Low-cost, multislice CT detector with multiple operating modes

Details Low-cost, multislice CT detector with multiple operating modes Low-cost, multislice CT detector with multiple operating modes

2000-12-12

2004-03-02

Church, Craig E. (Department: 2882)

X-ray or gamma ray systems or devices

Specific application

Computerized tomography

C250S370090

Reexamination Certificate

active

06700948

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates generally to detectors for computed tomography (CT) imaging systems, and more particularly to optimizations of such detectors for medical and other applications and to imaging systems using such optimized detectors.
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. In another mode of operation of the CT imaging system, a helical scan is used to obtain projection data for images.
More particularly, and referring to
FIGS. 1 and 2
, one known computed tomograph (CT) imaging system embodiment
10
includes a gantry
12
representative of a “third generation” CT scanner. Gantry
12
has an x-ray source
14
that projects a beam of x-rays
16
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. In at least one embodiment of the present invention, detector array
18
is fabricated in a multi-slice configuration. Each detector element
20
produces an electrical signal that represents the intensity of an impinging x-ray beam. As the x-ray beam passes through a patient
22
, the bean is attenuated. During a scan to acquire x-ray projection data, gantry
12
and the components mounted thereon rotate about a center of rotation
24
.
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 to DAS 32, x-ray controller
28
and gantry motor controller
30
. In addition, computer
36
operates a table motor controller
44
which controls a motorized table
46
to position patient
22
in gantry
12
. Particularly, table
46
moves portions of patient
22
through gantry opening
48
.
Multiple slice detector arrays
18
increase the rate at which a scan of a given volume can be performed by acquiring data for several parallel image slices at the same time. For example, and referring to
FIGS. 3 and 4
, one known prior art detector array
18
includes a plurality of detector modules
50
. Each detector module includes an array of detector elements
20
. Particularly, each x-ray detector module
50
includes a plurality of scintillators
52
positioned above and adjacent corresponding photodiodes
54
, a semiconductor device
56
, and at least one flexible electrical cable
58
. Photodiodes
54
are either individual photodiodes or a multi-dimensional photodiode array. Photodiodes
54
are optically coupled to scintillators
52
and generate electrical outputs on lines
60
, wherein the outputs are representative of light output by corresponding scintillators
52
. Each photodiode
54
produces a separate electrical output
60
that is a measurement of the beam attenuation for a specific element
20
. Photodiode output lines
60
are, for example, physically located on one side of module
50
or on a plurality of sides of module
50
. As shown in
FIG. 4
, photodiode outputs
60
are located at top and bottom of the photodiode array.
Semiconductor device
56
includes two semiconductor switches
62
and
64
. Switches
62
and
64
each include a plurality of field effect transistors (FET) (not shown) arranged as a multidimensional array. Each FET includes an input line electrically connected to a photodiode output
60
, an output line, and a control line (not shown). FET output and control lines are electrically connected to flexible cable
58
. Particularly, one-half of photodiode output lines
60
are electrically connected to each FET input line of switch
62
with the remaining one-half of photodiode output lines
60
electrically connected to the FET input lines of switch
64
.
Flexible electrical cable
58
includes a plurality of electrical wires
66
connecting its ends. FET output and control lines are electrically connected to cable
58
. Particularly, each FET output and control line is wire bonded to a wire
66
of one end of cable
58
. FET output and control lines are wire bonded to wires
66
in the same manner as photodiode outputs (not shown) are wire bonded to the FET input lines (also not shown). Cables
58
are secured to detector module
50
using mounting brackets
68
and
70
.
Referring to
FIG. 5
, after mounting detector modules
50
into detector array
18
, unconnected cable
58
ends are coupled to DAS 32 so that an electrical path exists between photodiode
52
outputs and DAS 32, and so that FET control lines
72
are electrically connected to DAS 32 to enable semiconductor device FETs
74
. In a four-slice CT imaging system
10
using the prior art detector array
18
embodiment of
FIGS. 3
,
4
, and
5
, each column of detector module
50
is electrically connected to four DAS 32 channels, i.e., two channels within each flexible electrical cable
58
. (In general, an N channel system would have N channels connected to each column of detector module
50
, with N/2 channels within each flexible electrical cable
58
.) One exemplary channel is represented, in part, in FIG.
5
. DAS 32 is coupled across a rotating gantry
12
slip ring
76
to computer
36
and image reconstructor or processor
34
. Each detector element
20
includes a photodiode
54
that is coupled to a plurality of FETs
74
, only one of which i

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