Patent Number: 050330756
Section: description

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION FIGS. 1 and 2 show a preferred embodiment of a filter of the present invention generally indicated at 10 comprising a metal foil 12 preferably constructed of an elemental material selected from the group consisting of niobium, copper, silver, tin, iron, nickel, zinc, zirconium or molybdenum. A particularly suitable construction is niobium in a thickness of up to about 75 microns, preferably about 40 to 60 microns, the most preferable thickness of the niobium metal foil 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 filter material and its thickness 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 metal foil 12 when an X-ray beam contacts the 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. FIG. 2 shows a cross-section of the filter 10 of FIG. 1 illustrating clearly the relationship between the metal foil 12, the cardboard envelope 14 and the plastic encasing material 16. FIG. 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 voltage from a generator (also not shown) 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. Most commonly, the target is 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, focused and emitted from the X-ray apparatus 20 through a port 26. Port 26 usually comprises a window made of glass or plastic with an inherent filtration equivalent to about 0.5 mm of aluminum. In the typical applications, the X-ray beam emitted from 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, setting the collimator opening 34. The X-ray apparatus also has inherent and added filtration (not shown), usually equivalent 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 millimetres of the examination object. These very low energy X-rays do not contribute at all to the resolution of the radiograph, but rather merely contribute to 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 18, by sticking it onto either the port 26 of the tube 30 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. FIG. 4 shows generally the X-ray wavelength spectrum emitted from an X-ray apparatus of FIG. 3. The apparatus with a tungsten target and 3.5 mm of aluminum equivalent filtration 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 .ANG. and the characteristic K.alpha. and K.beta. radiations of tungsten of about 0.21 .ANG. and 0.18 .ANG. respectively. The solid line shows the wavelength spectrum of the the normal radiation X-ray beam emitted from the apparatus prior to filtration by a 50 micron niobium filter. The long dash line is the attenuation properties of the 50 micron niobium filter. Niobium with an atomic number of 41 has a K absorption edge at about 0.65 .ANG. and an L.sub.I absorption edge at about 4.58 .ANG. (not shown on the figure). The short dash line shows the wavelength spectrum of X-ray beam after passing through the niobium filter. There is a marked decrease in the X-ray wavelengths from about 0.25 .ANG. to just before the K absorption edge at 0.65 .ANG. wherein only about 3% of the incident normal radiation is not absorbed by the filter. Thereafter the normal radiation of the X-ray beam is attenuated such that effectively all of the radiation is absorbed. The choice of filter materials for the filters is dependent upon the requirements of the diagnostic technique as different techniques may require differing X-ray wavelength spectrums. For most medical and dental diagnostic techniques wherein the X-ray apparatus is operated at a peak voltage of between 55 keV and 110 keV, then any material whose major component is an element having an atomic number between 26 and 50 will be suitable for attenuating the X-rays beam. The elements having atomic numbers between 26 and 50 have K absorption edges between about 7 keV and 30 keV and hence in these kVP ranges will not exhibit appreciable K-edge phenomenon and hence will generally act as nonspecific filters. The choice of the filter materials is also dependent upon availability of the material in a form suitable for filter construction, preferably in a metal foil of a suitable thickness. Owing to the characteristics of these materials, particularly for those elements available as metal foils, relatively thin filters are required, varying between generally on the order of 200 microns and less, the preferred materials resulting in X-ray filters having thicknesses on the order of 30 to 120 microns, the most preferred materials resulting in X-ray filters having thicknesses on the order of 30 to 70 microns. This is illustrated in the following table which lists the preferred metal foil filter materials and the preferred thickness. ______________________________________ At. No Element Thick Range Thick Preferred ______________________________________ 26 Fe 50-250 125 27 Co 50-225 125 28 Ni 50-200 100 29 Cu 50-180 120 30 Zn 60-205 125 38 Sr 100-305 205 39 Y 55-165 100 40 Zr 35-105 70 41 Nb 25-75 50 42 Mo 20-60 40 43 Tc 15-50 35 44 Ru 15-45 30 46 Pd 15-40 30 47 Ag 15-45 30 48 Cd 20-50 35 49 In 20-60 40 50 Sn 20-55 35 ______________________________________ Those elements having atomic numbers between 26 and 50 which are not available as metal foils may be utilized by alloying them with one of the other materials. Particularly useful for alloying purposes is aluminum. Filters constructed in accordance with the present invention are easily adaptable to existing X-ray installations, thus resulting in reduced radiation exposure to the patient without significant increased cost. The filters also have the added benefit of reducing incident scattered radiation from the X-ray source, thereby reducing the levels of radiation to which operators of such equipment may be exposed. If it is desirous to remove from the X-ray beam, all radiation having energy near the K absorption edge of niobium without appreciably increasing the attenuation of the beam in the diagnostically important region (generally from about 0.15 .ANG. to about 0.4 .ANG.), then a combination filter can be utilized. The combination filter will contain one or more materials containing more than one element selected from the group consisting of aluminum and elements having atomic numbers between 26 and 50. The combination filter can be constructed by layering individual metal foils or by alloying the materials into a single foil. The selection of the materials and the elements comprising the materials will be dependent upon the desired spectrum of the X-ray beam which in turn will be dependent upon the particular application. As shown in FIG. 5 a combination filter of 25 microns of niobium and 50 microns of selenium is utilized. The keys to the curves are the same as in FIG. 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 filtered spectrum. As is clearly shown, by employing selenium with a K absorption edge of about 0.98 .ANG., in combination with niobium, substantially all of the X-rays with wavelengths greater than about 0.6 .ANG. are removed from the X-ray beam by the combination filter. Thus, in the example shown in FIG. 5, the combination of niobium and selenium is particularly useful for applications where it is desirous to have an X-ray beam with wavelengths less than about 0.4 .ANG.. If a harder beam is desired, i.e. one where the wavelengths are less than 0.3 .ANG. or 0.2 .ANG., then the filter material would be chosen to remove X-rays with wavelengths longer than this. For example, tin with a K absorption edge at about 0.42 .ANG. or indium with a K absorption edge at about 0.44 .ANG. or silver with K absorption edge at about 0.48 .ANG. would be useful. The above or other materials similar in attenuation properties would be used in combination with one or more materials having K absorption edges in the region of about 0.6 .ANG. to 1.0 .ANG. as for example materials from technetium to germanium in the periodic table. 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, the density and nm 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 6 pulse unit with a total filtration of 3.5 mm. aluminum 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 ENTRANCE DOSE kV RANGE WITHOUT WITH TEST % DOSE (kVP) TEST FILTER FILTER REDUCTION ______________________________________ 40 .9 mr/mas .22 mr/mas 75% 50 2.0 .55 72 60 3.4 1.21 64 70 5.0 2.1 58 80 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 kVP. EXAMPLE II This experiment was carried out using a General Electric 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 No. 06-526 The added filtration was 2.0 mm of aluminum making a total filtration of 3.5 mm of aluminum equivalent. 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--0.35 Seconds; kVP--80. A half value layer experiment was carried out, as well as a comparison of radiation dose obtained under; (a) Normal operation--with only the 3.5 mm aluminum/equivalent between source and the detector PA0 (b) exactly as in item (a), but with 100 microns of Yttrium added at the source in the field. PA0 (c) Exactly as in item (a), but with 50 microns of Niobium added at the source in the field. PA0 (d) Exactly as in item (a), but with 25 microns of Niobium added at the source in the field. ______________________________________ % DOSE REDUCTION ADDITIONAL (COMPARED OPERATION FILTRATION mR DOSE TO A) ______________________________________ (A) NORMAL OPERATION 0 262 1 mm 210 2 mm 176 3 mm 148 4 mm 124 5 mm 107 HALF VALUE LAYER = 3.7 mm Al (B) ADDITION OF 100 MICRONS OF YTTRIUM TO A 0 149 44 1 mm 128 39 2 mm 112 37 3 mm 95 36 4 mm 83 33 HALF VALUE LAYER = 4.85 mm Al (C) ADDITION OF 50 MICRONS OF NIOBIUM TO A 0 138 48 1 mm 118 44 2 mm 99 44 3 mm 83 44 4 mm 72 42 5 mm 64 40 HALF VALUE LAYER = 4.35 mm Al (D) ADDITION OF 25 MICRONS OF NIOBIUM TO A 0 175 34 1 mm 148 30 2 mm 125 29 3 mm 107 28 4 mm 91 27 5 mm 79 26 HALF VALUE LAYER = 4.25 mm Al ______________________________________ EXAMPLE III Tests were conducted utilizing water phantoms of 5 cm, 10 cm, 15 cm, and 20 cm in depth. A step wedge was placed in the water to provide a measurable optical density (O.D.). A Siemens Tridoros Optimatic 800 generator was used for testing using the 0.6 focal spot size. Testing was done using a Keithly 35055 digital dosimeter at 115 cm FFD. The HVL 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: __________________________________________________________________________ ADDITIONAL TUBE % DOSE PHANTOM FILTRATION EXPOSURE VOLTAGE DOSE REDUCTION __________________________________________________________________________ 5 cm 10 mAs 63 kV 28.4 mR 5 cm 0.05 mm Nb 10 mAs 63 kV 10.2 mR 64% 5 cm 0.05 mm Nb 12 mAs 63 kV 16 mR 44% 5 cm 4 mm Al 10 mAs 63 kV 10.2 mR 64% 10 cm 20 mAs 77 kV 94 mR 10 cm 0.05 mm Nb 20 mAs 77 kV 50 mR 47% 10 cm 0.05 mm Nb 25 mAs 77 kV 73 mR 22% 10 cm 3 mm Al 20 mAs 77 kV 51 mR 46% 15 cm 32 mAs 96 kV 283 mR 15 cm 0.05 mm Nb 32 mAs 96 kV 170 mR 40% 15 cm 0.05 mm Nb 40 mAs 96 kV 215 mR 24% 15 cm 3 mm Al 50 mAs 96 kV 172 mR 39% 20 cm 50 mAs 117 kV 715 mR 20 cm 0.05 mm Nb 50 mAs 117 kV 453 mR 37% 20 cm 0.05 mm Nb 64 mAs 117 kV 569 mR 20% 20 cm 3 mm Al 50 mAs 117 kV 460 mR 36% __________________________________________________________________________ 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. __________________________________________________________________________ UNFILTERED FILTERED % DOSE PROJECTION FFD kVP mA TIME DOSE DOSE REDUCTION __________________________________________________________________________ CERVICAL 40 70 100 .1 31 7 78 SPINE LATERAL 40 90 300 .2 556 264 54 LUMBAR SPINE FULL 72 90 300 .2 110 50 55 SPINE ABDOMEN 72 90 300 .2 110 50 55 __________________________________________________________________________ The films taken with the niobium filter were judged by an experienced radiologist and determined to have greater detail than the unfiltered films. EXAMPLE V Tests were run using a WEBER Dental x-ray unit at 70 kVP and 10 mA with the 50 micron niobium filter. It was found that to achieve equivalent contrast and film quality with the niobium filter, exposure times were increased 1.5 to 2 times the exposure for the aluminum filter alone. In normal operation with the aluminum filter, exposure times are generally 0.2 to 0.3 seconds, with the addition of the niobium filter they are 0.3 to 0.5 seconds. Dose reductions are shown in the following table: ______________________________________ EXPOSURE % DOSE FILTER TIME DOSE MR REDUCTION ______________________________________ Al 0.2 116 69% 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% Nb 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 Nb 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 make thereto without departing from the spirit of the invention or the scope of the appended claims.