Method for linearizing attenuation measurements taken by a spectrometry sensor

Patent metadata

Field	Value
Publication number	US-10162069-B2
Application number	US-201514677293-A
Country	US
Kind code	B2
Filing date	Apr 2, 2015
Priority date	Apr 7, 2014
Publication date	Dec 25, 2018
Grant date	Dec 25, 2018

Abstract

The invention relates to a method for linearizing attenuation measurements obtained by means of a direct conversion spectrometer. The spectrometer comprises a radiation source and a detector for detecting said radiation after it has passed through an object. An attenuation measurement is represented by a vector M d giving the attenuation of the radiation in a plurality N k of energy channels of the detector, said spectrometer being characterized by a response matrix Ψ. Said method estimates, by means of an iterative process, a vector M lin , called an equivalent linear attenuation vector, giving for each energy channel an attenuation linearly depending on the thickness of the material through which the radiation passes. Said method is applicable to the characterization of the material as well as to the reduction of artifacts in computed tomography.

First claim

The invention claimed is:

1. A method for measuring the attenuation of a radiation by generating an image of an object, comprising:
irradiating the object with a radiation source;
detecting with a direct conversion spectrometer that includes a detector, radiation emitted from the radiation source after it has passed through the object, said detection being carried out in a plurality N k of energy channels of the detector, actual attenuation of the material constituting the object being resolved according to a characteristic base of the attenuation in the material, μ n , n=1, . . . , N with N≥2;
said direct conversion spectrometer being characterized by a response matrix Ψ giving, for the plurality N k of energy channels of the detector, the probability that a photon emitted in an energy bin h=1, . . . , N h is detected in an energy channel,
said method including estimating a vector M lin , called an equivalent linear attenuation vector, giving for each energy channel an attenuation linearly depending on the thickness of the material through which the radiation passes, said method comprising
an initialization step wherein M lin is estimated by a vector M d giving an attenuation of the radiation in the plurality of energy channels,
and a succession of iterations, each iteration j providing an estimate {circumflex over (M)} lin (j) and comprising:
(a) a step of projecting the estimate {circumflex over (M)} lin (j−1) of M lin obtained at a previous iteration onto an image base Ψμ n , n=1, . . . , N, image by said response matrix Ψ of said characteristic base of the attenuation of the material, the projection of {circumflex over (M)} lin (j−1) onto the image base Ψμ n , n=1, . . . , N being performed by determining components ĉ 1 , . . . , ĉ N such that:

M
^

lin

⁡

(

j
-
1

)

=

∑

n
=
1

N

⁢

c
^

n

⁡

(
j
)

⁢
Ψ
⁢

⁢

μ
n

=

A
μ

⁢

c
^

⁡

(
j
)

;

(b) a step of determining an energy non linear deformation, T, of the components of the estimate {circumflex over (M)} lin (j−1) for obtaining a corresponding attenuation M r d (j) in different energy channels, in accordance with a non linear model of the spectrometer;
(c) a step of reverse deformation of the components of M d to provide a new estimate {circumflex over (M)} lin (j) of the equivalent linear attenuation M lin of said attenuation measurement, or of the components ĉ 1 , . . . , ĉ N of said equivalent linear attenuation M lin in said image base,
wherein the estimate of the equivalent linear attenuation M lin of said attenuation measurement, or the estimate of the components thereof, are provided as attenuation measurements, and
the method includes generating the image of the object based on at least the detected radiation and on the new estimate {circumflex over (M)} lin (j) of the equivalent linear attenuation M lin of said attenuation measurement.

2. The method for measuring the attenuation according to claim 1 , wherein the iterations are stopped when a predetermined number (j max ) of iterations is reached.

3. The method for measuring the attenuation according to claim 1 , wherein the iterations are stopped when a convergence criterion of the estimate of the equivalent linear attenuation is met.

4. The method for measuring the attenuation according to claim 1 , wherein the characteristic base of the material is a base of vectors μ Co , μ Ph where μ Co gives a linear attenuation coefficient of the radiation due to the Compton effect in the different energy bins and μ Ph gives the attenuation coefficient of the radiation due to the photoelectric effect in the different energy bins, a vector of actual attenuation coefficients μ i of the material in the different bins being obtained as a linear combination of the vectors μ Co , μ Ph .

5. The method for measuring the attenuation according to claim 1 , wherein the characteristic base of the material is a base of vectors μ n , n=1, . . . , N, relating to reference materials, each vector of this base giving actual attenuation coefficients in the different energy bins for a reference material.

6. The method for measuring the attenuation according to claim 1 , wherein in step (a) of an iteration j, a vector ĉ j of size N is determined giving the components of {circumflex over (M)} lin (j−1) in the image base Ψμ n , n=1, . . . , N.

7. The method for measuring the attenuation according to claim 6 , wherein the non-linear transformation T is determined by estimating actual attenuation in the material from {circumflex over (M)} i (j)=B μ ĉ j , where B μ is a matrix the columns of which consist of vectors μ n , n=1, . . . , N, and by calculating a diagonal matrix W j d such that W j d {circumflex over (M)} lin (j−1)=M r d (j) where M r d (j)=−ln(Ψ e −B μ ĉ j ).

8. The method for measuring the attenuation according to claim 7 , wherein in step (c) of the iteration j, the new estimate {circumflex over (M)} lin (j) of the equivalent linear attenuation is obtained by means of {circumflex over (M)} lin (j)=(W j d ) −1 M d .

9. The method for measuring the attenuation according to claim 5 , further providing a characterization of the material from the components ĉ 1 , . . . , ĉ N of said equivalent linear attenuation vector M lin in said image base.

10. The method for measuring the attenuation according to claim 9 , said characterization is a composition of the material in said reference materials.

11. A system for measuring the attenuation of a radiation by an object, comprising:
a radiation source configured to irradiate the object; and
a direct conversion spectrometer that includes a detecto

Assignees

• Commissariat Energie Atomique
• Commissariat Energie Atomique

Inventors

• Popa Emil
• Rebuffel Veronique

Classifications

• G06T12/10

  Image preprocessing, e.g. calibration, positioning of sources or scatter correction · CPC title

• G01T7/005 Primary

  calibration techniques (stabilization of spectrometer G01T1/40) · CPC title

• G06T11/005

  Physics · mapped topic