The present invention also relates to a device for measuring the bone mineral content of the skeleton in a part of the body according to the pre-characterising part of claim 4.

A disease rapidly on the increase throughout the world is osteoporosis, both in the industrialised world and in the developing countries. The disease mostly affects older women, but it has recently been discovered that osteoporosis can also affect younger persons and persons of both sexes.

In order to reduce the risk of this type of disease it is usual for older women, in particular, to be given medicine that has an inhibiting effect on decalcification of the skeleton.

In order to decide whether medication is required, devices are available for measuring the bone mineral content in any bone in the body suitable for this purpose.

In WO/96 21856 a device is described in which the measurements are performed by having a detector matrix exposed to X-ray radiation on two levels, so that the rays pass through a part of the body in which the bone mineral content is to be measured, whereupon the thickness of the body part is established and the detected radiation is analysed. In the analysis of the detected radiation account is taken of the thickness of the body part in order to determine the thicknesses of the main constituents that make up the body part, water, fat and bone mineral, in order thereby to arrive at a representation of the bone mineral content of the skeleton in the said parts of the body. The intensity of the X-ray radiation emitted is known in that measurements have been carried out on a reference object of known composition.

An object of the present invention is to increase the accuracy of measurements with bone mineral measuring devices according to the above.

In one embodiment this is achieved by means of a method of the type initially referred to, which has the characteristic features specified in the characterising part of claim 1.

Preferred embodiments have any or some of the characteristics specified in the subordinate claims 2 to 3.

In another embodiment this is achieved by means of a device of the type initially referred to, which has the characteristic features specified in the characterising part of claim 4.

Preferred embodiments have any or some of the characteristics specified in the subordinate claims 5 to 10 The method and the device according to the invention have a number of advantages compared to previously known techniques. The accuracy is increased in that the measurements are calibrated in several ways by means of the reference object.

Firstly, the intensities of the radiation emitted are measured, secondly the mean energy levels of the radiation emitted are measured and thirdly the size of the leakage currents in the detector medium is determined, so that the said leakage currents can be compensated for when determining the bone mineral content. Owing to the fact that the reference object forms part of the device according to the invention, there is no need to take separate measurements in order to determine the said intensities, mean energy levels and leakage currents. Furthermore the reference object is arranged in the device so that it does not interfere with measurement of a part of the body, rather the calibration measurements with the reference object are performed simultaneously with the actual measurement of the part of the body. The device according to the invention is very reliable in operation, since it does not contain any moving components; furthermore the cost of the components is low.

The invention will be described in more detail below with reference to the drawings, in which: Figure 1 shows a schematic diagram of the main components of the bone mineral measuring device Figure 2 shows a perspective view of a measuring unit contained in the bone mineral measuring device, the top of the casing of the unit having been removed.

The device according to the invention essentially comprises a measuring unit 1 and

an image and signal-processing unit 7. As will be seen from figure 1, the measuring unit 1 has a casing 2 adapted to the shape of a certain test object 6, in one side 2a of which a radiation source 3 is arranged for radiation with two energy levels, and in the second side 2b of which a radiation detector matrix 4 is arranged in order to detect the radiation emitted from the radiation source. The test object 6 comprises a part of the body to be measured, such as a heel bone or a forearm.

A first reference object 8a is arranged in the measuring unit 1. The said reference object is placed in the radiation path between the radiation source 3 and the detector matrix 4, so that when measuring the test object it is exposed to the radiation source 3, but not overlapping a space designed to receive the part of the body. The reference object 8a preferably has a permeability in the same order of magnitude as the part of the body. The reference object 8a is, for example, a 5 mm thick aluminium plate or a 10 mm thick plate of Perspex or a plate containing both of the components.

A second reference object 8b is also arranged in the measuring unit 1. This object is placed in the radiation path between the radiation source 3 and the detector matrix 4, so that, when measuring the test object, the first reference object 8a and the space designed to receive the part of the body are exposed to the radiation source 3, but without any overlap. The reference object 8b preferably has a predetermined radiation permeability distribution transversely to the radiation direction, the permeability being in the same order of magnitude as the part of the body. In one embodiment the reference object 8b is cuneiform so that it has a predetermined permeability gradient according to the length of the reference object 8b transversely to the radiation. In another embodiment the reference object 8b is made so that it has a number of sections of different material and consequently different permeability. In yet another embodiment the reference object is of stepped shape and therefore has a number of sections of different thickness and consequently different permeability.

The first reference object 8a, particularly in the latter two embodiments, may constitute a part of the second reference object 8b.

The radiation source 3 preferably consists of an X-ray tube that is capable of emitting photons with two separate energy levels, 30 kV and 75 kV. The said energy levels are used in order to determine the bone mineral content of the test object 6 and it is therefore important that the energy levels are well separated and well defined. The different energy levels may, for example, be achieved by allowing a generator, which can switch between energy levels, to power the X-ray tube 3. Another way of

producing X-ray radiation with two energy levels is to filter the radiation that is obtained from an X-ray source 3 with one level.

The radiation detector matrix 4 intended for detection of the radiation is arranged in an opposing position relative to the radiation source 3. The detector matrix 4 comprises a number of elements, arranged in matrix form, which can detect and quantify the radiation that strikes each point in a certain instance or over a certain time.

The X-ray tube 3 and the detector matrix 4 may either be stationary or may describe a linear movement over the test object 6 and the reference objects 8a and 8b. In both these cases a collimator 1 is arranged in connection with the X-ray tube 3, by means of which the diffusion of the X-rays can be restricted so that they coincide with the coverage of the detector plate 4.

In the stationary arrangement of the X-ray tube 3, a specially adapted collimator is fitted in front of the detector matrix. The collimator is designed so that it has as many holes bored through it as the radiation detector matrix situated behind has individual elements. The orientation of the holes is designed so that the holes diverge from the focal point of the radiation source and each of the holes is directed towards its point on the detector matrix. The functions of the collimator are to filter out scattered secondary radiation that can cause interference, to ensure that the rays that pass through the holes are parallel and to direct the permeated rays towards each respective point on the detector matrix. The collimator may be made of material that has such a high attenuation that only those rays that pass through the holes are detected by the detector matrix 4 situated behind.

In an alternative design, in which the X-ray tube 3 is made to perform a linear movement over the test object 6 and the reference objects 8a and 8b, the X-ray tube 3 is fitted to a mechanical arm. This is moved in relation to the part of the body to be measured and the reference objects 8a and 8b so as to scan the essential sections of the part of the body to be measured and the reference objects. If the detector 4 is designed to move with the X-ray tube, the detector matrix 4 may comprise a single detector element.

The measuring unit 1 further comprises a counter/amplifier 4a in order to quantify the radiation against the detector matrix 4. The counter 4a is designed so that it can

identify firstly what point on the detector matrix 4 the radiation is incident upon, and secondly how high the photon energy level is in the radiation. By using two "measuring windows"at the characteristic energy levels for the two generated voltages a more reliable result is obtained.

In the embodiment shown in figure 2, measurement of the bone mineral content of a heel bone is being carried out. The X-ray tube 3 is arranged to scan a first surface of approx. 10.0 x 15.0 cm, that is equal to the full size of the test object plus a second surface equal to a part or the whole of the size of the reference objects. When the test object to be measured, for example a foot, is placed in the casing 2, the X-ray tube 3 is directed towards one side of the foot, towards or in proximity to the heel section of the foot and towards the reference objects 8a and 8b. The size of the area scanned by the X-ray tube 3 is naturally influenced by the size of the part of the body to be measured.

In order to be able to clearly determine the principal components forming part of the test object, water, fat and bone mineral, a third measuring parameter is required. This is obtained by arranging clearance measuring elements 5 on either side of the part of the body to be measured 6, by means of which elements 5 the clearance on either side of the X-rayed test object 6 can be established. The thickness of the test object can thereby be calculated. The said elements 5 preferably comprise laser measuring circuits. In order that the placing of the test object in the casing will not lead to uncertainty in the measuring result, the clearance measuring elements 5 are arranged on either side of the test object.

The test object-the part of the body-essentially comprises three constituents: bone mineral (in the form of hydroxyapatite Cal0 (PO4) 6 (OH) 2), fat and water. The parameters relating to these constituents will be indexed below with b, f or s respectively.

For irradiating of the test object with X-rays on two energy levels, the following equations can be compiled: N)=No)exp(-mb)tbrb-msttsrs-mf)tfrf)(1) N2 = No2exp (-mb2tbrb-ms2tsrs-mf2tfrf) (2)

The X-ray radiation from the lower energy (e. g. 35 kV) gives index 1 and the X-ray radiation from the higher energy (e. g. 70 kV) gives index 2. Ni is then the measured speed of calculation after passage through the test object 6 for the energy i; No ; is the speed of calculation that should theoretically be achieved if the test object 6 were not in the measuring unit for the energy i, that is to say the intensity of the X-ray radiation emitted; mus ils the coefficient of mass attenuation (cm2/g) for each constituent; tx is the thickness (cm) of each constituent and rx is the density of each constituent.

The total thickness of the object is determined with the aid of the clearance measuring elements on either side of the test object.

T=tb+ts+tf(3) where T represents the total thickness of the object and tb, ts and tf each represent the thickness of the respective constituent.

We now have a system of equations with three equations and 5 unknowns. The calculation speeds No, and No2 must therefore be established in equation (1) and (2) respectively. This can be achieved by using previously measured values. In one embodiment this is achieved by using the reference object 8a located in the radiation path at the side of the test object. In one case in which the reference object 8a is composed of aluminium (a in the equations below) and Perspex (p in the equations below) No, and No2 can be determined as: No=N)Fexp(-ma)takra-mp)tpkrp)(4) N02=N2Fexp(-ma2takra-mp2tp)jp)(5) where NIF and N2F are the calculation speeds, measured by the detector matrix, for the lower and higher energy respectively of the radiation after passage through the reference object 8a with the known composition. Both the coefficients of mass attenuation as well as the densities and the thicknesses (tbk, tSk) are known quantities.

In this way the thicknesses of the various constituents can be solved from the equations (1) (2) and (3) for each element of the second part of the detector matrix, which relates to detection of the test object. From the result for each such element a representation can be made of the bone mineral content in the part of the body

exposed to the measurement.

As stated above, the coefficients of mass attenuation for the three constituents water, fat and bone mineral are known for each value of the energy level. If, however, the mean energy level of the radiation from the X-ray tube 3 deviates somewhat from the values determined, an error will be obtained in the values for the coefficients of mass attenuation. As is known to the person skilled in the art, the relationship between the mean energy level and the coefficient of mass attenuation can be described by means of an exponential curve. It is therefore possible, in order to increase the accuracy of the measurements by means of the second reference object 8b, to calibrate the coefficients of mass attenuation so that they coincide with those for the actual mean energy levels from the X-ray tube 3. The reference object 8b preferably has at least five different permeability levels. By adjusting the measured values for the different permeability levels with an exponential curve, it is consequently possible to determine the actual mean energy levels from the X-ray tube 3. The coefficients of mass attenuation are thereafter selected on the basis of the mean energy levels determined. Determining the mean energy levels in this way also provides a check that the X-ray tube 3 is functioning as intended and keeps the mean energy levels to a suitable level.

On the basis of the detected values from the second reference object it is moreover possible to determine the size of the leakage currents in the detector 4. By determining the intensity of the detected radiation for a number of permeability levels it is possible to determine the appearance of the exponential curve, which describes the permeability in relation to the detected intensity. Ideally the detected intensity should fall to zero when the permeability diminishes before falling to zero. If, when measuring, this is not the case, it is possible to compensate for the errors present due to these leakage currents.

The image and signal-processing unit 7, which is preferably a personal computer, is used for this processing of the results obtained by the detector matrix 4 and the clearance measuring elements 5. There is preferably also access to databases with standards that are calculated from measured results from a larger number of different individuals. The value obtained in the measurement is compared with the value stored in the database in order to decide whether skeletal decalcification exists.

The computer is equipped with software adapted to the purpose, by means of which combinations of the information from the detector matrix and information from the clearance measuring elements are used, so that all constituents of the foot: fat, water and bone mineral, can be detected with a high degree of accuracy.

A keyboard is used in order to communicate with the computer unit, so that the operator can control the various possible operations. For displaying the result there is preferably a display screen on which images of the measured object can be presented.

The reference objects 8a, 8b may be suitably concealed in the foot inlet.