X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
2001-09-25
2004-01-13
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C378S098800
Reexamination Certificate
active
06678350
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable
REFERENCE TO MICROFICHE APPENDIX
Not Applicable
BACKGROUND OF THE INVENTION
The present invention relates to digital radiography, and more particularly, to digital radiography signal processing techniques for improving signal to noise ratios of an image generated from a bi-chromatic x-ray beam.
X-ray images of non-homogeneous material simultaneously display areas of different attenuation. In some cases, the attenuation differences are significant. For example, chest images simultaneously display areas of great attenuation (e.g., mediastine, spine) and areas of little attenuation (e.g., lungs). Optimum x-ray imaging of these two differing anatomical structures requires different parameters of x-ray flux. Specifically, areas of higher attenuation are better visualized by utilizing lower energy photons, whereas areas of lower attenuation are better visualized by utilizing higher energy photons. For this reason, chest images are generally taken with x-ray photons having a broad energy spectrum or, as it is done in certain procedures recently developed, heat images are taken with two narrow band x-ray pulses that have different average energies.
In both cases, the signal to noise ratio of the images so obtained is lower than the signal to noise ratio of an image obtained with a single narrow band x-ray pulse of the same dosage. The reason for the decreased signal to noise ratio is related to the fact that x-ray photons of different energies generate a different average amount of charge. As a result, the variance of the measured charge signal is affected by both the statistical variance of the photons, and the variance of the charge generated of them. This is known as “Schwank effect.”
The following description quantifies the decrease of the signal to noise ratio of an image taken with a bichromatic (i.e., two energy) x-ray beam, with respect to the signal to noise of an image taken with a monochromatic beam, assuming that the two beams have the same number of x-ray photons.
Monochromatic X-ray Beam: Single Image
In the case of a single image I produced from a monochromatic (i.e., one energy level) x-ray beam, the signal S and noise &sgr; of the image I is given by:
S=&agr;N&sgr;=&agr;{square root over (N)}
(
S
/&sgr;)
2
=N
where N is the average number of x-ray photons detected (i.e., the average photon flux per unit area), and &agr; is the average number of electrons generated per photon. The parameter &agr; is characterized by a single value at least in first approximation, because the energy of the monochromatic x-ray photons has a narrowband spectrum.
Bichromatic Beam: Single Image
The average number of detected x-ray photons N may be partitioned into two groups as follows: N
1
photons generate &agr;
1
carriers/photon and N
2
photons generate &agr;
2
carriers/photon, such that N
1
+N
2
=N. The N
1
photons are relatively narrowband with a single mean energy E
1
, and the N
2
photons are relatively narrowband with a single mean energy E
2
, with E
1
not equal to E
2
. The signal S and noise &sgr; of the image I is given by:
S
=
α
1
N
1
+
α
2
N
2
σ
=
α
1
2
N
1
+
d
2
2
N
2
(
S
/
σ
)
2
=
(
α
1
N
1
+
α
2
N
2
)
2
α
1
2
N
1
+
α
2
2
N
2
=
(
N
1
+
xN
2
)
2
N
1
+
x
2
N
2
id
.
,
and
x
=
α
2
/
α
1
The signal to noise ratio (S/&sgr;)
2
is a function of x, i.e., (S/&sgr;)
2
=ƒ(x).
For x=1, i.e., &agr;
1
=&agr;
2
, the case 2) reduces to the case 1) in which the x-ray beam has monochromatic energy.
It can also be shown that f(x=1) is a maximum of the function f(x), because
ⅆ
f
(
x
)
ⅆ
x
=
0
for
x
=
1
,
and
ⅆ
2
f
(
x
)
ⅆ
x
2
<
0
for
x
=
1
This means that the signal to noise ratio of the monochromatic case is higher than that of a bichromatic case, when the number of photons is equal in both cases.
It can also be shown that in general, the value (S/&sgr;)
2
for the monochromatic case is greater than the value (S/&sgr;)
2
for multi-chromatic beams, including the case of a broadband x-ray beam comprising x-ray photons having a broad energy spectrum. This result is known to those in the art as the “Shwank Effect.”
It is an object of the present invention to substantially overcome the above-identified disadvantages and drawbacks of the prior art.
SUMMARY OF THE INVENTION
The foregoing and other objects are achieved by the invention which in one aspect comprises a method of improving the signal to noise ratio associated with the output of a digital x-ray detector receiving bi-chromatic x-ray energy. The method includes acquiring a first image from the detector corresponding to x-ray energy at a first energy level, and scaling the first image with a first scaling factor so as to produce a scaled first image. The method further includes acquiring a second image from the detector corresponding to x-ray energy at a second energy level, and scaling the second image with a second scaling factor so as to produce a second scaled image. The method also includes combining the first scaled image and the second scaled image so as to form a compensated image.
In another embodiment of the invention, the first scaling factor is substantially equal to the product of a ratio and a fixed constant. The ratio is &agr;
2
/&agr;
1
, where &agr;
2
is the average number of carriers per photon generated by the x-ray energy at the second energy level, and &agr;
1
is the average number of carriers per photon generated by the x-ray energy at the first energy level. The second scaling factor is substantially equal to the fixed constant.
In another embodiment of the invention, combining the first and second scaled images further includes adding the first scaled image to the second scaled image.
In another aspect, the invention comprises a system for improving the signal to noise ratio associated with the output of a digital x-ray detector receiving bi-chromatic x-ray energy. The system includes a first multiplier for multiplying a first scaling factor by a first image acquired from the detector so as to produce a scaled first image. The first image corresponds to x-ray energy at a first energy level. The system also includes a second multiplier for multiplying a second scaling factor by a second image acquired from the detector so as to produce a scaled second image. The second image corresponds to x-ray energy at a second energy level. The system further includes a combiner for combining the first scaled image with the second scaled image so as to produce a composite image.
In another embodiment of the invention, the first scaling factor is substantially equal to the product of a ratio and a fixed constant. The ratio includes the average number of carriers per photon generated by the x-ray energy at the second energy level, divided by the average number of carriers per photon generated by the x-ray energy at the first energy level. The second scaling factor is substantially equal to the fixed constant.
In another embodiment of the invention, the combiner includes an adder, such that the combiner adds the first scaled image to the second scaled image to produce the composite image.
In another aspect, the invention comprises a system for improving the signal to noise ratio associated with an output of a digital x-ray detector that receives bi-chromatic x-ray energy and produces an electrical signal representative of the x-ray energy. The system includes an x-ray source for generating x-ray energy at a first and second energy level, and directing the x-ray energy through an object and toward the detector, so as to project a two-dimensional image of the object onto a surface of the detector. The system further includes an image processor for controlling the x-ray source to cyclically produce x-ray energy alternating betw
Coffin James G.
Dolazza Enrico
Analogic Corporation
Bruce David V.
Kiknadze Irakli
LandOfFree
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