U.S. patent application number 09/908356 was filed with the patent office on 2003-02-13 for methods and apparatus for fov-dependent aliasing artifact reduction.
Invention is credited to Hsieh, Jiang.
Application Number | 20030031289 09/908356 |
Document ID | / |
Family ID | 25425653 |
Filed Date | 2003-02-13 |
United States Patent
Application |
20030031289 |
Kind Code |
A1 |
Hsieh, Jiang |
February 13, 2003 |
Methods and apparatus for FOV-dependent aliasing artifact
reduction
Abstract
In one aspect, there is provided a method for reducing aliasing
artifacts in a computed tomography system. The method includes
scanning an object with a computed tomography system to acquire a
projection data set of measured views; synthesizing additional
views of the projection data set utilizing view interpolation; and
filtering and backprojecting the projection data set utilizing a
weighting function dependent upon parameters R.sub.f and R.sub.t,
where radius R.sub.f around an isocenter of the CT imaging system
is selected in accordance with a low artifact criterion and R.sub.t
is a parameter defining a transition region outside of radius
R.sub.f around the isocenter of the CT imaging system.
Inventors: |
Hsieh, Jiang; (Brookfield,
WI) |
Correspondence
Address: |
John S. Beulick
Armstrong Teasdale LLP
Suite 2600
One Metropolitan Sq.
St. Louis
MO
63102
US
|
Family ID: |
25425653 |
Appl. No.: |
09/908356 |
Filed: |
July 18, 2001 |
Current U.S.
Class: |
378/4 ; 378/207;
378/901 |
Current CPC
Class: |
Y10S 378/901 20130101;
A61B 6/032 20130101; G06T 11/005 20130101; A61B 6/5258 20130101;
G01N 23/046 20130101; G01N 2223/419 20130101 |
Class at
Publication: |
378/4 ; 378/901;
378/207 |
International
Class: |
G21K 001/12; H05G
001/60; A61B 006/00; G01N 023/00 |
Claims
What is claimed is:
1. A method for reducing aliasing artifacts in a computed
tomography system, said method comprising: scanning an object with
a computed tomography system to acquire a projection data set of
measured views; synthesizing additional views of the projection
data set utilizing view interpolation; and filtering and
backprojecting the projection data set utilizing a weighting
function dependent upon parameters R.sub.f and R.sub.t, where
radius R.sub.f around an isocenter of the CT imaging system is
selected in accordance with a low artifact criterion and R.sub.t is
a parameter defining a transition region outside of radius R.sub.f
around the isocenter of the CT imaging system.
2. A method in accordance with claim 1 wherein the weighting
function is different for the measured views and the synthesized
views.
3. A method for reducing aliasing artifacts in a computed
tomography system, said method comprising: scanning an object with
a computed tomography (CT) imaging system to acquire a projection
data set of measured views; reconstructing a first image utilizing
the measured views to produce an image having a central,
essentially artifact-free zone; interpolating views between
measured views to produce a projection data set having interpolated
views; reconstructing a second image utilizing both the
interpolated views and the measured views; and producing a blended
image from the first image and the second image utilizing a
weighting function selected to smoothly transition from the first
image to the second image in a transition region surrounding an
isocenter of the CT imaging system.
4. A method in accordance with claim 3 wherein the first image is
written P.sub.1 and the second image is written P.sub.2, the first
image and second image are combined using weighting functions
w.sub.1(i, j) and w.sub.2(i, j), and the blended image P(i, j) is
written: P(i, j)=w.sub.1 (i, j)P.sub.1(i, j)+w.sub.2 (i,
j)P.sub.2(i, j) where: w.sub.1(i, j)=1-w.sub.2(i, j) and 5 w 2 ( i
, j ) = { 0 r R f r - R f R t - R f R f < r R t 1 r > R t
.
5. A method in accordance with claim 3 further comprising
determining a radius R.sub.f of the central, essentially
artifact-free zone.
6. A method in accordance with claim 5 wherein the CT imaging
system has a data acquisition system (DAS), and R.sub.f is a
function of a sampling rate of the DAS and a scan speed of the CT
imaging system.
7. A method in accordance with claim 6 wherein R.sub.f is a
distance from the isocenter providing a sampling distance in an
azimuthal direction of 50.pi./984 cm.
8. A method in accordance with claim 3 wherein said interpolating
views between measured views comprises synthesizing views uniformly
in all directions.
9. A method for reducing aliasing artifacts in a computed
tomography system, said method comprising: scanning an object with
a computed tomography (CT) imaging system to acquire a projection
data set of measured views; reconstructing a first image utilizing
the projection data of measured views set to produce an image
having a central, essentially artifact-free zone; interpolating
views between the measured views to produce a projection data set
having interpolated views; reconstructing a second image utilizing
the interpolated views; and producing a blended image from the
first image and the second image utilizing a weighting function
selected to smoothly transition from the first image to the second
image in a transition region surrounding an isocenter of the CT
imaging system; and further wherein the first image is written
P.sub.1(i, j), the second image is written P.sub.2(i, j), pixel (i,
j) is a distance r from the isocenter, and images P.sub.1(i, j) and
P.sub.2(i, j) are combined by weighting P.sub.1(i, j) and
P.sub.2(i, j) with weights w.sub.1(i, j) and w.sub.2 (i, j),
respectively, where a sum of weights w.sub.1(i, j) and w.sub.2 (i,
j) is a constant, and w.sub.2 (i, j) increases in magnitude
relative to w.sub.1(i, j) at least within a transition region in a
vicinity of a radius R.sub.f of the central, essentially
artifact-free zone.
10. A method in accordance with claim 9 wherein 6 w 1 ( i , j ) = 1
- w 2 ( i , j ) and w 2 ( i , j ) = { 0 , r R f r - R f 2 ( R t - R
f ) R f < r R t 0.5 r > R t , where r is a distance of pixel
(i, j) from the isocenter; and R.sub.t is a parameter that
determines a transition region.
11. A computed tomographic (CT) imaging system having a rotating
gantry, a detector array on said rotating gantry, and a radiation
source on said rotating gantry configured to project a beam of
radiation towards said detector array through an object to be
imaged; said system configured to: scan an object to acquire a
projection data set of measured views; synthesize additional views
of said projection data set utilizing view interpolation; and
filter and backproject said projection data set utilizing a
weighting function dependent upon parameters R.sub.f and R.sub.t,
where radius R.sub.f around an isocenter of said CT imaging system
is selected in accordance with a low artifact criterion and R.sub.t
is a parameter defining a transition region outside of radius
R.sub.f around said isocenter of said CT imaging system.
12. A system in accordance with claim 11 wherein said weighting
function is different for said measured views and said synthesized
views.
13. A computed tomographic (CT) imaging system having a rotating
gantry, a detector array on said rotating gantry, and a radiation
source on said rotating gantry configured to project a beam of
radiation towards said detector array through an object to be
imaged; said system configured to: scan an object to acquire a
projection data set of measured views; reconstruct a first image
utilizing said measured views to produce an image having a central,
essentially artifact-free zone; interpolate views between measured
views to produce a projection data set having both interpolated
views and said measured views; reconstruct a second image utilizing
both said interpolated views and said measured views; and produce a
blended image from said first image and said second image utilizing
a weighting function selected to smoothly transition from said
first image to said second image in a transition region surrounding
an isocenter of said CT imaging system.
14. A system in accordance with claim 13 wherein said first image
is written P.sub.1 and said second image is written P.sub.2, said
first image and said second image are combined using weighting
functions w.sub.1(i, j) and w.sub.2(i, j), and said blended image
P(i, j) is written: P(i, j)=w.sub.1(i, j)P.sub.1(i, j)+w.sub.2(i,
j)P.sub.2(i, j) where: w.sub.1(i, j)=1-w.sub.2(i, j) and 7 w 2 ( i
, j ) = { 0 r R f r - R f R t - R f R f < r R t 1 r > R t
.
15. A system in accordance with claim 13 further configured to
determine a radius R.sub.f of the central, essentially
artifact-free zone.
16. A system in accordance with claim 15 wherein said CT imaging
system has a data acquisition system (DAS), and R.sub.f is a
function of a sampling rate of said DAS and a scan speed of said CT
imaging system.
17. A system in accordance with claim 16 wherein R.sub.f is a
distance from said isocenter providing a sampling distance in an
azimuthal direction of 50.pi./984 cm.
18. A system in accordance with claim 13 wherein to interpolate
said views between said measured views, said system is configured
to synthesize views uniformly in all directions.
19. A computed tomographic (CT) imaging system having a rotating
gantry, a detector array on said rotating gantry, and a radiation
source on said rotating gantry configured to project a beam of
radiation towards said detector array through an object to be
imaged; said system configured to: scan an object to acquire a
projection data set of measured views; reconstruct a first image
utilizing said measured views to produce an image having a central,
essentially artifact-free zone; interpolate views between said
measured views to produce a projection data set of interpolated
views; reconstruct a second image utilizing said interpolated
views; and produce a blended image from said first image and said
second image utilizing a weighting function selected to smoothly
transition from said first image to said second image in a
transition region surrounding an isocenter of said CT imaging
system; and further wherein said first image is written P.sub.1(i,
j), said second image is written P.sub.2(i, j), pixel (i, j) is a
distance r from said isocenter, and said system is configured to
combine said images P.sub.1(i, j) and P.sub.2(i, j) by weighting
P.sub.1(i, j) and P.sub.2(i, j) with weights w.sub.1(i, j) and
w.sub.2 (i, j), respectively, where a sum of said weights
w.sub.1(i, j) and w.sub.2(i, j) is a constant, and w.sub.2 (i, j)
increases in magnitude relative to w.sub.1(i, j) at least within a
transition region in a vicinity of a radius R.sub.f of said
central, essentially artifact-free zone.
20. A system in accordance with claim 19 wherein 8 w 1 ( i , j ) =
1 - w 2 ( i , j ) and w 2 ( i , j ) = { 0 , r R f r - R f 2 ( R t -
R f ) R f < r R t 0.5 r > R t , where r is a distance of
pixel (i, j) from said isocenter; and R.sub.t is a parameter that
determines a transition region.
21. A computer configured to: read a projection data set of
measured views obtained by scanning an object with a computed
tomographic (CT) imaging system; synthesize additional views of the
projection data set utilizing view interpolation; and filter and
backproject the projection data set utilizing a weighting function
dependent upon parameters R.sub.f and R.sub.t, where radius R.sub.f
around an isocenter of the CT imaging system is selected in
accordance with a low artifact criterion and R.sub.t is a parameter
defining a transition region outside of radius R.sub.f around the
isocenter of said CT imaging system.
22. A system in accordance with claim 21 wherein said weighting
function is different for said measured views and said synthesized
views.
23. A computer configured to: read a projection data set of
measured views obtained by scanning an object with a computed
tomographic imaging system; reconstruct a first image utilizing the
measured views to produce an image having a central, essentially
artifact-free zone; interpolate views between the measured views to
produce a projection data set having interpolated views;
reconstruct a second image utilizing said interpolated views; and
produce a blended image from said first image and said second image
utilizing a weighting function selected to smoothly transition from
said first image to said second image in a transition region
surrounding an isocenter of the CT imaging system.
24. A computer in accordance with claim 23 wherein to reconstruct
said second image, said computer is configured to utilize the
measured views in addition to said interpolated views; and said
first image is written P.sub.1 and said second image is written
P.sub.2, said first image and said second image are combined using
weighting functions w.sub.1(i, j) and w.sub.2(i, j), and said
blended image P(i, j) is written: P(i, j)=w.sub.1(i, j)P.sub.1(i,
j)+w.sub.2(i, j)P.sub.2(i, j) where: w.sub.1(i, j)=1-w.sub.2(i, j)
and 9 w 2 ( i , j ) = { 0 r R f r - R f R t - R f R f < r R t 1
r > R t .
25. A computer in accordance with claim 23 further configured to
determine a radius R.sub.f of the central, essentially
artifact-free zone.
26. A computer in accordance with claim 25 wherein the CT imaging
system has a data acquisition system (DAS), and said computer is
configured to determine R.sub.f as a function of a sampling rate of
the DAS and a scan speed of the CT imaging system.
27. A computer in accordance with claim 26 wherein R.sub.f is a
distance from the isocenter providing a sampling distance in an
azimuthal direction of 50.pi./984 cm.
28. A computer in accordance with claim 23 wherein to interpolate
said views between the measured views, said computer is configured
to synthesize views uniformly in all directions.
29. A computer in accordance with claim 23 wherein said first image
is written P.sub.1(i, j), said second image is written P.sub.2(i,
j), pixel (i, j) is a distance r from the isocenter, and wherein
said computer is configured to combine said images P.sub.1(i, j)
and P.sub.2(i, j) by weighting P.sub.1(i, j) and P.sub.2(i, j) with
weights w.sub.1(i, j) and w.sub.2 (i, j), respectively, where a sum
of said weights w.sub.1 (i, j) and w.sub.2(i, j) is a constant, and
w.sub.2(i, j) increases in magnitude relative to w.sub.1(i, j) at
least within a transition region in a vicinity of a radius R.sub.f
of said central, essentially artifact-free zone.
30. A computer in accordance with claim 29 wherein 10 w 1 ( i , j )
= 1 - w 2 ( i , j ) and w 2 ( i , j ) = { 0 , r R f r - R f 2 ( R t
- R f ) R f < r R t 0.5 r > R t , where r is a distance of
pixel (i, j) from the isocenter; and R.sub.t is a parameter that
determines a transition region.
31. A computer readable medium having instructions recorded thereon
configured to instruct a computer to: read a projection data set of
measured views obtained by scanning an object with a computed
tomographic (CT) imaging system; synthesize additional views of the
projection data set utilizing view interpolation; and filter and
backproject the projection data set utilizing a weighting function
dependent upon parameters R.sub.f and R.sub.t, where radius R.sub.f
around an isocenter of the CT imaging system is selected in
accordance with a low artifact criterion and R.sub.t is a parameter
defining a transition region outside of radius R.sub.f around the
isocenter of said CT imaging system.
32. A computer readable medium in accordance with claim 31 wherein
said weighting function is different for said measured views and
said synthesized views.
33. A computer readable medium having instructions recorded thereon
configured to instruct a computer to: read a projection data set of
measured views obtained by scanning an object with a computed
tomographic imaging system; reconstruct a first image utilizing the
measured views to produce an image having a central, essentially
artifact-free zone; interpolate views between the measured views to
produce an projection data set having interpolated views;
reconstruct a second image utilizing said interpolated views; and
produce a blended image from said first image and said second image
utilizing a weighting function selected to smoothly transition from
said first image to said second image in a transition region
surrounding an isocenter of the CT imaging system.
34. A computer readable medium in accordance with claim 33 where to
reconstruct a second image, said computer readable medium has
recorded thereon instructions configured to instruct the computer
to utilize the measured views in addition to said interpolated
views; and wherein said first image is written P.sub.1 and said
second image is written P.sub.2, said first image and said second
image are combined using weighting functions w.sub.1 (i, j) and
w.sub.2(i, j), and said blended image P(i, j) is written: P(i,
j)=w.sub.1(i, j)P.sub.1(i, j)+w.sub.2(i, j)P.sub.2(i, j) where:
w.sub.1(i, j)=1-w.sub.2(i, j) and 11 w 2 ( i , j ) = { 0 r R f r -
R f R i - R f R f < r R i 1 r > R i .
35. A computer readable medium in accordance with claim 33 further
having instructions recorded thereon configured to instruct a
computer to determine a radius R.sub.f of the central, essentially
artifact-free zone.
36. A computer readable medium in accordance with claim 35 wherein
the CT imaging system has a data acquisition system (DAS), and said
computer readable medium further has instructions recorded thereon
configured to instruct a computer to determine R.sub.f as a
function of a sampling rate of the DAS and a scan speed of the CT
imaging system.
37. A computer readable medium in accordance with claim 36 wherein
R.sub.f is a distance from the isocenter providing a sampling
distance in an azimuthal direction of 50.pi./984 cm.
38. A computer readable medium in accordance with claim 33 wherein
to interpolate said views between the measured views, said computer
readable medium has instructions recorded thereon configured to
instruct a computer to synthesize views uniformly in all
directions.
39. A computer readable medium in accordance with claim 33 wherein
said first image is written P.sub.1(i, j), said second image is
written P.sub.2(i, j), pixel (i, j) is a distance r from the
isocenter, and wherein said computer readable medium has recorded
thereon instructions configured to instruct the computer to combine
said images P.sub.1 (i, j) and P.sub.2(i, j) by weighting
P.sub.1(i, j) and P.sub.2(i, j) with weights w.sub.1(i, j) and
w.sub.2(i, j), respectively, where a sum of said weights w.sub.1(i,
j) and w.sub.2(i, j) is a constant, and w.sub.2(i, j) increases in
magnitude relative to w.sub.1(i, j) at least within a transition
region in a vicinity of a radius R.sub.f of said central,
essentially artifact-free zone.
40. A computer readable medium in accordance with claim 39 wherein
w.sub.1(i, j)=1-w.sub.2(i, j) and 12 w 2 ( i , j ) = { 0 , r R f r
- R f 2 ( R i - R f ) R f < r R i 0.5 r > R i , where r is a
distance of pixel (i, j) from the isocenter; and R.sub.t is a
parameter that determines a transition region.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to methods and apparatus
for computed tomographic (CT) imaging of an object, and more
particularly to method and apparatus for reducing artifacts and
increasing spatial resolution at high scan rates in sampling rate
limited CT imaging systems.
[0002] To avoid view aliasing artifacts, data-sampling rates must
be increased proportionately for faster CT scan speeds. As an
example, if a data acquisition system (DAS) sampling rate is
sufficient at 984 Hz for a 1.0 second scan speed, the DAS sampling
rate must be at least 984/.times. for a CT scanner that rotates at
x seconds per revolution. Accordingly, the DAS sampling rate must
be at least 1968 Hz for a 0.5 second scan speed. In one known
scanning system, the DAS sampling rate is limited to 1408 Hz. For a
fixed number of projections, the sampling density in the azimuthal
direction decreases with an increase in distance from an
iso-center. Thus, at 0.5 second scan speeds, this is a view
deficiency in such systems caused by a lack of adequate samples in
an azimuthal direction. This view deficiency results in aliasing
artifacts and reduced spatial resolution.
BRIEF SUMMARY OF THE INVENTION
[0003] There is therefore provided, in one aspect, a method for
reducing aliasing artifacts in a computed tomography system. The
method includes scanning an object with a computed tomography
system to acquire a projection data set of measured views;
synthesizing additional views of the projection data set utilizing
view interpolation; and filtering and backprojecting the projection
data set utilizing a weighting function dependent upon parameters
R.sub.f and R.sub.t, where radius R.sub.f around an isocenter of
the CT imaging system is selected in accordance with a low artifact
criterion and R.sub.t is a parameter defining a transition region
outside of radius R.sub.f around the isocenter of the CT imaging
system.
[0004] In another aspect, a method for reducing aliasing artifacts
in a computed tomography system is provided that includes scanning
an object with a computed tomography (CT) imaging system to acquire
a projection data set of measured views; reconstructing a first
image utilizing the measured views to produce an image having a
central, essentially artifact-free zone; interpolating views
between measured views to produce a projection data set having
interpolated views; reconstructing a second image utilizing the
interpolated views; and producing a blended image from the first
image and the second image utilizing a weighting function selected
to smoothly transition from the first image to the second image in
a transition region surrounding an isocenter of the CT imaging
system.
[0005] In still another aspect, there is provided a computed
tomographic (CT) imaging system having a rotating gantry, a
detector array on the rotating gantry, and a radiation source on
the rotating gantry configured to project a beam of radiation
towards the detector array through an object to be imaged. The
system is configured to scan an object to acquire a projection data
set of measured views; synthesize additional views of the
projection data set utilizing view interpolation; and filter and
backproject the projection data set utilizing a weighting function
dependent upon parameters R.sub.f and R.sub.t, where radius R.sub.f
around an isocenter of the CT imaging system is selected in
accordance with a low artifact criterion and R.sub.t is a parameter
defining a transition region outside of radius R.sub.f around the
isocenter of the CT imaging system.
[0006] In yet another aspect, there is provided a computed
tomographic (CT) imaging system having a rotating gantry, a
detector array on the rotating gantry, and a radiation source on
the rotating gantry configured to project a beam of radiation
towards the detector array through an object to be imaged. The
system is configured to scan an object to acquire a projection data
set of measured views; reconstruct a first image utilizing the
projection data set of measured views to produce an image having a
central, essentially artifact-free zone; interpolate views between
measured views of the projection data set of measured views to
produce a projection data set having interpolated views;
reconstruct a second image utilizing the interpolated views; and
produce a blended image from the first image and the second image
utilizing a weighting function selected to smoothly transition from
the first image to the second image in a transition region
surrounding an isocenter of the CT imaging system.
[0007] In still another aspect, there is provided a computer
configured to read a projection data set of measured views obtained
by scanning an object with a computed tomographic (CT) imaging
system; synthesize additional views of the projection data set
utilizing view interpolation; and filter and backproject the
projection data set utilizing a weighting function dependent upon
parameters R.sub.f and R.sub.t, where radius R.sub.f around an
isocenter of the CT imaging system is selected in accordance with a
low artifact criterion and R.sub.t is a parameter defining a
transition region outside of radius R.sub.f around the isocenter of
the CT imaging system.
[0008] In yet another aspect, there is provided a computer readable
medium having recorded thereon instructions configured to read a
projection data set of measured views obtained by scanning an
object with a computed tomographic imaging system; reconstruct a
first image utilizing the projection data set of measured views to
produce an image having a central, essentially artifact-free zone;
interpolate views between measured views to produce a projection
data set having interpolated views; reconstruct a second image
utilizing the interpolated views; and produce a blended image from
the first image and the second image utilizing a weighting function
selected to smoothly transition from the first image to the second
image in a transition region surrounding an isocenter of the CT
imaging system.
[0009] It will be observed that embodiments of the present
invention result in reduced aliasing artifacts and increased
spatial resolution for scan rates that would otherwise require
increased data acquisition system sampling rates.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a pictorial view of a CT imaging system.
[0011] FIG. 2 is a block schematic diagram of the system
illustrated in FIG. 1.
[0012] FIG. 3 is a simplified flow chart representative of one
embodiment of the present invention.
[0013] FIG. 4 is a simplified flow chart representative of another
embodiment of the present invention.
[0014] FIG. 5 is a pictorial representation of imaging blending
showing two image regions and solid lines representing acquired
projection data and dashed lines representing synthesized data
samples.
[0015] FIG. 6 is a simplified flow chart representative of a
suitable adaptive method for synthesizing views of an object.
DETAILED DESCRIPTION OF THE INVENTION
[0016] As used herein, an element or step recited in the singular
and preceded with the word "a" or "an" should be understood as not
excluding plural said elements or steps, unless such exclusion is
explicitly recited. Furthermore, references to "one embodiment" of
the present invention are not intended to be interpreted as
excluding the existence of additional embodiments that also
incorporate the recited features.
[0017] Also as used herein, the phrase "reconstructing an image" is
not intended to exclude embodiments of the present invention in
which data representing an image is generated but a viewable image
is not. However, many embodiments generate (or are configured to
generate) at least one viewable image.
[0018] Referring to FIGS. 1 and 2, a computed tomography (CT)
imaging system 10 is shown as including a gantry 12 representative
of a "third generation" CT scanner. Gantry 12 has an x-ray
radiation source 14 that projects a beam of x-ray radiation 16
toward a detector array 18 on the opposite side of gantry 12.
Detector array 18 is formed by detector elements 20 that together
sense the projected x-rays that pass through an object 22, for
example a medical patient. Each detector element 20 produces an
electrical signal that represents the intensity of an impinging
x-ray beam and hence the attenuation of the beam as it passes
through patient 22. During a scan to acquire x-ray projection data,
gantry 12 and the components mounted thereon rotate about a center
of rotation 24. In one embodiment, detector array 18 is fabricated
in a multi-slice configuration. In a multi-slice configuration,
detector array 18 has a plurality of rows of detector elements or
cells 20, only one of which is shown in FIG. 2. One or more
additional rows of detector elements 20 in such configurations are
arranged parallel to the illustrated row, and each row is
transverse to the translation direction of patient 22 (i.e., the
z-axis or patient axis).
[0019] Rotation of gantry 12 and the operation of x-ray source 14
are governed by a control mechanism 26 of CT system 10. Control
mechanism 26 includes an x-ray controller 28 that provides power
and timing signals to x-ray source 14 and a gantry motor controller
30 that controls the rotational speed and position of gantry 12. A
data acquisition system (DAS) 32 in control mechanism 26 samples
analog data from detector elements or cells 20 and converts the
data to digital signals for subsequent processing. An image
reconstructor 34 receives sampled and digitized x-ray data from DAS
32 and performs high speed image reconstruction. The reconstructed
image is applied as an input to a computer 36 which stores the
image in a mass storage device 38. Computer 36 also receives
commands and scanning parameters from an operator via console 40
that has a keyboard. An associated cathode ray tube display 42
allows the operator to observe the reconstructed image and other
data from computer 36. The operator supplied commands and
parameters are used by computer 36 to provide control signals and
information to DAS 32, x-ray controller 28 and gantry motor
controller 30. In addition, computer 36 operates a table motor
controller 44 which controls a motorized table 46 to position
patient 22 in gantry 12. Particularly, table 46 moves portions of
patient 22 through gantry opening 48. In a helical scan as
performed in one embodiments of the present invention, table 46
moves while projection data is being collected and gantry 12 is
rotating. The "helical pitch" is a measure of the amount of
movement of table 46 per rotation of gantry 12. In an axial scan as
performed in one embodiment of the present invention, table 46 is
stationary while projection data is being acquired and gantry 12 is
rotating.
[0020] In one embodiment, computer 36 includes a device 50 for
reading and writing onto removable media 52. For example, device 50
is a floppy disk drive, a CD-R/W drive, or a DVD drive.
Correspondingly, media 52 is either a floppy disk, a compact disk,
or a DVD. Device 50 and media 52 are used in one embodiment to
transfer acquired projection data from imaging system 10 to another
computer for further processing, or in another embodiment to input
machine readable instructions that are processed by computer
36.
[0021] View aliasing is caused mainly by a lack of adequate samples
in an azimuthal direction. For a fixed number of projections, a
sampling density in the azimuthal direction decreases with an
increase in distance from an isocenter. For example, although a 984
Hz sampling rate is insufficient to ensure adequate sampling
density near edges of a 50 cm FOV for a 0.5 scan speed, this rate
is still sufficient to provide adequate sampling within a 25 cm
FOV. That this rate is sufficient can be shown by observing that
for a 1 second scan speed, an azimuthal sampling distance between
adjacent views at the edge of a 50 cm FOV for a 1 second scan is
50.pi./984. With the same DAS 32 sampling rate, a 0.5 second scan
speed provides 492 views per revolution. Therefore, a sampling
distance in an azimuthal direction at an edge of a 25 cm FOV is
25.pi./492=50.pi./984. This sampling density is identical to a
sampling density for a 1 second scan, which is sufficient to
provide acceptably low view aliasing. As a result, a 0.5 second
scan provides an artifact-free zone for a 0.5 second scan inside a
center 25 cm FOV. In other words, no aliasing artifact correction
is required inside the center 25 cm FOV.
[0022] Thus, in one embodiment and referring to FIG. 3, an alias
artifact reduction algorithm is provided that is field of view
(FOV) and reconstruction algorithm dependent. In particular, an
object 22 is scanned 54 with CT imaging system 10 to acquire a
projection data set of measured views. Additional views of the
projection data set are synthesized 56 utilizing view
interpolation. The projection data set, including the interpolated
views and the measured views, is filtered and backprojected 58
utilizing a weighting function dependent upon parameters R.sub.f
and R.sub.t. Here, R.sub.f is a radius around an isocenter 24 of CT
imaging system 10 selected in accordance with a low artifact
criterion, and R.sub.t is a parameter defining a transition region
outside of radius R.sub.f. In one embodiment, the weighting
function is different for measured views and synthesized views of
the projection data set.
[0023] In another embodiment and referring to FIG. 4, an object is
scanned 60 with a CT imaging system 10 to acquire a projection data
set of measured views. A first image is reconstructed 62 utilizing
the measured views to produce an image having a central,
essentially artifact-free zone. Views between measured views of the
projection data set are interpolated 64 to produce a projection
data set having interpolated views. A second image is reconstructed
66 utilizing the interpolated views. (Interpolated views are not
included in the reconstruction of the first image.) A blended image
is produced 68 from the first image and the second image utilizing
a weighting function selected to smoothly transition from the first
image to the second image in an annular transition region
surrounding isocenter 24 of CT imaging system 10. In one
embodiment, a radius R.sub.f of the central, essentially
artifact-free zone is determined based on the sampling rate of DAS
32, the scan speed (i.e., gantry 12 rotation rate) of imaging
system 10, and the image reconstruction algorithms used (i.e.,
higher resolution kernels permit higher frequency contents to be
preserved).
[0024] For example and referring to FIG. 5, let R.sub.f be the
radius of the aliasing artifact-free (or nearly artifact-free)
zone. For the measured views, represented by solid lines 70, a
first image P.sub.1 is reconstructed. A view interpolation is
performed, for example, by synthesizing views 72 uniformly in all
directions or by synthesizing views utilizing an adaptive
method.
[0025] The projection data set of interpolated views produced
either adaptively or by uniform synthesis is used to reconstruct a
second image P.sub.2. The two images P.sub.1 and P.sub.2 are
combined using weighting functions written as:
P(i, j)=w.sub.1(i, j)P.sub.1(i, j)+w.sub.2(i, j)P.sub.2(i, j),
[0026] where the sum of weights w.sub.1(i, j) and w.sub.2(i, j) is
a constant, for example, unity [i.e., w.sub.1(i, j)=1-w.sub.2(i,
j)], and w.sub.2(i, j) increases in magnitude relative to
w.sub.1(i, j) at least within a transition region in a vicinity of
radius R.sub.f. For example, with the normalization w.sub.1(i,
j)=1-w.sub.2(i, j), one suitable function w.sub.2(i, j) is written:
1 w 2 ( i , j ) = { 0 , r R f r - R f 2 ( R t - R f ) R f < r R
t 0.5 r > R t .
[0027] Here, r is a distance of pixel (i, j) from isocenter 24 and
R.sub.tis a parameter that determines transition region 74, which
is dependent upon DAS 32 sampling rate, reconstruction kernel, and
scan speed. Parameter R.sub.t, in one embodiment, is determined
experimentally.
[0028] In another embodiment, a similar method is applied directly
as part of the backprojection. In this embodiment, it is not
necessary to generate two separate images. Instead, an additional
weighting function is used during backprojection. The weighting
function is a function of R.sub.f and R.sub.t. The weighting
functions for measured views and synthesized views are
different.
[0029] In another embodiment, second image P.sub.2 is reconstructed
utilizing both the measured views and the interpolated views.
Images P.sub.1 and P.sub.2 are combined using weighting functions
w.sub.1(i, j) and w.sub.2(i, j) written differently from those
above. The resulting image P(i, j) is written:
P(i, j)=w.sub.1(i, j)P.sub.1(i, j)+w.sub.2(i, j)P.sub.2(i, j)
[0030] where:
w.sub.1(i, j)=1-w.sub.2(i, j)
[0031] and 2 w 2 ( i , j ) = { 0 r R f r - R f R t - R f R f < r
R t 1 r > R t .
[0032] An example of a suitable adaptive method for synthesizing
views is shown in flow chart 76 of FIG. 6, which illustrates steps
executed by CT system 10 (shown in FIG. 1) in one embodiment to
compensate for view aliasing artifacts. The method illustrated in
FIG. 6 can be practiced by DAS 32 (shown in FIG. 2), image
reconstructor 34 (shown in FIG. 2), or computer 36 (shown in FIG.
2), or by a combination thereof. Generally, a processor in at least
one of DAS 32, reconstructor 34, and computer 36 is programmed to
execute the process steps described below.
[0033] Referring specifically to FIG. 6, and when performing a scan
78, a set of raw scan data is acquired. The raw scan data is
pre-processed to generate a set of projection data
p(.gamma.,.beta.). As explained above, view aliasing occurs when
scanned object 22 changes along a plurality of view directions,
where each view is determined by a projection angle, .beta..
Therefore, after collecting the projection data, p(.gamma.,
.beta.), a high frequency variation for each view in the projection
data 80 is determined according to the relationship: 3 ( , ) = p (
, ) ,
[0034] where p(.gamma., .beta.) is the measured projection after
proper pre-processing, .gamma. is the fan angle, .beta. is the
projection angle, and .xi.(.gamma., .beta.) represents the
high-frequency variation of the projection.
[0035] Variations in views collected during a scan can be caused,
for example, by patient table 46. Patient table 46 is formed with
multiple flat segments, and projection data for the multiple
segments changes from view to view. To avoid view variation, a
weighting function 82 is applied to high-frequency variation
estimate, .xi.(.gamma.,.beta.), to exclude the influence of table
46.
[0036] Although many weighting functions can be used, in one
embodiment, the boundaries of the weight function,
.gamma..sub.h(.beta.) and .gamma..sub.l(.beta.), are determined
based on the location at which the projection intensity exceeds a
fraction of the peak value for view .beta.. Weighting function,
w(.gamma., .beta.), is zero outside the boundaries and reaches
unity at the center region. The transition from zero to unity is a
smooth function. For example, an exemplary weighting function,
w(.gamma., .beta.), can be described by the following relationship:
4 w ( , ) = { - l ( ) l ( ) < l ( ) + 1 l ( ) < h ( ) h ( ) -
h ( ) - < h ( ) 0 otherwise ,
[0037] where .eta. represents the width of a transition region.
[0038] In addition to applying weighting function 82, because
aliasing artifacts come from the highest frequency contents in the
projection, for each view, the maximum value of the frequency
content is determined according to the following relationship:
.epsilon.(.beta.)=f[.vertline..xi.(.gamma., .beta.)w(.gamma.,
.beta.).vertline.],
[0039] where f is a function representing a maximum frequency
value. In another embodiment, f is a function representing the
average of a plurality of top N maximum values to reduce the
influence of noise. A preliminary aliasing index,
.epsilon.(.beta.), 84 is then determined based on the maximum
frequency contents from the weighted data. The amount of aliasing
artifacts present in a reconstructed image depends on the frequency
contents of the projection, and also on a projection view sampling
rate, a reconstruction field of view, and a reconstruction filter
kernel. The lower the number of views collected during a 2.pi.
rotation of gantry 12 (shown in FIGS. 1 and 2), the more likely is
the occurrence of the presence of an aliasing artifact. Similarly,
for the reconstruction region that is further away from an
isocenter, the likelihood of aliasing artifacts increases. The
reconstruction filter kernel will influence the aliasing artifact
in a similar manner, with the higher resolution kernels producing
more aliasing. Therefore, the preliminary index, .epsilon.(.beta.),
needs to be scaled 86 by a number of views per rotation, v; a
reconstruction location, R; and a reconstruction filter kernel,
.phi., according to the relationship:
.chi.(.beta.)=g(v.sup.-1,R,.phi.).epsilon.(.beta.)
[0040] To further reduce the influence of noise, the scaled
aliasing index is passed through a low pass filter 88.
[0041] The scaled and filtered aliasing index, .chi.(.beta.), is
then utilized to reduce aliasing artifacts 90 by determining a
region and a number of synthesized views required to minimize
potential aliasing artifact. The higher the value of the scaled
aliasing index, .chi.(.beta.), the higher the likelihood of the
occurrence of aliasing artifacts. Therefore, a greater number of
synthesized views should be generated in regions where the scaled
aliasing index is high as compared to regions where the scaled
aliasing index is low. The synthesized views can be generated using
any one of many known techniques, such as interpolation.
[0042] In another embodiment, to minimize aliasing artifact, more
contribution, i.e., a higher weight, is placed on the synthesized
views if the scaled aliasing index is high in a particular region.
In a further alternative embodiment, the aliasing index may be used
to predict sampling rates as a function of the projection angle,
when DAS 32 (shown in FIG. 2) sampling rate is dynamically
adjusted. For instance, the value of the aliasing index increases
as DAS 32 sampling rate increases, therefore, a particular sampling
rate corresponds to a particular range of aliasing indices.
[0043] Computer 36, image reconstructor 34, and/or DAS 32 of
imaging system 10, either alone or in combination, provide the
processing power necessary to perform the computational steps
described above in at least one embodiment of the present
invention. Instructions for performing the computational steps are
stored in an associated memory, such as mass storage device 38,
read only or read/write memory (not shown separately in FIG. 1), or
media 52.
[0044] In at least one embodiment of the present invention, a
computer system separate from imaging system 10 (for example, a
workstation, not shown in the figures) is provided to reconstruct
images using projection data acquired by imaging system 10. In
these embodiments, acquired projection data and corresponding
cardiac phase information is transferred from imaging system 10 to
the separate computer system via a network (not shown) or suitable
media 52. As a free-standing, separate computer system, these
embodiments do not require a rotating gantry, a radiation source,
or a detector array of their own. Also, these embodiments are
configured to read or input projection data previously acquired by
a CT imaging system. In other respects, they are configured in
manners similar to the other apparatus embodiments discussed
herein.
[0045] Other embodiments of the present invention include
machine-readable media 52 having recorded thereon instructions
configured to instruct a computer system to perform steps of one or
more of the methods described herein.
[0046] The above-described embodiments of the present invention
will be seen to be effective in reducing aliasing artifacts and
increasing spatial resolution at scan rates that would otherwise
require increased data acquisition system sampling rates.
[0047] While the invention has been described in terms of various
specific embodiments, those skilled in the art will recognize that
the invention can be practiced with modification within the spirit
and scope of the claims.
* * * * *