U.S. patent application number 15/639044 was filed with the patent office on 2018-01-04 for method for optimizing radiation beam intensity profile shape using dual multiple aperture devices.
The applicant listed for this patent is THE JOHNS HOPKINS UNIVERSITY, Philips Healthcare. Invention is credited to Reuven Levinson, Aswin John Mathews, Joseph Webster Stayman.
Application Number | 20180001111 15/639044 |
Document ID | / |
Family ID | 60806006 |
Filed Date | 2018-01-04 |
United States Patent
Application |
20180001111 |
Kind Code |
A1 |
Stayman; Joseph Webster ; et
al. |
January 4, 2018 |
METHOD FOR OPTIMIZING RADIATION BEAM INTENSITY PROFILE SHAPE USING
DUAL MULTIPLE APERTURE DEVICES
Abstract
The present invention is directed to multiple aperture devices
(MADs) for beam shaping in x-ray imaging. Two or more of these
binary filters can be placed in an x-ray beam in series to permit a
large number of x-ray fluence profiles. However, the relationship
between particular MAD designs and the achievable fluence patterns
is complex. The present invention includes mathematical and
physical models that are used within an optimization framework to
find optimal MAD designs. Specifically, given a set of target
fluence patterns, the present invention finds, for example, a dual
MAD design that is a "best fit" in generating the desired fluence
patterns. This process provides a solution for both the design of
MAD filters as well as the control actuation that is required
(relative motion between MADs) that needs to be specified as part
of the operation of a MAD-based fluence field modulation
system.
Inventors: |
Stayman; Joseph Webster;
(Baltimore, MD) ; Mathews; Aswin John; (Baltimore,
MD) ; Levinson; Reuven; (Haifa, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
THE JOHNS HOPKINS UNIVERSITY
Philips Healthcare |
Baltimore
Haifa |
MD |
US
IL |
|
|
Family ID: |
60806006 |
Appl. No.: |
15/639044 |
Filed: |
June 30, 2017 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62356690 |
Jun 30, 2016 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 23/00 20130101;
G21K 1/10 20130101; G01N 2223/316 20130101; A61N 5/1077
20130101 |
International
Class: |
A61N 5/10 20060101
A61N005/10; G01N 23/00 20060101 G01N023/00 |
Goverment Interests
GOVERNMENT RIGHTS
[0002] This invention was made with government support under
5U1EB018758 awarded by the National Institutes of Health. The
government has certain rights in the invention.
Claims
1. A device for beam shaping in imaging comprising: two or more
multiple aperture devices placed in series, wherein each of the
multiple aperture devices have a design and each of the multiple
aperture devices are configured to have motion relative to others
of the multiple aperture devices; and wherein the design and the
motion of the multiple aperture devices is predetermined to
generate a predetermined fluence pattern or the motion of the
multiple aperture devices is determined on-the-fly to generate a
fluence pattern.
2. The device of claim 1 wherein the design and relative motion are
chosen using mathematical models.
3. The device of claim 1 wherein the design and relative motion are
chosen using physical models.
4. The device of claim 1 wherein the design and relative motion are
chosen using mathematical and physical models.
5. The device of claim 1 wherein the predetermined fluence pattern
is based on a single target object.
6. The device of claim 1 wherein the predetermine fluence pattern
is based on a group of target objects.
7. The device of claim 1 wherein the multiple aperture device
comprises bars.
8. The device of claim 7 further comprising design characteristics
taking the form of thickness of each bar.
9. The device of claim 7 further comprising design characteristics
taking the form of position of each bar relative to one
another.
10. The device of claim 7 further comprising design characteristics
taking the form of the frequency of the bars.
11. A method for beam shaping in imaging comprising: placing two or
more multiple aperture devices in series, wherein each of the
multiple aperture devices have a design and each of the multiple
aperture devices are configured to have motion relative to others
of the multiple aperture devices; and generating a fluence
pattern.
12. The method of claim 11 further comprising predetermining the
design and the motion of the multiple aperture devices to generate
a predetermined fluence pattern.
13. The method of claim 11 further comprising determining the
motion of the multiple aperture devices on-the-fly to generate a
fluence pattern.
14. The method of claim 11 further comprising programming the
design and relative motion with one chosen from a group consisting
of using mathematical models, physical models, or a combination of
the two.
15. The method of claim 11 further comprising basing the
predetermined fluence pattern on a single target object.
16. The method of claim 11 further comprising basing predetermine
fluence pattern on a group of target objects.
17. The method of claim 11 further comprising the multiple aperture
device comprising bars.
18. The method of claim 11 further comprising design
characteristics taking the form of thickness of each bar.
19. The method of claim 11 further comprising design
characteristics taking the form of position of each bar relative to
one another.
20. The method of claim 11 further comprising design
characteristics taking the form of the frequency of the bars.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 62/356,690 filed on Jun. 30, 2016, which is
incorporated by reference, herein, in its entirety.
FIELD OF THE INVENTION
[0003] The present invention relates generally to medical imaging.
More particularly, the present invention relates to a method for
optimizing radiation beam intensity profile shape using dual
multiple aperture devices.
BACKGROUND OF THE INVENTION
[0004] X-ray computed tomography has found widespread clinical
utility; however, increasing concerns about the risks associated
with ionizing radiation have driven the search for exposure
reduction strategies. While many algorithmic strategies for
producing better images at lower exposures have been developed,
there has been relatively little research on innovative
hardware-based dose reduction methods. Dose to an individual
patient is naturally tied to the particular exposure settings of a
CT scanner; however, finding minimum dose strategies is both
complex due to the dependence on patient size, anatomical site,
etc. and, currently, somewhat limited due to the relative
inflexibility of modern CT scanners to control the distribution of
x-rays used to image a patient.
[0005] Typical clinical scanners permit coarse control of the x-ray
beam through exposure settings (tube current and voltage), and many
systems have tube current modulation hardware that permits
variation of exposure as a function of rotation angle and table
position. Control of the spatial distribution of the x-ray beam is
typically very limited and is achieved through the introduction of
a bow-tie filter. Some systems allow selection from a small number
(typically three or fewer) bow-tie filters based on patient size.
Typical filters attenuate x-rays at large fan angles to achieve
higher fluence levels in the center of the patient (where the
attenuation is highest) and lower fluence at the edges (where
attenuation is low). Unfortunately, such static beam shaping is
limited and cannot account for variability in the width/size of the
patient as a function of angle and table position. Similarly,
static bow-tie filters can be sensitive to positioning since a
well-centered patient is presumed.
[0006] Fluence-field modulated (FFM) CT is an area of active
research that seeks strategies for dynamic modulation of the
spatial distribution of the x-ray beam. Successful implementation
of FFM-CT increases acquisition flexibility permitting dose
reduction objectives as well as novel data collection strategies
(e.g., region-of-interest scans). A number of different FFM
strategies have been proposed including the use of heavy metal
compounds on paper, digital beam attenuators, piece-wise linear
dynamic bowties, and fluid filled attenuators. Due to the severe
operational requirements within a CT scanner (e.g. limited space,
high rotation speeds, accelerations, etc.), the design of dynamic
FFM-CT is a challenge.
[0007] Accordingly, there is a need in the art for a method for a
method for optimizing radiation beam intensity profile shape using
dial multiple aperture devices.
SUMMARY OF THE INVENTION
[0008] The foregoing needs are met, to a great extent, by the
present invention which provides a method for a device for beam
shaping in imaging having two or more multiple aperture devices
placed in series. Each of the multiple aperture devices have a
design and each of the multiple aperture devices are configured to
have motion relative to others of the multiple aperture devices.
The design and the motion of the multiple aperture devices is
predetermined to generate a predetermined fluence pattern.
[0009] In accordance with an aspect of the present invention, the
design and relative motion are chosen using mathematical models.
The design and relative motion can also be chosen using physical
models. Alternately, the design and relative motion are chosen
using mathematical and physical models. Alternately, the motion of
the multiple aperture devices is chosen according to real time data
from the imaging scanner. The predetermined fluence pattern is
based on a single target object or a group of target objects. The
multiple aperture device includes bars. Design characteristics take
the form of thickness of each bar, position of each bar relative to
one another, and the frequency of the bars.
[0010] In accordance with another aspect of the present invention,
a method for beam shaping in imaging includes placing two or more
multiple aperture devices in series. Each of the multiple aperture
devices has a design and each of the multiple aperture devices are
configured to have motion relative to others of the multiple
aperture devices. The method also includes generating a fluence
pattern.
[0011] In accordance with yet another aspect of the present
invention, the method includes predetermining the design and the
motion of the multiple aperture devices to generate a predetermined
fluence pattern. The method further includes determining the motion
of the multiple aperture devices on-the-fly to generate a fluence
pattern. The method includes programming the design and relative
motion with one chosen from a group of using mathematical models,
physical models, or a combination of the two. The method includes
basing the predetermined fluence pattern on a single target object.
Alternately, the predetermine fluence pattern can be based on a
group of target objects. The multiple aperture device can include
bars. Design characteristics can take the form of thickness of each
bar. Design characteristics take the form of position of each bar
relative to one another. Design characteristics can also take the
form of the frequency of the bars.
BRIEF DESCRIPTION OF THE DRAWING
[0012] The accompanying drawings provide visual representations,
which will be used to more fully describe the representative
embodiments disclosed herein and can be used by those skilled in
the art to better understand them and their inherent advantages. In
these drawings, like reference numerals identify corresponding
elements and:
[0013] FIG. 1 illustrates a schematic diagram of fluence modulation
using Dual MAD filters.
[0014] FIG. 2 illustrates a graphical view of simulated CT geometry
and phantom.
[0015] FIG. 3 illustrates a graphical view of parameterization of
the MAD design.
[0016] FIG. 4 illustrates graphical views of normalized target
fluence patterns at the MAD plane to flatten the fluence through
the phantom at the detector plane, and achievable fluence patterns
using the dual MAD setup.
[0017] FIGS. 5A-5D illustrate graphical views of solutions to the
dual MAD design optimization.
[0018] FIG. 6 illustrates a graphical view of post-filtering
fluence profiles at the MAD plane with a full range of control
actuation (displacements of MAD1 with respect to MAD0).
[0019] FIG. 7 illustrates a graphical view of tube current
modulation with and without MAD filters.
[0020] FIGS. 8A and 8B illustrate graphical views of the fluence
profiles received at the detector with no phantom scanner.
[0021] FIGS. 9A and 9B illustrate graphical views of the projection
data received at the detector with the phantom in the scanner.
[0022] FIG. 10A illustrates an image view of filtered
backprojection of a phantom with no MAD filters and dual MAD
filters. FIG. 10B illustrates noise images with no MAD filter and
Dual MAD filter.
DETAILED DESCRIPTION
[0023] The presently disclosed subject matter now will be described
more fully hereinafter with reference to the accompanying Drawings,
in which some, but not all embodiments of the inventions are shown.
Like numbers refer to like elements throughout. The presently
disclosed subject matter may be embodied in many different forms
and should not be construed as limited to the embodiments set forth
herein; rather, these embodiments are provided so that this
disclosure will satisfy applicable legal requirements. Indeed, many
modifications and other embodiments of the presently disclosed
subject matter set forth herein will come to mind to one skilled in
the art to which the presently disclosed subject matter pertains
having the benefit of the teachings presented in the foregoing
descriptions and the associated Drawings. Therefore, it is to be
understood that the presently disclosed subject matter is not to be
limited to the specific embodiments disclosed and that
modifications and other embodiments are intended to be included
within the scope of the appended claims.
[0024] The present invention is directed to multiple aperture
devices (MADs) for beam shaping in x-ray imaging. Two or more of
these binary filters can be placed in an x-ray beam in series to
permit a large number of x-ray fluence profiles. However, the
relationship between particular MAD designs and the achievable
fluence patterns is complex. The present invention includes
mathematical and physical models that are used within an
optimization framework to find optimal MAD designs. Specifically,
given a set of target fluence patterns, the present invention
finds, for example, a dual MAD design that is a "best fit" in
generating the desired fluence patterns. This process provides a
solution for both the design of MAD filters as well as the control
actuation that is required (relative motion between MADs) that
needs to be specified as part of the operation of a MAD-based
fluence field modulation system.
[0025] The conceptual operation of a MAD filter is illustrated in
FIG. 1. FIG. 1 illustrates a schematic diagram of fluence
modulation using Dual MAD filters. The device comprises thin bars
of a highly attenuating material (e.g. tungsten) of varying widths
and spacing. On a fine scale the MAD acts as a binary filter,
either completely blocking or passing the X-ray beam using
alternating bars and slots. In this fashion, one can concentrate
the amount of x-rays spatially by varying the thickness of the
blockers locally. The pitch (spacing between blockers) of the MAD
device may be designed to minimize high-frequency patterns at the
detector. For example, if the focal spot of the x-ray source is
assumed to be a rectangle, the MAD pitch may be placed at the first
null frequency associated with the focal spot blur MTF. In this
fashion, the fine bar pattern of the MAD device is blurred out and
is not visible at the detector. Desirable (lower frequency) spatial
modulation associated with the variable bar width is still
achievable.
[0026] A single fluence pattern can be obtained with a single MAD
device. With multiple MADs in series, capable of moving with
respect to each other, a range of fluence patterns can be obtained
since it is the composition of two binary filters. Moreover, small
relative displacement of the MADs with respect to each other can
induce large changes in the fluence pattern. Because small
actuations have a large effect on the x-ray distribution, speed and
acceleration requirements can be reduced for device. Similarly,
because these filters do not rely on variable attenuation using a
low atomic number material, the filters can be made very thin and
compact.
[0027] Fluence optimization for a single target object is discussed
as an example, herein; however, the approach may be extended to
classes of objects. Specifically, the known object in the
simulation study was chosen to be an anthropomorphic phantom body
of uniform material (acrylic), as illustrated in FIG. 2. FIG. 2
illustrates a graphical view of simulated CT geometry and phantom.
This digital phantom emulates commercially available physical
phantoms (QRM GmbH, Morehendorf, Germany) that will be used in
subsequent presentation of the invention.
[0028] The system geometry was chosen to emulate a CT scanner's
source-to-detector distance and also geometry achievable in a
flat-panel-based experimental test bench that is available for
subsequent experiments. The exemplary implementation of the present
invention includes 360 degree rotation, in steps of 0.5 degree. To
create projection data for MAD design and analysis, a polyenergetic
forward model and Spektr, a computational tool for x-ray spectral
analysis, corresponding to a tube voltage of 100 kVp with
additional filtration (2 mm of Al, 0.2 mm of Cu) were used. The
model also includes fluence adjustments to accommodate divergent
beam effects.
[0029] In order to design a set of MAD filters, the location and
dimensions of many MAD attributes must be specified. The elements
of a dual MAD design are identified in the illustration in FIG. 3.
FIG. 3 illustrates a graphical view of parameterization of the MAD
design. Specifically, the free design parameters include: 1)
b.sub.0(x), the thickness of each bar as a function of position in
MAD0 that locally blocks X-rays; 2) b.sub.1(x), the analogous bar
function for MAD1; 3) (x), a local offset function that specifies
the position of individual bars in MAD1 relative to MAD0; and 4)
the MAD pitch (e.g., the spacing interval between bars). The MAD
pitch may be designed independently of other parameters based on
the first null frequency of the focal spot, magnified to the MAD
plane. For a rectangular focal spot size, f.sub.s, the optimal MAD
pitch is
m = f s .times. ( 1 - SMD SDD ) ##EQU00001##
For nonrectangular focal spots, one can similarly find a null or
minimal pass frequency to enforce smooth fluence profiles.
Additionally, even though FIG. 3 shows MAD0 and MAD1 to be parallel
with identical pitch, each of the flat MADs have a slightly
different pitch and the bars/slots must be focused to the source
due to the diverging x-ray beam.
[0030] The last parameter that is important for design is the
control parameter .DELTA., which denotes the relative offset
between MAD0 and MAD1. This is the one-dimensional actuation that
controls the fluence profile enforced by the MAD filters. In
general, this parameter must be part of the design process as well,
and is a function of the CT rotation angle and/or table position,
which is denoted as .DELTA.(.theta.).
[0031] With MAD pitch specified, the remaining parameters:
b.sub.0(x), b.sub.1(x), and .DELTA.(.theta.) are sought. These
values can be determined analytically using an "endpoint" design to
match two desired profiles by considering the minimum and maximum
blocking conditions of a dual MAD system. While this approach is
attractive due to its closed-form solution, it fails to provide
best fit solutions for a wide range of desired fluence patterns.
Instead a nonlinear, nonconvex optimization is proposed herein as a
part of the present invention.
[0032] To facilitate optimization, the dual MAD design is further
parameterized using a low-dimensional set of basis functions. For
example, rather than have a parameter for every bar width in MAD0,
it is assumed that neighboring bar widths vary smoothly as a
function of position. Specifically, the parameters are represented
with a small set of Fourier coefficients, c.sub.p(.omega.) such
that
p(x)=m/(1+e.sup.-{circumflex over (p)}(x)){circumflex over
(p)}(x).sup.-1[c.sub.p(.omega.)]
where p(x) is one of {b.sub.0(x), b.sub.1(x), .delta.(x), and
.DELTA.(.theta.)}. Thus, the optimization will focus on finding the
optimal coefficients: c.sub.b1(.omega.), c.sub.b2(.omega.),
c.sub..delta.(.omega.), and c.sub..DELTA.(.omega.) which are
functions of the spatial (or, for D, angular) frequencies selected
for the basis set.
[0033] To define the optimization objective, a model of the fluence
output is constructed, which is a function of the design and
actuation values and can be written in terms of the original
parameters or vectors of low-dimensional Fourier coefficients:
M(b.sub.0(x),b.sub.1(x),.delta.(x);.DELTA.(.theta.))M(x,.theta.;c.sub.b1-
,c.sub.b2,c.sub..delta.,c.sub..DELTA.)
Note that M is a function of spatial location (e.g., a fluence
profile) as well as rotation angle.
[0034] Using this model, the following optimization:
{ c ^ b 1 , c ^ b 2 , c ^ .delta. , c ^ .DELTA. } = arg min .theta.
x .di-elect cons. P t ( .theta. , x ) t 0 ( .theta. ) - M ( x ,
.theta. ; c b 1 , c b 2 , c .delta. , c .DELTA. ) M 0 ( .theta. ) 2
##EQU00002##
where t(.theta.,x) denotes desired fluence patterns as a function
of rotation angle. The objective is computed as the mean squared
error between the desired and modeled fluence patterns over all
projections that intersect the phantom (or patient). As such,
x-rays passing outside the phantom (e.g. not contributing to dose)
will be ignored in the optimization process. Also note that both
the modeled and desired fluence patterns are normalized by
M.sub.0(.theta.)=.SIGMA..sub.xM(x,.theta.;.) and
t.sub.0(.theta.)=.SIGMA..sub.xt(.theta.,x)
respectively. This normalization concentrates the design process on
achieving the proper fluence shape. The magnitude of the profile
can be adjusted post-design through exposure settings and tube
current modulation. While there are many potential desired fluence
patterns that one might seek including those that enforce minimum
peak variance, combined noise and dose objectives, or maximize
task-based detectability, fluence patterns that flatten the signal
and homogenize noise in projection data are the focus to show the
utility of the present invention. Covariance Matrix Adaptation
Evolution Strategy (CMA-ES) was chosen for the optimization. CMA-ES
requires no derivative computations and is well-suited to nonlinear
nonconvex optimization since a population of solutions is employed
to avoid local optima. The objective function was implemented and
the profile modeling function in efficient C++ code including
parallelized computation of objective function values (over the
population) using OpenMP. The CMA-ES algorithm was initialized to
the output of the end-point design process. A population size of 16
was selected and the stopping criteria for optimization was to stop
if successive function evaluations differ less than 10.sup.-12 or
10,000 iterations was computed. All the MAD design parameters were
constrained to lie between 0 and a single pitch, m, to avoid
nonphysical and periodic solutions (e.g. beyond a single cycle of
actuation).
[0035] To generate desired fluence patterns, the phantom was
rotated 360 degrees in steps of 0.5 degrees. The fluence is
simulated at the detector plane, and the fluence profile at the MAD
plane required to flatten this fluence is computed using the
methodology of Section IIB. The MAD parameterization used 8 Fourier
coefficients for each MAD feature (4 total), and only symmetric
basis functions were employed to enforce symmetric MAD designs. A
subset of these target fluence profiles are shown in FIG. 4. FIG. 4
illustrates graphical views of normalized target fluence patterns
at the MAD plane to flatten the fluence through the phantom at the
detector plane, and achievable fluence patterns using the dual MAD
setup. The fluence required is normalized such that the sum is
unity. The fluence obtainable with the dual MADs using the CMA-ES
optimization is also plotted.
[0036] The designed fluence profiles very closely match the desired
fluence pattern suggesting that a dual MAD system can match a range
of fluence profiles and, in this case, substantially flatten the
fluence profiles at the detector for this phantom. For the fluence
profiles with a flat top, the achievable CMA-ES profiles show
fluctuation on the flat edge. The narrow fluence profiles show
slight misalignment error, which is potentially correctable by
shifting both MADs together. Such analysis is the topic of ongoing
investigations and will likely be important for asymmetric beam
profiles (e.g., for miscentered patients, off axis targets,
etc.).
[0037] The optimized MAD design parameters are shown in FIGS.
5A-5D. FIGS. 5A-5D illustrate graphical views of solutions to the
dual MAD design optimization. FIG. 5A illustrates a bar width
function, b.sub.0(x) for MAD0; FIG. 5B illustrates a barwidth
function b.sub.1(x) for MAD1; FIG. 5C illustrates a local offset
function .delta.(x); and FIG. 5D illustrates the actuation control,
.DELTA.(.theta.), as a function of rotation angle. Note, because
the bar widths are directly proportional to the amount of local
blockage and consequently inversely proportional to the local
fluence, the effect of the MAD0 filter alone is not unlike a
traditional bowtie (e.g. more fluence in the center of the field
and less at the edges). The MAD1 design is almost the opposite
(when acting alone). The bar widths in both MADs span the range of
approximately 50 .mu.m to 800 .mu.m. Such designs are largely
within the constraints of modern tungsten sintering technology,
though features<100 .mu.m can present some challenges (such
constraints can potentially be integrated into the design process).
The local offset function, .delta.(x), is predominantly negative,
meaning that the MAD1 bars are located to the left of the center
position in each MAD period.
[0038] The actuation control shown in FIG. 5D illustrates that MAD1
is displaced between 0.15 mm to 0.4 mm as the projection angle
changes from 0 to 360 degrees. This minimal movement of the MADs
causes the large change in the fluence patterns seen in FIG. 4 and
can be attributed to the relatively small MAD pitch. From an
implementation standpoint, the potential mechanical advantage is
the fast switching speed of the MAD fluence profiles as the CT
gantry spins around the patient. The smooth profile of the
displacement also reduces the acceleration requirements on the
actuator. Though not done here, one could integrate specific
acceleration limits as part of the optimization.
[0039] It is interesting to note that the design of the previous
sections only utilizes part of the actuation control range. FIG. 6
shows the full range of fluence patterns achievable as the second
MAD is moved with respect to the first MAD within a single MAD
pitch (e.g. one cycle). Recall, that for the selected phantom, only
fluence profiles between MAD1 displacements of 0.1 to 0.4 were
used. FIG. 6 illustrates a graphical view of post-filtering fluence
profiles at the MAD plane with a full range of control actuation
(displacements of MAD1 with respect to MAD0).
[0040] However, from the fluence map, it is clear that much sharper
fluence patterns can be obtained by changing the displacement to
0.7 mm. This potentially enables other applications such as
region-of-interest fluence modulation and suggests additional
design flexibility for larger classes of profiles (e.g. more
complex objects, multiple classes, etc.).
[0041] Although a variety of fluence patterns have been
demonstrated, practical application and fitting to the desired
fluence profiles requires proper scaling. This scaling can be
achieved through tube current modulation (TCM). Typical Automatic
Exposure Control (AEC) seeks to provide a constant fluence at the
center of the detector. This strategy was applied for the no filter
scenario. For the MAD scenario, the same strategy of providing
constant fluence at the central detector pixel was applied, through
the Dual MAD and phantom. For comparison between the no filter and
MAD filtered scenarios, the total fluence (i.e., the number of
simulated photons) incident on the phantom is constant for the two
approaches. Specifically, TCM is scaled to enforce a total of
100,000 photons incident on the phantom.
[0042] FIG. 7 shows the TCM required to convert the fluence
generated by the MADs to the required target fluence. FIG. 7
illustrates a graphical view of tube current modulation with and
without MAD filters. The solid line is without MAD and the dashed
line is with MAD. Without the MAD filter, the TCM is largest when
the path length of Xrays through the phantom is largest. The dual
MAD filter has maximum attenuation when the fluence profile is
narrow. Therefore, more photons are required at 0 and 180 degrees
to flatten the fluence with MAD than at 90 or 270 degrees. The MAD
requires higher scaling and modulation to generate the same number
of photons incident on the phantom.
[0043] FIGS. 8A and 8B and 9A and 9B show the fluence profiles with
and without the phantom in the field of view for the no filter and
MAD filtered scenarios (TCM is used in both cases). In FIGS. 8A and
8B, the no filter scenario can only modulate the per view number of
photons through TCM while the dual MAD filter can customize both
the shape and intensity of the beam. In FIGS. 9A and 9B, the
post-object fluence is more uniform across object projections (the
design goal) than the no filter, TCM-only scenario. FIGS. 8A and 8B
illustrate graphical views of the fluence profiles received at the
detector with no phantom scanner. FIG. 8A illustrates no filters
and AEC, and FIG. 8B illustrates using optimally actuated and
designed dual MAD filters and AEC. The space occupied by the
phantom is shown by a dotted line. Units are in photons. FIGS. 9A
and 9B illustrate graphical views of the projection data received
at the detector with the phantom in the scanner. FIG. 9A
illustrates using no filters and AEC, and FIG. 89B illustrates
using optimally actuated and designed dual MAD filters and AEC. The
space occupied by the phantom is shown by a dotted line. Units are
in photons.
[0044] With Poisson noise added to the projection data in FIGS. 9A
and 9B, filtered backprojection reconstructions were performed for
both filtering scenarios. Results are shown in FIGS. 10A and 10B.
FIG. 10A illustrates an image view of filtered backprojection of a
phantom with no MAD filters and dual MAD filters. FIG. 10B
illustrates noise images with no MAD filter and Dual MAD filter.
Both methods show approximately the same average noise level (as
expected due to an equal number of incident photons). However, much
greater noise uniformity exists in the MAD filtered image. This is
significant if a minimum noise level is prescribed to obtain
sufficient image quality. The TCM-only case will require more
incident photons (hence larger dose) to obtain the same minimum
noise level over the entire image.
[0045] While these initial results suggest that dual MAD filters
can successfully achieve a broad class of fluence patterns, this
invention can possibly be extended to larger classes of fluence
patterns (e.g. different size patients). Similarly, parallel
efforts are working to fabricate physical MAD devices and evaluate
performance in an experimental CT system.
[0046] The present invention can be carried out and/or supported
using a computer, non-transitory computer readable medium, or
alternately a computing device or non-transitory computer readable
medium incorporated into the imaging device. Indeed, any suitable
method of calculation known to or conceivable by one of skill in
the art could be used. It should also be noted that while specific
equations are detailed herein, variations on these equations can
also be derived, and this application includes any such equation
known to or conceivable by one of skill in the art.
[0047] A non-transitory computer readable medium is understood to
mean any article of manufacture that can be read by a computer.
Such non-transitory computer readable media includes, but is not
limited to, magnetic media, such as a floppy disk, flexible disk,
hard disk, reel-to-reel tape, cartridge tape, cassette tape or
cards, optical media such as CD-ROM, writable compact disc,
magneto-optical media in disc, tape or card form, and paper media,
such as punched cards and paper tape.
[0048] The computing device can be a special computer designed
specifically for this purpose. The computing device can be unique
to the present invention and designed specifically to carry out the
method of the present invention. Imaging devices generally have a
console which is a proprietary master control center of the imager
designed specifically to carry out the operations of the imager and
receive the imaging data created by the imager. Typically, this
console is made up of a specialized computer, custom keyboard, and
multiple monitors. There can be two different types of control
consoles, one used by the operator and the other used by the
physician. The operator's console controls such variables as the
thickness of the image, the amount of tube current/voltage,
mechanical movement of the patient table and other radiographic
technique factors. The physician's viewing console allows viewing
of the images without interfering with the normal imager operation.
This console is capable of rudimentary image analysis. The
operating console computer is a non-generic computer specifically
designed by the imager manufacturer for bilateral (input output)
communication with the scanner. It is not a standard business or
personal computer that can be purchased at a local store.
Additionally this console computer carries out communications with
the imager through the execution of proprietary custom built
software that is designed and written by the imager manufacturer
for the computer hardware to specifically operate the hardware.
[0049] The many features and advantages of the invention are
apparent from the detailed specification, and thus, it is intended
by the appended claims to cover all such features and advantages of
the invention which fall within the true spirit and scope of the
invention. Further, since numerous modifications and variations
will readily occur to those skilled in the art, it is not desired
to limit the invention to the exact construction and operation
illustrated and described, and accordingly, all suitable
modifications and equivalents may be resorted to, falling within
the scope of the invention. While exemplary embodiments are
provided herein, these examples are not meant to be considered
limiting. The examples are provided merely as a way to illustrate
the present invention. Any suitable implementation of the present
invention known to or conceivable by one of skill in the art could
also be used.
* * * * *