U.S. patent application number 09/307400 was filed with the patent office on 2002-03-21 for volumetric computed tomography system for cardiac imaging.
Invention is credited to ACHARYA, KISHORE C., FOX, STANLEY H., HE, HUI DAVID, HSIEH, JIANG, HU, HUI, SUN, YI.
Application Number | 20020034276 09/307400 |
Document ID | / |
Family ID | 23189596 |
Filed Date | 2002-03-21 |
United States Patent
Application |
20020034276 |
Kind Code |
A1 |
HU, HUI ; et al. |
March 21, 2002 |
VOLUMETRIC COMPUTED TOMOGRAPHY SYSTEM FOR CARDIAC IMAGING
Abstract
The present invention, in one form, is an imaging system for
generating images of an entire object. In one embodiment, a
physiological cycle unit is used to determine the cycle of the
moving object. By altering the rotational speed of an x-ray source
as a function of the object cycle, segments of projection data are
collected for each selected phase of the object during each
rotation. After completing a plurality of rotations, the segments
of projection data are combined and a cross-sectional image of the
selected phase of the object is generated. As a result, minimizing
motion artifacts.
Inventors: |
HU, HUI; (WAUKESHA, WI)
; HSIEH, JIANG; (BROOKFIELD, WI) ; FOX, STANLEY
H.; (BROOKFIELD, WI) ; ACHARYA, KISHORE C.;
(BROOKFIELD, WI) ; HE, HUI DAVID; (WAUKESHA,
WI) ; SUN, YI; (WAUKESHA, WI) |
Correspondence
Address: |
JOHN S BEULICK
ARMSTRONG TEASDALE LLP
ONE METROPOLITAN SQUARE
SUITE 2600
ST. LOUIS
MO
631022740
|
Family ID: |
23189596 |
Appl. No.: |
09/307400 |
Filed: |
May 7, 1999 |
Current U.S.
Class: |
378/8 ;
378/4 |
Current CPC
Class: |
A61B 6/541 20130101;
G01N 23/046 20130101; G01N 2223/419 20130101; A61B 6/027 20130101;
G01N 2223/612 20130101 |
Class at
Publication: |
378/8 ;
378/4 |
International
Class: |
G21K 001/12; H05G
001/60; A61B 006/00; G01N 023/00 |
Claims
1. A method for generating an image of an object using a computed
tomography (CT) imaging system, the imaging system including at
least one x-ray detector array and at least one rotating x-ray
source projecting an x-ray beam, said method comprising the steps
of: identifying a physiological cycle of the object, the cycle
comprising a plurality of phases; selecting at least one phase of
the object; collecting at least one segment of projection data for
each selected phase of the object during each rotation of each
x-ray source; generating a projection data set by combining the
projection data segments; and generating a cross-sectional image of
the entire object from the projection data set.
2. A method in accordance with claim 1 wherein generating a
projection data set by combining the projection data segments
comprises the step of generating a projection data set for each
selected phase of the object by combining the projection data
segments collected for the selected phase.
3. A method in accordance with claim 1 wherein collecting at least
one segment of projection data for each selected phase of the
object during each rotation of each x-ray source comprises the
steps of: rotating each x-ray source a plurality of rotations; and
emitting an x-ray beam from each x-ray source toward each x-ray
detector array from a plurality of projection angles.
4. A method in accordance with claim 3 wherein collecting at least
one segment of projection data for each selected phase of the
object during each rotation of each x-ray source further comprises
the step of collecting each segment of projection data for the
selected phase from a different projection angle.
5. A method in accordance with claim 4 wherein collecting each
segment of projection data for the selected phase from different
projection angle comprises the step of altering a rotational speed
of each x-ray source.
6. A method in accordance with claim 5 wherein emitting an x-ray
beam from each x-ray source toward each x-ray detector array from
plurality of projection angles comprises the step of emitting an
x-ray beam from each x-ray source for a determined imaging temporal
period.
7. A method in accordance with claim 6 wherein the rotational speed
of each x-ray source is determined in accordance with: 4 V G = 1 (
T C ( n * R t ) ) (in revolutions per second), where: Tc is the
cardiac cycle time in seconds; n is an integer constant; and Rt is
the determined imaging temporal period in seconds.
8. A method in accordance with claim 6 wherein collecting at least
one segment of projection data for each cycle of the object
comprise the step of altering the rotational speed of each x-ray
source in accordance with: 5 V G = ( 180 + n 180 * w ) ,(in
rotations per second) where: w is a period of a physiological cycle
(in seconds), .UPSILON. is a projection angle range for a complete
projection data set (in degrees), and n is a selected integer
number of cycles to collect a complete projection data set.
9. A method in accordance with claim 6 wherein collecting at least
one segment of projection data for each cycle of the object
comprise the step of altering the rotational speed of each x-ray
source in accordance with: 6 V G = ( 360 + n 360 * w ) ,(in
rotations per second) where: w is a period of a physiological cycle
(in seconds), .UPSILON. is a projection angle range for a complete
projection data set (in degrees), and n is a selected number of
cycles to collect a complete projection data set.
10. A method in accordance with claim 1 wherein the imaging system
includes a first x-ray source, a second x-ray source, a first
detector array and a second detector array.
11. A method in accordance with claim 1 wherein selecting at least
one phase of the object comprises the steps of: selecting a first
selected phase of the object; and selecting a second selected phase
of the object.
12. A method in accordance with claim 1 wherein identifying a
physiological cycle of the object comprises the step of identifying
a physiological cycle of a heart including a systolic phase and a
diastolic phase.
13. A method in accordance with claim 1 wherein identifying a
physiological cycle of the object comprises the step of identifying
a physiological cycle of a respiratory system.
14. A method in accordance with claim 1 wherein generating a
projection data set by combining the projection data segments
comprises the steps of: rotating each x-ray source a plurality of
projection angles; collecting projection data for a plurality of
projection angles using each detector array; and rebinning the
projection data for each selected phase of the object.
15. A method in accordance with claim 1 wherein collecting at least
one segment of projection data for each selected phase of the
object during each rotation of each x-ray source comprises the
steps of: detecting an arrhythmic cycle of the object; and
collecting replacement projection data for the projection data
collected during the arrhythmic cycle.
16. A computed tomography (CT) imaging system for generating an
image of an object, said imaging system including at least one
x-ray detector array and at least one rotating x-ray source
projecting an x-ray beam, said imaging system configured to:
identify a physiological cycle of the object, the cycle comprising
a plurality of phases; allow an operator to select at least one
phase of the object; collect at least one segment of projection
data for each selected phase of the object during each rotation of
each said x-ray source; generating a projection data set by
combining said projection data segments; and generate a
cross-sectional image of the entire object from said projection
data set.
17. An imaging system in accordance with claim 16 wherein to
generate a projection data set by combining said projection data
segments, said imaging system configured to generate said
projection data set for each said selected phase of the object by
combining said projection data segments collected for the selected
phase.
18. An imaging system in accordance with claim 16 wherein to
collect at least one segment of projection data for each selected
phase of the object during each rotation of each said x-ray source,
said imaging system configured to: rotate each said x-ray source a
plurality of rotations; and emitting an x-ray beam from each said
x-ray source toward each said x-ray detector array from a plurality
of projection angles.
19. An imaging system in accordance with claim 18 wherein to
collect at least one segment of projection data for each selected
phase of the object during each rotation of each said x-ray source,
wherein, said imaging system further configured to collect each
segment of projection data for the selected phase from a different
projection angle.
20. An imaging system in accordance with claim 19 wherein to
collect each segment of projection data for the selected phase from
different projection angle, said system configured to alter a
rotational speed of each said x-ray source.
21. An imaging system in accordance with claim 20 wherein to emit
an x-ray beam from each said x-ray source toward each said x-ray
detector array from plurality of projection angles, said imaging
system configured to emit an x-ray beam from each said x-ray source
for a determined imaging temporal period.
22. An imaging system in accordance with claim 21 wherein the
rotational speed of each x-ray source is determined in accordance
with: 7 V G = 1 ( T C ( n * R t ) ) (in revolutions per second),
where: Tc is the cardiac cycle time in seconds; n is an integer
constant; and Rt is said determined imaging temporal period in
seconds.
23. An imaging system in accordance with claim 21 wherein to
collect at least one segment of projection data for each cycle of
the object, said imaging system configured to alter the rotational
speed of each said x-ray source in accordance with: 8 V G = ( 180 +
n 180 * w ) ,(in rotations per second) where: w is a period of a
physiological cycle (in seconds), .UPSILON. is a projection angle
range for a complete projection data set (in degrees), and n is a
selected integer number of cycles to collect a complete projection
data set.
24. An imaging system in accordance with claim 21 wherein to
collect at least one segment of projection data for each cycle of
the object, said imaging system configured to alter the rotational
speed of each said x-ray source in accordance with: 9 V G = ( 180 +
n 180 * w ) ,(in rotations per second) where: w is a period of a
physiological cycle (in seconds), .UPSILON. is a projection angle
range for a complete projection data set (in degrees), and n is a
selected number of cycles to collect a complete projection data
set.
25. An imaging system in accordance with claim 16 wherein said
imaging system includes a first x-ray source, a second x-ray
source, a first detector array and a second detector array.
26. An imaging system in accordance with claim 16 wherein to allow
an operator to select at least one phase of the object, said
imaging system configured to: allow the operator to select a first
selected phase of the object; and allow the operator to select a
second selected phase of the object.
27. An imaging system in accordance with claim 16 wherein to
identify a physiological cycle of the object, said imaging system
configured to identify a physiological cycle of a heart comprising
a systolic phase and a diastolic phase.
28. An imaging system in accordance with claim 16 wherein to
identify a physiological cycle of the object, said imaging system
configured to identify a physiological cycle of a respiratory
system.
29. An imaging system in accordance with claim 16 wherein to
generate a projection data set by combining the projection data
segments, said imaging system configured to: rotate each said x-ray
source a plurality of projection angles; collect projection data
for a plurality of projection angles using each said detector
array; and rebin the projection data for each selected phase of the
object.
30. An imaging system in accordance with claim 16 wherein to
collect at least one segment of projection data for each selected
phase of the object during each rotation of each said x-ray source,
said imaging system configured to: detect an arrhythmic cycle of
the object; and collect replacement projection data for the
projection data collected during said arrhythmic cycle.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to computed tomography (CT)
imaging and more particularly, to generating images of a moving
object.
[0002] In at least one known CT system configuration, an x-ray
source projects a fan-shaped beam which is collimated to lie within
an X-Y plane of a Cartesian coordinate system and generally
referred to as the "imaging plane". The x-ray beam passes through
the object being imaged, such as a patient. The beam, after being
attenuated by the object, impinges upon an array of radiation
detectors. The intensity of the attenuated beam radiation received
at the detector array is dependent upon the attenuation of the
x-ray beam by the object. Each detector element of the array
produces a separate electrical signal that is a measurement of the
beam attenuation at the detector location. The attenuation
measurements from all the detectors are acquired separately to
produce a transmission profile.
[0003] In at least one known type of imaging system, commonly known
as a computer tomography (CT) system, the x-ray source and the
detector array are rotated with a gantry within the imaging plane
and around the object to be imaged so that the angle at which the
x-ray beam intersects the object constantly changes. A group of
x-ray attenuation measurements, i.e., projection data, from the
detector array at one gantry angle is referred to as a "view". A
"scan" of the object comprises a set of views made at different
gantry angles during one revolution of the x-ray source and
detector. In an axial scan, the projection data is processed to
construct an image that corresponds to a two dimensional slice
taken through the object.
[0004] One method for reconstructing an image from a set of
projection data is referred to in the art as the filtered
backprojection technique. This process converts the attenuation
measurements from a scan into integers called "CT numbers" or
"Hounsfield units", which are used to control the brightness of a
corresponding pixel on a cathode ray tube display.
[0005] To reduce the total scan time required for multiple slices,
a "helical" scan may be performed. To perform a "helical" scan, the
patient is moved while the data for the prescribed number of slices
is acquired. Such a system generates a single helix from a one fan
beam helical scan. The helix mapped out by the fan beam yields
projection data from which images in each prescribed slice may be
reconstructed. In addition to reduced scanning time, helical
scanning provides other advantages such as improved image quality
and better control of contrast.
[0006] In helical scanning, and as explained above, only one view
of data is collected at each slice location. To reconstruct an
image of a slice, the other view data for the slice is generated
based on the data collected for other views. Helical reconstruction
algorithms are known, and described, for example, in C. Crawford
and K. King, "Computed Tomography Scanning with Simultaneous
Patient Translation," Med. Phys. 17(6), November/December 1990.
[0007] In order to generate images of a rapidly moving object, such
as a heart, known imaging systems have minimized motion artifacts,
caused by the movement of the heart, by utilizing a high rotational
speed gantry or by incorporating electron beam technology. However,
the high speed gantry system significantly increases the force
applied to the x-ray source and the detector affecting performance
of the system. The electron beam technology requires a very complex
design that significantly increases the cost of the scanner. As a
result, few system are capable of generating images of a moving
heart without generating images containing significant motion
artifacts.
[0008] To generate images of a moving object, it is desirable to
provide an imaging system which gathers segments of projection data
of a selected phase of the object so that by combining the segments
motion artifacts are minimized. It would also be desirable to
provide such a system which generates a cross-sectional image of
the entire object for a selected phase of the object.
BRIEF SUMMARY OF THE INVENTION
[0009] These and other objects may be attained by a CT imaging
system that generates images of an entire object of interest using
segments of projection data collected from a plurality of
projection angles for a selected phase of the object. In accordance
with one embodiment of the present invention, the imaging system
includes at least one rotating x-ray source and at least one
detector array. A physiological cycle unit, or circuit, is utilized
to generate a physiological cycle signal of the object. The cycle
signal represents the time period of each cycle of the object
including a plurality of phases. To generate an image of the object
for a selected phase, an operator selects at least one phase of the
object. For each selected phase of the object, at least one segment
of projection data is collected during each rotation of each x-ray
source.
[0010] More specifically, each segment of projection data is
generated, or collected, by emitting an x-ray beam toward an x-ray
detector array for a determined imaging temporal period for each
selected phase during each rotation. Particularly, as each x-ray
source is rotated, an x-ray beam is emitted for the determined
imaging temporal period. As a result, a segment of projection data
is collected via each detector array. Each segment represents a
small range of angular positions. By altering a rotational speed of
each x-ray source, segments of projection data are collected from
different projection angles as each x-ray source is rotated. More
particularly, the rotational speed of each x-ray source is altered
so that each segment of projection data for each selected phase of
the object is collected from a different projection angle, or range
of projection angles. By completing a plurality of rotations of
each x-ray source, projection data is collected for a projection
angle range of (180 degrees plus a fan angle).
[0011] To generate an image of the selected phase of the object,
the segments of projection data collected from the different
projection angles are combined. More specifically, the collected
segments for a selected phase of the object are combined into a set
of projection data for the selected phase. The projection data set
is then used to reconstruct a cross-sectional image of the object
for the selected phase.
[0012] In alternative embodiments, the imaging system collects
segments of projection data for a plurality of phases of the object
during each rotation of each x-ray source. More specifically, after
selecting a plurality of phases, at least one segment of projection
data is collected for each selected phase of the object during each
rotation of each x-ray source.
[0013] The above described imaging system generates images of a
moving object by gathering segments of projection data for a
selected phase of the object so that motion artifacts are
minimized. In addition, the imaging system generates
cross-sectional images of the entire object for each selected phase
of the object.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a pictorial view of a CT imaging system.
[0015] FIG. 2 is a block schematic diagram of the system
illustrated in FIG. 1.
[0016] FIG. 3 is a illustration of a physiological cycle of a
heart.
DETAILED DESCRIPTION OF THE INVENTION
[0017] Referring to FIGS. 1 and 2, an imaging system 10 is shown as
a "third generation" computed tomography (CT) imaging system
including a gantry 12 having at least one rotating x-ray source 16
that projects from a focal spot 18 a beam of x-rays 20 toward a
detector array 22. X-ray beams 20 extend from source 16 along a
beam plane 24. Beam plane 24, generally referred to as the "fan
beam plane", contains the centerline of focal spot 18 and the
centerline of beam 20 of each source 16. Each x-ray beam 20 is
collimated by a collimator (not shown) to lie within in an X-Y
plane of a Cartesian coordinate system and generally referred to as
an "imaging plane". Each detector array 22 is formed by an array of
detector elements 26 which together sense the projected x-rays that
pass through a medical patient 28. Detector array 22 may be a
single slice detector or a multislice detector. Each detector
element 26 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 28. During a scan to acquire
x-ray projection data, gantry 12 and the components mounted thereon
rotate about a center of rotation, or iso-center, 30.
[0018] Rotation of gantry 12 and the operation of each x-ray source
16 are governed by a control mechanism 34 of CT system 10. Control
mechanism 34 includes an x-ray controller 36 that provides power
and timing signals to each x-ray source 16 and a gantry motor
controller 38 that controls the rotational speed and position of
gantry 12. More specifically, altering the signals supplied to
x-ray controller 36 determines when and for how long x-ray beam 20
is emitted from each x-ray source 16. Similarly, the rotational
speed of gantry 12 is determined, or altered, by supplying the
appropriate signals to gantry motor controller 38. A data
acquisition system (DAS) 40 in control mechanism 34 samples analog
data from detector elements 26 and converts the data to digital
signals for subsequent processing. A sampling rate of DAS 40 is
adjustable, or variable, so that the rate at which the data
supplied from elements 26 may be increased or decreased. An image
reconstructor 42 receives sampled and digitized x-ray data from DAS
40 and performs high speed image reconstruction. The reconstructed
image is applied as an input to a computer 44 which stores the
image in a mass storage device 46.
[0019] Computer 44 also receives commands and scanning parameters
from an operator via console 48 that has a keyboard. An associated
cathode ray tube display 50 allows the operator to observe the
reconstructed image and other data from computer 44. The operator
supplied commands and parameters are used by computer 44 to provide
control signals and information to DAS 40, x-ray controller 36 and
gantry motor controller 38. In addition, computer 44 operates a
table motor controller 52 which controls a motorized table 54 to
position patient 28 in gantry 12. Particularly, table 54 moves
portions of patient 28 through a gantry opening 56.
[0020] In one embodiment, system 10 includes a synchronization
unit, or circuit 100 to identify or determine, a physiological
cycle of the object, i.e., a heart. More specifically and in one
embodiment, circuit 100 is coupled to computer 44 and generates a
physiological cycle signal representative of the heart including a
plurality of phases of the object, e.g., a systole and a diastole
phases. System 10 utilizes the physiological signal to synchronize
the timing of the emission of x-ray beam 16, the collection rate of
projection data segments using DAS 40, and the rotational speed of
gantry 12 so that an image of the heart is generated for the
determined, or selected, phase of a heart cycle.
[0021] More specifically and in one embodiment, circuit 100
measures, or detects, the electrical activity of the heart of
patient 28 to identify or determine the cardiac phase signal for
each cycle of a patient's heart. In one embodiment, an output
signal of at least one electrode (not shown) attached to patient 28
is supplied to an electronic amplifier (not shown) within circuit
100 which generates the cardiac phase signal. For example, and as
shown in FIG. 3, the cardiac cycle signal waveform illustrates one
cardiac cycle including the systole condition, or phase, and a
diastole condition or phase, of the heart. The portion of the
signal which is labeled Q, R and S is referred to as the QRS
complex, in which the R-feature, or R-wave, is the most prominent,
highest amplitude, feature of the entire signal. In one embodiment,
the cardiac cycle determines the period of each heart cycle and the
timing of each phase of the heart. The amount of time required for
the heart to complete one cardiac cycle is identified as a cardiac
period, w, and typically is defined as beginning with a R-wave and
continuing until the occurrence of the next R-wave. In other
embodiments, the cardiac cycle signal may be generated by an EKG
subsystem or heart monitoring device as known in the art.
[0022] In one embodiment, system 10 is configured to generate a
volumetric image of an entire object within patient 28, for example
a heart (not shown), by collecting at least one portion, or
segment, of a projection data set during each rotation of gantry
12. After collecting the entire projection data set, the projection
data segments are combined and a reconstruction algorithm is used
to generate the volumetric tomographic image of the heart. More
specifically and in one embodiment, each segment of projection data
is collected for a determined time period, or temporal window for a
pre-determined, or operator selected, phase of the heart during
rotation of gantry 12. After completing a plurality rotations of
each x-ray source 16, a reconstruction algorithm stored within
image reconstructor 42 combines the projection data segments and
generates a volumetric cross-sectional image of the heart for the
determined phase.
[0023] More specifically and in one embodiment, system 10 utilizes
circuit 100 to identify the physiological cycle of the object,
i.e., a heart and a plurality of phases of the heart. An operator
then selects at least one phase of the heart to image using the
physiological signal. For example, the operator utilizes console 48
to select a systole phase of the heart. For each selected phase of
the object, at least one segment of projection data is collected
during each rotation of each x-ray source 16.
[0024] More specifically, each segment of projection data, in one
embodiment, is generated, or collected, by emitting an x-ray beam
20 toward an x-ray detector array 22 for a determined imaging
temporal period, R.sub.t, during each selected phase. Particularly,
during each rotation of each x-ray source 16, an x-ray beam is
emitted for the determined imaging temporal period. The projection
data collected via each detector array 22 during the temporal
period represents a range of angular positions. Specifically,
utilizing the cycle signal supplied from circuit 100, the emission
of x-ray beam 16 is limited to the nominal period of Rt at the
defined phase of each cycle. More specifically, utilizing the
physiological cycle signal, the emission of x-ray beam 16 is turned
on and off by altering a signal supplied to x-ray controller 36. In
one embodiment, the imaging temporal period is in a range of 10 mS
to 50 mS.
[0025] The projection data segment acquired from detector array 22
during the emission of x-ray beam 20 represents a small, or
limited, portion of angular positions within the time duration of
Rt. Particularly, the cycle signal is utilized by DAS 40 to alter
the sampling rate of each detector array 22 so that the outputs of
elements 26 are sampled only during the period of emission of x-ray
beam 20. During each subsequent rotation of each x-ray source 16, a
segment of projection data is collected for a different radial, or
projection, angle for each selected phase.
[0026] In one embodiment, a rotational speed of each x-ray source
16 is altered, or determined, so that at least one segment of
projection data is collected during each rotation of each x-ray
source 16. More specifically and in one embodiment, the rotational
speed of each x-ray source 16 is altered so that each segment of
projection data for each selected phase of the object is collected
from a different projection angle, or range of projection angles.
In one embodiment, by altering the signals supplied to gantry motor
controller 38, each x-ray source 16 is rotated a plurality of
rotations so that projection data is collected for a projection
angle range of (180 degrees plus a fan angle). For example,
segments of projection data for each selected phase of the heart
are collected from a projection angle range of 225 degrees. As a
result of each segment representing an approximate 18 degree change
in projection angle, a complete set of projection data may be
acquired in about 13 to 20 seconds, less than a single
breath-holding time, depending upon the imaging requirements.
[0027] More particularly and in one embodiment, the rotational
speed of each x-ray source 16, in rotations per second, is
determined in accordance with: 1 V G = 1 ( T C ( n * R t ) ) ,
[0028] where:
[0029] Tc is the cardiac cycle time in seconds;
[0030] n is an integer constant; and
[0031] Rt is the determined imaging temporal period in seconds.
[0032] For example, if n equals one, Tc equals 1 second and Rt
equals 0.05 seconds, the rotating speed of each x-ray source 16 is
approximately 1.05 revolutions per second or 0.95 revolutions per
second. As a result, a complete set of projection data segments is
collected in 13 to 20 seconds depending on image quality
requirements.
[0033] In one embodiment, the time required to collect a complete
set of projection data may be reduced by altering the rotational
speed of each x-ray source 16 in accordance with: 2 V G = ( 180 + n
180 * w ) ,
[0034] (in rotation per second)
[0035] where:
[0036] w is the period of a physiological cycle (in seconds),
[0037] .UPSILON. is the projection angle range for a complete
projection data set (in degrees), and
[0038] n is a selected integer number of cycles to collect a
complete projection data set.
[0039] In an alternative embodiment, the time required to collect a
complete set of projection data may be reduced by altering the
rotational speed of each x-ray source 16 in accordance with: 3 V G
= ( 180 + n 180 * w ) ,
[0040] (in rotations per second)
[0041] where:
[0042] w is the period of a physiological cycle (in seconds),
[0043] .UPSILON. is the projection angle range for a complete
projection data set (in degrees), and
[0044] n is a selected integer number of cycles to collect a
complete projection data set.
[0045] Using this method of altering the rotational speed of each
x-ray source 16, the data span within each physiological cycle is
improved from (.UPSILON./(360*V.sub.G)) seconds to
(.UPSILON./(360*V.sub.G*n)) seconds, and improvement of a factor of
n. For example, utilizing this method in a single slice CINE CT
mode, a projection data set representing one slice of projection
data is collected in (n*t) seconds. The rotation speed of each
x-ray source 16 may also be altered in accordance with this method
in a Helical mode to obtain projection data to generate a
volumetric data in one scan of the object.
[0046] More specifically and in one embodiment, in a multi-slice
helical CT scan mode, projection data is collected using a
relatively fast table speed. Using multiple rows of each detector
array 22, fast volume coverage is achieved. Particularly, z
resolution degradation is minimized when the speed of table 54, as
determined by table controller 52, is altered so that the table
speed, s, is determined in accordance with:
(i*d)/w,
[0047] where,
[0048] d is a detector row spacing of detector array 22, and
[0049] i is an integer.
[0050] For example, where d equals 5 mm, w equals 0.8 seconds and i
equals 3, the z resolution degradation is minimized when the speed
of table 54 is 18.95 mm/second.
[0051] In one embodiment, after collecting the segments of
projection data, the segments are combined into a projection data
set and a cross-sectional image of the object is generated from the
projection data set. More specifically, a projection data set is
generated for each selected phase of the object by combining the
projection data segments collected for the selected phase. For
example, where a first selected phase is a systole phase of the
heart, a first projection data set is generated by combining the
projection data segments collected from the plurality of projection
angles for the systole phase of the heart. In a similar manner, a
separate projection data set is then generated for each additional
selected phase of the object. For example, a second projection data
set may be generated by combining the projection data segments
collected from the diastole phase of the heart.
[0052] In one embodiment, the projection data set generated for a
selected phase is utilized to generate a cross-sectional image of
the entire object. More specifically and in one embodiment, a
cross-sectional, or volumetric tomographic, image is generated,
using a reconstruction algorithm stored in image reconstructor 42,
for each selected phase of the object. Each projection data set is
used to generate a separate cross-sectional image of the entire
object for each selected phase of the object. For example, using
system 10 in a single cardiac phase mode, where the operator
selects only a first phase to generate an image, the first set of
projection data is used to generate a cross-sectional image of the
entire heart in the first phase.
[0053] In one embodiment, circuit 100 also monitors each
physiological cycle to determine if an arrhythmic, or abnormal,
condition exists to determine whether the segments of projection
collected are valid. More specifically and in one embodiment,
circuit 100 determines an average cycle period by measuring the
time period of a selected number of cycles. For each completed
cycle of the object, circuit 100 compares the average cycle period
to a cycle period for the completed cycle. If the completed cycle
period exceeds the average cycle period plus or minus a tolerance,
the completed cycle is identified as an arrhythmic cycle. The
arrhythmic cycle causes the collected segments of projection data
to not represent the selected phase. In one embodiment, the
segments collected during the arrhythmic cycle are not used and
replacement segments of projection data are collected.
[0054] In an alternative embodiment defined as a multi-phase
cardiac mode, projection data sets are generated from a plurality
of selected phases of the object during each rotation of each x-ray
source 16. More specifically, segments of projection data are
collected as described above except that each x-ray source 16 is
turned on a plurality of times during each rotation so that
segments of projection data are collected from plurality of
selected phases of the object during each rotation. In another
embodiment of the multi-phase cardiac mode, segments of projection
data are collected from a plurality of segments for each selected
phase of the object during each rotation of each x-ray source
16.
[0055] In the multi-phase cardiac imaging mode, projection data is
collected for a plurality of selected cardiac phases during each
rotation of gantry 12 so that separate images are generated for
each selected cardiac phase. More specifically and in one
embodiment, the multi-phase cardiac imaging mode operates similar
to the single phase cardiac imaging mode except separate images are
generated for a plurality of phases of the heart. Initially, the
user determines, or selects, a plurality of cardiac phases to be
imaged. By altering the amount of time between each selected phase,
images of different phases of the heart may be generated, for
example for a systolic and a diastolic phase. After selecting a
plurality of phases, gantry 12 is rotated as described above. For
each rotation of gantry 12, x-ray beam 20 is emitted from source 14
toward detector array 22 for each selected cardiac phase and a
segment of projection data is collected by detector array 22 for
each selected cardiac phase. Particularly and as described above,
utilizing circuit 100, the rotational speed of gantry 12 and the
sampling rate of DAS 40 are altered so that projection data is
collected for the plurality of cardiac phases. After collecting a
complete set of projection for each cardiac phase as described
above, the reconstruction algorithm generates a volumetric image of
each selected phase of the heart.
[0056] Utilizing the above described mode and by increasing the
rotational speed of gantry 12, either the image temporal resolution
or the total scan time for a given organ coverage is significantly
improved. For example, if gantry 12 is rotated so that gantry 12
completes two complete rotation during a single cardiac cycle, the
temporal resolution is improved by 50% versus a system completing
one rotation per cardiac cycle.
[0057] In yet another embodiment of the present invention, an image
of cardiac wall motion may be generated by acquiring multiple
segments of projection data during a short period of time, for
example four seconds. In an alternative embodiment, continuous
segments of projection data is collected and the data is rebinned
for different phases of a cardiac cycle.
[0058] The above described imaging system generates images of a
moving object by gathering segments of projection data for a
selected phase of the object so that motion artifacts are
minimized. In addition, the imaging system generates
cross-sectional images of the entire object for each selected phase
of the object.
[0059] From the preceding description of various embodiments of the
present invention, it is evident that the objects of the invention
are attained. Although the invention has been described and
illustrated in detail, it is to be clearly understood that the same
is intended by way of illustration and example only and is not to
be taken by way of limitation. For example, imaging system may be
configured as a "fourth generation" system having at least one
rotating x-ray source and at least one fixed position detector
array. Accordingly, the spirit and scope of the invention are to be
limited only by the terms of the appended claims.
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