U.S. patent application number 09/745678 was filed with the patent office on 2002-06-27 for digital x-ray imager alignment method.
This patent application is currently assigned to GE Medical Systems Global Technology Company, LLC. Invention is credited to Granfors, Paul Richard, Kwasnick, Robert Forrest.
Application Number | 20020080922 09/745678 |
Document ID | / |
Family ID | 24997765 |
Filed Date | 2002-06-27 |
United States Patent
Application |
20020080922 |
Kind Code |
A1 |
Kwasnick, Robert Forrest ;
et al. |
June 27, 2002 |
Digital X-ray imager alignment method
Abstract
In a digital x-ray imager alignment method, a low exposure x-ray
image of an object is taken. The dose is sufficient to create an
image of an object and alignment bars on an antiscatter grid. The
relative position of the alignment bars on the image is measured.
The relative angle of the detector to the x-ray source is adjusted.
This adjustment brings the grid into alignment with the x-ray
source. A diagnostic x-ray exposure image of the object is then
taken.
Inventors: |
Kwasnick, Robert Forrest;
(Palo Alto, CA) ; Granfors, Paul Richard;
(Sunnyvale, CA) |
Correspondence
Address: |
WILLIAM COLLARD
COLLARD & ROE, P.C.
1077 NORTHERN BOULEVARD
ROSLYN
NY
11576
US
|
Assignee: |
GE Medical Systems Global
Technology Company, LLC
|
Family ID: |
24997765 |
Appl. No.: |
09/745678 |
Filed: |
December 22, 2000 |
Current U.S.
Class: |
378/205 ;
378/147; 378/154 |
Current CPC
Class: |
G06T 3/00 20130101; A61B
6/4291 20130101; A61B 6/587 20130101; A61B 6/488 20130101; A61B
6/06 20130101 |
Class at
Publication: |
378/205 ;
378/147; 378/154 |
International
Class: |
A61B 006/08 |
Claims
What is claimed is:
1. A method for aligning a digital x-ray imaging system comprising
an x-ray source, an antiscatter grid, and a detector, the grid
being attached to the detector and disposed between the detector
and an object under study, the grid having a front side and a back
side and at least one pair of substantially x-ray opaque alignment
bars, one bar of said pair attached to the front side and the other
bar of said pair attached to the back side, the method comprising
the steps of: (a) taking a low exposure non-diagnostic x-ray image
of the object with a dose sufficient to create an image of the
alignment bars on the object; (b) measuring the relative position
on the image of the alignment bars; (c) adjusting the relative
angle of the detector to the x-ray source to bring the grid into
alignment with the x-ray source; and (d) taking a diagnostic x-ray
exposure image of the object.
2. The method according to claim 1 wherein the low exposure x-ray
image is taken with an x-ray dose about 0.1% to 1% of the x-ray
dose used for the diagnostic x-ray dose image.
3. The method according to claim 1 wherein the antiscatter grid is
formed from alternating strips of x-ray opaque material and x-ray
transmissive material.
4. The method according to claim 3 wherein the x-ray opaque
material is lead and the x-ray transmissive material is selected
from the group consisting of plastic, aluminum, fiber and air.
5. The method according to claim 1 wherein the measuring step is
performed by computer algorithm.
6. The method according to claim 1 wherein the measuring step is
performed manually.
7. The method according to claim 1 further comprising taking a
second low exposure x-ray image of the object prior to the step of
taking a diagnostic x-ray exposure image in order to confirm
alignment.
8. The method according to claim 1 wherein the adjustment of the
relative angle of the detector to the x-ray source is the
arctangent of the distance d1 between the alignment bars in the
image divided by the thickness of the antiscatter grid.
9. The method according to claim 1 wherein the alignment bars are
made from lead.
10. The method according to claim 1 wherein the alignment bars are
made from tungsten.
11. A method for aligning a digital x-ray imager comprising the
steps of: (a) providing a portable digital x-ray imaging system
comprising an x-ray source, an antiscatter grid, and a planar
detector, the grid being attached to the detector and disposed
between the detector and an object under study, the grid having
front and back surfaces separated by a distance d2 and at least one
substantially x-ray opaque alignment bar on each of said surfaces;
(b) taking a first low level x-ray exposure to create an image of
the object and of the alignment bars on the object; (c) measuring
the distance d1 between the alignment bars on the image; (d)
adjusting the relative angle of the detector to the x-ray source by
the arctangent of the distance d1 between the bar divided by the
distance d2; (e) taking a second low level x-ray exposure to
confirm alignment of the grid with the x-ray source; and (f) taking
a diagnostic x-ray exposure image of the object.
12. The method according to claim 11 wherein the low level x-ray
exposures are taken with an x-ray dose about 0.1% to 1% of the
x-ray dose used for the diagnostic x-ray exposure image.
13. The method according to claim 11 wherein the antiscatter grid
is formed from alternating strips of x-ray opaque material and
x-ray transmissive material.
14. The method according to claim 11 wherein the x-ray opaque
material is lead and the x-ray transmissive material is selected
from the group consisting of plastic, aluminum, fiber and air.
15. The method according to claim 11 wherein the measuring step is
performed by computer algorithm.
16. The method according to claim 11 wherein the measuring step is
performed manually.
17. The method according to claim 11 wherein the alignment bars are
made from lead.
18. The method according to claim 11 wherein the alignment bars are
made from tungsten.
19. The method according to claim 1 further comprising repeating
steps (a) through (c) until satisfactory alignment is achieved
prior to the step of taking a diagnostic x-ray exposure image.
20. The method according to claim 11 further comprising repeating
steps (b) through (d) until satisfactory alignment is achieved
prior to the step of taking a diagnostic x-ray exposure image.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to x-ray imaging. More
particularly, an alignment method is provided for a digital x-ray
imager suitable for use in medical diagnostic applications.
[0003] 2. The Prior Art
[0004] In x-ray imaging, it is generally desirable to minimize
x-ray exposure to the amount required to achieve acceptable image
quality. For medical diagnostic imaging the goal is to keep the
x-ray exposure of the patient to the minimum needed. For other
applications such as industrial inspection or veterinary studies,
x-ray source (tube and generator) life is limited. Here again it is
desirable to use no more exposure than needed.
[0005] The x-ray imaging set up usually comprises at least an x-ray
source, a patient (or other object under study), and a detector.
The origin of x-rays within the x-ray source is generally on the
order of 1 mm. The distance is small relative to the distance
between the source and the detector (on the order of 1 m).
[0006] It is desirable to have an x-ray quality that is as good as
possible to enhance the utility of the x-ray image. Often
antiscatter grids are used to improve x-ray image quality.
Typically, the grid and detector are attached to form a
detector/grid assembly. The antiscatter grid is generally formed
from alternating strips of x-ray opaque material and x-ray
transmissive materials. Lead may be used as the x-ray opaque
material and plastics, aluminum or fiber may be used as the x-ray
transmissive material. Two dimensional arrangements of x-ray opaque
and transmissive materials (or air) are possible also. In either
arrangement, the grid substantially transmits unscattered (primary)
x-rays and substantially absorbs x-rays scattered by the patient or
object under study, thereby preventing the scattered x-rays from
degrading image quality. This function is accomplished by aligning
the strips parallel to the primary x-rays. In some applications,
for example, portable x-ray examinations, a patient is examined
while prone in a hospital bed. In these applications, the
detector/grid is not mechanically aligned perpendicular to the
source, which can degrade the grid's functionality.
[0007] Dr. Heber MacMahon, at the University of Chicago, uses a
technique involving alignment bars. In his technique, small
alignment bars are attached to the front and back sides of the
antiscatter grid. The alignment bars may be about 1 mm by 10 mm in
area and 0.1 to 1 mm in thickness. The bars are substantially
aligned with each other on opposite sides of the same septa of the
grid. In other words, they are disposed on each side of the grid
and overlap at least one of the spaces formed by the x-ray
transmissive grid parts. The detector typically comprises either
film or photostimulable phosphor. If the detector is not aligned
correctly, the two alignment bars appear in different positions in
the image. Misalignment correlates with lower quality images.
[0008] However, this alignment information is obtained only after
the detector is removed and the latent image is developed. Hence,
it cannot be used to improve the image in situ. At best, the
alignment information can be used only to indicate after the fact
whether the image quality is good. Additional images, resulting in
more x-ray exposure, may need to be taken if the image quality is
poor.
[0009] Hence, a method using alignment information to improve the
first diagnostic image is still needed. Also needed is a method
that reduces the amount of x-ray exposure required to achieve
acceptable image quality.
BRIEF SUMMARY OF THE INVENTION
[0010] A digital x-ray imager method is provided for use, for
example, in medical diagnostic applications. The method is
particularly useful in planar detectors used in portable imaging
applications. In such applications, the detector surface is not
mechanically constrained to be perpendicular to the central x-ray
source beam. The method requires only an insignificant increase in
x-ray exposure. The method also achieves a substantially optimal
quality diagnostic image in digital x-ray imaging systems. A
digital x-ray imaging system is provided including an x-ray source
and a detector. An antiscatter grid is attached to the detector and
disposed between the detector and the object under study, for
example, a patient. The grid has at least one pair of substantially
x-ray opaque alignment bars. Preferably, the antiscatter grid has
alternating strips of x-ray opaque material (lead) and x-ray
transmissive material (for example, plastic, aluminum, fiber or
air). The grid has front and back surfaces on which the alignment
bars are disposed, one of each pair on each surface.
[0011] A low exposure non-diagnostic x-ray image is taken with a
dose sufficient to create an image of the alignment bars on the
object. For example, the low x-ray dose may be about 0.001 to 0.01
of that used for the diagnostic x-ray image. The relative position
of the alignment bars on the image is measured, for example,
manually or by a computer algorithm. The relative angle of the
detector to the x-ray source is adjusted to align the grid with the
x-ray source. The adjustment required is the arc tangent of the
distance between the alignment bars in the image divided by the
antiscatter grid thickness. A diagnostic x-ray exposure image is
then taken of the object. Alternatively, a second low exposure
image may be taken to confirm alignment prior to taking the
diagnostic x-ray exposure image.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Other objects and features of the present invention will
become apparent from the following detailed description considered
in connection with the accompanying drawings. It should be
understood, however, that the drawings are designed for the purpose
of illustration only and not as a definition of the limits of the
invention.
[0013] In the drawings, wherein similar reference characters denote
similar elements throughout the several views:
[0014] FIG. 1 is a schematic diagram of a projection x-ray imaging
device incorporating an antiscatter grid with alignment bars used
in the present invention;
[0015] FIG. 2 is a perspective view of the antiscatter grid with
alignment bars of FIG. 1; and
[0016] FIG. 3 is a schematic diagram of the path of exemplary
x-rays through the antiscatter grid to the detector with the
alignment bars of FIG. 1 showing misalignment of x-ray source and
antiscatter grid.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0017] Referring to FIG. 1, a digital radiation imager system 10,
such as a medical x-ray projection imaging system, is shown. FIG. 1
shows a schematic diagram, not drawn to scale, of a typical system.
The projection x-ray imaging system 10 comprises a radiation point
source 12, typically an x-ray source. The imaging system 10 also
includes a radiation detector/antiscatter grid assembly 14
including a radiation detector 16 and an antiscatter grid 18. The
origin of x-rays within x-ray source 12 is generally small relative
to the distance between source 12 and detector 16. The distance
between source 12 and detector 16 is on the order of 1 m. The
origin of x-rays is on the order of 1 mm in spatial extent.
Detector 16 includes a radiation detector panel (not shown) that
converts incident radiation into electrical signals. The detector
elements of the detector panel are typically arranged in a
two-dimensional array. The elements are coupled to a signal
processing circuit 40 and from there to an image analysis and
display circuit 42. The surface of the detector/antiscatter grid
assembly 14 may not be constrained by mechanical means to be
perpendicular to the central ray of x-ray source 12.
[0018] Antiscatter grid 18 is attached to detector 16 and disposed
between detector 16 and an object, for example, a patient 20 under
study. FIG. 2 is a perspective view of a representative portion of
antiscatter grid 18. As shown in FIG. 2, antiscatter grid 18
comprises alternating strips of x-ray opaque material 22 and x-ray
transmissive material 24. X-ray opaque material 22 may be lead.
X-ray transmissive material 24 may be plastic, aluminum, fiber or
air. A typical grid may have dimensions of about 41 cm by 41 cm by
2 mm thick with about 78 lead strips per cm. Each lead strip is
about 1.3 mm tall by 28 .mu.m wide and the spacing between the lead
strips for the x-ray transmissive portion of the grid is about 100
.mu.m.
[0019] Antiscatter grid 18 includes at least one pair of
substantially x-ray opaque alignment bars 26, preferably disposed
on front surface 28 and back surface 30 of grid 18. They are
positioned at opposite sides of the same septa, or, more generally,
aligned along the line defined by an x-ray beam 32. Preferably, the
alignment bars are positioned towards the edge of the detector
where they do not interfere with the central region of the image.
Alignment bars 26 are preferably lead but may be another
substantially x-ray opaque material such as tungsten. Alignment
bars 26 are generally 1 mm by 10 mm in area and 0.1 to 1 mm in
thickness. Bars 26 are disposed parallel to and preferably
substantially centered on opposite sides of the same septa 24 of
grid 18. Bars 26 generally will be much wider than grid septa 24.
Preferably one pair of bars 26 are provided on the front 28 and
back 30 surfaces of grid 18, but additional pairs of alignment bars
26 may be used. In particular, it is useful to have two pairs of
alignment bars on opposite edges of the detector in order to more
accurately determine the angular alignment when the source is not
exactly at the focal spot of the grid. Each pair of alignment bars
26 is positioned precisely so that the image of bars 26 is
superimposed when the central x-ray beam is aligned with grid
18.
[0020] In performing an alignment, a low exposure x-ray image of
the object 20 is taken. The dose could be as low as a few
microRoentgen exposure to the detector. This dose is sufficient to
create an image of alignment bars 26 on the object. Generally,
lower dose is desirable when imaging human subjects. Typically, the
dose is about 1% and may be as low as 0.1% of that used for the
diagnostic image. This dose is sufficient with digital x-ray
detectors to observe alignment bars 26. The low exposure x-ray
image is preferably taken after preliminary alignment of
detector/grid assembly 14 to x-ray source 12. The low exposure
x-ray image is observed and the relative position of alignment bars
26 is measured on the image. This measurement can be done manually
or automatically. For example, a computer algorithm could be
readily designed by one skilled in the art to take the required
measurement. The positions of source 12 and/or detector/grid
assembly 14 are then adjusted to bring grid 18 into alignment with
x-ray source 12. A second low exposure image may then be taken to
confirm alignment. If alignment is not confirmed, additional
adjustment and confirming images may be performed. Thereafter, the
high exposure diagnostic image of object 20 is taken. The high
exposure is typically several hundred microRoentgen exposure to the
detector.
[0021] FIG. 3 illustrates a preferred method of determining angular
misalignment, and thus the adjustment required by either source 12
or detector/grid assembly 14. (By way of illustration, the bars are
shown as being in the center of the grid so that alignment bars 26
are at the same relative position on grid 18.) FIG. 3 illustrates
an exemplar pair of x-ray beams 32a, 32b. One beam of the pair
passes through the top alignment bar, and the other beam passes
through the same part of the bottom indicator bar. One alignment
bar 26 is disposed on front surface 28 and another alignment bar 26
is disposed on back surface 30 of antiscatter grid 18. Hence the
distance d2 between the bars is the thickness of antiscatter grid
18. Beam 32a creates an image of bar 26 on front surface 28, and
beam 32b creates an image for bar 26 on back surface 30. As shown
in FIG. 3, the low dose radiation creates an image of bars 26 that
shows misalignment of the x-ray source and the grid, i.e. the
x-rays are not perpendicular to grid 18 but rather are misaligned
by an angle .alpha.. The misalignment results in an image of the
alignment bars with a separation of distance d1 between them in the
image. The angular misalignment, and thus the adjustment required
of the relative angle of detector 16 to source 12 may be easily
determined. The adjustment is the arctangent of the distance d1
between the bars in the image divided by the antiscatter grid
thickness d2. In proper alignment, the image of the alignment bars
overlap each other to form a single line in the image (d1=0).
[0022] Thus, as described above, a series of low-level exposures is
used for the alignment. This alignment method is useful to align a
digital x-ray detector to the x-ray source in various systems. The
method is particularly useful in portable imaging applications. In
many such applications, the detector surface is not constrained by
mechanical means relative to the x-ray source. Thus, the detector
surface is not mechanically constrained to be perpendicular to the
central ray of the x-ray source, but rather is aligned during the
imaging procedure. An x-ray antiscatter grid is provided in
proximity to and attached to the detector. The grid is disposed
between the detector and the patient or object under study. The
grid has small, substantially x-ray opaque, alignment bars on both
its front and back surfaces. A low x-ray dose image is taken and
the positions on the image of the alignment bars are measured. Then
the x-ray source, and/or the detector/grid assembly, is moved into
alignment before the diagnostic high exposure image is taken.
[0023] While preferred embodiments of the present invention have
been shown and described, it is to be understood that many changes
and modifications may be made thereunto without departing from the
spirit and scope of the invention as defined in the appended
claims.
* * * * *