X-ray CT apparatus and method thereof
View Patent ↗In an X-ray CT apparatus 1 and an X-ray CT method, the thickness of an object to be inspected is computed on the basis of the number of transmitted X-rays in a specific energy range set above and below the K-absorption edge of an X-ray contrast medium serving as the object to be inspected, and a CT image is reconstructed on the basis of the computed thickness of the object to be inspected. Such X-ray CT apparatus 1 and X-ray CT method can generate an X-ray CT image stably and independently of the size of the object to be inspected and of X-ray tube voltage (X-ray energy distribution).
1. An X-ray CT apparatus, comprising:
an X-ray irradiating unit that irradiates X-rays onto a subject;
an X-ray measuring unit that opposes said X-ray irradiating unit across said subject and measures the number of transmitted X-rays within a specific energy range that corresponds to an object to be inspected inside said subject, from among the transmitted X-rays that pass through said subject;
a thickness computing unit that computes the thickness of said object to be inspected on the basis of the number of said transmitted X-rays measured by said X-ray measuring unit; and
an image reconstructing unit that reconstructs a CT image on the basis of the thickness of said object to be inspected as computed by said thickness computing unit,
wherein said object to be inspected is an X-ray contrast medium,
said specific energy range is set above and below the K-absorption edge of said X-ray contrast medium,
said thickness computing unit obtains the thickness t I of said object to be inspected on the basis of a ratio Φ1/Φ2 between the number of transmitted X-rays within a predetermined energy range Φ1 that is smaller than the K-absorption edge of said object to be inspected, and the number of transmitted X-rays within a predetermined energy range Φ2 that is greater than the K-absorption edge of said object to be inspected,
said image reconstructing unit uses, as projection data, the thickness of said object to be inspected which is computed by said thickness computing unit, performs convolution of said projection data and a predetermined reconstruction function, and generates a CT image of said subject through back projection of the convolution result,
the ratio Φ1/Φ2 being given by the following formula:
ln
ϕ
1
ϕ
2
=
1
-
(
μ
1
_
(
E
1
)
-
μ
1
_
(
E
2
)
)
·
t
I
-
(
μ
W
_
(
E
1
)
-
μ
W
_
(
E
2
)
)
·
t
W
.
wherein overbarred μ I (E n ) and overbarred μ W (E n ) are the mean attenuation coefficients of said object to be inspected and water, respectively, for X-rays of an energy range E n (n=1, 2), and t I and t w are the thickness of said object to be inspected and water, respectively.
2. The X-ray CT apparatus according to claim 1 ,
wherein said X-ray measuring unit comprises a detection medium that generates charge on account of energy imparted by transmitted X-rays that pass through said subject; and a plurality of electrodes disposed in said detection medium at positions removed by mutually different distances from an incidence end of said transmitted X-rays on said detection medium.
3. The X-ray CT apparatus according to claim 1 ,
wherein said X-ray measuring unit comprises a plurality of detection media arrayed in a propagation direction of said transmitted X-rays that pass through said subject, such that charge is generated at each of said detection media on account of energy imparted by said transmitted X-rays, and said detection media act as absorption bodies of said transmitted X-rays, as a result of which there varies the thickness of the absorption bodies through which said transmitted X-rays pass before arriving at said respective detection media.
4. The X-ray CT apparatus according to claim 1 ,
wherein said X-ray contrast medium is any from among an iodine contrast medium, a barium contrast medium and a gold contrast medium.
5. The X-ray CT apparatus according to claim 1 ,
wherein said X-ray irradiating unit irradiates filtered X-rays, resulting from filtering X-rays through a filter, onto the subject.
6. An X-ray CT method, comprising:
an X-ray irradiation step of irradiating X-rays onto a subject excluding a living organism;
an X-ray measurement step of measuring the number of transmitted X-rays within a specific energy range that corresponds to an object to be inspected inside said subject, from among the transmitted X-rays that pass through said subject;
a thickness computing step of computing the thickness of said object to be inspected on the basis of the number of said transmitted X-rays measured by said X-ray measuring step; and
an image reconstruction step of reconstructing a CT image on the basis of the thickness of said object to be inspected as computed in said thickness computing step,
wherein said object to be inspected is an X-ray contrast medium,
said specific energy range is set above and below the K-absorption edge of said X-ray contrast medium,
in said thickness computing step, the thickness t I of said object to be inspected is obtained on the basis of a ratio Φ1/Φ2 between the number of transmitted X-rays within a predetermined energy range Φ1 that is smaller than the K-absorption edge of said object to be inspected, and the number of transmitted X-rays within a predetermined energy range Φ2 that is greater than the K-absorption edge of said object to be inspected,
in said image reconstruction step, the thickness of said object to be inspected, which is computed in said thickness computing step, is used as projection data, said projection data and a predetermined reconstruction function are convoluted, and a CT image of said subject is generated through back projection of the convolution result,
the ratio Φ1/Φ2 being given by the following formula:
ln
ϕ
1
ϕ
2
=
1
-
(
μ
1
_
(
E
1
)
-
μ
1
_
(
E
2
)
)
·
t
I
-
(
μ
W
_
(
E
1
)
-
μ
W
_
(
E
2
)
)
·
t
W
.
wherein overbarred μ I (E n ) and overbarred μ W (E n ) are the mean attenuation coefficients of said object to be inspected and water, respectively, for X-rays of an energy range E n (n=1, 2), and t I and t w are the thickness of said object to be inspected and water, respectively.