CA1250062A - Radiation reduction filter for use in medical diagnosis - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

In accordance with the present invention, there is provided an X-ray filter comprised of a niobium metal foil having a maximum thickness of about 75 microns or niobium metal foil in combination with additional filtering foils. Preferably the niobium and/or combination foils are encased in a thin plastic sheet. The plastic sheet provides for protection of the foil filter during handling as well as some absorption of the secondary radiation emitted from the foil when it is contacted by the X-ray beam. As a result of the construction immediately above, the filter of the present invention filters energy from the X-ray beam which is usually absorbed by the examination object and does not contribute to the radiographic image of the examination object.

Description

- ~.Z5(~(~62 RADIATION REDUCTION FILTER F~R USE IN MEDICAL DIAGNOSIS

FIELD OF T~IE INVENTION

This invention relates to X-ray radiography and fluoroscopy and particularly to filters for limiting the radiation dosage to an examination object during medical diagnosis.

BACKGROUND OF THE INVENTION
X-rays are produced in an X-ray tube as a result of high speed electrons striking a target material. The electron strikes and penetrates the surface layers of the target and through interaction or collision with the molecules of the target, the energy of the electron is imparted to the electrons in the target.

: If, in striking the target, the energy of the electronis dissipated through a series of many collisions with the outer atomic electrons of the target molecules, then the energy either ~ 20 contributes to heating of the target or is released as visible light. An electron may, after a series of many collisions, also emerge from the target as a back-scattered electron. Energy losses as a result of a series of many collisions are the most common with the sollisions contributing to target heating and hence reduced X-ray tube life.

The electron may also have a radiative collision, giving up part or sometimes all of its energy to photon. These collisions result in the production of photons of an energy less than or equal to the energy given up by the electron.

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If the energy of the electron is sufficient to collide with and eject an electron from the inner K-shell of the target atom, then the excited target atom, when the electrons in the outer shells drop into the vacant inner shells, will return to its ground state and a photon will be emitted. The energies of these transitions are dependent upon the atoms comprising the target material and hense the energies of the photons emitted are characteristic of the target atom. This radiation is known in the art as the characteristic X-ray radiation and includes radiation produced with the X-ray tube only when the energy of the electron is above the level required to dislodge the K-electron of the target atom.

The energy of the X-ray is directly related to the energy given up by the electron in the collision with the target molecules. As it is well known that the relationship hetween $he wavelength of a photon and its energy is expressed by the Duane-Hunt equation, = 12 4 A~

this process results in X-ray of various wavelengths which constitute what is known in the art as the continuous X-ray spectrum.

The ability of the X-rays to penetrate an examination object depends on the wavelength or energy of the X-ray photons as well as the composition of the exam;nation object - ~e. its chemical elements, thickness and density. With respect to the wavelength or energy of the X-rays, generally the penetration

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ability is inversely proportional to wavelength or directlyproportional to energy. Thus, short wavelength Ihigh energy) X-rays have a greater penetrating ability than long wavelength (low energy) X-rays. With respect to the chemical elements making up the examination object, generally, the higher the atomic number of the element, the less the penetration of the X-ray beam.
However, at wavelengths or energy levels near the absorption edges of the elements, these generalizations do not hold true as, there are discontinuities in the degree of absorption of the X-ray beam at these points. With respect to the thickness and density of an examination object, generally, the thicker and denser the object the greater its ability to absorb X-rays and thus fewer pass through the object. It is the combination of these factors which allows for the diFferential diagnosis of radiographs. Thus, the selection of the operating parameters of the X-ray apparatus during medical diagnosis depends upon the examination object, its chemical composition, thickness and density. For more descriptions of the above, reference can be made to textbooks of medical physics or radiology.
As low energy X-rays do not normally contribute to the resolution of the method but are merely absorbed and scattered by the examination object, it is highly desirable to remove such X-rays from the X-ray beam prior to the beam contacting the examination object. These low energy X-rays are usually removed from the X ray beam through the use of attenuators or filters.
Similar to the effects on examination objects, the attenuating abilities oF a filter are dependent upon the chemical composition, density and thickness of the mater;al making up the filter. These

3~ relationships are represented by the following equation:

I = 1o e- x where I is the intensity of the radiation transmitted, I<, is the intensity of the incident radiation, e is the base of natural logarithum,~l is the mass attenuation co-efficient for the chemical element comprising the filter material, pis the density of the filter material 3 and x is the thickness of the filter material. Of the above factors, all except the attenuation co-efficient ~ are independent of the frequency or energy of the incident radiation. The attenuation co-efficient varies with the energy of the incident radiation and is related to the atomlc number of the chemical element of the filter material. These co-efficients have been experimentally determined and can be found in tables published for example in UCRL 5017~ by W.H. McMaster et al. available from the National Technical Information Service, Springfield, Va., 22151.

For many years the most common means of filtration has been through the use of aluminum filters. As an exampleg U.SO
patent 2,225,940 discloses a wedge shaped aluminum filter and/or copper filter whereby the degree of filtration is adjustable depending upon the thickness of the wedge which is brought into the path of the X-ray beam. Additionally, U.S. patent 3,976,889 discloses the use of variable thicknesses of aluminum filters in dental X-rays to vary exposure levels. Almost all commercial X-ray uni~s have some inherent fil~ration equivalent to about 1 to 1.5 mm of Al and those designed for medical and/or dental applications, utilize additional aluminum filtration.

The use oF filters other than Al to filter low energy X-rays from an ~-ray beam was the subject of U.S. patent

4,499,591, wherein a 127 micron thick yttrium filter was employed to filter the X-ray beam such that energies below 20 keV were eliminated from the beam. This has also been disclosed in Heinrick and Schuster, "Reduction of Patient Dose by Filtration in Paediatric Fluoroscopy and Fluorography" Ann. Radiol. (1976) 19:
57-66, where the authors utilized a molybdenum filter of 100 microns to remove radiation below 20 keV from the X-ray beam.
In X-ray crystalography and diffraction studies, it is useful to have relatively homogeneous, monochromatic X-ray beams.
Filter materials have been used for producing these relatively homogeneous X-ray beams by limiting the range of wavelengths of the X-ray beam. Thus, in U.~. patent 1,624,4~39 the use of a filter with a slightly lower atomic weight than the X-ray tube target has been found to produce an X-ray beam of suitable relative homogeneity for use in X~ray crystallography. This patent discloses, in a preferred embodiment, the use o~ a zirconium filter with a molybdenum target. The use of filters of the same material as the target has also been shown to result in an X-ray beam of relative homogeneity. U.S. patent 3,515,874 discloses the use of molybdenum for both a target and filter, particularly for mammography where it has been found that the energy level of the K-alpha line emitted from a molydbenum target is ideal for resolution of tumors in mammography applications.

As seen from the above, it is apprecia~ed that there is a risk involved when dealing with diagnostic X-rays due to the harmful effects of unnecessary radiation dosayes. Therefore,

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there is a need for an efficient X-ray filter to reduce such dosages and which is compatible with existing X-ray equipment.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided an X-ray filter comprised of a niobium metal foil having a maximum thickness of about 75 microns or a niobium metal foil in combination with additional filtering foils. Preferably the niobium and/or combinakion foils are encased in a thin plastic sheet. The plastic sheet provides for protection of the foil filter during handling as well as some absorption of the secondary radiation emitted from the foil when it is contacted by the X~ray beam.
As a result of the construction immediately above, the filter of the present inven~ion filters energy from the X-ray beam which is usually absorbed by the examination object and does not contribute to the radiographic image of the examination object.

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A further fedture of the present invention is the provision of a filter material which operates to produce an X-ray beam which approaches the characteristics of a homogeneous X-ray beam. These and other features of the present invention will be appreciated from the detailed description of the preferred embodiments of the invention which Follow.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention are shown in the accompanying drawings in which, Figure 1 shows a perspective view of a filter contructed in accordance with the present invention;

Figure 2 is a sectional view of the filter of figure 1, Figure 3 is an elevational view of an X~ray diagnostic apparatus with the filter of the present invention in place;

Figure 4 is an X-ray wavelength spectrum of the typical ~ apparatus of figure 3, showing both filtered and unfiltered .~ spectrum;

Figure 5 is an X-ray wavelength spectrum of the apparatus of figure 3, showing the unfiltered and the filtered spectrum wherein a filter of a second embodiment of the present invention has been utilized:

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DETAILED DESCRIPTION OF THE INVENTION

Figures 1 and 2 show a preferred embodiment of a filter of the present invention generally indicated at 10 comprising a niobium metal foil 12 in a thickness of up to about 75 microns, preferrably 40 to 60 microns, the most preferrable thickness being about 50 microns. This metal foil is encased in a coloured cardboard 14 wherein the colour can be used as an identifying means for the thickness of the filter or the application in which the filter is to be utilized. Overlying and encasing the filter 12 and cardboard envelope 14 is a plastic covering 16 which serves as a protective covering to the filter. Additionally the combination of the cardboard 14 and the plastic covering 16 serves to absorb some of the secondary radiation emitted from the niobium metal foil 12 when an X-ray beam contacts the niobium metal foil and also reduces or eliminates the exposure of the metal foil to air, thereby reducing oxidation. Attached to one side of the filter 10 is a means for attaching the filter to the X-ray unit shown in the figures as a strip of double sided tape 18. The method of attaching the filter to an X-ray apparatus is discussed below.

Figure 2 shows a cross-section of the filter 10 of figure 1 illustrating clearly the relationship between the niobium metal foil 12, the cardboard envelope 14 and the plastic encasing material 16.

Figure 3 illustrates an X-ray generating apparatus 20 of typical lead based construction. The apparatus comprises an X-ray tube 30 with a cathode 22 and a rotating anode 24. Located within the cathode is a filament (not shown) which when heated by an electric current produces a cloud of electrons around the cathode. When high vol~age from a generator is applied across the cathode 22 and the anode 24, the electrons in the cloud surrounding the cathode are accelerated as a beam towards the anode 24 which is comprised of a metallic material suitable as a target. Mos~ co~monly, the targets are constructed of tungsten. When the electron beam strikes the target material, the energy of the electron beam is absorbed by the target material and results in the production of X-rays as explained hereinabove. Owing to the construction of the anode 24, the X-ray beam is~ to a large degree, focussed and emitted from the X-ray apparatus 20 through a port 26. Port 26 usually comprises a window made of glass or plastic wi~h an inherent filtration equivalent to about 0,5mm of Al. In the typical applications, the X-ray beam emitted ~rom the tube is focused through the use of a collimator 28. The purpose of collimator 28 is to direct the X-ray beam to cover only the area required in exposure of the examination object. This is achieved through adjustment of diaphrams 32 and 36~ s~tting th~ collimator opening 34. The X-ray apparatus also has inherent and added filtration, usually eqivalent to 2.5 to 3.5 mm aluminum to remove, from the beam, very low energy X-rays which would be generally absorbed within the first few milimetres of the examination object. These very low energy X-rays do not contribute at all to the resolution of the radiograph, but ra~her merely contribute ~o increase the exposure dose of the examination object 42. The X-ray beams, once they pass through the examination object 42, are detected by a radiation detecting device as for example, an image intensifier 38 or directly on a radiographic film 40.

Filter 10 is shown attached in the apparatus between the port 26 of the tube 30 and the collimator 28. The filter is attached to the apparatus using the double sided tape, by sticking it onto either the port of the tube or the additional aluminum filtration. Alternatively, in those applications where this may not be possible, i.e. in some dental applications, it may be fixed in the opening of the collimator.

Figure 4 shows generally the X-ray wavelength spectrum emitted from an X-ray apparatus of figure 3. The appartus with a lS tungsten target was operated at an accelerating voltage of 80 KVP
thereby resulting in production of a continuous spectrum with a minimum wavelength of about 0.15 A and the characteristic K and K riadiations of tungsten at about 0.21 A and 0.18 A
respectively. The solid line shows the wavelength spectrum of the X-ray beam emitted from the apparatus prior to filtration by a 50 micron niobium ~ilter. The long dash line is the attenuation properties of the 50 micron niobium filter. Niobium with an atomic number of ~1 has a K absorption edge at about 0.65 A.
Thus, the attenuation of the beam by the filter increases up to the K absorption edge when it then drops and then increases again up to the L absorption edge at about 4.58 A (not shown on the figure). The short dash line shows the wa~elength spectrum of the X-ray beam af~er passing through the niobium filter. There is a marked decrease in the X-ray wavelengths from abou~ 0.25 A to the K absorption edge a~ 0.65 ~ wherein only about 3~ of the incident radiation is not absorbed by the filter.

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If it is desirous to remove from the X-ray beam, that radiation above the K absorption edge of niobium without appreciably increasing the attenuation of the beam in the diagnostically important region (generally from about 0.15 to about 0.4 ~) then a combination filter can be utilized as shown in figure 5. In this situation a combination filter o~ 25 microns of niobium and 50 microns of selenium is utilized. The keys to the curves are the same as in figure 4 where the solid line is the unfiltered spectrum, the long dash line is the attenuation profile of the combination filter and the short dash line is the filkered spectrum. As can be clearly shown by employing selenium with a K
absorption edge of about 0.98 AJ in combination with niobium, substantially all of the X-rays with wavelengths greater than about 0.6 A are removed from the X-ray beam by the combination filter.

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The choice of filter materials for the combination filters are dependent upon the requirements of the diagnostic technique as different techniques may require differing X-ray wavelength spectrums. Thus~ in the example shown in figure 5~ the combination of niobium and selenium is particularly useful for applica~ions where it is desirous to have an X-ray beam with wavelengths less than about 0.4 A. If a harder beam is desired~
i.e. one where the wavelengths are less than 0.3 A~or 0.2 A~ then the filter material would be chosen to remove X~rays with wavelengths longer than this. For example, tin with a K
absorpkion edge at about 0.42 A or indium with a K absorption edge at about 0.44 A or silver with K absorption edge at about Or49 A
would be useful. The above or other materials similar in - 30 attenuation properties ~ould be used in combination with one or more materials having K absorption edges in the region of about 0.6 A to 1.0 A as for example materials from technetium to germanium in the periodic table. The choice of the filter materials would be dependent upon availability of the material in a form suitable for filter construction. The preferred thickness of the selected materials is dependent upon the density and attenuation co-efficients as discussed above. Generally the total thickness of the filter should be chosen such that the product obtained by multiplying together the thickness and the denslty is in the range 0.035 to 0.055 g/cm2, rnost preferably 0.04 to 0.05 g/cm2. In a combination filter the sum of the products for each of the individual elements should be in the above ranges.
Preferably for a combination filter comprising two separate elements, each of the individual elements should be in the range of 0.0175 to 0.0275 g/cm2 and most preferably from 0.02 to 0.025 g/cm2 .

The use of a filter of the present invention will be illustrated further in the following examples:

EXAMPLE I

A 50 micron niobium filter encased in plastic was placed at the face of the collimator of a 3 phase ~ pulse unit with a total filtration of 3.5 mm. Al equivalent~

Entrance doses were measured using a Victoreen exposure meter. A series of radiographs were taken of phantoms with and without the niobium filter. In order to achieve identical optical density in the radiographs the exposure for the filtered radiographs was increased slightly by 8 to 10%. The dose reduction values have been corrected for the slight increase in exposure.

TABLE I - MEASURED ENTR~NCE DOSE

KV RAN6E WITHOUT TEST WITH TEST ~ DOSE
(KVP) FILTER FILTER REDU~TIO~
.9mr/mas ~22mr/mas 75%
2.0 .55 72 3.4 ~.21 64 5.0 2.1 5

6.7 3.1 54 .

TABLE I shows a significant reduction in entrance dose between measurements taken with and without the niobium filter. This dose reduction is most marked for the lower KYP.

EXAMPLE II
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This experiment was carried out using a General Electric - 20 Three Phase Generator and an automatic beam limiting device with an inherent filtration of 1.5 mm equivalent of aluminum at 150 ~ kvp. The radiation detection device used was a Rad Check Plus, ; Model #06-526~ The added filtration was 2.0 mm of aluminum, making a total filtration of 3.5 mm of aluminum/equi~alent. Since the majority of X-ray examinations are carried out between 75 to 100 kvp, the generator was used at the following settings; mA -200; TIME - .35 Seconds; KYP - 80.

A half value layer experiment was carr;ed out, as well as a comparison of radiation dose obtained under;

:

1',2~0G2 a) Normal operation - with only the 3.5 mm aluminum/equivalent between source and the detector.
b) Exactly as in item a), but with 100 microns of Yttrium added at the source in the field.
c) Exactly as in item a), but with 50 microns of Niobium added at the source in the field.
d) Exactly as in item a), but with 25 microns of Niobium added at the source in the field.

10 OPERATION ADDITIONAL mR DOSE g DOSE
FILTRATION REDUCTION O
_ _ ~COMPARED TO A) A) NORMAL

lmm 210 2mm 176 3mm 148 4mm 124 5mm 107 HALF VALUE LAYER = 3.7mm Al B) ADDITION OF

YTTRIUM TO A

lmm 128 39 2mm 112 37 3mm 95 36 4mm 83 33 HALF VALUE LAYER = 4.85mm Al i2 OPERATION ADDITIONAL mR DOSE % DOSE
FILTRATION REDUCTION
(COMPARED TO A) C) ADDITION OF 50 MICRONS OF NIOBIUM
TO A~

lmm 118 44 2mm 99 44 3mm 83 44 4mm 72 42 5mm 64 40 HALF VALUE LAYER = 4.35mm Al D) ADDITION OF 25 MICRONS OF NIOBIUM
` TO A

lmm 148 30 2mm 125 29 3mm 107 28 4mm 91 27 5mm 79 26 HALF VALUE LAYER = 4.25mm Al EXAMPLE III

Tests were conducted util~zing water phantoms of 5 cm, lO cm, 15 cm, and 20 cm in depth. A step wedge was placed in the ~2~6~
water to provide a measureable optical density (O.D.). A Siemens Tridoros Optimatic 800 generator was used for testing using the 0.6 focal spot size. Testing was done usins a Keithly 35055 digital dosimeter at 115 cm FFD. The HYL measured before testing was 3.8 mm Al at 80 kV. A 50 micron Niobium filter added to the 3.8 mm Al outside the collimator window. The results are as follows:

PHANTOM ADDITIONAL EXPOSURE TUBE DOSE ~ DOSE
FILTRATION YOLTAGE REDUCTION

5 cm water 10 mAs 63 kV 28.4 mR
5 cm water 0.05 mm Nb 10 mAs 63 kY 10.2 mR 64 5 cm water 0.05 mm Nb 12 mAs 63 kY 16 mR 44 5 cm water 4 mm Al 10 mAs 63 kY 10.2 mR 64 }O cm water 20 mAs 77 kY 94 mR
10 cm water 0.05 mm Nb 20 mAs 77 kY 50 mR 47 10 cm water 0.05 mm Nb 25 mAs 77 kV 73 mR 22~
10 cm water 3 mm Al 20 mAs 77 kY 51 mR 46%

15 cm water 32 mAs 96 kV 283 mR
15 cm water 0.05 mm Nb 32 mAs 96 kV 170 mR 40 15 cm water 0.05 mm Nb 40 mAs 96 kV 215 mR 24 15 cm water 3 mm Al 32 mAs 96 kV 172 mR 39 20 cm water 50 mAs 117 kV 715 mR
20 cm water O.Q5 mm Nb 50 mAs 117 kV 453 mR 37 20 cm water 0.05 mm Nb 64 mAs 117 kV 569 mR 20 20 cm water 3 mm Al 50 mAs 117 kV 460 mR 36 V~2 EXAMPLE IV

A series of spine and abdomen radiographs were taken under conditions shown in the following table. Measurement of dose was with a Capintec Dosimeter.

PROIECTION FFD KYP mA TIME UNFILTERED FILTERED % DOSE
l)OSE DOSE REDUCTION
CER~ICAL
SPINE 40 70 100 .1 31 7 78 LU~BAR
SPINE 40 90 300 .2 556 264 54 FULL
SPINE 72 90 300 .2 110 50 55 ABDO~IEN 72 90 300 .2 110 50 55 ,`Ch ,"~qS
The films ~!re taken with the niobium filter had greater detai1 enhanced by the increased homogeneity of the beam.

EXA~IPLE Y

Tests were run using a WEBER Dental X-ray unit a~ 70 KVP
and 10 mA with the 50 micron Nb filter. It was found that to achieve equivalent contrast and film quality with the Nb filter, exposure times were increased 1.5 to 2 times the exposure for the Al filter alone. In normal operation with the Al filter, exposure times are generally 0.2 to 0.3 seconds, wlth the Nb fllters they are 0.3 to 0.5 seconds. Dose reductions are shown ln the followlng table:

z FILTER EXPOSURE DOSE MR ~ DOSE
TIME REDUCTION

Al 0.2 116 69g Nb 0 2 36 Al 0.2 116 50.9 Nb 0.3 57 Al 0.2 116 37.9 Nb 0.4 72 Al 0.3 171 66.7 ~Ib 0.3 57 Al 0.3 171 57.9 Nb 0.4 72 Al 0.3 171 50.3 Nb 0.5 85 Al 0.3 171 30.4 Nb 0.6 102 Thus, at ordinary operating situations, the 50 micron ~b filter results in 30 to 50~ dose reductions to the patient.

Although various preferred embodiments of the present invention have been described herein in detail, it will be appreciated by those skilled in the art, that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A filter for use with an X-ray apparatus for medical diagnosis wherein the X-ray apparatus includes an X-ray source and an examination object subjected to X-ray beams from the source;

said filter comprising at least a niobium metal foil having a maximum thickness of about 75 microns.

2. A filter as claimed in claim 1 including additional filtering foils in combination with said niobium metal foil with said filter maintaining a maximum thickness of about 75 microns.

3. A filter as claimed in claim 1 encased in a transparent cover.

4. A filter as claimed in claim 3 wherein said transparent cover comprises a flexible plastic.

5. A filter as claimed in claim 3 including a colour backdrop within said transparent cover, the colour in said backdrop identifying specific information regarding the filter.

6. A filter as claimed in claim 5 wherein said colour backdrop comprises a cardboard material and wherein said transparent cover comprises a flexible plastic resulting in overall flexibility of said filter.

7. A filter as claimed in claim 1 wherein said niobium metal foil has a preferred thickness within the range of about 25 to about 75 microns.

8. A filter as claimed in claim 7 wherein said niobium metal foil has a thickness of about 50 microns.

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