Patent Application
20140177809
X-ray tubes for the production of x-rays for imaging including the medical imaging of human patients are typically provided with filters which condition the x-ray beam used for imaging. These filters attenuate certain x-rays in the beam to better suit the x-ray beam to a particular imaging task. For instance, for the medical imaging of human patients the softer x-rays which are likely to be absorbed by the tissue of the patient are filtered out of the beam. Some conditioning requires the use of metal filters which are adhered to the x-ray tube. For instance, in x-ray tubes for the medical imaging of human patients a 75 micron sheet of pure copper is adhered to the aluminum window which allows the x-rays to exit the tube for imaging. In some cases this copper sheet is protected by a polymer sheet which is in turn affixed to the aluminum window such that the x-rays used for imaging pass through the thickness of the copper sheet.
In one embodiment an x-ray system includes x-ray tube containing a cathode which supplies electrons and an anode which can be maintained at high positive electrical potential to the cathode and has a target area which is impacted by electrons from the cathode when the positive potential is maintained generating x-rays. The x-ray tube is enclosed in a radiation resistant casing. The radiation resistant casing has a window which allows some of the x-rays generated at the target area to exit the system. The window has a vapor deposited layer of a filtering metal of sufficient thickness to effectively condition the x-rays passing through the window and located to intercept the x-rays passing through the window.
Another embodiment includes a process for the construction of an x-ray system. The x-ray system includes one or more conditioning filters having an x-ray tube. The x-ray tube includes a cathode which supplies electrons and an anode which can be maintained at high positive electrical potential to the cathode and has a target area which is impacted by electrons from the cathode when the negative potential is maintained generating x-rays which is enclosed in a radiation resistant casing. The casing has a window which allows some of the x-rays generated at the target area to exit the system. The window has one or more layers of a filtering metal of sufficient thickness to effectively condition the x-rays passing through the window and located to intercept the x-rays passing through the window. A layer of filtering metal is vapor deposited on a surface of the window.
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The layer of filtering metal may be deposited by any of the known physical or chemical vapor deposition methods with the modification that the deposited layer is sufficiently thick to effectively condition the x-rays passing through it. Vapor deposition methods provide for a reproducible deposit of a layer of reasonably uniform composition and thickness so that the x-ray conditioning behavior is fairly consistent within each window and between multiple windows manufactured to the same specification. This deposition technique minimizes the formation of mixtures of the substrate material, i.e. the surface of the window on which the layer is being deposited, and the metal being deposited. This facilitates the design of windows with particular conditioning characteristics as these characteristics can be reproducibly predicted from the identity and thickness of the filtering layer without having to account for the effect of unintended mixtures of materials.
Suitable physical vapor deposition techniques include those involving creating a vapor of the metal to be deposited under reduced pressure of an inert gas and causing the vapor to condense on the substrate which will carry the layer by the application of an electrical potential. In one embodiment a magnetron is used to generate the vapor. In one embodiment the surface to which the layer is to be applied is cleaned by bombardment of ionized atoms of the inert gas before exposing this surface to the vapor. It is convenient to use an inert gas whose atomic number is reasonably close to that of the metal being deposited. For instance it is convenient to use argon when creating a layer of copper.
Suitable chemical vapor deposition techniques include those involving a chemical reaction which results in a vapor of the metal to be deposited being condensed on the substrate which will carry the layer. It is convenient to create a vapor of a chemical compound involving the desired metal and then to release the metal from the compound creating a vapor of the metal itself.
The layer of vapor deposited metal should be thick enough to cause a significant attenuation of the x-rays passing through it. Conditioning the x-ray beam with a filter removes or at least greatly reduces the presence of certain components of the x-ray spectrum generated by the impact of electrons on the anode target. For instance, X-rays used for the medical imaging of human patients are commonly filtered through a thick copper layer to remove all or a substantial portion of the “softer” x-rays which are likely to be absorbed by the tissue of the human patient as opposed to passing through this tissue. In one embodiment the vapor deposited metal layer is at least about 10 microns in thickness. In another embodiment it is between about 10 and 200 microns in thickness. In a further embodiment it is between about 50 and 150 microns in thickness. It may be convenient to employ a thickness of about 75 microns, particularly if the deposited metal is copper. The thickness may be readily tailored to achieve a desired x-ray conditioning effect.
The thickness of the layer of vapor deposited metal should be fairly uniform and reproducible between windows carrying such layers. In one embodiment the thickness is within plus or minus 2 microns of the nominal thickness intended. Thus for this embodiment a layer with a target thickness of 33 microns the thickness observed across a number of windows carrying such layers should be between 31 and 35 microns. This may be contrasted to the plus or minus seven micron tolerance common when the filter is formed from a rolled sheet material as opposed to a vapor deposited layer.
The layer may be of any metal which is amenable to one or more vapor deposition techniques and has desirable x-ray conditioning properties. These metals include Aluminum, Copper, Molybdenum, Tin, Titanium, Tungsten and Zirconium. It is not necessary that the metal have good cold or hot workability or ductility.
More than one layer may be deposited on the window to condition the x-rays. By varying the identity and thickness of multiple layers of vapor deposited metal conditioning effects can be tailored to meet particular needs. In one embodiment a layer of copper is vapor deposited followed by a layer of carbon and then followed by a layer of titanium. In this case one of the filtering layers was not a metal but it was sandwiched between two vapor deposited metals. In one embodiment the copper layer is about 50 microns in thickness while that of the carbon is about 25 microns and that of the titanium is about 40 microns.
The layer is deposited on a surface of the window through which the x-ray beam used for imaging passes in exiting the radiation resistant casing. In one embodiment this is the external surface of the window. Because the vapor deposited metal forms a good bond with the surface on which it is deposited it is also possible to place it on the interior surface of the window which faces the dielectric oil circulation path without undue concern that the oil will wick between the layer and the window surface.
The window may be constructed of any of the materials commonly used to allow the emission of an x-ray beam from an x-ray system with a radiation resistant casing. In one embodiment the window is fabricated of an appropriate material and has an appropriate thickness to contribute to the conditioning of the x-ray beam for its intended use. In one embodiment the window is fabricated of high purity aluminum. In one embodiment the window is inset such that its exterior surface is closer to the interior of the system than the exterior surface of the adjacent portion of the radiation resistant casing into which it is placed. This provides a recess which protects the surface of the vapor deposited layer from handling damage and also minimizes the thickness of the dielectric oil circulation path which passes in front of the window. This in turn minimizes the probability that turbulence or air bubbles in the dielectric oil will cause x-ray artifacts when the system is used for imaging.
The combination of the vapor deposited layer or layers and the window conditions the x-ray beam exiting the radiation resistant casing for imaging a particular type or class of target. In one embodiment the x-ray beam is conditioned for the medical imaging of human patients.
A copper layer of about 33 microns was generated on a high purity aluminum window fixture using a magnetron based physical vapor deposition process to yield composite suited to serve as a window for the radiation resistant casing enclosing an x-ray tube and to condition the x-rays passing through it for medical imaging of human patients. The window fixture was ultrasonically cleaned in alcohol for 5 minutes . and all surfaces of window fixture that were not to be coated with copper were masked. The window fixture was then placed in a vacuum chamber which was then evacuated to 3.0×10−6 Ton. The window fixture was held in vacuum at less than 3.0×10−6 Ton for one hour. The window fixture was then subject to 2 minutes of Argon ion scrubbing at 2.0 kV and 89 mA in a 17.5 mTorr Argon atmosphere. A copper vapor was supplied to the coating chamber by energizing a magnetron to 500 Watts for 60 minutes using a ramp rate of 8 seconds. A Torus Magnetron system from Kurt J Lesker™ Vacuum (Product Number TM3FS10XBS) with a 3″ diameter target was used. A 500VDC bias voltage was applied to the window fixture. The chamber pressure was adjusted to about 5 mTorr so that the magnetron plasma current and voltage were 1.59A and 256V, respectively. The deposition rate on the fixture was in excess of about 0.11 Å/sec. After about 60 minutes a copper layer of 33 microns had been generated.
An elemental analysis of a cross section of a composite prepared in the manner described in the coating example at various distances from the aluminum/copper junction was performed by scanning electron microscopy (SEM) using a sampling square of 1.6 microns. The results are reported in the table below.
The results show that the degree of intermixing of the aluminum and copper is minimal enough that the two layers can function effectively in conditioning x-rays as distinct layers.
It is important to note that the construction and arrangement of an x-ray system as described herein is illustrative only. Although only a few embodiments of the present invention have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g. variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. For example, elements shown as integrally formed may be constructed of multiple parts or elements and vice versa, the position of elements may be reversed or otherwise varied, and the nature of number of discrete elements or positions may be altered or varied. Accordingly, all such modifications are intended to be included within the scope of the present invention to be included within the scope of the present invention as defined in the appended claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the exemplary embodiments without departing from the scope of the present inventions as expressed in the appended claims.
