BONE EVALUATION METHOD

Abstract: A method for evaluating cancellous bone from a density partern of the cancellous bone obtained by irradiating the cancellous bone with a radiation wherein trabeculae of the cancellous bone are analyzed based on a power spectrum obtained by a Fourier transformation of the density pattern is provided. Furthermore, a method for evaluating cancellous bone from the density pattern of the cancellous bone obtained by irradiating the cancellous bone with radiation, wherein the density pattern is determined by irradiating the radiation sub­ stantially vertically against the main trabecula of the can­ cellous bone and the degree of the bone atrophy of the can­ cellous bone is evaluated by using the area (ΣGS) of the density pattern and/or the value (ΣGS/D) obtained by dividing the area (ΣGS) by the bone width (D) of the cancellous bone.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bone evaluation method. More specifically, it relates to a bone evaluation method in which a cancellous bone is evaluated based on a bone density pattern (i.e. a graph obtained through scanning the bone or the X-ray film and plotting the intensity of transmitted photon beam or X-ray) of the cancelous bone. For the evaluation of bones, the bone density pattern is transformed to power spectrum through the algorithm of the Fourier transformation. 2. Description of the Related Art The human bones are generally subdivided into cortical bones and cancellous bones. The cortical bone is dense osseous tissue and is represented by the diaphysis of appendicular skelton in the form of pipes. On the other hand, cancellous bone consists of trabeculae. Examples of such cancellous bones are present in the epiphysial portions of long shaft bones, spine, carpal bones, calcanei, tali, and tarsal bones. However, since the cancellous bones have a larger surface in contact with soft tissue containing vasculatures, the cancellous bones shown higher metabolic turn over and are predisposed to rapid changes under bone diseases or treatments. The so-called MD method is known as a method for evaluating cortical bones, for example, growth conditions of human skelton, bone age, or kinds of bone diseases such as osteoporosis and osteomalacia, as disclosed in, for example, "Kotsu Taisha" (i.e., Bone Metabolism) 13 , pp 187-195 (1980) and 14 , pp 91-104, (1981). The improved MD method is disclosed in, for example, "Kotsu Keitai Keisoku" (i.e., "Bone Morphometry") 5 , 36-46 (1985). On the other hand, a method is known of evaluating the cancellous bone whereby the bone is evaluated by analysing changes in the trabeculae from a plane X-ray of the cancellous bones. That is, the distribution of the trabeculae of the cancellous bones depends upon the stress distribution applied to the bones, and they contain thick trabeculae supporting the body weight (i.e., principal trabeculae) and thin trabeculae linking the thick trabeculae (i.e., sub-trabeculae). When the bone volume is decreased due to the diseases such as osteoporosis, commonly the sub-trabeculae are first absorbed and the principal trabeculae are retained in a relatively good condition. Accordingly, various methods for evaluating the degree of bone atrophy were developed utilising the above-mentioned common occurrence. For example, the Itami's classification for the spine, the Singh's classification for the neck of the femur, and the so-called calcaneal index for calcanei are known (see Osteoporosis (Kiso and Rinsho), T. Fujita et al ., pages 331-337, published on November, 1983 by Kyowakikaku Tsushin; J. Bone and Joint Surg., Vol. 65 B, No. 2, p 195-198 (1983); and J. Bone and Joint Surg., Vol. 52 A, 457 (1970)). US-A-3944830 discloses a method and apparatus for human bone density determination. According to this disclosure, in vivo γ- or x-ray radiation of two different photon energies is scanned across the human body part to be examined. For the two photon energies, the logarithms of the intensities of the radiation transmitted through the human body part are determined simultaneously. The logarithm values obtained are processed on line such that the ratio of these values is changed by a predetermined factor. The processed values are subtracted from each other to produce a simultaneous output value which is integrated to give an examination result proportional to the bone tissue content expressed as weight per length unit in a direction perpendicular to the scanning direction while being independent of the surrounding soft tissue. However, all these methods evaluate the degree of bone atrophy by the visual observation of plane X-ray photographs and, therefore, are unsatisfactory from an objective standpoint. Furthermore, the accuracy of these methods is insufficient only to distinguish minor changes in the bone diseases, and thus, there is a strong need to develop a method for objectively and quantitatively evaluating the conditions of cancellous bones. SUMMARY OF THE INVENTION Accordingly, the objects of the present invention are to eliminate the above-mentioned disadvantages of the conventional bone evaluation methods and to provide a novel bone evaluation method capable of objective and quantitative evaluation of cancellous bones such as calcanei and spines with a good reproducibility. Other objects and advantages of the present invention will be apparent from the following description. In accordance with the present invention, there is provided a method for evaluating cancellous bones by the steps of: obtaining a density pattern of the cancellous bone by measuring the intensity of transmitted radiation through the cancellous bone wherein trabeculae of the cancellous bone are analyzed based on a power spectrum obtained by a Fourier transformation of the density pattern. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the description set forth below with reference to the accompanying drawings, wherein: Fig. 1 illustrates an X-ray photographic image of a calcaneus; Fig. 2 illustrates a bone density pattern obtained from the X-ray photographic image shown in Fig. 1; Fig. 3 illustrates a power spectrum obtained by a Fourier transformation of the bone density pattern; DESCRIPTION OF THE PREFERRED EMBODIMENTS A first aspect of the present invention will now be explained. According to the present invention, when a cancellous bone is evaluated, a bone density pattern is first determined by measuring the photodensity of an X-ray photograph of the cancellous bone or by scanning the cancellous bone with photon beam. For the evaluation of the cancellous bone, calcanei, the spine, carpal bones, talus, tarsal bones, the epiphysial portions of long bones, and the like are most suitable. The X-ray photograph can be taken by any conventional X-ray photographic method and the desired bone density pattern can be obtained by measuring the photodensity of the resultant X-ray photograph. The bone pattern can be determined, for example, according to a conventional MD method. That is, the photodensity of the X-ray photograph is measured, together with the X-ray photograph of an aluminum step wedge consisting of, for example, 20 steps each having a minimum height of 1 mm and a maximum height of 20 mm), or an aluminum slope, by means of a densitometer. Alternatively, the X-ray photograph of the bone is read, together with that of the aluminum step wedge or aluminum slope, by a video camera, to determine the bone density pattern. When determining the bone density pattern, preferably the photo density of the bone taken by an X-ray photograph is read substantially vertically against the main trabecula of the cancellous bone. This is because, when the X-ray photograph is read substantially vertically against the main trabecula of the cancellous bone, a density pattern most preferably reflecting the principal trabeculae of the cancellous bone can be obtained. As an example, the bone evaluation method according to the present invention will now be explained in the case of a calcaneus. Figure 1 schematically illustrates an X-ray photographic image of a calcaneus. The density pattern is determined, to obtain a wave spectrum most accurately reflecting the main trabecula of the calcaneus, as follows. That is, a line a is drawn, for example, between the center of the points A and B and the center of the points C and D and the photodensity, for example, at a 10 mm portion in the center portion of line a is read, and thus, a density pattern is obtained. The density pattern most preferably reflecting the main trabecula is as shown in Fig. 2. However, it should be noted that the line a can be drawn, for example, between the center of the points A and B and the point C of the bony prominence. On the other hand, the bone density pattern can be obtained from the cancellous bone by photon absorptiometry. According to the photon absorptiometory method, a photon beam is used instead of an X-ray and the amount of the photon transmitted through the bone is quantitatively counted by a detector (see "Science" Vol. 142, pp 230 (1963)). In this method, the cross-section of the cancellous bone is scanned by a photon and the count numbers of the photon transmitted through the bone are plotted as an image as shown in Fig. 2. The bone density pattern is then transformed, by a Fourier transformation, to obtain a power spectrum. Before processing with Fourier transformation, it is preferable to subtract the direct current component from the bone density pattern. More preferably, in order to eliminate the effects from heterogeneous aspect of the cross-sectional shape of the bone, the pattern is subtracted by a broad curve approximating the bone density pattern, which is obtained, for example, by processing the pattern with the least square method. The Fourier transformation can be effected directly from the density pattern obtained above, but can also be effected after converting the maximum and the minimum values to 1.0 and 0, respectively, or after subtracting the direct current component from the pattern. In the Fourier transformation, the density pattern is developed, as a periodic function f(x), to a Fourier series: Thus, the Fourier factors A n , B n are determined. Then, a power spectrum C n is determined as follows: C n = √ A n ² + B n ² These operations can be carried out by using the algorism of fast Fourier transformation by a computer. The power spectrum thus obtained reflects the increase or decrease of the trabeculae of the cancellous bone. A power spectrum as illustrated in Fig. 3 is obtained from Fig. 2 as follows. The density measurements of the 10 mm portions are made, for example, 10 times (preferably more than 10 times) by shifting the line a slightly in the parallel direction in Fig. 1, and are then subjected to the Fourier transformation. To obtain the density pattern of the 10 mm portion in Fig. 1, 256 samples of density data are read. Preferably, as large a number of data as possible is read for precise Fourier transformation. Thus, each C n value is obtained. After summing up these results, the curves shown in Fig. 3 can be obtained. In Fig. 3, power spectra C n (i.e., solid line) obtained from the density patterns of calcanei of healthy persons and those C n (i.e., dotted line) obtained from the density patterns of senile osteoporosis patients are illustrated. As is clear from Fig. 3, the C n values of the healthy persons and those of the patients are remarkably different in the region nearby the point where n = 10 (i.e., C₁₀). Accordingly, the trabeculae of cancellous bones can be analyzed by comparing the special C n values, e.g., C₁₀ values, after obtaining the C n values as mentioned above. Thus, the types of bone diseases, and the changes and progresses thereof, can be evaluated. To make the evaluation from the power spectra more objective, the bone evaluation can be effected by comparing the ratio of the sum of the power spectra from n = 5 to n = 20 to that from n = 40 to n = 127. That is, in the case of senile osteoporosis, the ratio of the sum of the power spectra from n = 5 to n = 20 to that from n = 40 to n = 127 is remarkably small when compared to the ratio in the case of healthy persons. Furthermore, the bone can be more objectively and quantitatively evaluated by combining the power spectrum obtained from the bone density pattern as mentioned above with the bone density distribution obtained from, for example, any conventional MD method. As explained above, according to the present invention, the bone evaluation method capable of a more objective and more reliable evaluation of cancellous bones, especially calcanei, is provided. The present invention will now be further illustrated by, but is by no means limited to, the following Example. Example An X-ray image was obtained by taking an X-ray photograph of the calcaneus of a woman aged 55 with osteoporosis. From this X-ray image, the auxiliary line a was drawn as illustrated in Fig. 1. The bone density pattern illustrated in Fig. 2 was obtained by reading the photodensity of the central 10 mm portion of the auxiliary line a with a densitometer and 256 samplies were recorded. Similar operations were repeated 10 times by slightly shifting the auxiliary line a in a parallel direction to obtain the density patterns. The above-mentioned density patterns were subjected to Fourier transformation to obtain the C n values. The C n values were summed and the results of the Fourier transformation were plotted as shown in Fig. 3. In the same manner as mentioned above, the C n values were obtained from the calcanei of healthy persons. The results are shown in Fig. 3. As clear from Fig. 3, the power spectrum values of the patients with senile osteoporosis are lower than those of the healthy person in the region nearby n = 10. Furthermore, the ratios of the sum of the power spectra from n = 5 up to n = 20 to that of from n = 40 to n = 127 were about 0.07 in the case of the patients with senile osteoporosis and about 0.12 in the case of the healthy persons. Thus, it is clear that the cancellous bone can be evaluated by the power spectra obtained from the wave spectra of the Fourier transformation.

Claims: A method for evaluating cancellous bone from a density pattern of the cancellous bone obtained by irradiating the cancellous bone with a radiation wherein trabeculae of the cancellous bone are analyzed based on a power spectrum obtained by a Fourier transformation of the density pattern. A method as claimed in claim 1, wherein the density pattern is derived from the photodensity of the X-ray photograph obtained by irradiating the cancellous bone with an X-ray. A method as claimed in claim 1, wherein the density pattern is derived from photon absorptiometry obtained by irradiating the cancellous bone with a photon beam. A method as claimed in claim 1, wherein the radiation was irradiated substantially vertically against the principal trabeculae of the cancellous bone. A method as claimed in claim 1, wherein the power spectrum is obtained from the Fourier transformation after subtracting a direct current component from the density pattern. A method as claimed in claim 5, wherein the direct current component stands for a straight line or a broad curve approximating the bone density pattern. A method as claimed in claim 2, wherein the density pattern is obtained based on the image obtained from the cancellous bone or based on the photograph obtained from the cancellous bone. A method as claimed in claim 1, wherein the cancellous bone is a calcanei, spine, carpal bone, tarsal bone, talus or an epiphsial portion of long bone.

Inventors: HIRANO, YOSHIO, UOTANI, YASUHIRO, YAMASHITA, GENTARO
Applicants: TEIJIN LIMITED
Application year: 1986
Main classification: G01N21
IPC codes: G01N23