JPH07284020A - Bone density measuring method - Google Patents

Abstract

PURPOSE:To automatically extract a measurement area so as to make it suitable for the measurement of bone density by performing the setting of a measurement object area and the calculation of a correction value for which reproducibility is high based on the anatomical shape of a femur center end part. CONSTITUTION:Since it is considered that the anatomical shape of a femur part in image pickup is approximately the same in the case of the same patient as long as a substantial change such as the fracture of a bone or the like and a large difference in a photographing posture is not present around the femur, the measurement object area 20 is set based on the anatomical shape. That is, without preparing a difference picture 54 in addition to measuring the bone density of the femur center end part, the area measured by a tentative difference picture 52 and a binary picture 53 is extracted and a bone salinity amount is obtained by the cumulative density value of the tentative difference picture 52 and the correction value F. Since BMCN=BMCN '-F is attained when the cumulative density value in the tentative difference picture 52 of an N part 22 is defined as BMCN' for instance and the bone density BMDN becomes BMDN=BMD'-F when a tentative average density value is defined as BMDN' similarly for the bone density, only a threshold density value (t) is used so as to distinguish the bone and soft part tissues.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for automatically measuring bone mineral content and bone density using X-ray images, and more particularly to DEXA (DuA
al Energy X-ray Absorptiometry) method for automatic measurement of the central end of the femur.

[0002]

2. Description of the Related Art Most of the fractures due to osteoporosis occur in the vertebral body or the central end of the femur. The central end of the femur here means the neck of the femur, the greater trochanter, and the ward.
It is a triangle, and the central end of the femur is a generic term including the central end, a part of the shaft connected to the greater trochanter, and the pelvis connected to the femoral neck. Since these parts are surrounded by soft tissues composed of muscles and fats, when measuring bone density, DEXA method or DPA (Dual Photo) is used.
n Absorptiometry) method is used.

The main purposes of bone density measurement are to diagnose whether or not osteoporosis is present and to observe the therapeutic effect on osteoporosis. Especially in observing the therapeutic effect, high reproducibility of measured values is required. Without sufficient reproducibility, it is not possible to determine whether the change in the measured value over several measurements is a therapeutic effect or a device variation. Usually, a highly reproducible measurement method that can observe changes in bone mineral content, which is said to be several percent per year, is required.

In order to realize high reproducibility, (1) setting reproducibility of a region for measuring bone density, and (2) setting reproducibility of a region for correcting deviation of measurement values due to fat (hereinafter referred to as correction region). , Two must be met.

As shown in FIG. 2, the correction area 21 is a soft tissue portion in a rectangular measurement target area 20 that normally surrounds the central end of the femur, and the correction value is calculated from the density value of the correction area 21.

Conventionally, the operator designates the measurement target region 20, and based on this, the correction region 21 is set, the correction value is calculated,
The measurement area is extracted, the area of each part, the amount of bone mineral, and the bone density are measured. As shown in FIG. 2, there are three measurement regions, namely, the neck 22, the greater trochanter 23, and the Ward triangle 24 (hereinafter referred to as N, T, and W, respectively) at the central end of the femur. (THE BONE 1991.9 vol5 No3 p73-79).

[0007]

Conventionally, since the operator designates and determines the measurement target region 20, the setting reproducibility of the correction region 21, which is influenced by this, has not become high. Therefore, even if the measurement region is set to have high reproducibility, the reproducibility of the measured value is not connected.

It is an object of the present invention to perform both the setting of a measurement target area and the extraction of a measurement area in the bone density measurement of the central end of the femur with high reproducibility based on the anatomical shape of the femur. The purpose of the present invention is to provide a method for measuring bone density performed in.

[0009]

In order to achieve the above object, the present invention includes a femur from either the left or right pelvis, which is usually photographed when measuring the bone density of the central end of the femur, as shown in FIG. Two X images of the high and low energy of the part
A temporary difference image 52 is created from the line images 50 and 51 using a predetermined coefficient. Next, based on the anatomical shape of the central end of the femur and the density value changes in the vertical and horizontal directions of the temporary difference image 52 associated with the shape, the measurement target region 20 surrounding the central end of the femur 20 Is set, the correction value F is calculated from the density value of the correction region 21 which is the region of the soft tissue portion in the measurement target region 20, and the bone density is measured from the correction value F and the temporary difference image 52. The difference image 54 is created. Further, the main axis of the shaft of the femur is calculated from the difference image 54, the measurement area of the central end of the femur, that is, the N section, the T section, and the W section are extracted with the main axis as a reference, and the difference in the area of each section is calculated. The bone mineral content is calculated based on the sum of the density values of the image 54,
The bone density is calculated based on the average density value.

As another means, without creating the difference image 54, the N portion, the T portion, and the W portion are extracted on the temporary difference image 52, and the density of the temporary difference image 52 in the area of each portion is extracted. The bone mineral density can be calculated from the sum of the values and the correction value F, and the bone density can be calculated from the average density value and the correction value F. Further, the length from the connecting portion of the pelvis and the femur to the N portion or the T portion, the amount of bone mineral of the central femoral end included in the measurement target region, the bone density, and the like can be calculated.

[0011]

The anatomical shape of the femur in the photographed image is considered to be almost the same for the same patient unless there is a significant change such as a fracture or a large difference in the photographing posture around the femur. Since the above-mentioned means sets the measurement target region 20 based on this anatomical shape, this region can always be set with high reproducibility for the same patient. For example, even if the imaging position is displaced in the vertical direction or the horizontal direction, if at least the measurement region is captured, the measurement target region 20 is determined based on the anatomical shape without being affected by the displacement of the image. It is possible to set.

According to the present invention, as shown in FIG. 5, a temporary difference image 52 created from the captured L image 50 and H image 51 and a threshold density value t for distinguishing between bone and soft tissue are used. The measurement target region 20 as shown in FIG. 2 is set based on the vertical and horizontal density value changes of the difference image 52.

For example, in FIG. 2, a longitudinal boundary 201 between the femur and the soft tissue has a bone density of 1 and a density of the soft tissue or air when the threshold density value t is 0 or less. Is 0, the cumulative value in the vertical direction of the image of the part without bone is 0, and the cumulative value is not 0 when the part of bone is included. Therefore, the boundary between the position where the cumulative value in the vertical direction is 0 and the position where the cumulative value is not 0 can be set as the boundary 201. Further, the lateral boundary 202 of the connecting portion between the pelvis and the femur is
On the provisional difference image 52, the bone in the connected portion is thin and thus has a slightly low density, and the pelvis above the connected portion has a high density as compared with the surrounding area because the bone is thick. Therefore, the boundary 202 is a position where the cumulative density value or the average density value in the horizontal direction in the temporary difference image 52 is maximized and the cumulative density value is decreased → increased at a position slightly lower than that.
Can be set as Since the bone at the lower end of the temporary difference image 52 is the shaft, the left and right end points p′25,
q'26 can be extracted. By extracting this end point, the position and rough size of the femoral center end can be determined, and based on this, the boundaries 201-204 described above can be determined.
The range for calculating the cumulative value when setting is determined. In the measurement target area 20 set by the above method, an area having a threshold density value less than t is set as a correction area 21. The measurement area is the N section 22, the T section 23, and the W section 24 shown in FIG.

The N portion 22 is a region which is perpendicular to the central axis CD32 of the femoral neck shown in FIG.
Therefore, calculation of the central axis CD32 is necessary to extract the N portion 22. However, since the femoral neck has a short length and the contour shape is not always smooth, it is problematic in terms of accuracy to directly calculate the central axis CD32 from the neck contour. Therefore, in the present invention, the central axis CD32 of the neck is calculated based on the central axis AB31 of the femoral shaft shown in FIG.

Radiology 87: p904-90
7, According to November 1966, central axis AB31 and CD3
The angle α33 formed by 2 is about 127 °. Central axis AB31
Can be accurately calculated because the contour is sufficiently long and the contour has a smooth shape. In this way, first, AB31 is calculated, and after setting CD 'forming 127 ° with AB31,
By calculating the CD32 by adding the correction to the CD ', the CD
32 can be accurately set.

The T portion 23 is adjacent to the N portion 22 and is extracted as a region which does not touch the shaft. Although the boundary between the T portion 23 and the shaft cannot be clearly set, it is appropriate to set the boundary above the central axis CD32 of the neck, and therefore it is predetermined with respect to the central axis CD32 of the neck. Angle β7
A straight line passing through the center of gravity G of the N portion 22 with an inclination of 1 is defined as a boundary. Alternatively, a straight line passing through the center of gravity of the N portion 22 at an inclination that forms a predetermined angle β′72 with respect to the central axis AB31 of the shaft is set as a boundary. As mentioned above, the central axis AB of the shaft
Since 31 and the central axis CD32 of the neck are accurately calculated, the boundary set based on this can also be accurately set.

The W portion 24 is located around the adjacent portion of the N portion 22 and the T portion 23 where a large amount of cortical bone is present, has a large amount of cancellous bone, and is located between the trabecular bones, and thus has a relatively low density. From this, the position having the lowest average density around the adjacent portion between the N portion 22 and the T portion 23 is set as the W portion 24. The area of the W portion 24 is determined in advance, and the shape is set based on the inclination of the N portion and the inclination of the central axis CD32. As described above, since each measurement region is set based on the shaft center axis AB31 and the neck center axis CD32 that can be set with high accuracy, the region setting reproducibility can be increased.

[0018]

EXAMPLE This example is a method of measuring the amount of bone mineral and the bone mineral density at the central end of the femur using the DEXA method.

FIG. 4 is an example of a system configuration for implementing the present invention. The functional outline of each unit will be described below. The X-ray generator 41 alternately generates fan-beam-shaped X-rays with two kinds of high and low voltages. The X-rays pass through the subject 40, are detected by the X-ray detector 42, and are respectively converted by the AD converter 43. The generator 41 and the detector 42 work together to perform shooting while moving in the shooting section, and the two types of finally obtained images are sent to the image processing device 44. In the image processing device 44,
The bone mineral content and bone density measuring processes described below are performed. The display device 45 displays images, operation messages, measurement results, etc., and the operation console 46 is equipped with a keyboard, a mouse, a trackball, etc., and gives instructions and inputs. External storage device 47
Stores image data, and the output device 48 is the display device 45.
Make a hard copy of the display content of.

A method of measuring the amount of bone mineral and the bone mineral density at the central end of the femur will be described below with reference to the flowchart of the processing procedure of FIG.

Step 10: Creation of Temporary Difference Image S'52 (FIG. 5) Images photographed with two kinds of high and low voltages (hereinafter, L image 50,
H image 51), and a constant r, a correction value R (H
(i, j)), the temporary difference image S′52 is converted to S ′
(i, j) = (L '(i, j) -r * H' (i, j)) * R
Create as (H '(i, j)). Here, since the L image 50 and the H image 51 are theoretically shifted by 0.5 pixel depending on the photographing method, images L'and H'where at least one image is aligned by the cubic convolution Cubic Convolution method or the like are used. Further, when the beam hardening phenomenon occurs in the image L ', the correction is also performed. The beam hardening correction can be obtained as a function of the H image 51 like the correction value R.

The image photographed by the DEXA method is an integral image, and the difference image S54 created for measuring the bone density is an image in which the density of soft tissue is 0, and the pixel density of the bone part is larger than 0, The pixel density is the bone density as it is. If there is no fat, the soft tissue is almost the same as water, so S'52 created here is used as the difference image S54.
Can be used as However, in reality, the effects of fat occur. If the fat amount is large, the pixel density will decrease, so it is necessary to obtain a correction value F based on the fat amount for each data and correct S′52.

Step 11: Setting the measurement target area 20 and the correction area 21 (FIGS. 2 and 5) The correction value F is the average density of the soft tissue around the measurement area.
Calculate as the value to be set. A correction area 21 for obtaining the correction value F is set. To set the correction area 21, it is necessary to set the measurement target area 20. First, a binary image 53 for distinguishing a soft tissue and a bone portion from S'52 is created, for example, as 1 if the pixel density of S'is greater than or equal to a predetermined threshold density value t, and as 0 if it is less than t. . Threshold concentration value t
Is used to distinguish between soft tissue and bone. However, this process alone produces sesame salt-like noise in the soft tissue part and bone, so noise removal is performed and the final 2
The value image 53 is set. Noise removal methods include smoothing and determining from the density value around the point of interest whether the point of interest is an isolated point and removing it.

Next, the boundaries 201 to 204 of the measurement target area 20 are set using S'52 and the binary image 53. The boundary 201 can be determined from the change in the vertical cumulative density value of the binary image 53, and the boundary 202 can be determined from the change in the horizontal cumulative density value of S′52. Similarly, the boundary 203 is
It is determined from the cumulative density value of S'52 in the vertical direction. These boundaries 201 to 203 are determined from the cumulative density value. The cumulative range is obtained by finding the left and right end points p'25, q'26 of the shaft from the horizontal line below the imaging position, and from the position of the end points of the femur. A cumulative range is set based on the anatomical shape. In addition, since the inflection points p _{2 to} p ₇ , q ₂ , q ₃ can also be obtained from the binary image 53 based on the anatomical shape of the femur, the cumulative range is set also by using those positions. be able to.

As a simple method, the boundary 204 can be set at the bottom of the photographing position, or can be set at a position separated from the boundary 202 by a predetermined distance. However, in this method, the measurement target area 20 has an absolute size, and therefore the relative size and setting position of the measurement target area 20 vary depending on the difference in the imaging conditions. Therefore, as a method of relatively setting the measurement target region 20 regardless of the imaging condition, for example, p ₂ is detected, and the boundary 204 is set at a position where the distance between p ₂ and the boundary 202 is doubled. Further, a region in the measurement target region 20 in which the density value of the binary image 53 is 0 is set as a correction region 21.

By the above method, the measurement target area 20 and the correction area 21 can be set relatively constant even if the object is displaced due to the photographing conditions or the size is different. As another method, the left and right end points p′2 of the shaft
5, q'26, from the inflection point _{p 2 ~p 7, q 2,} q 3, it is also possible to directly set the boundaries 201-204. For example,
The boundary 201 is the position of the inflection points p ₂ and p ₃ , the boundary 202 is the position above the position of the inflection points p ₄ and p ₅ in the vertical direction between p ₂ and p ₄ , and the boundary 203 is the inflection point. The position of the point q ₃ is set as a reference, and the boundary 204 is set by the above method.

Step 12: Calculation of Correction Value F In S'52, the average density value F of the correction area 21 is obtained.

Step 13: Creation of a difference image S54 A difference image S54 is created by correcting the deviation of the density value due to fat from S'52 and the correction value F so that the pixel density at a certain position of the bone becomes the bone density at that position. To do.

S (i, j) = S '(i, j) -F Step 14: Measurement Area Extraction Using the binary image 53, the points p ₁ and p at which the boundary of the measurement target area 20 and the contour of the femur contact Calculate ₈ , q ₁ , and q ₄ . Less than,
A method of extracting the N portion 22, the T portion 23, and the W portion 24 at the central end of the femur will be described in order.

(1) Extraction of N part 22 (FIG. 6) (a) Setting of the central axis AB31 of the shaft Midpoints of p ₁ and p ₂ of the contour surrounding the femur p _x , midpoints of q ₁ and q ₂ q _x is obtained, each of the approximate straight lines is obtained by using the contour coordinates between p ₁ and p _x and the contour coordinates between q ₁ and q _x , and the center line is set as the central axis AB31. Alternatively, the region p ₁
p _x q _x q inertia of the main shaft calculated from all the coordinates within ₁ may be the same as the central axis AB31. Also, p _x is p ₁
And not limited to a middle point between the p _2, it may be set anywhere in between, of course p ₁ and p _2. The same applies to q _x . However, since the shaft expands as p _x approaches p ₂ and q _x approaches q ₂ , it is better to set p _x q _x at a position where the shaft is stable.

(B) Setting of the central axis CD32 of the femoral neck and extraction of the N portion 22 The inclination of a straight line forming an angle α33 counterclockwise with the central axis AB31 is set as the initial inclination a ₁ of the central axis CD32. . The angle α33 is 125 ° to 130 °, and is 127 here.
To °. Next, the most constricted paired coordinates P _N and Q _N between the neck contours p _{6 to} p ₇ and q _{2 to} q ₃ are detected, and the inclination a ₂ perpendicular to the line segment P _N Q _N is obtained, and a An inclination a between ₁ and a ₂ is the inclination of the central axis CD32. Centering on the midpoint of the line segment P _N Q _N ,
Line segments N ₁ N ₂ and N ₃ N ₄ are set at a predetermined distance k from the center with a slope b perpendicular to the slope a, and a region surrounded by the line segment and the contour is referred to as an N portion 22. Further, when the set position of the N portion 22 is inappropriate, the position of the N portion is corrected by moving in parallel in the direction perpendicular to the inclination a. When the setting position of the N portion 22 is inappropriate, for example, N ₄ is set to the p ₅ side from p ₆ or N ₁ is set to the p ₈ side from p ₇ or N ₂ is set to the q ₄ side from q ₃ This is the case. The corrected center of gravity G of the N portion 22 is obtained, and the straight line passing through G at the inclination a is set as the central axis CD32.

(2) Extraction of the T portion 23 (FIG. 7) The intersection of the central axis CD32 and the line segment N ₃ N ₄ is designated as T ₂ . The intersection point between the central axis CD32 passing through T ₂ and the straight femoral contour that forms the angle β71 in the clockwise direction is defined as T _1, and the region surrounded by the contour between T ₁ T ₂ N ₄ and T ₁ and N ₄ is defined as T23. To do. Angle β71 is 5 °
It is about 20 °, and is 10 ° here. Alternatively, T ₁ may be an intermediate contour position between C and p ₂ , or T ₂ may be the intersection of the line segment T ₁ G and the line segment N ₃ N ₄ . As still another method, the intersection of the straight line passing through T ₂ and the outline of the femur is tilted at an angle β′72 with the central axis AB31 of the shaft in the clockwise direction, and T ₁ is extracted, and the T portion 23 is extracted. Angle β'7
2 is about 60 ° to 70 °.

(3) Extraction of the W section 24 A position where the average density of the difference image 54 calculated at some points in the periphery becomes the lowest within a circular area centered on T ₂ and having a radius of T ₂ G. W ₁ is obtained, and the rectangular region W portion 24 is extracted so as to have a predetermined area Z centering on W ₁ . W part 24
The inclination of each side of is set to be parallel to the central axis CD32 and the line segment N ₃ N ₄ . Here to region seeking W ₁ it is may not be a circular, W ₁ may be a pixel density without requiring the average concentration. Further, the W portion 24 may be circular instead of rectangular.

Step 15: Calculation of Bone Mineral Amount and Bone Density The total number of coordinates in each region in the difference image 54 of the N part 22, T part 23 and W part 24 extracted above is defined as the area, and the cumulative density value is the bone mineral density. The amount and the average density value are used as the bone density. Further, a region surrounded by the contours p _{1 to} p _{5 to} N ₁ , the line segment N ₁ N ₂ , the contours q _{1 to} q _{2 to} N ₂ and the line segment p ₁ q ₁ is defined as a region of the entire femoral central end portion. Calculate area, bone mineral density, and bone density. Alternatively, the region surrounded by the contours p _{1 to} p ₈ , the line segment p ₈ q ₄ , the contours q _{1 to} q _4, and the line segment p ₁ q ₁ may be the region of the entire femoral central end portion.
Further, as other measured values, the length of the line segment CD32 and the length of the line segment T ₂ D can be calculated and used as an index for diagnosing the risk of fracture together with the bone density.

According to the flowchart of FIG. 1, in addition to the method of measuring the bone density of the central end of the femur, the measurement area is extracted from the temporary difference image 52 and the binary image 53 without creating the difference image 54. The bone mineral content can be calculated from the cumulative density value of the temporary difference image 52 and the correction value F. For example, N part 22
Assuming that the cumulative density value in the provisional difference image 52 is BMCN ′, the bone mineral content BMCN of the N portion 22 is BMCN = BM.
It becomes CN'-F. Similarly for the bone density, if the average density value in the temporary difference image 52 of the N portion 22 is BMDN ′,
The bone density BMDN of the N portion 22 is BMDN = BMDN′−
It becomes F. Other areas can be similarly obtained.

Further, although the binary image 53 is created in the embodiment, it is possible to perform processing by using only the temporary difference image 52 and the threshold density value t for distinguishing between bone and soft tissue.
It is not necessary to create the binary image 53.

[0037]

As described above, the present invention can automatically set the measurement target area, calculate the highly reproducible correction value, and extract the measurement area based on the anatomical shape of the central end of the femur. It is effective as a method for measuring bone density.

[Brief description of drawings]

FIG. 1 is a processing flowchart of bone density measurement according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of image processing according to an embodiment of the present invention.

FIG. 3 is an explanatory view of the main axis of the femoral shaft and the main axis of the femoral neck.

FIG. 4 is an explanatory diagram showing an example of a system configuration for realizing the present invention.

FIG. 5 is an explanatory diagram of creating a difference image according to an embodiment of the present invention.

FIG. 6 is an explanatory diagram of an extraction method of an N unit 22.

FIG. 7 is an explanatory diagram of a method of extracting a T portion 23.

[Explanation of symbols]

20 ... Measurement target area, 22 ... N section, 23 ... T section, 25,
26 ... Left and right end points p ′ of shaft, 31 ... Main axis of shaft, 32, 33 ... Main axis of femoral neck, 50 ... L image, 5
1 ... H image, 54 ... Difference image.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mitsuya Kumamoto 1-1-14 Kanda, Chiyoda-ku, Tokyo Inside Hitachi Medical Co., Ltd.

Claims (9)

[Claims] 1. A method of creating a difference image from an X-ray image including the central end of the femur captured by two types of energy, high and low, and measuring the bone density of the neck of the femur based on the difference image. A bone density measuring method comprising a process of automatically setting a measurement target region surrounding a central end of a femur and extracting the region of the femoral neck. 2. The femoral neck region extraction process according to claim 1, wherein the main axis of the shaft of the femur is calculated, and the thigh is set with reference to an inclination forming a predetermined angle α from the inclination of the main axis. A bone density measuring method including a process of calculating a principal axis of a bone neck. 3. The bone density measuring method according to claim 2, wherein the angle α is 125 ° to 130 ° counterclockwise with respect to the main axis of the shaft. 4. A method of creating a difference image from an X-ray image including the central end of the femur photographed with two types of energy, high and low, and measuring the bone density of the greater trochanter part based on the difference image. A bone density measuring method comprising a process of automatically setting a measurement target region surrounding the central end of the femur and extracting the region of the greater trochanter. 5. The area extracting process of the greater trochanter part according to claim 4, wherein the main axis of the femoral neck is calculated, and the inclination forming a predetermined angle β from the inclination of the main axis is used as a reference. A bone density measuring method including a process of calculating a boundary line that separates the region of the greater trochanter and the shaft. 6. The bone density measuring method according to claim 5, wherein the angle β is 5 ° to 20 ° clockwise with respect to the main axis of the femoral neck. 7. The area extracting process of the greater trochanter part according to claim 4, wherein the main axis of the shaft of the femur is calculated, and an inclination forming a predetermined angle β ′ from the inclination of the main axis is used as a reference.
A bone density measuring method including a process of calculating a boundary line that separates the region of the greater trochanter and the shaft. 8. The bone density measuring method according to claim 6, wherein the angle β ′ is 60 ° to 70 ° clockwise with respect to the main axis of the shaft. 9. The process for automatically setting the measurement target region according to claim 1, wherein the left and right end points of the shaft of the femur are extracted from the lower end portion of the difference image, and the femoral central end is extracted based on the end points. A bone density measuring method including a process of setting the position and size of a part.