X-ray CT system to generate tomographic phase contrast or dark field exposures
View Patent ↗An x-ray CT system that generates tomographic phase contrast or dark field exposures, has at least one grating interferometer with three grating structures arranged in series in the radiation direction, with a modular design of the second and third grating structures. The distance between the first grating structure of the x-ray source and the second grating structure (fashioned as a phase grating) of the respective grating/detector modules is adapted, depending on the fan angle, corresponding to a period of the grating structure of the x-ray source projected onto the grating detector module at a respective fan angle (φ i ).
1. An x-ray CT system comprising:
a source/detector system that generates tomographical phase contrast or dark field exposures, said source/detector system being mounted on a gantry rotatable around a system axis,
said source/detector system comprising an x-ray source having a grating structure that emits bands of emission maxima and emission minima of a generated x-ray radiation arranged like a grating, with a grating period (p 0 ), that emits a beam fan of x-rays widening in two places with a max fan angle (2xφ max ) in the rotation plane of the gantry and a Z-angle (2xξ max ) perpendicular to the rotation plane of the gantry;
said source/detector system comprising a detector arrangement with a plurality of grating/detector modules arranged in parallel, each module comprising, one after another in the beam direction, at least one phase grating (G 1i ) that generates an interference pattern, an analysis grating (G 2i ) with a directly connected detector with a plurality of detector elements to determine phase (x 0 ), average radiation intensity (I med ) and amplitude (I amp ) of the average intensity of the radiation per detector element given relative displacement of one of the upstream grating structures, the grating lines of all grating structures (G 0 , G 1i , G 2i ) being aligned parallel to one another and parallel to the system axis; and
a distance (I) between the grating structure (G 0 ) of the x-ray source and the phase grating (G 1,i ) of the respective grating/detector modules being adapted depending on the fan angle corresponding to a period (p 0i ') of the grating structure (G 0 ) of the x-ray source that is projected onto the grating/detector module at the respective fan angle (φ i ).
2. An x-ray CT system according to claim 1 , wherein the grating structure (G 0 ) of the x-ray source is flat, and wherein the grating/detector modules are arranged, with regard to the distance (I i ) between the grating structure (G 0 ) of the x-ray source and the phase grating (G 1i ), with
I 0 =I 0 * cos(φ i ),
wherein I 0 corresponds to the distance between the x-ray source (G 0 ) and the phase grating (G 10 ) at the fan angle φ 0 =0°.
3. An x-ray CT system according to claim 1 wherein the grating periods (p 1 , p 2 ) of the grating structures (G 1 , G 2 ) satisfy the following condition per grating/detector module, depending on the fan angle (φ 1 ):
p
1
(
φ
i
)
=
2
p
2
1
+
p
2
p
0
cos
(
φ
i
)
,
wherein p 0 corresponds to the grating period of the grating structure G 0 of the x-ray source, p 1 (φ i ) corresponds to the grating period of the i-th phase grating G 1,i at the fan angle φ i , and p 2 corresponds to the grating period of the analysis grating G 2 .
4. An x-ray CT system according to claims 1 wherein the distance (d(φ i )) between phase grating (G 1,i ) and analysis grating (G 2,i ) satisfies the following condition per grating/detector module, depending on the fan angle (φ i ):
d
(
φ
i
)
=
1
0
D
cos
(
φ
i
)
1
0
cos
(
φ
i
)
-
D
,
with
D
=
p
1
2
8
λ
wherein I 0 corresponds to the distance between the grating structure (G 0 ) of the x-ray source and the phase grating (G 1,0 ) at the fan angle φ 0 =0°, p 1 corresponds to the grating period of the phase grating (G 1,i ) and λ corresponds to the wavelength of the x-ray radiation to which the system is tuned.
5. An x-ray CT system according to claim 1 wherein the detector of the source/detector system comprises a row of grating/detector modules aligned in the direction of the fan angle.
6. An x-ray CT system according to claim 1 wherein the detector of the at least one source/detector system comprises at least two rows of grating/detector modules lined up in the direction of the fan angle.
7. An x-ray CT system according to claim 1 wherein the grating structure (G 0 ) of the x-ray source comprises a flat focal spot, and an absorption grating in the beam path.
8. An x-ray CT system according to claim 1 wherein the grating structure (G 0 ) of the x-ray source comprises a plurality of focal spots formed in bands and arranged parallel to one another.
9. An x-ray CT system according to claim 8 , wherein the x-ray source comprises a rotating anode having a rotating body with a cylindrical surface as an anode surface having a plurality of depressions therein, said rotation body of the rotating anode consisting of a first material, and a second material overlaid on the surface in the region of the depressions.
10. An x-ray CT system according to the claim 9 , wherein the second material has a higher mass number than the first material.
11. An x-ray CT system according to the claim 10 , wherein the second material is tungsten.
12. An x-ray CT system according to claim 9 wherein the depressions are aligned parallel to, or with a component parallel to, the rotation axis of the rotating anode, and comprising a synchronization unit that synchronizes scanning of the detector with rotation of the rotating anode to cause scanning of the detector and thus an integration interval of a measurement to occur when the rotating depressions of the rotating anode are respectively in congruent position.
13. An x-ray CT system according to claim 12 , comprising a rotation rate sensor that triggers scanning of the detector, mounted on the rotating anode.