High pitch reconstruction of multislice CT scans

Details High pitch reconstruction of multislice CT scans High pitch reconstruction of multislice CT scans

2000-08-15

2002-04-30

Bruce, David V. (Department: 2882)

X-ray or gamma ray systems or devices

Specific application

Computerized tomography

C378S901000

Reexamination Certificate

active

06381297

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates generally to computed tomography (CT) imaging and more particularly to methods and apparatus for generating CT imaging data using a multi-slice imaging system.
In at least one known computed tomography (CT) imaging 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.
In known third generation CT systems, 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, or view 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. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection 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.
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 in the z-axis synchronously with the rotation of the gantry, while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a 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 reducing scan time, helical scanning provides other advantages such as better use of injected contrast, improved image reconstruction at arbitrary locations, and better three-dimensional images.
The x-ray beam is projected from the x-ray source through a pre-patient collimator that defines the x-ray beam profile in the patient axis, or z-axis. The collimator typically includes x-ray-absorbing material with an aperture therein for restricting the x-ray beam. In at least one known CT imaging system, a scanning mode and corresponding reconstruction method are implemented for 3:1 and 6:1 helical pitches. The 6:1 helical pitch mode is referred to as a “high speed” mode because volume coverage is large, and scanning is faster along z-axis than in the 3:1 helical pitch mode. However, the scanning and reconstruction techniques used for this high speed mode have not been found suitable for scanning at greater helical pitches, for example, 8:1 or higher. One of several reasons that these techniques have not been found suitable is that the 6:1 high speed mode uses conjugate sampling pairs that are, in general, no longer valid at pitches of 8:1 or more.
For explaining the problems of the known high speed mode, it will be helpful to define a number of variables and their relationship to the geometry of a CT imaging system. Let &bgr;
k
, k=1, . . . , 4 represent projection angles at which detector rows k intersect a plane of reconstruction. Also, let &bgr;
k−
, k=1, . . . , 4 represent projection angles of conjugate samples for &bgr;
k
that are &pgr; earlier, so that &bgr;
k−
=&bgr;
k
−&pgr;−2&ggr;. Similarly, let &bgr;
k+
represent projection angles of conjugate samples for &bgr;
k
that are &pgr; later so that &bgr;
k+
=&bgr;
k
+&pgr;−2&ggr;.
In fan beam geometry, the detector angle, &ggr;, is defined as an angle formed by any ray with respect to an isoray
50
, as illustrated in FIG.
4
. More particularly, &ggr;
m
=max(|&ggr;|) represents a maximum fan angle. Referring to
FIGS. 5 and 6
, the four adjacent graphs
52
,
54
,
56
,
58
represent the four adjacent detector rows of one known CT imaging system. Graph
52
represents the weighting region for detector row
1
. Graphs
54
,
56
, and
58
are for detector rows
2
,
3
, and
4
, respectively. The labeled regions in the graphs (R
1
, R
2
, . . . , R
4
) are regions in which weighting functions are applied to the projection samples. Outside these regions, all weights are equal to zero. Therefore, projection data outside these regions is not needed.
In each graph
52
,
54
,
56
,
58
, horizontal axis
60
represents the detector angle, &ggr;, and vertical axis
62
represents the projection angle, &bgr;. Therefore, samples corresponding to a fan beam at a particular view angle are represented by horizontal lines in the graphs. Referring to
FIG. 5
, a lower boundary for region R
1
(corresponding to detector row
1
) represents conjugate samples of &bgr;
3
. Therefore, the boundary is defined by &bgr;
3−
.
As shown in
FIG. 5
, in high speed acquisition at 6:1, an iso-ray of &bgr;
3−
intersects detector row
1
when row
1
is one detector-row-width away from &bgr;
1
, where &bgr;
1
is a projection angle at which row
1
crosses a plane of reconstruction. For a 6:1 helical pitch, a table of the CT imaging system travels six times a thickness of a detector in a gantry rotation of 2&pgr;. Therefore, it takes 2&pgr;/6=&pgr;/3 to travel a single detector thickness. (In
FIG. 5
, &pgr;/3 thickness is
1
division of the vertical axis.) The angular span for R
2
, R
3
and R
4
is &pgr;/3, and corresponds to a detector thickness. The lower right region defined by &bgr;
3−
of R
1
(detector row
1
) is nearly 2&pgr;/3 away from &bgr;
1
, or almost twice a detector thickness away from a point at which detector row
1
intersects the plane of reconstruction. Thus, samples acquired far away from a true sample location are used to estimate an ideal sample, which adversely affects the accuracy of the estimation. This same problem also applies for regions R
2
′ (for detector rows
2
and
4
) and R
1
for detector row
3
.
FIG. 6
represents a corresponding high-speed mode weighting pattern for 8:1 helical reconstruction, showing that the problem becomes even worse at this higher pitch.
Furthermore, the known 6:1 high speed mode reconstruction relies upon the existence of certain conjugate samples. In particular, and referring to
FIG. 5
samples from rows
2
and
4
are used to perform interpolations in high speed mode, as are samples from rows
3
and
1
. However, this mode is not suitable for scanning at helical pitches of 8:1 or higher. Referring to
FIG. 6
, it is clear that at these higher pitches, &bgr;
4−
and &bgr;
1
lines for row
1
intersect. Similarly, lines for &bgr;
1+
and &bgr;
4
for row
4
intersect. Because lines &bgr;
4−
and &bgr;
1+
should carry a weight of zero and &bgr;
1
and &bgr;
4
, should carry a weight of 1, the weights for the intersecting points cannot be determined.
Another reason that the known high speed mode has not been found suitable for 8:1 and higher pitches is that an imaging system employing a 6:1 helical pitch is configured so that &bgr;
1
, &bgr;
2
, &bgr;
3
, and &bgr;
4
are spaced &pgr;/3 apart, while 2&ggr;
m
is slightly less than &pgr;/3. When an 8:1 helical pitch is employed, &bgr;
1
, &bgr;
2
, &bgr;
3
, and &bgr;
4
are spaced &pgr;/4

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