a number of locations or areas in the image data,
(b) deriving one or more features from the variation of the determined PTD-values, preferably in the longitudinal direction of the bone, and
an estimation procedure in which the skeletal status of the vertebrate is estimated on the basis of the one or more derived features and optionally other features related to the bone of the vertebrate and a predetermined relationship between the features and reference skeletal status parameters.
As indicated above, in the present context “skeletal status” is not equalled to “bone quantity”, such as Bone Mineral Density, as a loss of trabecular elements may lead to a disproportionate loss of biomechanical competence.
In fact, it is contemplated that a better estimate of skeletal status may be obtained by considering pertinent properties of cor
Boudreau Leo
Choobin M. B.
Torsana Osteoporosis Diagnostics A/S
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