WO2003092465A2

WO2003092465A2 - Bone densitometry and mammography

Info

Publication number: WO2003092465A2
Application number: PCT/IB2003/001577
Authority: WO
Inventors: Gideon Jacobus Johannes Joubert
Original assignee: Csir
Priority date: 2002-05-02
Filing date: 2003-04-25
Publication date: 2003-11-13
Also published as: WO2003092465A3; AU2003224348A8; AU2003224348A1

Abstract

A method of determining bone density includes placing a bone-containing target between attenuation measurement means and at least one radioisotope gamma ray source having an electromagnetic radiation spectrum with at least two characteristic peak penetrating emissions at different energies, and exposing the target to penetrating electromagnetic radiation from the source. The attenuation or relative attenuation of the at least two peak electromagnetic radiation emissions through the target is measured, and the bone density is determined from the attenuation measurements.

Description

BONE DENSITOMETRY AND MAMMOGRAPHY

THIS INVENTION relates to bone densitometry and mammography.

According to one aspect of the invention, there is provided a method of determining bone density, the method including placing a bone-containing target between attenuation measurement means and at least one radioisotope gamma ray source having an electromagnetic radiation spectrum with at least two characteristic peak penetrating emissions at different energies; exposing the target to penetrating electromagnetic radiation from the source; measuring the attenuation or relative attenuation of the at least two peak electromagnetic radiation emissions through the target; and determining the bone density from the attenuation measurements.

The method may include making corrections for background Compton scatter.

According to another aspect of the invention, there is provided a bone densitometer which includes at least one radioisotope gamma ray source having an electromagnetic radiation spectrum with at least two characteristic peak penetrating emissions at different energies; and measurement means for measuring the attenuation or relative attenuation of the at least two peak emissions through a bone-containing target.

The bone densitometer may include radiographic image capturing means for capturing a radiographic image of the bone-containing target.

The bone densitometer may also include a source of X-rays in addition to the radioisotope gamma ray source. Advantageously, the source of X-rays, e.g. an X-ray tube, can be used with the radiographic image-capturing means to produce a quality X- ray image of the bone-containing target. Instead, if the bone densitometer does not include an X-ray source in addition to the radioisotope gamma ray source, the radiographic image capturing means may be used with the radioisotope gamma ray source to provide a radiographic image of the bone-containing target.

The radioisotope gamma ray source may have a diameter of up to about 25 mm, e.g. between about 18 mm and about 25 mm.

The radioisotope gamma ray source may have a gamma ray emission surface area which is larger than the projected area of the gamma ray emission surface, as disclosed in WO 00/55866.

The radioisotope gamma ray source may have a characteristic peak X-ray emission at one or more energies, in addition to gamma ray emissions.

The bone densitometer may include collimator means for projecting the electromagnetic emissions from the radioisotope gamma ray source in a beam of desired geometry. The collimator means may be configured to project the emissions from the radioisotope gamma ray source in a cone beam geometry or a fan beam geometry or a pencil beam geometry.

Preferably, the gamma ray source includes as a major radioisotope constituent, a radioisotope having a half-life of at least one year. In a preferred embodiment of the invention, the radioisotope gamma ray source includes americium- 241 as a major radioisotope constituent. The americium-241 may be present in the form of a hydroxide. Instead, the americium-241 may be in metal form and may be electroplated on a substrate. The radioisotope gamma ray source may include europium as a minor radioisotope constituent. The americium-241 containing source may include a window which substantially prevents the characteristic 17 keV peak X-ray emissions of americium-241 , or more accurately the neptunium daughter product of americium, from escaping from the source. According to a further aspect of the invention, there is provided a method of obtaining a mammograph, the method including subjecting a breast to a spectrum of electromagnetic radiation having a peak penetrating emission at about 17 keV and emitted from a radioisotope gamma ray source which includes americium-241 as a major radioisotope constituent; and capturing a radiographic image of the breast.

According to yet a further aspect of the invention, there is provided an apparatus for obtaining a mammograph, the apparatus including a radioisotope gamma ray source which includes americium-241 as a major radioisotope constituent and which has an electromagnetic radiation spectrum which includes a characteristic peak X-ray emission at about 17 keV; and radiographic image capturing means for capturing a mammograph image of a breast.

The radiographic image capturing means may include an intensifying screen- film combination. Advantageously, the screen-film combination may include a fast screen, as disclosed in US 5746943.

The radiographic image capturing means may include an electromagnetic radiation converter for converting incident X-rays or gamma rays into electrical signals, e.g. the DirectRay (trade name) detector array supplied by Hologic, that comprises an amorphous selenium coating over a thin-film-transistor matrix with associated readout electronics. Instead, the converter may comprise a scintillator for converting X-rays or gamma rays into visible light, e.g. a thallium doped caesium iodide scintillator, in combination with a converter for converting the visible light into electrical charge, e.g. a silicon photodiode. A commercially available flat panel scintillator may be used as is, or it may be adapted for use with the particular radioisotope gamma ray source, e.g. by changing the dopant and/or thickness of the caesium iodide layer. Suitable flat panel scintillators are sold by, for example, GE Medical, Canon and Trixell, for example.

The radiographic image capturing means may include a screen for converting incident X-rays or gamma rays into visible and/or UV light, and a Charge Coupled Device (CCD) camera or a Back Entrance Charge Coupled Device (BCCD) camera, for displaying the captured image on a monitor. The radiographic image capturing means may be configured to employ frame addition, or, preferably, frame integration to enhance the quality of the captured image.

The apparatus may include collimator means. The collimator means may be configured to project the electromagnetic radiation emissions from the radioisotope gamma ray source in a cone beam geometry. When the collimator means is configured to project the electromagnetic radiation emissions from the radioisotope gamma ray source in a cone beam geometry, the radiographic image capturing means may include a plurality of scintillation detectors (e.g. caesium iodide scintillation detectors which include photomultiplier tubes), arranged in a pattern in a plane, e.g. a square array or a hexagonal close packed arrangement, and an apertured shield located between said plane and the radioisotope gamma ray source. Each detector may be at least about 92 % efficient in detecting photons of the energy of the X-rays or gamma rays from the radioisotope gamma ray source and/or the additional X-ray source, and may have a nominal diameter less than about 13 mm e.g. about 6 mm.

The apertured shield may be parallel to the plane of the detectors and typically have apertures of a size less than about one third of that of the area of a scintillation detector. Instead, the shield may be in the form of a rotating disc or oscillating strip. In another embodiment, it is envisaged that the shield may comprise a plurality of rotating cylinders with a plurality of passages arranged perpendicular to the axis of rotation. Thus, as it will be appreciated, in such a device, either the apertured shield, or preferably the plurality of detectors, is displaceable relative to the other. When the shield is stationary, it can be used as a support for an object to be radiographed. The shield may include a stainless steel bed, with an X-ray and/or gamma ray shielding material located beneath the steel bed and facing the scintillation detectors. When the shield is not stationary, an extra table above the vibratory shield is required to support an object to be radiographed.

Data collection via the plurality of scintillation detectors typically comprises observed count rates in up to eight counting windows for the detectors for up to four photo peaks, with each window being gated to accumulate counts in a specific energy range. The display of an image based on data collected in this manner may comprise direct display of gross count rates using conventional methods for converting it to pixels for display on a monitor screen, or displayed count rates after background correction using the so-called two window method known to those skilled in the art of background correction in gamma ray counting, or data displayed only after raw count rates are corrected for drift resulting from various causes known to those skilled in the art of gamma ray counting, such as drift in high voltage, or data displayed only after the data is subjected to additional digital image processing to enhance selected features, using methods known to those skilled in the art of digital image processing and which may include the use of different colour coding.

The collimator means may be configured to project the electromagnetic radiation emissions from the radioisotope gamma ray source in a fan beam. When the collimator means is configured to project the emissions from the radioisotope gamma ray source in a fan beam, the radiographic image capturing means may include a plurality of scintillation detectors. The scintillation detectors may be arranged in a linear array of at most three rows of detectors, and may be arranged either in a square packing or a hexagonal closed packing arrangement.

The mammography apparatus and the bone densitometer may include a plurality of radioisotope gamma ray sources and the radiographic image capturing means may include a plurality of associated detectors arranged in a pattern in a plane. They may further include collimator means configured to project the electromagnetic radiation emissions from the radioisotope gamma ray sources in pencil beams. The sources may be mounted in a shielding material and recessed into the shielding material, so that a passage extending from a source to the surface of the shielding material defines a collimator.

The radioisotope gamma ray source of the mammograph apparatus may be optimised to maximise its 17 keV emission and may have a window of beryllium or graphite. In a preferred embodiment of the invention, the window is graphite, with the americium-241 radioisotope material being encapsulated in graphite. The graphite window may have a thickness of up to about 2 mm. Advantageously, a graphite window has excellent resistance to corrosion and good electromagnetic radiation transmission at low energy. The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings and the example.

In the drawings, Figure 1 shows a section through a radioisotope gamma ray source for a bone densitometer and/or a mammograph apparatus in accordance with the invention;

Figure 2 shows schematically a section through a portion of radiographic image capturing means of a bone densitometer and/or a mammograph apparatus in accordance with the invention and comprising a plurality of scintillation detectors and an apertured shield; and

Figure 3 shows a plan view of a portion of the apertured shield of Figure 2.

Referring to Figure 1 of the drawings, reference numeral 10 generally indicates a radioisotope gamma ray source useful as part of a bone densitometer and/or a mammograph apparatus in accordance with the invention. The source 10 includes a circular cylindrical graphite body 12 defining a dome-shaped graphite window 14 with a thickness of about 2 mm. A graphite plug 16 is screwed into the body 12. A layer 18 of americium-241 hydroxide is sandwiched between the graphite window 14 and an optional backing layer 20. The backing layer 20, if present, typically consists of a metal, such as silver, zirconium or europium. Graphite powder 22 fills the void between the plug 16 and the backing layer 20.

If desired, an external thread may be provided on the graphite body 12, allowing it to be screwed into a shielding body, such as a lead or tungsten alloy shielding body. Such a shielding body can simultaneously act as a protective shield against accidental window fracture and as a collimator, although drop tests without a protective shield confirmed that the source 10 can be safely dropped from a height of up to about 2 m.

The source 10 emits 26 keV and 60 keV gamma rays and also 17 keV neptunium L-X-rays. As will be appreciated, by admixing rare earth or other materials with the americium hydroxide, the source 10 can also emit K-X-rays of energy in the range of about 22 keV to about 50 keV. Referring to Figures 2 and 3 of the drawings, a portion of radiographic image capturing means is shown and generally indicated by reference numeral 30. The radiographic image capturing means includes a plurality of scintillation detectors 32

(only five of which are shown) and a shield 34 having apertures 40. Only a portion of the shield 34 is shown.

Each scintillation detector 32 includes a caesium iodide or sodium iodide scintillator 36 and a photomultiplier tube 38 or any other suitable solid state detector. The detectors 32 are arranged in a square grid matrix, corresponding to the arrangement of the apertures 40 in the shield 34 as shown in Figure 3. The scintillators 36 each have a diameter of about 6 mm. The apertures 40 in the shield 34 each have a diameter of about 2 mm.

The scintillation or solid state detectors 32 are displaceable, in a plane parallel to the shield 34, relative to the shield 34 in a predetermined pattern. In this way the limited number of detectors 32 can be used to generate a finer grid of spectral data points for display on a monitor as an X-ray image. The shield 34 is used as a support to support an object of which a radiographic image is to be obtained.

In use, data for display of a radiographic image is collected by observing count rates in one or two windows per photo peak. However, there can be as many as four peaks and eight counting windows. The display of an image captured in this manner may be by any suitable means, e.g. direct display of gross count rates using conventional methods for converting it to pixels for display on a monitor screen.

Example

A radioisotope source similar to the source 10, but containing americium-241 and europium was used to measure electromagnetic radiation attenuation through various bone-containing body parts, in accordance with the invention. The source 10 emitted 26 keV and 60 keV gamma rays originating from the americium-241 , a K-X-ray originating from neptunium (the daughter product from americium) at 17 keV, and a 40 keV X-ray originating from the europium. Table 1 shows the results, where lo indicates incident intensity and I indicates emergent intensity. Table 1

For bone densitometry, it is necessary to determine the X- or gamma ray absorbence or attenuation values, i.e. absorbence (A)= . n(l/I₀). These results are shown in Table 2.

Table 2

As will be noted, the more negative A becomes the stronger the X- or gamma ray is absorbed. It is also noticeable that it is easier to take X-rays of a hand or finger than X-rays of a skull or lower back. Furthermore, due to a relatively large Compton background scatter at 40 keV, the calculated absorbence requires background correction to be accurate. As the measurements were made by using a sodium iodide detector, it is believed that this problem can be lessened by using caesium iodide detectors, or possibly bismuth germinate or other suitable solid state detectors.

The bone density for the middle finger, for example, can be determined from the absorption measurements using conventional techniques known to those skilled in the art and by considering the following:

At 60 keV A_flnger=-0.4549=-p_fingerM_fingerD, where D is the thickness of the finger (D=2.5 cm) p is density and μ is mass attenuation coefficient. Substituting yields Pfinger finger =0.18196.

If two energies are used, say 60 keV and 26 keV, then the experimental value of Pfinger Pfing_er calculated from Table 2 is 0.21644 for 26 keV because absorption is stronger at 26 keV, i.e. this is because μfi_nger at 26 keV is larger than μf_inger at 60 keV. Although various methods are used to calculate BMD (bone mineral density), some perform measurement for tissue only, i.e. the part right next to the finger bone. This was not done for the Example, but it may easily be done, which then enables measurement of PtissueMtissue-

Note that one may write: Pfinge.Mfinge.=PtissuePtissue⁺PboneMbone, i.e.

0.18196=PtissuePtissue60⁺PboneMbone60 and 0.21644=Ptis_SuePtissue26⁺PboneMbone26- If measurement on tissue gives

and

then

and

i.e. data for bone only emerge. If these measurements were made on a standard phantom of known p_bone_> one can solve for standard μtissue26 and μtissueθo-

The use of these experimental μtissue26 and μtissueθo in later measurements then enables p_bone to be calculated. Two independent values is better than one, i.e. dual energy measurement is more reliable. Furthermore, measurement of line spectra as produced by radioisotope sources are more accurate than measurement of broad energy spectra produced by X-ray tubes.

Pressure is mounting worldwide for reducing radiation dose per X-ray examination, such as bone densitometry or mammography. From calculations done by the Applicant, the 17, 26 and 60 keV electromagnetic radiation originating from a source which includes americium-241 , transfers a minimum of its energy to man when exposed to the electromagnetic radiation, i.e. the lowest possible dose per X-ray examination. The Applicant has also performed comparative dose measurements confirming that doses from X-ray machines exceed those from an americium radioisotope source. The Applicant is thus of the opinion that an americium-241 radioisotope source is optimum for use in bone densitometry and mammography. In the case of bone densitometry, the fact that the americium-241 radioisotope source emits an electromagnetic radiation spectrum with at least two characteristic peak penetrating emissions at different energies, is a particular advantage. The 17 keV emission of americium-241 is well suited for mammography, whilst limiting radiation dose.

Claims

CLAIMS:

1. A method of determining bone density, the method including placing a bone-containing target between attenuation measurement means and at least one radioisotope gamma ray source having an electromagnetic radiation spectrum with at least two characteristic peak penetrating emissions at different energies; exposing the target to penetrating electromagnetic radiation from the source; measuring the attenuation or relative attenuation of the at least two peak electromagnetic radiation emissions through the target; and determining the bone density from the attenuation measurements.

2. The method as claimed in claim 1 , which includes making corrections for background Compton scatter.

3. A bone densitometer which includes at least one radioisotope gamma ray source having an electromagnetic radiation spectrum with at least two characteristic peak penetrating emissions at different energies; and measurement means for measuring the attenuation or relative attenuation of the at least two peak emissions through a bone-containing target.

4. The bone densitometer as claimed in claim 3, which includes radiographic image capturing means for capturing a radiographic image of the bone-containing target.

5. The bone densitometer as claimed in claim 3 or claim 4, which includes a source of X-rays in addition to the radioisotope gamma ray source.

6. The bone densitometer as claimed in any one of claims 3 to 5 inclusive, in which the radioisotope gamma ray source has a diameter of up to about 25 mm.

7. The bone densitometer as claimed in any one of claims 3 to 6 inclusive, in which the radioisotope gamma ray source has a characteristic peak X-ray emission at one or more energies, in addition to gamma ray emissions.

8. The bone densitometer as claimed in any one of claims 3 to 7 inclusive, in which the gamma ray source includes as a major radioisotope constituent, a radioisotope having a half-life of at least one year.

9. The bone densitometer as claimed in any one of claims 3 to 8 inclusive, in which the radioisotope gamma ray source includes americium-241 as a major radioisotope constituent.

10. The bone densitometer as claimed in claim 9, in which the radioisotope gamma ray source includes europium as a minor radioisotope constituent.

11. The bone densitometer as claimed in claim 9 or claim 10, in which the americium-241 containing source includes a window which substantially prevents the characteristic 17 keV peak X-ray emissions of americium-241 from escaping from the source.

12. A method of obtaining a mammograph, the method including subjecting a breast to a spectrum of electromagnetic radiation having a peak penetrating emission at about 17 keV and emitted from a radioisotope gamma ray source which includes americium-241 as a major radioisotope constituent; and capturing a radiographic image of the breast.

13. An apparatus for obtaining a mammograph, the apparatus including a radioisotope gamma ray source which includes americium-241 as a major radioisotope constituent and which has an electromagnetic radiation spectrum which includes a characteristic peak X-ray emission at about 17 keV; and radiographic image capturing means for capturing a mammograph image of a breast.

14. The apparatus as claimed in claim 13, in which the radiographic image capturing means includes an intensifying screen-film combination.

15. The apparatus as claimed in claim 13, in which the radiographic image capturing means includes an electromagnetic radiation converter for converting incident X-rays or gamma rays into electrical signals.

16. The apparatus as claimed in claim 13, in which the radiographic image capturing means includes a screen for converting incident X-rays or gamma rays into visible and/or UV light, and a Charge Coupled Device (CCD) camera or a Back Entrance Charge Coupled Device (BCCD) camera, for displaying the captured image on a monitor.

17. The apparatus as claimed in claim 13, which includes collimator means configured to project the electromagnetic radiation emissions from the radioisotope gamma ray source in a cone beam geometry, the radiographic image capturing means including a plurality of scintillation detectors, arranged in a pattern in a plane, and an apertured shield located between said plane and the radioisotope gamma ray source.

18. The apparatus as claimed in claim 17, in which the apertured shield is parallel to the plane of the detectors and has apertures of a size less than about one third of that of the area of a scintillation detector.

19. The apparatus as claimed in claim 17 or claim 18, in which the apertured shield, or the plurality of detectors, is displaceable relative to the other.

20. The apparatus as claimed in claim 13, which includes collimator means configured to project the electromagnetic radiation emissions from the radioisotope gamma ray source in a fan beam, the radiographic image capturing means including a plurality of scintillation detectors arranged in a linear array of at most three rows of detectors, either in a square packing or a hexagonal closed packing arrangement.

21. The apparatus as claimed in claim 13, which includes a plurality of radioisotope gamma ray sources and in which the radiographic image capturing means includes a plurality of associated detectors arranged in a pattern in a plane, the apparatus further including collimator means configured to project the electromagnetic radiation emissions from the radioisotope gamma ray sources in pencil beams.

22. The apparatus as claimed in any one of claims 13 to 21 inclusive, in which the radioisotope gamma ray source is optimised to maximise its 17 keV emission and has a window of graphite.

23. A method of determining bone density as claimed in claim 1 , substantially as herein described and illustrated.

24. A bone densitometer as claimed in claim 3, substantially as herein described and illustrated.

25. A method of obtaining a mammograph as claimed in claim 12, substantially as herein described and illustrated.

26. An apparatus for obtaining a mammograph as claimed in claim 13, substantially as herein described and illustrated.

27. A new method of determining bone density, a new bone densitometer, a new method of obtaining a mammograph, or a new apparatus for obtaining a mammograph, substantially as herein described.