Method of estimating the BMD of a bone using radiogrammetry

Details Method of estimating the BMD of a bone using radiogrammetry Method of estimating the BMD of a bone using radiogrammetry

2001-12-04

2004-07-13

Imam, Ali (Department: 3737)

Surgery

Diagnostic testing

Detecting nuclear, electromagnetic, or ultrasonic radiation

Reexamination Certificate

active

06763257

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of estimating the BMD of a bone by using radiogrammetry.
DESCRIPTION OF THE BACKGROUND ART
Today, Bone Mineral Density (BMD) is most often estimated using Dual X-ray Absorptiometry (DEXA) where two images are taken at two different X-ray energies and where BMD is derived from the image resulting from a subtraction of the two images. It is also possible to estimate BMD using Radiographic Absorptiometry (RA), where the X-ray absorption in the bone is used for determining the BMD of the bone. However, the use of that method requires the use of a calibration wedge in order to determine what grey level in the image corresponds to which thickness of e.g. Aluminium. This is required in order to take into account differences in film type, developing methods, radiation doses, and radiation energy used.
The use of this calibration wedge, though, incorporates the disadvantage that forgetting it or using the wrong wedge will render the image data totally useless, where after the patient will have to be irradiated once again.
BRIEF SUMMARY OF THE INVENTION
It is an object of the invention to provide a method of using standard X-ray images for providing BMD data without having to introduce standard wedges or other elements for use in the calculation.
The present invention relates to the use of values normally derived in radiogrammetry. Until now, radiogrammetry has mainly been used for more coarse predictions, such as predictions of fracture risk and the like. These types of predictions merely relate to the determination of whether a derived value is below or above a given threshold value. Exactly how the derived value generally correlates with e.g. BMD has not been in focus and has not been important, as long as the correlation in the vicinity of the threshold value is acceptable.
In radiogrammetry, the usual manner is to start out with an X-ray image of the bone and therefrom determine the overall width, w, of the bone and either the thickness, t, of the cortical shell of the bone, or the inner width, w
l
, of the bone. Obviously, w=2t+w
l
.
As seen in “Bone Densitometry and Osteoporosis”, H. K. Genant, G. Guglielmi and M. Jergas, Springer, 1998, chapter 14, different manners have been used in the estimation of e.g. the fracture risk of a bone:
the combined cortical thickness (CCT): w−w
l
∝t,
the cortical index: (w−w
l
)/w∝t/w,
the cortical area: w
2
−w
l
2
∝t(w−t), and
the relative cortical area: (w
2
−w
l
2
)/w
2
∝(t/w)×(1−t/w).
However, as has also been acknowledged by the prior art, these values have been found to correlate moderately at best with BMD measured using e.g. DEXA.
It is an object of the present invention to provide a novel manner of estimating the BMD of a bone—a manner in which a standard X-ray image may be used and a standard manner of deriving t, w, and/or w
l
, may be used, but which has been found to correlate better with e.g. a DEXA-BMD measurement of the bone.
Thus, in a first aspect, the invention relates to a method for estimating the Bone Mineral Density (BMD) of a bone, the method comprising:
obtaining two-dimensional image data comprising information relating to the cortical bone of at least a part of the bone, the image data being data obtained by exposing at least the part of the bone to electromagnetic radiation,
determining a thickness, t, of the cortical bone,
estimating the BMD of the bone as:
BMD=
const.
A
×t,
 where const.
A
is determined from a calibration based on values of t and corresponding BMD values for one or more bones.
In the present context, we will denote BMD determined using the method of the invention DXR-BMD.
In the present context, const.
A
may be a constant value used for e.g. a specific bone (typically of a predetermined type and a predetermined part thereof but for all persons in a predetermined group of persons, such as all Caucasian women.
A major reason for the existence of const.
A
with those properties is that it has been found that a number of bones in the human vertebrate have very similar shapes—except perhaps for a scaling factor—for large groups of persons, such as all Caucasian women. This means that across a predetermined group of persons, no geometric factor needs to be taken into account in order to adapt the measurement across the group.
It is contemplated that a different value for const.
A
will be suitable for e.g. Asian women or Caucasian men.
Also, it has been found that the density of non-porous bone is surprisingly constant. Or rather, the natural porosity of the dense bone is quite constant. This is also a reason why the same const.
A
may be used for a whole group of persons. However, it has been found that the natural porosity of bone varies slightly with age. Thus, in order to obtain an even better BMD measure, it may be desired to have const.
A
depend on a porosity determined from the image data.
On the other hand, there are factors which relate to the individual person and which are quite useful in a determination of BMD. Factors of these types are age, ethnicity and sex. As to ethnicity and sex, different const.
A
's may be provided for different ethnicities and sexes—or an expression for const.
A
may be provided which depends on these values and is thus adapted to take that into account. As to age, entering the age and in that manner taking the variation of bone porosity with age into account will provide this correction without requiring determination of porosity from the image data.
In these situations, the values or expressions for const.
A
may be derived on the basis of the values of t and corresponding BMD values for one or more bones as well as information relating to one or more of the factors—where the factors are those relating to the person having one of the one or more bones.
Preferably, const.
A
will not depend on any other measure derived from the image data.
A scaling factor derives from different distances from an X-ray source to the bone and from the bone to an image forming means, such as a CCD and an X-ray film. Thus, before deriving t, the image data may be scaled. This scaling may be considered a normalisation of the image data.
In order to obtain a suitable reproducibility, it is preferred that the method comprises:
determining a direction at least substantially perpendicular to a longitudinal axis of the bone,
determining the thickness, t, of the cortical bone along the direction.
In one situation, t may be determined along a single line extending in the direction.
If a better reproducibility of the determination is desired, values of t may be determined for a plurality of lines extending in the direction and being positioned at different positions along the longitudinal direction of the bone, and the BMD may be determined on the basis of t values corresponding to the individual lines. In that situation, the BMD may be determined as: const.
A
times a mean value of the t values corresponding to the individual lines.
In the above, first aspect, the correlation with t has been found sufficient especially when t is small. However, a second order correction of t
2
/w has been found to make the BMD determination better for larger values of t.
Thus, in a second aspect, the invention relates to a method for estimating the Bone Mineral Density (BMD) of a bone, the method comprising:
obtaining two-dimensional image data comprising information relating to the cortical bone of at least a part of the bone, the image data being data obtained by exposing at least the part of the bone to electromagnetic radiation,
determining a thickness, t, of the cortical bone,
determining an overall width, w, of the bone,
estimating the BMD of the bone as:
BMD=
const.
B
×t
(1
−t/w
),
 where const.
B
is determined from a calibration where pairs of (t,w) have been calibrated to corresponding BMD values for one or more bones.
The above considerations relating to const.
A
will also apply to const.
B
.
Also, in order to inc

Affiliated with

Also associated with

Rating

Method of estimating the BMD of a bone using radiogrammetry does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Method of estimating the BMD of a bone using radiogrammetry, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of estimating the BMD of a bone using radiogrammetry will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3255436