X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2000-12-28
2002-07-02
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S004000, C378S901000, C382S131000
Reexamination Certificate
active
06415013
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to multislice computed tomography (CT) imaging apparatus and methods, and more particularly to methods and apparatus for backprojecting attenuation data acquired during a scan to form an image.
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.
More specifically, and referring to
FIGS. 1 and 2
, one known computed tomography (CT) imaging system
10
includes a gantry
12
representative of a “third generation” CT scanner. Gantry
12
has an x-ray source
14
(or more generally, a radiation source
14
) that projects a beam of x-rays
16
(or more generally, a beam of radiation) toward a detector array
18
on the opposite side of gantry
12
. Detector array
18
is formed by detector elements
20
which 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
. Detector array
18
may be fabricated in a single slice or multislice configuration. In a multislice configuration, detector array
18
has a plurality of rows of detector elements
20
, only one of which is shown in FIG.
2
.
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
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 t o DAS
32
, x-ray controller
28
and gantry motor controller
30
. In addition, computer
36
operates a table motor controller
44
which control s a motorized table
46
to position patient
22
in gantry
12
. Particularly, table
46
moves portions of patient
22
through gantry opening
48
.
In embodiments of imaging system
10
that employ detector arrays
18
having only a single row, only fan-beam backprojection is utilized for reconstruction of images.
Referring to the planar geometry of
FIG. 3
, the basic geometry for backprojecting a set of corrected and filtered detector samples R(&bgr;
i
,&ggr;
j
) for a particular pixel is shown. The pixel is located at coordinates (x
k
,Y
l
) and the gantry angle is &bgr; (for view number i). D is the distance from x-ray source
14
to isocenter
24
, and &ggr; is the angle between a ray passing through isocenter
24
and a ray passing through the pixel.
Fan-beam backprojection of the detect or samples R(&bgr;
i
,&ggr;
j
) in one known imaging system is accomplished in three steps for each pixel. First, the value of the filtered detector pixel must be calculated for the ray passing through the pixel. The angle &ggr; is calculated, using interpolation between filtered detector samples that lie to each side of the angle &ggr; to yield a precise filtered projection value R(&bgr;
i
,&ggr;). Second, the distance L
1
between x-ray source
14
and the pixel (x
k
,Y
l
) is calculated, and finally the value R(&bgr;
l
, &ggr;)/(L
1
)
2
is calculated and added to the pixel value.
To simplify these calculations, an alternative coordinate system defined by s and t is used rather than x and y. The s axis lies a long a line from source
14
to isocenter
24
while the t axis passes through isocenter
24
at a right angle to the s axis. Thus, the s and t coordinates are simply the x and the y coordinates rotated by the gantry angle. Values for s are positive from isocenter
24
towards detector array
18
and negative from isocenter
24
towards source
14
. Values for t are positive for positive &ggr; and negative for negative &ggr;.
The value of &ggr; using this coordinate system is written:
γ
=
tan
-
1
(
t
D
+
s
)
.
Rarely will &ggr; exactly equal one of the discrete y
j
values that correspond to one of the detector samples R(&bgr;
i
,&ggr;
j
). Instead, &ggr; will usually lie between two &ggr;
j
values. In this general case, linear interpolation is used to approximate the actual value of R(&bgr;
i
,&ggr;).
Using the Pythagorean theorem, the value of L is written:
L={square root over (t
2
+L +(
D+s
+L )
2
+L )}.
The values of &ggr; and L are calculated directly by a pipeline CORDIC processor of a type described in U.S. Pat. No. 4,896,287 and in J. E. Volder, “The CORDIC Trigometric Computing Technique,”
IRE Transactions on Electronic Computers,
September 1959, pp. 330-334. The CORDIC (COordinate Rotation DIgital Computing) algorithm is an efficient algorithm that computes certain transcendental functions. The algorithm is time-efficient because it replaces multiplication and division operations by shift operations, leaving additions as the only costly computation. The CORDIC processor accepts and inputs two values A and B and makes the following computations:
&thgr;=tan
−1
(
B/A
);
and
r=k{square root over (A
2
+B
2
+L )};
where
k
=
∏
i
=
0
n
-
2
1
+
2
-
2
i
,
through a series of vector rotations of angle and scaling steps. By substituting pixel coordinate t for input B and the pixel coordinate D+s for input A, the CORDIC processor computes the values of &ggr; and kL directly. Inputs t and D+s are prescaled by dividing by k in one embodiment so that the desir
Hsieh Jiang
Metz Stephen W.
Wang Sharon X.
Armstrong Teasdale LLP
Dunn Drew A.
GE Medical Systems Global Technology Company LLC
Horton Esq. Carl B.
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