Method of making x-ray beam hardening filter and assembly

An x-ray beam hardening filter and method for making the same is disclosed. According to an embodiment, the method comprises etching a plurality of regularly spaced pits into a surface area of a sheet having an x-ray beam hardening quality, aligning the sheet to a support member and bonding the sheet to the support member. An x-ray beam hardening filter can be made and used which is not only compact and useful in diagnostic x-ray imaging, but which is capable of shaping an x-ray energy spectrum envelope in a highly controllable manner.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention pertains to x-ray beam hardening filters and methods for 
making the same. 
2. Background 
X-ray sources used in medical imaging are typically polychromatic, that is, 
the x-ray source produces x-ray photons with varying energies. For 
example, an x-ray source capable of producing a 120 keV photon will 
typically produce an x-ray beam having a mean energy of only one-third to 
one-half of the peak energy. Given that the mean energy is roughly 
one-half to one-third of the peak energy, many of the photons that 
comprise an x-ray beam will be characterized by energy levels below the 
mean energy. 
A problem with lower energy photons is that they do not contribute to the 
construction of the radiographic image. Many of the lower energy photons, 
for example those with energies less than 20 keV, may be absorbed in the 
object under investigation; these lower energy photons only contribute to 
harmful patient radiation. Therefore, it is desirable to filter the lower 
energy x-ray photons from the x-ray beam. 
It is known to use filters to remove lower energy photons from the x-ray 
beam. One form of filtration is inherent filtration. Inherent filtration 
results from the absorption of x-ray photons as they pass through the 
x-ray tube and its housing. Such filtration varies with the composition, 
or lining of the x-ray tube and housing, as well as the length of the 
x-ray tube and housing. Inherent filtration, which is measured in aluminum 
equivalents, typically varies between 0.5 and 1.0 mm aluminum equivalent. 
A second form of filtration is added filtration. Added filtration is 
achieved by placing an x-ray attenuator or filter in the path of the x-ray 
beam. Most materials have the property of attenuating the lower energy 
photons more strongly than higher energy photons. When lower energy x-ray 
beams strike the added filter they are absorbed. By adding a filter to the 
x-ray beam path, lower energy x-ray photons can be absorbed, thereby 
reducing the unnecessary radiation created by the lower energy x-ray 
photons. Because the lower energy x-ray photons are preferentially removed 
from the x-ray beam, the mean energy of the x-ray beam is increased. 
Increasing the mean energy of the x-ray beam is referred to as "hardening" 
of the x-ray beam. 
Objects to be x-rayed vary in thickness and composition. Thus, it is 
desirable to control the amount of filtration that occurs. Some x-ray 
systems, having a relatively small diameter x-ray source, often use a 
filter consisting of a thin sheet of aluminum or aluminum and copper. The 
filter is placed in the path of the x-ray beam, either manually or by an 
electromechanical actuator. Because of the small diameter of the x-ray 
source, the filter and filter control mechanism can be made compact. 
However, when a large-area x-ray source (e.g., having a diameter of 
approximately 25 cm or larger) is used in an x-ray imaging system and if 
added filtration is used, the beam hardening filter inserted into the path 
of the x-ray beam would be as large as the overall x-ray source in order 
to cover the entire source. Furthermore, the mechanical travel of the 
filter to insert it into the path of the x-ray beam would also be about 
the same as the size of the x-ray source (e.g., 25 cm) or the filter. 
Using a conventional x-ray hardening filter, for example one that slides 
in a parallel plane to the surface of the x-ray source, on a large-area 
x-ray source would involve a large mechanical actuator assembly and would 
add undesirable bulk to the x-ray imaging system. 
SUMMARY OF THE INVENTION 
The present invention comprises a method for making a novel x-ray beam 
hardening filter and assembly, comprising etching a plurality of pits into 
a sheet having an x-ray hardening quality, aligning the sheet and a 
support member to a reference point, and bonding the sheet to a support 
member. In another embodiment, the method further comprises, aligning the 
plurality of pits with a plurality of collimator apertures, reaming the 
plurality of pits to a finished size, and removing burs from the support 
member. 
As a result of the method for making the x-ray beam hardening filter 
disclosed herein, an x-ray beam hardening filter comprising a beam 
hardening sheet, the beam hardening sheet having a plurality of pits, and 
a support member can be made and used which is useful in diagnostic x-ray 
imaging, especially for filtering harmful x-ray radiation which does not 
contribute to an x-ray image.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The specification and drawings described in detail herein are related to 
copending U.S. patent application Ser. Nos. 09/167,639, 09/167,399, filed 
on the same day herewith, and U.S. Pat. No. 5,610,967, all of which are 
incorporated herein by reference in their entirety. 
FIG. 1 depicts a preferred construction of an x-ray beam hardening filter 
100. The x-ray beam hardening filter 100 comprises a filter plate or 
"support member" 110, as it is referred to herein, and a sheet having a 
beam hardening quality. As used herein, the sheet having a beam hardening 
quality is referred to as a "beam hardening sheet" 120. The beam hardening 
sheet 120 preferably comprises a plurality of pits. The areas of the beam 
hardening sheet without pits are configured to cause certain energy levels 
of x-ray radiation from a polychromatic x-ray beam incident thereon to be 
absorbed (or filtered), whereas the plurality of pits are configured to 
not to filter the x-ray radiation. The x-ray beam hardening filter 100 
therefore is capable of shaping the energy spectrum envelope of the 
polychromatic x-ray beam such that certain energy levels of harmful 
radiation are selectively removed. 
The support member 110 is preferably manufactured from stainless steel. 
Furthermore, the support member 110 is initially larger than washer-shaped 
article depicted in FIG. 1, for it includes an etching plate 140, which 
can be removed after bonding a beam hardening sheet 120 to the support 
member 110, or, later still, after aligning the x-ray beam hardening 
filter 100 to a collimator assembly. 
The outer diameter of the relevant portion of the support member 110 is 
approximately 10.27 inches, while the inner diameter of the support member 
110 is approximately 9.800 inches. The upper and lower portions of the 
support member 110, that is bottom portion 150 and top portion 160, have a 
flattened edge 112 extending inward from the outer diameter to a distance 
4.512 inches from the x-centerline 102. The side portion 155 also has a 
flattened portion 112 which extends inward from the outer diameter to a 
distance of 4.512 inches from the y-centerline 104. 
The outer edge of the support member 110 is defined by a number of 
connector openings 180 that permit unobstructed movement of the x-ray beam 
hardening filter 100 within (or over) a collimator (described in greater 
detail below with reference to FIG. 4). Both the top and bottom edges, 160 
and 150, of the support member 110 comprise direction guides 192 which 
guide the motion of the support member in straight path. The direction 
guides 192 have a width of 0.110 inches. 
A receiver, or an "actuator aperture" 194, as it is referred to herein, is 
formed on the top edge 160 of the support member 110. The actuator 
aperture 194 surrounds an actuator (not shown) which provides a force to 
move that support member 110 in the straight path defined by direction 
guides 192. The bottom edge 150 of the support member 110 does not have an 
actuator aperture 194. The bottom edge 150 instead has a rectangular 
shaped opening 152. Within the rectangular shaped opening 152 is a break 
away alignment tab 154. Two additional alignment tabs 154 are also 
depicted in FIG. 1. 
FIG. 2 depicts the support member 110 without the beam hardening sheet 120. 
FIG. 3A depicts the top edge 160 of the support member 110, and FIG. 3B 
depicts the bottom edge 150 of the same. Actuator aperture 194 and 
alignment slot 172 are depicted in the top edge 160. Alignment slot 172 is 
0.110 (.+-.0.002) circular mils. It is preferred that the alignment slot 
172 is within 0.002 inches of the true position of the apertures 156 in 
the break away tabs 154. The actuator aperture 194 preferably has a 
generally rectangular shape with a height of approximately 0.220 inches, a 
width of approximately 0.695 inches, and rounded comers with a radius of 
approximately 0.046 inches. At approximately 0.520 inches from the left 
side of the rectangle (as depicted in FIG. 3A), near both the top and 
bottom edges of the rectangle, two circular extensions are carved from the 
actuator aperture 194. The radius of the two circular extensions is 0.175 
inches. The actuator aperture 194 can vary in size and shape, however, it 
is important that it still allow for movement of an actuator therein, the 
actuator used to move the beam hardening filter 100 into or out of the 
path of a polychromatic x-ray beam. 
FIG. 3B depicts the bottom edge 150 of the support member 110. The 
rectangular ledge 152 carved from the support member 110 is begins 
approximately 0.338 inches from left of the y-centerline 104 and down 
approximately 4.623 inches from the intersection of the x- and 
y-centerlines 102 and 104. An alignment tab 154 connects to two sides of 
the ledge 152. The alignment tab 154 is configured to break away from the 
support member 110. An alignment aperture 156, measuring 0.047 circular 
mils, is located on the alignment tab 154. Similar alignment apertures 156 
are located on the left and right side of the support member 110 on the x- 
and y-centerlines 102 and 104. 
FIG. 3C depicts a break away tab 154 and alignment aperture 156 which is 
located on the right side 155 of the support member 110. The break away 
tab 154 has a radius of 0.100 inches, which is the same as the radius of 
the alignment tab 154 depicted with reference to FIG. 3B. Again, an 
alignment aperture 156 is located at the center point of the alignment tab 
154. 
Returning again to FIG. 1, according to a presently preferred embodiment, a 
method for making the x-ray beam hardening filter comprises the steps 
described below. First, a plurality of areas having a different x-ray 
absorption quality than the beam hardening sheet 120 are chemically etched 
into the surface of the beryllium (Be) and copper (Cu) beam hardening 
sheet 120. The result of the etching is a plurality of pits 130 that are 
regularly spaced about the surface area of the beam hardening sheet 120. 
The pits 130 are preferably 0.036 (.+-.0.002) circular mils, and are spaced 
and shaped according to the parameters defined in Table 1. Furthermore, 
the pits 130 are symmetrical with the x- and y- centerlines 102 and 104 
respectively, with a center of a single pit placed at the intersection of 
the centerlines 102 and 104. Thus, according to a preferred embodiment, 
the plurality of pits form a multidimensional array of uniformly sized and 
spaced pits in the surface area of the beam hardening sheet 120. 
TABLE 1 
______________________________________ 
Beam Hardening Filter Pit Spacing (inches & circular mils) 
Sheet Reduction 
Thickness 
Level Hole Pitch 
Hole Size 
______________________________________ 
0.004 0.990636 0.89703 0.036 (.+-.0.002) 
0.008 0.990043 0.89650 0.036 (.+-.0.002) 
______________________________________ 
An advantage of the present invention is that when uniformly spaced and 
sized pits 130 are employed, and they are spaced according to Table 1 
above, then the movement of the beam hardening sheet need only be a 
distance approximately equal to one-half the hole pitch, or the spacing 
between two adjacent pits in the beam hardening sheet. In other 
embodiments, movement of the beam hardening sheet 120 may follow a curved 
path and the movement can be restricted to approximately three times the 
distance between two adjacent areas of equal x-ray absorption. This unique 
feature allows for a minimal amount of movement of the beam hardening 
sheet 120 to vary the x-ray absorption quality of the beam hardening 
filter 100. 
In the next step, the support member 110 and the beam hardening sheet 120 
are aligned. The alignment is accomplished with the aid of one or more 
alignment elements. In a preferred embodiment, the beam hardening sheet 
120 is first placed on a surface (e.g., a jig) and support member 110 is 
placed over it. The beam hardening sheet 120 and the support member are 
aligned to a reference position, namely the alignment slots 170 (having a 
diameter of 0.125 inches) which are formed into the etching blank 140 and 
the beam hardening sheet 120. 
Once the beam hardening sheet 120 and the support member 110 are aligned, 
they are bonded together. The bonding step comprises applying a 95% tin 
and a 5% silver brazing paste between the top of the beam hardening sheet 
120 and the bottom of the support member 110, followed by heating the 
brazing paste to approximately 480 F in a hydrogen atmosphere. Preferably, 
none of the solder overlaps any of the pits 130. To accomplish this, the 
brazing paste may be blown from the active area of the sheet before the 
step of heating with a fan. Furthermore, the beam hardening sheet 120 and 
support member 110 are clamped together to prevent movement which may 
cause misalignment before the step of bonding. 
It is important not to overheat the brazing paste, and consequently the 
x-ray beam hardening filter, because there is a chance it will warp. 
Furthermore, the heating step is preferably performed in a furnace. 
According to one embodiment, the x-ray beam hardening filter 100 components 
(e.g., beam hardening sheet 120 and support member 110) are electroplated 
before the step of bonding. 
Now that the beam hardening sheet 120 has been bonded to the support member 
110, another alignment step is performed. Referring to FIG. 4, the x-ray 
beam hardening filter 100 is placed over a collimator 404 such that the 
pits 130 align with collimator apertures 436 in the collimator 404. The 
alignment is facilitated again by alignment slots 170, which can be placed 
over a jig or alignment pins, alignment slots 172, through which an 
alignment pin 408 can pass, as well as with the aid of alignment apertures 
156 in alignment tabs 154. 
Once the pits 130 are aligned, the direction guides 192 are reamed to their 
preferred size. A final inspection is made of the alignment of the pits 
130 with the collimator apertures 436. If alignment is confirmed, then the 
alignment slots 172 are machined and the etching blank 140 and alignment 
tabs 154 are removed from the support member 110. 
The x-ray beam hardening filter 100 can then be removed from the collimator 
404. Burs are preferably ground from the edges of the support member 110. 
A lubricant is applied to the surfaces of the finished x-ray beam 
hardening filter 100. According to one embodiment, a dry film lubricant is 
used. A presently preferred dry film lubricant is Dicronite.RTM. made by 
Dricronite.RTM. Dry Lube Northwest, and which is available from CLS, Inc, 
in Santa Clara, Calif. 
Turning again to FIG. 4, one or more x-ray beam hardening filters 416 are 
placed within a collimator assembly 400. Mounting pins 412 tie the 
collimator 404 to the collimator cover 432. Spacers, e.g., spacer 428, 
create a void between the collimator 404 and the collimator cover 432 in 
which the one or more beam hardening filters 416 can move, aided by an 
actuator 420 having a cam bearing 424, while pressure is maintained around 
the collimator cover 432 and collimator 404. 
In the foregoing specification, the invention has been described with 
reference to specific embodiments thereof. It will be evident, however, 
that various modifications and changes may be made thereto without 
departing from the broader spirit and scope of the invention. For example, 
the dimensions and sizes of the various components can be altered and 
different materials substituted for the construction thereof. Furthermore, 
the spacing of the pits does not have to be uniform, nor do the pits 
themselves need to be of a uniform size or shape. The specification and 
drawings are, accordingly, to be regarded in an illustrative, rather than 
a restrictive sense.