Scalable detector for computed tomograph system

A multislice detector array producing an alterable quantity of slices and slice resolutions. In one embodiment, the detector array includes a detector housing, a plurality of detector modules, and a collimator. Each detector module includes a plurality of photodiodes arranged in an array of rows and columns, a switch apparatus electrically coupled to the photodiode output signals, and a decoder. The collimator is configured to separate X-ray beams so that only the focal X-ray beams are impinged upon the detector modules.

FIELD OF THE INVENTION 
This invention relates generally to computed tomograph (CT) imaging and, 
more particularly, to detectors utilized in connection with CT systems. 
BACKGROUND OF THE INVENTION 
In at least some computed tomograph (CT) imaging system configurations, 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. X-ray sources typically include x-ray 
tubes, which emit the x-ray beam at a focal spot. X-ray detectors 
typically include a collimator for collimating x-ray beams received at the 
detector. A scintillator is located adjacent the collimator, and 
photodiodes are positioned adjacent the scintillator. 
Multislice CT systems are used to obtain data for an increased number of 
slices during a scan. Known multislice systems typically include detectors 
generally known as 3-D detectors. With such 3-D detectors, a plurality of 
detector elements form separate channels arranged in columns and rows. 
Each row of detectors forms a separate slice. For example, a two slice 
detector has two rows of detector elements, and a four slice detector has 
four rows of detector elements. During a multislice scan, multiple rows of 
detector cells are simultaneously impinged by the x-ray beam, and 
therefore data for several slices is obtained. 
Multislice detectors may have multiple detector elements in the X and Z 
directions to increase spatial resolution. These elements can be separated 
by narrow gaps of only a few mils between adjacent elements. The gaps are 
filled with a light reflecting material. The detector elements could 
accept off-axis, or scattered, x-ray beams which decrease contrast 
resolution. 
Accordingly, it would be desirable to provide a detector array that 
collimates and separates the x-ray beams toward individual detector 
elements. In addition, it is desirable to provide a detector array 
collimator that protects the gaps between the elements from x-rays so that 
radiation damage of the reflecting material is minimized. It is also 
desirable to provide a detector array collimator that reduces penetration 
of the x-rays towards the photodiodes. 
SUMMARY OF THE INVENTION 
These and other objects may be attained by a detector array, which in one 
embodiment, enables modification of the quantity of slices and slice 
resolution, or slice thickness. The detector array includes a detector 
housing, a plurality of detector modules and a collimator. Each detector 
module is mounted to the detector housing and includes a photodiode array 
optically coupled to a scintillator array. The photodiode array includes a 
plurality of photodiodes arranged in rows and columns. The collimator is 
aligned and positioned adjacent to the scintillator array and separates 
the X-ray beams so that the X-ray beams that pass through the collimator 
correspond to the scintillator array. 
Each detector module further includes a switch apparatus and a decoder. The 
switch apparatus is electrically coupled between the photodiode output 
lines and a CT system data acquisition system (DAS). The switch apparatus, 
in one embodiment, is an array of field effect transistors (FETs) and 
alters the number of slices and the thickness of each slice by allowing 
each photodiode output line to be enabled, disabled, or combined with 
other photodiode output lines. 
In one embodiment, each detector module is fabricated by depositing, or 
forming, the photodiode array, the switch apparatus, and the decoder on a 
substrate. Each photodiode output line is electrically connected to the 
switch apparatus. The switch apparatus output and decoder control lines 
are then electrically coupled to the first end of a flex cable. After 
installing the detector modules into the detector array, the second end of 
the flex cable is electrically connected to the DAS. 
The collimator is fabricated by spacing and securing together a plurality 
of plates so that the longitudinal axis of each plate extends parallel to 
the longitudinal axis of the other plates, and each plate is focally 
aligned. In one embodiment, one wire is then extended the length of the 
collimator perpendicular to the longitudinal axis plates forming a 
plurality of sections. The number of sections corresponds to the size of 
the photodiode array so that the X-ray beams are separated to correspond 
to the number of photodiode array rows and columns. 
The above described detector array enables selection of the number of 
slices of data to be electrically transmitted for each rotation of the CT 
system. In addition, the detector collimator allows the X-ray beams to be 
separated so that only the focal X-ray beams are transmitted to the 
scintillator array resulting in more accurate scan data. Additionally, the 
detector modules allows the slice thickness to be selected to produce 
various slice resolutions. As a result, the configuration of the detector 
module can be altered to accommodate the specific needs and requirements 
of a test.

DETAILED DESCRIPTION 
Referring to FIGS. 1 and 2, a computed tomography (CT) imaging system 10 is 
shown as including 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 modules 20 which together sense the 
projected x-rays that pass through a medical patient 22. Each detector 
module 20 produces electrical signals that represent the intensity of 
impinging x-ray beams and hence the attenuation of the beams as they pass 
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. 
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 modules 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 a gantry opening 48. 
As shown in FIGS. 3 and 4, detector array 18 includes a plurality of 
detector modules 20 secured to an arch shaped detector housing 50. Each 
detector module 20 includes a multidimensional photodiode array 52 and a 
multidimensional scintillator array 56 positioned in front of and adjacent 
to photodiode array 52. One photodiode array that may be used is described 
in copending U.S. patent application Ser. No. (15-CT-4631), entitled, 
Photodiode Array For A Scalable Multislice Scanning Computed Tomography 
System, which is assigned to the present assignee and hereby incorporated 
herein, in its entirety, by reference. One scintillator array that may be 
used is described in copending U.S. patent application Ser. No. 
(15-CT-4513), entitled, Scintillator For A Multi-slice Computed Tomograph 
System, which is assigned to the present assignee and hereby incorporated 
herein, in its entirety, by reference. Detector array 18 also includes a 
collimator 54 positioned in front of and adjacent scintillator array 56 to 
collimate x-ray beams 16 before such beams impinge upon scintillator array 
56. Photodiode array 52 includes a plurality of photodiodes 60 which are 
optically coupled to scintillator array 56. Photodiodes 60 generate 
electrical output signals 62 representative of the light output by each 
scintillator of scintillator array 56. 
Referring to FIG. 5, collimator 54 includes a plurality of plates 64 and at 
least one wire 66. Plates 64 are spaced and secured together so that the 
longitudinal axis of each plate 64 extends substantially parallel to the 
longitudinal axis of each adjacent plate 64. Plates 64 are inserted in 
slots (not shown) located in housing 50 and bonded at the top and bottom 
of plates 64. Plates 64 and wire 66 are made, in one embodiment, of 
tungsten. Wire 66 extends the length of collimator 54 substantially 
perpendicular to the longitudinal axis of plates 66 and is inserted in 
horizontal slots (not shown) in plates 64 and bonded. 
Plates 64 and wire 66 create a plurality of sections (not shown) with each 
section having an active area and an inactive area (not shown). The active 
areas are approximately equal in size and separate X-rays 16 so that only 
the focal x-ray beams are allowed to pass through collimator 54 to 
scintillator array 56. Inactive areas prevent non-focal x-rays beams from 
impinging upon scintillator array 56 and photodiode 52. The number of 
sections is dependent on the size of scintillator array 56 and photodiode 
array 52. The area of scintillator array 56 directly below wire 66 is 
protected from impinging x-ray beams 16. For example, wire 66 may be 
positioned above each scintillator array gap (not shown) to protect 
reflective material from radiation damage and reduce penetration of x-ray 
beams 16 toward photodiode array 52. In one embodiment, the number of 
collimator wires 66 is one greater than the number of rows in scintillator 
array 56 so that each gap is protected. 
For example, in a sixteen slice mode of operation, detector array 18 
includes fifty-seven detector modules 20. Each detector module 20 includes 
a photodiode array 52 and scintillator array 56, each having an array size 
of 16 .times.16 so that array 18 has 16 rows and 912 columns (16.times.57 
modules). As a result, collimator 54 includes seventeen wires 66 and 913 
plates 64 allowing 16 simultaneous slices of data to be collected with 
each rotation of gantry 12. Additional examples include, a two slice mode 
of operation including three wire 66; and a four slice mode of operation 
including five wires 66. Additional modes beyond those described are 
possible. 
Detector module 20 also includes a switch apparatus 68 electrically coupled 
to a decoder 72. Switch apparatus 68 is a multidimensional semiconductor 
switch array of similar size as photodiode array 52. In one embodiment, 
switch apparatus 68 includes an array of field effect transistors (not 
shown) with each field effect transistor (FET) having an input, an output, 
and a control line (not shown). Switch apparatus 68 is coupled between 
photodiode array 52 and DAS 32. Particularly, each switch apparatus FET 
input is electrically connected to a photodiode array output 62 and each 
switch apparatus FET output is electrically connected to DAS 32, for 
example, using flexible electrical cables 74 and 76. Cables 74 and 76 are 
secured to detector module 20 with respective mounting blocks 80A and 80B. 
Decoder 72 controls the operation of switch apparatus 68 to enable, 
disable, or combine photodiode outputs 62 in accordance with a desired 
number of slices and slice resolutions for each slice. Decoder 72, in one 
embodiment, is a decoder chip or a FET controller as known in the art. 
Decoder 72 includes a plurality of output and control lines coupled to 
switch apparatus and computer 36. Particularly, the decoder outputs are 
electrically connected to the switch apparatus control lines to enable 
switch a apparatus 68 to transmit the proper data from the switch 
apparatus inputs to the switch apparatus outputs. The decoder control 
lines are electrically connected to the switch apparatus control lines and 
determine which of the decoder outputs will be enabled. Utilizing decoder 
72, specific FETs within switch apparatus 68 are enabled, disable, or 
combined so that specific photodiode outputs 62 are electrically connected 
to CT system DAS 32. In one embodiment defined as a 16 slice mode, decoder 
72 enables switch apparatus 68 so that all rows of photodiode array 52 are 
electrically connected to DAS 32, resulting in 16 separate, simultaneous 
slices of data being sent to DAS 32. Of course, many other slice 
combinations are possible. 
For example, decoder 72 may also select from other multiple slice modes, 
including one, two, and four slice modes. As shown in FIG. 6, by 
activating the appropriate decoder control lines, switch apparatus 68 can 
be configured in the four slice mode so that data is collected from four 
slices of one or more rows of photodiode array 52. Depending upon the 
specific configuration of switch apparatus 68 as defined by decoder 
control lines, various combinations of photodiode outputs 62 can be 
enabled, disabled, or combined so that the thickness of each slice may be 
1, 2, 3, or 4 rows. Additional examples include, a single slice mode 
including one slice with slices ranging from 1 row to 16 rows thick; and a 
two slice mode including two slices with slices ranging from 1 row to 8 
rows thick. Additional modes beyond those described are possible where the 
total number of photodiode array element rows, or pixels per channel, is 
equal to the number of slices or FET outputs times the number of rows per 
slice. For example, in a 4 slice mode of operation using 4 rows per slice, 
photodiode and scintillator arrays 52 and 56 include at least 16 rows of 
elements and switch apparatus 68 includes at least 4 FET outputs. In one 
embodiment, for example, each row is 1.25 mm wide. 
In one embodiment and referring to FIG. 7, switch apparatus 68 and decoder 
72 are combined into a FET array 104. FET array 104 includes a plurality 
of field effect transistors (FET) (not shown) arranged as a 
multidimensional array. In one embodiment, two semiconductor devices 106 
and 108 are utilized so that one-half of photodiode output lines 62 are 
connected to device 106 and one-half of photodiode output lines 62 are 
connected to device 108. FET arrays 106 and 108 each include respective 
input lines 110 and 112, output lines 114 and 116, and control lines (not 
shown). Internal to device 106, input lines 110 are electrically connected 
to the switch apparatus input lines, output lines 114 are electrically 
connected to the switch apparatus output lines, and decoder output lines 
are electrically connected to FET control lines. Switch 108 is internally 
configured identical to switch 106. 
In fabrication of detector module 20, photodiode array 52 including 
scintillator array 56 and FET arrays 106 and 108 are deposited, or formed, 
on substrate 200 so that photodiode outputs 62 are adjacent arrays 106 and 
108. Photodiode outputs 62 are then connected to inputs 110 and 112 of 
respective FET arrays 106 and 108. Particularly, one-half of photodiode 
outputs 62 are wire bonded to FET array inputs 110 and one-half of 
photodiode outputs 62 are wire bonded to respective FET array inputs 112 
so that each output 62 is electrically connected to a FET input line. 
Photodiode outputs are wire bonded to FET input lines using various wire 
bonding techniques, including, for example, aluminum wire wedge bonding 
and gold wire ball bonding as known in the art. First ends of flexible 
electrical cables 74 and 76 are then electrically connected and secured to 
FET arrays 106 and 108. FET array output and control lines are 
electrically connected to cables 74 and 76. Particularly, each FET array 
output line 114 and 116 is wire bonded to a wire of respective cables 74 
and 76. Detector module 20 is completed by securing first ends of cables 
74 and 76 with respective mounting blocks 80A and 80B. 
After fabricating detector modules 20 as described above, detector modules 
20 are mechanically mounted onto housing 50 so that scintillator arrays 56 
are positioned adjacent to collimator 54 and form array 18. Second ends of 
cables 74 and 76 of each detector module 20 are then electrically 
connected to CT system DAS 32. 
In operation, the operator determines the number of slices and thickness of 
each slice. The appropriate configuration information is transmitted to 
the array control lines to configure switch apparatus 68 using decoder 72. 
As X-ray beams 16 are projected toward detector array 18, collimator 54 
allows only the focal X-ray beams to impinge upon detector modules 20. As 
a result, data for the selected configuration is transmitted to DAS 32. 
The above described detector array enables selection of the number of 
slices of data to be electrically transmitted for each rotation of the CT 
system. In addition, the detector collimator allows the X-ray beams to be 
separated so that only the focal X-ray beams are transmitted to the 
scintillator array resulting in more accurate scan data. Additionally, the 
detector modules allows the slice thickness to be selected to produce 
various slice resolutions. As a result, the configuration of the detector 
module can be altered to accommodate the specific needs and requirements 
of a test. 
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. Accordingly, 
the spirit and scope of the invention are to be limited only by the terms 
of the appended claims.