Medical image processing apparatus and medical image processing method
There are a medical image processing apparatus and a medical image processing method that are capable of maintaining the estimation accuracy of noise intensity even though a system noise ratio rises. A medical image processing apparatus includes a difference image generating unit that divides projection data obtained by applying radiation to an examinee to create a difference image between tomographic images reconstructed for every divided piece of projection data; a local variance computing unit that computes local variance in the difference image; and a noise estimation unit that corrects the local variance using a correction function found in advance to estimate noise intensity of a tomographic image of the examinee. The correction function includes system noise.
1. A medical image processing apparatus comprising:
a difference image generating unit that divides projection data obtained by applying radiation to an examinee to create a difference image between tomographic images reconstructed for every divided piece of projection data;
a local variance computing unit that computes local variance in the difference image; and
a noise estimation unit that corrects the local variance using a correction function, the correction function having been found in advance, to estimate noise intensity of a tomographic image of the examinee,
wherein the correction function includes system noise.
2. The medical image processing apparatus according to claim 1 , wherein when the local variance is VarD, the noise intensity is VarC, a distance from a rotation center of a scanner is s, and the correction function is α(s) and β(s), the noise intensity VarC is computed from
VarC
=
VarD
α
(
s
)
+
α
(
s
)
2
+
β
(
s
)
·
VarD
.
3. The medical image processing apparatus according to claim 2 , wherein the correction functions α(s) and β(s) are found for each of imaging conditions based on projection data of a phantom filled with a uniform substance.
4. The medical image processing apparatus according to claim 1 , wherein the noise estimation unit creates a noise intensity map by mapping the noise intensity, and displays the noise intensity map together with a tomographic image of the examinee.
5. The medical image processing apparatus according to claim 4 , wherein a noise reduction process is applied to the tomographic image of the examinee based on noise intensity of a ROI set in the tomographic image of the examinee.
6. The medical image processing apparatus according to claim 1 , wherein the system noise is noise generated in a detector circuit in a course of detection of the radiation to conversion of a digital signal.
7. A medical image processing apparatus comprising:
a difference image generating unit that divides projection data obtained by applying radiation to an examinee to create a difference image between tomographic images reconstructed for every divided piece of projection data;
a local variance computing unit that computes local variance in the difference image; and
a noise estimation unit that corrects the local variance using a correction function found in advance to estimate noise intensity of a tomographic image of the examinee,
wherein the correction function includes system noise, and
wherein the system noise is noise generated in a detector circuit in a course of detection of the radiation to conversion of a digital signal.
8. The medical image processing apparatus according to claim 7 , wherein when the local variance is VarD, the noise intensity is VarC, a distance from a rotation center of a scanner is s, and the correction function is α(s) and β(s), the noise intensity VarC is computed from
Var
C
=
Var
D
α
(
s
)
+
α
(
s
)
2
+
β
(
s
)
·
Var
D
.
9. The medical image processing apparatus according to claim 8 , wherein the correction functions α(s) and β(s) are found for each of imaging conditions based on projection data of a phantom filled with a uniform substance.
10. The medical image processing apparatus according to claim 7 , wherein the noise estimation unit creates a noise intensity map by mapping the noise intensity, and displays the noise intensity map together with a tomographic image of the examinee.
11. The medical image processing apparatus according to claim 10 , wherein a noise reduction process is applied to the tomographic image of the examinee based on noise intensity of a ROI set in the tomographic image of the examinee.
12. A medical image processing method comprising:
a difference image creating step in which projection data obtained by applying radiation to an examinee is divided to create a difference image between tomographic images reconstructed for every divided piece of projection data;
a local variance computing step wherein local variance is computed in the difference image; and
a noise estimating step wherein the local variance is corrected using a correction function, the correction function having been found in advance, to estimate noise intensity of a tomographic image of the examinee,
wherein the correction function includes system noise.
13. The medical image processing method according to claim 12 , wherein the system noise is noise generated in a detector circuit in a course of detection of the radiation to conversion of a digital signal.
14. The medical image processing method according to claim 12 , wherein when the local variance is VarD, the noise intensity is VarC, a distance from a rotation center of a scanner is s, and the correction function is α(s) and β(s), the noise intensity VarC is computed from
Var
C
=
Var
D
α
(
s
)
+
α
(
s
)
2
+
β
(
s
)
·
Var
D
.
15. The medical image processing method according to claim 14 , wherein the correction functions α(s) and β(s) are found for each of imaging conditions based on projection data of a phantom filled with a uniform substance.
16. The medical image processing method according to claim 12 , further comprising:
creating a noise intensity map by mapping the noise intensity; and
displaying the noise intensity map together with a tomographic image of the examinee.
17. The medical image processing method according to claim 16 , further comprising:
applying a noise reduction process to the tomographic image of the examinee based on noise intensity of a ROI set in the tomographic image of the examinee.