U.S. patent application number 13/145128 was filed with the patent office on 2011-11-24 for method and apparatus for large field of view imaging and detection and compensation of motion artifacts.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS N.V.. Invention is credited to Matthias Bertram, Christoph Neukirchen, Colas Schretter.
Application Number | 20110286573 13/145128 |
Document ID | / |
Family ID | 41820756 |
Filed Date | 2011-11-24 |
United States Patent
Application |
20110286573 |
Kind Code |
A1 |
Schretter; Colas ; et
al. |
November 24, 2011 |
METHOD AND APPARATUS FOR LARGE FIELD OF VIEW IMAGING AND DETECTION
AND COMPENSATION OF MOTION ARTIFACTS
Abstract
A method and apparatus are provided to improve large field of
view CT image acquisition by using at least two scanning
procedures: (i) one with the radiation source and detector centered
and (ii) one in an offset configuration. The imaging data obtained
from both of the scanning procedures is used in the reconstruction
of the image. In addition, a method and apparatus are provided for
detecting motion in a reconstructed image by generating a motion
map that is indicative of the regions of the reconstructed image
that are affected by motion artifacts. Optionally, the motion map
may be used for motion estimation and/or motion compensation to
prevent or diminish motion artifacts in the resulting reconstructed
image. An optional method for generating a refined motion map is
also provided.
Inventors: |
Schretter; Colas; (Aachen,
DE) ; Bertram; Matthias; (Aachen, DE) ;
Neukirchen; Christoph; (Aachen, DE) |
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS
N.V.
EINDHOVEN
NL
|
Family ID: |
41820756 |
Appl. No.: |
13/145128 |
Filed: |
December 23, 2009 |
PCT Filed: |
December 23, 2009 |
PCT NO: |
PCT/IB2009/055951 |
371 Date: |
July 19, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61146093 |
Jan 21, 2009 |
|
|
|
Current U.S.
Class: |
378/4 ;
382/131 |
Current CPC
Class: |
G06T 2211/412 20130101;
G06T 2211/40 20130101; A61B 6/488 20130101; G06T 2207/10116
20130101; G06T 7/20 20130101; A61B 6/032 20130101; G06T 2207/20032
20130101; G06T 11/005 20130101; A61B 6/4452 20130101; A61B 6/587
20130101; A61B 6/4085 20130101; A61B 6/4233 20130101; G06T 5/20
20130101; A61B 6/542 20130101; A61B 6/5217 20130101 |
Class at
Publication: |
378/4 ;
382/131 |
International
Class: |
A61B 6/03 20060101
A61B006/03; G06K 9/00 20060101 G06K009/00 |
Claims
1. An apparatus for acquiring tomographic projection data at a
plurality of angular positions relative to an object disposed in an
examination region, the apparatus comprising: a radiation source; a
radiation sensitive detector which detects radiation emitted by the
source that has traversed the examination region; and a
reconstructor; wherein the apparatus is adapted to perform at least
two scanning procedures of an object; wherein at least a first
scanning procedure is a centered geometry scanning procedure,
wherein a center of the radiation sensitive detector is aligned
with a center of rotation of the source and detector; wherein at
least a second scanning procedure is an offset geometry scanning
procedure; wherein a center of the radiation sensitive detector is
displaced by a distance of approximately half a width of the
detector or more from the center of rotation of the source and
detector; wherein projection data is acquired during the at least
two scanning procedures including centered geometry projection data
during the centered geometry scanning procedure and offset geometry
projection data during the offset geometry scanning procedure; and
wherein the reconstructor reconstructs the projection data acquired
during the at least two scanning procedures together to generate
volumetric data indicative of the object.
2. (canceled)
3. The apparatus of claim 1, wherein the reconstruction performed
by the reconstructor combines the projection data acquired during
the at least two scanning procedures to form a data set
corresponding to an imaging scan performed by a single virtual
detector in one position with respect to the source.
4. The apparatus of claim 1, wherein the radiation sensitive
detector is a flat detector, and wherein the center of the
radiation sensitive detector is transversely displaced from the
center of rotation in the transaxial plane during the at least one
offset geometry scanning procedure.
5. (canceled)
6. The apparatus of claim 1, wherein a faded weighting technique
and an averaging technique are applied to an overlap region of the
projection data acquired during the at least two scanning
procedures during reconstruction.
7. The apparatus of claim 1, wherein a lower dose of radiation is
administered during the at least one offset geometry scanning
procedure than during the at least one centered geometry scanning
procedure.
8. The apparatus of claim 7, wherein the dose of radiation
administered during the at least one offset geometry scanning
procedure is less than half of the radiation administered during
the at least one centered geometry scanning procedure.
9. The apparatus of claim 1, wherein the apparatus is a cone-beam
computed tomography imaging device.
10. The apparatus of claim 1, further comprising a mechanical drive
for moving the radiation sensitive detector with respect to the
radiation source.
11. The apparatus of claim 1, further comprising an image
processor, a user interface and a user input, and wherein the image
processor processes the volumetric data for display on the user
interface.
12. A computed tomography imaging method, comprising the steps of:
performing at least two scanning procedures of an object,
including: acquiring projection data during at least one centered
geometry scanning procedure in which a center of the radiation
sensitive detector is aligned with a center of rotation of the
source and detector; and acquiring projection data during at least
one offset geometry scanning procedure in which a center of the
radiation sensitive detector is displaced from the center of
rotation of the source and detector by a distance of approximately
half a width of the detector or more; acquiring projection data
during the at least two scanning procedures including centered
geometry projection data during the centered geometry scanning
procedure and offset geometry projection data during the offset
geometry scanning procedure; and reconstructing the projection data
acquired during the at least two scanning procedures together to
generate volumetric data indicative of the object.
13. (canceled)
14. The method of claim 12, wherein the reconstruction combines the
projection data acquired during the at least two scanning
procedures to form a data set corresponding to an imaging scan
performed by a single virtual detector in one position with respect
to the source.
15. The method of claim 12, wherein the radiation sensitive
detector is a flat detector, and further comprising transversely
displacing the center of the radiation sensitive detector from the
center of rotation in the transaxial plane during the at least one
offset geometry scanning procedure.
16. (canceled)
17. The method of claim 12, wherein a faded weighting technique and
an averaging technique are applied to an overlap region of the
projection data acquired during the at least two scanning
procedures during reconstruction.
18. The method of claim 12, further comprising administering a
lower dose of radiation during the at least one offset geometry
scanning procedure than during the at least one centered geometry
scanning procedure.
19. The method of claim 18, wherein the dose of radiation
administered during the at least one offset geometry scanning
procedure is less than half of the radiation administered during
the at least one centered geometry scanning procedure.
20. An apparatus for generating a motion map, the apparatus
comprising: a radiation source; a radiation sensitive detector
which detects radiation emitted by the source that has traversed an
examination region; and a reconstructor an image processor; wherein
the radiation source and the radiation sensitive detector are used
to acquire projection data at a plurality of angular positions
relative to an object disposed in the examination region; wherein
the reconstructor is used to generate a reference image from the
projection data; wherein reference projection data is obtained from
a forward projection of the reference image; wherein differences
between the acquired projection data and the reference projection
data are computed to determine line integral differences; and
wherein the image processor uses the line integral differences to
generate a motion map indicative of the regions of a corresponding
image reconstructed from the projection data that are affected by
motion.
21. The apparatus of claim 20, wherein the image processor applies
a windowing process to refine the motion map.
22. The apparatus of claim 20, wherein the image processor applies
a normalization process to refine the motion map.
23. The apparatus of claim 20, wherein the image processor applies
a volumetric median filter to refine the motion map.
24. The apparatus of claim 20, wherein the image processor applies
a Gaussian blur to refine the motion map.
25. The apparatus of claim 20, wherein the apparatus is a cone-beam
computed tomography imaging device.
26. The apparatus of claim 20, wherein the image processor
processes the volumetric data for display on a user interface.
27. The apparatus of claim 20, wherein the motion map indicates the
amount of motion correction to be applied to an image.
28. The apparatus of claim 27, wherein the image processor uses the
motion map in a motion-compensated image reconstruction.
29. The apparatus of claim 28, wherein the image processor performs
a reconstruction as a weighted average between a motion corrected
reconstruction and a reconstruction not corrected for motion,
wherein the weights are provided by the motion map.
30. The apparatus of claim 27, wherein the motion displacement in a
motion-corrected reconstruction is adapted according to the motion
map.
31. A method for generating a motion map, the method comprising the
steps of: acquiring projection data at a plurality of angular
positions relative to an object disposed in an examination region;
reconstructing from the projection data to generate a reference
image; obtaining reference projection data from a forward
projection of the reference image; computing differences between
the acquired projection data and the reference projection data to
determine line integral differences; and using the line integral
differences to generate a motion map indicative of the regions of a
corresponding image reconstructed from the projection data that are
affected by motion.
32. The method of claim 31, further comprising the step of applying
a windowing process to refine the motion map.
33. The method of claim 31, further comprising the steps of
refining the motion map by normalizing the motion map, applying a
volumetric median filter to the motion map, and applying a Gaussian
blur to the motion map.
34. The method of claim 31, further comprising the step of using
the motion map in conjunction with the corresponding image
reconstructed from the projection data to detect regions of the
reconstructed image that are affected by motion.
35. The method of claim 31, further comprising the step of using
the motion map in conjunction with a motion correction technique to
compensate for the effects of motion in the corresponding image
reconstructed from the projection data.
36. The method of claim 35, further comprising compensating for
motion only in regions of the corresponding image that are
indicated to have been affected by motion by the motion map.
37. The method of claim 35, further comprising compensating for
motion by applying a weighted value of motion correction to regions
of the corresponding image reconstructed from the tomographic
projection data, the weighted value being calculated for each
region based upon a quantitative amount of motion indicated for
each image region by the motion map.
Description
[0001] The present application relates generally to the imaging
arts. In one embodiment, it provides a method and apparatus for
imaging large objects. In another embodiment, it provides for the
detection and compensation of motion artifacts when reconstructing
tomographic images. The application subject matter finds use at
least with computed tomography (CT) imaging, and more particularly
with flat detector cone-beam computed tomography (CBCT) imaging,
and will be described with particular reference thereto. However,
it also has more general application with other imaging methods and
in other arts.
[0002] A conventional CT imaging device includes an x-ray source
and an x-ray sensitive detector disposed on opposite sides of an
examination region. A human patient or other object to be examined
is supported in the examination region by a suitable support. The
source emits x-ray radiation which transverses the examination
region and is detected by the detector as the source and detector
rotate about a center of rotation. A CT imaging device capable of
having an offset geometry includes an x-ray source and an x-ray
sensitive detector that may be transversely displaced from the
center of rotation in the transaxial plane in certain
configurations. Such offset geometry CT imaging devices are
desirable because they allow for an increased field of view or
allow for the use of a smaller sized detector.
[0003] However, existing offset geometry CT imaging devices may not
adequately accommodate certain large objects, such as an obese
patient. In part that is because x-ray source and detector offsets
may deteriorate the quality of the reconstructed image.
Furthermore, attenuation correction during reconstruction benefits
from full anatomical coverage, which may not be possible even with
large offsets.
[0004] Furthermore, the quality of images obtained from CT imaging
devices, especially slowly rotating CBCT imaging devices, is also
frequently degraded by uncontrolled patient movement, such as the
patient's failure to hold his or her breath, intestinal
contractions, nervous shaking, natural cyclic motion, heartbeat,
respiration, or other forms of motion. Currently, iterative
algorithmic motion compensation methods are used to improve image
quality for images that contain motion artifacts. While such
methods are capable of improving image quality for certain types of
motion, the motion compensation effects accomplished by these
methods are often inaccurate and they also can introduce artifacts
into image regions that have not been affected by any motion.
[0005] It is desirable to provide a method and apparatus that
permit a larger field of view than current CT imaging devices with
offset geometries and that alleviate the artifacts that typically
occur in reconstructed images obtained from existing CT imaging
devices with large offset geometries. Further, it is also desirable
to provide a method and apparatus for detecting image regions that
are affected by motion artifacts when reconstructing tomographic
images and for providing motion estimation and motion compensation
to prevent such motion artifacts in the resulting reconstructed
image.
[0006] Aspects of the present invention address these matters, and
others.
[0007] According to one aspect of the present invention, a method
and apparatus are provided to improve large field of view CT image
acquisition using two scanning procedures: (i) one with the
radiation source and detector centered and (ii) one with the
detector being offset. In accordance with this aspect, a large
field of view can be achieved that can accommodate larger objects
than can currently be accommodated by existing CT imaging devices
with offset geometries. In addition, as the imaging data from both
of the scanning procedures is used in the reconstruction of the
image, the artifacts that typically occur with reconstruction of
imaging data obtained from existing CT imaging devices with large
offset geometries can be avoided because of the large overlap
between virtual detectors in opposite viewing directions.
[0008] According to another aspect of the present invention, a
method and apparatus are provided for the detection and
compensation of motion artifacts when reconstructing tomographic
images. In accordance with this aspect, a method and apparatus for
creating a motion map is provided. The motion map is utilized to
indicate which image regions may be corrupted by motion artifacts
and/or for motion compensation to prevent motion artifacts in the
reconstructed tomographic image.
[0009] Still further aspects of the present invention will be
appreciated by those of ordinary skill in the art upon reading and
understanding the following detailed description. Numerous
additional advantages and benefits will become apparent to those of
ordinary skill in the art upon reading the following detailed
description of preferred embodiments.
[0010] The invention may take form in various components and
arrangements of components, and in various process operations and
arrangements of process operations.
[0011] The drawings are only for the purpose of illustrating
preferred embodiments and are not to be construed as limiting the
invention.
[0012] FIG. 1 is a transaxial view of a centered CT acquisition
geometry according to an embodiment of the invention;
[0013] FIG. 2 is a transaxial view of an offset CT acquisition
geometry according to an embodiment of the invention;
[0014] FIG. 2A is a transaxial view of a virtual detector which
results from combining the data gathered from the centered geometry
of FIG. 1 and the offset geometry of FIG. 2;
[0015] FIG. 3 is an imaging system according to an embodiment of
the invention; and
[0016] FIG. 4 depicts an imaging method according to an embodiment
of the invention;
[0017] FIG. 5 depicts a method for detecting motion according to an
embodiment of the invention;
[0018] FIGS. 6A and 6B depict optional methods for refining a
motion map in accordance with an embodiment of the present
invention;
[0019] FIG. 7 is exemplary image generated by a software program
depicting a motion map in accordance with an embodiment of the
present invention;
[0020] FIG. 8 is an exemplary image generated by a software program
depicting a motion-corrupted reconstructed image without any motion
correction;
[0021] FIG. 9 is an exemplary image generated by a software program
depicting the reconstructed image of FIG. 8 after a global motion
correction; and
[0022] FIG. 10 is an exemplary image generated by a software
program depicting the reconstructed image of FIG. 8 after a local
motion correction.
[0023] One aspect of the present invention is directed generally to
a method and apparatus for CT image acquisition, and more
particularly to a method and apparatus for providing a large field
of view ("FOV") with improved image quality by utilizing at least
two scanning procedures taken by a CT image apparatus. At least one
scan is taken with the radiation source and detector of the CT
image apparatus in a centered geometry and at least one scan is
taken with the detector and/or source in an offset geometry. The
image data obtained from the at least two scanning procedures is
then combined to produce a reconstructed image.
[0024] FIG. 1 depicts an exemplary centered geometry 100 for a CT
imaging apparatus. The exemplary centered geometry 100 has an x-ray
source 102, such as an x-ray tube, and an x-ray sensitive detector
104, such as a flat panel area detector array extending in the
transverse and axial directions. As illustrated in FIG. 1, the
center of rotation 114 may also serve as the center of the
transverse field of view (FOV) 118.
[0025] However, the center of rotation 114 is not necessarily
always aligned with the center of the transverse FOV 118 in every
application. As illustrated, an object support 110 supports the
object 108 under examination in an examination region 106. A
central ray or projection 116 of the x-ray beam 112 is
perpendicular to the detector center 119, which is aligned with the
center of rotation 114.
[0026] The x-ray source 102 and the x-ray sensitive detector 104
rotate about the center of rotation 114. The source 102 and
detector 104 are generally mounted to a rotating gantry (not shown)
for rotation about the examination region 106. In some embodiments,
however, the source 102 and detector 104 may remain at a constant
angular position while the object 108 is moved and/or rotated to
produce the requisite angular sampling. While the figures and
description are focused on the use of flat panel detectors, arcuate
detectors or detectors having yet other shapes may also be used.
Furthermore, while the figures and discussion focus on a CT system
in which the source 102 is a point source, other alternatives are
contemplated. For example, the source 102 may be a line source.
Gamma and other radiation sources may also be used. Multiple
sources 102 and detectors 104 may also be provided, in which case
corresponding sets of sources and detectors may be offset angularly
and/or longitudinally from one another.
[0027] In FIG. 1, the x-ray source 102 and detector 104 of the
exemplary centered geometry 100 are depicted in two opposing
positions in the transaxial plane, position A in solid lines and
position B in dotted lines. In position B, the x-ray source 102 and
detector 104 are rotated 180 degrees about the center of rotation
114 from position A. As both the x-ray source 102 and detector 104
of the exemplary centered geometry 100 are centered with respect to
the center of rotation 114, the central ray 116 of the x-ray beam
112 and the detector center 119 are aligned with the center of
rotation 114 when the x-ray source 102 and detector 104 are in both
position A and position B.
[0028] FIG. 2 depicts an exemplary offset geometry 200 for an
imaging apparatus. The detector center 119 of the detector 104 of
the exemplary offset geometry 200 is transversely displaced or
offset from the center of rotation 114 in the transaxial plane by a
distance D. As described previously in connection with the centered
geometry 100, the x-ray source 102 and the x-ray sensitive detector
104 of the offset geometry 200 rotate about the center of rotation
114. In FIG. 2, the x-ray source 102 and detector 104 of the
exemplary offset geometry 100 are depicted in two opposing
positions in the transaxial plane, position A in solid lines and
position B in dotted lines. In position B, the x-ray source 102 and
detector 104 are rotated 180 degrees about the center of rotation
140 from position A. As illustrated in FIG. 2, the detector center
119 is offset from the center of rotation 114 in the transaxial
plane by a distance D in both position A and position B.
[0029] The transverse FOV 218 of the offset geometry 200 is larger
than the transverse FOV 118 of the centered geometry 100. The
detector center 119 may be offset from the center of rotation 114
in the transaxial plane by various distances in different
embodiments of the present invention by varying the distance D. For
example, the detector center 114 may be offset from the center of
rotation 119 by a distance D between 0 and 35 centimeters or
greater. The distance D may approximate, or even exceed, the
transverse half-width of the detector, so that there is a "hole"
222 in the center of the transverse FOV 218. The distance D may be
varied in multiple ways to customize the size of the transverse FOV
218. The detector 104 may be shifted to vary the size of the
transverse FOV 118 by any suitable means. For example, the detector
104 may be moved in various directions relative to the rotating
gantry and the center of rotation 114 either manually by a human
user or by a mechanical drive. It can be shifted linearly, as is
useful with a flat panel detector, or rotationally, as is useful
for a curved detector. While the exemplary offset geometry 200
described includes a centered source and an offset detector, it
should be understood that additional CT imaging device geometries,
which include an offset source or an offset source and an offset
detector are contemplated.
[0030] FIG. 2A depicts an overlay of the exemplary centered
geometry 100 and the exemplary offset geometry 200. In FIG. 2A, the
x-ray source 102 and detector 104 of the exemplary centered
geometry 100 and exemplary offset geometry 200 are overlaid each
other in two opposing positions in the transaxial plane, position A
in solid lines and position B in dotted lines. The area of the
detector 104 of the exemplary centered geometry 100 in position A
that overlaps with the detector 104 of the exemplary offset
geometry 200 in position A is indicated by the cross-hatched
section 220 in FIG. 2A. Likewise, there is also an overlapping
region 220 between the detector 104 of the exemplary centered
geometry 100 in position B and the detector 104 of the exemplary
offset geometry 200 in position B. During image reconstruction, the
projection data obtained from the exemplary centered geometry 100
and exemplary offset geometry 200 can be combined together, as if
they were measured by a single larger virtual detector V. This may
be accomplished, for example, with faded weighting and/or averaging
the projection data obtained in the overlapping region 220. In
additional embodiments, projection data may not be obtained from a
centered geometry and an offset geometry, but, rather, projection
data may be obtained from two different offset geometries. For
example, projection data could be obtained from a scan taken with
the detector center 114 offset from the center of rotation 119 by a
first distance D and a second set of projection data could be
obtained from another scan taken with the detector center 114
offset from the center of rotation 199 by a second distance D.
[0031] FIG. 3 depicts a CT imaging system 300 suitable for use with
the exemplary centered geometry 100 and offset geometry 200
described above. The CT imaging system 300 includes a CT data
acquisition system 302, a reconstructor 304, an image processor
306, a user interface 308, and a user input 310. The CT data
acquisition system 302 includes the source 102 and detector 104,
which are mounted to a rotating gantry 312 for rotation about the
examination region. Circular or other angular sampling ranges as
well as axial, helical, circle and line, saddle, or other desired
scanning trajectories are contemplated. The embodiment of the CT
imaging device system 300 illustrated in FIG. 3 includes a drive
318, such as a microstep motor, that provides the requisite force
required to move the source 102 and/or detector 104.
[0032] The reconstructor 304 reconstructs the data generated by the
data acquisition system 302 using reconstruction techniques to
generate volumetric data indicative of the imaged subject.
Reconstruction techniques include analytical techniques such as
filtered backprojection, as well as iterative techniques. The image
processor 306 processes the volumetric data as required, for
example for display in a desired fashion on the user interface 308,
which may include one or more output devices such as a monitor and
printer and one or more input devices such as a keyboard and
mouse.
[0033] The user interface 308, which is advantageously implemented
using software instructions executed by a general purpose or other
computer so as to provide a graphical user interface ("GUI"),
allows the user to control or otherwise interact with the imaging
system 300, for example by selecting a desired FOV configuration or
dimension, initiating and/or terminating scans, selecting desired
scan or reconstruction protocols, manipulating the volumetric data,
and the like.
[0034] A user input 310 operatively connected to the user interface
308 controls the operation of the CT data acquisition system 302,
for example to carry out a desired scanning protocol, optionally
position the detector 104 and/or the source 102 so as to provide
the desired FOV, and the like.
[0035] An exemplary imaging process 400 according to one aspect of
the present invention is illustrated in FIG. 4. In step 402, the CT
imaging system 300 is utilized to take at least one scan of the
imaged subject with the source 102 and detector 104 in the centered
geometry 100 to acquire projection data at a sufficient plurality
of angular positions about the examination region 106. In step 404,
at least one scan is taken by the CT imaging system 300 with the
source 102 and detector 104 in an offset geometry 200. The order of
steps 402 and 404 may be reversed, as the first scan(s) may be
taken with the CT imaging system 300 in the offset geometry 200
followed by scan(s) with the CT imaging system 300 in the centered
geometry 100. As discussed above, the detector 104 and/or the
source 102 of the offset geometry 200 may be offset by a variety of
distances D from the center of rotation 114 in the transaxial
plane. In additional embodiments, one or more scanning procedures
may be conducted with the detector 104 and/or the source 102 offset
from the center of rotation 114 in the transaxial plane by a
different distance D in each scan.
[0036] As shown in FIG. 4, centered geometry projection data 406 is
obtained from the centered geometry scan(s) of step 402 and offset
geometry projection data 408 is obtained from the offset geometry
scan(s) of step 404. The reconstructor 304 reconstructs the
centered geometry projection data 406 and offset geometry
projection data 408 at step 410 using known reconstruction
techniques currently used in connection with offset geometry CT
imaging devices to generate volumetric data indicative of the
imaged subject 108, i.e., reconstructed image data 412. During
reconstruction, the centered geometry projection data 406 and the
offset geometry projection data 408 are pair-wised stitched
together using the overlapping region between the projection data
406 and 408 resulting from the overlapping region 220 of the
detector 104 for the registration of the projection data 406 and
408 with each other. Faded weighting and/or averaging may be
optionally applied in the overlap regions of the centered geometry
projection data 406 and the offset geometry projection data 408
during the reconstruction process. The combined reconstruction of
the projection data 406 and 408 emulates a single scan with the
large virtual detector V illustrated in FIG. 2A.
[0037] The reconstructed image data 412 obtained from step 410 is
processed by the image processor 306. The resulting reconstructed
image is displayed on the user interface 308 at step 414.
[0038] Existing CT imaging devices with offset geometries often
suffer from image quality problems due to a limited data redundancy
between opposite viewing directions, especially if the detector
offset is large. Insufficient redundancies can visibly degrade
image quality during reconstruction. These image degrading effects
encountered with existing CT imaging devices utilizing offset
geometries are largely avoided with the apparatus and method
disclosed herein, because an even larger field of view can be
achieved while significant redundancy between opposite virtual
enlarged views is nevertheless guaranteed. Specifically, an
"overlap" between opposite virtual enlarged views of half the
actual detector width can easily be achieved, minimizing the
likelihood and effect of artifacts occurring as a result of
approximations made in the reconstruction for an off-center
geometry.
[0039] The fact that the image acquisition method disclosed herein
involves the usage of at least two scanning operations provides
certain freedom with the distribution of radiation dosage during
the scanning procedures. Different levels of radiation dosage may
be associated with each of the scan(s) of step 404 and 402 as
desired by the operator of the CT imaging device 300. For example,
the offset geometry scan(s) of step 404 may be adapted to deliver
less than half of the radiation dosage that is used in connection
with the centered geometry scan(s) of step 402. Such dosage
techniques can result in a better contrast-to-noise ratio being
obtained for the centered geometry scan(s) of step 402. At the same
time, the border areas of the imaged subject scanned by the offset
geometry scan(s) of step 404, which are less relevant for medical
diagnosis but useful for attenuation correction, will be exposed to
relatively less radiation. In this manner, the radiation dosage
delivered to a patient during the scanning procedures of steps 402
and 404 can be tailored to be generally equivalent or less than the
radiation dosage delivered to a patient during a single scan with a
wide detector, such as those used in helical CT imaging.
[0040] Another aspect of the present invention is directed
generally to a method and apparatus for the detection, estimation
and/or compensation of motion artifacts encountered when
reconstructing tomographic images. In accordance with this aspect,
a method and apparatus are provided for generating a motion map.
The motion map is utilized to indicate which image regions may be
corrupted by motion artifacts and/or for motion estimation and
motion compensation to prevent or diminish motion artifacts in the
reconstructed tomographic image.
[0041] An exemplary method 500 of detecting motion in reconstructed
tomographic images according to one aspect of the present invention
is illustrated in FIG. 5. At step 502, the CT imaging system 300 is
used to obtain a set of acquired projection data 504 of the imaged
subject 108. At step 506, tomographic reconstruction is applied to
this acquired projection data 504 using known reconstruction
techniques, such as filtered backprojection (FBP), to generate a
reconstructed image (i.e., the "reference" image) 508. The
reference image 508 may have artifacts as a result of object
movement during the scanning process. At step 510, known forward
projection techniques are applied to the previously reconstructed
reference image 508 to derive reference projection data 512.
Although the computation of reference projection data by the
forward projection of a reconstructed image is a conventional
aspect of iterative image reconstruction, it should be understood
by those skilled in the art that the accuracy of image space
interpolations and the possible truncation of projections are two
important potential issues that may need to be addressed during
this process. Furthermore, if the reference image 508 is
reconstructed using the classical Feldkamp-Davis-Kress (FDK)
algorithm, cone beam artifacts may corrupt the reference
projections and hence should be accounted for.
[0042] Next, at step 514, the line integral differences 516 between
the acquired projection data 504 and the reference projection data
512 are computed. Any such differences likely result from artifacts
caused by movement of the object during the imaging scan 502. The
line integral differences 516 between the acquired projection data
504 and the reference projection data 512 are computed
independently for each pair of corresponding projections from the
acquired projection data 504 and reference projection data 512. A
data correction step could be optionally employed at this stage
using, for example, the Helgason-Ludwig conditions or other similar
data correction measures to correct any data inconsistencies. The
line integral differences 516 represent an isolation of motion that
occurred during the scanning procedure 502 in projection space.
[0043] At step 518, tomographic reconstruction is applied to the
absolute values of the line integral differences 516 using known
reconstruction techniques, such as filtered backprojection (FBP).
The resulting image that is generated is a motion map 520, which is
representative of the regions of the image 508 which are corrupted
by motion that occurred during the scanning procedure 502. Thus the
motion map 520 represents an isolation of the motion that occurred
during the scanning procedure 502 in image space. The motion map
520 could be adapted to be a binary motion map that simply
indicates whether or not motion exists in a given image voxel.
Alternatively, a refined motion map 520 could indicate the
amplitude of the motion that exists in any given image voxel. An
exemplary motion map 520 is illustrated in FIG. 7. It should be
understood by those skilled in the art that the exemplary method
500 of generating a motion map 520 may be an iterative process in
additional embodiments.
[0044] An optional exemplary method 600 for refining the motion map
520 in accordance with an embodiment of the present invention is
illustrated in FIG. 6A. At step 602, the line integral differences
516 may be processed or refined, such as for example by windowing,
normalization, or filtering, to produce pre-processed line integral
differences 604. A windowing refinement is a non-linear mapping of
input values to modified output values, where input values below a
given minimum value and above a given maximum value are ignored or
set to zero. As a special form of windowing, thresholding may be
applied, where input values below a given threshold are set to zero
and values above the threshold are set to one. Another sort of
refinement is normalization, wherein the line integral differences
are transformed to values between 0 and 1 to standardize and
simplify subsequent mathematical calculations. Yet another sort of
refinement is to apply a volumetric median filter, a Gaussian blur,
or some other filtering process. In one exemplary embodiment, the
size of the neighborhood for the volumetric median filter and the
size of the convolution kernel for the Gaussian blur are set to
3.times.3.times.3. The pre-processing refinement 602 may also
involve other kinds of image processing in additional
embodiments.
[0045] The pre-processed line integral differences 604 are
reconstructed using known reconstruction techniques, such as
filtered backprojection (FBP), at step 606. The resulting image
that is generated is a refined motion map 608, that has been
windowed, normalized, filtered, or otherwise refined. The refined
motion map 608 could be adapted to be either a binary motion map
that simply indicates whether or not motion exists in a given image
voxel or the refined motion map 608 could indicate the amplitude of
the motion that exists in any given image voxel.
[0046] Another optional exemplary method 610 for refining the
motion map 520 in accordance with an embodiment of the present
invention is illustrated in FIG. 6B. At step 612, the line integral
differences 516 are reconstructed using known reconstruction
techniques, such as filtered backprojection (FBP). The resulting
image that is generated is an initial motion map 614. The initial
motion map 614 is then processed or refined at step 616, such as
for example by windowing, normalization, filtering, to produce a
post-processed refined motion map 618. For example, in one
exemplary embodiment the initial motion map 614 is thresholded at
150 Hounsfield units (HU). Such processing or refining of the
motion map 614 serves to remove "reconstruction noise" or other
inconsistencies in the data and to avoid streaking
[0047] A motion map such as the motion map 520, 608 or 618 has
multiple uses. For example, the motion map can be used as a
reference by a radiologist or other individual performing the
imaging process to indicate which voxels of a particular
reconstructed image could potentially contain reconstruction
artifacts due to motion, e.g., regions of an image with potential
motion artifacts that cause them to be unsuitable for diagnosis or
localization. In this manner, the motion map serves as a
reliability indicator to be used in conjunction with a
reconstructed image, as it supplies information about the location
of in-scan motion present in the reconstructed image.
[0048] In addition, the motion map can be combined with a motion
estimation and compensation scheme to apply local motion correction
during image reconstruction. Conventional global motion
compensation techniques are applied universally to the entire image
during the reconstruction process. This can result in artifacts
being introduced into regions of the reconstructed image which were
not affected by any motion. As a result, in practice, these global
motion compensation methods can corrupt static regions of
reconstructed images with artifacts resulting from incorrect motion
compensation.
[0049] However, the use of the motion map in conjunction with local
motion correction prevents the application of motion compensation
in static regions where no motion occurred during the scanning
procedure. This can prevent artifacts in such static regions. For
example, the motion map could be used as a "blending map" with
motion correction techniques being applied only in those areas
which are indicated to have experienced motion based upon the
motion map. Furthermore, the motion map could also be used as a
"weighting map." Under this approach, the motion map would be used
to determine a "weighted" amount of motion correction that would be
applied to any given image voxel, which would be an adjusted value
between zero motion correction being applied and, at most, the
amount of motion correction that would be applied under current
conventional global motion correction techniques. Still further
uses and applications of the motion map will be appreciated by
those of ordinary skill in the art. To further illustrate the
application of the motion compensation techniques described herein,
an exemplary motion-corrupted image generated by a software program
is illustrated in FIG. 8. FIG. 9 is a reconstruction of the image
in FIG. 8 that has undergone global motion correction. FIG. 10 is a
reconstruction of the image in FIG. 8 that has undergone local
motion correction using a motion map.
[0050] The aforementioned functions, such as for example, selecting
a desired FOV configuration or dimension, initiating and/or
terminating scans, selecting desired scan or reconstruction
protocols, manipulating the volumetric data, and the like, can be
performed as software logic. "Logic," as used herein, includes but
is not limited to hardware, firmware, software and/or combinations
of each to perform a function(s) or an action(s), and/or to cause a
function or action from another component. For example, based on a
desired application or needs, logic may include a software
controlled microprocessor, discrete logic such as an application
specific integrated circuit (ASIC), or other programmed logic
device. Logic may also be fully embodied as software.
[0051] "Software," as used herein, includes but is not limited to
one or more computer readable and/or executable instructions that
cause a computer or other electronic device to perform functions,
actions, and/or behave in a desired manner. The instructions may be
embodied in various forms such as routines, algorithms, modules or
programs including separate applications or code from dynamically
linked libraries. Software may also be implemented in various forms
such as a stand-alone program, a function call, a servlet, an
applet, instructions stored in a memory, part of an operating
system or other type of executable instructions. It will be
appreciated by one of ordinary skill in the art that the form of
software is dependent on, for example, requirements of a desired
application, the environment it runs on, and/or the desires of a
designer/programmer or the like.
[0052] The systems and methods described herein can be implemented
on a variety of platforms including, for example, networked control
systems and stand-alone control systems. Additionally, the logic,
databases or tables shown and described herein preferably reside in
or on a computer readable medium, such as a component of the
imaging system 300. Examples of different computer readable media
include Flash Memory, Read-Only Memory (ROM), Random-Access Memory
(RAM), programmable read-only memory (PROM), electrically
programmable read-only memory (EPROM), electrically erasable
programmable read-only memory (EEPROM), magnetic disk or tape,
optically readable mediums including CD-ROM and DVD-ROM, and
others. Still further, the processes and logic described herein can
be merged into one large process flow or divided into many
sub-process flows. The order in which the process flows herein have
been described is not critical and can be rearranged while still
accomplishing the same results. Indeed, the process flows described
herein may be rearranged, consolidated, and/or re-organized in
their implementation as warranted or desired.
[0053] The invention has been described with reference to the
preferred embodiments. Modifications and alterations may occur to
others upon reading and understanding the preceding detailed
description. It is intended that the invention be constructed as
including all such modifications and alterations insofar as they
come within the scope of the appended claims or the equivalents
thereof.
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