U.S. patent application number 11/867998 was filed with the patent office on 2008-05-22 for optimized aperture selection imaging computed tomography system and method.
Invention is credited to Sean A. Graham, David A. Jaffray, Jeffrey H. Siewerdsen.
Application Number | 20080118032 11/867998 |
Document ID | / |
Family ID | 39416941 |
Filed Date | 2008-05-22 |
United States Patent
Application |
20080118032 |
Kind Code |
A1 |
Graham; Sean A. ; et
al. |
May 22, 2008 |
OPTIMIZED APERTURE SELECTION IMAGING COMPUTED TOMOGRAPHY SYSTEM AND
METHOD
Abstract
The invention provides a method and imaging system for operating
imaging computed tomography using a radiation source and a
plurality of detectors to generate an image of an object. The
method includes: defining a desired image characteristics; and
performing calculations to determine the pattern of fluence to be
applied by the radiation source, to generate said desired image
quality or characteristics. Then, the radiation source is
modulated, to generate the intended pattern of fluence between the
beam source and the object to be imaged. The desired image
characteristics can comprise at least one of desired levels of
contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR), and
may provide at least one of: desired image quality in at least one
defined region of interest; and at least one desired distribution
of said image quality.
Inventors: |
Graham; Sean A.; (Etobicoke,
CA) ; Jaffray; David A.; (Etobicoke, CA) ;
Siewerdsen; Jeffrey H.; (Toronto, CA) |
Correspondence
Address: |
BERESKIN AND PARR
40 KING STREET WEST, BOX 401
TORONTO
ON
M5H 3Y2
omitted
|
Family ID: |
39416941 |
Appl. No.: |
11/867998 |
Filed: |
October 5, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60828481 |
Oct 6, 2006 |
|
|
|
Current U.S.
Class: |
378/152 ;
378/145; 378/153 |
Current CPC
Class: |
G21K 1/04 20130101; G21K
1/046 20130101 |
Class at
Publication: |
378/152 ;
378/145; 378/153 |
International
Class: |
G21K 1/04 20060101
G21K001/04 |
Claims
1. A method for operating imaging computed tomography using a
radiation source and a plurality of detectors to generate an image
of an object, the method comprising the steps of: a) defining
desired image characteristics; b) performing calculations to
determine the pattern of fluence to be applied by the radiation
source, to generate said desired image characteristics; and c)
modulating the radiation source to generate said pattern of fluence
between the beam source and the object to be imaged.
2. The method of claim 1, wherein said desired image
characteristics comprise at least one of desired levels of
contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR).
3. The method of claim 2, wherein said desired image
characteristics providing at least one of: desired image quality in
at least one defined region of interest; and at least one desired
distribution of said image quality.
4. The method of claim 1 wherein performing said calculations
comprises solving an inverse problem using an iterative
solution.
5. The method of claim 4, wherein said calculations being performed
include considering at least one of: i) The dependence of the image
quality on primary fluence transiting through the object. ii) The
dependence on scatter fluence to the detector. iii) The dependence
upon scattered dose to the object and its dependence on
.phi.(.theta.,u,v). iv) The exposure dependent DQE of the detector
DQE(.phi.(.theta.,u,v)).
6. The method of claim 3, including constraining the calculations
by a dose distribution constraint.
7. A method as claimed in claim 4 including defining a region of
interest from a library of population modules or at least one
previously acquired image of the object. [alternatively a pre-scan
of the image can be conducted]
8. The method of claim 7 comprising performing said calculations to
determine the pattern of fluence by solving the inverse problem
according to the equation: m(u,v)I(u,v)=G.sup.-1[C({right arrow
over (r)})] [7] where C({right arrow over (r)}) is and image metric
defining the desired image characteristic, I (u, v) represents the
intensity of the radiation applied to the object from the source
and m(u, v) represents the modulation of the radiation by the
object, and where G.sup.-1 is an operator which relates the image
metric C({right arrow over (r)}) to the applied radiation
intensities, and wherein the method further comprises iteratively
solving the equation: min{.parallel.C({right arrow over
(r)})-C.sub.i({right arrow over (r)}).parallel.} where, for each
step i, the image metric C.sub.i({right arrow over (r)}) is
calculated and compared to the desired quantity C({right arrow over
(r)}).
9. The method of claim 8, including lower and upper bounds on the
image metric, whereby in the reconstructed image: C({right arrow
over (r)}).ltoreq.{circumflex over (f)}({right arrow over
(r)}).ltoreq. C({right arrow over (r)}) [8] where C({right arrow
over (r)}) and C({right arrow over (r)}) are lower and upper bounds
respectively of the desired of C({right arrow over (r)}) at each
point {right arrow over (r)}.
10. The method of claim 9, including optimizing image
characteristics and dose iteratively according to:
min{.parallel.C({right arrow over (r)})-C.sub.i({right arrow over
(r)}).parallel.+w.parallel.D({right arrow over (r)})-D.sub.i({right
arrow over (r)}).parallel.} wherein .parallel.(C({right arrow over
(r)})-C.sub.i({right arrow over (r)}).parallel. represents optimal
image quality, .parallel.D({right arrow over (r)})-D.sub.i({right
arrow over (r)}).parallel. represents optional patient dose, and w
is weighting given to the dose.
11. The method of claim 10, including providing different weighting
to the image characteristics and the patient dose.
12. The method of claim 11, including weighting at least one of the
image quality and the patient dose across individual voxels
according to: min{.parallel.W.sub.C({right arrow over
(r)})(C({right arrow over (r)})-C.sub.i({right arrow over
(r)})).parallel.+w.parallel.W.sub.D({right arrow over
(r)})(D({right arrow over (r)})-D.sub.i({right arrow over
(r)})).parallel.}
13. The method of claim 1, including providing temporal modulation
of the radiation source.
14. The method of claim 1, including providing spatial modulation
of the radiation source.
15. The method of claim 1, including both spatial and temporal
modulation of the radiation source.
16. The method of claim 14 or 15, comprising providing a temporal
modulator comprising a plurality of individual elements adapted to
absorb radiation, and moving these elements to provide desired
temporal modulation
17. An imaging system, the system comprising: a) a radiation source
for directing a beam at an object to be imaged; b) a modulator
placed between said beam source and the object to be imaged; and c)
a computer for performing calculations based on the desired
distribution of image quality to determine the pattern of fluence
to be applied by said temporal modulator.
18. An imaging system as claimed in claim 17, wherein the modulator
comprises at least one of a spatial modulator and a temporal
modulator.
19. An imaging system as claimed in claim 17, wherein the modulator
comprises a plurality of individual elements, each being
substantially impervious to the radiation and being movable between
an open position and a closed position, whereby open positions of
the individual elements define an aperture permitting passage of
the beam from the radiation source.
20. An imaging system as claimed in claim 19, wherein the
compensator is configured as a louvre compensator comprising a
first set of substantially parallel louvres extending in one
direction and a second set of parallel louvres extending in another
direction, whereby by selected positioning of at least one louvre
of the first set in the open position and at least one louvre of
the second set in the open position an aperture is defined for
passage of the beam of radiation.
21. An imaging system as claimed in claim 17, wherein the
compensator comprises a plurality of pairs of elements, the
elements in each pair being elongate and extending end to end, and
the pairs of elements being arranged in a side by side abutting
relationship, wherein, with all the pairs of elements arranged with
the ends of the elements abutting one another, the compensator is
closed, and with at least one pair of elements open so that the
ends of the elements of that pair are spaced apart, an aperture is
defined for the beam of radiation.
Description
FIELD
[0001] This invention relates generally to the field of computed
tomography (CT) and more particularly to an optimized aperture
selection imaging CT (OASCT) system and method utilizing
compensating filters to modulate the fluence pattern applied during
image acquisition for specific distributions of dose and image
noise.
BACKGROUND
[0002] The following paragraphs are not an admission that anything
discussed in them is prior art or part of the knowledge of persons
skilled in the art.
[0003] Current imaging practice attempts to acquire high image
quality throughout a scanned volume, though some focus is now being
directed at more patient specific methods of imaging. It is
recognized that many imaging tasks only require elevated image
quality in smaller volumes while low image quality would be
sufficient throughout the remainder of the imaged volume. The
development of techniques to perform region of interest (ROI)
imaging (see R. Chityala, K. R. Hoffmann, S. Rudin, and D. R.
Bednarek, "Region of interest (ROI) computed tomography (CT):
Comparison with full field of view (FFOV) and truncated CT for a
human head phantom," Proc. SPIE Physics of Medical Imaging 5745,
583-590 (2005) and C. J. Moore, T. E. Marchant, and A. M. Amer,
"Cone beam CT with zonal filters for simultaneous dose reduction,
improved target contrast and automated set-up in radiotherapy,"
Phys Med Biol 51, 2191-2204 (2006)) are a step towards acquiring
images that provide varying image quality through the reconstructed
volume. However, there remains a need for further improvements to
be made by having the ability to optimally modulate the x-ray
fluence patterns applied during imaging in a more patient specific
fashion.
[0004] Many technologies have been developed for the purpose of
improving external beam radiation therapy by imaging patients in
the treatment position. These systems, which include CT imagers
placed on rails in the treatment room (see K. Kuriyama, H. Onishi,
N. Sano, et al. "A new irradiation unit constructed of self-moving
gantry-CT and linac," Int J Radiat Oncol Biol Phys 55, 428-35
(2003)), Tomotherapy (see T. R. Mackie, T. Holmes, S. Swerdloff, et
al. "Tomotherapy: a new concept for the delivery of dynamic
conformal radiotherapy." Med Phys 20, 1709-19 (1993)), and imaging
CT systems mounted on the gantries of conventional linear
accelerators have the potential to improve radiation therapy
targeting. One example of a CT imaging system is cone-beam CT (see
D. A. Jaffray, J. H. Siewerdsen, J. W. Wong, and A. A. Martinez,
"Flat-panel cone-beam computed tomography for image-guided
radiation therapy," Int J Radiat Oncol Biol Phys 53, 1337-1349
(2002)) and another example is scanning-beam CT (see E. G. Solomon,
B. P. Wilfley, M. S. Van Lysel, A. W. Joseph, and J. A. Heanue,
"Scanning-beam digital x-ray (SBDX) system for cardiac
angiography," in Medical Imaging 1999: Physics of Medical Imaging
(SPIE, New York, 1999), Vol. 3659, pp. 246-257; T. G. Schmidt, J
Star-Lack, N. R. Bennett, S. R. Mazin, E. G. Solomon, R Fahrig, N.
J, Pelc, "A prototype table-top inverse-geometry volumetric CT
system." Medical Physics, June 2006 33(6), pp. 1867-78). With this
improvement comes the possibility of reducing planned treatment
volumes (PTVs), increasing the sparing of normal tissues and
increasing the dose to tumors. The contents of all these
publications are hereby incorporated by reference.
[0005] Also, a large quantity of work has been accomplished to
improve the ability of systems designed for image guided radiation
therapy to improve patient outcome. For the case of cone-beam CT,
there is a large interest in developing flat-panel detectors with
improved performance (dynamic range, spatial resolution) and
removing the effects of scattered x-rays reaching the detector. It
has now been shown that implementing compensating filters into
imaging CT systems has the potential to play a large role in
reducing scatter that reaches the detector, as well as scatter
within the patient delivering unnecessary patient dose.
[0006] Accordingly, there is a need for an imaging system to
optimize image quality in the most clinically relevant regions of
an image, while reducing dose to the patient by reducing the
fluence intensity outside defined regions of interests.
INTRODUCTION
[0007] The following introduction is intended to introduce the
reader to this specification but not to define any invention. One
or more inventions may reside in a combination or sub-combination
of the apparatus elements or method steps described below or in
other parts of this document. The inventor does not waive or
disclaim his rights to any invention or inventions disclosed in
this specification merely by not describing such other invention or
inventions in the claims.
[0008] In accordance with a first aspect of the present invention,
there is provided a method for operating imaging computed
tomography using a radiation source and a plurality of detectors to
generate an image of an object, the method comprising the steps
of:
[0009] (a) defining desired image characteristics;
[0010] (b) performing calculations to determine the pattern of
fluence to be applied by the radiation source, to generate said
desired image characteristics; and
[0011] (c) modulating the radiation source to generate said pattern
of fluence between the beam source and the object to be imaged.
[0012] A second embodiment of the present invention provides an
imaging system, the system comprising:
[0013] (a) a radiation source for directing a beam at an object to
be imaged;
[0014] (b) a modulator placed between said beam source and the
object to be imaged; and
[0015] (c) a computer for performing calculations based on the
desired distribution of image quality to determine the pattern of
fluence to be applied by said temporal modulator.
[0016] The present invention can be applied to any suitable object.
It is expected that it will be commonly used to examine a human or
animal body.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] For a better understanding of the present invention, and to
show more clearly how it may be carried into effect, reference will
now be made, by way of example, to the accompanying drawings which
show some examples of the present invention, and in which:
[0018] FIG. 1 is a block diagram of an example implementation of
the imaging CT system of the present invention;
[0019] FIG. 2 is an illustrative block diagram of the imaging
geometry being imaged by the imaging CT system of FIG. 1; and
[0020] FIG. 3 is a flow chart illustrating the general process
steps for optimal modulation determination;
[0021] FIG. 4 shows a mathematical phantom used to model the
present invention;
[0022] FIGS. 5a and 5b show two desired SNR images;
[0023] FIG. 6 shows a graph of a cost function;
[0024] FIG. 7 shows a modulation function as a function of gantry
angle and position;
[0025] FIGS. 8a, 8b, 8c and 8d show, respectively, theoretical SNR
with no modulation, SNR after optimization with uniform W.sub.SNR,
image acquired with no modulation, and image acquired using
modulation pattern;
[0026] FIGS. 9a and 9b show, respectively, the SNR distribution and
the image acquired with W.sub.SNR tripled in regions of higher
SNR;
[0027] FIGS. 9c and 9b show, respectively, the SNR distribution and
the image acquired with W.sub.SNR tripled in regions of higher SNR,
using the SNR from FIG. 5b;
[0028] FIG. 10a shows a first embodiment of temporal compensation
scheme, comprising a louvre compensator; and
[0029] FIG. 10b shows the louvre compensation of FIG. 10a in a
partial open position;
[0030] FIG. 10c shows the louvre compensation of FIGS. 10a, b in
use;
[0031] FIG. 11a shows an example of another temporal compensation
scheme, comprising a multi-leaf compensator; and
[0032] FIG. 11b shoes the multi-leaf compensation of FIG. 11a in
use.
[0033] It will be appreciated that for simplicity and clarity of
illustration, elements shown in the figures have not necessarily
been drawn to scale. For example, the dimensions of some of the
elements may be exaggerated relative to other elements for clarity.
Further, where considered appropriate, reference numerals may be
repeated among the figures to indicate corresponding or analogous
elements.
DETAILED DESCRIPTION
[0034] Various apparatuses or methods will be described below to
provide an example of an embodiment of each claimed invention. No
embodiment described below limits any claimed invention and any
claimed invention may cover apparatuses or methods that are not
described below. The claimed inventions are not limited to
apparatuses or methods having all of the features of any one
apparatus or method described below or to features common to
multiple or all of the apparatuses described below. It is possible
that an apparatus or method described below is not an embodiment of
any claimed invention. The applicants, inventors and owners reserve
all rights in any invention disclosed in an apparatus or method
described below that is not claimed in this document and do not
abandon, disclaim or dedicate to the public any such invention by
its disclosure in this document.
[0035] The present invention has the potential to decrease dose to
patients by concentrating image quality on desired regions of
interest (ROIs) or distributions of image quality. An iterative
optimization process is utilized to design patterns of modulation
to be applied during imaging to acquire images as near as possible
to those desired. This optimizing process can account for numerous
parameters of the imaging CT system, including the efficiency of
the detector, the presence of x-ray scatter reaching the detector,
and the constraints of the modulator used to form the intensity
modulated fluence patterns.
[0036] Reference is first made to FIG. 1, which illustrates an
imaging CT system 10 made in accordance with a preferred embodiment
of the present invention. Imaging CT system 10 can be any method of
CT imaging, such as a cone-beam CT system or a scanning-beam CT
system. It can also be an inverse-geometry volumetric system, as
disclosed in the paper by T. G. Schmidt et al. noted above. Note
that configurations of the present invention are not limited to
x-ray sources or x-ray radiation and are applicable to other
imaging systems, although the configuration of CT imaging systems,
utilize an x-ray source and x-ray radiation.
[0037] Imaging CT system 10 comprises of an x-ray source 12, a
modulator 14, an object to be imaged 16, an array of detectors 18,
and a computer 20. Both x-ray source 12 and array of detectors 18
are placed on a rotational gantry (not shown) and are able to
continuously rotate around the object to be imaged 16, so that the
angle at which x-ray beam 13 intersects with the object to be
imaged 16 constantly changes. The Modulator 14 is a device placed
between the x-ray source 12 and the object to be imaged 16 for
effecting the desired fluence pattern as determined by computer 20.
Detector array 18 is formed by a plurality of detector rows (not
shown) including a plurality of detector elements (not shown) which
together sense the radiation that passes through the object to be
imaged 16. In operation, x-ray source 12 emits x-ray beams 13
through modulator 14 towards the object to be imaged 16 so that the
array of detectors 18 can detect the x-ray fluence passing through
the object to be imaged 16.
[0038] The resulting signals at the array of detectors 18 are then
sampled by a data measurement system (not shown) to build up a
projection, and subsequently a reconstructed volume. Note that the
optimized aperture selection CT system and method can be
implemented for any number of imaging geometries, source-detector
trajectories, or reconstruction algorithms, such as cone-beam CT or
scanning-beam CT.
[0039] Computer 20 is the computational engine of imaging CT system
10 which generates the operational parameters of modulator 14 to
control the pattern of fluence to be applied during image
acquisition based on a desired distribution of
contrast-to-noise-ratio (CNR) (as will be discussed further below).
Computer 20 makes use of either previously acquired patient images
22 to define regions of interest (ROIs) or a library of population
models 24 to define a distribution of desired image quality.
[0040] Referring now to FIGS. 2 and 3, the general process steps
100 for determining optimized fluence patterns through modulation
will be described for the imaging geometry 50 shown. Both the
theory behind the design of imaging CT system 10 and its practical
applications will be described in detail below.
[0041] At step 102, the process begins with an estimate of the
object to be imaged 16 provided to computer 20. Object to be imaged
16 is described by attenuation function .mu.({right arrow over
(r)}) 52 where {right arrow over (r)} is the position of the voxels
in the volume. Projection images of the object 52 are acquired by
first directing a two-dimensional x-ray beam I.sup.o (u,v) 54
towards the object at each angle .theta..sub.i 58 to determine the
detected x-ray fluence I.sub..theta..sub.i (u,v) 56 after passage
through the object. The variables u and v represent the pixel
matrix of the x-ray detector in use. In this work v=v(z) and
u=u(x,y) where x, y, and z are the dimensions of the object being
imaged The x-y plane, or imaging plane, is the plane where the
x-ray beam 54 projected by x-ray source is collimated to lie. The
projections, without any modulation applied to the x-ray beam, are
given by the following:
P.sub..theta..sub.i(u,v)=-1n(I.sub..theta..sub.i/I.sup.o) [/1]
[0042] In the present system and method, a modulation function
m.sub..theta..sub.i (u,v) is introduced to provide modulated
fluence patterns during imaging, and is effected in imaging CT
system 10 through modulator 14. The modulation function, with
values in the interval [0,1], describes the percentage of the
incident two-dimensional x-ray beam 54 to be directed at the
scanned object for each pixel (u,v) and each angle .theta..sub.i
58. Where the modulation factor is 1, this would be equivalent to
imaging without any modulating filter placed in the beam.
Introduction of this modulation factor causes the x-ray fluence
incident on the scanned object 54 to be m.sub..theta..sub.i
(u,v)I.sup.o(u,v), and the detected fluence through the object 56
to be m.sub..theta..sub.i (u,v)I.sub..theta..sub.i(u,v). From these
values the modulated projection images can be determined as:
P.sub..theta..sub.i.sup.m(u,v)=1n(m.sub..theta..sub.i(u,v)I.sup.o(u,v))--
1n(m.sub..theta..sub.i(u,v)I.sub..theta..sub.i(u,v))=-1n(I.sub..theta..sub-
.i/I.sup.o) [2]
and it is seen that imaging with modulated fluence patterns has no
effect on the expected value of the projections for this idealized
case.
[0043] The effect of the modulation is only seen when the noise in
the projections is investigated. Assuming that the x-ray fluence is
Poisson distributed, then the variance of the x-ray fluence through
the object 52 will be given by the expected value of the fluence,
.sub..theta..sub.i(u,v) For the modulated fluence patterns the
variance will be m.sub..theta..sub.i(u,v) .sub..theta..sub.i(u,v).
This leads to variances in the projections of
var { P .theta. i ( u , v ) } = 1 I _ .theta. i ( u , v ) [ 3 ]
##EQU00001##
for the unmodulated case, and
var { P .theta. i m ( u , v ) } = 1 m .theta. i ( u , v ) I _
.theta. i ( u , v ) [ 4 ] ##EQU00002##
for the modulated fluence patterns. So, although the modulation
function does not affect the expected value of the projections, it
does affect the noise in the projections.
[0044] The projections can be used to form volumetric
reconstructions. For a parallel beam geometry with no scatter or
energy dependence the reconstructed image can be found with the
formula
f ( x , y , z ) = .pi..tau. M proj i = 1 M proj k P .theta. i m ( u
, v ) h ( x cos .theta. i + y sin .theta. i - k .tau. ) [ 5 ]
##EQU00003##
where M.sub.proj is the total number of projection images, .tau. is
the sampling interval of the object, and h is the inverse Fourier
transform of the filtering function. The filtering of the
projection takes place in the u(x,y) dimension of the projections,
and is performed for each value of v(z). The expected value of the
reconstruction is not affected by the modulation function, but the
variance of the reconstructed image depends on the variance of the
projections, given by the formula:
var { f ( x , y , z ) } = ( .pi. .tau. M proj ) 2 i = 1 M proj k 1
m .theta. i ( u , v ) I _ .theta. i ( u , v ) h 2 ( x cos .theta. i
+ y sin .theta. i - k .tau. ) [ 6 ] ##EQU00004##
So it is evident that depending on the selection of the modulation
function m.sub..theta..sub.i(u,v) there can be a variation in noise
across a reconstructed volume. As such, the object of the present
invention is then to determine the modulation function that is
optimal for a desired imaging task.
[0045] At step (104), the desired distribution can be defined.
Given some metric C({right arrow over (r)}) describing image
characteristics (e.g. contrast-to-noise ratio (CNR) or
signal-to-noise ratio (SNR) in a volumetric image, computer 20
determines a modulation function m.sub..theta..sub.i(u,v) which can
be applied to x-ray intensities incident on the scanned object 52
to obtain an image which falls within a specified range from
C({right arrow over (r)}). An example of an image characteristic is
the contrast-to-noise ratio (CNR), where the CNR distribution in
the body for CT is dependent upon both the constraints of the
object 52 and the fluence pattern applied 54 in the generation of
the CT image, namely CNR({right arrow over (r)})=f(.mu.({right
arrow over (r)}), I.sub..theta..sub.i(u,v)). The CNR ({right arrow
over (r)}) would be designed according to the object 52 and the
anticipated location of the object 52 at the time of imaging.
[0046] The necessary modulation can be found by solving the inverse
problem
m(u,v)I(u,v)=G.sup.-1[C({right arrow over (r)})] [7]
where G.sup.-1 is an operator which relates the image metric
C({right arrow over (r)}) to the applied radiation intensities.
This will result in a reconstructed image {circumflex over
(f)}({right arrow over (r)}) where
C({right arrow over (r)}).ltoreq.{circumflex over (f)}({right arrow
over (r)}).ltoreq. C({right arrow over (r)}) [8]
with C(r) and C({right arrow over (r)}) being the lower and upper
bounds respectively desired of C({right arrow over (r)}) at each
point {right arrow over (r)}. This accounts for the fact that the
desired C({right arrow over (r)}) may not be obtainable with the
possible modulation combinations. For example, if a matrix
containing the desired image quality was 65.times.65 pixels, and
180 projections were desired, this would result in a modulation
factor matrix of size 65.times.180 (a total of 11,7000 values to be
optimized). However, it is noted that one could cut the amount of
processing required by using the symmetry of the desired image
quality patterns optimized for the number of angles required to
determine the modulation factor, reducing the problem to only 5,850
values.
[0047] An upper bound on C({right arrow over (r)}) is necessary to
limit the dose applied during image acquisition, while the lower
bound is necessary if sufficient image quality is to be obtained.
Variable image quality can be defined in different regions of the
image depending on the imaging task.
[0048] Careful characterization of the imaging CT system 10 is
necessary to find the relationship between m.sub..theta..sub.i(u,v)
and C({right arrow over (r)}). In order to plan the fluence
patterns that will lead to the desired image, it is necessary to
take various quantities, that are also modulated by
m.sub..theta..sub.i(u,v), into account such as: the dose in the
scanned object where D({right arrow over (r)})=D(.mu.({right arrow
over (r)}),I.sup.o(u,v),m.sub..theta..sub.i(u,v)), the scattered
radiation inherent to imaging CT systems I.sup.s(.mu.({right arrow
over (r)}),I.sup.o(u,v),m.sub..theta..sub.i(u,v)), and the exposure
dependent detective quantum efficiency of the detector
DQE(.nu.,.mu.({right arrow over
(r)}),D/proj,I.sup.o(u,v),m.sub..theta..sub.i(u,v)). The
computational engine of computer 20 comprises a model for
dependence of CNR({right arrow over (r)}) and D({right arrow over
(r)}) on I.sub..theta..sub.i(u,v), including the above mentioned
quantities.
[0049] It is not expected that it will be possible to determine an
analytical solution to the inverse problem when taking account of
the numerous dependencies. The constraints of the problem will be
satisfied by computer 20 determining a numerical solution to the
problem at step (106).
[0050] An iterative solution could have a form
min{.parallel.C({right arrow over (r)})-C.sub.i({right arrow over
(r)}).parallel.} [9]
where with each step i the image metric C.sub.i({right arrow over
(r)}) is calculated from the given properties of the imaging CT
system 10 and compared to the desired quantity C({right arrow over
(r)}). Changes to the fluence modulating function
m.sub..theta..sub.i(u,v) can be applied so that C.sub.i({right
arrow over (r)}) approaches C({right arrow over (r)}). For every
iterative step this process will require determining the value of
C.sub.i({right arrow over (r)}) given appropriate inputs. The
determination of C.sub.i({right arrow over (r)}) can be
accomplished by applying pre-determined look-up tables which
contain information involved in the relationship between
m.sub..theta..sub.i(u,v) and C({right arrow over (r)}). With more
flexibility available for the choice of m.sub..theta..sub.i(u,v) it
becomes necessary to create more complicated look up tables.
[0051] Additionally it is possible to optimize multiple properties
of the imaging CT system 10. For example, a modulation function
could be found to achieve both an optimal image quality,
.parallel.(C({right arrow over (r)})-C.sub.i({right arrow over
(r)}).parallel., and an optimal patient dose, .parallel.D({right
arrow over (r)})-D.sub.i({right arrow over (r)}).parallel., and an
appropriate weighting could combine the two to determine the
optimal modulation to apply to the fluence patterns, resulting in
an iterative solution of the form
min{.parallel.C({right arrow over (r)})-C.sub.i({right arrow over
(r)}).parallel.+w.parallel.D({right arrow over (r)})-D.sub.i({right
arrow over (r)}).parallel.} [10]
[0052] Another possible addition to this optimization would be to
not only weight the relative importance of image quality and dose
across the entire image, but to also weight the importance of dose
and image quality in individual voxels. This would require a matrix
of weights for image quality, W.sub.C({right arrow over (r)}), and
for dose, W.sub.D({right arrow over (r)}), giving a final form for
the iterative solution of
min{.parallel.W.sub.C({right arrow over (r)})(C({right arrow over
(r)})-C.sub.i({right arrow over
(r)})).parallel.+w.parallel.W.sub.D({right arrow over
(r)})(D({right arrow over (r)})-D.sub.i({right arrow over
(r)})).parallel.} [11]
[0053] Although the parameters of x-ray scatter reaching the
detector and the energy dependence of the x-rays used for imaging
have been let out of the formulation discussed above, it should be
apparent to one skilled in the art on how to modify the above
formulas to account for these parameters.
[0054] In an alternate embodiment, computer 20 of imaging CT system
10 could potentially use a small library of general modulation
factors that are designed for certain anatomical regions. This
would shorten the optimization process 100 as described above when
performed for specific patients.
[0055] Finally, at step (108), once the proper modulation function
is determined by computer 20 using the method described above,
modulation can be applied during image acquisition. There are
various possibilities for the construction of the modulator 14. A
main consideration is whether to use a modulator 14 that operates
with spatial modulation or temporal modulation.
[0056] A modulator 14 that spatially modulates would consist of a
shaped material that uses differing thicknesses of the material to
absorb differing percentages of the primary x-rays. One example of
a simple spatially modulating filter is a Cu Compensator, where the
modulator has a shape that is thicker for outer detector rows and
thinner for inner detector rows. As a result of this shape the
x-rays corresponding to the outer detector rows undergo greater
filtering than the x-rays corresponding to the inner detector rows
(see U.S. Pat. No. 6,647,095, Jiang Hsieh). For imaging CT system
10 the modulator 14 would ideally be able to have a different
optimized shape for each angle that a projection image is acquired
at. One of the potentially problematic aspects of the spatially
modulated approach is the energy dependent absorption of the x-rays
by the modulator 14. As has already been shown (see S. A. Graham,
D. J. Moseley, J. H. Siewerdsen, and D. A. Jaffray, "Compensators
for dose and scatter management in cone-beam CT" Med Phys
(submitted)) spectral hardening from shaped filters placed in the
beam can cause artifacts in reconstructed volumetric images. If
this problem cannot be addressed it may be necessary to investigate
alternate approaches.
[0057] Temporal modulation is a possibility for avoiding problems
associated with the energy dependent properties of the x-rays used
for imaging. Rather than consisting of a material that partially
absorbs incident x-rays a temporal modulator would be constructed
of a material that absorbs most, if not all, of the incident
photons. The modulation would be provided by having the modulator
14 block the x-rays for different amounts of time while moving
across the projection image. FIG. 10 illustrates an embodiment of a
temporal modulating filter, called a louvre compensator, where the
material contains louvres that can be independently turned to
create small field sizes during imaging. A combination of many of
these small fields would provide the intensity-modulated pattern.
FIG. 11 illustrates another embodiment, namely a multi-leaf
compensator, where the material is made of small individual
`leaves` that slide across the field-of-view to create intensity
modulated patterns. This approach would be similar to dynamic MLC
IMRT (see P. Keall, Q. Wu, Y. Wu, and J. O. Kim, "Dynamic MLC
IMRT," in Intensity-modulated radiation therapy: The state of the
art. Edited by J. R. Palta and T. R. Mackie. Medical Physics
Publishing, Madison, 2003), the contents of which are hereby
incorporated by reference. It should be noted that both compensator
examples could be constructed with any number of louvres or leaves
depending on how coarse or fine a modulation pattern is desired.
Although temporal modulation removes the complication of the energy
dependent x-ray spectrum, there are other possible obstacles to be
addressed. One possible issue is that the edges of the leaves in
the modulator 14 may cause artifacts in the images that cannot be
easily removed. There may also be difficulties in constructing a
modulator 14 capable of moving the leaves with speeds high enough
to modulate the fluence pattern during a projection, which takes
place in a time on the order of 10 ms.
III. Demonstration of Optimized Aperture Selection CT
[0058] A demonstration of the ability to optimize fluence patterns
to arrive at a desired image was performed in Matlab.TM.. Optimized
fluence patterns were determined for a circular mathematical
phantom containing three simulated `nodules` 30 of slightly
different attenuation, in a body 32, as shown in FIG. 4. The
optimization for determining the optimized fluence patterns was
performed on a mathematical phantom without any simulation of
surrounding soft tissue structure. This was done because when using
this technique on patients we would not know the exact location of
all soft tissue structures. It was decided that the optimization
should be performed on a uniform object to avoid the changes in SNR
that would be introduced by the change in attenuation. If the
imaged area was to include regions with large variation in
attenuation (i.e. bone or lung tissue) it is expected that these
tissues would need to be included in the optimization.
[0059] The optimization routines available in Matlab were not able
to manage the large number of variables to be optimized, requiring
an alternative method to be used. A simple simulated annealing code
was written to find modulated fluence patterns that provided low
values of the cost function being minimized. The simulated
annealing algorithm proceeds towards an optimized solution by
randomly selecting a new solution that is near the current
solution, and then comparing the two. If the cost function that is
being minimized decreases with the new solution it is accepted and
the algorithm can proceed to the next iteration. If, on the other
hand, the cost function increases, the new solution is accepted
with a probability:
Pr = exp ( - .DELTA. CF T ) [ 12 ] ##EQU00005##
where .DELTA.CF is the change in the cost function, and T is the
current unitless "temperature" of the system (if the cost function
were a measure of the energy of the system, then unitless
temperature would be replaced by kBT where kB is the Boltzmann
constant and T is a temperature measured, for example, in Kelvin).
For the simulations shown here a geometric temperature decrease was
used so that the unitless temperature for an iteration i+1 was
given by:
T.sub.i+1=.alpha.T.sub.i [13]
where Ti is the temperature in the previous iteration, and .alpha.
is a constant with a value between 0 and 1. This constant was
chosen to be 0.9998 to provide very slow cooling of the system.
[0060] Two different examples of the desired SNR, SNR.sub.D are
shown in FIGS. 5a and 5b. Both figures have SNR values of 30, 15,
5, and 0. The SNR value of 30 is represented by the lightest nodule
40a in the phantom and the SNR value of 0 is represented by the
dark area 46a outside the phantom. In FIG. 5a the SNR was designed
to be 15 at the skinline 42a and 5 throughout the rest of the
phantom, indicated at 44a. While in FIG. 5b most of the phantom is
defined as an SNR 15, indicated at 42b, with a region at the bottom
of the phantom designed to be a region where less dose is desired,
indicated at 44b. Both desired SNR images were used to determine
optimal fluence patterns for the mathematical phantom. The matrices
containing the desired SNR values were 65.times.65 pixels, and 180
projections were desired of the phantom, resulting in a modulation
factor matrix of size 65.times.180 (a total of 11,700 values to be
optimized). Using the symmetry of the SNR patterns optimized for
the number of angles required to optimize the modulation factor
over could be cut in half, reducing the problem to 5,850 values to
be optimized. The initial value of the modulation factor was chosen
to be one everywhere, which would be equivalent to imaging without
any modulating filter placed in the beam. The cost function for
iteration i was described by
CF i = ( x , y ( W SNR ( SNR i - SNR D ) ) 2 ) ( x , y ( W SNR (
SNR o - SNR D ) ) 2 ) + w ( D i ) ( D o ) [ 14 ] ##EQU00006##
[0061] The matrix W.sub.SNR weighted the SNR difference in each
pixel differently before the sum in each pixel was calculated.
Although the dose across the image could be similarly weighted, in
this case only the total dose absorbed by the phantom was used. The
dose and totaled SNR difference were normalized by their initial
values to facilitate comparison between the values. The value of w
to weight the sum of the two normalized values was set at one to
provide equal weighting between reducing dose and providing the
desired SNR. This also results in a cost function with an initial
value of two, as shown in FIG. 6.
[0062] As illustrated in FIG. 6 the cost functions tended to have
an initial sharp decrease followed by a slow decrease. The cost
function, which began with a value of two, was reduced to a value
of 0.5 in approximately 20 iterations. This is because the initial
modulation provided the highest dose possible. Beginning the
optimization with a solution that is nearer to an optimized
solution removes the sharp decrease at the beginning of the
optimization process. Implementing OASCT could potentially use a
small library of general modulation factors that are designed for
certain anatomical regions. This would shorten the optimization
process when performed for specific patients.
[0063] For the SNR distribution shown in FIG. 5a the optimization
process determined a value for m.sub..theta..sub.i(u,v) (FIG. 7)
using equal weighting on all SNR values (W.sub.SNR equal to one).
The right hand portion of FIG. 7 indicates a scale indicative of
the value of the modulation function, m.sub..theta..sub.i(u,v) in
the range [0,1]. The main portion of FIG. 7 shows the variation of
the modulation function as a function of gantry angle, shown on the
horizontal axis, and positioned across the image, shown along the
vertical axis. As shown in FIG. 7, the value of
m.sub..theta..sub.i(u,v) corresponding to where low SNR is desired
had a value of approximately 0.04. For other positions, there is a
band of higher value modulation function, which shifts following a
sine waveform as shown in FIG. 7. Thus, at either side of FIG. 7,
for gantry angles of 0 degrees and 180 degrees, this higher value
modulation function is found at approximately k.tau.=0. It shifts
downwards towards k.tau.=approx. 10, for the gantry angle 90
degrees. This is so that the desired SNR values will be achieved as
closely as possible.
[0064] Applying this modulation gave images with distinct patterns
of SNR (FIGS. 8a, 8b, 8c, 8d). FIG. 8a illustrates the theoretical
SNR in an unmodulated case. FIG. 8b illustrates the SNR after the
optimization process with uniform W.sub.SNR. FIG. 8c illustrates
the image acquired with no modulation and FIG. 8d illustrates the
image acquired using the modulated pattern. The theoretical SNR
shown is based on the evaluation of equations 5 and 6. The desired
SNR was not achieved, likely because what was defined as the
desired SNR was impossible to achieve given the constraints of the
system. FIG. 8b shows SNR values of approximately 19, 8.3, and 6.5
at the locations where the SNR was defined to be 30, 15, and 5.
FIG. 8a, with no modulation applied, had an SNR of approximately 30
across the image. The relative doses in the unmodulated and
modulated cases were 1 and 0.15 respectively. The CNR of the
nodules was 6.6.+-.1.2 in the unmodulated case, and decreased to
3.2.+-.0.9 when modulation was applied. The cost function was
decreased from 2 to 0.082.
[0065] If the weighting W.sub.SNR, is changed on the SNR a
different m.sub..theta..sub.i(u,v) will be found. Performing the
same optimization, but changing W.sub.SNR to be 3 where the SNR is
desired to be 30, and keeping it as 1 everywhere else, provides a
optimization with higher dose, and less noise where we desire high
SNR. FIG. 9a shows the SNR distribution when W.sub.SNR is tripled
and in this case the relative dose is increased to 0.21, the SNR
(where it had a desired value of 30) was approximately 24, and the
CNR of the nodules was 3.9.+-.0.7. FIG. 9b shows the image acquired
when the W.sub.SNR is tripled in the region of higher SNR.
[0066] For the optimization using the SNR from FIG. 5b, W.sub.SNR
was set at 3 for the areas where SNR was desired to be 30 and 5.
W.sub.SNR was one where SNR was desired to be 15. FIG. 9c shows the
SNR distribution when W.sub.SNR is tripled and for this case the
SNR achieved was approximately 21, 7.8, and 5.9 for the regions
that were desired to be 30, 15, and 5. The relative dose was 0.18
and the CNR of the nodules was 3.7.+-.0.7. FIG. 9d shows the image
acquired when the W.sub.SNR is tripled for the desired SNR shown in
FIG. 5b.
[0067] OASCT has the potential to greatly decrease dose to patients
by concentrating image quality on desired regions of interest
(ROIs). It will allow the prescription of desired image quality and
dose throughout a volume, and an iterative optimization process
will design patterns of modulation to be applied during imaging to
acquire images as near as possible to those desired. This
optimization process can account for numerous parameters of the
imaging system, including the efficiency of the detector, the
presence of x-ray scatter reaching the detector, and the
constraints of the modulator used to form the intensity modulated
fluence patterns. As mentioned above, there are various
possibilities for constructing the modulator, using either a
spatial or temporal compensating filter. For OASCT a spatial
modulator would ideally be able to have a different optimized shape
for each angle that a projection image is acquired at.
[0068] The simulation detailed above demonstrates the potential of
this method, but more advanced work may be needed to be performed
to determine how a real system may respond to the application of
OASCT. The use of Monte Carlo methods (see G. Jarry, S. A. Graham,
D. J. Moseley, et al. "Characterization of scattered radiation in
kV CBCT images using Monte Carlo simulations," Med Phys.
(submitted)) is a possibility for investigating OASCT. This would
allow realistic modeling of OASCT, with the additional benefit of
being able to choose which properties are included so that they may
be studied individually (as opposed to experimental imaging CT
measurements where it may be difficult to separate the causes and
effects of different properties).
[0069] The mathematical formulation helps to demonstrate how
modulation can be used to alter the noise in projections and
reconstructed volumes. However, the formulas used are for parallel
beam geometry, but the OASCT imaging system can be implemented for
any number of imaging geometries, source-detector trajectories, or
reconstruction algorithms. Also left out of the formulation are
quantities such as the x-ray scatter reaching the detector and the
energy dependence of the x-rays used for imaging. Although these
omissions may affect the results in equations 5 and 6 it is
expected that modulated fluence patterns still have the ability to
provide the desired optimized images. The optimization process to
determine the modulated fluence patterns will be a mathematical
optimization rather than an exact inversion so that equations
similar to 5 and 6 are not necessary to implement OASCT.
[0070] Reference will now be made to FIG. 10 and details of a
louvre compensator. This compensator comprises two sets of louvres
110, 112 extending perpendicularly to one another and overlapping
so that rotation of individual louvres may be used to select a
desired opening. The louvres are formed from a material that
absorbs substantially all the x-rays incident on them, so that the
effective x-ray beam is the opening in the louvre compensator.
[0071] FIG. 10b shows one simple opening scheme where one louvre
110a in the first set of louvres and another louvre 110b in the
second set are both rotated through 90 degrees so as, in effect to
provide two open slots running perpendicularly to one another. The
individual louvres 110a, 110b will be located in the middle of
these slots but their dimensions are such that they will have no
significant effect on the x-ray beam as it passes through each slot
thus formed.
[0072] As indicated in FIG. 10c, an x-ray beam originates as a
cone-beam from source 114 and is instant on the louvre compensator
110, 112. Due to the open configurations of the individual louvres
110a, 112a, an approximately square aperture is provided, that
permits an x-ray beam 116 of square, conical shape to extend
towards and through a body indicated schematically at 118. The beam
passes through the body and is detected at a detector.
[0073] Referring to FIG. 11, this shows an alternative compensator
scheme, with a compensator indicated schematically at 130. Here,
the compensator 130 includes a plurality of individual pairs of
elements indicated for one pair 132a, 132b. These elements 132a,
132b are movable in and out from a central plane as indicated by
the arrows 136, so as to define the shape and area of an aperture
134.
[0074] Referring to FIG. 11b, with a selected aperture 134 set for
the compensator 130, an x-ray source 138 is then arranged, to pass
a beam through the aperture 134. This generates a beam of the
desired shape as indicated at 140. The shaped beam 140 then passes
through a body indicated schematically at 142, to impinge on a
detector 144.
[0075] It will be understood that, either instead of or as well as,
the temporal modulators shown in FIGS. 10 and 11, one or more
spatial moderators can be used. A spatial moderator will provide
some fixed modulation, and may result in some beam hardening.
[0076] Accordingly, it is shown that it is possible to design an
imaging CT system with gantry angle dependent compensation, capable
of achieving desired image quality in defined ROIs and
distributions.
[0077] While certain features of the invention have been
illustrated and described herein, many modifications,
substitutions, changes, and equivalents will now occur to those of
ordinary skill in the art. It is, therefore, to be understood that
the appended claims are intended to cover all such modifications
and changes as fall within the true spirit of the invention.
* * * * *