Apparatus and method for image reconstruction for a synthetic aperture gamma ray imager

1. An imaging system, comprising:

a platform having mounted thereon a coded-aperture imaging device and positioned to receive radiation over a baseline; and

a computer configured to:

acquire a plurality of far-field datasets over the baseline, the plurality of far-field datasets comprising data received via the coded-aperture imaging device;

form a preliminary image based on the acquired plurality of far-field datasets; and

apply an expectation maximization (EM) algorithm to the preliminary image;

wherein the EM algorithm includes an ordered subset algorithm.

2. The imaging system of claim 1 , wherein the computer is configured to form the preliminary image using a single iteration ordered subsets expectation maximization (OSEM) algorithm.

3. The imaging system of claim 1 , wherein the computer, in being configured to apply the EM algorithm, is configured to apply the EM algorithm after formation of the preliminary image to enhance a contrast of the near-field image.

4. The imaging system of claim 1 , wherein the computer is configured to back-project the acquired plurality of far-field datasets to form the preliminary image.

5. The imaging system of claim 4 , wherein the computer, in being configured to back-project, is configured to back-project each far-field dataset in list mode as a single-photon operation.

6. The imaging system of claim 4 , wherein the computer, in being configured to back-project, is configured to aggregate and back-project a subset of the plurality of far-field datasets as a single spatial response.

7. The imaging system of claim 1 , wherein the plurality of far-field datasets are acquired with the platform positioned over N baseline segments along the baseline, wherein N is an integer ≧2.

8. The imaging system of claim 1 , wherein the received radiation is from a gamma source.

9. The imaging system of claim 1 , further comprising a mobile vehicle having the computer and the platform mounted thereon.

10. The imaging system of claim 1 , wherein the position sensitive radiation detector is an Anger camera.

11. The imaging system of claim 1 , wherein the coded-aperture imaging device comprises:

a position-sensitive detector; and

a mask having a defined pattern of apertures therein such that each acquired far-field dataset is a shadow of the mask formed at positions on the detector.

12. The imaging system of claim 11 , wherein the detector includes a two-dimensional array of elements configured to be stimulated in one dimension as a function of azimuthal angle and in a second dimension as a function of elevation angle.

13. A method of imaging comprising:

acquiring, via a position-sensitive detector, a plurality of far-field images, each far-field image comprising a measured distribution of photons on the position-sensitive detector acquired along an imaging baseline;

reconstructing a near-field image using the plurality of far-field images; and

enhancing contrast using at least one iteration of an expectation maximization (EM) reconstruction algorithm;

wherein the far-field images are ordered as subsets of data.

14. The method of imaging of claim 13 , comprising reconstructing the near-field image using a single iteration ordered subsets expectation maximization (OSEM) algorithm.

15. The method of imaging of claim 13 , comprising transporting the position-sensitive detector to a plurality of imaging locations over the imaging baseline; and

wherein the imaging baseline is defined by motion of a vehicle by which the position-sensitive detector is transported.

16. The method of imaging of claim 13 , wherein acquiring the plurality of far-field images comprises acquiring the measured distribution of photons on the position-sensitive detector in both an azimuthal direction and an elevational direction.

17. The method of imaging of claim 13 , comprising reconstructing the near-field image by back-projecting each far-field image in a list mode.

18. The method of imaging of claim 13 , comprising reconstructing the near-field image by back-projecting each far-field image in an aggregated mode from multiple consecutive counts to form a single spatial response.

19. A computer readable storage medium having stored thereon a program, which, when executed by a processor, causes the processor to:

acquire a plurality of real-valued images of a radiation source from a coded-aperture imaging device over a synthetic aperture and located in a far-field of the radiation source;

reconstruct a near-field image of the source using the real-valued images; and

enhance contrast of the reconstructed near-field image using an ordered subsets expectation maximization (OSEM) algorithm.

20. The computer readable storage medium of claim 19 , wherein the program causes the processor to reconstruct the image in one of a single-photon operation and an aggregated operation.

21. The computer readable storage medium of claim 20 , wherein, for C measured counts of a single-photon operation, the list-mode back-projection resulting therefrom is expressed as a sum:

I

⁡

(

x

,

y

)

=

∑

i

=

1

C

⁢

⁢

r

⁡

(

arctan

⁡

(

x

-

x

i

y

-

y

i

)

-

θ

i

,

d

i

)

;

wherein:

(x i ,y i ) is a location of the imaging device at the time of measurement of the i th count;

θ i is the angular orientation of the platform at that moment; and

d i is the detector that recorded the i th count.

22. The computer readable storage medium of claim 20 , wherein, for an aggregated operation, the aggregated back-projection is expressed as a sum:

I

a

⁡

(

x

,

y

)

=

∑

s

=

1

S

⁢

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∑

i

=

1

C

s

⁢

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r

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(

arctan

⁡

(

x

-

x

s

y

-

y

s

)

-

θ

s

,

d

i

)

;

wherein

S is the number of baseline segments defined for the aggregation scheme;

C s is the number of counts in each segment s;

(x s ,y s ) is the nominal location of the segment, defined by:

x

s

=

x

∑

n

=

1

s

-

1

⁢

⁢

C

n

+

x

∑

n

=

1

s

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C

n

2

⁢

⁢

and

⁢

⁢

y

s

=

y

∑

n

=

1

s

-

1

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C

n

+

y

∑

n

=

1

s

⁢

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C

n

2

;

 and

θ s is the orientation angle of segment “s.”

23. The computer readable storage medium of claim 19 , wherein the synthetic aperture is defined by a plurality of positions from which the imaging device acquires the plurality of real-value images.

24. The computer readable storage medium of claim 19 , wherein the program causes the processor to reconstruct the image using a single iteration OSEM algorithm.