X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2002-10-10
2004-05-11
Glick, Edward J. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S098900, C378S158000, C378S159000
Reexamination Certificate
active
06735273
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to an X-ray computed tomography apparatus with multi-spectra beam hardening correction.
2. Description of the Prior Art
The attenuation p that X-radiation generated by an X-ray source experiences in a transirradiated subject is measured in X-ray computed tomography. It is determined from the X-ray intensity I
0
incident onto the subject and from the intensity I that is registered in a detector arranged in the beam path following the subject, according to the following equation:
p=
−1
n
(
I
/0) (1).
In the case of mono-energetic radiation, the following applies for a homogeneous subject with the attenuation coefficient &mgr; and the transirradiated subject thickness d:
p=&mgr;d
(2).
The X-ray attenuation thus increases linearly with the subject thickness.
In fact, however, an X-ray tube emits polychromatic X-radiation with the energy distribution S(E). The attenuation is then calculated according to the following equation:
p=∫·∫&mgr;
(
E
)
S
(
E
)
dEdx
(3).
Even when the subject is homogeneous, the X-ray attenuation produced by the subject is thus no longer linearly dependent on the transirradiated subject thickness. Since &mgr;E usually decreases toward higher energies, the “energy center of gravity” shifts toward higher energies, namely all the more the greater the transirradiated subject thickness is. This effect is referred to as beam hardening.
In image reconstruction methods that are standard in CT technology, a linear change of the X-ray attenuation with the subject thickness is assumed for homogeneous subjects. The overall attenuation p of a beam on its path through a subject composed of partial subjects i with attenuation coefficient &mgr;
i
and thickness d
i
then derives from:
p=&Sgr;
i
(&mgr;
i
d
i
) (4)
The deviations from this assumption caused by the beam hardening lead to data inconsistencies and, thus, to image errors. Typical image errors caused by beam hardening are key artifacts in large, homogeneous subjects and line or bar artifacts in CT images with a high proportion of bone or contrast agent. Current correction methods often have the principal goal or eliminating key artifacts and stripe artifacts in subjects with high attenuation, for instance in shoulder and pelvis exposures. These corrections usually ensue with what is referred to as polynomial correction, whereby a corrected attenuation value p
c
is calculated from a detected measured attenuation value p
M
by insertion into a polynomial with predetermined coefficients an according to the following equation:
p
c
=&Sgr;
[n=0.1 . . . N]
(
a
n
P
M
n
) (5).
The coefficients an are acquired, for example, by measuring the attenuation values of uniform absorbers (for example, Plexiglas® bars) given N different thicknesses.
It has been shown that improved correction methods are needed for the correction of locally limited bar and line artifacts as well as unsharp bone-tissue transitions as particularly occur given skull exposures (another known stripe artifact, for example, is what is referred to as the Hounsfield stripe between the petrous bones). An approach has thereby proven beneficial wherein the length of the “base material” that the X-ray beam leading to a measured value has traversed in the body of the patient under examination is individually estimated for each measured attenuation value. In medical examinations, bone substance and soft tissue or, respectively, water, which has spectral attenuation properties similar to soft tissue, are usually selected as base materials. A method referred to as the two-spectra method, for example, is known from the pertinent literature for estimating the base material lengths traversed by an X-ray beam. In this method, two measured values with respectively different spectral energy distribution of the X-ray, which is equivalent to a different average energy of the X-ray, are registered. Given known attenuation coefficients &mgr;
W
(E
1
) and &mgr;
W
(E
2
) of water at the average spectral energies E
1
and E
2
and &mgr;
K
(E
1
) and &mgr;
K
(E
2
) of bone at these average energies, the following, approximate estimate is possible for the measured attenuation values p(E
1
) and p(E
2
) obtained given there energies E
1
and E
2
:
p
(
E
1
)
=d
W
·&mgr;
W
(
E
1
)
+d
K
·&mgr;
K
(
E
1
) (6a)
p
(
E
2
)
=d
W
·&mgr;
W
(
E
2
)
+d
K
·&mgr;
K
(
E
2
) (6b).
The water and bone lengths d
W
and d
K
can then be estimated from these equations.
Corrected measured values p
c
(E
1
) or, respectively, p
c
(E
2
) can now be respectively determined in the following way for the average spectral energies E
1
and E
2
:
p
c
(
E
1
)
=p
(
E
1
)
+f
E1
(
d
W,
d
K
) (7a)
p
c
(
E
2
)
=p
(
E
2
)
+f
E2
(
d
W,
d
K
) (7b)
The correction values f
E1
and f
E2
are taken from tables that were determined in advance either computationally or empirically for the average spectral energies E
1
and E
2
.
Further information about the above two-spectra method can be found, for example, in the following publications:
1) P. M. Joseph, R. D. Spiftal, Journal of Computer Assisted Tomography, 1978, Vol.2, p.100;
2) P. C. Johns, M. Yaffe, Medical Physics, 1982, Vol.9, p.231;
3) G. H. Glover, Medical Physics, 1982, VOl.9, p.860;
4) A. J. Coleman, M. Sinclair, Physics in Medicine and Biology, 1985, Vol.30, No.11, p.1251.
In order to register measured values at two different average energies in a conventional CT apparatus, two successive revolutions of the X-radiator around the patient must be implemented. In the second revolution, a different beam prefiltering or a different tube voltage is used compared to the first revolution. A disadvantage of such a procedure, however, is that the measured results can exhibit inconsistencies due to patient movement or contrast agent flow.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a computed tomography apparatus which avoids the aforementioned disadvantages of conventional computed tomography systems.
This object is achieved in accordance with the principles of the present invention in an X-ray computed tomography apparatus having an X-ray radiator with at least two spring foci between which the X-ray radiator is switched to alternatingly emit X-ray beams respectively from the foci, a radiation detector disposed in the X-ray beams, with an examination subject adapted to be disposed between the X-ray radiator and the radiation detector, the radiation detector having a number of detection channels, a beam filter arrangement disposed in said X-ray beams preceding said subject, the beam filter arrangement having regions of different filter characteristics respectively allocated to the foci to give the X-ray beams respectively different energies that the subject, and wherein the X-ray radiator irradiates a slice of the subject with the X-ray beams from a number of projection angles with the X-ray beams, after attenuation by the subject, being incident on detector channels of the radiation detector in a projection range. The radiation detector, for each detector channel in the projection range, generates at least two measured projection values for the respective X-ray beams at different energies from the foci. The measured projection values are supplied to an electronic and evaluation reconstruction unit connected to the radiation detector, which determines a beam hardening-corrected projection value for each of the measured projection values, and reconstructs a tomographic image of the slice of the examination subject using these corrected projection values.
The different regions of the filter arrangement achieve the aforementioned different energy-influencing filter characteristics by being of different filter material, or being of the same filter material but having respectively different thickness profiles in the regions.
The varying material
Flohr Thomas
Ohnesorge Bernd
Glick Edward J.
Ho Allen C.
Schiff & Hardin LLP
Siemens Aktiengesellschaft
LandOfFree
X-ray computed tomography apparatus and multi-spectra... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with X-ray computed tomography apparatus and multi-spectra..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and X-ray computed tomography apparatus and multi-spectra... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3186058