X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-12-15
2001-09-18
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C250S370110
Reexamination Certificate
active
06292529
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of detector arrays and more specifically to the field of X-ray detector arrays in CT scanner applications.
Computed tomography (CT) X-ray scanners are used in a variety of applications. For example, such scanners are used in X-rays in medical diagnostic applications and for X-ray baggage inspection in airport security systems. For the most part, a CT scanner includes at least one X-ray source and a series of X-ray detectors. The detectors are disposed diametrically opposite the X-ray source on a rotating disk. During rotation the X-ray source emits X-rays which pass through the object being scanned and ultimately impinge on the detectors. Given that the original signal characteristics of the emitted X-rays are known, by measuring the attenuated signals arriving at the detectors, the electronics determines the density distribution in the object. Algorithms for determining an object's density based on such signal measurements are well known in the art.
In most CT systems, the X-ray detectors each first translate the received X-ray signal into an optical signal and then translate the optical signal into an electrical signal, which is processed by electronics forming part of each system. The electronics then process the electrical signals in accordance with specific application algorithms. A detector of this type often is made of a light emitting scintillating element (e.g., a scintillator crystal) paired with an optical detector or “photo-detector” (e.g., a photodiode). The scintillator crystal receives the X-ray signal and responsively generates an optical signal (e.g., blue light). The optical signal from each crystal is then detected by its corresponding photodiode, which responsively generates an electrical signal that is a function of the original X-ray flux received by the scintillator crystal. A typical detector array takes the form of a two-dimensional (i.e., m×n) array of detectors, or m×n scintillator crystal and photodiode pairs. It is important in such a detector array that light emitted from one scintillator crystal is not sensed by adjacent photodiodes which are adjacent to the intended photodiode with which the light emitting scintillator crystal is paired. Such light leakage, referred to as “optical cross-talk”, causes inaccuracies in the measurement (e.g., noisy signals, erroneous detections by adjacent detectors, artifacts, etc.) and, therefore, in the X-ray system overall.
One X-ray detector array of the prior art is described in pending U.S. patent application Ser. No. 08/948450, assigned to Analogic Corporation of Peabody, Mass. and incorporated herein by reference. As shown in
FIGS. 1-4
of the present application, the prior art X-ray detector system includes a large number of relatively small individual detector elements, or scintillator crystal/photodiode pairs, arranged in a two-dimensional (2-D) array. The detector array incorporates a multi-functional structure comprising a set of alignment grids which function both to align each individual scintillator crystal with a corresponding photodiode and also to isolate the individual photodiode/crystal pairs from one another to prevent optical cross-talk. Overall, the detector array is substantially stable under the typical operating conditions of the CT scanning system, which include vibration and/or temperature fluctuations.
As illustrated schematically in
FIG. 1
, a substrate
12
provides the basic structural support of the prior art detector array. Photodiodes
14
are arranged on the substrate in a 2-D array. As an example, a single m×n array may comprise 72 photodiodes arranged in six rows of twelve photodiodes each (i.e., a 6×12 array). The substrate
12
also includes a signal transmission arrangement
16
for transmitting electrical signals generated by the photodiodes to a signal processing subsystem
20
for image reconstruction. The signal transmission arrangement
16
can include electrically conductive circuit paths printed into the substrate, or an electrically conductive interconnect layer
17
attached to the substrate. Electrically conductive leads
19
from each photodiode to one or more of the paths complete an electrical connection between each photodiode and the signal processing means
20
.
A scintillator crystal assembly
18
is positioned over the photodiode array and includes a number of scintillator crystals
22
and alignment grids for arranging the crystals in a 2-D array which corresponds to the photodiode array. Each of the scintillator crystals
22
is substantially aligned and interfaced with a corresponding photodiode
14
and is also substantially optically isolated from surrounding crystals. As shown in
FIG. 2
, at least one of the alignment grids
24
is substantially planar and includes a number of cells or openings
26
. Each of the cells
26
is of a sufficient dimension to receive and substantially align with a scintillator crystal
22
, as shown in FIG.
3
. Another alignment grid
28
is optically opaque and substantially rigid 2-D grid, having a significant thickness relative to grid
24
. It also includes a number of cells
26
′ corresponding to the cells of the first alignment grid
24
. Each of the cells of the alignment grid
28
is substantially aligned with a corresponding cell of the planar alignment grid
24
and thus with a scintillator crystal
22
. Optical opacity and dimensional stability are critical features of the alignment grids.
The alignment grids
24
and
28
provide a structural framework for the scintillator crystals
22
in the detector array which ensures the correct alignment of the crystals with corresponding photodiodes
14
and provides dimensional stability to the crystal assembly. As shown in
FIGS. 2 and 3
, the cells
26
,
26
′ of the respective alignment grids
24
,
28
are each sized to accommodate and align a single scintillator crystal with a corresponding photodiode
14
. The 2-D alignment grid
28
includes walls which extend above the photodiode array and electrical interconnections
17
on the substrate. The walls are positioned directly in between photodiode detectors and establish individual wells or cells for each scintillator crystal.
The cell width of the 2-D grid is sufficiently large to accommodate the bonding of wire
19
from the detection side of photodiode
14
, the traversal of the wire
19
down the side of the photodiode, and bonding of the wire to the electrical layer
17
. Because the wire leads
19
from the photodiodes may be relatively fragile, effort is taken to protect them from damage. The 2-D alignment grid
28
additionally serves as a standoff between the photodiode array and a support for the scintillator crystal assembly so that the crystals
22
cannot rest directly on corresponding underlying photodiodes
14
and wire leads
19
, which would likely cause damage to the relatively fragile wire leads
19
. Therefore, the height and width of grid
28
is at least as great as the height and width of the combined photodiode
14
and wire lead
19
, as shown in FIG.
4
.
As is also shown in
FIG. 4
, the scintillator crystals
22
are surrounded on all sides, other than the side closest to a corresponding photodiode
14
, by an optically reflective material
30
, like paint, foil, or surface deposition layers. The region between a scintillator crystal
22
and a corresponding photodiode
14
is filled with an optically transmissible medium
34
(e.g., epoxy) to facilitate transmission of light from the crystal to the photodiode.
As will be appreciated by those skilled in the art, manufacture of the prior art X-ray detector array tends to be complex and labor intensive, due to the precautions necessary to insure its reliable construction. For example, each wire
19
must be bonded to the photodiode, and carefully looped, such that the wire when bonded to a circuit path on the substrate
12
does not touch the photodiode wall or grid
28
. Additionally, alignment grid
28
is itsel
Botka Alexander T.
Marcovici Sorin
Analogic Corporation
Kim Robert H.
McDermott & Will & Emery
Thomas Courtney
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