U.S. patent application number 09/681785 was filed with the patent office on 2002-12-05 for x-ray detector image quality test techniques.
Invention is credited to Farrokhnia, Farshid, Kump, Kenneth S., Langler, Donald F., Vafi, Habib.
Application Number | 20020181661 09/681785 |
Document ID | / |
Family ID | 24736797 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020181661 |
Kind Code |
A1 |
Vafi, Habib ; et
al. |
December 5, 2002 |
X-RAY DETECTOR IMAGE QUALITY TEST TECHNIQUES
Abstract
An x-ray system (10) include a digital detector (400) that
defines two regions: a first region (404) suitable for generating
data useful for creating a patient x-ray image and a second region
(406) less suitable for generating such data than the first region.
A source (20) transmits x-rays through a phantom (420) located
between the source and the second region (406) so that the detector
(400) generates test data in the second region. A processor (302)
measures at least one parameter in response to the test data and
stores a value of the parameter at one point of time. The processor
compares the first value with a second value of the one parameter
generated at a later second point in time. The processor also
generates a result signal representing the results of the
comparison.
Inventors: |
Vafi, Habib; (Brookfield,
WI) ; Farrokhnia, Farshid; (Brookfield, WI) ;
Langler, Donald F.; (Brookfield, WI) ; Kump, Kenneth
S.; (Waukesha, WI) |
Correspondence
Address: |
MCANDREWS HELD & MALLOY, LTD
500 WEST MADISON STREET
SUITE 3400
CHICAGO
IL
60661
|
Family ID: |
24736797 |
Appl. No.: |
09/681785 |
Filed: |
June 5, 2001 |
Current U.S.
Class: |
378/207 ;
378/98.8 |
Current CPC
Class: |
A61B 6/583 20130101 |
Class at
Publication: |
378/207 ;
378/98.8 |
International
Class: |
G01D 018/00 |
Claims
1. In an x-ray system comprising a digital detector defining a
first region suitable for generating data useful for creating a
patient x-ray image and a second region less suitable for
generating such data than said first region, apparatus for testing
the detector comprising: a source of x-rays; a phantom located
between said source and at least a portion of the second region so
that said detector generates detector test data in at least a
portion of the second region in response to said x-rays; and a
processor arranged to measure at least one parameter responsive to
at least a portion said test data, store a first value of said one
parameter at one point of time, make a comparison of said first
value with a second value of said one parameter generated at a
second point in time later than said first point of time; and
generate a result signal representing the results of said
comparison.
2. Apparatus, as claimed in claim 1, wherein said first and second
parameters comprise modulation transfer functions.
3. Apparatus, as claimed in claim 1, wherein said processor is
arranged to generate said result signal when said first and second
values fall within a predetermined tolerance.
4. Apparatus, as claimed in claim 1, and further comprising a
communication module arranged to transmit said result signal to a
remote location.
5. Apparatus, as claimed in claim 1, wherein said detector
comprises a solid-state detector.
6. Apparatus, as claimed in claim 1, and further comprising a
display arranged to display at least said first value.
7. Apparatus, as claimed in claim 1, wherein said processor is
arranged to generate qualified test data in response to said
detector test data and to measure said at least one parameter
responsive to said qualified test data.
8. Apparatus, as claimed in claim 7, wherein said processor is
arranged to generate said qualified test data by generating
statistical values in response to said qualified data based on one
or more of mean values, minimum values, maximum values and standard
deviation values, and comparing the statistical values with one or
more limits.
9. Apparatus, as claimed in claim 1, wherein the second region
comprises a margin region.
10. In an x-ray system comprising a digital detector defining a
first region suitable for generating data useful for creating a
patient x-ray image and a second region less suitable for
generating such data than said first region, a source of x-rays, a
phantom located between said source and at least a portion of the
second region so that said detector generates detector test data in
at least a portion of the second region in response to said x-rays,
and a processor, a method for testing the detector comprising:
measuring at least one parameter responsive to at least a portion
of said test data; storing a first value of said one parameter at
one point of time; comparing said first value with a second value
of said one parameter generated at a second point in time later
than said first point of time; and generating a result signal
representing the results of said comparison.
11. A method, as claimed in claim 10, wherein said first and second
parameters comprise modulation transfer functions.
12. A method, as claimed in claim 10, wherein said generating
comprises generating said result signal when said first and second
values fall within a predetermined tolerance.
13. A method, as claimed in claim 10, and further comprising
transmitting said result signal to a remote location.
14. A method, as claimed in claim 10, wherein said detector
comprises a solid-state detector.
15. A method, as claimed in claim 10, and further comprising
displaying at least said first value.
16. A method, as claimed in claim 10, and further comprising:
generating qualified test data in response to said detector test
data; and measuring said at least one parameter responsive to said
qualified test data.
17. A method, as claimed in claim 16, wherein said generating said
qualified test data comprises: generating statistical values in
response to said qualified data based on one or more of mean
values, minimum values, maximum values and standard deviation
values, and comparing the statistical values with one or more
limits.
18. A method, as claimed in claim 10, wherein the second region
comprises a margin region.
Description
BACKGROUND OF INVENTION
[0001] This invention relates to x-ray detectors and more
specifically relates to techniques for testing such detectors.
[0002] Almost all image quality evaluation methods rely on placing
off-the-shelf or custom-made x-ray phantoms in the field of view.
Some methods use image processing and analysis tools to
automatically detect regions of interest in the acquired image of
the phantom. These methods have a significant advantage over
"manual" methods that rely heavily on human operators to perform
these measurements. These methods also provide more consistent and
objective measurements.
[0003] However, automating the analysis of the image of the phantom
does not result in full automation of the image quality evaluation,
because, like the "manual" methods, they still require intervention
by a human operator to place the x-ray phantom(s) in the field of
view. Experience has shown that human operators are not inclined to
take the time to place the x-ray phantom in the field of view. As a
result, detector problems may go undetected for some time. X-ray
images generated while the detector problems go undetected can
result in degraded image quality.
[0004] This invention addresses these problems and provides a
solution.
SUMMARY OF INVENTION
[0005] The preferred embodiment is useful in an x-ray system
comprising a digital detector defining a first region suitable for
generating data useful for creating a patient x-ray image and a
second region less suitable for generating such data than the first
region. In such an environment, the detector can be tested by
providing a source of x-rays and a phantom located between the
source and at least a portion of the second region so that the
detector generates detector test data in at least a portion of the
second region in response to the x-rays. At least one parameter is
measured in response to at least a portion the test data. A first
value of the one parameter is stored at one point of time. A
comparison is made of the first value with a second value of the
one parameter generated at a second point in time later than the
first point of time. A result signal representing the results of
the comparison is generated.
[0006] By using the foregoing techniques, the detector can be
tested without human intervention, thereby insuring more reliable
and timely testing than has been possible in the past.
BRIEF DESCRIPTION OF DRAWINGS
[0007] FIG. 1 is a schematic block diagram of an exemplary form of
x-ray system employing a preferred embodiment of the invention.
[0008] FIG. 2 is a schematic top plan view of the detector shown in
FIG. 1 illustrating different regions of the detector and also
schematically illustrating a preferred form of phantom made in
accordance with the invention.
[0009] FIG. 3 is a schematic, fragmentary, side elevational view of
the phantom shown in FIG. 2.
[0010] FIG. 4 is an enlarged fragmentary top plan view of the
phantom shown in FIG. 3 together with adjacent portions of the
detector shown in FIGS. 1 and 2.
[0011] FIG. 5 is graph illustrating an exemplary plot of modulation
transfer function versus spatial frequency of phantom grids.
DETAILED DESCRIPTION
[0012] Referring to FIG. 1, a preferred form of x-ray imaging
system 10 made in accordance with the invention comprises an x-ray
tube 20 that generates x-rays from a focal spot 22 and directs the
x-rays in relationship to a central axis CA. A digital image
detector 400 detects the x-rays in a well-known manner. A
collimator 320 includes collimator blades shown schematically in
FIG. 1.
[0013] A calibration processor 302 includes communication interface
or module 304, a keyboard 305, a central processing unit (CPU) 306,
a memory 308 and a display unit 309, such as a computer monitor,
all coupled by a bus 307 as shown. The processor may include, for
example, a microprocessor, digital signal processor,
microcontroller or various other devices designed to carry out
logical and arithmetic operations. Signals corresponding to an
x-ray image are read from detector 400 by readout electronics 312.
The design and operation of most of the components with numbers
greater than 300 are described in more detail in application Ser.
No. 09/342,686, filed Jun. 29, 1999, in the names of Kenneth S.
Kump et al., entitled "Apparatus And Method For x-ray Collimator
Sizing And Alignment," assigned to General Electric Company and
incorporated by reference in its entirety into this
specification.
[0014] Communication interface 304 is coupled through a modem 340
and a network 342, such as the Internet, to a computer system 344
at a remote location 346. Maintenance personnel at location 346
monitor computer system 344 to determine if detector 400 requires
repair or maintenance.
[0015] FIG. 2 is a top plan view of detector 400 that defines an
outer periphery 402 and an inner region 404 that is suitable for
generating data useful for creating a patient x-ray image. Between
region 404 and periphery 402 is a margin region 406 less suitable
for generating data useful for creating a patient x-ray image than
region 404. Region 406 is typically about 2-3 millimeters (mm)
wide. Within region 406 is a generally rectangular saw tooth strip
phantom 420.
[0016] A fragment of phantom 420 is shown in FIG. 3. Phantom 420
comprises a frame 422 substantially transparent to x-rays and
identical regions of interest (ROIs) or coupons 424 that absorb
x-rays. The ROIs are separated by identical distances of about 10
mm and have dimensions of about 10 by 2 mm.
[0017] Referring to FIG. 4, phantom 420 may be located in one of at
least two different positions. For example, phantom 420A is located
directly under cover 430 of detector 400. The location of phantom
420A has the advantage of being accessible for replacement and
service. However, phantom 420B may be positioned more accurately
than phantom 420A by being located inside a sealed metal box or
cabinet 440. As shown in FIG. 4, phantom 420B is located below an
aluminum graphite cover 442 and above a scintillator 444. An
amorphous silicon array 446 is located below scintillator 444 and
is carried by a glass substrate 448. A seal 450 is provided between
cover 442 and array 446 to protect scintillator 444.
[0018] FIG. 5 illustrates an x-ray image of a tungsten coupon
sub-phantom and a modulation transfer function (MTF) curve computed
based on the upper edge profile of the tungsten coupon. FIG. 5 also
shows how the edge profile of a rectangular tungsten coupon can be
used to compute MTF. The coupon illustrated in FIG. 5 is about 30
mm by 30 mm. The vertical axis in FIG. 5 indicates modulation
strength and the horizontal axis indicates spatial frequency of the
tungsten coupons. The profiles of vertical and horizontal edges of
the coupon can be used to compute MTF in horizontal and vertical
directions, respectively. The coupon is deliberately positioned at
a slightly rotated angle with respect to the top surface of
detector 400 to avoid the edge points from lining up along a row or
column.
[0019] In general, phantom 420 is used to conduct a self-test of
certain image quality (IQ) parameters of solid state digital x-ray
detector 400. Phantom 420 is located in margin region 406 of
detector 400. Image data from pixels in margin 406 of the detector
is created when x-rays are transmitted through phantom 420 to
detector 400. A certain number of rows and columns of data in the
margin 406 are read out but are not displayed. This is because the
process used to make the detector panels does not always result in
uniform deposition of the cesium iodide (x-ray scintillator) on the
edges (e.g., region 406), compared to the rest of the panel (e.g.,
region 404).
[0020] By planting small x-ray phantoms, such as phantom 420, in
these unused margins (e.g., margin 406), it is possible to compute
certain image quality parameters. For example, a narrow "edge"
phantom as shown in FIGS. 3 and 4 can be used to compute the
modulation transfer function (MTF), at every exposure as
illustrated in FIG. 5. Alternatively, the noise power spectrum or
contrast to noise ratio, can be calculated in this margin
region.
[0021] Specifically, for MTF, an edge-based method of computation
can be utilized, based on edge profiles along the diagonal side of
each saw tooth of the type shown in FIG. 3.
[0022] Usually, measuring IQ parameters of a x-ray detector
involves placing a known x-ray phantom in the field of view,
acquiring an image and then processing it to compute the IQ
parameters. The use of implanted sub-phantoms, such as phantom 420,
inside the detector 400 eliminates the need for an external
phantom, and more importantly the need for an operator to place the
phantom.
[0023] In addition to providing the necessary sub-phantoms, such as
phantom 420, and an image, a "qualifying" algorithm is used. This
algorithm is executed by CPU 306 and ensures that the image data
being received in margin 406 are of good enough quality. That is,
the x-ray field must be uniform (or be correctable) and the
detector quality must be adequate. This is important since patients
will be imaged simultaneously while the self-test of the detector
is being conducted. This is a feature which limits the amount of
x-ray radiation received by the patient. Technologists using good
practice will collimate to the interesting patient anatomy. We are
relying on the scattered radiation and occasionally "raw"
(un-attenuated) radiation to expose phantom 420 in margin region
406. The qualifying algorithm computes simple statistics in the
region of phantom 420 or the parts of region 406. For example, the
mean, minimum, maximum, and standard deviation of gray levels
(counts) can be determined. These values are compared to predefined
limits to determine if the image data is valid for subsequent
calculation. Additional details about the qualifying algorithm are
as follows: Step 1) There is first a need to define which ROIs are
acceptable for computation. An initial "Pre-calibraion" to select
ROIs with acceptably low number of bad pixels, minimum conversion
factor (CF), and define a response correlation to the known good
area in the region of the detector suitable for creating a patient
x-ray image is required. This is conducted once per detector
calibration which may occur, for example, roughly yearly.
[0024] Step 2) Of the ROIs deemed acceptable in step 1, for each
exposure, there are additional acceptance criteria such as: minimum
contrast between x-ray absorbing and x-ray transparent areas, and
minimum signal count. Only the ROIs passing both step 1 and step 2
criteria will be used in the calculation.
[0025] After the image data is qualified, CPU 306 executes another
algorithm to analyze the data and produce summary data, such as MTF
data. Additional details about the MTF algorithm are as follows:
Calculate MTF by a) Starting with the 12.sup.th row or column in
from the edge of the panel, for an ROI, record the signal response
vs the location of the edge; b) Increment until all rows or columns
crossing the edge of the imbedded phantom have been sampled; c)
Fourier transform the data set; d) Extract the frequency
coefficients; e) Normalize the data for each frequency and adjust
per the correlation defined in step 1; f) Repeat steps a-e for each
acceptable ROI; and g) average all the ROI results.
[0026] This summary data is then placed into log files in memory
308 that can be actively "swept" using remote diagnostic equipment
embodying computer system 344. Alternatively, the process may
proactively call-out to a remote host 344 (at on-line-center) to
report its data. This may be done on a scheduled timeline, or when
particular events occur (e.g.: values fall below certain
pre-defined levels indicating failure or imminent failure).
However, as a self-test, what is important is detection of any
variations in the MTF on the edges, not the absolute MTF. The
creation of summary reports includes the appending of new
qualifying data to the "log" files. The data includes a parameter,
such as MTF. A process may be included which compares the new data
or parameter (or results from trending of current plus previous
data) to predefined or calculated parameter thresholds that were
previously stored. CPU 306 generates a result signal indicating the
results of the comparison. When these thresholds are exceeded, a
result signal or a message is sent to remote computer system 344
via modem 340 and the Internet to indicate a problem or status. For
example, a message indicating a problem may be sent if the MTF
summary data describing an MTF curve like the one shown in FIG. 5
from a previous year is more than 10 percent different from current
summary data describing a current MTF curve. All of the foregoing
data and parameters may be displayed on display 309.
[0027] Using implanted sub-phantoms, such as phantom 420, in the
unused margins of the detector (e.g., region 406) allows testing
and evaluation of certain parameters of the detector during a
normal patient image acquisition. This self-test capability can be
used to collect IQ data during every "scan". Analyzing the data
over time can be used to identify possible change or degradation of
IQ of the detector in a pro-active fashion. This design results in
further automation of image quality evaluation of solid state x-ray
detectors. It eliminates or minimizes the reliance on human
operators to perform the IQ evaluation on a regular basis, making
it possible to be truly pro-active in servicing it.
[0028] Those skilled in the art will recognize that the preferred
embodiments may be altered and modified without departing from the
true spirit and scope of the invention as defined in the
accompanying claims.
* * * * *