X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
2000-06-05
2002-06-18
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C378S098110
Reexamination Certificate
active
06408049
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to radiographic and tomographic imaging, and, more particularly, to estimating and reducing scatter in digital radiographic and tomographic imaging and to an improved digital X-ray detector used for same.
A typical prior art radiographic or computed tomography (CT) imaging system
10
of the so-called third generation is shown in FIG.
1
. Imaging systems of the type disclosed in
FIG. 1
are described in further detail in Principles of Computerized Tomographic Imaging, by Avinash C. Kak & Malcolm Slaney, IEEE Press, 1988. As shown in
FIG. 1
, the imaging system
10
includes a source
12
, such as an x-ray source, transmitting primary signals to an object
14
, such as a patient, positioned on a support
16
, such as a table. Some of the primary signals pass through the object
14
and the support
16
, and are detected by detector array
18
. Detection of the primary signals by detector array
18
is controlled by data acquisition component
19
.
A characteristic of a third-generation CT imaging system is that the source
12
and the detector array
18
containing collimating plates are focally aligned with each other and are both controlled by a common controller
20
to move in tandem with each other while maintaining their established focal alignment. Focal alignment means that the collimating plates of the detector array
18
point toward the source
12
. The controller
20
typically controls on/off states and motion of the source
12
and the detector array
18
, based upon instructions issued by the CT system computer
22
. The CT system computer
22
also controls the data acquisition component
19
.
Once the x-ray signals are detected, data acquisition component
19
converts the detected signals into digital data supplied to the CT system computer
22
. The CT system computer
22
then processes according to well-known techniques the digital data, stores the processed digital data in system memory
24
, and displays the processed digital data on display
26
.
When high-resolution digital area detectors are used in radiographic and tomographic imaging applications, one concern that commonly arises is the problem of scatter corruption of the primary signal. Scatter, or scatter corruption, reduces low-contrast detectability and resolution in both radiographic images and reconstructed tomographic images. Further, scatter signals can cause streaks between highly attenuating objects in reconstructed tomographic images, thus masking the shape of the objects
14
. Unlike with lower resolution linear detectors, it is impossible to collimate appropriately each detector element in an area detector since the dimension of individual detector elements can be an order of magnitude smaller than their linear detector counterparts. As a result, it is necessary to devise methods or schemes to either reduce scatter or correct for scatter.
An approach of the prior art in the field of radiography has been to use a collimating grid positioned over the detector. Although this grid covers part of the detector, it is continuously moved during the x-ray exposure interval so as not to produce discernible artifacts.
Another approach offered in the prior art has been to estimate scatter in regions of the imager that are not shadowed by the patient. A constant value of scatter is then subtracted from all pixels in the image. Although this approach has some benefit for radiographic applications, it usually produces severe artifacts in tomographic applications. In general, using area detectors for tomographic applications is a fairly new technology; hence, no standard methods to reduce/correct scatter have been devised. As used herein, “area detectors” generally refer to detectors having rows and columns of pixels that are connected together to provide readout by rows and columns of the pixels in the array, and typically implies a large number (e.g., more than 5 rows, and commonly an array of a size 1000×1000 pixels or detector elements) of pixels in the array; multi-row detectors typically refer to a small number (e.g., 5 or less) pixel rows that are arranged to provide an imaging signal, and a “single row” detector refers to one row of pixels disposed to provide the imaging signal.
In most x-ray imaging systems using area detectors, the x-ray detector that is used to measure the intensity of the x-ray beam, or primary signal, that remains after passing through the object (or patient) does not completely absorb the remaining x-ray flux from the x-ray beam. As a result, a slab of attenuating material called a beam-stop
28
is usually placed directly behind the detector
18
panel, as shown in FIG.
2
. The use of a beam-stop
28
is discussed in “Measurement of Scatter Fractions in Clinical Bedside Radiography,” Radiology 1992, 183:857-861. The slab of attenuating material (or beam-stop)
28
is usually made from an x-ray absorbing material such as lead or tungsten and is used to reduce the residual x-ray intensity that is not detected by the area detector
18
to a near zero value. Since a part of the x-ray flux is not absorbed by the area detector
18
, some of the dose applied to the patient
14
is not used for diagnostic purposes, a health concern for radiologists.
Since x-ray scatter, resulting from the interaction of the primary x-ray beam P
1
(as shown in
FIG. 2
) with the object
14
within the imaging system
10
, may be a significant fraction of the detected x-ray signal in the area detector
18
, a method and apparatus for estimating and reducing the scatter signal would be useful. If x-ray data collected contain a significant amount of scatter, computed tomography (or CT) reconstructions of the object will contain noticeable artifacts that will limit their utility for diagnostic purposes.
It is known in the art that hardware collimation significantly reduces the scatter signal measured with linear detector arrays
18
. A detector array
18
comprises cells
30
. In general, for these types of detector arrays
18
in a third-generation CT imaging machine
10
, each cell
30
of the detector array
18
includes individual detector elements
18
-n and may also include collimator plates
32
as shown in
FIG. 3. A
typical area detector array
18
shown in
FIGS. 1-3
includes, for example, 1K×1K (1,024×1,024) of detector elements
18
-n or cells
30
detecting the x-ray beam. In the detector cell
30
shown in
FIG. 3
, collimator plates
32
are included. However, collimator plates
32
are not necessarily included in other detector cells
30
of area detector arrays
18
. As shown in
FIG. 3
, a collimator plate
32
, made of thin lead or tungsten and which is focally aligned to the x-ray source
12
, is placed between each detector element
18
-n in detector array
18
; these plates
32
attenuate the scatter signal while allowing the primary signal to be detected as desired. The dimensions of the collimator plates
32
affect the amount of scatter that is rejected by the imaging geometry of the imaging system
10
. For area detector technology, it is not possible to collimate each detector element
18
-n (also referred to as cell
30
) in area detector array
18
because of increased resolution of individual detector elements
18
-n or cells
30
used in area detector array
18
. The thickness of the collimator plates
32
required to appropriately attenuate the scattered radiation would be such that the plates
32
would cover most, if not all, of the active area of the individual detector elements
18
-n or cells
30
in area detector array
18
. Therefore, other methods are needed to appropriately characterize and correct for scatter.
BRIEF SUMMARY OF THE INVENTION
The present invention provides an x-ray imaging system and method which images an object by transmitting primary signals through the object. In the present invention, a collimator is placed between two detectors of the x-ray imaging system. The collimator reduces respective scatter components of total signals measured in one of th
Edic Peter Michael
Ishaque Ahmad Nadeem
Cabou Christian G.
Ingraham Donald S.
Kiknadze Irakli
Kim Robert H.
LandOfFree
Apparatus, methods, and computer programs for estimating and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus, methods, and computer programs for estimating and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus, methods, and computer programs for estimating and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2944234